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A newly released, novel ionospheric data set of global gridded vertical total electron content (VTEC) is introduced in this paper. This VTEC data set, provided by Los Alamos National Laboratory (LANL), is derived from very high frequency (VHF; defined as 30–300 MHz) broadband radio‐frequency (RF) measurements of lightning made by U.S. Department of Defense sensing systems on board Global Positioning System (GPS) satellites. This paper presents the new data set (LANL VTEC), discusses the errors inherent in VHF TEC estimation due to ionospheric dispersion, and compares the LANL VTEC to two community standard VTEC gridded products: Jet Propulsion Laboratory's Global Ionospheric Model (JPL GIM) and the CEDAR community's Open Madrigal VTEC gridded measurements of L‐band GNSS (global navigation satellite systems) TEC. We find that the LANL VTEC data have an offset of 3 TECU from CEDAR Madrigal GNSS VTEC and a full‐width‐half‐maximum (FWHM) of 6 TECU. In comparison, the offset between LANL VTEC and the JPL GIM model is −3 TECU, but with a FWHM of 5 TECU. We also compare to Jason‐3 VTEC measurements over the ocean, finding an offset of less than 0.5 TECU and an FWHM of <5 TECU. Because this technique uses a completely different methodology to determine TEC, the sources of errors are distinct from the typical ground‐based GNSS L‐band (GHz) TEC measurements. Also, because it is derived from RF lightning signals, this data set provides measurements in regions that are not well covered by ground‐based GPS measurements, such as over oceans and over central Africa. Plain Language Summary: The ionosphere is a region of the atmosphere that is very important in communications between ground and satellite. For that reason, decades of scientific effort have been put toward developing models of the ionosphere so that we can more accurately predict what the state of the ionosphere is at any given location and time. A general product produced by many of these models is the vertical total electron content (VTEC), which is the vertically integrated electron density at a particular location at a particular time. The majority of these models use measurements of TEC from ground‐based receivers or instruments, meaning that abundant measurements that go into the models often lack data from over the oceans or in technologically limited regions of the world (e.g., Africa). Here, we present a new VTEC data set that is derived from lightning strokes detected with U.S. Department of Defense sensing systems on GPS satellites. Because the data set uses naturally occurring lightning for its source, it does not have the same limitations as ground‐based TEC measurements and can provide an additional source of validation data for ionospheric models. We introduce the data set and compare it with community‐accepted VTEC models and measurements. Key Points: New global gridded vertical total electron content (VTEC) data set derived from radio‐frequency measurements from lightning emissions during the entire year of 2018 has been releasedVTEC product adds new global coverage in areas with few global navigation satellite systems receivers (ocean and continental Africa)The VTEC values, generated from very high frequency radio‐frequency data, show bias in Madrigral data set and differences to Jet propulsion laboratory's global ionospheric model model‐derived VTEC [ABSTRACT FROM AUTHOR]

Physics-based Data Assimilation (DA) has been shown to be a powerful technique for specifying and predicting space weather. However, it is also known that different data assimilation models simulating the same geophysical event can display different space weather features even if the same data are assimilated. In this study, we used our Multimodel Ensemble Prediction System (MEPS) of DA models to elucidate the similarities and differences in the individual DA model reconstructions of the mid-low latitude ionosphere when the same data are assimilated. Ensemble model averages were also obtained. For this ensemble modeling study, we selected the quiet/storm period of 16 and 17 March 2013 (equinox, solar medium). Five data assimilation models and one physics-based model were used to produce an ensemble mean output for Total Electron Content (TEC), ionospheric peak density (NmF2), and ionospheric peak height (hmF2) for latitudes less than 60° and all longitudes. The data assimilated included ground-based Global Positioning Satellite TEC and topside plasma densities near 800 km altitude derived from the COSMIC (Constellation Observing System for Meteorology, Ionosphere, and Climate) satellites. Both a simple average and a weighted average of the models were used in the ensemble averaging in order to determine if there was an improvement of the ensemble averages over the individual models. [ABSTRACT FROM AUTHOR]

