Search

Ionospheric electron density (IED) and vertical total electron content (VTEC) are two of the most important characteristics that interpret the ionosphere. Today, satellite geodesy plays an important role in providing these data. Apart from monitoring the ionosphere by means of Global Navigation Satellite System observations, predictions of these ionospheric characteristics have also been taken into consideration. The majority of prediction methods in the ionosphere are focused on VTEC prediction. The continuity equation in the ionosphere is a spatiotemporal partial differential equation that can be used to predict both IED and VTEC. In this study, we address this issue during May 8, 2016, which was a day of high geomagnetic activity. For this purpose, first, high-accuracy IED grids with a spatial resolution of 0.5° × 0.5° in longitude and latitude, 50 km in altitude, and a temporal resolution of 10 min are prepared. These IED grids are called analysis grids and are derived by assimilation of GPS-derived VTECs into the background IED grids provided by international reference ionosphere. Second, the analysis grids can be utilized as the initial and boundary values in the continuity equation to predict IED for the next 3 h with a step of 10 min. The analysis grids are constructed over the Iran region, and the performance of the continuity equation in the ionosphere prediction is investigated during the strongest geomagnetic storm of 2016. Finally, the accuracy of the predictions has been evaluated in two ways. First, analysis grids with high accuracy are available for each epoch in which the prediction is presented. On average, the root mean square (RMS) of the differences between the predicted IEDs and the analysis IEDs is 1.66 × 1010 el/m3 for 1-h predictions and 5.33 × 1010 el/m3 for 3-h predictions. Second, by numerical integration of the predicted grids in the vertical direction, VTECs are estimated and compared with VTEC values of test data that were already known. On average, the RMS of their difference was 1.29 total electron content unit (TECU) for 1-h predictions and 2.57 TECU for 3-h predictions. An acceptable accuracy for both IED and VTEC prediction by a continuity equation has been obtained up to 3 h on the most active ionospheric day. [ABSTRACT FROM AUTHOR]

The global navigation satellite system (GNSS) ionospheric tomography technique can reconstruct the 3D images of ionospheric electron density (IED) in response to geomagnetic storms. In a common practice, the absolute IED values are first reconstructed with tomography and then the IED disturbances are calculated for the study of geomagnetic storm. However, the ionosphere disturbances derived from the conventional method are often contaminated by the initial value error and the slant total electron content (STEC) uncertainty. Therefore, this paper proposes an improved ionospheric tomography method for a direct inversion of global IED disturbance. In this improved method, IED disturbances are tomographically reconstructed from the zero initial value with detected STEC residuals. The global ionosphere maps (GIMs) released by the Center for Orbit Determination in Europe (CODE) are used to generate virtual STEC residuals for compensating the uneven and sparse distribution of the global GNSS network. The success rates for disturbance detection of the improved method under different simulation conditions are in the range of 80–90 %. The ionosphere shows a strong reaction to geomagnetic storms with significant IED disturbances in the altitude of 100–600 km. Based on the tomographic IED disturbance values, this paper fully analyzes the evolution of global IED disturbances during the geomagnetic storm on 25–26 August 2018. The ionospheric disturbances are slightly delayed compared with the occurrence of the geomagnetic storm. IED disturbances first appeared in the polar and low latitude regions at the altitude of 200–400 km and gradually expanded worldwide. As the geomagnetic storm reached the peak, the ionospheric disturbances were at their strongest with duration of 3–5 h. In general, the improved tomography method can retrieve the accurate global IED disturbances and clearly show positive and negative disturbances caused by the geomagnetic storm. [ABSTRACT FROM AUTHOR]

Ionospheric topside O+ ${O}^{+}$ diffusive flux is derived using Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) radio occultation data, to investigate its global distribution and also its role in winter nighttime enhancement (WNE) of electron density. The flux of the winter hemisphere maintains downward throughout the night. It is much larger between 30° $30{}^{\circ}$ and 50° $50{}^{\circ}$ geomagnetic latitudes and keeps increasing until 22:00–00:00 LT. It peaks at 60° $60{}^{\circ}$W and 60° $60{}^{\circ}$E–120° $120{}^{\circ}$E geographic longitudes during the December solstice, and at 180° $180{}^{\circ}$E during the June solstice. These features are similar to those of WNE in NmF2. Furthermore, the derived flux is applied as the upper boundary condition to run the Thermosphere Ionosphere Electrodynamics General Circulation Model (TIEGCM). The simulated spatial‐temporal variations of WNE are consistent with the observations. The results indicate that downward plasma diffusion from the plasmasphere is the major mechanism of WNE, and the simulation quantifies its contribution. Plain Language Summary: Solar radiation ionizes the atmosphere to produce the ionosphere. However, ionospheric electron density in the midlatitude of the winter hemisphere has been observed to increase at night with the absence of photoionization, which is referred to as winter nighttime enhancement (WNE). Many studies have suggested that the plasma causing WNE comes from the overlying plasmasphere via downward diffusion, but so far the global distribution of ionospheric topside diffusive flux has not been systematically examined because it cannot be measured directly. In this study, the topside O+ ${O}^{+}$ diffusive flux is derived based on observational data. The flux is downward at night and varies with geographical location and local time. These characteristics are similar to those of WNE. Furthermore, the theoretical model Thermosphere Ionosphere Electrodynamics General Circulation Model (TIEGCM) is used to conduct a modeling of WNE, with the upper boundary condition modified by incorporating the derived diffusive flux. WNE is well reproduced, providing direct evidence that downward plasma diffusion is the major mechanism of WNE. This study provides new insight into the physical processes in the nighttime ionosphere, and has implication for future development and improvement of ionospheric models. Key Points: Global distribution of ionospheric topside O+ diffusive flux is derived for the first time using COSMIC radio occultation dataThe Thermosphere Ionosphere Electrodynamics General Circulation Model simulation driven by the derived flux effectively reproduces the midlatitude electron density enhancement in winter nighttimeFirst global‐scale evidence indicating downward plasma diffusion as the dominant mechanism for electron density enhancement is provided [ABSTRACT FROM AUTHOR]

We report on the recent earthquakes (EQs) that occurred, with the main shock on 6 February 2023, principally in the central southern part of Turkey and northwestern Syria. This region is predisposed to earthquakes because of the tectonic plate movements between Anatolian, Arabian, and African plates. The seismic epicenter was localized at 37.08°E and 37.17°N with depth in the order of 10 km and magnitude Mw7.8. We use Graz's very-low-frequency VLF facility (15.43°E, 47.06°N) to investigate the amplitude variation in the Denizköy VLF transmitter, localized in the Didim district of Aydin Province in the western part of the Anatolian region in Turkey. Denizköy VLF transmitter is known as Bafa transmitter (27.31°E, 37.40°N), radiating at a frequency of 26.7 kHz under the callsign TBB. This signal is detected daily by the Graz facility with an appropriate signal-to-noise ratio, predominantly during night observations. We study in this analysis the variations of TBB amplitude and phase signals as detected by the Graz facility two weeks before the earthquake occurrence. It is essential to note that the TBB VLF transmitter station and the Graz facility are included in the preparation seismic area, as derived from the Dobrovolsky relationship. We have applied the multi-terminators method (MTM), revealing anomalies occurring at sunset and sunrise terminator occasions and derived from the amplitude and the phase. Minima and maxima of the TBB signal are linked to three terminators, i.e., Graz facility, TBB transmitter, and EQ epicenter, by considering the MTM method. We show that the significant anomalies are those linked to the EQ epicenter. This leads us to make evident the precursor seismic anomaly, which appears more than one week (i.e., 27 January 2023) before EQ occurrence. They can be considered the trace, the sign, and the residue of the sub-ionospheric propagation of the TBB transmitter signal disturbed along its ray path above the preparation EQ zone. We find that the sunrise–sunset anomalies are associated with tectonic regions. One is associated with the Arabian–African tectonic plates with latitudinal stresses in the south–north direction, and the second with the African–Anatolian tectonic plates with longitudinal stresses in the east–west direction. The terminator time shift anomalies prior to EQ are probably due to the lowering (i.e., minima) and raising (i.e., maxima) of the ionospheric electron density generated by atmospheric gravity waves. [ABSTRACT FROM AUTHOR]

A novel model called PyIRI was recently developed. It constructs the ionospheric electron density for the entire day and on the entire global grid in one computation, which has a very low computational overhead. PyIRI introduced a novel approach to the computation of the global and diurnal functions and their matrix multiplication with Consultative Committee on International Radio (CCIR) coefficients or the International Union of Radio Science (URSI) coefficients, that enabled this global approach for the density specification. Since the International Reference Ionosphere‐based Real‐Time Assimilative Model (IRTAM) produces coefficients in a similar format as CCIR/URSI coefficients, the PyIRI computational approach was extended to work with IRTAM coefficients. This technical note describes the PyIRTAM software and provides usage examples. The PyIRTAM tool is made publicly available through PyPI and GitHub. Plain Language Summary: A novel Python module called PyIRTAM was developed in addition to the previously released PyIRI model. The PyIRI computational approach was extended to work with Real‐Time Assimilative Model (IRTAM) coefficients. This enables global approach to the density specification, utilizing computation of the global and diurnal functions and their multiplication with coefficients in the matrix form. PyIRTAM tool is made publicly available through PyPI and GitHub. Key Points: Global approach for the IRTAM coefficients was implemented in PythonPython tool for making rapid global ionospheric electron density estimates was developedPyIRTAM calculates 24‐hr global electron densities from IRTAM coefficients in a few seconds [ABSTRACT FROM AUTHOR]

Due to factors such as the uneven distribution of ground receiving stations and the lack of effective observation rays, Global Positioning System (GPS)‐based computerized ionospheric tomography (CIT) is typically of low quality and requires additional data sources. Recently, Faraday rotation angle (FRA) retrieval using the Phased Array L‐band Synthetic Aperture Radar (PALSAR) full‐pol data have emerged as a reliable technique for ionospheric detection. Similar to the total electron content (TEC), the FRA is the integral effect of the electron density and geomagnetic field, with the geomagnetic field being accurately estimated by the International Geomagnetic Reference Field (IGRF) model. Therefore, this paper proposes a 3‐D secondary CIT algorithm by integrating PALSAR and GPS data: first, the product of the electron density values obtained from GPS‐based CIT and the magnitude of geomagnetic field in corresponding voxel obtained from IGRF is used as the initial value. Then, the iterative algorithm is improved by using the FRA obtained from PALSAR data, rather than TEC, as the input for the second iteration, avoiding the approximation error caused by converting FRA into TEC. The geomagnetic field information is then separated by using the IGRF model, and the reconstructed spatial distribution is finally obtained. Experimental verification shows that the FRA can compensate for the lack of GPS observation rays to a certain extent and improve the accuracy of the reconstructed electron density. The results also indicate that the PALSAR can provide an effective and feasible data source for CIT. Plain Language Summary: The paper proposes a 3‐D secondary CIT algorithm by integrating Phased Array L‐band Synthetic Aperture Radar (PALSAR) and Global Positioning System (GPS) data: first, the product of the electron density values obtained from GPS‐based CIT and the magnitude of geomagnetic field in corresponding voxel obtained from IGRF is used as the initial value. Then, the iterative algorithm is improved by using the FRA obtained from PALSAR data, rather than TEC, as the input for the second iteration. This avoids the approximation error caused by converting FRA into TEC. The geomagnetic field information is then separated by using the IGRF model, and the reconstructed spatial distribution is finally obtained. Experimental verification shows that the FRA can compensate for the lack of GPS observation rays to a certain extent and improve the accuracy of the reconstructed electron density. This indicates that PALSAR data can provide an effective and feasible data source for CIT. Key Points: A 3‐D secondary computerized ionospheric tomography algorithm by integrating Phased Array L‐band Synthetic Aperture Radar and Global Positioning System (GPS) data is proposed for the first timeExperimental verification shows that the Faraday rotation angle can compensate the lack of GPS observation rays and improve the accuracy of the reconstructed [ABSTRACT FROM AUTHOR]

