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In addition to being driven by tidal winds, the sporadic E (Es) layers are modulated by gravity waves (GWs), although the effects are not yet comprehensively understood. In this article, we discuss the effects of mesoscale GWs on the Es layers determined by using a newly developed model, MISE-1D (one-dimensional Model of Ionospheric Sporadic E), with low numerical dissipation and high resolution. Driven by the wind fields resolved by the high-resolution version of the Whole Atmosphere Community Climate Model with thermosphere and ionosphere extension (WACCM-X), the MISE-1D simulation revealed that GWs significantly influence the evolution of the Es layer above 100 km but have a very limited effect at lower altitudes. The effects of GWs are diverse and complex, generally including the generation of fluctuating wavelike structures on the Es layer with frequencies similar to those of the GWs. The mesoscale GWs can also cause increases in the density of Es layers, or they can disperse or diffuse the Es layers and increase their thickness. In addition, the presence of GWs is a key factor in sustaining the Es layers in some cases.

Abstract The strongest geomagnetic storm in the preceding two decades occurred in May 2024. Over these years, ground‐based observational capabilities have been significantly enhanced to monitor the ionospheric weather. Notably, the newly established Sanya incoherent scatter radar (SYISR) (Yue, Wan, Ning, & Jin, 2022, https://doi.org/10.1038/s41550‐022‐01684‐1), one of the critical infrastructures of the Chinese “Meridian Project,” provides multiple parameter measurements in the upper atmosphere at low latitudes over Asian longitudies. Unique ionospheric changes on superstorm day 11 May were first recorded by the SYISR experiments and the geostationary satellite (GEO) total electron content (TEC) network over the Asian sector. The electron density or TEC displayed wavelike structures rather than a regular diurnal pattern. Surprisingly, two humps, a common feature in the daytime equatorial ionization anomaly structure, disappeared. The SYISR observations revealed that multiple wind surges accompanied the downward phase propagation caused by atmospheric gravity waves (AGWs) originating from auroral zones. Meanwhile, strong upward and large downward drifts were respectively observed in the daytime and around sunset. The Thermosphere‐Ionosphere Electrodynamics Global Circulation Model (TIEGCM) simulations demonstrated that abnormal ionospheric changes were attributed to meridional wind disturbances associated with AGWs and recurrent penetration electric fields corresponding to larger Bz southward excursions and disturbance dynamo. The complicated interplay between AGWs and disturbance electric fields contributed to this unique ionospheric variation.

Abstract Through applying a 4‐MHz linear frequency modulation waveform, which has high range resolution and signal intensity, we successfully detected for the first time the ionospheric 150‐km echo enhancement at 430–450 MHz of the Ultra‐High‐Frequency (UHF) band using the newly built Sanya Incoherent Scatter Radar (SYISR). The obtained low signal enhancement (less than 0.5 dB) explains why previous UHF experiments did not detect them. We also found that our measured fine structure shows a much wider forbidden region than previous results and covers a much larger altitudinal and local time region. In comparison with recent upper‐hybrid instability theory and simulation, our results confirmed the predicted higher altitude occurrence, wider gaps between enhancements, the turn corner feature around sunrise, and perhaps the weak enhancement, which provide an independent evaluation of the newly proposed mechanism in UHF band. Future UHF experiments could further improve the physical understanding of 150‐km echo phenomenon.

Abstract Low latitude ionosphere experiences complex dynamical and electrodynamical processes, which make the spatiotemporal variations of the corresponding electron density complicated and therefore influence trans‐ionosphere radio communications. The monitoring of low latitude dynamical drivers, such as neutral wind and ionospheric electric field, is essential for both dynamic mechanism investigations and applications. The Sanya Incoherent Scatter Radar Tristatic System (SYISR‐TS) was proposed with the main objective of low latitude ionospheric monitoring and investigation and has been successfully developed over the past decade. The system consists of the Sanya (18.3°N, 109.6°E) trans‐receiving main station with key parameters of ∼1,600 m2 antenna aperture, >4 MW peak power,

Abstract The F2‐peak plasma frequency (foF2) and the height of the F2 peak (hmF2) are two of the most important parameters for any ionospheric model, as well as radio propagation studies and applications. In this study, we have developed empirical models to capture the most significant variations of foF2 and hmF2. The derived empirical models (referred to as the USTC models within this study) are specified through global ionosonde and reanalysis data based on the International Reference Ionosphere (IRI) Consultative Committee on International Radio (CCIR) method and Constellation Observindg System for Meteorology, Ionosphere, and Climate (COSMIC) observations based on the empirical orthogonal function analysis, respectively. The USTC models are validated against the IRI CCIR model prediction. The comparison results revealed that the empirical foF2 model performs better in capturing the foF2 variations than the IRI CCIR model, which can overcome the underestimation of the IRI CCIR model at low latitudes. Although the IRI CCIR model overestimation at middle latitudes is addressed by the empirical hmF2 model, the visible differences between the model predictions and ionosonde observations still exist at low latitudes, which could be attributed to the significant difference between COSMIC and ionosonde hmF2 measures.

