Thermospheric Model Research Articles

Improving the estimation of thermospheric neutral density via two-step assimilation of in situ neutral density into a numerical model

AbstractNeutral thermospheric density is an essential quantity required for precise orbit determination of satellites, collision avoidance of satellites, re-entry prediction of satellites or space debris, and satellite lifetime assessments. Empirical models of the thermosphere fail to provide sufficient estimates of neutral thermospheric density along the orbits of satellites by reason of approximations, assumptions and a limited temporal resolution. At high solar activity these estimates can be off by 70% when comparing to observations at 12-hourly averages. In recent decades, neutral density is regularly observed with satellite accelerometers on board of low Earth orbiting satellites like CHAMP, GOCE, GRACE, GRACE-FO, or Swarm. When assimilating such along-track information into global models of thermosphere–ionosphere dynamics, it has been often observed that only a very local sub-domain of the model grid around the satellite’s position is updated. To extend the impact to the entire model domain we suggest a new two-step approach: we use accelerometer-derived neutral densities from the CHAMP mission in a first step to calibrate an empirical thermosphere density model (NRLMSIS 2.0). In a second step, we assimilate—for the first time—densities predicted for a regular three-dimensional grid into the TIE-GCM (Thermosphere Ionosphere Electrodynamics General Circulation Model). Data assimilation is performed using the Local Error-Subspace Transform Kalman Filter provided by the Parallel Data Assimilation Framework (PDAF). We test the new approach using a 2-week-long period containing the 5 April 2010 Geomagnetic storm. Accelerometer-derived neutral densities from the GRACE mission are used for additional evaluation. We demonstrate that the two-step approach globally improves the simulation of thermospheric density. We could significantly improve the density prediction for CHAMP and GRACE. In fact, the offset between the accelerometer-derived densities and the model prediction is reduced by 45% for CHAMP and 20% for GRACE when applying the two-step approach. The implication is that our approach allows one to much better ’transplant’ the precise CHAMP thermospheric density measurements to satellites flying at a similar altitude. Graphical Abstract

Energy Deposition by Mesoscale High‐Latitude Electric Fields Into the Thermosphere During the 26 October 2019 Geomagnetic Storm

AbstractMesoscale high‐latitude electric fields are known to deposit energy into the ionospheric and thermospheric system, yet the energy deposition process is not fully understood. We conduct a case study to quantify the energy deposition from mesoscale high‐latitude electric fields to the thermosphere. For the investigation, we obtain the high‐latitude electric field with mesoscale variabilities from Poker Flat Incoherent Scatter Radar measurements during a moderate geomagnetic storm, providing the driver for the Global Ionosphere and Thermosphere Model (GITM) via the High‐latitude Input for Mesoscale Electrodynamics framework. The HIME‐GITM simulation is compared with GITM simulations driven by the large‐scale electric field from the Weimer model. Our modeling results indicate that the mesoscale electric field modifies the thermospheric energy budget primarily through enhancing the Joule heating. Specifically, in the local high‐latitude region of interest, the mesoscale electric field enhances the Joule heating by up to five times. The resulting neutral temperature enhancement can reach up to 50 K above 200 km altitude. Significant increase in the neutral density above 250 km altitude and in the neutral wind speed are found in the local region as well, lagging a few minutes after the Joule heating enhancement. We demonstrate that the energy deposited by the mesoscale electric field transfers primarily to the gravitational potential energy in the thermosphere.

Modeling Solar Eclipses at Extreme Ultra Violet Wavelengths and the Effects of Nonuniform Eclipse Shadow on the Ionosphere‐Thermosphere System

AbstractThe impacts of solar eclipses on the ionosphere‐thermosphere system particularly the composition, density, and transport are studied using numerical simulation and subsequent model‐data comparison. We introduce a newly developed model of a solar eclipse mask (shadow) at extreme ultraviolet (EUV) wavelengths—PyEclipse—that computes the corresponding shadowing as a function of space, time, and wavelength of the input solar image. The current model includes interfaces for Solar Dynamics Observatory and Geostationary Operational Environmental Satellites EUV telescopes providing solar images at nine different wavelengths. We show the significance of the EUV eclipse shadow spatial variability and that it varies significantly with wavelength owing to the highly variable solar coronal emissions. We demonstrate geometrical differences between the EUV eclipse shadow compared to a geometrically symmetric simplification revealing changes in occultation vary ±20%. The EUV eclipse mask is validated with in situ solar flux measurements by the PRoject for Onboard Autonomy 2/Large Yield Radiometer instrument suite showing the model captures the morphology and amplitudes of transient variability while the modeled gradients are slower. The effects of spatially EUV eclipse masks are investigated with Global Ionosphere Thermosphere Model for the 21 August 2017 eclipse. The results reveal that the modeled EUV eclipse mask, in comparison with the geometrically symmetric approximation, causes changes in the Total Electron Content in order of ±20%, 5%–20% in F‐region plasma drift, and 20%–30% in F‐region neutral winds.

