Particle precipitation characteristics in the dayside four‐sheet field‐aligned current structure

Abstract

We study the statistical characteristics of four‐sheet structures of large scale field‐aligned currents (FACs) with an emphasis on particle precipitation collocated with each FAC sheet observed in dayside using magnetic field and particle precipitation data from the Defense Meteorological Satellite Program (DMSP). A total of 3247 four‐sheet events were identified by applying an automatic procedure to the magnetic field data. The occurrence frequency of the events was less than 10% in each magnetic local time (MLT) sector for both hemispheres. The selected events were classified into two groups (type W and type M) with respect to the polarity of FACs. The polarity of the most equatorward type W (type M) FAC sheet is upward (downward). Type W (type M) events are observed predominantly in the morning (evening) sector and display a clear interplanetary magnetic field (IMF) BY dependence. Type W (type M) events appear for negative (positive) IMF BY in the Northern Hemisphere with the opposite dependence in the Southern Hemisphere. For each FAC sheet for type W events, we examined the occurrence of different precipitation regions identified with another automatic procedure. The most equatorward FAC sheet is collocated predominantly with central plasma sheet precipitation except for the prenoon sector, where boundary plasma sheet (BPS) precipitation has the highest occurrence. For the second and the third equatorward sheets, boundary layer‐like precipitation, BPS near dawn and low‐latitude boundary layer near noon, is dominant. In the most poleward sheet, mantle precipitation was observed frequently near noon, while BPS precipitation was dominant at earlier MLTs. These results, especially precipitation characteristics near noon, are consistent with our interpretation of the four‐sheet structures, which is based on IMF BY dependence of convection patterns consisting of lobe, merging, and viscous cells.