Accurate estimation of thermospheric neutral density is vital for atmospheric drag compensation. Actual measurements of thermospheric neutral density are rare, and often limited to specific altitude ranges. Numerical models are often used as a substitute, sometimes in conjunction with data assimilation schemes.During geomagnetic storms, having an accurate representation of the thermosphere-ionosphere (TI) is vital, since climatological models cannot accurately reproduce the system response. Recently, the physics-based Coupled Thermosphere Ionosphere Plasmasphere electrodynamics (CTIPe) model has been shown to provide accurate global estimates of neutral density when used in conjunction with the Thermosphere Ionosphere Data Assimilation scheme (TIDA).This approach adds the model inputs to the state vector and better accounts for the strongly forced nature of the TI. In this study, we expand on previous work by demonstrating the use of TIDA-CTIPe for neutral density estimation over a much broader time interval, covering multiple geomagnetic events.We demonstrate the capability to improve global estimates of neutral density by assimilating measurements in a narrow altitude range, from the CHAMP, GRACE and SWARM missions. Additionally, we demonstrate TIDA's capability to improve the thermospheric neutral density by assimilating different data types, such as COSMIC-2 derived TEC. Finally, we discuss the need for near-real-time data for potential forecasting applications.

Having a comprehensive understanding of the ionosphere's irregular behavior and its response to solar activity is crucial for satellite communication and navigation applications. The sun's extreme ultraviolet (EUV) and ultraviolet (UV) radiation are the primary sources of energy for the Earth's thermosphere and ionosphere (TI). To understand the global response of TI parameters (e.g., O/N2, and the peak electron density (Nmax)) to changes in solar irradiance, various data have been used. These include the Global-Scale Observations of the Limb and Disk (GOLD) ultraviolet imaging spectrograph, solar radio flux F10.7, predictions from the Coupled Thermosphere Ionosphere Plasmasphere electrodynamics (CTIPe) model, and International Global Navigation Satellite System Service total electron content maps (TEC). The comparison between these measurements shows that the CTIPe model successfully reproduces the behavior of the low- and mid-latitude ionosphere during both low and high solar activity. The study also investigated the delayed ionospheric TEC response against solar flux variations within the 27-day solar modulation. It was observed that the delay is less than one day, which was also confirmed in model simulations. Furthermore, the model simulations showed that the ionospheric time delay is significantly affected by various physical processes such as diffusion, photodissociation, solar and geomagnetic activities, and wave dynamics.

We compare Joule Heating rates as derived from ground-based magnetic field and all-sky camera data, from Low Earth Orbit satellite data (ESA Swarm) and from a MHD simulation (GUMICS-5) with each other in a case study of an auroral arc system. The observational estimates of Joule Heating rates provide information on regional scales and with high spatial resolution (10–100 km). Their comparison with global MHD results is conducted for a quiet time interval of a few minutes, just before a magnetic substorm. Analysis of the ground-based observations yields electric field with dominating North-South component pointing towards the arcs and having maxima values in the range 20–35 V/km. Combining these values with Pedersen conductance estimates from optical data (5–10 S) yields Joule Heating rates in the range 2.5–3.5 mW/m2. Swarm electric field measurements are consistent in their direction and intensity with the ground-based estimates. They also show that heating is increased particularly in the region where the conductance is low. The total amount of Joule heating in the area between the Swarm A and C satellite footprints while crossing the all-sky camera field of view is estimated to be 46 MW and the total amount energy dissipation during the 80 s overflight is around 3.6 GJ (1000 kWh). GUMICS-5 estimate of the peak Joule Heating in the magnetic local time sector of the arc system is smaller than that from the ground-based data with a factor of 2.9. Comparisons of GUMICS-5 results with Space Weather Modelling Framework (SWMF), shows that the latter gives on average larger heating rates being thus more consistent with our regional observations. However, both MHD-codes yield smaller Joule Heating rates around the time of the arcs and during the following substorm than the CTIP-e code. CTIP-e has a more detailed description of ionosphere-thermosphere interactions than the MHD-codes and its convection electric field is enhanced with a randomly varying additional component mimicking small scale structures. GUMICS-SWMF comparisons of global Joule Heating patterns in the Northern polar area reveal that the two simulations have significant differences in their spatial distribution of heating rates. Main cause for these deviations is the difference in the derivation of ionospheric Pedersen conductance. Our results emphasize the fact that future estimates of the global energetics in the magnetosphere–ionosphere–thermosphere system require better knowledge on ionospheric conductivities, both by new measurement concepts and by better understanding on the background physics controlling conductivity variations.

