场向电流极性和密度异常事件统计研究

Statistical study of abnormal polarity and density of field-aligned currents events

本文利用Swarm卫星2015—2016年高精度的磁场矢量数据,将晨昏地方时扇区高纬场向电流(Field-Aligned Currents,FACs)事件按极性和电流密度分为四类,并首次比较研究了四类FACs事件的时空分布特征及其影响因素,研究发现:极性正常事件(晨侧靠极侧电流元向下流入电离层,靠赤道侧电流元向上流出电离层,昏侧电流极性相反,即传统意义上的R1和R2FACs)发生率约为70%,其中R1FACs强于R2事件的发生率为R1FACs弱于R2的3~5倍;极性异常事件(与传统的R1和R2FACs流向相反,两片电流元定义为R1*和R2*FACs)发生率约占30%,其中R1*<R2*的发生率约为R1*>R2*的1.5~2.5倍.进一步分析发现极性正常事件主要发生在南向IMF Bz期间,与重联电场相关性较好,净电流密度随着重联电场和电离层电导率的增加而增加.其中R1<R2事件通常位于IMF Bz北向偏转期间,与亚暴过程有关.同时,极性异常事件通常发生于北向IMF Bz期间.相对于极性正常事件,极性异常事件所处磁纬向极侧偏移4°.在北半球,晨侧R1*>R2*事件通常发生在IMF By<0期间,昏侧事件主要发生在IMF By>0期间,而R1*<R2*事件所对应IMF By极性相反,南半球情形与北半球相反.

By using high resolution magnetic field data from Swarm A and C satellites during years of 2015 and 2016,we have investigated the spatial and temporal distribution of field-aligned currents(FACs)in the dawn-dusk sector by differentiating their polarity and relative current density for the first time.The normal polarity events consist of the poleward part flowing into the ionosphere and the equatorward part flowing out of the ionosphere in the dawn sector,which is reversed in the dusk sector.They are traditional R1 and R2 FACs.The occurrence frequency of the normal polarity event is about 70%,and the occurrence frequency of events with R1 FACs stronger than R2 is about a fact of 3~5 larger than those with R1 FACs weaker than R2.The abnormal polarity events,which are opposite to the flow directions of traditional R1 and R2 FACs,are termed as R1*and R2*FACs.They account for about 30%,and the occurrence frequency of R1*<R2*is about a fact of 1.5~2.5 larger than that of R1*>R2*.The normal polarity events are associated with the southward IMF Bz,which have a strong correlation with the merging electric field,and the net current density increases with the merging electric field andionospheric conductivity.The R1<R2 events are related to the northward turning of IMF Bzand substorm process.Since the abnormal events are usually observed during periods of northward IMF Bz,the latitude of these events is about 4°higher than those of normal events.In the northern hemisphere,the R1*>R2*events tend to occur when IMF By<0 in the dawn sector,and the dusk events mainly occur during the period of IMF By>0,while the R1*<R2*events correspond to the opposite polarity of IMF By and the situation in the southern hemisphere is opposite to that in the northern hemisphere.