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The boundary between the thermosphere and exosphere is often given the simplified description of being a separation between highly collisional continuum mechanics and a collisionless domain. The realistic smooth transition through this space has historically presented a challenge to model as the assumptions used to simplify the Boltzmann equation in fluid models are invalidated at higher altitudes. A lack of rigorous modeling of the region limits the ability to understand the dynamics of light atmospheric species. This manuscript describes the dynamics present in a two‐way coupled fluid‐particle atmospheric model extending from the mesosphere through the exosphere with a smooth transition between fluid and particle domains. This model is used to examine the coupled nature of the thermosphere and exosphere using the fluid simulation TIME‐GCM and the direct simulation Monte Carlo simulation Monaco. The coupled model allows for examination of the thermosphere circulation and exosphere transport mechanisms, as well as their impacts on the distribution of hydrogen. In this analysis, upper transport regions in the exosphere are revealed and distinguished from lower transport regions during June solstice. Furthermore, coupling allows TIME‐GCM to account for effects of lateral exospheric transport of hydrogen, altering its upper boundary condition and consequentially the spatial distribution of hydrogen throughout the thermosphere. Finally, it is asserted that a self‐consistent hydrogen exobase distribution is necessary to constrain other analytical extrapolation techniques used to predict the vertical hydrogen profile in the exosphere. Plasma interactions are excluded from this study to isolate neutral dynamics. Plain Language Summary: With increasing altitude, the density of the atmosphere decreases. High above the ground, the gas can no longer be described as a continuous fluid. Instead, the motion of individual atoms is important. This transition occurs near 350–500 km (220–310 miles) in altitude, between the thermosphere and exosphere, making this region difficult to simulate. A common solution is to use a direct simulation Monte Carlo model to describe the motion of atoms in the exosphere and a fluid model to describe gas circulation in the thermosphere. However, previous studies have imposed unrealistic conditions on such combined models and not allowed the thermosphere to react to exospheric gas motion. This study describes the results of allowing a simulation of each region to be informed by the other. A focus is placed on neutral hydrogen, which comprises an increasing fraction of the gas at high altitudes, influences Earth's reaction to solar events, and undergoes atmospheric escape. With the new model, hydrogen motion in the exosphere can be fully realized. This motion affects the upper boundary of the thermospheric model and its hydrogen distribution. Finally, simplified methods for finding hydrogen density in the exosphere are compared to examine what type of errors may result. Key Points: A two‐way coupled thermosphere‐exosphere model permits analysis of hydrogen's competing thermospheric circulation and exospheric transportCoupling alters TIME‐GCM's hydrogen density distribution through transport between exosphere and thermosphere modelsA self‐consistent exobase hydrogen density is needed to constrain extrapolation methods predicting exospheric densities above fluid models [ABSTRACT FROM AUTHOR]

In this paper, the equatorial thermosphere anomaly (ETA) is investigated using accelerometer measurements to determine whether the feature is density‐dominated, wind‐dominated, or some combination of the two. An ascending‐descending accelerometry (ADA) technique is introduced to address the density‐wind ambiguity that appears when interpreting the ETA in atmospheric drag acceleration analyses. This technique separates ascending and descending acceleration measurements to determine if a wind's directionality influences the interpretation of the observed ETA feature. The ADA technique is applied to accelerometer measurements taken from the Challenging Minisatellite Payload mission and has revealed that the ETA is primarily density‐dominated from 9:00 to 16:00 local time (LT) near 400 km altitude, with the acceleration perturbations behaving similarly between 2003 and 2004 across all seasons. This finding suggests that the perturbations in the acceleration due to in‐track wind perturbations are small compared to the perturbations due to mass density, while indicating that the formation mechanisms across these local times are similar and persistent. The results also revealed that in the terminator region at 18:00 LT the acceleration perturbations deviate appreciably between ascending and descending passes, indicating different or multiple processes occurring at this local time compared to the 9:00–16:00 LT ascribed to the ETA. These results help constrain ETA formation theories to specific local times and thermospheric property responses without the use of supplemental wind measurements, while also indicating regions where in‐track winds cannot always be neglected. Plain Language Summary: Earth's thermosphere, a layer in the upper atmosphere, is a highly variable region that contributes to satellite drag in low Earth orbit. The equatorial thermosphere anomaly (ETA) is a perturbation feature that produces increases and decreases in drag accelerations across the lower latitudes of the thermosphere. Direct measurements of the thermosphere are often limited, so satellite instruments, such as accelerometers, have been used to analyze the acceleration that a satellite experiences due to atmospheric drag to indirectly measure thermospheric properties. However, the measured drag acceleration is dependent on both mass density and wind, making it difficult to determine whether a change in drag across the ETA is due to either property. This has implications on what mechanisms are responsible for its formation. This work introduces an ascending‐descending accelerometry technique to exploit the fact that while acceleration perturbations produced by mass density are independent of satellite direction, those arising from winds can either be positive or negative depending on the relative spacecraft motion. By separating satellite orbits into ascending (south‐to‐north) and descending (north‐to‐south) passes, it can be determined whether drag acceleration perturbations are due to mass density, wind, or a mixture of the two, helping constrain possible formation mechanisms. Key Points: Accelerometer observations of the Equatorial Thermosphere Anomaly (ETA) cannot distinguish between in‐track density and wind perturbationsA new technique is applied to observations determining that the ETA, occurring between 9 and 16 LT, is a density‐dominated featureThe ETA demonstrates repeatable annual behavior for each season, suggesting a common and persistent formation mechanism [ABSTRACT FROM AUTHOR]

