Laboratory observation of parallel electrostatic electric field around X line in 3D guide field magnetic reconnection

The parallel electric field has been identified as the primary mechanism of energy conversion in guide field magnetic reconnection. In 2D reconnection, the parallel electric field around the X line can be balanced by gradients of agyrotropic pressure. But this mechanism is important only when the current sheet thickness is at the scale of electron Larmor radius [1]. However, additional mechanisms especially in 3D, such as the recently detected anomalous resistivity due to Lower Hybrid Drift Waves [2], are needed to explain the observed large parallel electric field around the X line of reconnection with a modest guide field in both satellite observations and laboratory experiments [1,2]. Here, we present experimental evidence of yet another 3D mechanism based on a parallel electrostatic electric field due to the gradient of electron thermal pressure along the X line in guide field reconnection in MRX. The preliminary results show that the total parallel electric field at the X line is about 2 times of the reconnection electric field from the magnetic flux annihilation rate when there is a significant guide field. In contrast, the total electric field along the out-of-plane direction around X line in anti-parallel reconnection, is comparable to the reconnection electric field. This inferred large parallel electric field in guide field reconnection is consistent with space observations [1]. The future plan to identify the nature of this 3D structure in guide field reconnection will also be discussed.