The Global-scale Observations of Limb and Disk (GOLD) imagers scan the Earth's Thermosphere-Ionosphere (TI) in the far ultraviolet wavelengths. Measurements from GOLD daylit spectrum are used to retrieve the column integrated atomic oxygen to molecular nitrogen density ratio (O/N-2) over about one fourth of the globe. The present investigation assesses the impact of assimilating GOLD disk O/N-2 on the Whole Atmosphere Community Climate Model with thermosphere-ionosphere eXtension (WACCMX) using the Data Assimilation Research Testbed (DART) ensemble adjustment Kalman filter. Two Observing System Simulation Experiments (OSSEs) are performed, and improvements are quantified by calculating root mean square error (RMSE) and bias with respect to a truth run. In addition to solar and geomagnetic forcing, we introduced gravity wave forcing perturbations to increase the ensemble spread, which has not previously been applied in ensemble assimilation. One of the OSSEs assimilates only the lower atmosphere (LA, <100 km) observations, referred to as LA experiment, and the second assimilates GOLD O/N-2 and LA observations, referred to as Whole Atmosphere (WA) experiment. The WA-analysis O/N-2 RMSE and bias are about 60% and 87% better compared to LA-analysis. Also, the O/N-2 RMSE and bias for the WA-analysis are about 23% and 54% better compared to WA 1-hr forecasts. The improvement in WA electron column density (ECD), a model equivalent of Total Electron Content (TEC), is about 24% compared to the LA experiment. These results demonstrate that the assimilation of GOLD O/N-2 improves both the thermosphere and ionosphere in a whole atmosphere data assimilation system.

To understand the global response of thermospheric-ionospheric (TI) parameters to variations in solar irradiance measurements from the Global-Scale Observations of the Limb and Disk (GOLD) ultraviolet imaging spectrograph, solar radio flux F10.7, predictions from the Coupled Thermosphere Ionosphere Plasmasphere electrodynamics (CTIPe) model, and International Global Navigation Satellite System Service total electron content maps (TEC) have been used. Various parameters such as GOLD O/N-2, O-2, and the nighttime peak electron density (Nmax) have been compared with the CTIPe model simulations. The GOLD observed Nmax shows a number of significant features including a winter anomaly and an equatorial ionization anomaly. The comparison with solar proxies showed that the GOLD Q(EUV) correlates very well with the EUV observations compared to the F10.7 index. The study also examined the relationship between the solar proxies and Nmax on different time scales and found that Nmax responded significantly to Q(EUV) at both medium- and long-term timescales. Furthermore, a low correlation between Nmax in the equatorial region and solar proxies was found. A delayed ionospheric TEC response against solar flux variations within the 27-day solar rotation was investigated. This ionospheric delay of TEC with respect to solar flux was observed to be less than 1 day, which was reproduced in model simulations. The current study has shown that the GOLD observations can be used to investigate the delayed ionospheric response and to gain a better understanding of the influence of solar activity on the TI system.