MSIS‐UQ: Calibrated and Enhanced NRLMSIS 2.0 Model With Uncertainty Quantification

AbstractThe Mass Spectrometer and Incoherent Scatter radar (MSIS) model family has been developed and improved since the early 1970's. The most recent version of MSIS is the Naval Research Laboratory (NRL) MSIS 2.0 empirical atmospheric model. NRLMSIS 2.0 provides species density, mass density, and temperature estimates as function of location and space weather conditions. MSIS models have long been a popular choice of thermosphere model in the research and operations community alike, but—like many models—does not provide uncertainty estimates. In this work, we develop an exospheric temperature model based in machine learning that can be used with NRLMSIS 2.0 to calibrate it relative to high‐fidelity satellite density estimates directly through the exospheric temperature parameter. Instead of providing point estimates, our model (called MSIS‐UQ) outputs a distribution which is assessed using a metric called the calibration error score. We show that MSIS‐UQ debiases NRLMSIS 2.0 resulting in reduced differences between model and satellite density of 25% and is 11% closer to satellite density than the Space Force's High Accuracy Satellite Drag Model. We also show the model's uncertainty estimation capabilities by generating altitude profiles for species density, mass density, and temperature. This explicitly demonstrates how exospheric temperature probabilities affect density and temperature profiles within NRLMSIS 2.0. Another study displays improved post‐storm overcooling capabilities relative to NRLMSIS 2.0 alone, enhancing the phenomena that it can capture.

Description and comparison of 21st century thermosphere data

The quality and distribution in time and space of available atmospheric observations are crucial for the accuracy of semi-empirical thermosphere models. However, datasets can be inconsistent, and their qualities and resolutions are often unequal. The main thermospheric density datasets of this century are briefly described and then compared to each other when possible in order to quantify differences. Total mass densities used in the comparisons include all high-resolution CHAMP, GRACE and GOCE data, Swarm A, daily-mean Stella, global daily mean TLE densities, and the SET HASDM density database. The temperature data from TIMED-SABER are also reviewed.The recently updated daily-mean TLE densities (TLE2021) are 2–10 % smaller on average than the previous version (TLE2015). The differences are not constant offsets per altitude level, but fluctuations of up to 5 % are present. Compared to HASDM densities for 6 altitudes from 250 to 675 km, TLE2021 is 15–20 % smaller at 250 km, and then the difference diminishes with altitude to reach the same average value at 575 km. These mean differences also fluctuate by a few percent on time scales of months, to 10 % over half a solar cycle at 575 km. The TLE2021 and HASDM densities are larger than the accelerometer-inferred CHAMP, GRACE and GOCE densities and average offsets are 10–15 % and 10–20 %, respectively. The comparison to Swarm-A and Swarm-B showed mean offsets of 10 % and less, with significant positive trends seen in the comparison with HASDM. Finally, largest differences are found for Stella and HASDM at 800 km, up to 45 % with strong semiannual variations.This study clearly shows that the available density data cannot be simply assimilated or combined without first accurately calibrating the data. The HASDM database is a valuable asset due to its considerable coverage in space and time, but its uncertainty and true resolution are not well understood and are still being evaluated. Data compatibility requires employing physically accurate and harmonized aerodynamic force models in the density derivation procedure, which is presently not achieved. The accuracy of the procedure, independent of the quality of the instrument (GNSS receiver, ground-based orbit determination, or accelerometer), inevitably decreases with altitude due to weakening of the drag signal to noise ratio.The TIMED-SABER instrument provides measurements of pressure and temperature in the lower thermosphere. SABER temperature uncertainty is well-known. The SABER dataset now exceeds twenty years and has been continuously operating that entire time. It was ingested in NRLMSIS 2.0 and comparisons show the much-improved fit in comparison with NRLMSISE-00. The lower thermosphere temperatures significantly modify density at higher altitudes, and its measurement is essential for modeling and assessment.