Prediction of ionospheric state is a critical space weather problem. We expand on our previous research of medium‐range ionospheric forecasts and present new results on evaluating prediction capabilities of three physics‐based ionosphere‐thermosphere models (Thermosphere Ionosphere Electrodynamics General Circulation Model, TIE‐GCM; Coupled Thermosphere Ionosphere Plasmasphere Electrodynamics Model, CTIPe; and Global Ionosphere Thermosphere Model, GITM). The focus of our study is understanding how current modeling approaches may predict the global ionosphere for geomagnetic storms (as studied through 35 storms during 2000–2016). Prediction approach uses physics‐based modeling without any manual model adjustment, quality control, or selection of the results. Our goal is to understand to what extent current physics‐based modeling can be used in total electron content (TEC) prediction and explore uncertainties of these prediction efforts with multiday lead times. The ionosphere‐thermosphere model runs are driven by actual interplanetary conditions, whether those data come from real‐time measurements or predicted values themselves. These model runs were performed by the Community Coordinated Modeling Center (CCMC). Jet Propulsion Laboratory (JPL)‐produced global ionospheric maps (GIMs) were used to validate model TEC estimates. We utilize the True Skill Statistic (TSS) metric for the TEC prediction evaluation, noting that this is but one metric to assess predictive skill and that complete evaluations require combinations of such metrics. The meanings of contingency table elements for the prediction performance are analyzed in the context of ionosphere modeling. Prediction success is between about 0.2 and 0.5 for weak ionospheric disturbances and decreases for strong disturbances. We evaluate the prediction of TEC decreases and increases. Our results indicate that physics‐based modeling during storms shows promise in TEC prediction with multiday lead time. Key Points: Physics‐based predictions of the total ionospheric content are evaluated for 35 stormsPrediction success is estimated below 0.5 and is with line with other geospace model predictionsPhysics‐based modeling shows promise in TEC prediction with multiday lead timeIt is important to evaluate all elements of the contingency matrix and to understand the role of each element in evaluation metrics [ABSTRACT FROM AUTHOR]

Forecasting conditions in the thermosphere and ionosphere is a key outcome expected from space weather research. In this work, we perform numerical simulations using the first‐principles models Global Ionosphere‐Thermosphere Model (GITM) and Thermosphere‐Ionosphere Electrodynamics General Circulation Model (TIE‐GCM) to address the reliability of thermospheric‐ionospheric forecasts. When considering forecasts applicable to periods of geomagnetic activity, careful consideration is required of model inputs, which largely determine how the models will simulate disturbed conditions. We adopt an approach to drive the models with solar wind parameters and the 10.7 cm solar radio flux. This aligns our investigation with recent research and operational activities to forecast solar wind conditions on the Earth a few days in advance. In this work, we examine a weak geomagnetic storm, the June 2012 high‐speed‐stream event, for which we drive GITM and TIE‐GCM with observed solar wind and F10.7 values. We find general agreement between the simulations and observation‐based Global Ionospheric Maps of the total electron content (TEC) response. However, overestimated TEC response is found in the middle to low latitudinal region of the American sector and surrounding areas for both GITM and TIE‐GCM during similar time periods. By conducting numerical modeling experiments and comparing the modeling results with observational data, we find that the overestimated TEC response can be almost equally attributed to the solar wind driving and F10.7 driving during the June 2012 event. We conclude that the accuracy of the high‐latitude electric field and the solar irradiance is crucial to reproduce the TEC response in forecastable‐mode modeling. Key Points: Forecastable global thermosphere‐ionosphere modeling is carried out for a weak geomagnetic stormThe modeled daytime middle‐ to low‐latitude TEC response is primarily driven by the solar wind condition on the first day of the stormOn later days of the storm the solar irradiance plays a comparable role as the solar wind in determining the modeled daytime TEC response [ABSTRACT FROM AUTHOR]

As part of its International Capabilities Assessment effort, the Community Coordinated Modeling Center initiated several working teams, one of which is focused on the validation of models and methods for determining auroral electrodynamic parameters, including particle precipitation, conductivities, electric fields, neutral density and winds, currents, Joule heating, auroral boundaries, and ion outflow. Auroral electrodynamic properties are needed as input to space weather models, to test and validate the accuracy of physical models, and to provide needed information for space weather customers and researchers. The working team developed a process for validating auroral electrodynamic quantities that begins with the selection of a set of events, followed by construction of ground truth databases using all available data and assimilative data analysis techniques. Using optimized, predefined metrics, the ground truth data for selected events can be used to assess model performance and improvement over time. The availability of global observations and sophisticated data assimilation techniques provides the means to create accurate ground truth databases routinely and accurately. Key Points: A working team has been established to develop a process for validation of auroral precipitation and electrodynamics modelsValidation of auroral electrodynamic parameters requires generation of ground truth data sets for selected eventsCurrent observations and data assimilation techniques continue to improve the accuracy of global auroral electrodynamic specification [ABSTRACT FROM AUTHOR]