In this study, we used the Sanya Incoherent Scatter Radar (SYISR) to observe the altitude profiles of traveling ionospheric disturbances (TIDs) during a moderate magnetic storm from 13 to 15 March 2022. Three TIDs were recorded, including two large-scale TIDs (LSTIDs) and one medium-scale TID (MSTID). These LSTIDs occurred during the storm recovery phase, characterized by periods of ~110–155 min, downward phase velocities of 22–60 m/s, and a relative amplitude of 17–25%. A nearly vertical front was noted at ~350–550 km, differing from AGW theory predictions. This structure is more attributed to the combined effects of sunrise-induced electron density changes and pre-sunrise uplift. Moreover, GNSS observations linked this LSTID to high-latitude origins, indicating a connection to polar magnetic storm excitation. However, the second LSTID was observed at lower altitudes (150–360 km) with a higher elevation angle (~17°). This LSTID, observed by the SYISR, was absent in the GNSS data from mainland China and Japan, suggesting a potential local source. The MSTID exhibited a larger relative amplitude of 29–36% at lower altitudes (130–210 km) with severe upward attenuation. The MSTID may be related to atmospheric gravity waves from the lower atmosphere. AGWs are considered to be the perturbation source for this MSTID event.

Abstract On 3 February 2022, 38 satellites launched by SpaceX re‐entered the atmosphere and were subsequently destroyed. An investigation found that a minor geomagnetic storm occurred on 3–4 February 2022 led to a neutral density enhancement and large atmospheric drag. To better understand the responses of the thermosphere to geomagnetic storms, the method proposed by Li et al. (2023, https://doi.org/10.1029/2022ja030988) was employed to extract exospheric temperature (Tex) from ionosonde electron density profiles (∼150–200 km) in Beijing (geolocation: 39.56°N; 116.2°E; geomagnetic location: 30.16°N; 172.08°W) station. The retrieved Tex was plugged into the NRLMSISE‐00 model to calculate the corresponding neutral density. Derived results showed a ∼2%–7% enhancement in Tex and a ∼15%–38% enhancement in neutral density at 430 km. The relative deviation in neutral density on the satellites’ orbital trajectory ranges from ∼10% (210 km) to ∼35% (500 km) on 3 February, and from ∼13% (210 km) to ∼60% (500 km) on 4 February. Furthermore, the neutral density reproduced the variations observed by the SWARM‐C satellite fairly well both on quiet and disturbed days. These results suggest that even a minor geomagnetic storm can cause significant changes in neutral temperature and neutral density at middle latitudes. Additionally, the application of our inversion method, combined with the global, long‐term and real‐time ionospheric observations from ionosondes, provides an opportunity to improve the capability of thermosphere forecasting and nowcasting.

The dune aurora, at a scale of ~30 kilometers, was reported recently using ground camera. The small-scale dune aurora occurs on the duskside and exhibits a monochromatic oscillation in the auroral emission, implying fundamental energy conversions. However, whether the dune auroras correspond to atmospheric waves or are associated with magnetospheric dynamics should be determined. This paper reports a dune aurora that occurred during a storm; further, we demonstrate that it was the substructure of the sawtooth aurora that was generated by plasmapause surface waves. Conjugate observations in the magnetospheric source region suggest that the exohiss waves, which are periodically modulated by the plasmapause surface wave-excited ultralow frequency wave, might be responsible for the generation of the dune aurora. Most reported dune aurora events have occurred simultaneously with sawtooth auroras, suggesting that both are plasmapause-driven cross-scale auroral structures.

Abstract The presence of grand minima, characterized by significantly reduced solar and stellar activity, brings a challenge to the understanding of solar and stellar dynamo. The Maunder Minimum (1645–1715 AD) is a representative grand solar minimum. The cyclic variation of solar activity, especially the cycle length during this period, is critical to understand the solar dynamo but remains unknown. By analyzing the variations in solar activity‐related equatorial auroras recorded in Korean historical books in the vicinity of a low‐intensity paleo‐West Pacific geomagnetic anomaly, we find clear evidence of an 8‐year solar cycle rather than the normal 11‐year cycle during the Maunder Minimum. This result provides a key constraint on solar dynamo models and the generation mechanism of grand solar minima.

