Geomagnetic jerks based on the midnight mean of the geomagnetic field from geomagnetic networks of China
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摘要: 为探索地磁长期变化中地磁急变事件的识别方法，分析地磁急变的特征，本文基于多个地磁台站子夜均值数据，利用线性拟合方法计算了地磁场X，Y和Z三个分量的年变率，对近年来发生的地磁急变事件进行了识别和分析。结果显示：Y分量能对分析时段内已报道的地磁急变事件进行很好的识别，其中1999年的地磁急变事件，在我国区域内发生的时间可能为1998年，此外2017年可能存在一个新的地磁急变事件；Z分量年变率整体变化平缓，2001年和2013年前后发生两次显著的地磁年变率变化，并且分别早于2003年和2014年两次显著的地磁急变事件时间，这与下地幔的高电导率层对不同分量地磁信号从核幔边界传播至地表过程中的延迟作用有关；X分量年变率出现多次地磁急变事件特征，其变化与Dst指数年变率变化具有相关性，可利用其去除X分量年变率中存在的外部空间电流体系影响成分，更可靠地辅助Y分量对地磁急变事件进行识别。总体上，地磁子夜均值数据年变率的空间分布与基于第12代国际地磁参考场（IGRF12）模型计算的地磁数据年变率的空间分布所呈现的变化特征在总趋势上具有一致性，表明地磁台站子夜均值数据能够反映我国区域地球主磁场的变化特征，而分别由子夜均值数据和IGRF12模型计算的2003年Y分量年变率空间分布均存在的显著局部特征，可能与地磁急变事件的区域特征有关。Abstract: In order to explore the identification method of geomagnetic jerks and analyze their characteristics, we identified and analyzed the geomagnetic jerks in recent years based on the secular variation of geomagnetic field X, Y and Z components, which were calculated by linear fitting method based on the midnight mean of geomagnetic field from geomagnetic networks of China. The results show that the occurrence time of the reported geomagnetic jerks can be accurately identified in the analysis period by Y component, the time of the jerk occurred in 1999 may be 1998 in China, and a new jerk may occur in 2017. The secular variation of Z component is gentle, and two significant changes in secular variation took place near the year 2001 and 2013, which are early than the two big geomagnetic jerks in 2003 and 2014, respectively. This is related to the high conductivity layer of the lower mantle, which can delay the propagation of geomagnetic signals from the core-mantle boundary to the surface. The secular variation of X component presents features of many geomagnetic jerks and has a certain correlation with the secular variation of Dst index, which can be used to remove the influence component of the external space current system from the X component. Then the X component can more reliably assist the Y component in recognizing the geomagnetic jerks. However, the spatial distribution of the secular variation of the geomagnetic midnight mean show consistent variation charac-teristics with that of the geomagnetic data calculated based on the IGRF12 model on the general trend, indicating that the midnight mean of geomagnetic stations can reflect the variation characteristics of the main magnetic field in China. Moreover, the spatial distribution of secular variation of Y component in 2003 showed a significant local characteristics, which may be related to the regional characteristics of the geomagnetic jerks.
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Keywords:
- geomagnetic jerk /
- secular variation /
- midnight mean /
- Dst index
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图 1 中国地磁台网地磁台站分布图
Figure 1. Distribution of the geomagnetic observatories from geomagnetic networks of China
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图 2 年尺度（实线）与半年尺度（虚线）Y 分量年变率的对比
Figure 2. Comparison of the Y component secular variation of annual scale （solid line） with that of semi-annual scale （dotted line）
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图 3 所有台站Y （a），Z （b）和X （c）三个分量的年变率曲线
Figure 3. The secular variation of Y （a），Z （b） and X （c） components of all geomagnetic observatories
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图 4 2003年和2014年地磁子夜均值X，Y和Z三分量的年变率空间分布
（a）2003年X分量；（b）2003年Y分量；（c）2003年Z分量；（d）2014年X分量；（e）2014年Y分量；（f）2014年Z分量
Figure 4. Spatial distribution of the secular variation of X，Y and Z components of the geomagnetic midnight mean in 2003 and 2014
（a） X component in 2003；（b） Y component in 2003；（c） Z component in 2003；（d） X component in 2014；（e） Y component in 2014；（f） Z component in 2014
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图 5 基于IGRF12模型数据计算得到的2003年和2014年地磁三分量年变率的空间分布
（a）2003年X分量；（b）2003年Y分量；（c）2003年Z分量；（d）2014年X分量；（e）2014年Y分量；（f）2014年Z分量
Figure 5. Spatial distribution of the secular variation of X，Y and Z components of the geomagnetic midnight mean calculated by the IGRF12 model in 2003 and 2014
（a） X component in 2003；（b） Y component in 2003；（c） Z component in 2003；（d） X component in 2014；（e） Y component in 2014；（f） Z component in 2014
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图 6 1995—2018年16:00—18:00 Dst指数的均值（黑线）及其年均值（红线）的时序变化
Figure 6. The variation of average Dst index （black curves） from 16:00 to 18:00 and its annual mean （red curves） during 1995−2018
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图 7 基于IGRF12模型计算的7个台站位置的X分量年变率及其均值曲线
Figure 7. The secular variation of X component at seven station positions calculated based on IGRF12 model and their means
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图 8 X分量年变率与参考背景场的对比（a）和去除背景场后的X分量年变率与Dst指数年变率的对比（b）
Figure 8. Comparison of X component secular variation with reference background field （a） and comparison of X component secular variation excluding background field with Dst secular variation （b）
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