As societal dependence on transionospheric radio signals grows, space weather impact on these signals becomes increasingly important yet our understanding of the effects remains inadequate. This challenge is particularly acute at high latitudes where the effects of space weather are most direct and no reliable predictive capability exists. We take advantage of a large volume of data from Global Navigation Satellite Systems (GNSS) signals, increasingly sophisticated tools for data‐driven discovery, and a machine learning algorithm known as the support vector machine (SVM) to develop a novel predictive model for high‐latitude ionospheric phase scintillation. This work, to our knowledge, represents the first time an SVM model has been created to predict high‐latitude phase scintillation. We use the true skill score to evaluate the SVM model and to establish a benchmark for high‐latitude ionospheric phase scintillation prediction. The SVM model significantly outperforms persistence (i.e., current and future scintillation are identical), doubling the predictive skill according to the true skill score for a 1‐hr lead time. For a 3‐hr lead time, persistence is comparable to a random chance prediction, suggesting that the memory of the ionosphere in terms of high‐latitude plasma irregularities is on the order of, or shorter than, a few hours. The SVM model predictive skill only slightly decreases between the 1‐ and 3‐hr predictive tasks, pointing to the potential of this method. Our findings can serve as a foundation on which to evaluate future predictive models, a critical development toward the resolution of space weather impact on transionospheric radio signals. Plain Language Summary: Society is increasingly dependent on radio signals, particularly those from the Global Navigation Satellite Systems (GNSS), and the technologies (e.g., navigation and financial transactions) that they enable. The integrity and reliability of these signals is threatened by their travel from the GNSS satellites to the ground, which includes passage through a charged region between 100 and 1,000 km known as the ionosphere. Disturbances to the ionosphere from solar energy, or space weather, cause variations in GNSS signals that adversely affect the dependent systems and technologies. Currently, the effect of the ionosphere on these signals cannot be reliably predicted, and the challenge is particularly important at latitudes above 45° where space weather impacts are most direct. We have compiled a large volume of data from the regions important to space weather (i.e., from the Sun to the Earth) to develop a novel machine learning model capable of skillfully predicting disruptions to GNSS signals at high latitudes. To our knowledge, this model is the first of its kind. We find that the new model is capable of more accurate predictions than current methods and position this model as a benchmark on which future predictive models can be measured. Key Points: Support vector machine yields predictive performance roughly double that of persistence; the benefit increases with longer prediction timesElectron precipitation information is critical to the prediction of high‐latitude phase scintillationA benchmark is established for high‐latitude ionospheric phase scintillation using the robust true skill score (TSS) metric [ABSTRACT FROM AUTHOR]

For the first time we derive an analytical expression for energy deposition by propagating monochromatic Alfvén waves in the collisional ionosphere‐thermosphere (IT). Based on empirical models of IT parameters, we show an approximately 10% increase in efficiency of energy deposition at high latitudes and about 150 km altitude by Alfvén waves at 5 Hz as compared to static fields of the same magnitude. We analyze the dependence of the wave energy deposition on geomagnetic activity through a Global Ionosphere‐Thermosphere Model (GITM) simulation of IT conditions during the 13–14 October 2016 geomagnetic storm. The peak Alfvén wave contribution occurs in the E layer and lower part of the F layer. Wave‐induced heating is shown to increase in the daytime and postdusk high‐latitude F layer during the storm due to increased wave‐modified conductivity. We estimate that the Alfvén wave contribution to the IT energy budget can reach about 30% of the value of static Joule heating during a strong storm and that the actual percentage increase is dependent on latitude and altitude. This effect carries important implications for ionospheric dynamics, especially for density enhancement in the daytime cusp, heating in the vicinity of auroral arcs and ion outflow. Key Points: An analytical expression for energy deposition by propagating Alfvén waves in the collisional ionosphere‐thermosphere is derivedThe relative efficiency of energy deposition rate of Alfvén wave (up to 5 Hz in frequency) to static field is estimated to be ∼10% at high latitudes and below 250 km altitudeWe show that Alfvén wave energy deposition can reach about 30% of the value of static Joule heating during a strong storm [ABSTRACT FROM AUTHOR]