Assessment of Using Field‐Aligned Currents to Drive the Global Ionosphere Thermosphere Model: A Case Study for the 2013 St Patrick’s Day Geomagnetic Storm

AbstractIn this study, field‐aligned currents (FACs) obtained from the Active Magnetosphere and Planetary Electrodynamics Response Experiment data set have been used to specify the high‐latitude electric potential in the Global Ionosphere Thermosphere Model (GITM). The advantages and challenges of the FAC‐driven simulation are investigated based on a series of numerical experiments and data‐model comparisons for the 2013 St Patrick's Day geomagnetic storm. It is found that the cross‐track ion drift measured by the Defense Meteorological Satellite Program satellites can be well reproduced in the FAC‐driven simulation when the electron precipitation pattern obtained from Assimilative Mapping of Ionospheric Electrodynamics (AMIE) technique is used in GITM. It is also found that including the neutral wind dynamo properly is very important when using FACs to derive the high‐latitude electric field. Without the neutral wind dynamo, the cross‐polar‐cap potential and hemispheric integrated Joule heating could be underestimated by more than 20%. Moreover, the FAC‐driven simulation is able to well reproduce the ionospheric response to the geomagnetic storm in the American sector. However, the FAC‐driven simulation yields relatively larger data‐model discrepancies compared to the AMIE‐driven GITM simulation. This may result from inaccurate Joule heating estimations in the FAC‐driven simulation caused by the inconsistency between the FAC and electron precipitation patterns. This study indicates that the FAC‐driven technique could be a useful tool for studying the coupled ionosphere and thermosphere system provided that the FACs and electron precipitation patterns can be accurately specified.

Specifying Satellite Drag Through Coupled Thermosphere‐Ionosphere Data Assimilation of Radio Occultation Electron Density Profiles

AbstractThe largest obstacle to managing satellites in low Earth orbit (LEO) is accurately forecasting the neutral mass densities that appreciably impact atmospheric drag. Empirical thermospheric models are often used to estimate neutral densities but they struggle to forecast neutral densities during geomagnetic storms when they are highly variable. Physics‐based models are thus increasingly turned to for their ability to describe the dynamical evolution of neutral densities. However, these models require observations to constrain dynamical state variables to be able to forecast mass densities with adequate fidelity. The LEO environment has scarce neutral state observations. Here, we demonstrate, in simulated experiments, a reduction in orbit errors and neutral densities using a physics‐based, data assimilation approach with ionospheric observations. Using a coupled thermosphere‐ionosphere model, the Thermosphere Ionosphere Electrodynamics General Circulation Model, we assimilate Constellation Observing System for Meterology, Ionosphere, and Climate electron density profiles (EDPs) derived from radio occultation (RO) observations. We use the EDPs to directly update neutral states, improving errors for neutral temperature by 70% and neutral winds by 20%. Updated neutral temperature and neutral winds additionally improve helium composition errors by 60% and 40%, respectively. Improved neutral density estimates correspond to a reduction in orbit errors of 1.2 km over 2 days, a 70% reduction over a no‐assimilation control, and a 29 km improvement over 9 days. This study builds on the results of our earlier work to further develop and demonstrate the potential of using a vast and growing RO data source, with a physics‐based model, to overcome our limited number of neutral observations.

Impact of Soft Electron Precipitation on the Thermospheric Neutral Mass Density During Geomagnetic Storms: GITM Simulations

AbstractIn this study, the impact of improving soft (0.1–1 keV) electron precipitation on the F‐region neutral mass density has been evaluated using the Global Ionosphere Thermosphere Model (GITM). Two types of electron energy spectra having the same total energy flux and average energy but different spectral shapes have been used to specify the electron precipitation in GITM. One is the Maxwellian spectrum and the other is from an empirical model, Auroral Spectrum and High‐Latitude Electric field variabilitY (ASHLEY), which provides stronger (up to 2–3 orders of magnitude) soft electron precipitations than the Maxwellian spectrum. Data‐model comparisons indicate that the storm‐time orbital averaged neutral density can be increased by 10%–40% and is more consistent with the observation if the non‐Maxwellian ASHLEY spectrum is used. This study reveals the importance of accurate soft electron precipitation specifications in the whole auroral zone to improving the F‐region neutral mass density estimations.

