Upper atmosphere differences between northern and southern high latitudes: The role of magnetic field asymmetry
Upper atmosphere differences between northern and southern high latitudes: The role of magnetic field asymmetry
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DOI：
10.1002/jgra.50554
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发表时间：
2013-09-01
影响因子：
2.8
通讯作者：
Cnossen, Ingrid
中科院分区：
文献类型：
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作者：
Foerster, Matthias;Cnossen, Ingrid
地球主磁场的非偶极部分构成了两个半球之间的显着差异。除了北半球 (NH) 和南半球 (SH) 的磁通密度和模式不同之外，南半球 (SH) 的不变磁极与地理极点之间的偏移量也比北半球 (SH) 更大。我们利用全球数值模拟研究了这种磁场不对称性对高纬度热层和电离层的影响，并将我们的结果与最近的观测结果进行了比较。虽然对高纬度等离子体对流的影响很小，但对中性风环流的影响却很大。北半球跨极中性风和离子漂移速度普遍大于南半球，且半球差异呈现半日变化。北半球的中性风涡度同样比南半球更大，随着太阳活动的增强，这种差异可能会变得更大。相比之下，SH极区中性风的空间变异较大，半球差异表现出较强的半日变化。其相位与半球星等差的半日变化的相位相似。半球离子漂移和中性风震级的差异很可能部分是由SH近极区较大的磁通密度引起的，部分是由于SH中不变极和地理极之间较大的偏移引起的，而空间方差的差异可能只是由后者引起的。我们得出的结论是，磁场在强度和方向上的不对称性，在高纬度高层大气中的中性风和等离子体漂移中建立了显着的半球差异，这有助于解释用簇/电子漂移仪器（EDI）和挑战微型卫星有效载荷（CHAMP）观察到的半球差异。
The nondipolar portions of the Earth's main magnetic field constitute substantial differences between the two hemispheres. Beside the magnetic flux densities and patterns being different in the Northern Hemisphere (NH) and Southern Hemisphere (SH), also the offset between the invariant magnetic and the geographic poles is larger in the SH than in the NH. We investigated the effects of this magnetic field asymmetry on the high-latitude thermosphere and ionosphere using global numerical simulations and compared our results with recent observations. While the effects on the high-latitude plasma convection are small, the consequences for the neutral wind circulation are substantial. The cross-polar neutral wind and ion drift velocities are generally larger in the NH than the SH, and the hemispheric difference shows a semidiurnal variation. The neutral wind vorticity is likewise larger in the NH than in the SH, with the difference probably becoming larger for higher solar activity. In contrast, the spatial variance of the neutral wind is considerably larger in the SH polar region, with the hemispheric difference showing a strong semidiurnal variation. Its phase is similar to the phase of the semidiurnal variation of the hemispheric magnitude differences. Hemispheric differences in ion drift and neutral wind magnitude are most likely caused partly by the larger magnetic flux densities in the near-polar regions of the SH and partly by the larger offset between the invariant and geographic pole in the SH, while differences in spatial variance are probably just caused by the latter. We conclude that the asymmetry of the magnetic field, both in strength and in orientation, establishes substantial hemispheric differences in the neutral wind and plasma drift in the high-latitude upper atmosphere, which can help to explain observed hemispheric differences found with the Cluster/Electron Drift Instrument (EDI) and the Challenging Minisatellite Payload (CHAMP).