Altitude variation of the topside ionospheric electron density or its scale height has been widely investigated in the past. However, as the oxygen ion (O+) is an important indicator for separating the topside ionosphere and plasmasphere, the altitude variation of O+, which has not been well investigated, is crucial to understand the topside ionosphere. In this study, we provided analysis on how the O+ scale height varies with altitude under different solar and geomagnetic activities, by using 12‐year measurements from the incoherent scatter radar (ISR) located at Jicamarca. Constant scale height Chapman (CSC) as well as Linearly Varying Chapman (LVC) functions are used to reconstruct the O+ profile. The corresponding scale heights of O+ based on both approaches have been compared. The O+ profile derived from LVC function shows better agreement with the ISR measurements than that from CSC function. We found that the O+ scale height increases with increasing solar/geomagnetic activity, and its height gradient varies significantly with local time, reaching a maximum of 0.05 at sunrise (around 06:00 local time) and a minimum of about −0.08 at noon (around 12:00 LT). We further investigated possible drivers causing the O+ scale height variations at topside ionosphere, based on simulations from the SAMI2 physics‐based model. The model results show that the solar extreme ultra‐violet (EUV) radiation plays a key role in the positive gradient of O+ scale height observed around sunrise, while the vertical plasma drift caused by E × B significantly contributes to the negative gradient observed around noon. Plain Language Summary: The topside ionosphere consists mainly of O+, H+ and He+ ions, and the proportion of these ions varies at different altitudes. The O+ ions are important for distinguishing between the upper ionosphere and the plasmasphere which contains mainly light ions. It is therefore critical to comprehend how O+ concentrations change with altitude. While in the past very few studies have been performed focusing on altitude variation of O+, the characteristics and the potential physical mechanisms are poorly understood. Using Jicamarca ISR measurements and simulation results from SAMI2 model, we found that the O+ ion scale height varies linearly with altitude. The effects of solar UV radiation and E × B induced vertical plasma drift are critical for the altitudinal variation of O+ scale height. This study will significantly enhance our understanding of the topside ionospheric physical processes, to improve the development of the empirical models for the topside ionosphere forecasting. Key Points: Scale height of O+ in the topside ionosphere has been investigated comprehensively based on ISR measurements over one solar cycleReconstructed O+ profile from LVC function agrees better with observations than that from commonly used CSC functionSimulations from SAMI2 show that solar EUV radiation and E × B drift are the main contributors to cause altitude variation of O+ [ABSTRACT FROM AUTHOR]

Undifferenced and Uncombined Precise Point Positioning (UPPP) currently stands as a prominent research area, where the integration of high-precision ionospheric products holds the potential to substantially enhance accuracy and convergence performance in UPPP. Presently, the majority of external ionospheric constraints for UPPP rely on Global Ionospheric Maps (GIM). However, the accuracy and resolution of GIM fall short, imposing significant limitations on the positioning performance of UPPP. Consequently, this paper introduces the application of Computerized Ionospheric Tomography based on Machine Learning (CIT-ML) to enhance UPPP performance (PPP-CIT-ML). In this approach, we convert the three-dimensional electron density of CIT-ML into vertical total electron content (VTEC), and then it is compiled into ionospheric grid files essential for UPPP. Simultaneously, the traditional ionospheric tomography methods based on the improved algebraic reconstruction technique (IART) is also employed for UPPP (PPP-CIT-IART), alongside the ionospheric grid files broadcasted by the Center for Orbit Determination in Europe (CODE) for UPPP (PPP-CODE), where PPP-CIT-IART and PPP-CODE are used as the reference methods to test the performance of PPP-CIT-ML. In Static and Kinematic UPPP, compared to PPP-CODE, PPP-CIT-IART demonstrated average improvements in positioning accuracy and convergence performance by over 10% and 7%, respectively. PPP-CIT-ML showed average improvements in positioning accuracy and convergence performance by over 26% and 28%, respectively. The extrapolated ionospheric electron density (IED) applied to UPPP (ECIT-ML-PPP) and compared with PPP-CODE displayed average improvements in positioning accuracy by over 21% and convergence performance by over 26%. Compared with PPP-CIT-IART, ECIT-ML-PPP displayed average improvements in positioning accuracy and convergence performance by over 6% and 21%, respectively. These findings highlight that ionospheric error correction information obtained through ionospheric tomography significantly enhances the positioning accuracy and convergence performance of UPPP. Moreover, the performance enhancement achieved through machine learning-based ionospheric tomography is more pronounced. This study provides preliminary validation for the feasibility of applying machine learning-based ionospheric tomography results to navigation positioning. [ABSTRACT FROM AUTHOR]

During earthquake preparation, a seismogenic electric field is generated on the Earth's surface, which then penetrates the atmosphere, reaching several hundred kilometers above the lithosphere and causing perturbations in the ionospheric electron density. These seismo-ionospheric anomalies are typically identified using statistical techniques, such as running averages or inter-quartile range (IQR), with the anomalous zones analyzed from Global Ionospheric Maps (GIMs). This study used ASK-VTEC technique for detecting preseismic signatures, based on the daily mean of vertical total electron content (AVTEC) and its standard deviation (SVTEC). The spatial distribution of vertical electron density (VTEC) is mapped using Ordinary Kriging (OrK) interpolation instead of GIMs. To validate and assess the performance of this approach, the M9.0 Tohoku earthquake, which occurred on March 11, 2011, was chosen as a case study. The results show that ionospheric anomalies were observed on May 8, 2011, three days before the earthquake. A significant anomalous zone was detected southwest of the epicenter between 06:00 and 10:00 UTC. These findings are consistent with previous studies, and further, the VTEC spatial distribution maps generated by the OrK interpolation technique outperformed GIMs on a local scale. Additionally, the analytical process is less complex compared to conventional methods. Therefore, the pre-seismic signature detection technique proposed in this study offers a viable alternative for investigating VTEC anomalies prior to earthquake events. [ABSTRACT FROM AUTHOR]

This study aims to examine the impact of lower atmospheric forcing on upper atmospheric variability using the Thermosphere‐Ionosphere‐Electrodynamics General Circulation Model (TIEGCM). We conducted numerical experiments comparing induced variability due to Hough Mode Extension (HME) tides constrained by winds and temperatures from Ionospheric Connection Explorer‐Michelson Interferometer for Global High‐Resolution Thermospheric Imaging (ICON‐MIGHTI) observations. Our model comparisons focus on the changes in the composition of the thermosphere‐ionosphere and the delayed ionospheric response to the 27‐day solar EUV flux variations during periods of low solar activity. We report the results of model simulations with and without tidal forcing at the approximate 97 km lower boundary of the TIEGCM. The differences led to changes in thermosphere‐ionosphere parameters such as electron density, peak electron density, and the O/N2 $O/{N}_{2}$ ratio. The results show that the impact of tidal forcing is mainly observed in the low‐ and mid‐latitude regions, affecting the correlation between O/N2 $O/{N}_{2}$ and NmF2. This change in correlation affects the amount of ionospheric delay. When tidal forcing is included, the modeled delay improves compared to the observed delay during low solar activity. The spatial variation of ionospheric delay due to induced tidal effects highlights the importance of understanding lower atmospheric forcing in thermosphere‐ionosphere models. This is crucial for predicting and understanding the ionospheric response to solar flux. Key Points: The variations in thermospheric‐ionospheric parameters caused by tidal forcing have been examined using TIEGCM simulationsLower atmospheric forcing influences the ionospheric delay during low solar activityAt the 27‐day time scale, the ionospheric delay in electron density against solar flux variations is about 19 hr [ABSTRACT FROM AUTHOR]

Fluctuations in the ionospheric electron density cause distortions in the Global Navigation Satellite Systems (GNSS) signals recorded on the ground. The examination of these distortions reveal some of the physical conditions under which the electron density fluctuations develop as well as their physical characteristics. Several studies have investigated the correlation between the rate of change of the total electron content (ROTI) $(ROTI)$ and amplitude and phase scintillation indices S4 ${S}_{4}$ and σΦ ${\sigma }_{{\Phi }}$, respectively. These studies stipulate that ROTI $ROTI$ could be used as a proxy for scintillation indices. The link between the scintillation indices and the variations in TEC $TEC$ is investigated both theoretically and empirically. Our study shows that the second derivative (the Laplacian) of the TEC $TEC$ provides a better diagnosis of the nature of the interaction of trans‐ionospheric radio signals with ionospheric irregularities. In the refractive case, the second derivative of TEC $TEC$ fluctuations vanishes. In the diffractive limit, we show that the amplitude scintillation index and the standard deviation of the second derivative of TEC $TEC$ are linearly dependent. The theoretical results are empirically validated with measurements of GNSS radio signals propagating through the auroral ionospheric region and recorded by ground receivers of the Canadian High Arctic Ionospheric Network (CHAIN). The present study suggests that the use of ROTI $ROTI$ as a proxy for scintillation occurring in the polar and auroral regions must be taken with caution. Key Points: Refraction and diffraction of trans‐ionospheric radio signals is empirically examinedThe amplitude scintillation index and the standard derivation of the second derivative of the Total Electron Content are linearly dependentA zero laplacian of the Total Electron Content is an indication of refraction [ABSTRACT FROM AUTHOR]

The Martian ionosphere‐thermosphere (I‐T) coupling is variable due to complex variations of the driving factors such as atmospheric tides and crustal magnetic fields. In this study, variability of the I‐T coupling in longitude structures was investigated using a series of data segments of the MGS ionospheric measurements. Measurements in each data segment can cover different longitudes, and the solar forcing and local solar time just change a little. Ionospheric and thermospheric longitude variations are statistically correlated. Ionospheric peak electron density (NmM2) decreases while ionospheric main peak height (hmM2) increases with increasing neutral scale height (Hn) along longitudes. These correlated longitude variations are consistent with the photochemical coupling that Hn longitude disturbances induce ionospheric longitude structure through photochemical processes. Statistically, NmM2 is a better indicator than hmM2 for the Hn disturbances in the lower thermosphere. Hn longitude variation intensity is a crucial factor affecting the photochemical I‐T coupling in longitude structures; it is closely related to NmM2 longitude variation intensity and tends to decline with increasing altitudes. The I‐T coupling in longitude structures tends to decline near the terminator, which is in line with the declining longitude variation of Hn with increasing altitudes since hmM2 significantly increases near the terminator. Moreover, it tends to enhance at high solar activity level due to increased photoionization rate. The I‐T coupling in longitude structures also shows seasonal dependence, as seasonal variation of hmM2 can affect the Hn longitude variation intensity nearby the ionospheric main peak. Key Points: The I‐T coupling in longitude structures declines near the terminator and enhances at high solar activity level, and it depends on seasonLongitude variation intensity of neutral temperature is crucial for the I‐T coupling; it tends to decline with increasing heightsIonospheric peak electron density can better indicate thermospheric temperature disturbance related to non‐migrating tides than peak height [ABSTRACT FROM AUTHOR]