Low- and Mid-Latitude Ionospheric Response to the 2013 St. Patrick’s Day Geomagnetic Storm in the American Sector: Global Ionosphere Thermosphere Model Simulation

In this study, the low-and mid-latitude ionospheric response to the main phase of the 2013 St. Patrick’s Day geomagnetic storm in the American sector on the dayside has been investigated using the ground-based measurements and the Global Ionosphere Thermosphere Model (GITM). First, it is found that the observed ionospheric response can be well reproduced by GITM when it is driven by the electric potential and electron precipitation patterns derived from the Assimilative Mapping of Ionospheric Electrodynamics (AMIE) technique. The AMIE-driven GITM simulation also significantly improves the data-model comparison as compared with the simulation driven by the high-latitude empirical models. Second, it is found that the transport process associated with the neutral wind is largely responsible for the observed ionospheric response. Specifically, the traveling atmospheric disturbances (TADs) propagating from the opposite hemisphere play an important role in the formation of the negative storm phase at low and middle latitudes. Third, it is found that the asymmetric negative storm phases occurred at the nominal equatorial ionization anomaly (EIA) peak region in the afternoon sector are mainly attributed to the interaction of the TADs launched in different hemispheres with different phase speeds. Specifically, stronger Joule heating deposited in the northern hemisphere (NH) generates TADs with faster phase speeds than those launched in the southern hemisphere (SH). Consequently, the locations where the TADs originated from the different hemispheres interact are asymmetric about the geomagnetic equator, leading to the formation of asymmetric ionospheric negative storm phases. This study highlights the importance of accurately specifying high-latitude electrodynamic forcings in global I-T simulations and provides a new insight into the cause of the interhemispheric asymmetry phenomena during geomagnetic storms.

Orbit-localised thermosphere density prediction using a Kalman filter based calibration of empirical models

Accurate estimation of thermosphere mass density is critical to determining how satellite orbits evolve over time and thus to planning and managing space missions. Empirical thermosphere models are commonly employed for this purpose, but have substantial uncertainties. In this work, a Kalman filter method for calibrating these models along a particular satellite’s trajectory is described. This method was applied to calibrate the NRLMSISE-00, JB2008, and DTM-2020 models with respect to densities measured by the Swarm-C and GOCE satellites. Substantial improvements in root mean squared density residuals were obtained using the technique when compared with either uncalibrated model output or calibration using a linear regression on previous data. Further improvement was obtained by combining estimates from different models using a best linear unbiased estimator method.

Exospheric Temperature Measured by NASA-GOLD Under Low Solar Activity: Comparison With Other Data Sets.

Exospheric temperature is one of the key parameters in constructing thermospheric models and has been extensively studied with in situ observations and remote sensing. The Global‐scale Observations of the Limb and Disk (GOLD) at a geosynchronous vantage point provides dayglow limb images for two longitude sectors, from which we can estimate the terrestrial exospheric temperature since 2018. In this paper, we investigate climatological behavior of the exospheric temperature measured by GOLD. The temperature has positive correlations with solar and geomagnetic activity and exhibits a morning‐afternoon asymmetry, both of which agree with previous studies. We have found that the arithmetic sum of F10.7 (solar) and Ap (geomagnetic) indices is highly correlated with the exospheric temperature, explaining ∼64% of the day‐to‐day variability. Furthermore, the exospheric temperature has good correlation with thermospheric parameters (e.g., neutral temperature, O2 density, and NO emission index) sampled at various heights above ∼130 km, in spite of the well‐known thermal gradient below ∼200 km. However, thermospheric temperature at altitudes around 100 km is not well correlated with the GOLD exospheric temperature. The result implies that effects other than thermospheric heating by solar Extreme Ultraviolet and geomagnetic activity take control below a threshold altitude that exists between ∼100 and ∼130 km.