The solar minimum period between solar cycles 23 and 24 has generated extensive study, in part because the changes in the upper atmosphere and ionosphere were so dramatic. Thermospheric density during 2008–2009, at the benchmark 400‐km altitude, was shown to be approximately 30% lower than the previous solar minimum during 1996, in observations and in model simulations. Ionospheric densities were also estimated to be lower, by 10% to 20%. However, earlier analyses using data from the global network of Global Navigation Satellite System receivers did not find any significant reduction in global mean total electron content. Recent work reexamining those data using a limited set of receivers, but with consistent processing, identified a 19% reduction (Emmert et al., 2017, https://doi.org/10.1002/2016JA023680). In this study, we compare model simulations of the thermosphere‐ionosphere system during the 2008–2009 solar minimum to its predecessor and estimate a 16% reduction in global mean total electron content. This is consistent with the neutral thermosphere density change seen in model simulations and in satellite drag observations. In the model simulations, the primary driver is a 10% reduction in solar extreme ultraviolet irradiance, with a smaller contribution from lower geomagnetic activity, and a very small decrement due to increase in atmospheric carbon dioxide. Plain Language Summary: The Sun goes through 11‐year activity cycles, best known with regard to the number of sunspots. At solar minimum, when sunspots mostly vanish, the extreme ultraviolet region of the solar spectrum also diminishes. This causes lower temperature and density in the ionosphere and upper atmosphere above 100 km, where the extreme ultraviolet radiation is absorbed. Past solar minimum periods have seemed basically similar with regard to extreme ultraviolet radiation, and other manifestations of solar activity. However, the solar minimum during 2008–2009 was longer and had fewer sunspots than its predecessor during 1996, or than any minimum period in the past century. The thermosphere was also lower in density, and the ionosphere also appeared to be lower in density, but there were some contradictory measurements. Recent reanalysis of ionospheric measurements found better agreement with the thermospheric measurements. Modeling studies attempting to explain the changes in the ionosphere and thermosphere have suggested that they were caused by a decrease of about 10% in solar extreme ultraviolet radiation. Here we show that the model results and the ionospheric measurements are now in good agreement. This demonstrates that solar minima are not all the same and may have implications for understanding the Sun during extended periods of very low activity. Key Points: Reanalysis of ionospheric electron content by Emmert et al. () found it lower during the 2008‐2009 solar minimum than the 1996 minimumModel simulations showed that both ionospheric electron content and thermospheric neutral density were lower during 2008‐2009 than in 1996Thermosphere and ionosphere reductions during the 2008‐2009 solar minimum were consistent in observations and in model simulations [ABSTRACT FROM AUTHOR]

In this brief note we explore the role of the neutral atmosphere in magnetosphere–ionosphere coupling. We analyze momentum balance in the ion rest frame to form hypotheses regarding the role of neutral momentum in the lower ionosphere during geomagnetic storms. Neutral momentum that appears in the ion rest frame is likely the result of momentum imparted to ionospheric ions by solar wind flow and the resultant magnetospheric dynamics. The resulting ionneutral collisions lead to the existence of an electric field. Horizontal electron flow balances the momentum supplied by this electric field. We suggest a possible role played by the neutral atmosphere in generating field-aligned currents due to local auroral heating. Our physical interpretation suggests that thermospheric neutral dynamics plays a complementary role to the high-latitude field-aligned currents and electric fields resulting from magnetospheric dynamics. [ABSTRACT FROM AUTHOR]

We revisit three complex superstorms of 19-20 November 2003, 7-8 November 2004, and 9-11 November 2004 to analyze ionosphere-thermosphere (IT) effects driven by different solar wind structures associated with complex interplanetary coronal mass ejections (ICMEs) and their upstream sheaths. The efficiency of the solar wind-magnetosphere connection throughout the storms is estimated by coupling functions. The daytime IT responses to the complex driving are characterized by combining and collocating (where possible) measurements of several physical parameters (total electron content or TEC, thermospheric infrared nitric oxide emission, and composition ratio) from multiple satellite platforms and ground-based measurements. A variety of metrics are utilized to examine global IT phenomena at ~1 h timescales. The role of direct driving of IT dynamics by solar wind structures and the role of IT preconditioning in these storms, which feature complex unusual TEC responses, are examined and contrasted. Furthermore, IT responses to ICME magnetic clouds and upstream sheaths are separately characterized. We identify IT feedback effects that can be important for long-lasting strong storms. The role of the interplanetary magnetic field By component on ionospheric convection may not be well captured by existing coupling functions. Mechanisms of thermospheric overdamping and consequential ionospheric feedback need to be further studied. [ABSTRACT FROM AUTHOR]

