Abstract Quasi‐thermal noise (QTN) spectroscopy is an important tool for plasma diagnostics in space missions, where the antenna geometry affects the measured voltage spectrum. Existing antenna response models mainly focus on linear dipole antennas, whereas the influence of finite opening angles in V‐shaped antennas has not been fully characterized. In this work, we develop a generalized response‐function model for symmetric V‐shaped dipole antennas with arbitrary opening angles. The formulation incorporates the wave‐vector projections along the two antenna arms and naturally recovers the classical linear‐dipole limit and short‐antenna asymptotic behavior. Comparisons with the projected‐equivalent linear‐dipole approximation demonstrate that the effective‐length description is valid only in the low‐wavenumber regime, whereas finite‐wavenumber deviations arise from the full two‐arm geometry. The dependence of the response function on the opening angle is investigated, showing systematic variations in the peak amplitude and characteristic wavenumber. The effects of finite center gaps and realistic current redistribution are further incorporated using an AWAS‐type current model. Solar Orbiter (SolO)‐like parameters are adopted to demonstrate the impact of antenna geometry on the impedance and absolute QTN spectrum. These results provide a generalized framework for interpreting QTN measurements with V‐shaped spacecraft antennas and highlight the importance of retaining the full antenna geometry beyond simple projected‐length approximations.
Abstract We improved the Model for Analyzing the Terrestrial Exosphere (MATE), a kinetic exosphere model, by incorporating solar radiation pressure and Coriolis force arising from the Earth's revolution around the Sun, in addition to Earth's gravity, to investigate their impacts on the terrestrial exospheric hydrogen density. Under constant and uniform exobase conditions, we examined the governing force effects. Solar radiation pressure enhances the exospheric density and induces a day‐night asymmetry that increases with radial distance, whereas the Coriolis force has a minor effect (<10%). In separate simulations, we applied globally averaged exobase conditions derived from NRLMSIS‐2.0, covering a realistic solar activity range (F10.7 = 70–340). The density response to solar activity exhibits a distinct radial dependence. At 2 R E , the density peaks during solar minimum and reaches its minimum during solar maximum, closely following the exobase hydrogen density. In contrast, beyond 6 R E , the density is lowest during solar minimum and peaks at moderate solar activity (F10.7 ∼ 160), suggesting that the exobase temperature plays a dominant role in controlling the outer exosphere. Lastly, we simulated one synodic solar rotation during the solar minimum using time‐varying and non‐uniform MSIS exobase conditions. The resulting day‐night asymmetry is weaker than in the uniform exobase case because the dayside solar EUV heating increases the hydrogen population reaching higher altitudes, partially compensating for the asymmetry produced by solar radiation pressure. Although geomagnetic activity remained quiet, a weak disturbance (Ap∼11) enhanced the exospheric density by up to 21%, highlighting geomagnetic activity as an important driver of short‐term exospheric variability.
Abstract Utilizing GNSS, ionosonde, meteor radar, and multiple satellite observations, this study investigates the enhancement of nighttime mid‐latitude ionospheric periodic perturbations associated with medium‐scale traveling ionospheric disturbances (MSTIDs) over China sector during the 2020/2021 major sudden stratospheric warming (SSW). GNSS measurements reveal a sustained increase in nighttime TEC perturbations, characterized by mesoscale structures with predominantly southwestward propagation. Concurrent plasma‐density fluctuations observed by Swarm and CSES‐1, together with enhanced spread F in ionosonde measurements, show that the enhanced variability was detected by multiple independent instruments. The perturbation enhancement was accompanied by intensified sporadic‐E (Es) activity. Meteor‐radar observations, JAWARA reanalysis, and ICON HME results further show enhanced semidiurnal tidal activity, particularly SW2, together with increased vertical ion convergence near the observed Es‐layer heights. These concurrent changes support SSW‐related lower‐atmospheric preconditioning, while the correspondence between enhanced Es activity and F‐region perturbations is consistent with possible Es–F‐region electrodynamic coupling. Geomagnetic forcing also likely contributed: the largest daily perturbation enhancement occurred near the weak geomagnetic storm on 12 January, whereas elevated activity persisted during several comparatively quiet intervals. Regional gravity‐wave activity did not show a sustained enhancement sufficient to explain the persistent ionospheric response. Overall, the observations suggest contributions from both SSW‐related lower‐atmospheric variability and episodic geomagnetic forcing, whereas their relative roles and possible coupling remain unresolved.
Abstract Medium‐scale traveling ionospheric disturbances (MSTIDs) are important wave‐like perturbations in the ionosphere. Although some previous studies have been conducted, the source of winter daytime MSTIDs remains unclear. Using joint observations from the Chinese Dual Auroral Radar Network (CN‐DARN) of the Chinese Meridian Project Phase II, the Atmospheric Infrared Sounder (AIRS), and ERA5 Reanalysis data, this study provides observations linking stratospheric polar vortex variability, gravity wave (GW) activity, and daytime MSTIDs over the Asian mid‐latitudes. We analyzed an exceptionally strong polar vortex event in February 2025. The observation of pronounced MSTIDs activity by CN‐DARN was accompanied by remarkably intense gravity wave activity during the same event. In contrast, we examined the same period in 2024, which was a typical polar vortex breakdown event. We found that GW activity was weak, and the occurrence rate of MSTIDs was very low. The analysis results show that daytime MSTIDs propagated predominantly southward with propagation azimuths ranging from 129° to 177° (measured clockwise from geographic north), horizontal phase velocities of 110–218 m/s, and periods of 20–24 min during February 2025. Comparative analysis suggests that polar vortex activity is likely a source of wintertime daytime MSTIDs. We performed WACCM‐X simulations to investigate daytime neutral winds at 200 km altitude during different polar atmospheric activities in winter. The simulation results indicate that stronger northwestward neutral winds occur at middle and high latitudes during polar vortex events than during polar vortex breakdown events, which favors the propagation of southward MSTIDs. This study establishes a link between the polar vortex, GWs, and MSTIDs in the Asian sector, advancing the understanding of MSTID sources and characteristics.
Abstract Near the peak of Solar Cycle 25 in 2024, the Earth was impacted by major solar eruptive events that triggered dramatic geomagnetic storm activity in May and October. For these storms, we present a regional analysis of measured proton and electron flux within the South Atlantic Anomaly (SAA), a region in which low‐Earth orbit spacecraft are exposed to enhanced radiation hazard. We use data from the well‐studied instruments onboard CSES‐01 and MetOp‐C, and additionally analyze new electron data from MSS‐1A and proxy proton measurements from Swarm‐B. MSS‐1A provides unique measurements of high‐energy electron enhancements of the inner belt, while Swarm‐B provides a novel capability to resolve storm‐time proton dynamics. Compared to the October Storm, the May storm was more intense, with higher‐energy proton injection extending to lower L‐shells, longer persistence and clearer proton loss. Electron fluxes show strong energy‐dependent enhancements during May and more limited changes during October. We compare these flux data with in situ measurements of satellite hazard: single‐event‐upsets (SEUs) on a swarm. Despite strong storm‐time particle injections at higher L‐shells, only limited variations in SEU occurrence are observed at low L‐shells, consistent with the limited penetration of storm‐time proton fluxes to the Swarm orbital altitude of approximately 450–500 km.