Abstract On 3 February 2022, at 18:13 UTC, SpaceX launched and a short time later deployed 49 Starlink satellites at an orbit altitude between 210 and 320 km. The satellites were meant to be further raised to 550 km. However, the deployment took place during the main phase of a moderate geomagnetic storm, and another moderate storm occurred on the next day. The resulting increase in atmospheric drag led to 38 out of the 49 satellites reentering the atmosphere in the following days. In this work, we use both observations and simulations to perform a detailed investigation of the thermospheric conditions during this storm. Observations at higher altitudes, by Swarm‐A (∼438 km, 09/21 Local Time [LT]) and the Gravity Recovery and Climate Experiment Follow‐On (∼505 km, 06/18 LT) missions show that during the main phase of the storms the neutral mass density increased by 110% and 120%, respectively. The storm‐time enhancement extended to middle and low latitudes and was stronger in the northern hemisphere. To further investigate the thermospheric variations, we used six empirical and first‐principle numerical models. We found the models captured the upper and lower thermosphere changes, however, their simulated density enhancements differ by up to 70%. Further, the models showed that at the low orbital altitudes of the Starlink satellites (i.e., 200–300 km) the global averaged storm‐time density enhancement reached up to ∼35%–60%. Although such storm effects are far from the largest, they seem to be responsible for the reentry of the 38 satellites.

Abstract We used dense global navigation satellite system data from China to track the propagation of traveling ionospheric disturbances (TIDs) triggered by the 2022 January 15 Tonga volcanic eruption. We identified two TIDs originating from the eruption. One, which has been reported widely by a number of recent investigations, had a velocity of ∼361 m/s. However, another long‐distance propagating TID with a velocity of ∼264 m/s has not been widely discussed. The velocities of these TIDs coincide with previous simulation results of gravity‐wave L0 and L1 modes. We propose that these TIDs were caused by the L0 and L1 ducted modes of gravity waves excited by the volcanic eruption. However, the L1 mode is usually negligible due to its weak amplitude in comparison with that of the L0 mode. The enormous energy release resulted in a stronger amplitude of the L1 mode, which induced a detectable TID in the ionosphere.

Climate change is characterized by global surface warming associated with the increase of greenhouse gas population since the start of the industrial era. Growing evidence shows that the upper atmosphere is experiencing appreciable cooling over the last several decades. The seminal modeling study by Roble and Dickinson (1989) suggested potential effects of increased greenhouse gases on the ionosphere and thermosphere cooling which appear consistent with some observations. However, several outstanding issues remain regarding the role of CO2, other important contributors, and impacts of the cooling trend in the ionosphere and thermosphere: for example, (1) what is the regional variability of the trends? (2) the very strong ionospheric cooling observed by multiple incoherent scatter radars that does not fit with the prevailing theory based on the argument of anthropogenic greenhouse gas increases, why? (3) what is the effect of secular changes in Earth’s main magnetic field? Is it visible now in the ionospheric data and can it explain some of the regional variability in the observed ionospheric trends? (4) what is the impact of long-term cooling in the thermosphere on operational systems? (5) what are the appropriate strategic plans to ensure the long-term monitoring of the critical space climate?

Abstract The upper boundary height of the traditional community general circulation model of the ionosphere‐thermosphere system is too low to be applied to the topside ionosphere/thermosphere study. In this study, the National Center for Atmospheric Research Thermosphere‐Ionosphere‐Electrodynamics General Circulation Model (NCAR‐TIEGCM) was successfully extended upward by four scale heights from 400–600 km to 700–1,200 km depending on solar activity, named TIEGCM‐X. The topside ionosphere and thermosphere simulated by TIEGCM‐X agree well with the observations derived from a topside sounder and satellite drag data. In addition, the neutral density, temperature, and electron density simulated by TIEGCM‐X are morphologically consistent with the NCAR‐TIEGCM simulations before extension. The latitude‐altitude distribution of the equatorial ionization anomaly derived from TIEGCM‐X is more reasonable. During geomagnetic storm events, the thermospheric responses of TIEGCM‐X are similar to NCAR‐TIEGCM. However, the ionospheric storm effects in TIEGCM‐X are stronger than those in NCAR‐TIEGCM and are even opposites at some middle and low latitudes due to the presence of more closed magnetic field lines. Defense Meteorological Satellite Program observations prove that the ionospheric storm effect of TIEGCM‐X is more reasonable. The well‐validated TIEGCM‐X has significant potential applications in ionospheric/thermospheric studies, such as the responses to storms, low‐latitude dynamics, and data assimilation.

Responses of atmospheric carbon dioxide (CO2) density to geomagnetic secular variation are investigated using the Whole Atmosphere Community Climate Model-eXtended (WACCM-X). Our ensemble simulations show that CO2 volume mixing ratios (VMRs) increase at high latitudes and decrease at mid and low latitudes by several ppmv in response to a 50% weakening of the geomagnetic field. Statistically significant changes in CO2 are mainly found above ~90 km altitude and primarily redetermine the energy budget at ~100–110 km. Our analysis of transformed Eulerian mean (TEM) circulation found that CO2 change is caused by enhanced upwelling at high latitudes and downwelling at mid and low latitudes as a result of increased Joule heating. We further analyzed the atmospheric CO2 response to realistic geomagnetic weakening between 1978 and 2013, and found increasing (decreasing) CO2 VMRs at high latitudes (mid and low latitudes) accordingly. For the first time, our simulation results demonstrate that the impact of geomagnetic variation on atmospheric CO2 distribution is noticeable on a time scale of decades.

