Abstract
This paper presents a multi-parameter global statistical model of extreme horizontal geomagnetic field fluctuations (dBH/dt), which are a useful input to models assessing the risk of geomagnetically induced currents in ground infrastructure. Generalised Pareto (GP) distributions were fitted to 1-min measurements of |dBH/dt| from 125 magnetometers (with an average of 28 years of data per site) and return levels (RL) predicted for return periods (RP) between 5 and 500 years. Analytical functions characterise the profiles of maximum-likelihood GP model parameters and the derived RLs as a function of corrected geomagnetic latitude,λ. A sharp peak in both the GP shape parameter and the RLs is observed at |λ| = 53° in both hemispheres, indicating a sharp equatorward limit of the auroral electrojet region. RLs also increase strongly in the dayside region poleward of the polar cusp (|λ| > 75°) for RPs > 100 years. We describe how the GP model may be further refined by modelling the probability of occurrences of |dBH/dt| exceeding the 99.97th percentile as a function of month, magnetic local time, and the direction of the field fluctuation,dBH, and demonstrate that these patterns of occurrence align closely to known patterns of auroral substorm onsets, ULF Pc5 wave activity, and (storm) sudden commencement impacts. Changes in the occurrence probability profiles with the interplanetary magnetic field (IMF) orientation reveal further details of the nature of the ionospheric currents driving extreme |dBH/dt| fluctuations, such as the changing location of the polar cusp and seasonal variations explained by the Russell-McPherron effect.
Highlights
1.1 GICs and magneto-ionospheric currentsLarge fluctuating magnetic fields arising from electrical currents in the ionosphere and magnetosphere can cause geomagnetically induced currents (GICs) in any ground-based infrastructure that contains long metal conductors
The outlier at k = 70° N is for the site at Utqiagvik, Alaska (BRW), which together with nearby sites in Alaska presents anomalously high values for jdBH =dtj (Fig. 2)
The most extreme ionospheric currents are likely to be associated with substorm expansions along auroral arcs near the edge of the auroral bulge and this interpretation was recently supported by Kozyreva et al (2018) who examined a number of substorms associated with the March 2015 geomagnetic storm, observing that the maximum 1-min fluctuations occurred near the edges of the auroral electrojets, the location for which were inferred from the magnitude of the change in BN
Summary
1.1 GICs and magneto-ionospheric currentsLarge fluctuating magnetic fields arising from electrical currents in the ionosphere and magnetosphere can cause geomagnetically induced currents (GICs) in any ground-based infrastructure that contains long metal conductors. The damaging effects of GICs have been reported in relation to high voltage (HV) electricity power networks (Boteler et al, 1998; Pirjola et al, 2000; Erinmez et al, 2002; Molinski, 2002; Thomson et al, 2010; Boteler & Pirjola, 2017), trans-oceanic cables (Root, 1979; Lanzerotti et al, 1995), railway signalling systems (Wik et al, 2009; Eroshenko et al, 2010; Qian et al, 2016), railway electrification systems (Liu et al, 2016), and pipelines (Boteler, 2000; Pirjola et al, 2000; Pulkkinen et al, 2001). To simplify the calculation of E, it is often assumed that both geomagnetic and geoelectric field perturbations result from a downward propagating plane wave and that the Earth
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