Parker Solar Probe Observations of Compound Reconnection Exhaust Boundaries and Mirror-Mode Structures in the Near-Sun Heliospheric Current Sheet

TL;DR

This study reports Parker Solar Probe observations of magnetic reconnection exhaust boundaries in the near-Sun heliospheric current sheet, revealing complex compound structures and mirror-mode phenomena that challenge classical shock models.

Contribution

It provides the first in situ characterization of reconnection exhaust boundaries with compound magnetic structures near the Sun, expanding understanding of energy conversion processes.

Findings

Reconnection exhaust boundaries are bounded by compound magnetic structures, not pure slow shocks.

Inner slow shocks exhibit rapidly changing Mach numbers and shock-normal angles.

Mirror-mode structures are generated within the exhaust due to high perpendicular temperatures.

Abstract

Magnetic reconnection is a fundamental physical process that can drive rapid conversion of magnetic energy into plasma bulk flows, thermal heating, and particle acceleration in space and astrophysical plasmas. Classical reconnection theory predicts that the Alfvenic reconnection exhausts are bounded by pairs of slow-mode shocks. However, identifying and characterizing these shocks through in situ spacecraft observations remains a challenge. Here we report Parker Solar Probe (PSP) observations of a reconnection exhaust embedded in the heliospheric current sheet (HCS) at a heliocentric distance of 12.2 R_O. The reconnection exhaust is bounded on both boundaries by compound magnetic structures rather than a pair of pure slow shocks. Each boundary consists of a rapidly evolving, steep inner slow shock, whose Mach numbers and shock-normal angles change significantly within several minutes, and an outer, gradual compound structure which comprises a slow shock and a rotational discontinuity. These slow shocks are quasi-perpendicular and are accompanied by enhanced proton perpendicular heating. Deep within the reconnection exhaust, high perpendicular temperature together with large plasma beta trigger mirror instability and generate mirror-mode structures. These observations provide new insights into the structure of reconnection exhaust boundaries and their role in energy conversion in the near-Sun plasma.