THE ENERGIZATION OF COLD IONS AROUND A PLASMOID IN THE MAGNETOTAIL

We study numerically the interaction of cold ionospheric ions in the t ail lobe with a plasmoid propagating steadily in the tailward directio n. We have found that significant energization of cold ions occurs at the encounter with a plasmoid, where three types of the interaction pr ocesses are taking place: First, ions show gradient and curvature drif t motions as the plasmoid approaches. These ions are accelerated up to similar to 1 keV by the cross-tail electric field accompanying with t he plasmoid motion (type-I energization). Second, some of the ions get large pitch angles after the type-I acceleration, and then mirror ref lected by the plasmoid. These ions are accelerated up to 10 keV after the reflection (type-II energization). Third, the motions of the ions near the plasmoid become highly non-adiabatic, because their gyroradii become comparable to the field-line curvature radius after the type-I energization. Consequently the energy distribution of these ions is b roadened in the trailing side of the plasmoid (type-III energization). These theoretical results are successfully compared with the recent G EOTAIL observation showing the dynamic interaction of cold ions with a plasmoid and their subsequent energization.