After days of intense solar activity, active region AR3664 launched seven CMEs toward Earth producing an extreme G5 geomagnetic storm commencing at 17:05 UT on 10 May 2024. The storm impacted power grids, disrupted precision navigational systems used by farming equipment, and generated aurora seen around the globe. The storm produced remarkable effects on composition, temperature, and dynamics in the Earth's thermosphere that were observed by NASA's Global-scale Observations of the Limb and Disk (GOLD) mission and are reported here for the first time. We use synoptic disk images of Sigma O/N2 and neutral temperature (at similar to 160 km) measured by GOLD to directly link dynamics resulting from the storm with dramatic changes in thermospheric composition and temperature. We observe a heretofore unseen spatial morphology simultaneously in Sigma O/N2, neutral temperature, and total electron content. Equator-to-pole temperature differences reach 400 K with high latitude peak neutral temperatures near 160 km exceeding 1400 K. On Saturday 10 May 2024, the sun launched a wave of energized plasma toward the Earth. A large disturbance in the Earth's magnetic field associated with the solar wind resulted in an extreme geomagnetic storm. The storm impacted power grids, disrupted navigational systems used by farming equipment, and produced aurora seen around the globe. The storm produced remarkable effects in the Earth's upper atmosphere that were observed by NASA's Global-scale Observations of the Limb and Disk (GOLD) mission. In this letter, we use images measured by GOLD to directly link atmospheric dynamics resulting from the May 10-12 superstorm with dramatic changes in composition, temperature, and global circulation in the Earth's upper atmosphere. We observe previously unseen structure in the upper atmosphere associated with equator-to-pole temperature differences exceeding 400 K. Peak neutral temperatures near 160 km exceed 1400 K at high latitudes. GOLD disk images of Sigma O/N2 and neutral temperature link storm time dynamics with changes in thermospheric composition and temperature We observe a previously unseen spatial morphology in Sigma O/N2, neutral temperature, and total electron content Peak equator-to-pole temperature differences exceed 400 K but relax to pre-storm conditions well before Sigma O/N2

A unique phenomenon {\textendash} merging of Equatorial Ionization Anomaly (EIA) crests, leading to an X-pattern (EIA-X) around the magnetic equator {\textendash} has been observed in the night-time ionospheric measurements by the Global-scale Observations of the Limb and Disk (GOLD) mission. A whole atmospheric general circulation model simulation reproduces this pattern. The pattern is also produced in an assimilative ionosphere model that assimilates slant Total Electron Content (slant-TEC) from Global Navigation Satellite System (GNSS) and Constellation Observing System for Meteorology, Ionosphere, and Climate 2 (COSMIC-2). Due to the observed similarity between measurements and simulations, the latter is used to diagnose this heretofore unexplained phenomenon. The simulation shows that the EIA-X occurs in the afternoon to evening sector at a longitude where the vertical drift is negative, which is a necessary but not sufficient condition. The simulation was performed under constant low-solar and quiescent-geomagnetic forcing conditions, therefore we suggest that one of the drivers of this phenomenon is from lower-atmospheric processes.

Abstract Assessing space weather modeling capability is a key element in improving existing models and developing new ones. In order to track improvement of the models and investigate impacts of forcing, from the lower atmosphere below and from the magnetosphere above, on the performance of ionosphere‐thermosphere models, we expand our previous assessment for 2013 March storm event (Shim et al., 2018, https://doi.org/10.1029/2018SW002034). In this study, we evaluate new simulations from upgraded models (the Coupled Thermosphere Ionosphere Plasmasphere Electrodynamics (CTIPe) model version 4.1 and the Global Ionosphere Thermosphere Model (GITM) version 21.11) and from the NCAR Whole Atmosphere Community Climate Model with thermosphere and ionosphere extension (WACCM‐X) version 2.2 including eight simulations in the previous study. A simulation from the NCAR Thermosphere‐Ionosphere‐Electrodynamics General Circulation Model version 2 (TIE‐GCM 2.0) is also included for comparison with WACCM‐X. TEC and foF2 changes from quiet‐time background are considered to evaluate the model performance on the storm impacts. For evaluation, we employ four skill scores: Correlation coefficient (CC), root‐mean square error (RMSE), ratio of the modeled to observed maximum percentage changes (Yield), and timing error (TE). It is found that the models tend to underestimate the storm‐time enhancements of foF2 (F2‐layer critical frequency) and TEC (Total Electron Content) and to predict foF2 and/or TEC better in North America but worse in the Southern Hemisphere. The ensemble simulation for TEC is comparable to results from a data assimilation model (Utah State University‐Global Assimilation of Ionospheric Measurements (USU‐GAIM)) with differences in skill score less than 3% and 6% for CC and RMSE, respectively.

