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.

Observations of long‐term field‐aligned flow of O+ ions near the equator during summer and winter

“…They resemble the "bouncing ion clusters" observed by Quinn and Mcllwain [1979] with the ATS 6. Of course, such bouncing clusters can also be produced by a parallel electric field of short duration and by a suddenly increased perpendicular electric field, as discussed by Mauk [1986] and Delcourt and Sauvaud [1994].…”

Modeling of inner plasma sheet and ring current during substorms

“…The three-dimensional particle code of Delcourt [Delcourt et al, 1990;Delcourt and Sauvaud, 1994] is used to calculate particle trajectories in order to set up the proton distributions on the nightside along a boundary at a constant equatorial 12 Earth radii (RE). This test-particle code computes the guiding center equation in the near-Earth region and the full equation of particle motion at larger distance.…”

“…Not surprisingly, there is no significant substorm-associated flux depletion at 2200 and 0000 MLT. The plasma sheet flux dropouts and enhancements during substorms have been previously modeled using our singleparticle code [Sauvaud et al, 1996;Delcourt and Sauvaud, 1994]. representing low-energy (1-5 keV) flux, green representing medium-energy (5-40 keV) flux, and blue representing highenergy (40-300 keV) flux.…”

“…Dipolarization in the tail and the breaking of adiabaticity could strongly influence the characteristics of the plasma being fed into the ring current growth region. To correct this, we sought to use the three-dimensional (3D) test particle code of D. Delcourt [Delcourt et al, 1990;Delcourt and Sauvaud, 1994] to generate this ring current outer boundary condition as an inner boundary result for a dipolarizing plasma sheet, fed by constant sources of boundary plasmas. In this study.…”

Shock-driven variation in ionospheric outflow during the 11 October 2001 moderate storm

“…Since then, the acceleration term has been included in many numerical models of ion motion and transport in the magnetosphere [e.g., Horwitz, 1987;Cladis, 1988;Delcourt et al, 1990;Swift, 1990;Delcourt and Sauvaud, 1994]. The mechanism of centrifugal acceleration is that charged particles are accelerated along a "rotating" magnetic field line due to E × B drift along the direction of change of the field direction [Northrop, 1963;Cladis, 1986;Horwitz et al, 1994].…”