Effects of the extreme-ultraviolet (EUV) late phase of solar flares on the ionosphere were rarely studied. Here we simulated such effects on the ionospheric electron density using an ionosphere−thermosphere coupled model driven by the realistic solar spectrum observed during the X1.8 flare on 2012 October 23. Global total electron content (TEC) observations and simulations showed that the dayside ionospheric TEC during the EUV late phase increased more than that of the flare's main phase. We examined the performance of the model for flares with EUV late phase. The results showed that the F-region electron density enhancement and recovery did not vary in the same pace as the temporal variations of the EUV late phase, and the presence of the EUV late phase prolonged the recovery of electron density by ∼9 hr. We also found that the enhancement in electron density was mainly determined by the chemical production, while the recovery of electron density was primarily controlled by the electric field transport effects. This study enhanced understanding of the intricate physical and photochemical processes governing Earth's space environment and similar planetary systems during solar flare events with EUV late phase. [ABSTRACT FROM AUTHOR]

Sprite halos are diffuse glow discharges in the D-region ionosphere triggered by the quasi-electrostatic (QES) fields of lightning discharges. A three-dimensional (3D) QES model is adopted to investigate the effect of ionospheric electron density on sprite halos. The electron density is described by an exponential formula, parameterized by reference height (h') and sharpness (β), and the local inhomogeneity has a Gaussian density distribution. Simulation results indicate that the reference height and steepness of the nighttime electron density affect the penetration altitudes and amplitudes of normalized electric fields, as well as the altitudes and intensities of the corresponding sprite halos optical emissions. A comparison of the daytime and nighttime conditions demonstrates that the daytime electron density profile is not favorable for generating sprite halos emissions. Furthermore, the pre-existing electron density inhomogeneities lead to enhanced local electric fields and optical emissions, potentially offering a plausible explanation for the horizontal displacement between sprites and their parent lightning, as well as their clustering. [ABSTRACT FROM AUTHOR]

This study investigated the latitudinal variations of post‐sunset enhancements in the ionospheric electron density during the geomagnetic quiet period in May 2021 with a combination of high‐precision ionospheric parameters obtained from four ionosondes, Beidou geostationary satellite (BD‐GEO) receiver network and Sanya incoherent scatter radar (SYISR). We identified four categories of post‐sunset enhancement phenomena (Types 1–4), each with unique spatial and temporal evolutions, yet uniformly accompanied by a decrease in hmF2. Measurements of plasma drift vector velocities from SYISR and hmF2 gradients across various latitudes provided pivotal insights, confirming that the ionospheric post‐sunset enhancements can result from downward plasma motion due to westward electric field, downward field‐aligned drift, or a combination of both. For Type 1, dominated by field‐aligned drift, plasma density enhancements not only intensify at low latitudes but may also extend to mid‐latitudes, exhibiting a distinct temporal delay with increasing latitude. In contrast, Type 4, primarily driven by the westward electric field, is characterized by modest increases in plasma density confined to localized low‐latitude regions, with no observable latitudinal time delay in the peak of enhancements. Types 2 and 3, which are subject to the combined influence of the westward electric field and field‐aligned drift, exhibit plasma density increases at certain low‐latitude areas, with Type 2 presenting a delayed pattern and Type 3 showing none with rising latitude. Meanwhile, neutral winds can partially account for the observed post‐sunset enhancement from low to middle latitudes. These findings offer new insights into the factors influencing ionospheric behavior after sunset. Key Points: Four types of distinctive latitudinal variations of post‐sunset enhancements were identified with multiple data sets during the quiet periodAccompanying the increase in the electron density at sunset, the decrease in hmF2 is favorable for the formation of post‐sunset enhancementDownward field‐aligned drift and westward electric field govern the spatial scale of post‐sunset enhancements at low and middle latitudes [ABSTRACT FROM AUTHOR]

Solar cycle variation of earth's atmosphere, particularly the ionosphere is of particular interest in the field of space science and space weather studies. In this article, we present the outcome of our detailed quantitative study on solar cycle variation of lower ionospheric properties using numerical investigation. First, we seek to model and compare the collective D‐region ionization rates (q $q$'s) due to the individual contributions from the ionizing sources, namely, (a) solar extreme ultraviolet (EUV) radiation including the Lyman‐α $\alpha $ irradiation and (b) solar X‐ray irradiation throughout the 24th $2{4}^{th}$ solar cycle (C24). Then, we compute the electron density Ne $\left({N}_{e}\right)$ profiles using the ionization rates. We report significant solar cycle variation in ionization rate and electron density profiles for the entire span of C24. We use the sunspot number (SPN) profile to substantiate the finer details of Ne ${N}_{e}$ profile during C24. For the segments of the D‐region above two different geographic coordinates, we report that Ne ${N}_{e}$ profiles show a consistent "dual peak" nature. It is very similar to the SPN profile during C24. As a next‐order validation, we compare our modeled Ne ${N}_{e}$ profiles with their IRI‐2020 counterpart (Ne,iri ${N}_{e,iri}$s). Their overall trends are found to be in agreement. Finally, we discuss the response of the D‐region in terms of Ne ${N}_{e}$ due to C24. The work lays the foundation for our upcoming studies on D‐region response to solar cycle variation with Very Low Frequency (VLF) observation. Plain Language Summary: The solar cycle is a periodic change in solar activity with a periodicity of about 11 years. An observable indicator of the solar cycle is the temporal variation in Sunspot Number (SPN). During the supposed maxima period of most of the observed cycles, two peaks have been detected in the sunspot data. The gap between these two peaks is named as Gnevyshev gaps. The D‐region is the lowermost part of the ionosphere with the altitude distribution from, roughly, 60–90 km. We develop a numerical model to study the long‐term variability of the D‐region ionosphere during solar cycle 24 (C24). The D‐region is maximally ionized by solar extreme ultraviolet (EUV) and X‐ray radiation. We estimate the net ionization rate (q) $(q)$ due to both of these two types of solar radiation in this region. We model the electron density Ne $\left({N}_{e}\right)$ profile and check the long‐term effects due to C24. We found, just like the "dual peak" in SPN, Ne ${N}_{e}$ profiles also show similar "dual peak" consistently across different parts of the D‐region above different geographic coordinates. We also investigate the altitude (h) $(h)$ dependency of ionization rate (q) $(q)$ and Ne ${N}_{e}$. We infer that the D‐region ionosphere varies appositely with solar activity phases during C24. Key Points: Variability of solar Lyman‐α and X‐ray due to solar cycle 24D‐region ionization in presence of solar Lyman‐α and X‐raySolar cycle 24 induced D‐region electron density variation [ABSTRACT FROM AUTHOR]

Energetic proton precipitation from the magnetosphere plays an important role in the magnetosphere‐ionosphere‐thermosphere coupling and energy transfer. Proton precipitation causes hydrogen emissions, such as Hβ (486.1 nm), and also triggers the excitation of other emission lines such as the blue‐line (427.8 nm) and the green‐line (557.7 nm). In light of the growing availability of ground‐based proton auroral measurements in recent years, we revisit the proton auroral modeling in this study, with more focus on the application for interpreting ground observations. An accurate simulation of these optical emissions requires a comprehensive understanding of particle transport and collisions in the upper atmosphere, where the simultaneous consideration of precipitating protons, newly generated energetic hydrogen atoms, and secondary electrons is critical. For this purpose, we couple a 3D Monte‐Carlo proton transport model and an electron transport model. The integrated model framework can compute the emission rates of most major auroral emission lines/bands resulting from proton precipitation, along with self‐consistent calculation of the ionospheric electron density variations. The model results show improved agreement with ground optical observations in terms of the Hβ yield and the green‐to‐Hβ ratio compared to previous model studies. Our new model is a valuable tool for quantifying excitation and ionization due to proton aurora. It has the potential to leverage ground observations to infer precipitating conditions at high altitudes and even for studying magnetospheric activity. Plain Language Summary: The terrestrial auroral display is caused by the collision of energetic particles from space with the Earth's atmosphere. Both energetic electrons and protons can enter the atmosphere and cause auroras, but their transport processes are different. The proton can exchange its charge with atmospheric neutral particles in a collision and become a neutral hydrogen atom, and the hydrogen atom can become a proton again in another collision with atmospheric particles. Both the protons and the hydrogens can ionize the atmospheric neutrals and produce secondary electrons. To model the proton‐induced auroras, the three components, protons, hydrogens, and secondary electrons, must be considered together with their different trajectories and transport in the atmosphere. In this study, we present such a coupled proton‐hydrogen‐electron transport model and simulate the resulting proton auroral intensities. The model results show reasonable agreement with existing ground‐based observations. Key Points: We present a model of coupled proton‐hydrogen‐electron transport in the atmosphere and the resulting auroral excitation and ionizationThe model results show improved agreement with the Hbeta yield reported in existing observations compared to previous modelsOur model can simulate the 557.7 nm emission in proton auroras. The modeled green‐to‐Hbeta ratio is compatible with the existing observation [ABSTRACT FROM AUTHOR]

• HF & VHF radars, making measurements of resident space objects, detect Doppler oscillations. • The Doppler oscillations are interpreted as waves propagating in the ionosphere. • Statistical analysis shows the waves have a frequency distribution in the 0.2–1 Hz range. • The waves are most likely observed during day with higher frequencies in weaker ionospheres. • The wave frequency and occurrence distributions suggest the most likely waves are EMIC waves. Radars performing observations of resident space objects (RSO) measure Doppler variations in wave events of several minutes duration, with frequencies in the 0.2–1 Hz range peaking near 0.5–0.7 Hz, consistent with variations in electron density induced by waves in the intervening ionosphere. The two mid-latitude radars used were a Very High Frequency (VHF) radar operating at 55 MHz, and a High Frequency (HF) radar operating at 30 MHz, both in southern Australia. The VHF radar wave observations exibited a peak in wave occurrence in the post-dawn sector (0600–1200 local solar time). The seasonal occurrence of the waves had a strong minimum during winter compared with the other seasons. Comparison between observations in 2018/19 (near solar minimum) and 2021/22 (mid-rise to cycle 25 peak) suggests wave occurrence is anti-correlated with sunspot number and hence with EUV and ionospheric strength. Generally the waves had higher frequencies during night than day, and low sunspot number than mid solar cycle, when there was a weaker ionosphere. Given the oscillation frequency of the wave events, the most likely geophysical phenomena that is consistent with the observations are electro-magnetic ion cyclotron (EMIC) plasma waves in the Pc1 (0.2–5 Hz) frequency range. No other candidate geophysical disturbance appears to fit the wave characteristics. The majority of geomagnetic field-guided transverse Pc1 EMICWs project from the outer magnetosphere down onto the polar ionosphere, where they can convert to compressional fast-mode waves propagating parallel to the Earths surface in the ionospheric F2 layer waveguide, both equatorwards to mid-latitudes and polewards. It is likely the majority of the Doppler oscillations in the Pc1 frequency range observed by the radars at mid-latitudes are these ducted compressional waves. However, sources of transverse EMICWs from the magnetosphere onto the mid-latitude ionosphere do exist, and these may cause some of the observed oscillations. The micro-physics of these compressional waves causing Doppler oscillations in radio observations is not inconsistent with the history of ionospheric Doppler measurements and theory, although the radar trans-ionospheric radio propagation and the observed waves being higher frequency than previous studies is different. If the observed Doppler oscillations are compressional EMICW in the ionospheric waveguide then several of the statistical results can be explained. The anti-correlation of the wave occurrence with sunspot number (and resultant ionospheric strength) can be attributed to lower ionospheric attenuation at the higher latitudes, between where geomagnetically field-guided transverse EMIC waves initially enter the high-latitude ionospheric waveguide from the magnetosphere above, and their observation at mid-latitudes. The observation of higher frequency waves during low sunspot number may also be explained by a source effect in the magnetosphere that preferentially selects the higher frequency field-guided transverse EMIC waves to propagate down to the ionosphere. Comparisons will be shown with results from ground and space based magnetometers of compressional waves in the waveguide, highlighting consistent results and areas where the measurement techniques differ. Theory suggests the radar Doppler measurements are far more sensitive to variations in in-situ ionospheric electron density than magnetic field variations. This agrees with existing literature which highlights the very strong contribution from the compressional component. Theory also suggests that the Doppler sensitivity of a radar to ionospheric electron density variations is frequency dependent, with lower frequencies being more sensitive. This is borne out by the measurements, with the HF radar being more sensitive than the VHF radar. [ABSTRACT FROM AUTHOR]

