Mountains versus valleys: Semiannual variation of geomagnetic activity
Mountains versus valleys: Semiannual variation of geomagnetic activity
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DOI：
10.1029/1999ja900439
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发表时间：
2000-02-01
影响因子：
2.8
通讯作者：
Ling, AG
中科院分区：
文献类型：
--
作者：
Cliver, EW;Kamide, Y;Ling, AG
地磁活动的半年变化通常归因于拉塞尔-麦克弗伦效应。在该图中，春分点附近南向磁场 B-s 的增强解释了 3 月和 9 月观测到的较高地磁活动。从相反的角度来看，我们认为大部分半年变化是由昼夜平分效应（基于太阳风流方向与地球偶极轴之间的 psi 角度）造成的，这种效应使得 B-s 耦合在至日时的效果较差（平均大约 25%）。因此，半年变化不仅仅归因于春分时的“造山”（B-s的产生），而且主要是夏至时的“挖谷”（耦合效率的损失）造成的。我们估计，后一种效应在 am 指数的日变化中清晰地显现出来，它对半年调制的影响类似，占 65%。分点假说的特征印记在时间微分 Dst 指数和 AE 指数的每小时/每月平均值中也很明显。
The semiannual variation in geomagnetic activity is generally attributed to the Russell-McPherron effect. In that picture, enhancements of southward field B-s near the equinoxes account for the observed higher geomagnetic activity in March and September. In a contrary point of view, we argue that the bulk of the semiannual variation results from an equinoctial effect (based on the psi angle between the solar wind flow direction and Earth's dipole axis) that makes B-s coupling less effective (by similar to 25% on average) at the solstices. Thus the semiannual variation is not simply due to "mountain building" (creation of B-s) at the equinoxes but results primarily from "valley digging" (loss of coupling efficiency) at the solstices. We estimate that this latter effect, which clearly reveals itself in the diurnal variation of the am index, is responsible for similar to 65% of the semiannual modulation. The characteristic imprint of the equinoctial hypothesis is also apparent in hourly/monthly averages of the time-differentiated Dst index and the AE index.