Nighttime meridional neutral wind responses to SAPS simulated by the TIEGCM:A universal time effect

The present work uses the Thermosphere Ionosphere Electrodynamics General Circulation Model(TIEGCM),under geomagnetically disturbed conditions that are closely related to the southward interplanetary magnetic field(IMF),to investigate how the nighttime poleward wind(30°–50°magnetic latitude and 19–22 magnetic local time)responds to subauroral polarization streams(SAPS)that commence at different universal times(UTs).The SAPS effects on the poleward winds show a remarkable UT variation,with weaker magnitudes at 00 and 12 UT than at 06 and 18 UT.The strongest poleward wind emerges when SAPS commence at 06 UT,and the weakest poleward wind develops when SAPS occur at 00 UT.A diagnostic analysis of model results shows that the pressure gradient is more prominent for the developing of the poleward wind at 00 and 12 UT.Meanwhile,the effect of ion drag is important in the modulation of the poleward wind velocity at 06 and 18 UT.This is caused by the misalignment of the geomagnetic and geographic coordinate systems,resulting in a large component of ion drag in the geographically northward(southward)direction due to channel orientation of the SAPS at 06 and 18 UT(00 and 12 UT).The Coriolis force effect induced by westward winds maximizes(minimizes)when SAPS commence at 12 UT(00 UT).The centrifugal force due to the accelerated westward winds shows similar UT variations as the Coriolis force,but with an opposite effect.

2020年4月20日磁暴恢复相期间舌状中性结构的垂直变化

在发生于2020年4月20日的地磁暴恢复相阶段,GOLD(Global-scale Observations of the Limb and Disk)成像仪在第112天(day of year,DOY 112)中低纬地区观测到氧原子(O)和氮气分子(N_(2))的柱密度比(ΣO/N_(2))的舌状中性结构(TON).TON结构一般指发生于中高纬且形成于两个ΣO/N_(2)暴时衰减结构之间的ΣO/N_(2)增强结构.热层-电离层电动力学大气环流模式(Thermosphere-Ionosphere-Electrodynamics General Circulation Model,TIEGCM)定性地模拟再现了在本次磁暴恢复相期间观测到的ΣO/N_(2)增强结构,并且发现这个结构在前一天(DOY 111)当地下午形成,通过中性风的输运被逐渐耗散.模拟结果呈现了不同高度O/N_(2)的TON结构的垂直变化,其强度和纬度范围有明显的高度依赖性,并且随磁暴演化不断变化.诊断分析表明:下沉流(downwelling)驱动的垂直输运首先导致较低纬(约30°N—70°N)O/N_(2)的增强,然后通过极向风驱动的水平输运将其向更高纬地区输运.在中低热层(约120~300 km高度),主导O/N_(2)的TON结构演化的中低纬极向风主要是由气压梯度力导致的,同时科里奥利力对极向风也有一定的正贡献.而在约300 km高度以上的高热层,极向风主要由气压梯度力和与其作用相反的垂直黏性力两项控制.

Case study of an Equatorial Plasma Bubble Event investigated by multiple ground-based instruments at low latitudes over China

Observational evidence is insufficient to understand how equatorial plasma bubbles(EPBs)form over low latitudes.The mechanism of plasma-density enhancement(formation of"plasma blobs")at low latitudes is in dispute.In this paper,we use data from multiple ground-based instruments(one all-sky airglow imager,five digisondes,and one Fabry–Perot interferometer)to investigate the evolution of an EPB event that occurred at low latitudes over China on the night of 06 December 2015(06-Dec-2015).We provide observational evidence that an enhanced equatorward wind most likely induced by a substorm could have initiated the Rayleigh–Taylor instability(RTI)that destabilized several EPB depletions in an upwelling region of a large-scale wave-like structure(LSWS)in the bottomside ionosphere.Those EPB depletions were forced to surge poleward,from nearly 10°to 19°magnetic latitude,two hours before midnight.Smaller-scale bifurcations evolved rapidly from tips of airglow depletions by a secondary E×B instability when the aforementioned substorm-induced southwestward wind blew through.During the growth phase of the EPB depletions,a westward polarization electric field inside the LSWS is likely to have compressed plasma downward,inducing the two airglow-type blobs observed in the bottomside ionosphere,by a mechanism of LSWS-blob connection that we propose.We also provide observational evidence of brightness airglow depletions.We find that an enhanced poleward wind associated with a passing-by brightness wave(BW)is likely to have transported plasma to fill the airglow depletions,which finally evolved into brightness airglow structures.This study investigates the physical processes accompanied by the EPB event and those two-airglow blobs observed at low-latitudes over China.