Tailward Expansion of the Plasma Sheet during a Substorm Using THEMIS Observations
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摘要: 利用THEMIS卫星观测结果，分析2008年3月13日10：40UT-12：10UT的一次中等亚暴事件在磁尾的全球演化过程.在该过程中，THEMIS的5颗卫星在午夜区附近沿x轴依次排列，离地心距离约8.7~13.2Re.亚暴触发开始后，磁场偶极化和等离子体片的膨胀依次被在磁尾不同位置的卫星观测到.等离子体尾向膨胀的平均速度约为140km·s-1.在此次亚暴事件中可观测到两种类型的偶极化.一种为偶极化锋面，其与爆发性整体流（BBF）密切相关；另一种为全球偶极化，其与等离子体片的膨胀密切相关.亚暴触发开始约7min后，THEMIS卫星在低中高纬都可以观测到Pi2脉动的发生，且Pi2脉动的振幅随着纬度的升高逐渐变大.此次亚暴事件中的离子整体流速度主要是由离子电漂移速度引起的，测得的电场为局地磁通量变化导致的感应电场.Abstract: A moderately intense substorm from 10:40UT to 12:10UT on 13 March 2008 observed by THEMIS probes is examined to analyze the global evolution of the substorm in the magnetotail. During this substorm, five THEMIS probes are arranged along the x-axis in the vicinity of the midnight, from the radical distance of about 8.7 to 13.2Re. Magnetic field dipolarization and plasma sheet expansions are all observed by different probes after the substorm onset. The average velocity of the plasma sheet expansions is about 140km·s-1. Two types of dipolarization are detected in this substorm. One type is dipolarization front which is associated with the Bursty Bulk Flow (BBF). The other type is global dipolarization which is associated with plasma sheet expansions.The occurrence of Pi2 pulsation can be observed at low, medium and high latitudes 7 minutes later from the substrom onset. The amplitude of Pi2 pulsation gradually increases with the increase of latitude. The ion bulk velocity in the substorm event is mainly caused by the ion drift velocity, and the measured electric field is the induced electric field generated by the change of the local magnetic flux.
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Key words:
- Substorm /
- Magnetic dipolarization /
- Plasma sheet expansion /
- Pi2 pulsation /
- Electric drift velocity
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