In situ Evidence of Breaking the Ion Frozen-in Condition via the Non-gyrotropic Pressure Effect in Magnetic Reconnection

TL;DR

This study provides in-situ evidence that non-gyrotropic ion pressure effects are the primary mechanism breaking the ion frozen-in condition during magnetic reconnection at the magnetopause.

Contribution

It identifies the off-diagonal components of the ion pressure tensor as the main factor unfreezing ion fluid, clarifying the physical process in magnetic reconnection.

Findings

Off-diagonal ion pressure tensor is responsible for breaking the ion frozen-in condition.

Non-gyrotropic pressure effects are linked to ion demagnetization.

Ion pressure tensor influences the reconnection electric field.

Abstract

For magnetic reconnection to proceed, the frozen-in condition for both ion fluid and electron fluid in a localized diffusion region must be violated by inertial effects, thermal pressure effects, or inter-species collisions. It has been unclear which underlying effects unfreeze ion fluid in the diffusion region. By analyzing in-situ THEMIS spacecraft measurements at the dayside magnetopause, we present clear evidence that the off-diagonal components of the ion pressure tensor is mainly responsible for breaking the ion frozen-in condition in reconnection. The off-diagonal pressure tensor, which corresponds to a nongyrotropic pressure effect, is a fluid manifestation of ion demagnetization in the diffusion region. From the perspective of the ion momentum equation, the reported non-gyrotropic ion pressure tensor is a fundamental aspect in specifying the reconnection electric field that controls how quickly reconnection proceeds.