The Influence of Substorms on Extreme Rates of Change of the Surface Horizontal Magnetic Field in the United Kingdom
The Influence of Substorms on Extreme Rates of Change of the Surface Horizontal Magnetic Field in the United Kingdom
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DOI：
10.1029/2018sw002148
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发表时间：
2019-06
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影响因子：
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通讯作者：
M. Freeman;C. Forsyth;I. J. Rae
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作者：
M. Freeman;C. Forsyth;I. J. Rae
我们研究了英国表面水平磁场变化率 R 在亚暴扩展和恢复阶段与其他时间相比有何不同的统计特性。 R 是根据三个 INTERMAGNET 天文台（Lerwick、Eskdalemuir 和 Hartland）以及 1996 年至 2014 年（近两个太阳周期）的 1 分钟磁场数据计算得出的。亚暴扩展和恢复阶段是使用 Electrojet 方法指数中的亚暴爆发和阶段从 SuperMAG Lower 指数中确定的。 R 的概率分布被分解为亚暴扩展和恢复阶段、增强对流区间或其他时间的类别。由此，我们发现所有极端 R 值（定义为第 99.97 个百分位数以上）的 54-56% 发生在亚暴扩展或恢复阶段。通过类似地分解出现大 R 值（>99%）的磁局部时间变化，我们推断出亚暴扩展和恢复阶段 21-25% 的大 R 可归因于亚暴电流楔引起的扰动极地 (DP)1 磁扰动。这相当于整个数据集中所有大 R 的 10-14%。这些结果与发生概率的渐进趋势一起，可能表明磁层对流引起的双单元 DP2 磁扰动是危险 R > 600 nT/min 的主要来源，可能对英国国家电网造成损害。因此，需要进一步研究来理解和模拟 DP2、其中尺度湍流结构和亚暴反馈，以便更好地理解和预测 GIC 对国家电网的影响。
We investigate how statistical properties of the rate of change R of the surface horizontal magnetic field in the United Kingdom differ during substorm expansion and recovery phases compared with other times. R is calculated from 1‐min magnetic field data from three INTERMAGNET observatories—Lerwick, Eskdalemuir, and Hartland and between 1996 and 2014—nearly two solar cycles. Substorm expansion and recovery phases are identified from the SuperMAG Lower index using the Substorm Onsets and Phases from Indices of the Electrojet method. The probability distribution of R is decomposed into categories of whether during substorm expansion and recovery phases, in enhanced convection intervals, or at other times. From this, we find that 54–56% of all extreme R values (defined as above the 99.97th percentile) occur during substorm expansion or recovery phases. By similarly decomposing the magnetic local time variation of the occurrence of large R values (>99th percentile), we deduce that 21–25% of large R during substorm expansion and recovery phases are attributable to the Disturbance Polar (DP)1 magnetic perturbation caused by the substorm current wedge. This corresponds to 10–14% of all large R in the entire data set. These results, together with asymptotic trends in occurrence probabilities, may indicate the two‐cell DP2 magnetic perturbation caused by magnetospheric convection as the dominant source of hazardous R > 600 nT/min that is potentially damaging to the U.K. National Grid. Thus, further research is needed to understand and model DP2, its mesoscale turbulent structure, and substorm feedbacks in order that GIC impact on the National Grid may be better understood and predicted.