Plasma sheet ion energization during dipolarization events

Abstract

The energization of plasma sheet particles during storm‐time dipolarization of the magnetospheric field lines is examined by means of three‐dimensional particle codes. In the direction perpendicular to the magnetic field, owing to appreciable temporal variations of the field within a cyclotron turn, particles can experience large magnetic moment changes. A detailed analysis of this effect reveals an “effective pick‐up velocity” at substorm onset, which depends upon the magnitude of the magnetic transition and delineates particle motions at nearly constant or highly variable magnetic moment. In the parallel direction, the large but short‐lived electric field yields an intense acceleration of the particles traveling in the equatorial region. It is demonstrated that significant decelerations can also be achieved off‐equator if the particles drift against the surging electric field. Such decelerations occur in regions where the curvature of the E × B paths substantially exceeds that of the magnetic field lines (namely, at relatively large L‐ shells). Systematic calculations of model plasma sheet distribution functions clearly exemplify the earthward injection of energetic particles during dipolarization events. Owing to phasing between particle motion and the transient electric field, the computed spectrograms display the formation of characteristic bouncing ion patterns with fine structures on fast time scales. To some extent, the simulations support the convection surge mechanism of particle acceleration put forward by Mauk [1986]. However, distinct collapse characteristics as well as the inclusion of nonadiabatic effects result in significantly higher energization rates, particularly at high mass‐to‐charge ratios.