On the relationship between the energetic particle flux morphology and the change in the magnetic field magnitude during substorms

Abstract

In this paper we examine the relationship between the energetic particle flux morphology and the change in the magnetic field magnitude (Δ|B|) during substorms in the near‐Earth (<10 RE) magnetotail. We use Δ|B| as an indicator of position relative to the cross‐tail current sheet and analyze five dispersionless energetic ion injections which are representative of a set of 167 events. The five sample injections provide us with cases which occurred both above the plane of the current sheet and in the plane of the current sheet. It is found that the particle flux morphologies of the events are organized by the sign of Δ|B| into two generic categories. These categories are two ends of a spectrum of energetic particle behavior observed by AMPTE CCE. Events during which Δ|B| was negative exhibited large particle flux decreases and equatorward directed gradient anisotropies during the growth phase. These observations are consistent with a thinning of the near‐Earth plasma sheet. The injections were composed of an Earthward streaming leading edge, followed by a (relatively) isotropic hot plasma sheet. This is consistent with particle energization in the central plasma sheet during local substorm onset, followed by an expansion of the plasma sheet which engulfs the satellite. Events during which Δ|B| was positive exhibited a relatively small particle flux decrease during the growth phase, and the injections were short‐lived. The ions did not exhibit a strong earthward field‐aligned anisotropy at the leading edge of the injection, in contrast to the Δ|B|<0 events. During one of these events, which we believe was an in situ observation of the disruption of the cross‐tail current sheet, the largest energetic particle fluxes were associated with high levels of |dB/dt|. The observations made during this event are consistent with turbulent disruption of the cross‐tail current, and possibly a tailward motion of the region of particle acceleration. This event, and the Δ|B|<0 events, represent the extrema of energetic particle behavior. A second event during which the magnetic field increased, but by a small amount, exhibited energetic particle behavior which had characteristics of both categories. We believe that this case is an example of an event in the transitional range of the spectrum of energetic particle behavior during substorms in the near‐Earth magnetotail.