Journal of Geophysical Research: Space Physics最新文献

Based on the curlometer method, we calculate the azimuthal component of the current density from nearly 19 years of Fluxgate Magnetometer (FGM) data of Cluster and investigate its latitudinal distribution in the dayside noon sector (09:00–15:00 magnetic local time, MLT). A crossing event in the noon meridian plane shows an unexpected eastward current at a geocentric distance of 8 R_E, away from the equator with latitudes of 30–40°. Further statistical results of the current distribution show that, the topology of the current can be radially divided into the inner and outer branches over the whole r_xy − z plane (r_xy = $��\sqrt{��x^2�+�y^2��}�$ and z are in solar magnetic (SM) coordinates), with the separation point of these two branches at a geocentric distance of about 8 R_E. The current variations of the inner and outer branches are different under different Kp, solar wind flow speed V_sw, and solar wind dynamic pressure P_dyn. It is shown that the current densities in both the inner and outer branches increase significantly with the Kp. High solar wind dynamic pressure enhances the current density of the outer branch, while high solar wind speed, on the contrary, enhances that of the inner branch. The formation of the outer branch may be related to the anisotropy of plasma pressure.

The terrestrial ionosphere is a birefringent medium that allows radio waves to propagate in two modes: the ordinary (O-mode) and the extraordinary (X-mode). A difference in the index of refraction of the two modes results in differential delay (mode delay) between the O- and X-modes of radio waves propagating through the ionosphere. Mode delays of transionospheric high frequency radio waves are determined using the Radio Receiver Instrument (RRI) onboard the Enhanced Polar Outflow Probe (e-POP) on the CASSIOPE/Swarm-E satellite. Experiments between RRI and the SuperDARN radar at Saskatoon, Canada show large variabilities in mode delays. The mode delays are nearly constant in some experiments but other experiments have large variations. The mode delay observations could not be explained by the distance between the e-POP satellite and the radar, the foF2 values along the satellite track, nor whether the satellite track was across or along a SuperDARN radar beam. A combination of RRI observations and ray-trace modeling are used to investigate the mode delay of transionospheric radio waves.

Electromagnetic ion cyclotron (EMIC) waves are believed to play an important role in the dynamics of the inner magnetosphere, including the ring current, the radiation belts and potentially, the cold plasma. In this work, we investigate their occurrence in the magnetosphere and the geomagnetic and solar wind conditions which lead to their excitation. We use an automated detection algorithm of EMIC waves observed by Van Allen Probes over the entire mission duration between 2012 and 2019. Consistent with earlier studies, we find that the H⁺ band occurrence maximizes in the dayside magnetosphere during enhancements of solar wind dynamic pressure. Both the H⁺ and He⁺ band are also generated along the duskside magnetosphere during disturbed geomagnetic conditions. In addition, to H⁺ and He⁺ bands commonly surveyed, we investigate the occurrence of H⁺ waves above and below 0.5 H⁺ gyrofrequency, as well as wave occurrence in the N⁺ and O⁺⁺ bands. Most H⁺ waves are observed in the band below 0.5f_H+. We find several events in the N⁺ band, indicative of their very low occurrence. The O⁺⁺ band is observed during disturbed geomagnetic conditions and high solar wind dynamic pressure at low L-shells. Its radial localization coincides with the O⁺⁺ torus. This study provides a comprehensive picture of EMIC wave distribution and insight into ion composition in the inner magnetosphere during variable geomagnetic conditions.

This study investigated the latitudinal variations of post-sunset enhancements in the ionospheric electron density during the geomagnetic quiet period in May 2021 with a combination of high-precision ionospheric parameters obtained from four ionosondes, Beidou geostationary satellite (BD-GEO) receiver network and Sanya incoherent scatter radar (SYISR). We identified four categories of post-sunset enhancement phenomena (Types 1–4), each with unique spatial and temporal evolutions, yet uniformly accompanied by a decrease in hmF2. Measurements of plasma drift vector velocities from SYISR and hmF2 gradients across various latitudes provided pivotal insights, confirming that the ionospheric post-sunset enhancements can result from downward plasma motion due to westward electric field, downward field-aligned drift, or a combination of both. For Type 1, dominated by field-aligned drift, plasma density enhancements not only intensify at low latitudes but may also extend to mid-latitudes, exhibiting a distinct temporal delay with increasing latitude. In contrast, Type 4, primarily driven by the westward electric field, is characterized by modest increases in plasma density confined to localized low-latitude regions, with no observable latitudinal time delay in the peak of enhancements. Types 2 and 3, which are subject to the combined influence of the westward electric field and field-aligned drift, exhibit plasma density increases at certain low-latitude areas, with Type 2 presenting a delayed pattern and Type 3 showing none with rising latitude. Meanwhile, neutral winds can partially account for the observed post-sunset enhancement from low to middle latitudes. These findings offer new insights into the factors influencing ionospheric behavior after sunset.