Climatological Statistics of Extreme Geomagnetic Fluctuations with Periods from 1 s to 60 min

Abstract

Using a global database of 125 magnetometers covering several decades we present occurrence statistics for fluctuations of the horizontal geomagnetic field (𝑑𝐵 ℎ /𝑑𝑡) exceeding the 99.97 th percentile (P99.97) for both ramp changes (Rn) and the root-mean-square (Sn) of fluctuations over periods, 𝜏, from 1 to 60 min and describe their variation with geomagnetic latitude and magnetic local time (MLT). Rates of exceedance are explained by reference to the magneto-ionospheric processes dominant in different latitude and MLT sectors, including ULF waves, interplanetary shocks, auroral substorm currents, and travelling convection vortices. By fitting Generalised Pareto tail distributions above P99.97 we predict return levels (RLs) for Rn and Sn over return periods of between 5 and 500 years. P99.97 and RLs increase monotonically with frequency (1/𝜏) (with a few exceptions at auroral latitudes) and this is well modelled by quadratic functions whose coefficients vary smoothly with latitude. For UK magnetometers providing 1-s cadence measurements, the analysis is extended to cover periods from 1 to 60 seconds and empirical Magnetotelluric Transfer functions are used to predict percentiles and return levels of the geoelectric field over a wide frequency range (2 × 10 −4 to 4 × 10 −2 Hz) assuming a sinusoidal field fluctuation. These results help identify the principal causes of field fluctuations leading to extreme geomagnetically induced currents (GIC) in ground infrastructure over a range of timescales and they inform the choice of frequency dependence to use with 𝑑𝐵 ℎ /𝑑𝑡 as a GIC proxy. Key points1. Occurrence rates of extreme geomagnetic fluctuations (𝑑𝐵 ℎ /𝑑𝑡) are mapped against magnetic local time, latitude, and season.2. The log frequency (or timescale) dependence of 𝑑𝐵 ℎ /𝑑𝑡 is well modelled by quadratic functions parameterised by geomagnetic latitude.3. Electric fields calculated at 3 UK sites peak at periods of 20 min at the 99.97 th percentile but 0.5-2 min for 1/100-year events.