Semi-annual, annual and Universal Time variations in the magnetosphere and in geomagnetic activity: 1. Geomagnetic data
Semi-annual, annual and Universal Time variations in the magnetosphere and in geomagnetic activity: 1. Geomagnetic data
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DOI：
10.1051/swsc/2020023
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发表时间：
2020-06-10
影响因子：
3.3
通讯作者：
Coxon, John C.
中科院分区：
文献类型：
--
作者：
Lockwood, Mike;Owens, Mathew J.;Coxon, John C.
我们研究地磁活动的半年变化，如在地磁指数am、aa(H)、AL、Dst 和针对 6 小时 MLT 扇区（中午、黎明、黄昏和午夜左右）导出的四个 a sigma 指数中检测到的变化。对于每一个，我们将半年变化的幅度（作为总体平均值的一部分）与根据行星际观测估计的磁层功率输入的相应变化 P-alpha 进行比较。我们证明，与 P-alpha 相比，地磁数据的半年度变化被放大，每个指数的因子都不同。最大的放大是 Dstindex（因子类似于 10），最小的是中午 MLT 扇区的 sigma 指数（sigma-noon，因子大约为 1.1）。通过按地心太阳赤道 (GSEQ) 参考系中行星际磁场 (IMF) 的 Y 分量（黎明-黄昏）的主导极性对数据进行排序，我们证明了 Russell-McPherron (R-M) 效应（其中 GSEQ 中的一个小的向南 IMF 分量通过地球偶极子倾斜转换为地球有效场）是 P(α) 和地磁指数半年变化的关键因素。然而，GSEQ 坐标系中 IMF 南向分量的变化导致磁层 P(α) 功率输入的变化比 R-M 效应更大，达两倍以上。我们表明，对于越来越大的地磁扰动，GSEQ 中向南的大磁场事件（已知通常与日冕物质抛射有关）所产生的 P(α) 成为主要驱动因素，而 R-M 效应的重要性下降，并且通常会降低 GSEQ 中最南向 IMF 的地磁效应：因此，大型风暴的半年变化表明，要么是平均条件对磁层进行了预处理，要么是在分点时的附加效应。我们确认 Dst 指数中非常大的 R-M 效应是因为在小型和中等活动级别而不是在大型风暴中产生了很大的效应。我们讨论了观测到的“春分”年时（F）-世界时（UT）地磁响应模式的影响、半年变化的波形和相位、六月和十二月至日响应之间的差异以及三月和九月春分峰值的振幅比率。我们还证实，地磁活动的 UT 变化是真正的全球响应。后面的论文将分析所描述的影响的起源和含义。
We study the semi-annual variation in geomagnetic activity, as detected in the geomagnetic indicesam, aa(H), AL, Dst and the four a sigma indices derived for 6-hour MLT sectors (around noon, dawn, dusk and midnight). For each we compare the amplitude of the semi-annual variation, as a fraction of the overall mean, to that of the corresponding variation in power input to the magnetosphere, P-alpha, estimated from interplanetary observations. We demonstrate that the semi-annual variation is amplified in the geomagnetic data compared to that in P-alpha, by a factor that is different for each index. The largest amplification is for theDstindex (factor similar to 10) and the smallest is for the a sigma index for the noon MLT sector (a sigma-noon, factor approximate to 1.1). By sorting the data by the prevailing polarity of theY-component (dawn-dusk) of the Interplanetary Magnetic Field (IMF) in the Geocentric Solar Equatorial (GSEQ) reference frame, we demonstrate that the Russell-McPherron (R-M) effect, in which a small southward IMF component in GSEQ is converted into geoeffective field by Earth's dipole tilt, is a key factor for the semi-annual variations in both P(alpha)and geomagnetic indices. However, the variability in the southward component in the IMF in the GSEQ frame causes more variability in power input to the magnetosphereP(alpha)than does the R-M effect, by a factor of more than two. We show that for increasingly large geomagnetic disturbances, P(alpha)delivered by events of large southward field in GSEQ (known to often be associated with coronal mass ejections) becomes the dominant driver and the R-M effect declines in importance and often acts to reduce geoeffectiveness for the most southward IMF in GSEQ: the semi-annual variation in large storms therefore suggests either preconditioning of the magnetosphere by average conditions or an additional effect at the equinoxes. We confirm that the very large R-M effect in theDstindex is because of a large effect at small and moderate activity levels and not in large storms. We discuss the implications of the observed "equinoctial" time-of-year (F) - Universal Time (UT) pattern of geomagnetic response, the waveform and phase of the semi-annual variations, the differences between the responses at the June and December solstices and the ratio of the amplitudes of the March and September equinox peaks. We also confirm that the UT variation in geomagnetic activity is a genuine global response. Later papers will analyse the origins and implications of the effects described.