2020年4月20日磁暴恢复相期间舌状中性结构的垂直变化

Vertical variations of a neutral'tongue'during the recovery phase of the geomagnetic storm on April 20,2020

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.

A'tongue'of neutral composition(TON)in O and N_(2) column density ratio(ΣO/N_(2))at middle-low latitudes was observed by the Global-scale Observations of the Limb and Disk(GOLD)on DOY(day of year)112 during the recovery phase of geomagnetic storm on April,202020.The TON was an area of positive∑O/N_(2) variations sandwiched by two regions of negative∑O/N_(2) variations.Thermosphere-Ionosphere-Electrodynamics General Circulation Model(TIEGCM)qualitatively reproduced the GOLD observed∑O/N_(2) enhancement during the storm recovery phase,and indicated that it was transported from local afternoon of the previous day(DOY 111).The simulations presented the vertical variations of the TON,and the magnitude and latitude range of the structure have a significant altitude dependency.The diagnostic analysis showed that the downwelling first resulted in the O/N_(2) enhancement at the lower latitudes of~30°N—70°N.Then the O/N_(2) enhancement was transported to higher latitudes by the horizontal advection driven by poleward winds.At middle-low thermosphere(~120 to 300 km),the poleward winds at middle-low latitudes that dominated the evolution of the TON structure were primarily caused by the pressure gradient force.The Coriolis force also contributed positively a little to the poleward winds.At higher thermosphere of above~300 km,the momentum forcing of the poleward winds was determined by the pressure gradient and the vertical viscosity force with opposite directions.