The distribution and direction of extreme geomagnetic fluctuations over 1-60 minute periods

We present a statistical climatological study of extreme fluctuations in the horizontal geomagnetic field (dB/dt), generated at the Earth's surface by extreme electrical currents in the ionosphere and magnetosphere. Applying techniques from Extreme Value Theory we predict the magnitude of |dB/dt| for return periods (RP) from 5 to 500 years. Results presented for both ramp changes and RMS variations over periods from 1-60 minutes show considerable variation in magnitude and rate of occurrence when considered as functions of magnetic latitude, local time, and season. The direction of extreme geomagnetic fluctuations is strongly isotropic at lower latitudes, but this changes with timescale. A technique for improving return level estimates by combining Generalised Pareto tail distributions from discrete directional sectors produces return levels that are higher at low geomagnetic latitudes but higher at mid to high latitudes.