The current state of the art in ionospheric characterization is to use global measurement of total electron content (TEC) data from space based and ground-based networks of GNSS receivers.Currently ground based GNSS receivers on only deployed on landmasses.Which leaves 70% of the earth's surface empty of persistent ionospheric measurements.Discussed here is a new capability that allows the deployment of GPS receivers on ocean buoys, a capability first deployed in 2018.Also discussed in a possible array deployment of receivers in a longitudinal array that will help ameliorate the dearth of data over ocean regions that hinders the comprehensive modeling and characterization of the ionosphere.Several immediate science questions that can be studied with this unique data set are presented.This data set can be used to study both small scale structure of the ionosphere as well as coupling between atmospheric regions.This buoy deployment architecture can be used with other sensor modalities, including all-sky imagers and interferometers either singularly or together creating larger virtual aeronomical and ionospheric observatory.

The thermosphere-ionosphere system changes significantly on various temporal scales due to the forcings from solar and geomagnetic processes, and the lower atmosphere. The 27-day variation caused by solar rotation is one of the most important modulating factors in the ionosphere. A robust feature in this context is the ionospheric lag of about 1-2 days in ionospheric parameters such as total electron content (TEC) and F2 layer peak electron density with respect to solar variations at the 27-day solar rotation period. Here, the ionospheric TEC provided by the International GNSS Service (IGS) and the Coupled Thermosphere Ionosphere Plasmasphere electrodynamics (CTIPe) model were used to understand the delayed ionospheric response and the underlying physics. The O/N2 measurements from the imaging ultraviolet spectrograph Global-Scale Observations of the Limb and Disk (GOLD), the Global Ultraviolet Imager (GUVI), were also analyzed during the 2019-2021 period of low solar activity. The comparative study shows that the model successfully reproduces the delayed response of the ionosphere during low solar activity. The observed and modeled O/N2 ratio was found to be positively correlated with the solar EUV proxy (GOLD QEUV), with a lag of about 2 days, indicating a contribution to the ionospheric lag in TEC.Furthermore, the CTIPe model simulations show that the ionospheric time delay is significantly affected by various physical processes such as diffusion, photodissociation, solar and geomagnetic activities, and wave dynamics.

Themospheric conditions during a minor geomagnetic event of 3 and 4 February 2022 has been investigated using disk temperature (T-disk) observations from Global-scale Observations of the Limb and Disk (GOLD) mission and model simulations. GOLD observed that the T-disk increases by more than 60 K during the storm event when compared with pre-storm quiet days. A comparison of the T-disk with effective temperatures (T-eff, i.e., a weighted average based on airglow emission layer) from Mass Spectrometer Incoherent Scatter radar version 2 (MSIS2) and Multiscale Atmosphere-Geospace Environment (MAGE) models shows that MAGE outperforms MSIS2 during this particular event. MAGE underestimates the T-eff by about 2%, whereas MSIS2 underestimates it by 7%. As temperature enhancements lead to an expansion of the thermosphere and resulting density changes, the value of the temperature enhancement observed by GOLD can be utilized to find a GOLD equivalent MSIS2 (GOLD-MSIS) simulation-from a set of MSIS2 runs obtained by varying geomagnetic ap index values. From the MSIS-GOLD run we found that the thermospheric density enhancement varies with altitude from 15% (at 150) to 80% (at 500 km). Independent simulations from the MAGE model also show a comparable enhancement in neutral density. These results suggest that even a modest storm could impact the thermospheric densities significantly and GOLD data can be used to improve the empirical and assimilative models of the thermosphere.