The ionosphere-thermosphere (IT) energy partitioning for the interplanetary coronal mass ejection (ICME) storms of 16-19 March 2013 and 2015 is estimated with the Global Ionosphere-Thermosphere Model (GITM), empirical models and proxies derived from in situ measurements. We focus on auroral heating, Joule heating, and thermospheric cooling. Solar wind data, F10.7, OVATION Prime model and the Weimer 2005 model are used to drive GITM from above. Thermospheric nitric oxide and carbon dioxide cooling emission powers and fluxes are estimated from TIMED/SABER measurements. Assimilative mapping of ionospheric electrodynamics (AMIE) estimations of hemispheric power and Joule heating are presented, based on data from global magnetometers, the AMPERE magnetic field data, SSUSI auroral images, and the SuperDARN radar network. Modeled Joule heating and auroral heating of the IT system are mostly controlled by external driving in the March 2013 and 2015 storms, while NO cooling persists into the storm recovery phase. The total heating in the model is about 1000 GW to 3000 GW. Additionally, we intercompare contributions in selected energy channels for five coronal mass ejection-type storms modeled with GITM. Modeled auroral heating shows reasonable agreement with AMIE hemispheric power and is higher than other observational proxies. Joule heating and infrared cooling are likely underestimated in GITM. We discuss challenges and discrepancies in estimating and global modeling of the IT energy partitioning, especially Joule heating, during geomagnetic storms. [ABSTRACT FROM AUTHOR]

We present the first complex network theory-based analysis of high-latitude total electron content (TEC) data, including dependencies on interplanetary magnetic field (IMF) clock angle and hemisphere. We examine several network measures to quantify the spatiotemporal correlation patterns in the TEC data for winter and summer months in 2016. We find that significant structure exists in the correlation patterns, distinguishing the dayside and nightside ionosphere, and specific features in the high latitudes such as the polar cap and auroral oval, including the cusp and ionospheric foot points of magnetospheric boundary layers. These features vary with the IMF, exhibiting a strong dependence on the north-south direction and generally larger variations during the winter months in both hemispheres. Our exploratory results suggest that network analysis of TEC data can be used to study characteristic ionospheric spatial scales at high latitudes, thereby extending the utility of these data. We explore mesoscale and large scale (greater than tens of kilometers and greater than hundreds of kilometers, respectively) as a function of winter/summer season, hemisphere, and IMF direction and conclude that the relative importance of different ionospheric scales is not a constant relationship. Together with an identification of important areas of future work, our findings provide a foundation for the application of network analysis techniques to ionospheric TEC. Our results suggest that network analysis can reveal new physical connections in the ionospheric system. [ABSTRACT FROM AUTHOR]

We have explored the use of COSMIC data to provide valuable scientific information on the ionospheric impacts of energetic particle precipitation during geomagnetic storms. Ionospheric electron density in the E region, and hence ionospheric conductivity, is significantly altered by precipitating particles from the magnetosphere. This has global impacts on the thermosphere-ionosphere because of the important role of conductivity on high-latitude Joule heating. Two high-speed stream (HSS) and two coronal mass ejection (CME) storms are examined with the COSMIC data. We find clear correlation between geomagnetic activity and electron density retrievals from COSMIC. At nighttime local times, the number of profiles with maximum electron densities in the E layer (below 200 km altitude) is well correlated with geomagnetic activity. We interpret this to mean that electron density increases due to precipitation are captured by the COSMIC profiles. These "E-layer-dominant ionosphere" (ELDI) profiles have geomagnetic latitudes that are consistent with climatological models of the auroral location. For the two HSS storms that occurred in May of 2011 and 2012, a strong hemispheric asymmetry is observed, with nearly all the ELDI profiles found in the Southern, less sunlit, Hemisphere. Stronger aurora and precipitation have been observed before in winter hemispheres, but the degree of asymmetry deserves further study. For the two CME storms, occurring in July and November of 2012, large increases in the number of ELDI profiles are found starting in the storm's main phase but continuing for several days into the recovery phase. Analysis of the COSMIC profiles was extended to all local times for the July 2012 CME storm by relaxing the ELDI criterion and instead visually inspecting all profiles above 50- magnetic latitude for signatures of precipitation in the E region. For 9 days during the July 2012 period, we find a signature of precipitation occurs nearly uniformly in local time, although the magnitude of electron density increase may vary with local time. The latitudinal extent of the precipitation layers is generally consistent with auroral climatology. However, after the storm main phase on 14 July 2012 the precipitation tended to be somewhat more equatorward than the climatology (by about 5-10° latitude) and equatorward of the auroral boundary data acquired from the SSUSI sensor onboard the F18 DMSP satellite. We conclude that, if analyzed appropriately, high-latitude COSMIC profiles have the potential to contribute to our understanding of MI coupling processes and extend and improve existing models of the auroral region. [ABSTRACT FROM AUTHOR]

