CHARACTERISTICS OF THE ION PRESSURE TENSOR IN THE EARTHS MAGNETOSHEATH

AMPTE/IRM satellite data are used to examine characteristics of the io n pressure tenser in the earth's magnetosheath. The eigenvalues and pr incipal axes of the pressure tenser are computed, and the directions o f the principal axes are compared to the direction of the independentl y measured magnetic field B. When the pressure tenser is anisotropic, as is usually the case in the magnetosheath, one of its eigenvalues is observed to be distinguishable from the other two, which are about eq ual to one another. Thus, the eigenvector associated with the distingu ishable eigenvalue defines an axis of symmetry of the pressure tenser. The symmetry axis is generally not parallel to B. New features of the plasma distribution function are revealed by using the actual eigenva lues of the pressure tenser rather than the usual p(perpendicular to) and p(parallel to) where perpendicular to and parallel to denote direc tions perpendicular and parallel to B. Introduction