Density as a controlling factor for seasonal and altitudinal variations of the auroral particle acceleration region
Density as a controlling factor for seasonal and altitudinal variations of the auroral particle acceleration region
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DOI：
10.1029/2002ja009786
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发表时间：
2003-07-29
影响因子：
2.8
通讯作者：
Mukai, T
中科院分区：
文献类型：
--
作者：
Morooka, M;Mukai, T
[1]本文报道了由Akebono(EXOS-D)卫星观测到的极光上空高纬度对准粒子加速区的统计特性。秋野经常同时观测到向下加速的电子和向上加速的离子，这表明卫星处于平行加速区内。我们发现了三个主要的粒子加速区域，其中两个位于正午和午后的日侧，一个位于午夜前。日边区平均总场向电位差约为600V(夏季午后区为800V)，粒子加速事件的发生频率在夏季较大。在午夜前区，平均总场向电位差约为4kV，冬季粒子加速事件的发生频率约为前者的两倍。这项研究的一个重要结果是显示了平行加速区的高度分布。在白天地区，夏季80%以上的离子加速出现在6000公里以上，冬季70%以上的电子加速出现在8000公里以下。在午夜前区，夏季有90%的离子加速在6000公里以上，冬季有90%以上的电子加速在6000公里以下。我们用加速区周围等离子体密度的季节变化来讨论这些加速高度的季节变化，在冬季密度较低，在较低的海拔引起加速。Akebono没有观察到随着季节和海拔的变化，势降底部的密度和势差之间的关系发生了任何变化，这支持了加速高度密度的重要性。极光电子加速的发生频率可能受极光场线周围等离子体密度的季节变化控制。
[1] This paper reports on the statistical properties of the field-aligned particle acceleration region at high latitudes above the auroral region observed by the Akebono (EXOS-D) satellite. Akebono often observes downward accelerated electrons and upward accelerated ions at the same time, indicating that the satellite is inside the parallel acceleration region. We found three major regions of particle acceleration, two located in the dayside in the prenoon and postnoon sectors and one in the premidnight sector. In the dayside regions the average total field-aligned potential difference is about 600 V ( 800 V in the summer postnoon region), and the occurrence frequency of the particle acceleration events is larger in the summer. In the premidnight region the average total field aligned potential difference is about 4 kV, and the occurrence frequency of the particle acceleration events is two times larger in the winter. An important result in this study is showing the altitude profile of the parallel acceleration region. In the dayside regions, more than 80% of the ion acceleration is observed above 6000 km in the summer, and more than 70% of the electron acceleration is observed below 8000 km in the winter. In the premidnight region, 90% of the ion acceleration is above 6000 km and more than 90% of electron acceleration is below 6000 km in summer and winter, respectively. We discuss these seasonal changes of the acceleration altitude in terms of seasonal variations of the ambient plasma density at altitude of the acceleration region, with a lower density during the winter, causing acceleration at lower altitude. The importance of the density at the altitude of acceleration is supported by the fact that Akebono does not observe any change of the relationship between the density at the bottom of the potential drop and the potential difference with changing season and altitude. Also the occurrence frequency of auroral electron acceleration is likely to be controlled by the seasonal changes of the ambient plasma density on auroral field lines.