On 22 January 2024, at 18 UT, a strong earthquake (EQ), Mw = 7, occurred with the epicenter at 41°N, 79°E. This seismic event generated a complex response, the elements of which correspond to the concept of lithosphere–atmosphere–ionosphere coupling through electromagnetic processes. While flying over the EQ area on the night-ide of the Earth, the tandem of low-orbiting Swarm satellites observed small-scale irregularities in the plasma density with an amplitude of ~1.5 × 104 el/cm3, which are likely associated with the penetration of the coseismic electric field into the ionosphere. The local anomaly was detected against the background of a global increase in total electron content, TEC (although geomagnetic indices remained quiet), since the moment of EQ coincided with the ionospheric response to a solar flare. In the troposphere, specific humidity decreased while latent heat flux and aerosol optical depth increased, all exhibiting the co-located disturbances that can be attributed to the effect of increased air ionization rates, resulting in greater electrical conductivity in the near-Earth boundary layer. Anomalies started developing over the epicenter the day before and maximized on the day of the main shock and aftershocks. [ABSTRACT FROM AUTHOR]

In this study, we present the first results of detecting ionospheric irregularities using non-typical GPS observations recorded onboard the Geostationary Operational Environmental Satellites (GOES) mission operating at ~35,800 km altitude. Sitting above the GPS constellation, GOES can track GPS signals only from GPS transmitters on the opposite side of the Earth in a rather unique geometry. Although GPS receivers onboard GOES are primarily designed for navigation and were not configured for ionospheric soundings, these GPS measurements along links that traverse the Earth's ionosphere can be used to retrieve information about ionospheric electron density. Using the radio occultation (RO) technique applied to GPS measurements from the GOES–16, we analyzed variations in the ionospheric total electron content (TEC) on the links between the GPS transmitter and geostationary GOES GPS receiver. For case-studies of major geomagnetic storms that occurred in September 2017 and August 2018, we detected and analyzed the signatures of storm-induced ionospheric irregularities in novel and promising geostationary GOES GPS observations. We demonstrated that the presence of ionospheric irregularities near the GOES GPS RO sounding field of view during geomagnetic disturbances was confirmed by ground-based GNSS observations. The use of RO observations from geostationary orbit provides new opportunities for monitoring ionospheric irregularities and ionospheric density. [ABSTRACT FROM AUTHOR]

This work reports the development of the first version of the E‐region Prompt Radio Occultation Based Electron Density (E‐PROBED) Model. This is an empirical model of E‐region electron density (Ne) between 90 and 120 km developed using radio occultation measurements from the COSMIC‐1 mission. This first version captures more than 80% of the observed variability in monthly‐mean latitude‐local time‐altitude E‐region Ne profiles but it does not account for longitudinal variability at constant local‐time. This work also reports a validation of E‐PROBED simulations through comparisons with ionosondes and incoherent scatter radar (ISR) E‐region Ne profiles. E‐PROBED generally agrees with these ground‐based observations during day‐time. During night‐time, there is a large disparity between E‐PROBED and ISR values. Finally, this work compares E‐PROBED with E‐region Ne simulated by the International Reference Ionosphere (IRI) and the Specified Dynamics—Whole Atmosphere Community Climate Model with Ionosphere/Thermosphere eXtension (SD‐WACCM‐X). One of the main differences amongst these models is on the simulation of variabilities that cannot be attributed to photoionization. IRI barely simulates any variability not driven by photoionization. Both E‐PROBED and SD‐WACCM‐X simulates variability not driven by photoionization. Another main difference is in the absolute magnitude of night‐time E‐region Ne values. Both IRI and SD‐WACCM‐X are substantially lower than E‐PROBED. This work first concludes that E‐PROBED can conveniently provide E‐region Ne latitude—local time variabilities and structures that COSMIC‐1 observes. This work also concludes that E‐region Ne have significant non‐photoionization driven variabilities. Plain Language Summary: Navigation and communication systems reliant on sending signals into space require accurate predictions from empirical models of ionospheric electron density (Ne) values because the frequency of the appropriate signals to use are a function of Ne. Empirical models currently struggle the most with the ionospheric E‐region (between 90 and 120 km) because most of these models were constructed using sparsely sampled E‐region Ne data from ground‐based observations. Recently, E‐region Ne profiles were finally retrieved from Global Navigation Satellite Systems Radio Occultation (GNSS RO) measurements. This work reports on a new empirical model of monthly‐mean E‐region Ne that uses global observations of E‐region Ne from GNSS RO. The model is called the E‐region Prompt Radio Occultation Based Electron Density (E‐PROBED) Model, and it captures more than 80% of the observed variability in monthly‐mean latitude‐local time‐altitude E‐region Ne profiles. It agrees well with both ground‐based observations and first principles Physics‐based model simulations. These findings suggest that E‐PROBED makes a viable alternative E‐region Ne empirical model for those interested in an E‐region Ne model constrained by satellite observations. Key Points: The first version of E‐region Prompt Radio Occultation Based Electron Density Model (E‐PROBED) is developedE‐PROBED agrees with ground‐based observations during day‐time but not during night‐timeE‐PROBED agrees with SD‐WACCM‐X that E‐region Ne have significant non‐photoionization‐driven variabilities [ABSTRACT FROM AUTHOR]

Pulsating proton auroras are often attributed to periodic proton precipitation. However, how the proton precipitation is periodically generated in the magnetosphere remains an open issue. Utilizing multi‐point space‐borne and ground‐based observations, this study proposed a potential mechanism responsible for pulsating proton precipitation and intermittent ionospheric electron density disturbances. On 8 September 2017, Pc4 ULF waves and electromagnetic ion cyclotron (EMIC) wave packets were simultaneously observed by Van Allen Probes (RBSP) in the inner magnetosphere. The EMIC wave packets were quasi‐periodically excited at the same frequency as the ULF waves, which resulted in 30–100 keV proton precipitation detected by Low‐Earth‐Orbit (LEO) POES satellites. Meanwhile, conjugate European Incoherent Scatter (EISCAT) radar on the ground observed E‐region electron density enhancements that intermittently appeared nearly at the same frequency as the EMIC wave packets in space. These observations together suggest that ULF waves in the magnetosphere are the ultimate driver that modulates quasi‐periodic EMIC waves to induce proton precipitation and pulsating disturbances in the ionosphere. Plain Language Summary: Pulsating proton auroras are a kind of auroral emission with a periodicity of tens of seconds. Although proton aurora has been known to be generally caused by energetic protons (at energies of tens of kiloelectron volts) precipitating from space into the upper atmosphere, the generation mechanism of its periodicity is still not well understood. This study explored a case study using various observations in space and on ground to analyze the magnetospheric source and ionospheric responses. It is found that the ultra‐low‐frequency (ULF) waves in the magnetosphere are able to modulate and trigger quasi‐periodic higher‐frequency waves called electromagnetic ion cyclotron (EMIC) waves. Furthermore, direct evidence is found in the electron density at about 120–140 km altitude, where proton aurora would occur, experiences intermittent disturbances, in good correlation with the quasi‐periodic EMIC waves. Therefore, we have found a causal link from the ULF waves to quasi‐periodic EMIC waves that subsequently induce pulsating ionospheric disturbances, an indication of pulsating proton precipitation and pulsating proton aurora. Key Points: We demonstrate quasi‐periodic EMIC waves modulated by ULF waves and subsequent pulsating disturbances in the ionosphereIonospheric pulsating disturbances should be associated with the pulsating proton precipitation induced by quasi‐periodic EMIC wavesThis study suggests a potential mechanism to explain pulsating proton aurora with a period of 1 and 2 min [ABSTRACT FROM AUTHOR]

This paper conducts a multi‐instrument and data assimilation analysis of the three‐dimensional ionospheric electron density responses to the total solar eclipse on 08 April 2024. The altitude‐resolved electron density variations over the continental US and adjacent regions are analyzed using the Millstone Hill incoherent scatter radar data, ionosonde observations, Swarm in situ measurements, and a novel TEC‐based ionospheric data assimilation system (TIDAS) with SAMI3 model as the background. The principal findings are summarized as follows: (a) The ionospheric hmF2 exhibited a slight enhancement in the initial phase of the eclipse, followed by a distinct reduction of 20–30 km in the recovery phase of the eclipse. The hmF2 in the umbra region showed a post‐eclipse fluctuation, characterized by wavelike perturbations of 10–25 km in magnitude and a period of ∼ ${\sim} $30 min. (b) There was a substantial reduction in ionospheric electron density of 20%–50% during the eclipse, with the maximum depletion observed in the F‐region around 200–250 km. The ionospheric electron density variation exhibited a significant altitude‐dependent feature, wherein the response time gradually delayed with increasing altitude. (c) The bottomside ionospheric electron density displayed an immediate reduction after local eclipse began, reaching maximum depletion 5–10 min after the maximum obscuration. In contrast, the topside ionospheric electron density showed a significantly delayed response, with maximum depletion occurring 1–2.5 hr after the peak obscuration. Plain Language Summary: On 8 April 2024, a total solar eclipse traversed across North America with a dense network of observational equipment in place, providing a great opportunity for analyzing ionospheric effects during the eclipse. This paper presents a multi‐instrument and data assimilation analysis of the three‐dimensional ionospheric electron density response to this solar eclipse, utilizing Millstone Hill incoherent scatter radar data, ionosonde observations, Swarm satellite in situ measurements, and a new TEC‐based ionospheric data assimilation system (TIDAS) over continental US and adjacent regions with the SAMI3 as the background model. The observations and SAMI3‐TIDAS data assimilation reveals the time‐evolving 3‐D spatial distribution of the ionospheric electron density during the eclipse, highlighting key features of altitude‐dependent ionospheric variation with significant discrepancies and time delays between the bottomside and topside ionosphere. Key Points: The altitude‐resolved Ne response to the solar eclipse in the 3‐D domain was effectively reconstructed by TIDAS‐SAMI3 data assimilationThe eclipse led to a substantial ionospheric Ne reduction of 20%–50%, with the maximum depletion occurring in the F region of 200–250 kmThe Ne showed a time‐delayed variation with increasing altitude, from 5 to 10 min in the bottomside to 1–2.5 hr in the topside ionosphere [ABSTRACT FROM AUTHOR]