Impacts of Lower Thermospheric Atomic Oxygen and Dynamics on the Thermospheric Semiannual Oscillation Using GITM and WACCM‐X

AbstractThe latitudinal and temporal variation of atomic oxygen (O) is opposite between the empirical model, NRLMSISE‐00 (MSIS) and the whole atmosphere model, whole atmosphere community climate model with thermosphere and ionosphere extension (WACCM‐X) at 97–100 km. Atomic Oxygen from WACCM‐X has maxima at solstices and summer mid‐high latitudes, similar to [O] from Sounding of the Atmosphere using Broadband Emission Radiometry (SABER). We use the densities and dynamics from WACCM‐X to drive the Global Ionosphere Thermosphere Model (GITM) at its lower boundary and compare it with the MSIS driven GITM. We focus on the differences in the modeling of the thermospheric and ionospheric semiannual oscillation (T‐I SAO). Our results reveal that driving GITM with WACCM‐X causes the T‐I SAO to maximize around solstices, opposite to when MSIS is used. This is because the global mixing in GITM during solstices is not strong enough to decrease the solstitial [O] densities below the equinoctial values between mesosphere and lower thermosphere (MLT) and upper thermosphere. Larger summer [O] in the MLT leads to the accumulation of [O] at lower latitudes in the thermosphere due to weaker meridional transport, which further increases the amplitude of the oppositely phased SAO. WACCM‐X itself has the right phase of SAO in the upper thermosphere but wrong at lower altitudes. The exact mechanisms that can correct the phase of T‐I SAO in GITM while using SABER‐like [O] in the MLT are currently unknown and warrant further investigation. We suggest mechanisms that can reduce the solstitial maxima in the lower thermosphere, for example, stronger interhemispheric meridional winds, stronger residual circulation, seasonal variations in eddy diffusion, and momentum from breaking gravity waves.

Scale Factors of the Thermospheric Density: A Comparison of Satellite Laser Ranging and Accelerometer Solutions

AbstractA major problem in the precise orbit determination (POD) of satellites at altitudes below 1,000 km is the modeling of the aerodynamic drag which mainly depends on the thermospheric density and causes the largest non‐gravitational acceleration. Typically, empirical thermosphere models are used to calculate density values at satellite positions but current thermosphere models cannot provide the required accuracy. Thus, unaccounted variations in the thermospheric density may lead to significantly incorrect satellite positions. For the first time, we bring together thermospheric density corrections for the NRLMSISE‐00 model in terms of scale factors with a temporal resolution of 12 hr derived from satellite laser ranging (SLR) and accelerometer measurements. Whereas, the latter are in situ information given along the satellite orbit, SLR results have to be interpreted as mean values along the orbit within the underlying time interval. From their comparison, we notice a rather similar behavior with correlations of up to 80% and more depending on altitude. During high solar activity, scale factors vary around 30% at low solar activity and up to 70% at high solar activity from the value one. In addition, we found the scaled thermospheric density decreasing stronger as the modeled density of NRLMSISE‐00. To check the reliability of the SLR‐derived scale factors, we compare the POD result of two different software packages, namely DOGS‐OC from DGFI‐TUM and GROOPS from IGG Bonn. Furthermore, a validation of our estimated scale factors with respect to an external data set proofs the high quality of the obtained results.

RMCSat: An F10.7 Solar Flux Index CubeSat Mission

The F10.7 solar flux index is a measure of microwave solar emissions at a wavelength of 10.7 cm or 2800 MHz. It is widely used in thermosphere and ionosphere models as an indicator of solar activity and is recorded at only one terrestrial observatory in Penticton, Canada during daylight hours. The lack of geographical and temporal coverage of F10.7 measurements and no external redundancy to the existing system has led to the development of the RMCSat mission, which seeks to demonstrate the feasibility of collecting microwave solar flux emissions from a space-based platform. RMCSat is the first CubeSat mission by the Royal Military College of Canada. It offers a training environment for personnel in space mission analysis and design, satellite assembly, integration and testing, and satellite operations. This paper introduces the mission concept and preliminary design of a space-based solution that captures solar density flux measurements during each orbit as the Sun passes through the boresight of the primary payload antenna. In addition to two channels recording the 2800 MHz frequency (2785 MHz and 2815 MHz), a third channel records 2695 MHz using the same calibration standard to determine if the United States Radio Solar Telescope Network (RSTN) could be leveraged to supplement the existing F10.7 solar flux measurements and improve solar flux approximations. The RMCSat mission, satellite design, and system budgets are demonstrated here as being viable. Future design considerations pertain to the payload antennas and achievable launch orbits.

