2004年11月磁暴期间环电流的演化过程——TC-2卫星中性原子成像仪观测结果

本文通过对TC-2卫星上搭载的中性原子成像仪(NUADU)在2004年11月发生的一次大磁暴期间观测到的一系列中性原子(ENA)图像的分析,试图给出环电流在磁暴期间的演化模式.研究表明,南向的行星际磁场(IMF)分量在离子从磁尾向内磁层注入和随后的环电流增长过程中起着关键的作用.IMF转为北向后,离子注入随即很快停止.在离子注入增强期间,离子的漂移路径是开放的,以致大量环电流离子从黄昏侧注入后快速地损失在黄昏至正午的磁层顶.所以,环电流往往在离子漂移路径从开放变为封闭后才达到最大强度,而不是在这之前,尽管那时的离子注入强度更大.在该磁暴主相期间,离子注入发生在17∶00~22∶00 LT范围内,形成极其不对称的环电流分布形态.而在恢复相期间,由于受大的IMF B_y分量的影响,离子注入区的地方时分布范围东向扩张.对称环电流在磁尾对流减小、离子漂移路径变为封闭形态之后形成.在磁暴恢复相后期,从ENA图像看环电流基本衰减到平静时期的水平,而Dst指数仍然显示较强的磁扰动,这说明越尾电流对Dst指数有很重要的影响.

Evolvement of the ring current during a magnetic storm:TC-2 neutral atom imager observations

A series of ENA images were obtained by the NUADU instrument aboard TC-2 during a strong, long duration magnetic storm that occurred in November 2004. This paper follows the evolution of the ring current during this magnetic storm. It is shown that the southward IMF B_z component plays a key role in facilitating ion injection from the magnetotail into the inner magnetosphere with consequent enhancement of the ring current. Ion injection stopped soon after the IMF turned from southward to northward. Also during ion injection episodes, the ion drift paths opened and ring current ions were lost at the noon to dusk magnetopause soon after they were injected at the dusk side region. As a result, the ring current intensified after the occurrence of an open-to-closed transformation of the ion drift paths, rather than at the stage when ion injection was strong. In the main phase of the storm, ion injection took place between 17∶00~22∶00 LT, leading to an extremely asymmetric ring current. In contrast in the recovery phase of the storm, ion injection region reached the post-midnight sector. We attributed this phenomenon to a significant positive IMF B_y component. A symmetrical ring current was formed after the drift paths changed back to a closed configuration due to a decrease in tail convection. When the storm entered its late recovery phase, the ring current faded away while the Dst still indicated a strong magnetic disturbance. This suggested an important contribution to Dst from the cross tail current.