Small-scale auroral spots (SsAS) are frequently observed, particularly on the nightside, yet their formation mechanisms remain unclear. Utilizing all-sky imager data from the Chinese Arctic Yellow River Station and radar measurements from the European Incoherent Scatter Scientific Association (EISCAT), we identified 65 SsAS events occurring near magnetic local noon from 2003 to 2009. We found that they are observed only in the 557.7 nm emission band. They can be classified into two types based on their morphology: sporadic SsAS and fence-like SsAS. The occurrence of these events shows clear dependence on the southward interplanetary magnetic field (IMF) Bz, with the fence-like SsAS exhibiting an even more pronounced dependence. Their occurrence also depends on the westward IMF By. Additionally, we found that most events (88%) are located on the poleward side of the auroral oval, and the drift motion of these SsAS is consistent with the motion direction of adjacent auroral structures, whose motion is strictly governed by the IMF By polarity. These observations indicate that generation of these dayside SsAS may be directly linked to the solar wind energy input through magnetopause reconnection. The EISCAT observations further show that SsAS are accompanied by pronounced electron density gradients. On the basis of these characteristics, we propose that the generation of SsAS may be related to phase mixing of Alfvén waves arising from the density gradients. This study helps elucidate the coupling chain by which solar wind energy is injected into the polar region and subsequently drives ionospheric responses.
Characterizing hourly wind variability in the mesosphere and lower thermosphere(MLT)remains challenging because direct observations are sparse and observational constraints on gridded products weaken rapidly toward the mesopause.Here we present an initial evaluation of January-March 2019 winds from a whole-atmosphere analysis produced by coupling the Whole Atmosphere Community Climate Model version 6(WACCM6)with the Next-generation Ensemble Data Assimilation System(NEDAS)using a two-stage framework.Below～80 km,we perform cross-product consistency checks against the European Centre for Medium-Range Weather Forecasts fifth-generation atmospheric reanalysis(ERA5),the Modern-Era Retrospective analysis for Research and Applications,version 2(MERRA-2),the specified-dynamics WACCM(SD-WACCM),and the Horizontal Wind Model 2014(HWM14)on a common grid.Above 76 km,hourly winds are independently evaluated against nonassimilated meteor radar observations over Wuhan(30.6°N,114.4°E).In the upper stratosphere and lower mesosphere,WACCM-NEDAS reproduces the dominant large-scale zonal-mean circulation and remains broadly consistent with ERA5 and MERRA-2,with the most coherent agreement against MERRA-2 through much of the 60-to 80-km layer.In the MLT,WACCM-NEDAS is closer to the meteor radar winds than is SD-WACCM,with full-sample correlations of 0.55 for the zonal wind and 0.67 for the meridional wind and height-resolved correlations reaching 0.7-0.8 in favorable layers.At a representative 90-km level,wavelet analysis further indicates that WACCM-NEDAS retains a broader short-period variability spectrum than does SD-WACCM and avoids the overly smoothed near-diurnal response of HWM14.A supplementary 2-to 6-d wavelet analysis during the 2018-2019 sudden stratospheric warming interval further shows that WACCM-NEDAS captures a 4-to 5-d enhancement consistent with the meteor radar spectra.These results indicate meaningful added value of WACCM-NEDAS for hourly MLT wind studies,although broader validation across additional stations,seasons,and dynamical conditions is still required.
The equatorial electrojet (EEJ) is a narrow eastward current that flows in the daytime equatorial E region. It is an important part of low-latitude ionospheric electrodynamics. This review presents a synthesis of recent advances in EEJ research. The main topics include historical observations, observation and inversion methods, local time and longitudinal variations, mid- and high-latitude forcing, solar activity forcing, and EEJ models. Ground-based and satellite observations show clear local time and longitudinal variations of the EEJ. These variations are mainly controlled by E-region conductivity, atmospheric tides, and geomagnetic field geometry. Among these factors, nonmigrating tides play a dominant role in the longitudinal pattern. Under disturbed conditions, mid- and high-latitude processes, such as geomagnetic storms, substorms, sudden changes in solar wind dynamic pressure, sudden stratospheric warming events, and subauroral polarization streams, can substantially modify the EEJ intensity and even reverse its direction through coupling processes. In addition, solar activity, including solar flares and solar eclipses, can modulate the EEJ primarily through rapid changes in ionospheric conductivity and associated electrodynamic adjustments. Although physics-based electrodynamic models, data-driven empirical models, and global numerical simulations have provided valuable insights across different scales, their quantitative consistency and capability of reproducing EEJ disturbances remain limited. This review points out several open issues. These include the combined effects of different driving factors, the nonlinear response of the EEJ, and the lack of reliable EEJ models during disturbed conditions. Finally, future studies should focus on combined observations, improved coupling models, and prediction of EEJ variability.
The medium-energy electron detector on board China’s second earthquake-monitoring satellite can measure the energy, flux, and direction of medium- and high-energy electrons within the satellite’s orbit. These data may provide the technical means for exploring new technologies and methods for earthquake monitoring and prediction. This work presents a description of the scientific objectives, key technical specifications, payload design, and results of ground calibration and verification for the payload.
The Earth's outer radiation belt contains a large number of relativistic(>500 keV)electrons,which can exhibit different pitch angle distributions(PADs).Whistler-mode chorus waves are considered an important factor governing the evolution of electron PADs,but their long-term and global effects remain insufficiently studied.This paper presents a statistical analysis using observations from the Van Allen Probes during 2013-2018.A pitch angle anisotropy index A is employed to characterize electron PADs,and its correlation with chorus waves is investigated.Statistical results show that when chorus waves intensify,the index A increases,implying that electrons tend to exhibit pancake PADs.In addition,the index A is found to be positively correlated with enhanced solar wind dynamic pressure or substorm activity.Our results demonstrate that chorus waves play an important role in accelerating relativistic electrons and reshaping their PADs,which contributes to a deeper insight into the physical mechanisms underlying the dynamic evolution of the outer radiation belt.