This short paper presents phase and coherence data from the cross-wavelet transform applied on longitudinally separated very high frequency (VHF) equatorial ionospheric scintillation observations over Christmas Island. The phase and coherence analyses were employed on a pair of scintillation observations, namely, the east-looking and westlooking VHF scintillation monitors at Christmas Island. Our analysis includes 3 years of peak season scintillation data from 2008, 2009 (low solar activity), and 2011 (moderate solar activity). In statistically significant and high spectral coherence regions of the cross-wavelet transform, scintillation observations from the east-looking monitor lead those from the west-looking monitor by about 20 to 60 (40±20) min (most frequent lead times). Using several years (seasons and solar cycle) of lead (or lag) and coherence information of the cross-wavelet transform, we envisage construction of a probability model for forecasting scintillation in the nighttime equatorial ionosphere. [ABSTRACT FROM AUTHOR]

We study solar wind-ionosphere coupling through the late declining phase/solar minimum and geomagnetic minimum phases during the last solar cycle (SC23) - 2008 and 2009. This interval was characterized by sequences of high-speed solar wind streams (HSSs). The concomitant geomagnetic response was moderate geomagnetic storms and high-intensity, long-duration continuous auroral activity (HILDCAA) events. The JPL Global Ionospheric Map (GIM) software and the GPS total electron content (TEC) database were used to calculate the vertical TEC (VTEC) and estimate daily averaged values in separate latitude and local time ranges. Our results show distinct low- and midlatitude VTEC responses to HSSs during this interval, with the low-latitude daytime daily averaged values increasing by up to 33 TECU (annual average of ~20 TECU) near local noon (12:00 to 14:00 LT) in 2008. In 2009 during the minimum geomagnetic activity (MGA) interval, the response to HSSs was a maximum of ~30 TECU increases with a slightly lower average value than in 2008. There was a weak nighttime ionospheric response to the HSSs. A well-studied solar cycle declining phase interval, 10-22 October 2003, was analyzed for comparative purposes, with daytime lowlatitude VTEC peak values of up to ~58 TECU (event average of ~55 TECU). The ionospheric VTEC changes during 2008-2009 were similar but ~60% less intense on average. There is an evidence of correlations of filtered daily averaged VTEC data with Ap index and solar wind speed. We use the infrared NO and CO2 emission data obtained with SABER on TIMED as a proxy for the radiation balance of the thermosphere. It is shown that infrared emissions increase during HSS events possibly due to increased energy input into the auroral region associated with HILDCAAs. The 2008-2009 HSS intervals were ~85% less intense than the 2003 early declining phase event, with annual averages of daily infrared NO emission power of ~3.3 × 1010W and 2.7 × 1010W in 2008 and 2009, respectively. The roles of disturbance dynamos caused by high-latitude winds (due to particle precipitation and Joule heating in the auroral zones) and of prompt penetrating electric fields (PPEFs) in the solar wind-ionosphere coupling during these intervals are discussed. A correlation between geoeffective interplanetary electric field components and HSS intervals is shown. Both PPEF and disturbance dynamo mechanisms could play important roles in solar wind-ionosphere coupling during prolonged (up to days) external driving within HILDCAA intervals. [ABSTRACT FROM AUTHOR]

