Application of Multivariate Maxwellian Mixture Model to Plasma Velocity Distribution.

Recent space plasma observations provide us three-dimensional velocity distributions which are found to have multiple peaks. We propose a method for analyzing such a velocity distribution via a multivariate Maxwellian mixture model whose each component represents each of the multiple peaks. The parameters of the model are determined through the EM algorithm. For an auto judgment of preferable number of components of the mixture model, we introduce a method of examining the number of extremum of the resulting mixture model. We show applications of our method to observations in the plasma sheet boundary layer and in the central plasma sheet in the terrestrial magnetosphere.