伴随着高速流的磁尾电流片拍动观测研究

磁尾电流片在磁尾动力学过程中起着重要作用.卫星观测表明磁尾电流片经常处于拍动状态.但磁尾电流片拍动的特性和产生机制至今仍然没有被完全弄清楚.本文主要利用欧洲空间局Cluster卫星数据,研究一个伴随高速离子流的电流片拍动事件.该电流片拍动事件具有很强的周期性.拍动的周期约是2min,磁场振荡幅度约为20nT.能量电子和离子的通量具有周期性增强和减弱的特征.电流密度X和Y分量也具有周期性的振荡,并且振荡周期与磁场振荡周期一致.通过对粒子流速矢量与电流矢量的分析,发现粒子运动具有涡旋的特征.因此可以推断,该磁尾电流片的拍动不是由磁尾等离子体片高速流产生的,而是与局地等离子体不稳定性有关.

Obervations of current sheet flapping associated with high speed flow in the magnetotail

The magnetotail current sheet frequently moves in the north-south direction,which named current sheet flapping. A statistical survey of Geotail data showed that the flapping motions of the current sheet might be related to the plasma bursty bulk flows BBFs. Current sheet flapping association with fast plasma flows in the plasma sheet and Pi2 pulsation was also observed by THEMIS observations. These observations also pointed out the close relationship between current sheet flapping and the Z component of the ion flow velocity. During the repeated crossings of the current sheet by the spacecraft, the ion velocity V_z showed obviously periodic oscillations. In this observation, the single case will be discussed to reveal that the associated observation BBFs may be the local ions flow. The Cluster FGM, CIS, PEACE and RAPID data will be used to analyze the current sheet flapping case. The CL soft will be used to calculate the current. In the magnetotail, on Oct 26 2002 the Cluster crosses southward the central current sheet in a distance of 19 R_E to Earth. The current sheet flapping case associated by high speed ions flow was observed by Cluster on Oct 26 2002. This flapping case periodic is very good. The FFT analyses show the period of the current sheet flapping is about 2 min, the amplitude is about 20 nT. The velocity values changed periodically. The X component of ions velocity reaches 400 km·s^-1. The Z component of ion velocity is up to 600 km·s^-1. The energy range from 50~100 keV electrons and 10 keV ions flux is enhanced during the spacecrafts across current sheet center. When the spacecrafts move to the current sheet boundary, the energy of ions and electrons flux is decreasing. The current density is also flapping periodically. The velocity vector distribution show the particles motion is not a single direction. Base on the velocity and current vector analyses, the local particles motion is vortex motion. This current sheet flapping is highly cyclical event and lasts for about 10 minutes. Through the particle and current density characters analyses, the high speed flows is local particles velocity. The present result indicated that the high speed ion flow is local particles motion, not come from far away magnetail. This result show the current sheet flapping may be induced by a self-circulation process not the BBF compressed the magnetosphere.