Hasegawa H, Denton R E, Nakamura T K M, Genestreti K J, Phan T D, Nakamura R, Hwang K-J, Ahmadi N, Shi Q Q, Hesse M, Burch J L, Webster J M, Torbert R B, Giles B L, Gershman D J, Russell C T, Strangeway R J, Wei H Y, Lindqvist P-A, Khotyaintsev Y V, Ergun R E, Saito Y
Institute of Space and Astronautical Science Japan Aerospace Exploration Agency Sagamihara Japan.
Department of Physics and Astronomy Dartmouth College Hanover NH USA.
J Geophys Res Space Phys. 2022 Jul;127(7):e2022JA030408. doi: 10.1029/2022JA030408. Epub 2022 Jul 7.
We present observations in Earth's magnetotail by the Magnetospheric Multiscale spacecraft that are consistent with magnetic field annihilation, rather than magnetic topology change, causing fast magnetic-to-electron energy conversion in an electron-scale current sheet. Multi-spacecraft analysis for the magnetic field reconstruction shows that an electron-scale magnetic island was embedded in the observed electron diffusion region (EDR), suggesting an elongated shape of the EDR. Evidence for the annihilation was revealed in the form of the island growing at a rate much lower than expected for the standard X-type geometry of the EDR, which indicates that magnetic flux injected into the EDR was not ejected from the X-point or accumulated in the island, but was dissipated in the EDR. This energy conversion process is in contrast to that in the standard EDR of a reconnecting current sheet where the energy of antiparallel magnetic fields is mostly converted to electron bulk-flow energy. Fully kinetic simulation also demonstrates that an elongated EDR is subject to the formation of electron-scale magnetic islands in which fast but transient annihilation can occur. Consistent with the observations and simulation, theoretical analysis shows that fast magnetic diffusion can occur in an elongated EDR in the presence of nongyrotropic electron effects. We suggest that the annihilation in elongated EDRs may contribute to the dissipation of magnetic energy in a turbulent collisionless plasma.
我们展示了磁层多尺度航天器在地球磁尾的观测结果，这些观测结果与磁场湮灭而非磁拓扑结构变化相一致，磁场湮灭导致了电子尺度电流片中磁场到电子的快速能量转换。对磁场重建的多航天器分析表明，一个电子尺度的磁岛嵌入在观测到的电子扩散区（EDR）中，这表明EDR呈拉长形状。湮灭的证据以该磁岛以远低于EDR标准X型几何结构预期的速率增长的形式显现出来，这表明注入EDR的磁通量并非从X点喷出或在磁岛中积累，而是在EDR中耗散。这种能量转换过程与重联电流片标准EDR中的过程形成对比，在标准EDR中，反平行磁场的能量大多转换为电子整体流动能量。全动力学模拟也表明，拉长的EDR会形成电子尺度的磁岛，在其中会发生快速但短暂的湮灭。与观测和模拟结果一致，理论分析表明，在存在非各向同性电子效应的情况下，拉长的EDR中会发生快速磁扩散。我们认为，拉长的EDR中的湮灭可能有助于无碰撞湍流等离子体中磁能的耗散。
