Periodic variation in the geomagnetic activity : a study based on the Ap index
Periodic variation in the geomagnetic activity : a study based on the Ap index
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DOI：
10.1029/92ja02200
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发表时间：
1993-06
影响因子：
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通讯作者：
A. Gonzalez;W. Gonzalez;S. Dutra;B. Tsurutani
中科院分区：
文献类型：
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作者：
A. Gonzalez;W. Gonzalez;S. Dutra;B. Tsurutani
用功率谱技术研究了1932-1982年51年AP地磁指数的月样本和日样本。总体而言，这些结果证实了先前关于地磁活动可能存在周期性的发现。然而，在我们看来，以下方面要么是新的，要么是与其他作者的解释略有不同。每月AP功率谱中约4年的周期与地磁活动变化中观察到的双峰结构有关[Gonzalez等人，1990]。日AP谱显示的几个峰值被解释为6个月周期的谐波和其他由太阳自转周期引起的峰值，这样两个傅立叶序列被认为是并列的。在这些序列中观察到了很强的太阳周期调制，特别是与太阳自转周期有关的周期调制，这种周期调制在太阳活动高峰期几乎消失。对季节变化的研究得到了叠加历元分析的补充。这一分析得到的轮廓似乎显示了6个月周期的多重来源，因此为这种众所周知的季节性变化寻找唯一原因似乎不现实。这一结论也得到了在短持续时间间隔(即8天)上超过给定强度级别的风暴发生的直方图的支持。根据这些直方图，对于大数据样本，风暴数量最多的日期分布在不同理论模型预测的日期周围，而对于较短的间隔，有时甚至可能不存在半年周期。因此，这些已知的机制将结合在一起，从而对随机产生的风暴源的地磁响应进行调制。研究还发现，对于最强烈的风暴范围(AP≥150nT)，7月份似乎存在一个额外的季节峰值，其幅度与赤道分峰值相当。在第21个太阳周期中，还检测到了158天左右的弱周期，与太阳活动中观察到的155天左右的周期有很好的相关性。
The monthly and daily samples of the Ap geomagnetic index for 51 years, 1932-1982, were investigated by means of the power spectrum technique. In general, the results confirm previous findings about possible periodicities in the geomagnetic activity. However, in our opinion the following aspects are either new or they are being interpreted somewhat differently than other authors have done. The period around 4 years in the monthly Ap power spectrum is associated to the double peak structure observed in the geomagnetic activity variation [Gonzalez et al., 1990]. Several of the peaks shown by the daily Ap spectrum are interpreted as harmonics of the 6-month period and other peaks as caused by the solar rotation periodicity, in such a way that the two series of Fourier sequences are consider to be juxtaposed. A strong solar cycle modulation is observed in these series, particularly in that related to the solar rotation period, which almost disappears for the solar maximum phase. The study of the seasonal variation was complemented by a superposed epoch analysis. The profiles resulting from this analysis seem to show a multiple origin of the 6-month periodicity, so that it does not seem realistic to search for a unique cause for this well-known seasonal variation. This conclusion is also supported by the histograms of the occurrence of storms above a given intensity level, taken over short duration intervals (i.e., 8 days). According to these histograms, for large data samples the dates with largest number of storms are spread out around those predicted by the different theoretical models, while for short intervals the semiannual periodicity may sometimes not even be present. Therefore these known mechanisms would combine to give a resulting modulation of the geomagnetic response to the randomly generated source of storms. It was also found that an additional seasonal peak seems to exist in July, with an amplitude comparable to those of the equinoctial peaks, for the range of the most intense storms (Ap ≥ 150 nT). A weak periodicity around 158 days, well correlated to that of about 155 days observed in the solar activity, has also been detected for some years during solar cycle 21.