Impacts of Different Causes on the Inter‐Hemispheric Asymmetry of Ionosphere‐Thermosphere System at Mid‐ and High‐Latitudes: GITM Simulations

AbstractIn this study, the Global Ionosphere Thermosphere Model is utilized to investigate the inter‐hemispheric asymmetry in the ionosphere‐thermosphere (I‐T) system at mid‐ and high‐latitudes (|geographic latitude| > 45°) associated with inter‐hemispheric differences in (a) the solar irradiance, (b) geomagnetic field, and (c) magnetospheric forcing under moderate geomagnetic conditions. Specifically, we have quantified the relative significance of the above three causes to the inter‐hemispheric asymmetries in the spatially weighted averaged E‐region electron density, F‐region neutral mass density, and horizontal neutral wind along with the hemispheric‐integrated Joule heating. Further, an asymmetry index defined as the percentage differences of these four quantities between the northern and southern hemispheres (|geographic latitude| > 45°) was calculated. It is found that: (a) The difference of the solar extreme ulutraviolet (EUV) irradiance plays a dominant role in causing inter‐hemispheric asymmetries in the four examined I‐T quantities. Typically, the asymmetry index for the E‐region electron density and integrated Joule heating at solstices with F10.7 = 150 sfu can reach 92.97% and 38.25%, respectively. (b) The asymmetric geomagnetic field can result in a strong daily variation of inter‐hemispheric asymmetries in the F‐region neutral wind and hemispheric‐integrated Joule heating over geographic coordinates. Their amplitude of asymmetry indices can be as large as 20.81% and 42.52%, which can be comparable to the solar EUV irradiance effect. (c) The contributions of the asymmetric magnetospheric forcing, including particle precipitation and ion convection pattern, can cause the asymmetry of integrated Joule heating as significant as 28.43% and 34.72%, respectively, which can be even stronger than other causes when the geomagnetic activity is intense.

The Impact of Assimilating Ionosphere and Thermosphere Observations on Neutral Temperature Improvement: Observing System Simulation Experiments Using EnKF

AbstractAccurate 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 (<200 km), relative to the MIT VTEC assimilation; and (c) simultaneously assimilating both observation types can better improve the quality of neutral temperature estimation over the global area during the whole data assimilation process. The current results demonstrate that assimilating GOLD observations is important to improve the forecast capability of the physical‐based model for the lower thermosphere states and can provide a possible reference for the joint assimilation of the ionosphere and thermosphere observations to better thermosphere specification.

Latitudinal and Seasonal Asymmetries of the Helium Bulge in the Martian Upper Atmosphere

AbstractIn the present study, we investigate the characteristics of helium (He) bulges in the Martian upper atmosphere using He densities and winds measured by the Neutral Gas and Ion Mass Spectrometer (NGIMS) aboard the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft. The observations are compared with those predicted by the Mars Global Ionosphere Thermosphere Model (M‐GITM). The results of the present study show that the nightside He bulge is a persistent feature of the Martian upper atmosphere in all seasons. The He densities inside the bulges are 1–2 orders of magnitude greater than those on the dayside. In solstices, the bulges are observed in the winter polar region which is in accordance with the model predictions. In equinoxes, however, the bulges are observed to extend from mid‐latitudes into the southern polar regions (>60°S), which is contrary to the model predictions at mid‐latitudes. These anomalous bulges are predominantly observed in the northern spring equinox and are 10–30 × greater than the modeled ones. During the autumnal equinox, the observed winds depart from the modeled winds. Furthermore, the observed winds point to the southern polar regions where the bulges are observed. Thus, the results of the present study indicate that in equinoxes the regions of local vertical advection, that are responsible for the formation of the bulges, are displaced toward the southern polar regions. The results of the present study point to the need of a larger wind database from NGIMS in southern polar region, particularly during equinoxes.

Estimation of the thermospheric density using ephemerides of the CYGNSS and Swarm constellations

