Anomalous Reconnection Layer at Earth's Dayside Magnetopause

Abstract

Observations by the Magnetospheric Multiscale spacecraft (MMS) of an unusual layer, located between the dayside magnetosheath and the magnetosphere, alternating with encounters with the magnetosheath during an extended time period between December 31, 2015 and January 01, 2016, when the interplanetary magnetic field was strongly southward and the Earth's dipole tilt large and negative, are presented. It appears to have been magnetically connected to both magnetosphere and magnetosheath. The layer appears to be located mostly on closed field lines and was bounded by a rotational discontinuity (RD) at its magnetosheath edge and by the magnetosphere on its earthward side. A separatrix layer, with heated magnetosheath electrons streaming unidirectionally along the field lines, was present sunward of the RD. We infer that the layer was started by a dominant reconnection site well north of the spacecraft and that it may have gained additional width, from a large drop in solar wind density and ram pressure, which preceded the beginning of the event by more than an hour. Relative to the magnetosheath, in which the magnetic field was strongly southward, this unusual layer was characterized by a less southward, more dawnward magnetic field of lower magnitude. The plasma density and flow speed in the region were lower than in the magnetosheath, albeit with Alfvénic jetting occurring at the magnetosheath edge as well as at the magnetospheric edge of the layer. The closing of the magnetic field lines requires the existence of another reconnection site, located southward/tailward of MMS.

MMS Observations of Storm‐Time Magnetopause Boundary Layers in the Vicinity of the Southern Cusp

“…We report an MMS crossing from the magnetosheath into magnetosphere, in‐between which it sampled multiple BLs with different properties, approximately at the peak ring current of a large geomagnetic storm near winter solstice. These wide magnetopause BLs associated with low Mach number storm‐time and large dipole tilt, and containing closed flux regions, are similar to those reported forming due to remote reconnection (Paschmann et al., 2021). The key new feature in our event is the entry into the magnetosphere with B z < 0, interpreted as MMS crossing from BL1 through the magnetopause either equatorward or tailward of the southern cusp.…”

“…Electrons (Figure 1d) in the magnetosheath have 100 eV peak energy, while in BL1 some intervals show higher-than-sheath energy electrons. Furthermore, both the plasma characteristics and magnetic field strength in BL1 (approximately half the magnetosheath value) are consistent with those in the BL produced by reconnection exhausts reported in Paschmann et al (2021).…”

“…One key new feature in the storm-time BL observations presented in Figure 1 and not in the report by Paschmann et al (2021) is the entry into the magnetosphere with B z < 0. Two possible scenarios may be admitted to account for the unusual magnetosphere entry: (1) from BL1 through the magnetopause equatorward of the southern cusp;…”

“…The key new feature in our event is the entry into the magnetosphere with B z < 0, interpreted as MMS crossing from BL1 through the magnetopause either equatorward or tailward of the southern cusp. In addition, Paschmann et al (2021) hypothesized that higher latitude reconnection occurred to produce closed field lines in the BL. In this study, observational evidence indicates consistency with a reconnection x-line near the southern cusp as MMS crossed from BL1 into the magnetosphere, supporting active reconnection nearby.…”

“…(2017) and Paschmann et al. (2021) examined low Mach number magnetopause BL traversals associated with storm‐time. The event discussed in Section 3 examines the transition from storm‐time BL into the exterior cusp magnetosphere for the first time.…”