Plasma structures of Kelvin-Helmholtz billows at the duskside flank of the magnetotail

A multiple magnetopause crossing event on 24 March 1995 is examined to investigate the structures of plasma in the boundary regions of the magnetotail. The event was observed by Geotail crossing the magnetopause at the duskside flank of the near-Earth magnetotail. The interplanetary magnetic field had been sustained northward for more than 6 h before the event. Repeated patterns of variations in the temperature and density of plasma suggest strongly the presence of Kelvin-Helmholtz billows on the magnetopause. In agreement with previous papers, a magnetospheric region, termed the magnetotail low-latitude boundary layer (T-LLBL), is identified where parallel anisotropic ions flow tailward with the speed slower than similar to 150 km/s. A new finding is that there is a region, termed the low-latitude interactive region (T-LLIR), filled with isotropic ions flowing tailward with the speed of similar to 150 km/s between the magnetopause and the T-LLBL. The T-LLIR was found on the surface of magnetospheric billows but seemed not to exist near the trough between billows. The energy spectrum of the ion momentum ("the momentum spectrum" for simplicity) is first introduced to examine the characteristics of these boundary regions. It is shown that the momentum spectra in different regions show different profiles. The profile in the T-LLIR looks like an intermediate of those obtained in the magnetosheath and the T-LLBL. Using a multivariate Maxwellian mixture model, it is found that the distribution function of ions in the T-LLIR is composed of two cold components and one hot component. A significant feature in this model is that the bulk speed of a hotter component is faster. In contrast, the bulk speeds of the three Maxwellian components are nearly equal to each other in the T-LLBL. These characteristics in the distribution functions are responsible for the differences in the momentum spectra. The role of the T-LLIR in momentum and particle transportation to the magnetosphere from the magnetosheath is discussed.