In this study, we investigate the intricate electrodynamics of the Earth's horizontal component of the geomagnetic field (ΔH) in response to two significant solar flares (SF) occurring on 03 July and 28 October 2021. These flares are classified as X1.59 and X1.0, respectively. It is noted that the ΔH follows the X‐ray variation during the SF, but there is a time lag of a few minutes between the X‐ray and ΔH. A possible explanation for the time lag is the neutral atmosphere and ionosphere coupling, via ion drag. Plain Language Summary: The ionospheric electron density and Earth's magnetic field are strong disturbed during solar flares (SFs) X‐class. In this paper we show that horizontal component of the magnetic field (ΔH) follows the X‐ray variation during the SF, but there is a time lag of a few minutes, between the X‐ray and ΔH. The time lag between the X‐ray and ΔH peaks vary from 1 to 8 min, the ion drag force between the neutral atmosphere and ionosphere may explain. Key Points: The synchronization of ΔH peaks and valleys from equatorial‐low‐mid latitudes indicates that the X‐ray and ΔH variations are relatedA visible time lag exists between the X‐ray burst peak and ΔH peaks and valleysIt is proposed the coupling between the neutral upper atmosphere and the ionosphere as a possible explanatory framework for this time lag [ABSTRACT FROM AUTHOR]

Powerful lightning strikes generate broadband electromagnetic signals. At Extremely Low Frequencies (ELF), the signal partly leaks into the ionosphere and produces whistlers that can be detected by satellites. Indeed, the satellites of the European Space Agency (ESA) Swarm Earth Explorer mission can detect those signals during 250 Hz burst‐mode acquisition campaigns of their Absolute Scalar Magnetometers (ASM). The dispersion of these whistlers depends on their propagation path and the distribution of ionization in the ionosphere crossed along that path. In this paper, we introduce a technique to derive a new measure of ionosphere electron content, the Total square‐Root Electron Content (TREC), using the arrival times of two frequencies of the whistler signal. We validate this approach by using data from ionosondes and from in situ measurements of the electron density at Swarm location. This technique brings new opportunities for sounding the ionosphere in regions poorly observed by other techniques. Plain Language Summary: A lightning strike generates an electromagnetic impulse that propagates within Earth's atmosphere and eventually leaks out into the ionosphere. As it propagates through the ionosphere toward low‐Earth orbiting (LEO) satellites, it gets converted into a so‐called whistler, with high frequencies arriving earlier than low frequencies. This frequency dispersion depends on the state of the ionosphere. Here, we analyse such whistler waves detected by magnetometers onboard the European Space Agency Swarm satellites to recover information about the state of the ionosphere below the satellites. We first introduce a new metric, the Total Root Electron Content (TREC), which quantifies the cumulative value of the square root of electron density along the path of the whistler. We next propose a method to recover the TREC from the analysis of the whistler dispersion. We finally validate this method by using independently derived ionospheric electron density profiles to infer expected TREC values. Our results show that whistlers detected by LEO satellites can be used to locally improve the widely used empirical International Reference Ionosphere model. Such whistler inferred TREC values could be used to sound the ionosphere above places difficult to sample with conventional measuring techniques, and help better model and understand the highly dynamic ionosphere. Key Points: Total square‐Root Electron Content (TREC) is a new measure of the ionospheric electron content for electromagnetic signals in the ELF bandA method to retrieve TREC from fractional‐hop whistlers in the ELF detected by the ESA Swarm mission is proposedThe method is validated using TREC computed with independently constrained electron density profiles close to the Swarm whistler locations [ABSTRACT FROM AUTHOR]

Abstract—Ground-based magnetometers and vertical ionospheric sounding stations were used to record specific variations in the geomagnetic field, caused by the perturbation in the lower ionospheric current systems, and specific variations in the upper ionospheric electron density after a strong volcanic eruption in Kamchatka, Russia, on April 10, 2023. The analysis of the measurements from two series of explosions has shown that the impact on the lower ionosphere is realized via both seismic Rayleigh waves (which are a source of acoustic waves propagating into the ionosphere), and atmospheric internal gravity waves generated by explosions. At distances up to a thousand km from the source, a repeatability of the pattern of ionospheric perturbations after each of the six volcanic explosions is discovered. At larger distances, signals from acoustic waves caused by the Rayleigh waves are clearly recorded in the ionosphere, whereas separation of the signals from atmospheric internal waves is difficult due to the influence of perturbations from other external sources. [ABSTRACT FROM AUTHOR]

The article "Ionosphere Monitoring with Remote Sensing Vol II." provides an overview of the importance of studying the Earth's ionosphere and its influence on space weather. It discusses the use of remote sensing and in situ instruments to investigate ionospheric features and anomalies. The article presents original research papers that contribute to the understanding of the ionosphere and its relevance for signal transmission and space weather studies. It also explores the use of artificial intelligence technology in ionospheric prediction models and the study of ionospheric anomalies associated with natural phenomena. The research findings highlight the complex nature of the ionosphere and its impact on communication and navigation systems. [Extracted from the article]

Based on a modified numerical model of the ionosphere and plasmasphere developed at the Institute of Solar–Terrestrial Physics (ISTP), Siberian Branch, Russian Academy of Sciences, the altitude profiles of the Ne electron density for the quiet and disturbed states of the thermosphere were calculated for the conditions of January 25, 2009, at a geographical point with the coordinates 52.4° N, 104.3° E (Irkutsk). The disturbed conditions were set by varying the temperature of neutral particles T in the thermosphere. At altitudes below 180 km and above 250 km, with an increase/decrease in T, an increase/decrease in Ne occurs. At altitudes of 180–250 km, the opposite picture is observed: an increase/decrease in T causes a decrease/increase in Ne. The opposite nature of the change in the Ne profile is associated with the influence of the ratio of concentrations of atomic oxygen and molecular nitrogen [O]/[N2] at the altitudes of region F. Quantitative estimates of the change in Ne at different altitudes with changes in the temperature of neutral particles were obtained. It has been established that a change in T by 1 K leads to a change in Ne by 0.2–0.3%. The modeling results are compared with observations of the peak electron density NmF2 obtained on the Irkutsk ionosonde during sudden stratospheric warming in January 2009. [ABSTRACT FROM AUTHOR]

The ionosphere is one of the important error sources that affect the communication of radio signals. The international reference ionosphere (IRI) model is a commonly used model to describe ionospheric parameters. The driving parameter IG12 of the IRI-2016 model was optimally updated based on GNSS-TEC data from 2015 and 2019. The electron density profiles and NmF2 calculated by the IRI-2016 model (upda-IRI-2016) driven by the updated IG12 value (IG-up) were evaluated for their accuracy using ionosonde observations and COSMIC inversion data. The experiments show that both the electron density profiles and NmF2 calculated by upda-IRI-2016 driven by IG-up show significant optimization effects, compared to the IRI-2016 model driven by IG12. For electron density, the precision improvement (PI) for both MAE and RMSE at the Beijing station exceed 31.2% in January 2015 and 16.0% in January 2019. While the PI of MAE and RMSE at the Wuhan station, which is located at a lower latitude, both exceed 32.5% in January 2015, both exceed 42.1% in January 2019, which is significantly higher than that of the Beijing station. In 2015, the PI of MAE and RMSE compared with COSMIC are both higher than 20%. For NmF2, the PI is greater for low solar activity years and low latitude stations, with the Wuhan station showing a PI of more than 11.7% in January 2019 compared to January 2015. The PI compared to COSMIC was higher than 17.2% in 2015. [ABSTRACT FROM AUTHOR]

Specification and forecast of ionospheric parameters, such as ionospheric electron density (Ne), have been an important topic in space weather and ionospheric research. Neural networks (NNs) emerge as a powerful modeling tool for Ne prediction. However, heavy manual adjustments are time consuming to determine the optimal NN structures. In this work, we propose to use neural architecture search (NAS), an automatic machine learning method, to mitigate this problem. NAS aims to find the optimal network structure through the alternate optimization of the hyperparameters and the corresponding network parameters within a pre‐defined hyperparameter search space. A total of 16‐year data from Millstone Hill incoherent scatter radar (ISR) are used for the NN models. One single‐layer NN (SLNN) model and one deep NN (DNN) model are both trained with NAS, namely SLNN‐NAS and DNN‐NAS, for Ne prediction and compared with their manually tuned counterparts (SLNN and DNN) based on previous studies. Our results show that SLNN‐NAS and DNN‐NAS outperformed SLNN and DNN, respectively. These NN predictions of Ne daily variation patterns reveal a 27‐day mid‐latitude topside Ne variation, which cannot be reasonably represented by traditional empirical models developed using monthly averages. DNN‐NAS yields the best prediction accuracy measured by quantitative metrics and rankings of daily pattern prediction, especially with an improvement in mean absolute error more than 10% compared to the SLNN model. The limited improvement of NAS is likely due to the network complexity and the limitation of fully connected NN without the time histories of input parameters. Plain Language Summary: Neural network (NN) models have garnered significant attention for their application in predicting physical parameters in the ionosphere, notably ionospheric electron density (Ne). In this study, we introduce a novel approach aimed at enhancing the performance of NN models by employing the advanced technique known as neural architecture search (NAS). Leveraging a data set spanning 16 years of Ne measurements obtained from the incoherent scatter radar located at the Millstone Hill observatory, we conduct a comprehensive analysis. This analysis encompasses training both manually calibrated NN models and NN models optimized via NAS. The NN models fine‐tuned through NAS achieve a notable improvement in their ability to predict Ne when compared to their manually adjusted counterparts. This improvement underscores the efficacy of NAS in optimizing neural network hyperparameters for ionospheric modeling. Furthermore, we delve into a thorough exploration of the factors contributing to the somewhat limited improvements observed in the context of our current data set. This investigation yields valuable insights and prompts valuable discussions on the potential avenues for further refinement in ionospheric prediction methodologies. Key Points: Neural architecture search (NAS) can automate hyperparameter optimization for neural networks (NN) to predict Ne at 350 km altitudeA total of 16‐year of data from incoherent scatter radar are used for NN models and their NAS counterparts, which outperformed IRI2016The limited improvement of NAS could be due to the network complexity and the lack of temporal mechanism of fully connected NNs [ABSTRACT FROM AUTHOR]

