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For magnetic reconnection to proceed, the frozen-in condition for both ionfluid and electron fluid in a localized diffusion region must be violated byinertial effects, thermal pressure effects, or inter-species collisions. It hasbeen unclear which underlying effects unfreeze ion fluid in the diffusionregion. By analyzing in-situ THEMIS spacecraft measurements at the daysidemagnetopause, we present clear evidence that the off-diagonal components of theion pressure tensor is mainly responsible for breaking the ion frozen-incondition in reconnection. The off-diagonal pressure tensor, which correspondsto a nongyrotropic pressure effect, is a fluid manifestation of iondemagnetization in the diffusion region. From the perspective of the ionmomentum equation, the reported non-gyrotropic ion pressure tensor is afundamental aspect in specifying the reconnection electric field that controlshow quickly reconnection proceeds.

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