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.

Dynamical changes in the ionosphere and thermosphere during geomagnetic storm times can have a significant impact on our communication and navigation applications, as well as satellite orbit determination and prediction activities. Because of the complex electrodynamics coupling processes during storms, which cannot be fully described with the sparse set of thermosphere–ionosphere (TI) observations, it is crucial to accurately model the state of the TI system. The approximation closest to the true state can be obtained by assimilating relevant measurements into physics-based models. Thermospheric mass density (TMD) derived from satellite measurements is ideal to improve the thermosphere through data assimilation. Given the coupled nature of the TI system, the changes in the thermosphere will also influence the ionosphere state. This study presents a quantification of the changes and improvement of the model state produced by assimilating TMD not only for the thermosphere density but also for the ionosphere electron density under storm conditions. TMD estimates derived from a single Swarm satellite and the Coupled Thermosphere Ionosphere Plasmasphere electrodynamics (CTIPe) physics-based model are used for the data assimilation. The results are presented for a case study during the St. Patricks Day storm 2015. It is shown that the TMD data assimilation generates an improvement of the model’s thermosphere density of up to 40% (measured along the orbit of the non-assimilated Swarm satellites). The model’s electron density during the course of the storm has been improved by approximately 8 and 22% relative to Swarm-A and GRACE, respectively. The comparison of the model’s global electron density against a high-quality 3D electron density model, generated through assimilation of total electron content, shows that TMD assimilation modifies the model’s ionosphere state positively and negatively during storm time. The major improvement areas are the mid-low latitudes during the storm’s recovery phase. Graphical Abstract

The behavior of the ionosphere is strongly dependent on solar activity conditions. Attempts have been made to understand the delayed response of the ionosphere at low and high solar activity in a systematic way. The total electron content provided by International GNSS Service (IGS) and Coupled Thermosphere Ionosphere Plasmasphere electrodynamics (CTIPe) was used to study the delay during 2008 and 2013 at the location 50 degrees N, 18 degrees E. The comparative study shows that the model successfully reproduces the ionospheric behavior during low and high solar activity. In addition, the CTIPe has been used for sensitivity studies with different model components. The analysis suggests that the model reproduction of the ionospheric time delay is significantly affected by various processes such as diffusion, photodissociation, solar and geomagnetic activities. To this end, several model runs were performed during low and high solar activity conditions. Furthermore, we have systematically studied the delayed response of the ionosphere during low, medium, and high solar activity and it was confirmed that the ionospheric delay increases with increasing solar activity. (C) 2022 COSPAR. Published by Elsevier B.V. All rights reserved.

&lt;p&gt;During geomagnetic storms, communication and navigation instruments can be dramatically affected by the rapid changes that occur in the upper atmosphere. The assimilation of data in physics-based models such as the Coupled Thermosphere Ionosphere Plasmasphere electrodynamics (CTIPe) model through and ensemble Kalman filter, can improve the representation of the thermosphere-ionosphere (TI) system. Due to the coupled nature of the TI system, the ionosphere is affected by, among others, changes in the neutral atmosphere. In this study, we investigate the capability of the CTIPe model to provide better estimates of the ionosphere by improving its specification of the thermosphere via data assimilation. Here, we assimilate thermospheric mass density (TMD) observations from the Swarm mission normalized to 400 km altitude during the 2015 St. Patrick&amp;#8217;s Day storm. The changes that occur in the ionosphere due to assimilation of TMD data are measured by means of the difference between the model results with and without assimilation. To measure the improvement gained with data assimilation, we compare with independent measurements of electron density along the orbit of GRACE (Gravity Recovery and Climate Experiment) satellite, that shows a reduction in the root mean square error (RMSE) by a 22% with respect to the non-assimilation run. The impact on the global scale is measured by comparing the CTIPe model results with the corresponding output of the 3D B-Spline electron density model. The results illustrate that the electron density equatorial region is the most affected region by assimilation of TMD, with an average RMSE reduction of 25% at the assimilation altitude of 400 km.&lt;/p&gt;

