Electron heating and energy inventory during asymmetric reconnection in a laboratory plasma

Electron heating and the energy inventory during asymmetric reconnection are studied in the laboratory plasma with a density ratio of about 8 across the current sheet. Features of asymmetric reconnection such as the large density gradients near the low-density-side separatrices, asymmetric in-plane electric field, and bipolar out-of-plane magnetic field are observed. Unlike the symmetric case, electrons are also heated near the low-density-side separatrices. The measured parallel electric field may explain the observed electron heating. Although large fluctuations driven by lower-hybrid drift instabilities are also observed near the low-density-side separatrices, laboratory measurements and numerical simulations reported here suggest that they do not play a major role in electron energization. The average electron temperature increase in the exhaust region is proportional to the incoming magnetic energy per an electron/ion pair but exceeds scalings of the previous space observations. This discrepancy is explained by differences in the boundary condition and system size. The profile of electron energy gain from the electric field shows that there is additional electron energy gain associated with the electron diamagnetic current besides a large energy gain near the X-line. This additional energy gain increases electron enthalpy, not the electron temperature. Finally, a quantitative analysis of the energy inventory during asymmetric reconnection is conducted. Unlike the symmetric case where the ion energy gain is about twice more than the electron energy gain, electrons and ions obtain a similar amount of energy during asymmetric reconnection.

本研究针对电流片两侧密度比约为8的实验室等离子体，开展了不对称磁重联过程中的电子加热与能量收支研究。观测到不对称磁重联的典型特征：低密度侧分界面附近存在强密度梯度、平面内电场不对称以及平面外磁场呈双极分布。与对称磁重联情形不同，低密度侧分界面附近同样存在电子加热现象。实测的平行电场可对观测到的电子加热现象作出解释。尽管低密度侧分界面附近也观测到由低混杂漂移不稳定性驱动的强涨落，但本研究的实验室测量结果与数值模拟均表明，此类涨落并未在电子增能过程中发挥主要作用。出流区内电子温度的平均增幅与每对电子/离子携带的入射磁能呈正比关系，且其标度超出了此前空间观测得到的标度律范围。该偏差可通过边界条件与系统尺度的差异加以解释。电子从电场中获得能量的分布特征表明，除X线附近的强能量增益外，还存在与电子抗磁电流相关的额外电子能量增益。该额外能量增益提升的是电子焓，而非电子温度。最后，本研究对不对称磁重联过程中的能量收支开展了定量分析。与对称磁重联情形下离子能量增益约为电子两倍的结果不同，不对称磁重联过程中电子与离子获得的能量量级相近。