Geotail observations of the dayside outer boundary region: Interplanetary magnetic field control and dawn‐dusk asymmetry

[1] On the basis of Geotail observations, we investigate the ion and electron behavior in the dayside low-latitude magnetosphere just inside the magnetopause in order to obtain clues for understanding the formation mechanism of the low-latitude boundary layer (LLBL). The dayside region is classified into several categories according to ion energy spectrum characteristics, electron pitch angle anisotropy, the north-south polarity of the interplanetary magnetic field (IMF), and the observed locations. An important category is the ion mixing region, which contains dense ions of the solar wind origin and hot magnetospheric ions simultaneously. We define this class as data samples for which both the number density and the ion flux at >10 keV exceed certain threshold values. Our statistics show that the ion mixing region thus automatically identified is encountered far more often when the IMF having a northward component lasts for about 4 hours or more. The ion mixing region under extended northward IMF is almost always accompanied by field-aligned, bidirectional electrons of a few hundreds eV, which energy is higher than that of typical magnetosheath electrons. The flux of >2 keV electrons, that is, electrons of the magnetospheric origin is significantly reduced in the mixing region as compared to that in the region earthward of the mixing region. These facts suggest that the plasma transport process operating for extended northward IMF periods plays an important role both for heating low-energy (presumably magnetosheath) electrons in field-aligned directions and for escape/cooling of the magnetospheric electrons. The mixing region exhibits a clear dawn-dusk asymmetry in the ion energy spectrum, supporting the idea that the transport/heating process of the entrant solar wind ions is different for different sides of the magnetosphere. On the basis of similarities/differences between these observed signatures and those having been found in the tail-flanks, relationships among the dayside and tail LLBLs under extended northward IMF are discussed.