Effect of By on neutral line ridges and dynamical source ordering

Abstract

In this paper we study the effect of a uniform magnetic field By in the cross‐tail y direction on the dynamics and distribution function of energetic ions in a current sheet model including a neutral line. Martin and Speiser (1988) have previously shown that a “ridge” in the velocity space distribution function is a remote neutral line signature in a two‐dimensional field without By. Our results show that as By is increased, using nominal tail parameters, there is very little change in the ridge signature for small By (up to about 4 times Bz). For intermediate values (up to about 10 times Bz) the ridge becomes observable further from the neutral line, while close to the X line the ridge is strongly modified. For large By (of the order of Bx) the ridge evolves into new neutral line signatures which depend strongly on whether the observation point is above or below the current sheet. These results are used to estimate By < 2 nT in the AMPTE/IRM event modeled by Speiser and Martin (1994), which is consistent with onboard magnetometer measurements. We further show that the cause of the new structures appearing at large By is the same source separation effect that produced the original ridge and that all these signatures are relatively insensitive to a Small parallel electric field. Finally, we point out a dynamical ordering which occurs at large By: Using high‐resolution simulations, we find a common boundary separating regions of positive and negative initial x, y, and z position, as well as initial pitch angle. This boundary is purely dynamical in nature and is independent of modeled source distributions.