Abstract. Simulations of the ionospheric response to solar flux changes driven by the twenty-seven days solar rotation have been performed using the global 3-D Coupled Thermosphere/Ionosphere Plasmasphere electrodynamics (CTIPe) physics- based numerical model. Using the F10.7 index as a proxy for solar EUV variations in the model, the ionospheric delay at the solar rotation period is well reproduced and amounts to about 1 day, which is consistent with satellite and in-situ measurements. From mechanistic CTIPe studies with reduced and increased eddy diffusion, we conclude that the eddy diffusion is a primary factor that influences the delay of the ionospheric total electron content (TEC). We observed the peak response time of atomic oxygen to the molecular nitrogen ratio to solar EUV flux changes quickly during the increased eddy diffusion compared with weaker eddy diffusion. These results suggest that an increase in the eddy diffusion leads to faster transport processes and an increased loss rates resulting in a decrease of the ionospheric time delay. Furthermore, we found that an increase in solar activity leads to an enhanced ionospheric delay. At low latitudes, the influence of solar activity is stronger, as EUV radiation drives ionization processes that lead to composition changes. Hence, the combined effect of eddy diffusion and solar activity lead to longer delay in the low and mid latitude region.

Global‐scale Observations of Limb and Disk (GOLD) disk measurements of far ultraviolet molecular nitrogen band emissions are used to retrieve temperatures ( Tdisk ), which are representative of lower thermospheric altitudes. The present investigation studies the response of lower thermospheric temperatures to geomagnetic activities of varying magnitudes. In this study, it has been observed that Tdisk increases over all latitudes in response to enhanced geomagnetic activity. The increase in temperature is proportional to the strength of the geomagnetic activity and is greater at higher latitudes. Temperature enhancements vary from 10s to 100s of Kelvins from low‐ to mid‐latitudes. Local time behavior shows that pre‐noon enhancements in temperatures, during relatively stronger geomagnetic activities, are greater compared to afternoon, which may be caused by the combined action of daytime dynamics and geomagnetic forcing. This study, thus, demonstrates the utility of GOLD Tdisk when investigating the effects of dynamical and external forcings in the thermosphere.

Global-scale Observations of Limb and Disk (GOLD) disk measurements of far ultraviolet molecular nitrogen band emissions are used to retrieve column integrated disk temperatures (Tdisk), which are representative of the lower-and-middle thermosphere.The present work develops a new approach to assimilate the Tdisk in the Whole Atmosphere Community Climate Model with thermosphereâ\euro?ionosphere eXtension (WACCMX) using the Data Assimilation Research Testbed (DART) ensemble adjustment Kalman filter.Nine days of data,1 to 9 November 2018, are assimilated.Analysis state variables such as thermospheric effective temperature (Teff, airglow layer integrated temperature), ratio of atomic oxygen to molecular nitrogen column densities (O/N2), and column electron content are compared with a control simulation that is only constrained up to ˜50 km.It is observed that assimilation of the GOLD Tdisk improves the analysis states when compared with the control simulation.The analysis and model states, particularly, Teff, O/N2, and Electron Column Density (ECD) are also compared with their measurement counterparts for a validation of the assimilation.Teff and O/N2 are compared with GOLD Tdisk and O/N2.While, the ECD is compared with ground based Total Electron Content (TEC) measurements from Global Navigational Satellite System (GNSS) receivers.Root Mean Square Error (RMSE) improvements in Teff and O/N2 are about 10.8% and 22.6%, respectively.The RMSE improvement in analyses ECD is about 10% compared to control simulation.