When ground-based radar and range-Doppler algorithm are used to image the nearside of the Moon, it is inevitable to encounter the problem of "north-south ambiguity". This is because when the range-Doppler imaging algorithm is used to image a rotating celestial body, the echoes of the two points conjugated at the apparent equator are superposed together and cannot be resolved in the range-Doppler image. We propose a solution to this problem based on the Sanya Incoherent Scatter Radar, namely the mosaic imaging technology of the northern and southern hemispheres of the near side of the Moon. In this technology, two independent experiments were carried out to separately illuminate the northern and southern hemispheres of the nearside of the Moon by adjusting the beam direction to a specific position. Finally, a complete Moon nearside map was obtained by combining the images of the northern and southern hemispheres. The results of experiments show that this technique can successfully get the Moon images, but there are still some defects that need to be improved.

Abstract The intense plasma irregularities within the ionospheric sporadic E (Es) layers at 90–130 km altitude have a significant impact on radio communications and navigation systems. As a result, the modeling of the Es layer is very important for the accuracy, reliability, and further applications of modern real‐time global navigation satellite system precise point positioning. In this study, we have constructed an empirical model of the Es layer using the multivariable nonlinear least‐squares‐fitting method, based on the S4max from Constellation Observing System for Meteorology, Ionosphere, and Climate satellite radio occultation measurements in the period 2006–2014. The model can describe the climatology of the intensity of Es layers as a function of altitude, latitude, longitude, universal time, and day of year. To validate the model, the outputs of the model were compared with ionosonde data. The correlation coefficients of the hourly foEs and the daily maximum foEs between the ground‐based ionosonde observations and model outputs at Beijing are 0.52 and 0.68, respectively. The model can give a global climatology of the intensity of Es layers and the seasonal variations of Es layers, although the Es layers during the summer are highly variable and difficult to accurately predict. The outputs of the model can be implemented in comprehensive models for a description of the climatology of Es layers and provide relatively accurate information about the global variation of Es layers.

In the past decades, the Incoherent Scatter Radar (ISR) has been demonstrated to be one of the most powerful instruments for ionosphere monitoring. The Institute of Geology and Geophysics at the Chinese Academy of Sciences was founded to build a state-of-the-art phased-array ISR at Sanya (18.3°N, 109.6°E), a low-latitude station on Hainan Island, named the Sanya ISR (SYISR). As a first step, a prototype radar system consisting of eight subarrays (SYISR-8) was built to reduce the technical risk of producing the entire large array. In this work, we have summarized the preliminary experimental results based on the SYISR-8. The amplitude and phase among 256 channels were first calibrated through an embedded internal monitoring network. The mean oscillation of the amplitude and phase after calibration were about 1 dB and 5°, respectively, which met the basic requirements. The beam directivity was confirmed by crossing screen of the International Space Station. The SYISR-8 was further used to detect the tropospheric wind profile and meteors. The derived winds were evaluated by comparison with independent radiosonde and balloon-based GPS measurements. The SYISR-8 was able to observe several typical meteor echoes, such as the meteor head echo, range-spread trail echo, and specular trail echo. These results confirmed the validity and reliability of the SYISR-8 system, thereby reducing the technical risk of producing the entire large array of the SYISR to some extent.

Abstract Using high‐resolution global navigation satellite system data with a temporal resolution of 1 s, we presented observations of short‐period concentric traveling ionospheric disturbances (CTIDs) triggered by the launch of the Long March 4B rocket at 03:26 UT 7 December 2014 in Taiyuan, China. The total electron content (TEC) perturbations observed approximately 22 min after the launch have amplitudes of ∼0.1–0.3 TEC units, horizontal phase velocities of ∼832–1,348 m/s, and durations of ∼600 s. With periods of ∼90–200 s, these disturbances are the shortest‐period TIDs excited during rocket launches reported thus far. The CTIDs began with a quick increase in the amplitude, followed by 5–6 periodic oscillations with decreasing amplitude. A continuous southward propagation of 5 wave fronts was observed, which propagated southward at a distance of ∼450 km in 600 s. A slow decrease in perturbation amplitude during propagation was found. According to the direction of horizontal velocity observed by different stations, we found that the source region of the CTIDs was located ∼140 km south of the launch site. The CTIDs were most likely generated after the ignition of second‐stage rockets above the mesopause region.

