  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.

Plasma Physics;Space Physics