A statistical study of the behavior of the electron temperature in ejecta

Ejecta, which are the interplanetary manifestations of coronal mass ejections, are identified by various signatures. It was first reported over 20 years ago that ejecta are generally associated with low plasma electron temperatures (Te). However, further case studies, in particular, recent studies of magnetic clouds (a subset of ejecta characterized by large rotations in the magnetic field direction), have shown that in some ejecta, Te is enhanced, not depressed, after the forward boundary is crossed. To clarify the general behavior of Te in ejecta, we have examined, in a statistical study, the behavior of Te in 95 ejecta in the near-Earth solar wind. Of these ejecta, 13 were magnetic clouds. We find that the behavior of Te in ejecta is variable. Most typically, Te fluctuates and may be enhanced (relative to the temperature in the ambient (unshocked) solar wind) in one region of the ejecta and depressed in another. This structure in Te is often reflected in the electron density (ne), with variations in Te being significantly anticorrelated with variations in ne. The behavior of Te contrasts with that of the proton temperature (Tp) in that Tp is depressed in most ejecta so that Te/Tp ≫ 1 in virtually all events. In addition, Tp shows no correlation or anticorrelation with the plasma density. Our study suggests that a criterion according to which Te/Tp \u3e 2 is a more appropriate, though still not unique, indicator of an ejecta than one based on depressions in Te