Simultaneous TRACERS and THEMIS Observations of Reversed Cusp Ion Dispersions and Dual-Lobe Reconnection

TL;DR

This study uses simultaneous TRACERS-2 and THEMIS observations to analyze cusp ion dispersions and reconnection events under different IMF conditions, revealing consistent plasma signatures despite varying IMF orientations.

Contribution

It demonstrates that tailward-of-cusp reconnection occurs under both northward and BX-dominated IMF, producing similar low-altitude cusp signatures.

Findings

Reversed cusp ion dispersion observed during reconnection events.

Similar plasma signatures found under different IMF conditions.

Reconnection can occur tailward of the cusp for various IMF orientations.

Abstract

We present observations from two consecutive TRACERS-2 orbits through the northern low-altitude cusp. During the first crossing, TRACERS-2 observed reversed cusp ion dispersion and sunward convection, consistent with magnetopause reconnection tailward of the cusp during this northward IMF interval. Simultaneous THEMIS-D observations at the equatorial magnetopause show heated magnetosheath plasma captured on closed field lines, with similar particle spectra as in in the low-altitude cusp, indicating that reconnection indeed occurred tailward of the cusp and in both hemispheres. When TRACERS-2 traversed the northern cusp again, 95 minutes later, the IMF was dominated by a negative BX component. Despite the different IMF conditions, TRACERS-2 recorded nearly the same cusp signatures as before, i.e., reversed ion dispersion and sunward convection. The observations indicate that tailward-of-cusp reconnection can occur for both northward and BX-dominated IMF and that these distinct IMF geometries can produce remarkably similar plasma and field signatures in the low-altitude cusp.