Study Of Abnormal Polarity And Density Of Field-aligned Currents Events

Field-Aligned Currents(FACs)are of pivotal importance for transporting mass,energy and momentum of the magnetosphere to the ionosphere,playing an outstanding role in the magnetosphere-ionosphere coupling system.By using the high resolution magnetic field data from Swarm A and C satellites(the dual-satellite),the spatial and temporal distribution of FACs in the high latitude ionosphere are investigated systematically.Coordinated with DMSP satellites,the morphological correspondence of high latitude FACs events and auroral particle precipitation as well as the source regions of these events are further discussed.The main results are summarized as follows:1.Based on the magnetic field data of the dual-satellite during 2015-2016(over36560 polar passes),we have investigated statistically the spatial and temporal distribution of the high latitude FACs events in the dawn and dusk sectors by differentiating their polarity and relative current density for the first time,their responses to Interplanetary Magnetic Field(IMF)and substorms are further studied.The results suggest:(1)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 about70%,and the occurrence frequency of events with R1 FACs stronger than R2 FACs is about a fact of 3~5 larger than those with R1 FACs weaker than R2 FACs.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*events is about a fact of 1.5~2.5 larger than that of R1*>R2* events.(2)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 and ionospheric conductivity.The R1>R2 events are related to the southward turning of IMF Bz,while the R1<R2 events prefer the northward turning of the IMF Bz.(3)The abnormal polarity events are usually observed during periods of the northward IMF Bz,the latitude of these events is about 4° higher than that of the 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.The flow pattern relationship with IMF By cited above is opposite to that in the southern hemisphere.(4)The normal polarity events correlate well with the substorms.The R1>R2events often occur during the substorm expansion phase,while the R1<R2events are more likely to occur during the substorm recovery phase.2.Simultaneous observations of the typical FACs events are performed by Swarm and DMSP during April to December 2014.By using the particle precipitation energy spectrum data and auroral emissions,the spatial distribution characteristics of these typical events are further analyzed.The magnetospheric source regions of these events are also discussed.The main results of the study are as follows:(1)The source regions of R2 FACs of the normal polarity events are the central plasma sheet and plasma sheet boundary layer in the dawn and dusk sectors,while R1 FACs mostly map to the plasma sheet boundary layer and the low-latitude boundary layer.The R2* FACs of the abnormal polarity events are originate from the plasma sheet boundary layer and the low-latitude boundary layer,and the R1* FACs map to the polar rain regions.(2)The normal polarity events are related with the southward IMF Bz.The abnormal polarity events are usually associated with the northward IMF Bz and the north?south and dawn-dusk asymmetries of these events are related to the polarity of the IMF By.Besides,the R1>R2 events often occur during the substorm expansion phase while the R1<R2 events prefer occuring during the substorm recovery phase.