In this study, we use multi‐instrument observations (all‐sky imager (ASI), global navigation satellite system (GPS) receivers, digisonde) to study the interaction of nighttime medium‐scale traveling ionospheric disturbances (MSTIDs) on 13 November 2018. The most attractive aspect of this event is that the interaction appeared between two dark bands both propagated southwestward. The airglow observations show that the latter band moved faster and caught up with the former, and these two bands merged into a new one. The propagating characteristics and morphology of the MSTIDs changed during the interaction process. The simulations from the Thermosphere‐Ionosphere‐Electrodynamics General Circulation Model (TIEGCM) suggested that the ionospheric background zonal winds and electron density distributions could play essential roles in the interaction of the MSTIDs. Moreover, the merging process might be associated with the electrostatic reconnection. Plain Language Summary: This study shows an interesting medium‐scale traveling ionospheric disturbance (MSTID) event. Two MSTID bands encountered during the propagation, the latter dark band caught up with the former band, and then, the two bands merged into one. In contrast, these dark bands kept propagating southwestward. The Thermosphere‐Ionosphere‐Electrodynamics General Circulation Model (TIEGCM) simulations obtain the wind and electron density data, which provide a possible explanation that the changing of ionospheric zonal winds and ionospheric electron density might result in the interaction of these dark bands. Moreover, the merging process might be connected with the electrostatic reconnection, which could be influenced by E × B plasma flows in opposite directions. Key Points: Two bands propagated southwestward, the latter band moved faster and caught up with the former, they interacted and merged into oneThe chasing behavior showed the velocity differences, possibly related to the ionospheric zonal wind and electron density distributionThe merging process of the two bands might be connected with electrostatic reconnection [ABSTRACT FROM AUTHOR]

Upward propagating waves of lower atmospheric origin play an important role in coupling terrestrial weather with space weather on daily to inter‐annual timescales. Quantifying their short‐term (<30 days) variability is a difficult challenge because simultaneous observations at multiple local times are needed to sample diurnal cycles. This study demonstrates and validates a short‐term estimation method of the DE3 non‐migrating tide at the equator and then applies the technique to three independent data sets: MIGHTI, SABER, and TIDI. We find that daily DE3 estimates from SABER, MIGHTI, and TIDI at equator agree well with correlation coefficients ranging between 0.76 and 0.85. The daily DE3 amplitude variability is typically ∼7 m/s in zonal winds and ∼3 K in temperature. We also find that daily MLT variations and F‐region ionospheric DE3 from COSMIC‐2 Global Ionospheric Specification (GIS) show a correlation of 0.55–0.65, suggesting that not all ionospheric variability can be attributed to the E‐region dynamo; however, increasing correlation with increasing time‐scale suggests that lower atmospheric variability has pronounced impact on the ionosphere on intra‐seasonal scales. We find that the MLT and the F‐region ionosphere exhibit strong coherent intra‐seasonal oscillations (residual amplitudes upto 50%–60%); their coherency with the MJO in 2020 suggests a possible modulation of the upward propagating DE3 tide related to this major tropical tropospheric weather pattern. In addition, we find stratospheric QBO signatures in the MLT DE3 on inter‐annual scales. This study offers fresh observational insights into the pivotal role of tropospheric weather in shaping variability in the coupled thermosphere‐ionosphere system. Plain Language Summary: A growing body of research unequivocally demonstrates the important role upward propagating waves from the lower atmosphere play in shaping the meteorology of the mesosphere, thermosphere and ionosphere from daily to inter‐annual time scales. Understanding these variations is crucial for space weather studies and applications, including radio wave propagation, satellite communication, and space orbital debris. Among these oscillations, the diurnal eastward propagating tide with zonal wave number 3 (DE3) holds particular importance. This oscillation, driven by expansive tropospheric weather systems, can attain substantial amplitudes in the upper atmosphere. The primary objective of this study is to demonstrate and validate a methodology facilitating the short‐term estimation of DE3 to understand the tidal weather of the upper atmosphere. Our approach starts by using a physics‐based model of the upper atmosphere to test and validate this new daily DE3 retrieval method. Next, we apply our tidal estimation method to extensive satellite‐based measurements of winds, temperatures, and ionospheric electron density. Lastly, we analyze the results to provide fresh observational insights into the pivotal role of tropospheric weather on Earth's near space environment from daily to inter‐annual time scales. Key Points: Demonstrated and validated a short‐term DE3 tidal estimation technique at the equatorDaily DE3 tidal amplitude variability at the equator is typically ∼7 m/s in zonal winds and ∼3 K in temperatureStrong intra‐seasonal variations in MLT region DE3 and F‐region ionosphere electron densities possibly due to the MJO are observed [ABSTRACT FROM AUTHOR]

This study uses Mars Atmosphere and Volatile EvolutioN observations of electron density and magnetic field for a period of four Martian years (MYs 33–36) (∼8 Earth years) to investigate the effects of Martian crustal magnetic fields on the distribution and variability of Mars' ionosphere. The results show a clear enhancement in electron density in the southern hemisphere in the region where the strong crustal magnetic fields are present with the longitudes between 120° and 240° (i.e., the central longitude), which is in agreement with previous studies. On the contrary, the corresponding northern hemisphere region in the central longitudes shows an exactly opposite behavior that the electron density is lower compared to the surrounding longitude regions. These effects are found to be primarily dayside phenomena. As opposed to dayside, the nightside electron density in the central longitudes are slightly reduced at altitudes below 200 km, compared to longitudes on its western and eastern sides. Above 200 km, the nightside effects are not very clear. Significant hemispheric asymmetry is observed in the longitude regions of enhanced crustal magnetic fields compared to other longitude regions during the daytime. This dayside south‐north asymmetry in the central longitude region is observed to be a constant feature across all seasons. However, on the nightside, the south‐north asymmetry remains more or less similar across all longitude regions, during all seasons implying a weakened control of the crustal fields over the nightside ionosphere. Even then, the southern hemisphere retains a stronger nightside ionosphere during all seasons except summer. Key Points: Variation in electron density with crustal field location is investigated for dayside and nightside conditions using Mars Atmosphere and Volatile EvolutioN dataNight‐side electron density is reduced in the crustal field region at altitudes below 200 kmLongitudes corresponding to crustal fields show strong dayside south‐north asymmetry in ne, across all seasons [ABSTRACT FROM AUTHOR]

This study focuses on the poorly known effect of polar cap patches (PCPs) on the ion‐neutral coupling in the F‐region. The PCPs were identified by total electron content measurements from the Global Navigation Satellite System (GNSS) and the ionospheric parameters from the Defense Meteorological Satellite Program spacecraft. The EISCAT incoherent scatter radars on Svalbard and at Tromsø, Norway observed that PCPs entered the nightside auroral oval from the polar cap and became plasma blobs. The ionospheric convection further transported the plasma blobs to the duskside. Simultaneously, long‐lasting strong upper thermospheric winds were detected in the duskside auroral oval by a Fabry‐Perot Interferometer (FPI) at Tromsø and in the polar cap by the Gravity Recovery and Climate Experiment satellite. Using EISCAT ion velocities and plasma parameters as well as FPI winds, the ion drag acting on neutrals and the time constant for the ion drag could be estimated. Due to the arrival of PCPs/blobs and the accompanied increase in the F‐region electron densities, the ion drag is enhanced between about 220 and 500 km altitudes. At the F peak altitudes near 300 km, the median ion drag acceleration affecting neutrals more than doubled and the associated median e‐folding time decreased from 4.4 to 2 hr. The strong neutral wind was found to be driven primarily by the ion drag force due to large‐scale ionospheric convection. Our results provide a new insight into ionosphere‐thermosphere coupling in the presence of PCPs/blobs. Plain Language Summary: This study investigates how the evolution of the polar cap patches (PCPs) affects the upper layer of the Earth's atmosphere, termed the thermosphere. PCPs are dense patches of charged particles that move from the dayside to the nightside of the high‐latitude ionosphere through the polar cap region. Using the measurements by multiple ground‐based instruments and satellites, this study found that PCPs can enhance the formation of strong upper thermospheric winds. The winds are primarily driven by the ion drag force due to the interactions between charged particles and neutral gases. The results show that because of the arrival of PCPs, which increase the F‐region electron densities in the auroral oval, the ion drag acceleration acting on the neutrals can more than double and the related time constant of the ion drag can be halved. Key Points: Transportation of polar cap patches (PCPs) and their development in the nightside auroral oval was observed by multiple instrumentsVery strong, long‐lasting westward upper thermospheric wind in the duskside oval was associated with large‐scale ionospheric convectionHigh electron density produced by PCPs increases the ion drag force that drives the upper thermospheric wind [ABSTRACT FROM AUTHOR]

The Vertical Scale Height (VSH) is a crucial parameter that describes the shape of the ionospheric electron density profile. Evidence suggests a complex relationship between VSH and the thermal structure and dynamics of the ionosphere. A statistical study was conducted on the VSH at low altitudes (175 km) and high altitudes (300 km) in the Martian ionosphere, using data from the MAVEN observations from 2014 to 2023. The results suggest that the influence of the crustal magnetic field on VSH175 is more pronounced than on VSH300. VSH175 shows a minor peak around −20° latitudes, which is more than 7% higher than the average value, and an increasing trend with latitude in the northern hemisphere. VSH300 is higher in the southern hemisphere than in the northern hemisphere, especially in summer, by approximately 42.1%. Regarding the local time variation of VSH, we observed an increasing trend from midnight to morning and a decreasing trend from dusk to midnight in almost all conditions. The local time variation of VSH also exhibits seasonal and latitudinal dependence. These variations have different levels of consistency with the gradient of the electron temperature (Te) and the collision frequency between charged particles and neutrals. Specifically, the correlation coefficient between VSH175 and the collision frequency between charged particles and neutrals reaches as high as 0.93 in the northern hemisphere winter and southern hemisphere summer. The correlation coefficient between VSH300 and the gradient of the Te reaches up to 0.72 in the southern hemisphere equinox. Plain Language Summary: The ionosphere is produced by photo‐ionization of the Martian upper atmosphere. The Vertical Scale Height (VSH) is a parameter that describes how quickly the electron density in the ionosphere changes with height. It is closely linked to the thermal structure and dynamics of the ionosphere. Our research focuses on how the crustal magnetic field affects the VSH on Mars and looks at variations with latitude and local time at low altitudes (175 km) and high altitudes (300 km) using MAVEN observations. We find that high VSH values at 175 km align with areas of strong crustal magnetic fields on Mars. There are significant latitudinal and daily variations in the VSH of the Martian ionosphere. Latitudinal variation is mainly manifested as hemispherical asymmetry, which may be related to Martian topography. The daily variations are influenced by the temperature gradient and the collision frequency between charged particles and neutrals. These effects also have dependences on altitude and season. Key Points: The crustal magnetic field of Mars has a significant effect on Vertical Scale Height (VSH) at low altitudesOur analysis revealed evident diurnal and latitudinal variations in the VSH of the Martian ionosphereThe thermal structure and dynamics contribute differently to local time variation of the VSH at different heights [ABSTRACT FROM AUTHOR]

This paper investigates ionospheric response characteristics from multiple perspectives based on globally distributed GNSS data and products, ionosonde data, FORMOSAT-7/COSMIC-2 occultation data, and Swarm satellite observations caused by the total solar eclipse of 8 April 2024 across North and Central America. The results show that both GNSS-derived TEC products have detected the ionospheric TEC degradation triggered by the total solar eclipse, with the maximum degradation exceeding 10 TECU. The TEC data from nine GNSS stations in the path of the maximum eclipse reveal that the intensity of ionospheric TEC degradation is related to the spatial location, with the maximum degradation value of the ionospheric TEC being about 14~23 min behind the moment of the maximum eclipse. Additionally, a negative anomaly of foF2 with a maximum of more than 2.7 MHz is detected by ionosonde. In the eclipse region, NmF2 and hmF2 show trends of decrease and increase, with percentages of variation of 40~70% and 4~16%, respectively. The Ne profile of the Swarm-A satellite is significantly lower than the reference value during the eclipse period, with the maximum negative anomaly value reaching 11.2 × 105 el/cm3, and it failed to show the equatorial ionization anomaly. [ABSTRACT FROM AUTHOR]