The plasma lines observed by Sanya incoherent scatter radar (SYISR) are dependent on the enhancement of Langmuir waves due to superthermal photoelectrons generated by solar EUV radiation. The plasma line power spectrum can be obtained using long-pulse and alternating-code transmission signals during the period from sunrise to noon almost every day. For the power spectrum of the long pulse, the CLEAN algorithm that has been applied in this field is used to verify the feasibility of this method for SYISR in only a few cases. However, it is difficult to deal with alternating code with such a low SNR using the general deconvolution method. The irreversible-migration filtering (IMF) method has been developed to separate signal noise from the measurements of the alternating code. Some experimental results from the SYISR measurements validate the excellent performance of the IMF method for alternating code. Additionally, an example observation of the electron density with a high time and range resolution is derived. The results show that plasma line detection can be a powerful new observational capability for SYISR as an ionospheric experimental mode for ionospheric calibration, when possible, which can be simultaneously measured with the ion line for constant radar calibration in the standard fitting of the ion line.

As the first advanced modular phase array incoherent scatter radar (ISR) established in the Eastern Hemisphere at low latitudes, Sanya ISR (SYISR) can measure the line-of-sight (LOS) velocity of ion drift in multiple directions, potentially yielding the spatial distribution of ionospheric plasma drift. Three beam-scanning modes are designed for plasma drift detection: meridian, zonal and cross-shaped (both meridian and zonal) plane, which will provide the distribution of plasma drift in latitude/longitude as well as altitude. The altitude profile of plasma drift and the first presented distribution of low latitude plasma drift in the meridian plane for March to May 2021 are inversed through LOS velocity using cross-shaped and meridian beam-scanning modes, respectively. A statistical correlation coefficient between the vpn and crest-to-trough ratio (CTR) of equatorial ionization anomaly (EIA) TEC and a case study of magnetic storm response in plasma drift show that the inversed plasma drift can be a good indicator in response to the changes in atmospheric tide and solar wind at different time scales and explain the corresponding ionospheric electron density variations at low and equatorial latitudes. This study proves that the SYISR-measured plasma drift is reliable and will play an important role in the atmosphere-ionosphere-magnetospheric coupling study in the East Asian region.

Abstract Geomagnetic storm can affect the performance of Global Navigation Satellite System (GNSS) precise positioning services. To mitigate the adverse effects of strong geomagnetic storms, we propose to establish the geometry‐free (GF) cycle slip threshold model based on ionospheric disturbance index rate of total electron content index to reduce the false detection rate of cycle slip in GNSS precise point positioning (PPP) during strong storm periods, thus improving the accuracy and reliability of GNSS PPP. The performance of our proposed model is validated by using 171 International GNSS Service (IGS) tracking stations data on 8 September 2017. The analysis indicates that compared with conventional PPP scheme, the proposed model can improve the positioning accuracy by approximately 14.0% (36.8%) and 23.1% (51.5%) in the horizonal and vertical components for global (high latitudes) stations. Furthermore, the availability of our proposed model is also validated by PPP experiments using 379 IGS tracking stations data during another strong storm occurred on 26 August 2018.

The radar constant calibration in incoherent scatter radar ion line processing is essential for the data quality and was not paid enough attention in previous studies. In this investigation, based on several experiments made by the newly built Sanya incoherent scatter radar (SYISR), we made and evaluated the ion line calibration by plasma line both in case study and statistically. The calibration factor had local time and altitude variations, due to the corresponding variations of the transmitted power, the radar gain, and the noise temperature. We obtained a mean calibration factor of 1.35 by the simultaneous measured plasma line and ion line electron density and applied it to a one-month ion line observation calibration. Through a co-located ionosonde measured foF2 evaluation, the calibration decreased the mean deviation from −1.92 MHz (−18%) to −0.33 MHz (−3%), which resulted in much better agreement between the ion line foF2 after calibration and the ionosonde results. The existed deviations between after calibration and ionosonde results were due to the uncertainties either in the used calibration factor or the ionosonde measurements. An empirical Te/Ti usage in raw electron density estimation and ignoring the seasonal and short-term variations of the effecting factors might influence the calibration performance. Using the to-be-completed SYISR Tristatic System, the performance of plasma line calibration technique is expected to be improved in the future.

Abstract Accurate specification of the thermosphere states is crucial to the low Earth orbit satellite operation. In this work, the impact of different ionosphere and thermosphere observing systems on the improvement of neutral temperature of the data assimilation model has been investigated by a series of observing system simulation experiments. The selected observations include the Global Navigation Satellite System total electron content (e.g., MIT vertical total electron content [VTEC]) and the daytime Global‐scale Observations of the Limb and Disk (GOLD) level‐2 disk temperature (Tdisk). Such observations are ingested into the coupled ionosphere and thermosphere model based on our developed ensemble Kalman Filter data assimilation systems on the basis of the ensemble Kalman filter algorithm and the National Center for Atmospheric Research Thermosphere Ionosphere Electrodynamics General Circulation Model. The main findings are as follows: (a) A considerable improvement of the neutral temperature estimation of the physical‐based model can be obtained in the global region by assimilating either the MIT VTEC or the GOLD Tdisk observations; (b) the assimilation of the GOLD can further contribute to temperature improvement in the lower thermosphere (

