Liou Kan, Sotirelis Thomas, Mitchell Elizabeth J
The Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland, 20723, USA.
Sci Rep. 2018 Nov 22;8(1):17230. doi: 10.1038/s41598-018-35091-2.
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.
源自磁层的高能粒子不断撞击极地地区的地球高层大气，产生被称为极光的光发射。最壮观的极光现象与被称为磁层亚暴（又称极光亚暴）的周期性事件有关。亚暴在夜侧磁层的闭合磁力线上启动。因此，人们普遍认为极光亚暴应该在两个半球的同一条磁力线上（即磁共轭）发生。然而，这一假设尚未得到统计验证。在这里，通过分析美国国家航空航天局（NASA）卫星“极地号”上的紫外线成像仪获取的2659次极光亚暴，我们发现了惊人的证据，即亚暴起始的平均位置并非共轭，而是表现出一种地理偏好，这很难用当前的亚暴理论来解释。在北半球，极光亚暴最常发生在加拿大的丘吉尔（约西经90°）和西伯利亚的哈坦加（约东经100°），其发生频率高达冰岛（约西经22°）的三倍。在南半球，亚暴在南极海洋的一个位置（约东经120°）发生得更为频繁，比在南极大陆上的发生频率高出约四倍。南北起始的纵向分布存在如此大的差异，这与人们普遍认为的北半球和南半球的亚暴应该是磁共轭的观点相悖。进一步分析表明，当更多的电离层处于黑暗状态时，这些亚暴事件发生得更为频繁。这些地理区域也与地球磁场最强的区域重合。这些事实表明，当电离层电导率较低时，极光亚暴发生得更为频繁，也许更为强烈。由于磁尾的大部分能量是通过行星际磁场和地球磁场的合并从太阳风获取的，人们普遍认为亚暴的发生是由太阳风和磁尾中的等离子体不稳定性外部控制 的。本研究结果有力地证明了电离层在亚暴发生过程中发挥着更积极的作用。