The typical atmospheric boundary layer (ABL) over the southeast (SE) Pacific Ocean is featured with a strong temperature inversion and a sharp moisture gradient across the ABL top. The strong moisture and temperature gradients result in a sharp refractivity gradient that can be precisely detected by the Global Positioning System (GPS) radio occultation (RO) measurements. In this paper, the Constellation Observing System for Meteorology, Ionosphere & Climate (COSMIC) GPS RO soundings, radiosondes and the high-resolution ECMWF analysis over the SE Pacific are analyzed. COSMIC RO is able to detect a wide range of ABL height variations (1-2 km) as observed from the radiosondes. However, the ECMWF analysis systematically underestimates the ABL heights. The sharp refractivity gradient at the ABL top frequently exceeds the critical refraction (e.g., -157 N-unit km-1) and becomes the so-called ducting condition, which results in a systematic RO refractivity bias (or called N-bias) inside the ABL. Simulation study based on radiosonde profiles reveals the magnitudes of the N-biases are vertical resolution dependent. The N-bias is also the primary cause of the systematically smaller refractivity gradient (rarely exceeding -110 N-unit km-1) at the ABL top from RO measurement. However, the N-bias seems not affect the ABL height detection. Instead, the very large RO bending angle and the sharp refractivity gradient due to ducting allow reliable detection of the ABL height from GPS RO. The seasonal mean climatology of ABL heights derived from a nine-month composite of COSMIC RO soundings over the SE Pacific reveals significant differences from the ECMWF analysis. Both show an increase of ABL height from the shallow stratocumulus near the coast to a much higher trade wind inversion further off the coast. However, COSMIC RO shows an overall deeper ABL and reveals different locations of the minimum and maximum ABL heights as compared to the ECMWF analysis. At low latitudes, despite the decreasing number of COSMIC RO soundings and the lower percentage of soundings that penetrate into the lowest 500-m above the mean-sea-level, there are small sampling errors in the mean ABL height climatology. The difference of ABL height climatology between COSMIC RO and ECMWF analysis over SE Pacific is significant and requires further studies. [ABSTRACT FROM AUTHOR]

The article presents a study which examines the effects of large-scale ionospheric structure on the accuracy of radio occultation (RO) retrievals of atmospheric parameters including temperature and refractivity. The study compares quiet and geomagnetically disturbed conditions using an ionospheric data assimilation model and a climatological model of ionosphere. It is found out that satellites in higher altitude orbits have lower retrieval bias.

Abstract: In the coming years, opportunities for remote sensing of electron density in the Earth’s ionosphere will expand with the advent of Galileo, which will become part of the global navigation satellite system (GNSS). Methods for accurate electron density retrieval from radio occultation data continue to improve. We describe a new method of electron density retrieval using total electron content measurements obtained in low Earth orbit. This method can be applied to data from dual-frequency receivers tracking the GPS or Galileo transmitters. This simulation study demonstrates that the method significantly improves retrieval accuracy compared to the standard Abel inversion approach that assumes a spherically symmetric ionosphere. Our method incorporates horizontal gradient information available from global maps of Total Electron Content (TEC), which are available from the International GNSS Service (IGS) on a routine basis. The combination of ground and space measurements allows us to improve the accuracy of electron density profiles near the occultation tangent point in the E and F regions of the ionosphere. [Copyright &y& Elsevier]

Solar soft X-ray (XUV) radiation is highly variable on all time scales and strongly affects Earth’s ionosphere and upper atmosphere; consequently, the solar XUV irradiance is important for atmospheric studies and for space weather applications. Although there have been several recent measurements of the solar XUV irradiance, detailed understanding of the solar XUV irradiance, especially its variability during flares, has been hampered by the broad bands measured in the XUV range. In particular, the simple conversion of the XUV photometer signal into irradiance, in which a static solar spectrum is assumed, overestimates the flare variations by more than a factor of two as compared to the atmospheric response to the flares. To address this deficiency in the simple conversion, an improved algorithm using CHIANTI spectral models has been developed to process the XUV Photometer System (XPS) measurements with its broadband photometers. Model spectra representative of quiet Sun, active region, and flares are combined to match the signals from the XPS and produce spectra from 0.1 to 40 nm in 0.1-nm intervals for the XPS Level 4 data product. The two XPS instruments are aboard NASA’s Solar Radiation and Climate Experiment (SORCE) and Thermosphere, Ionosphere, Mesosphere, Energetics, and Dynamics (TIMED) satellites. In addition, the XPS responsivities have been updated for the latest XPS data processing version. The new XPS results are consistent with daily variations from the previous simple conversion technique used for XPS and are also consistent with spectral measurements made at wavelengths longer than 27 nm. Most importantly, the XPS flare variations are reduced by factors of 2 – 4 at wavelengths shorter than 14 nm and are more consistent, for the first time, with atmospheric response to solar flares. Along with the details of the new XPS algorithm, several comparisons to dayglow and photoelectron measurements and model results are also presented to help verify the accuracy of the new XUV irradiance spectra. [ABSTRACT FROM AUTHOR]