Short-wave communication, operating within the frequency range of 3–30 MHz, is extensively employed for long-distance communication because of its extended propagation range and robustness. The ionosphere undergoes complex transformations when influenced by the geomagnetic field, evolving into an uneven and anisotropic electromagnetic medium. This complex property makes the transmission of electromagnetic fields within the ionosphere extremely complex, posing significant challenges for accurately evaluating electromagnetic scattering phenomena. To address the aforementioned challenges, this paper proposes a new method for calculating short-wave ionospheric scattering based on a complex anisotropic multilayer medium transmission matrix. Firstly, by utilizing the characteristic changes of ionospheric electron density with height, the ionization layer is divided into multiple horizontal thin layers, each with an approximately uniform electron density, forming a multilayer horizontal anisotropic structure. Subsequently, the scattering characteristics of electromagnetic waves in the ionosphere were calculated using the transmission matrix approach. The results calculated using this method are consistent with actual measurement values and superior to traditional short-wave ionospheric transmission calculation methods. [ABSTRACT FROM AUTHOR]

Long-term trend estimation in the peak height of the F2 layer, hmF2, needs the previous filtering of much stronger natural variations such as those linked to the diurnal, seasonal, and solar activity cycles. If not filtered, they need to be included in the model used to estimate the trend. The same happens with the maximum ionospheric electron density that occurs in this layer, NmF2, which is usually analyzed through the F2 layer critical frequency, foF2. While diurnal and seasonal variations can be easily managed, filtering the effects of solar activity presents more challenges, as does the influence of geomagnetic activity. However, recent decades have shown that geomagnetic activity may not significantly impact trend assessments. On the other hand, the choice of solar activity proxies for filtering has been shown to influence trend values in foF2, potentially altering even the trend's sign. This study examines the impact of different solar activity proxies on hmF2 trend estimations using data updated to 2022, including the ascending phase of solar cycle 25, and explores the effect of including the Ap index as a filtering factor. The results obtained based on two mid-latitude stations are also comparatively analyzed to those obtained for foF2. The main findings indicate that the squared correlation coefficient, r2 , between hmF2 and solar proxies, regardless of the model used or the inclusion of the Ap index, is consistently lower than in the corresponding foF2 cases. This lower r2 value in hmF2 suggests a greater amount of unexplained variance, indicating that there is significant room for improvement in these models. However, in terms of trend values, foF2 shows greater variability depending on the proxy used, whereas the inclusion or exclusion of the Ap index does not significantly affect these trends. This suggests that foF2 trends are more sensitive to the choice of solar activity proxy. In contrast, hmF2 trends, while generally negative, exhibit greater stability than foF2 trends. [ABSTRACT FROM AUTHOR]

Polar mesospheric summer echoes (PMSEs) are radar echoes that are measured in the upper atmosphere during the summer months and that can occur in several layers. In this study, we aimed to investigate the relationship between PMSE layers ranging from 80 to 90 km altitude and the solar cycle. We investigated 230 h of observations from the EISCAT very high frequency (VHF) radar located near Tromsø, Norway, from the years 2013, 2014 and 2015 during the solar maximum and the years 2019 and 2020 during the solar minimum and applied a previously developed classification model to identify PMSE layers. Our analysis focused on parameters such as the altitude, thickness and echo power in the PMSE layers, as well as the number of layers present. Our results indicate that the average altitude of PMSEs, the echo power in the PMSEs and the thickness of the layers are, on average, higher during the solar maximum than during the solar minimum. In the considered observations, the electron density at 92 km altitude and the echo power in the PMSEs are positively correlated with the thickness of the layers except for four multilayers at solar minimum. We infer that higher electron densities at ionospheric altitudes might be necessary to observe multilayered PMSEs. We observe that the thickness decreases as the number of multilayers increases. We compare our results with previous studies and find that similar results regarding layer altitudes were found in earlier studies using observations with other VHF radars. We also observed that the bottom layer in the different sets of multilayers almost always aligned with the noctilucent cloud (NLC) altitude reported by previous studies at 83.3 km altitude. Also, an interesting parallel is seen between the thickness of NLC multilayers and PMSE multilayers, where both NLCs and PMSEs have a similar distribution of layers greater than 1 km in thickness. Future studies that include observations over longer periods would make it possible to distinguish the influence of the solar cycle from possible other long-term trends. [ABSTRACT FROM AUTHOR]

Electron density enhancements in the ionospheric D‐region due to the precipitation of high‐energy electrons (>30 keV) have been measured as increases in cosmic radio noise absorption (CNA) using ground‐based riometers. CNA has been studied since the 1960s. However, there have been few studies of the spatiotemporal development of CNA at multi‐point ground stations distributed in longitude at subauroral latitudes, where plasma particles with a wide energy range are intermingled. In this study, we analyzed the longitudinal development of CNA steep increases using simultaneous riometer observations at six stations at subauroral latitudes in Canada, Alaska, Russia, and Iceland over 3 years from 2017 to 2020. The results revealed that the occurrence rate of steep increases in CNA was highest at midnight at 22‐08 magnetic local time (MLT), and lowest near dusk at 17–21 MLT. We also showed statistically that the CNA steep increases expanded eastward on the dawn side and westward on the dusk side. The CNA expansion velocity was slightly faster than the results of previous studies in the auroral zone. Correlation and superposed epoch analyses of CNA with solar wind and geomagnetic parameters revealed that CNA intensity was dependent on the Interplanetary Magnetic Field Bz, Interplanetary Electric Field Ey, SYM‐H index, and SME index. These results indicate that the CNA at subauroral latitudes is closely related to solar wind and geomagnetic activities, and its propagation characteristics correspond to the dynamics of high energy electrons in the inner magnetosphere. Plain Language Summary: The inner magnetosphere close to the Earth contains plasma particles with a wide range of energies. It is important to understand the dynamics of high‐energy electrons in the inner magnetosphere for safe space utilization and space weather forecast. This study focuses on the increase in electron density in the Earth's lower ionosphere caused by high‐energy electrons coming down into the atmosphere, based on measurement of cosmic radio noise absorption (CNA) in the ionosphere. Faint radio noise from external astronomical sources can show such absorption, and is slightly less intense, when the ionosphere is bombarded by such electrons. We studied the development in time and space of CNA at multiple ground stations at subauroral latitudes over 3 years. We found statistically that CNA was most likely to occur at midnight and expanded eastward on the dawn side and westward on the dusk side. We also found some correlation between CNA and solar wind and geomagnetic parameters. These results suggest that CNA is closely related to solar wind and geomagnetic activities and shows us the dynamics of high‐energy electrons in the inner magnetosphere. Key Points: This is the first statistical study of longitudinal development of cosmic noise absorption (CNA) using six riometers at subauroral latitudesThe occurrence rate of steep increases in CNA is high at 22‐08 magnetic local time (MLT) and low at 17–21 MLTCNA at subauroral latitudes is closely related to solar wind and geomagnetic activities [ABSTRACT FROM AUTHOR]

Decision makers must often choose how many sensors to deploy, of what types, and in what locations to meet a given operational or scientific outcome. An observation system simulation experiment (OSSE) is a numerical experiment that can provide critical decision support to these complex and expensive choices. An OSSE uses a "truth model" or "nature run" to simulate what an observation system would measure and then passes these measurements to an assimilation model. Then, the output of the assimilation model is compared to that of the truth model to assess improvement and the impact of the observation system. Orion Space Solutions has developed the OSSE tool (OSSET) to perform OSSEs for ionospheric electron density specification quickly and accurately. In this study, we use OSSET to predict the impact of adding commercial radio occultation total electron content (TEC) data to an assimilation model. We compare the OSSE's predictions to the real performance at a group of validation ionosondes and find good agreement. We also demonstrate the global assessments that are possible with the OSSET using the improvement in critical frequency specification as an example. From this, we find that commercial radio occultation data can improve the critical frequency specification by nearly 20% at high latitudes, which are not covered by COSMIC-2. The commercial satellites are in sun-synchronous orbits with constant local times, and this improvement is concentrated at these local times. [ABSTRACT FROM AUTHOR]

Continuously to a previous work on qualitatively investigating the probable electromagnetic interacting process among lithosphere, atmosphere and ionosphere, this work aims to quantitatively establish an electromagnetic coupling model among these three spheres prior to the Wenchuan earthquake. Firstly, a underground finite length electrical dipole in a half-space model has been employed to estimate the possible "energy source" for an observable 1.3 mV m-1 electrical field registered at 1440 km Gaobeidian station during the Wenchuan event. The result shows that the seismo-telluric current covers ~105–106 kA if the measuring frequency f = 0.01–10 Hz considered. The central magnitude of the vertical electrical field caused by the current at 0.01 Hz on the Earth's surface can be up to kV m-1. Then, this vertical field acts as an input into an electric field penetration model. It is shown that this field attenuates quickly at the atmosphere and completely vanishes at the top ionosphere and produces a 0.1 mV m-1 additional electrical field at the ionospheric bottom. Through the TIE-GCM, this additional electrical field causes 0.01 % ionospheric variations on electron density and TEC near the Wenchuan epicenter, as well as near its magnetically conjugated point. Further, the simulations have also been discussively performed on frequencies of 1 Hz and 10 Hz. The results demonstrate that the variations of electron density present their maximum values at the height of ~300–400 km and the varied percentages of ionospheric parameters have been beyond 10 %, the same magnitude as what has been registered during the Wenchuan shock. [ABSTRACT FROM AUTHOR]

The storm‐enhanced density (SED) is a large‐scale midlatitude ionospheric electron density enhancement in the local afternoon sector, which exhibits substantial spatial gradients and thus can impose detrimental effects on modern navigation and communication systems, causing potential space weather hazards. This study has identified a comprehensive list of 49 SED events over the continental US and adjacent regions, by examining strong geomagnetic storms occurring between 2000 and 2023. The ground‐based Global Navigation Satellite System (GNSS) total electron content and data from a new TEC‐based ionospheric data assimilation system were used to analyze the characteristics of SED. For each derived SED events, we have quantified its morphology by employing a Gaussian function to parameterize key characteristics of the SED, such as the plume intensity, central longitude, and half‐width. A statistical analysis of SEDs was conducted for the first time to characterize their climatological features. We found that the SED distribution exhibits a higher peak intensity and a narrower width as geomagnetic activity strengthens. The peak intensity of SED has maximum values around the equinoxes in their seasonal distribution. Additionally, we observed a solar cycle dependence in the SED distribution, with more events occurring during the solar maximum and declining phases compared to the solar minimum. SED plumes exhibit a sub‐corotation feature with respect to the Earth, characterized by a westward drift speed between 50 and 400 m/s and a duration of 3–10 hr. These information advanced the current understanding of the spatial‐temporal variation of SED characteristics. Key Points: A comprehensive list of 49 storm‐enhanced density (SED) events over the continental US was identified for periods of intense geomagnetic storms during 2000–2023A first‐time statistical analysis of SEDs demonstrates their geomagnetic dependence, seasonal distribution, and solar cycle variationSEDs demonstrate a sub‐corotate feature with respect to the Earth, with westward drifting speeds of 50–400 m/s and a duration of 3–10 hr [ABSTRACT FROM AUTHOR]