Modeling the neutral density in the upper atmosphere is a major challenge as it involves a comprehensive understanding of the complex coupling between the thermosphere and its environment. In addition, this system is strongly driven by solar activity, which is hardly predictable. This complexity limits the ability of thermospheric models to accurately estimate thermospheric characteristics, such as the neutral density. With the rapidly increasing number of satellites in orbit and the recent progress in tracking their trajectories, new techniques are being implemented to derive density estimates based on trajectory perturbations. However, most of these methods require the integration of complex and costly orbit determination algorithms, which greatly limit their applicability at a public level. The approach presented here provides density estimates along a spacecraft trajectory and only requires the implementation of an orbit propagator. More specifically, it corrects density predictions made by the NRLMSIS00e thermosphere model by optimizing the orbit fitting of modeled trajectories with observations under different thermospheric density conditions. The algorithm is applied to the Cyclone Global Navigation Satellite System and Swarm-B spacecraft trajectories during a period of varying geomagnetic activity. The study puts in evidence a bias in NRLMSIS00e estimates during moderate solar activity, corrected by the algorithm. In addition, the approach provides a better characterization of the rapid density enhancement resulting from a geomagnetic storm. Overall, the correlation with the Level 2 data is improved by 15% and the normalized root mean square error decreased by a factor ~2.3, compared to the estimations with NRLMSIS00e.

Azimuthal averaging–reconstruction filtering techniques for finite-difference general circulation models in spherical geometry

Abstract. When solving hydrodynamic equations in spherical or cylindrical geometry using explicit finite-difference schemes, a major difficulty is that the time step is greatly restricted by the clustering of azimuthal cells near the pole due to the Courant–Friedrichs–Lewy condition. This paper adapts the azimuthal averaging–reconstruction (ring average) technique to finite-difference schemes in order to mitigate the time step constraint in spherical and cylindrical coordinates. The finite-difference ring average technique averages physical quantities based on an effective grid and then reconstructs the solution back to the original grid in a piecewise, monotonic way. The algorithm is implemented in a community upper-atmospheric model, the Thermosphere–Ionosphere Electrodynamics General Circulation Model (TIEGCM), with a horizontal resolution up to 0.625∘×0.625∘ in geographic longitude–latitude coordinates, which enables the capability of resolving critical mesoscale structures within the TIEGCM. Numerical experiments have shown that the ring average technique introduces minimal artifacts in the polar region of general circulation model (GCM) solutions, which is a significant improvement compared to commonly used low-pass filtering techniques such as the fast Fourier transform method. Since the finite-difference adaption of the ring average technique is a post-solver type of algorithm, which requires no changes to the original computational grid and numerical algorithms, it has also been implemented in much more complicated models with extended physical–chemical modules such as the Coupled Magnetosphere–Ionosphere–Thermosphere (CMIT) model and the Whole Atmosphere Community Climate Model with thermosphere and ionosphere eXtension (WACCM-X). The implementation of ring average techniques in both models enables CMIT and WACCM-X to perform global simulations with a much higher resolution than that used in the community versions. The new technique is not only a significant improvement in space weather modeling capability, but it can also be adapted to more general finite-difference solvers for hyperbolic equations in spherical and polar geometries. Highlights. The ring average technique is adapted to solve the issue of clustered grid cells in polar and spherical coordinates with a finite-difference method. The ring average technique is applied to develop a 0.625∘×0.625∘ high-resolution TIEGCM and more complicated geoscientific models with polar and spherical coordinates as well as finite-difference numerical schemes. The high-resolution TIEGCM shows good capability in resolving mesoscale structures in the ionosphere–thermosphere (I–T) system.

Observational Evidence and Formation Mechanism of Low‐Density Cells in the Upper Thermosphere on September 8, 2017

AbstractThe low‐density cell structure in the high‐latitude thermosphere is referred to as the density depletion with respect to the adjacent area. Based on Gravity Recovery and Climate Experiment (GRACE) accelerometer data during the September 2017 geomagnetic storms, the thermospheric mass density at about 350 km are estimated and further investigated especially in the high‐latitude regions. At least two kinds of low‐density cells over the Southern Hemisphere (SH) are observed along the GRACE orbit. To understand the low‐density cell structures over the SH observed by GRACE, we investigate the underlying physical mechanism based on thermosphere‐ionosphere numerical simulations using Thermosphere‐Ionosphere Electrodynamic General Circulation Model and Global Ionosphere Thermosphere Model. According to the simulation results, the formation mechanism of the low‐density cell is attributed to the storm‐time vertical advection and horizontal velocity divergence driven by the auroral ion convection. The critical height of observable low‐density cells is shown to be not less than 350 km. The meridional spatial scale of observed low‐density cells over the SH are approximately or slightly larger than 1,500 km. Three low‐density cells, including two in the dawn sector and one in the night sector were observed about 1 hour after the direction of interplanetary magnetic field BY component reversed. The occurrence of thermospheric low‐density structure is essential to be included in the empirical model during geomagnetic storm time.