The ionospheric weather Vσ index maps are generated from GIM-TEC produced at the Jet Propulsion Laboratory (JPL) with the denser grids of 1 × 1° in latitude and longitude, respectively. Using sliding-window statistical analysis, moving TEC median μ for 15 preceding days with estimated variance bounds is obtained for each grid pixel of GIM-TEC. The ionosphere variability index, Vσ, is ΔTEC deviation from the median normalized by the standard deviation σ. Vσ index segmentation is introduced specifying TEC storm if an instant TEC is outside the bound of μ±2σ. Comparisons of Vσ variability of 1 × 1° GIM-TEC maps with normal GIM-TECs of 2.5 × 5° in latitude and longitude demonstrates the greater TEC storm activity with denser grids maps. The superposed epoch analysis is performed for 77 intense storms (Dst ≤ −100 nT) and 246 moderate storms (−100 < Dst ≤ −50 nT) during 2000–2017, with zero time, t 0 , at Dst storm peak. The probabilities of TEC storm occurrence both positive (Vσp index) and negative (Vσn) are evaluated for 24 h prior to t 0 and 48 h after t 0 for global scale and three specified latitude zones: North High (NH) magnetic latitudes λ > 50°, South High (SH) λ < −50°, and Equatorial Ionization Anomaly (EIA) Band between ±35° magnetic latitudes. The equinoctial positive Vσp index shows the maxima on March and September, Dst intensity peaks on April and October, and the negative Vσn peaks on May and November. The amplitude of Vσp for the intense storms exceeds that for the moderate storm by 2 times and exceeds the amplitude of Vσn by 1.5 times. Storm Enhanced Density (SED) is evident with the largest amplitude of Vσp at EIA Band exceeding Vσp for NH and SH zones by 3 times. The negative Vσn occurrence observed for selected zones during the intense storms is decreased by 5 times for the moderate storms. The normalized cross-correlation function is used to define lag (time delay) between peak of Vσp and Vσn and t 0 of Dst peak. The majority of positive Vσp indices (57.5%) are observed prior to Dst storm (lag varies from −1 to −6 h with Dst from −50 to −400 nT) so the TEC enhancement can serve as a precursor of Dst storm. The time delay for the negative Vσn is inversely related with Dst storm intensity (lag varies from 11 to 8 h with Dst from −50 to −400 nT). • Superiority of denser global TEC maps (1 × 1 deg grids) over standard IONEX GIM-TECs is demonstrated. • Hourly-seasonal hierarchy is established with the positive TEC storms preceeding Dst storm echoed by the negative TEC storm. • Storm Enhanced Density is dominant at Equatorial zone over North and South high latitude zones. • Intensity of ionosphere storms is dropped by 3–5 times from intense to moderate Dst storms. [ABSTRACT FROM AUTHOR]

Comprehensive measurements of the Martian atmosphere, especially high-resolution temperature and pressure fields of the lower atmosphere, are needed for the characterization of scientifically important Martian weather and climate features such as wind, dust storms, atmospheric tides, and seasonal hydrogen escape. In addition, spatial information about the atmospheric state is necessary for planning of future aero braking events and for prediction of various aspects of the entry, descent, and landing of spacecraft.While the individual measurements obtained from sensors such as infrared radiometers, spectrometers or radio occultation payloads onboard Mars Odyssey, Mars Express, or Mars Reconnaissance Orbiter, are processed on a routine basis to derive detailed information about the atmospheric state, inter-technique comparisons are beneficial to understand the specific sensor characteristics and to reduce the risk for misinterpretations. We will show how the combination of measurements based on the tomography principle can improve the understanding of atmospheric processes on Mars, especially of those, which are characterized by distinct horizontal temperature and pressure gradients.In contrast to Earth, horizontal temperature and pressure gradients on Mars can be much more severe. Based on end-to-end simulations we will show how and under which conditions it is possible to reconstruct these gradients from radio occultation and Mars Climate Sounder measurements using tomography principles. Successfully applied to real observations, the enhanced temperature and pressure retrievals can provide crucial information for improving our understanding of atmospheric dynamics, especially over complex terrain, at the day-night terminator, or the Northern polar vortex. [ABSTRACT FROM AUTHOR]