We report thermospheric exospheric temperature and composition responses on the 15 January 2022 Tonga volcanic eruption. The temperature and composition profiles are inversed from three ionosonde (MHJ45, EG931, FF051) observed electron density profiles (∼150–200 km) using our new method (Li, Ren, et al., 2023, https://doi.org/10.1029/2022ja030988). The retrieved exospheric temperatures all showed obvious eruption‐induced perturbations, with maximum disturbance magnitude of ∼200 K at MHJ45 and ∼100 K at EG931 and FF051. The temperature variations were related to eruption‐excited thermospheric waves and their propagation with different speeds. While column ∑O/N2 had no evident changes similar to temperatures, which were basically consistent with GOLD observations. In comparison, higher thermospheric O/N2 has larger eruption‐related changes, maybe due to the exponential increase of thermospheric wave amplitudes with height. The application of our inversion method, combined with continuous observations and global coverage of ionosonde data, provide a possibility to further investigate thermospheric responses to different geophysical conditions. Plain Language Summary: Extreme volcanic eruptions and resulted tsunami at 04:14:45 UT on 15 January 2022 generated a series of atmospheric waves, which can propagate out globally and up into the thermosphere. The ionosphere responses on this eruption, relative to thermosphere, have been reported a lot due to the large amounts of ionospheric observations. Here, we used the new method proposed by Li, Ren, et al. (2023), https://doi.org/10.1029/2022ja030988 to inverse daytime thermospheric parameters (neutral temperature and composition) from ionospheric electron density profiles (∼150–200 km). We selected ionosonde data at three stations (MHJ45, EG931, FF051) to verify the thermospheric responses during this eruption. The retrieved temperature at three stations showed the obvious eruption‐induced perturbations, but ∑O/N2 not, which were basically consistent with GOLD observations. However, O/N2 in higher thermosphere had larger eruption‐related changes. The comparison with GOLD observations and observed F2 layer peak electron densities verified the credibility of our inversion method again. Thus, the application of the method to the continuous and high‐covering ionosonde data provides a possibility to further investigate thermospheric responses to different geophysical conditions. Key Points: Inversed exospheric temperatures showed obvious eruption‐induced perturbations on the 15 January 2022 Tonga eruption∑O/N2 had no evident eruption‐induced changes similar to the temperature, neither in our inversion data nor in GOLD observationsIonosonde can expand the understanding of thermospheric responses to different geophysical conditions by our inversion method [ABSTRACT FROM AUTHOR]

This editorial titled "The future of space physics 2022" discusses the efforts of the space physics community in producing white papers for research on space physics, mission programs, and funding. It introduces a research topic called "The Future of Space Physics 2022," which contains 64 publications covering various topics in space physics. The document provides a summary of various concepts for new space missions in the field of heliophysics, including studying mesoscale dynamical structures in the solar wind and exploring the interaction between the heliosphere and the interstellar medium. It also emphasizes the importance of diversity and inclusion in the space community, the role of data science and computer simulations in heliophysics research, and the potential of citizen science in advancing space-physics research. The authors express their hope for the future of the heliophysics field and acknowledge the contributions of various individuals and organizations. [Extracted from the article]

As a fundamental physical process in the ionosphere, photoionization and the associated photoelectrons play vital roles in determining the ionospheric electron density and temperature for Earth and other planets with atmospheres such as Mars and Venus. The production and transport of ionospheric photoelectrons have been widely examined on Earth, but relatively less studied for other terrestrial planets, such as Mars and Venus. In this study, a two‐stream photoelectron transport model for Mars and Venus is constructed, in which the photoelectron fluxes, photoelectron heating rates, primary and secondary ionization rates are calculated. The simulated photoelectron fluxes agree with Mars Atmosphere and Volatile Evolution (MAVEN) observations at various altitudes, with the input of solar spectrum irradiance, electron density and temperature, neutral density and temperature observed by MAVEN. Moreover, by parametrically fitting the simulation results for various solar zenith angles and solar activities, we obtain empirical parameterized formulas for ionization and heating efficiencies which can potentially be adapted to planetary ionospheric models for the community. Key Points: A two‐stream model for Venus and Mars provides photoelectron flux, secondary ionization and photoelectron heating ratesThe photoelectron fluxes simulated by the two‐stream model agree with the Mars Atmosphere and Volatile Evolution observationsParameterized models of ionization and heating efficiencies are available for the community [ABSTRACT FROM AUTHOR]

Previous studies paid little attention to the ionospheric storm‐time altitudinal differences due to insufficiency of ionospheric measurements. In this work, multiple instrumental observations were used to investigate the ionospheric storm‐time response at low latitudes in the American and Asian‐Australian sectors during the May 2021 geomagnetic storm. The ground‐based Global Navigation Satellite Systems (GNSS) total electron content (TEC) and Low Earth Orbit (LEO) satellite topside TEC presented opposite (positive/negative) variations in the low‐latitude and equatorial region of both sectors during this storm. The electron density profiles from the Constellation Observing System for Meteorology, Ionosphere, and Climate‐2 (COSMIC‐2) and the Sanya Incoherent Scatter Radar showed a good agreement and well explained the opposite variations between GNSS TEC and LEO satellite topside TEC. The F2‐layer peak height (hmF2) and peak density (NmF2) displayed inverse variations, and the feature was present mostly in the regions between equatorial ionization anomaly crests. The combined modulation effects of the storm‐time zonal electric fields and the field‐aligned transports possibly resulted in the contrary variations of hmF2 and NmF2 in the low‐latitude and equatorial region, leading to the storm‐time altitudinal differences during this storm. Relatively, the storm‐time thermospheric composition disturbances might be a minor factor responsible for these differences. Plain Language Summary: The ionospheric storm‐time responses have been the research hotspot for nearly 80 years. When geomagnetic storms occur, the temperature, electron density, and electric field of the ionosphere are violently disturbed due to the enhanced energy coupled into the ionosphere, which are called ionospheric storms. The increases (decreases) of ionospheric electron density and total electron content are called positive (negative) storm during ionospheric storm. The ionospheric storm‐time responses present complex variations such as sectoral differences, latitudinal differences, and altitudinal differences. The altitudinal differences are mainly reflected in that the ionospheric storm‐time responses are inconsistent at different altitudes. This study focuses on the ionospheric storm‐time altitudinal differences at low latitudes during the May 2021 geomagnetic storm. In the low‐latitude and equatorial region, the ionospheric storm‐time electron densities at different altitudes show inconsistent variations. The storm‐time zonal electric fields and field‐aligned transports may play significant roles in the ionospheric storm‐time altitudinal differences in the low‐latitude and equatorial region during this storm. Key Points: The ionospheric storm‐time altitudinal differences were studied by comparing Global Navigation Satellite Systems (GNSS) total electron content (TEC), Swarm, COSMIC‐2, Sanya Incoherent Scatter Radar, Ionospheric Connection Explorer, and Global Scale Observations of the Limb and DiskThe ground‐based GNSS TEC and topside Low Earth Orbit TEC revealed opposite variations in both the Asian‐Australian and American sectorsStorm‐time zonal electric fields and field‐aligned transports possibly played key roles [ABSTRACT FROM AUTHOR]

Ionospheric density structures at low latitudes range in size from thousands of kilometers down to a few meters. Radio frequency (RF) signals, such as those from global navigation satellite systems, that propagate through irregularities suffer from rapid fluctuations in phase and intensity, known as scintillations. In this study, we use the high‐sample‐rate measurements of the Swarm Echo (CASSIOPE/e‐POP) satellite's GPS Occultation (GAP‐O) receiver taken after its antenna was re‐oriented to vertical‐pointing, simultaneously with e‐POP Ion Mass Spectrometer surface current observations as a proxy for plasma density, to obtain the spectral characteristics of GPS signal intensity and in‐situ irregularities at altitudes from 350 to 1,280 km. We show that the power spectra of both measurements can generally be characterized by a power law. In the case of density irregularities, the spectral index with the highest occurrence rate is around 1.7, which is consistent with previous studies. Also, all the power spectra of GPS signal intensity in this study show a single spectral index near 2. Moreover, roll‐off frequencies estimated in this work range from 0.4 to 2.5 Hz, which is significantly higher than Fresnel frequencies calculated from ground GPS receivers at low latitudes (between 0.2 and 0.45 Hz). Part of this increase is due to the 8 km/s orbital velocity of Swarm Echo near perigee. Another key difference is that variations in the GPS signals in this study are dominated by the topside ionosphere, whereas GPS signals received from ground are affected mostly by the relatively dense F‐region plasma in the 250–350 km altitudinal range. Plain Language Summary: Variations in ionospheric electron density, called irregularities, cause fluctuations in radio frequency signals traversing the ionosphere. The Swarm Echo satellite carries a high‐sample‐rate GPS receiver with an antenna normally pointing in the horizontal direction. In this study, the satellite was reoriented while it was passing the low latitude region so that the receiver antenna would point vertically in order to probe irregularities above the satellite at high resolution. Our spectral analysis of GPS signal intensity and plasma density variations measured in situ (on board the spacecraft) revealed power law behavior in both measurements, a well‐established feature of ionospheric turbulence. More specifically, our results show that the spectral index (which is the negative of the power law slope) of in‐situ density irregularities and GPS signal intensities are 1.7 and 2, respectively. The roll‐off frequencies in the GPS signal intensity results are considerably higher than ground‐based estimates. This difference is attributable to the satellite's high velocity (8 km/s) and the fact that, comparing to the ground estimates which are dominated by the dense plasma density between 250 and 350 km altitude, Swarm Echo observations are mostly affected by the upper reaches of the ionosphere. Key Points: The most frequently occurring spectral index of the in‐situ irregularity power spectra is 1.7The power spectra of GPS signal intensity show a single spectral index near 2, which is smaller than ground‐based weak scintillation valuesThe roll‐off frequencies in the signal intensity power spectra are 0.4–2.5 Hz, significantly exceeding ground‐based Fresnel frequency values [ABSTRACT FROM AUTHOR]

Space weather variations can significantly affect ionospheric electron density, which can, in turn, adversely affect various navigational and communication technologies. One such phenomenon is the prompt penetration electric field. The convective electric field from a magnetospheric origin penetrates the lower ionosphere during a geomagnetic storm. The electric field penetrating the ionosphere can challenge space-based technologies. Thus, understanding the convectional electric field from higher latitudes to the low-latitudinal region during geomagnetic disturbance is critical. The ionosphere over the Indian region consists of equatorial and low latitude dynamics, such as equatorial ionization anomaly, that are highly dynamic even during the quiet days. Therefore, understanding the effects of prompt penetration electric field observed in the Equatorial Electrojet is of utmost importance over the Indian region. In the current study, two geomagnetic storms, the St. Patrick's Day storm of 17th March 2015 and another low intense storm of 8th June 2014, were chosen to understand the effects of PPEF over equatorial ionization anomaly. The ionospheric density was enhanced during the relatively less intense geomagnetic storm compared to the St. Patrick's Day storm. In these two events, large-scale ionospheric irregularities were observed from ROTI values during the local daytime hours, but no ionospheric scintillation was detected (S4 index). [ABSTRACT FROM AUTHOR]