North-South Asymmetry in the Geographic Location of Auroral Substorms correlated with Ionospheric Effects.
North-South Asymmetry in the Geographic Location of Auroral Substorms correlated with Ionospheric Effects.
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DOI：
10.1038/s41598-018-35091-2
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发表时间：
2018-11-22
影响因子：
4.6
通讯作者：
Mitchell EJ
中科院分区：
文献类型：
--
作者：
Liou K;Sotirelis T;Mitchell EJ
源自磁层的高能粒子不断撞击地球两极地区的高层大气，产生被称为极光的光辐射。最壮观的极光显示与称为磁层亚暴（又名极光亚暴）的周期性事件有关。亚暴起源于夜侧磁层闭合磁力线上。因此，通常认为极光亚暴应该发生在同一场线上的两个半球（即，磁共轭）。然而，这样的假设还没有得到统计学的验证。在这里，通过分析2659极光亚暴获得的紫外线成像仪在船上的美国宇航局卫星“极地”，我们发现了令人惊讶的证据表明，平均位置的亚暴发作是不共轭，但显示出地理偏好，不能很容易地解释目前的亚暴理论。在北方半球（NH），极光亚暴最常发生在加拿大的丘吉尔（~90°W）和西伯利亚的卡坦加（~100°E），是冰岛（~22°W）的三倍。在南半球（SH），亚暴发生在南极海洋（~120°E）的位置更频繁，比南极大陆多4倍。南北爆发的纵向分布如此之大的差异与北半球和南半球的亚暴应该是磁共轭的普遍看法相矛盾。进一步的分析表明，这些亚暴事件发生得更频繁时，更多的电离层是黑暗的。这些地理区域也与地球磁场最大的区域相吻合。这些事实表明，当电离层传导率较低时，极光亚暴发生得更频繁，也许更强烈。由于大部分磁尾能量来自太阳风，通过行星际磁场和地球磁场的合并，人们通常认为亚暴的发生受到太阳风和磁尾等离子体不稳定性的外部控制。本研究结果为电离层在亚暴发生中起着更为积极的作用提供了有力的论据。
Energetic particles of magnetospheric origin constantly strike the Earth’s upper atmosphere in the polar regions, producing optical emissions known as the aurora. The most spectacular auroral displays are associated with recurrent events called magnetospheric substorms (aka auroral substorms). Substorms are initiated in the nightside magnetosphere on closed magnetic field lines. As a consequence, it is generally thought that auroral substorms should occur in both hemispheres on the same field line (i.e., magnetically conjugated). However, such a hypothesis has not been verified statistically. Here, by analyzing 2659 auroral substorms acquired by the Ultraviolet Imager on board the NASA satellite “Polar”, we have discovered surprising evidence that the averaged location for substorm onsets is not conjugate but shows a geographic preference that cannot be easily explained by current substorm theories. In the Northern Hemisphere (NH) the auroral substorms occur most frequently in Churchill, Canada (~90°W) and Khatanga, Siberia (~100°E), up to three times as often as in Iceland (~22°W). In the Southern Hemisphere (SH), substorms occur more frequently over a location in the Antarctic ocean (~120°E), up to ~4 times more than over the Antarctic Continent. Such a large difference in the longitudinal distribution of north and south onset defies the common belief that substorms in the NH and SH should be magnetically conjugated. A further analysis indicates that these substorm events occurred more frequently when more of the ionosphere was dark. These geographic areas also coincide with regions where the Earth’s magnetic field is largest. These facts suggest that auroral substorms occur more frequently, and perhaps more intensely, when the ionospheric conductivity is lower. With much of the magnetotail energy coming from the solar wind through merging of the interplanetary and Earth’s magnetic field, it is generally thought that the occurrence of substorms is externally controlled by the solar wind and plasma instability in the magnetotail. The present study results provide a strong argument that the ionosphere plays a more active role in the occurrence of substorms.
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