Contribution of the Hall current effect in the substorm current system

在地球磁尾薄等离子体层的磁重联中，流入的电子随磁场线流向扩散区，而流入的离子无法到达扩散区，停留在离子惯性长度的距离附近。电子和离子的相对运动产生电流，即霍尔电流。霍尔电流可以提高磁重联的速率，因此在观测和模拟中评估霍尔电流对磁重联的影响具有重要意义。利用Geotail航天器对磁尾中磁重联区附近的等离子体和磁场结构进行了研究。我们通过直接测量电流密度和由此产生的磁场变化，证明了四个霍尔电流环的存在。我们已经确定，最外层等离子体片的向外电流是由低能量的流入电子携带的。向内电流存在于邻近区域，就在向外电流的内部。此外，产生的偏转被限制在等离子体片/尾叶边界的薄层中。因此，每个电流回路都具有薄的双片电流结构。通过模拟，我们推导出了双片电流结构的厚度，发现其厚度是离子惯性长度的数量级。

In magnetic reconnection in the thin plasma sheet of the Earth's magnetotail, inflow electrons flow with the magnetic field line to the diffusion region, whereas inflow ions cannot reach the diffusion region and rest around a distance of the ion inertial length. The relative motion of the electrons and ions results in electric currents, that is, the Hall currents. The Hall currents can enhance the reconnection rate, so that it is important to evaluate the effects of the Hall currents in magnetic reconnection in observations as well as in simulations. We have investigated the plasma and magnetic field structure near the magnetic reconnection region in the magnetotail with the Geotail spacecraft. We have demonstrated the existence of the four Hall current loops with direct measurements of current density and with the resulting magnetic field By variations. We have firmly established that the outward currents in the outermost plasma sheet are carried with low-energy inflowing electrons. The inward currents exist in the adjacent region, just inside of the outward currents. Furthermore, the resulting By deflection is confined into the thin layer in the plasma sheet/tail lobe boundary. Hence, each current loop has the thin double-sheet current structure. With simulations, we have deduced the thickness of the double-sheet current structure and have found that its thickness is of the order of ion inertial length.