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 (Time History of Events and Macroscale Interactions during Substorms) 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 non-gyrotropic pressure effect in this event, 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.
Dai, L., Wang, C., Angelopoulos, V., & Glassmeier, K. H. (2015). In situ evidence of breaking the ion frozen-in condition via the non-gyrotropic pressure effect in magnetic reconnection. Annales Geophysicae, 33(9), 1147–1153. https://doi.org/10.5194/angeo-33-1147-2015