This study focuses on utilizing the increasing availability of satellite trajectory data from global navigation satellite system‐enabled low‐Earth orbiting satellites and their precision orbit determination (POD) solutions to expand and refine thermospheric model validation capabilities. The research introduces an updated interface for the GEODYN‐II POD software, leveraging high‐precision space geodetic POD to investigate satellite drag and assess density models. This work presents a case study to examine five models (NRLMSIS2.0, DTM2020, JB2008, TIEGCM, and CTIPe) using precise science orbit (PSO) solutions of the Ice, Cloud, and Land Elevation Satellite‐2 (ICESat‐2). The PSO is used as tracking measurements to construct orbit fits, enabling an evaluation according to each model's ability to redetermine the orbit. Relative in‐track deviations, quantified by in‐track residuals and root‐mean‐square errors (RMSe), are treated as proxies for model densities that differ from an unknown true density. The study investigates assumptions related to the treatment of the drag coefficient and leverages them to eliminate bias and effectively scale model density. Assessment results and interpretations are dictated by the timescale at which the scaling occurs. DTM2020 requires the least scaling (∼−7%) to achieve orbit fits closely matching the PSO within an in‐track RMSe of 7 m when scaled over 2 weeks and 2 m when scaled daily. The remaining models require substantial scaling of the mean density offset (∼30 − 75%) to construct orbit fits that meet the aforementioned RMSe criteria. All models exhibit slight over or under‐sensitivity to geomagnetic activity according to trends in their 24‐hr scaling factors. Plain Language Summary: This research aims to make use of data from well‐tracked satellites that orbit close to Earth where they still interact with the atmosphere via satellite drag. The goal is to use high‐quality orbit solutions of satellites to provide additional means of testing density models of the Earth's upper atmosphere. By updating a precise orbit software tool called GEODYN‐II, we demonstrate how a satellite's orbit is affected by the drag forces from different density models of the upper atmosphere, and how those orbits can be used to assess the accuracy of the models. Each density model produces an orbit fit that deviates from the precise orbit of a satellite, and these deviations are used as proxies for model error relative to the unknown true density. We tested five different models using precise orbit solutions from the ICESat‐2 satellite. Various assumptions related to the drag force are investigated and used to scale the model densities at different timescales. We find that all models benefit from scaling to produce orbit fits that more closely match the ICESat‐2 precise orbit. Key Points: Precision orbit determination solutions are expanded to study satellite drag and assess upper atmospheric density models using GEODYNA proof‐of‐concept case study assessment of density models is presented using ICESat‐2 precise science orbits and orbit fitsAssessment results are provided for empirical (MSIS2, DTM2020, JB2008) and physics‐based (TIEGCM, CTIPe) models for 14‐days in November 2018 [ABSTRACT FROM AUTHOR]