In the present work, the momentum fluxes of gravity wave (GW) around the mesopause are estimated, using the decadal continuous observations by meteor radar at Mohe (53.5°N, 122.3°E). Applying the Hocking’s (2005) approach with the modified-composite-day (MCD) analysis, the GW momentum fluxes of short-periods (less than 2 h) are estimated month by month. As the first step, several experiments are designed to evaluate the accuracy and uncertainty in the estimation. The results show that Mohe meteor radar has the ability to give reasonable estimations on the GW momentum fluxes at a height of 82–94 km, in which errors are generally less than 5 m2/s2. The uncertainty induced by different angular information of the detected meteor in each month achieves ~2 m2/s2. It is inferred that the variability of the GW momentum fluxes over 2 m2/s2 can be distinguished in the observation. The interannual variation of the estimated GW momentum fluxes show a significant enhancement in 2012, and a depression in 2013, with a fluctuation over ±10 m2/s2 at 82 km. However, no obvious quasi-biennial oscillation (QBO) -like signal has been found in the Lomb–Scargle periodogram.

Space objects around the Earth are a potential pollution source for ground-based radio observations. The Sanya incoherent scatter radar (SYISR) is a newly built active digital phased array, all solid-state transmitting and digital receiving incoherent scatter radar in Sanya (18.3°N, 109.6°E), with the main purpose of ionospheric monitoring and investigations. In this study, we presented the effect of the greatly increased number of space objects on ionospheric observations through SYISR. Firstly, we showed the space object pollution on the range-time-intensity (RTI), autocorrelation function (ACF)/power spectra, and ionosphere parameter of SYISR measurements. An altitude of around 600 km is the region where space objects occur most frequently. Then, we eliminated the space object pollution using the traditional smallest of constant-false-alarm-rate (SO-CFAR) algorithm. However, pollution from smaller space objects remains, whose reflected echo is comparable to or lower than the background ionosphere, which results in unrealistic retrieved ionospheric electron density. Furthermore, we quantitatively assessed the space object effect based on the current space object orbit database and simulation. The pollution should linearly increase with the increase in the number of space objects in the future. Among the space objects, whose radar cross section (RCS) and orbit information are now published, there still exist ~9000 (~37% of the total number) space objects, whose effect is difficult to eliminate. This study is beneficial to the data process of SYISR and has implications for similar types of ionospheric observations by radar.

Abstract Accurate ionospheric specification for the current and future is one of the key tasks in operational space weather. In this work, we have assessed the effect of a dense ground network consisting of different radio instruments either developed or under developing on ionosphere nowcasting and forecasting over China and adjacent region (0–60°N and 70–140°E) through observing system simulation experiments. The data assimilation system is an Ensemble Kalman filter (EnKF) ionosphere and thermosphere data assimilation algorithm. The National Center for Atmospheric Research Thermosphere Ionosphere Electrodynamics General Circulation Model is used as a background model. Effects of different observation types, including the slant total electron content from Beidou System geostationary satellite and Global Position System and electron density observations from ionosondes and incoherent scattering radar on the ionospheric nowcasting and forecasting, are examined by the accurate specification of ionosphere key parameters (total electron content and 3‐D electron density). We found that simultaneously assimilating different observation types can greatly improve the quality of ionosphere specification. Furthermore, updating the thermospheric state variables in the coupled thermosphere‐ionosphere forecast model in the assimilation step plays an important role in improving the ionosphere forecasting. The ionosphere forecasting capability can last longer (>24 hr) in each observing system simulation experiment due to the adjusted thermosphere states. This study can provide a reference for observing system design over the China region and future ionosphere forecasting operation.

The ionospheric sporadic E (Es) layer has a significant impact on the global positioning system (GPS)/global navigation satellite system (GNSS) signals. These influences on the GPS/GNSS signals can also be used to study the occurrence and characteristics of the Es layer on a global scale. In this paper, 5.8 million radio occultation (RO) profiles from the FORMOSAT-3/COSMIC satellite mission and ground-based observations of Es layers recorded by 25 ionospheric monitoring stations and held at the UK Solar System Data Centre at the Rutherford Appleton Laboratory and the Chinese Meridian Project were used to derive the hourly Es critical frequency (foEs) data. The global distribution of foEs with a high spatial resolution shows a strong seasonal variation in foEs with a summer maximum exceeding 4.0 MHz and a winter minimum between 2.0 and 2.5 MHz. The GPS/GNSS RO technique is an important tool that can provide global estimates of Es layers, augmenting the limited coverage and low-frequency detection threshold of ground-based instruments. Attention should be paid to small foEs values from ionosondes near the instrumental detection limits corresponding to minimum frequencies in the range 1.28–1.60 MHz.

