Locations of Night-side Particle Precipitation Boundaries Relative to R2 and R1 Currents

The objective of this study is to place observational constraints on the source location and physical processes of night-side auroral precipitation from the viewpoint of the global magnetosphere-ionosphere coupling. For this end we statistically compare the locations of various boundaries of ion and electron precipitation with those of large-scale field-aligned currents (FACs), i.e., R2 and R1 currents, observed by DMSP satellites. Night-side precipitation boundaries were identified based on criteria proposed by Newell et al. [1996], whereas large-scale FACs were identified by an automatic procedure developed by Higuchi and Ohtani [2000]. A total of 50,000+ orbits were examined. The results are summarized as follows. (1) The b2i boundary, which is defined as the point where the energy flux of >3 keV ions has a maximum and well agrees with the isotopic boundary of ion precipitation, is located in the middle of the R2 system throughout the night side (18 < MLT < 06). (2) Accelerated electron precipitation tends to be spatially confined in R1 currents at 18 < MLT < 24, whereas at 00 < MLT < 06 it is often observed in R2 currents as well as in R1 currents. (3) The zero-energy boundaries of ion and electron precipitation are very often located inside R2 currents at 18 < MLT < 24, whereas they are usually located equatorward of the equator edge of R2 currents at 00 < MLT < 06. We will discuss these results in terms of the magnetospheric configuration, M-I coupling, and convection.