Themospheric conditions during a minor geomagnetic event of 3 and 4 February 2022 has been investigated using disk temperature (Tdisk) observations from Global‐scale Observations of the Limb and Disk (GOLD) mission and model simulations. GOLD observed that the Tdisk increases by more than 60 K during the storm event when compared with pre‐storm quiet days. A comparison of the Tdisk with effective temperatures (Teff, 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 Teff 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. Plain Language Summary: High speed plasma from the sun can bring energy, momentum, and particles into the Earth's upper atmosphere triggering a geomagnetic storm, which disturbs the thermosphere‐ionosphere (TI) system. It is observed that even minor geomagnetic events can have a significant impact on the TI system. In this work we have investigated the neutral properties of the thermosphere during a minor geomagnetic event using NASA‐Global‐scale Observations of the Limb and Disk (GOLD) mission observations. By calibrating the Mass Spectrometer Incoherent Scatter radar version 2 empirical model based on the temperature change observed by GOLD, we have found that the thermospheric density increased by 15% (at 150) to 80% (at 500 km). These results demonstrates that the GOLD data can help in improving thermospheric empirical models. Also, they can help satellite‐drag and space weather forecasters to find ways to improve their products. Key Points: Global‐scale Observations of the Limb and Disk (GOLD) observed a ∼60 K rise in lower‐to‐middle thermospheric temperature during a minor geomagnetic storm on 3–4 February 2022GOLD‐informed MSIS calculations indicate an increase in density by 15% (at 150) to 80% (500 km), which agrees with Multiscale Atmosphere‐Geospace Environment estimationGOLD Tdisk can be used in empirical and data assimilative forecasting models to improve thermospheric specifications [ABSTRACT FROM AUTHOR]

On 03 February 2022, SpaceX launched 49 Starlink satellites, 38 of which re‐entered the atmosphere on or about 07 February 2022 due to unexpectedly high atmospheric drag. We use empirical model (NRLMSIS, JB08, and HASDM) outputs as well as solar extreme ultraviolet occultation and high‐fidelity accelerometer data to show that thermospheric density was at least 20%–30% higher at 210 km relative to the 9 days prior to the launch due to consecutive geomagnetic storms related to solar eruptions from NOAA AR12936 on 29 January 2022. We model the orbital altitude and in‐track position of a Starlink‐like satellite in a low‐drag configuration at 200 km during minor (G1) and extreme (G5) geomagnetic storms to show that an extreme storm would have at least a factor of two higher impact, with cumulative in‐track errors on the order of 10,000 km after a 5‐day duration extreme storm. Comparison of the JB08 and NRL MSIS models relative to the HASDM model during modeled historical minor and extreme geomagnetic storms shows that in‐track errors on the order of 100 km per day at 250 km, decreasing to cumulative errors on the order of 1 km per day at 550 km during geomagnetic storms. We conclude that full‐physics, data assimilative, coupled models of the magnetosphere and upper atmosphere, as well as new operational satellite missions providing "nowcasting" data to launch controllers, space traffic coordinators, and satellite operators, are needed to prevent similar—or worse—orbital system impacts during future geomagnetic storms. Plain Language Summary: On 03 February 2022, SpaceX launched 49 Starlink satellites into staging orbits at 210 km above sea level prior to raising them to their operational altitudes of 550 km. The pre‐launch space weather briefing included no information about an ongoing geomagnetic storm. Excessive atmospheric drag due to the geomagnetic storm resulted in 38 of the satellites re‐entering the atmosphere on or about 07 February 2022. We use both models and direct measurements of the atmospheric density during the event to show that density values were enhanced by 20%–30% at the 210 km staging altitude relative to values prior to the geomagnetic storm onset, while they were enhanced 90%–160% at higher altitudes. We recommend improving our ability to model the upper atmospheric response to geomagnetic storms to provide accurate forecasts and actionable "nowcasts" of conditions in low Earth orbit to launch controllers, space traffic managers, and satellite operators. Key Points: Geomagnetic storms can cause large variations in neutral density and satellite drag, especially at Very Low Earth Orbit altitudesData assimilative empirical model results and observations during the Starlink event show 20%–30% neutral density enhancements at 210 kmFull‐physics data assimilative models and real‐time measurements of thermospheric conditions can mitigate impacts on low Earth orbit operations [ABSTRACT FROM AUTHOR]

Describes the benefits and availability of leadership development for African-American male college students. Explored 80 college students' perceptions of leadership opportunities on predominantly White campuses. Results show that those men who were fraternity leaders were also involved in other campus organizations, thus indicating the transfer of skills to campuswide leadership. (EMK)