Statistics of extreme geomagnetic field fluctuations : Directionality, timescale dependence, and spatial correlations

Rogers, Neil and Wild, Jim and Eastoe, Emma (2021) Statistics of extreme geomagnetic field fluctuations : Directionality, timescale dependence, and spatial correlations. In: 17th European Space Weather Week, 2021-10-25 - 2021-10-29, Technology Innovation Centre.

Text (Poster - Rogers et al. - Euro Space Weather Wk 2021)
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Abstract

This paper presents the key findings of various statistical analyses predicting the likelihood of extreme rates of change of the geomagnetic field, dB/dt – an important indicator of Geomagnetically Induced Currents. The studies were conducted as part of a four-year UK research programme called Space Weather Impacts on Ground-based Systems (SWIGS). Using a global dataset of measurements from 125 magnetometers, each providing between 20 and 48 years of data, we fitted Generalised Pareto distributions to the upper tail of the probability distributions to predict the magnitude (or ‘return level’) of dB/dt expected over periods of between 5 and 500 years. These return levels were fitted to a model parameterised by geomagnetic latitude, magnetic local time, and season, and also found to depend on other factors such as the interplanetary magnetic field orientation and solar wind velocity. It was notable that for certain latitudes and local times, there is a strongly preferred compass direction for large geomagnetic fluctuations, indicative of the highly directional ionospheric or magnetospheric electric currents which induce the fluctuation – examples being the eastward Chapman-Ferraro currents at low latitudes associated with Sudden Commencements, or westward auroral electrojets in the auroral zones. A technique to improve return level estimates by combining Generalised Pareto tail distributions in discrete directional sectors produced estimates that were lower at low absolute corrected geomagnetic latitudes (λ < 40°) but significantly higher at mid to high latitudes (λ > 40°). We also found that return levels of extreme dB/dt were strongly dependent on the timescale of the fluctuation (between 1 s and 60 min) and the frequency dependence was well modelled by quadratic functions whose coefficients varied with latitude (λ). Finally, we assessed the spatial coherence of daily maxima of dB/dt, observing pairwise extremal dependences, and discovered strong asymptotic dependence between sites at low latitudes (λ < 55°) and/or in the polar cap region (λ > 80°), but a tendency towards asymptotic independence for sites paired with those at auroral latitudes.