Sanya incoherent scatter radar (SYISR) is a newly developed phased array incoherent scatter radar in the low latitudes of China located at Sanya (18.3°N, 109.6°E), Hainan Province. The main objective of SYISR is to observe the ionosphere. Given its frequency and power, it should have the capability to observe the troposphere. In this study, we show several tropospheric wind experiments that may indicate radar function expansion and capability verification, although observing the troposphere will not be an operation mode in the future. Reliable radar echoes were detected by SYISR up to 20 km with a turbulence scale of 0.35 m and a frequency of 430 MHz. Generally, both the geometric (GEO) method and the velocity azimuth display (VAD) method give similar wind profiles. Above 10 km, the discrepancy between the two methods becomes nonnegligible. For the same method, the discrepancy above 15–20 km among winds derived from different zenith angle measurements is nonnegligible. The VAD methods give more reasonable results at higher altitudes. The standard deviation of the difference (SYISR radar minus the reanalysis data ERA5) for zonal wind and meridional wind was 1.1 m/s and 0.78 m/s, respectively. During rainfall, we can distinguish the spectrum of rainfall and atmospheric turbulence from the power spectrum according to the spectral widths and Doppler frequency shifts.

Previous ground-based, radar lunar imaging experiments have usually employed the Range-Doppler (RD) algorithm. This algorithm performs in the frequency domain and has high computational efficiency. However, in the case of a long coherent integration time, the defocus phenomenon will appear, and the image will be smeared. This study proposes the use of the back projection (BP) algorithm to obtain focused lunar images to solve this problem. The BP algorithm is a time-domain algorithm which is frequently employed in synthetic aperture radar (SAR) imaging and can theoretically achieve the focused imaging of each pixel in an arbitrarily long coherent integration time. However, the largest drawback of this algorithm is its high computational complexity. Therefore, this study only applies this method to map local regions of the moon. We select Sanya incoherent scatter radar (SYISR) as the transmitting and receiving device and utilize the linear frequency modulation chirp pulse to transmit right-hand, circularly polarized electromagnetic waves and to receive left-hand, circularly polarized echoes. RD and BP algorithms are simultaneously adopted to image the Pythagoras crater region, and a contrastive analysis is performed. The results show that the BP algorithm can be well applied to a ground-based, radar lunar imaging experiment and that it has a better focusing performance, but the effect is not as obvious as expected. Thus, the processing method needs to be further improved. In addition, the computational efficiency of BP is very low, and certain fast algorithms need to be applied to improve it.

Electron density is a key parameter to characterize Martian ionospheric structure and dynamics. Based on the ephemeris and auxiliary information derived from the Spacecraft, Planet, Instruments, C-matrix, and Events (SPICE) toolkit, we calculated the bending angle of signal path from the frequency residuals measured by the Mars Express Radio Science Experiment (MaRS) of the Mars Express (MEX) mission under the assumption of a spherically symmetric ionosphere. We stratified the ionosphere into layers and assumed a linear change of bending angle between layers, to derive profiles in radial distance of refractivity with the optimized parameters of upper integral limit of 4890 km and baseline correction boundary of 3690 km. Meanwhile, we also compared the retrieved electron density profiles between the frequency residuals of the single-frequency and differential Doppler of the dual-frequency. In total, ~640 electron density profiles of Martian ionosphere between April 2004 and April 2015 were retrieved successfully. There are 24 profiles identified manually that exhibit an additional sporadic layer occurrence below the normal two-layers. We also found that the peak altitude of this layer increases with the main peak altitude.

The planet Earth is an integrated system, in which its multi-spheres are coupled, from the space to the inner core. Whether the space environment in short to long terms has been controlled by the earth’s interior process is contentious. In the past several decades, space weather and space climate have been extensively studied based on either observation data measured directly by man-made instruments or ancient data inferred indirectly from some historical medium of past thousands of years. The acquired knowledge greatly helps us to understand the dynamic processes in the space environment of modern Earth, which has a strong magnetic dipole and an oxygen-rich atmosphere. However, no data is available for ancient space weather and climate (>5 ka). Here, we propose to take the advantage of " space-diversity” to build a " generalized planetary space family”, to reconcile the ancient space environment evolution of planet Earth from modern observations of other planets in our solar system. Such a method could also in turn give us a valuable insight into other planets’ evolution.

Abstract In this work, we evaluated the quasi‐realistic ionosphere forecasting capability by an ensemble Kalman filter (EnKF) ionosphere and thermosphere data assimilation algorithm. The National Center for Atmospheric Research Thermosphere Ionosphere Electrodynamics General Circulation Model is used as the background model in the system. The slant total electron contents (TECs) from global International Global Navigation Satellite Systems Service ground‐based receivers and from the Constellation Observing System for Meteorology, Ionosphere and Climate are assimilated into the system, and the ionosphere is then predicted in advance during the quiet interval of 23 to 27 March 2010. The predicted ionosphere vertical TEC (VTEC) and the critical frequency foF2 are validated by the Massachusetts Institute of Technology VTEC and global ionosondes network, respectively. We found that the ionosphere forecast quality could be enhanced by optimizing the thermospheric neutral components via the EnKF method. The ionosphere electron density forecast accuracy can be improved by at least 10% for 24 hr. Furthermore, the Thermosphere, Ionosphere, Mesosphere Energetics and Dynamics/Global Ultraviolet Imager (TIMED/GUVI) [O/N2] observations are used to validate the predicted thermosphere [O/N2]. The validation shows that the [O/N2] optimized by EnKF has better agreement with the TIMED/GUVI observation. This study further demonstrates the validity of EnKF in enhancing the ionospheric forecast capability in addition to our previous observing system simulation experiments by He et al. (2019, https://doi.org/10.1029/2019JA026554).

Abstract We define a new thermospheric concept, the reference heights of O/N2, referring to a series of thermospheric heights corresponding to the fixed ratios of O to N2 number density. Here, based on Global Ultraviolet Imager (GUVI) limb measurement, we compare O/N2 column density ratio (∑O/N2) and the reference heights of O/N2. We choose the transition height of O and N2 (transition height hereafter), a special reference height at which O number density is equal to N2 number density, to verify the connection with ∑O/N2 during geomagnetically quiet periods. It is found that transition height and ∑O/N2 have noticeable negative correlation with correlation coefficient of ‐0.887. An empirical model of transition height (O/N2 model hereafter) is established based on nonlinear least‐squares‐fitting method. The considerable correlation (greater than 0.96), insignificant errors (less than 4%) and the great influencing weight of ∑O/N2 to reference heights indicate the validity of O/N2 model and the existence of quantitative relation between ∑O/N2 and transition height. Besides, it is verified that the similar quantitative relation also exists between ∑O/N2 and reference heights of other O/N2 values. Namely, using the O/N2 model coefficients, we can roughly get the whole altitude profiles of O/N2 within 6% precision for any given ∑O/N2.

Choi et al. (2019) analyzed the correlation between the ionospheric total electron content (TEC) and the Global Navigation Satellite System (GNSS) receiver differential code bias (DCB) and concluded that the long-term variations of the receiver DCB are caused by the corresponding variations in the ionosphere. Unfortunately, their method is problematic, resulting in conclusions that are not useful. The long-term variations of the Global Positioning System (GPS) DCBs are primarily attributed to the GPS satellite replacement with different satellite block series under the zero-mean constraint condition, rather than the ionospheric variability.

The global ionospheric maps (GIMs), generated by Jet Propulsion Laboratory (JPL) and Center for Orbit Determination in Europe (CODE) during a period over 13 years, have been adopted as the primary source of data to provide global ionospheric correction for possible single frequency positioning applications. The investigation aims to assess the performance of new NeQuick model, NeQuick 2, in predicting global total electron content (TEC) through ingesting the GIMs data from the previous day(s). The results show good performance of the GIMs-driven-NeQuick model with average 86% of vertical TEC error less than 10 TECU, when the global daily effective ionization indices (Az) versus modified dip latitude (MODIP) are constructed as a second order polynomial. The performance of GIMs-driven-NeQuick model presents variability with solar activity and behaves better during low solar activity years. The accuracy of TEC prediction can be improved further through performing a four-coefficient function expression of Az versus MODIP. As more measurements from earlier days are involved in the Az optimization procedure, the accuracy may decrease. The results also reveal that more efforts are needed to improve the NeQuick 2 model capabilities to represent the ionosphere in the equatorial and high-latitude regions.

Atmosperic profiles derived from Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) radio occultation (RO) measurements make up for the lack of operational radiosonde soundings with a high spatiotemporal distribution, and their performance over China is assessed in this paper. COSMIC-retrieved atmospheric wet profiles from 2014 to 2015 are compared to the contemporaneous radiosonde profiles from 120 stations, and the vertical mean differences are used. The results show that the vertical mean biases of temperature, pressure and vapor pressure are −0.10 K, 0.69 hPa and −0.01 hPa, respectively, and that for refractivity is 0.17 N. Moreover, the temperature differences are positively correlated with station altitude, yet both the pressure and vapor pressure differences are negatively correlated with station latitude, as is the refractivity difference. The large temperature difference arising from the Qinghai-Tibet Plateau (QTP) region may be associated with the complex topography of the area and the limitations in the background model used in the COSMIC profile retrieval. Furthermore, negative refractivity bias between COSMIC and radiosonde data occurs below 5 km and is large in wet southern China, with a value of less than 1%. This result may be related to more humid conditions and super-refraction.