Comparing extreme event statistics in Dst, SYM-H and SMR geomagnetic indices.

Extreme space weather events are rare, and quantifying their likelihood is challenging, often relying on geomagnetic indices obtained from ground-based magnetometer observations that span multiple solar cycles. The Dst index ring-current monitor, derived from an hourly average over four low-latitude stations, is a benchmark for extreme space weather events, and has been extensively studied statistically. We apply extreme value theory (EVT) to two geomagnetic ring current indices: SMR, (derived from up to 120 stations) and SYM-H (derived from 6 stations). EVT analysis reveals a divergence between the return level found for Dst, and those for SMR and SYM-H, that increases non-linearly with return period. For return periods below 10 years, hourly averaged SMR and SYM-H have return levels similar to Dst, but at return periods of 50 and 100 years, they respectively exceed that of Dst by about 10% and 15% (SYM-H) and about 7% and 12% (SMR). One minute resolution SMR and SYM-H return levels progressively exceed that of Dst; their 5, 10, 50 and 100 year return levels exceed that of Dst by about 10%, 12%, 20% and 25% respectively. Our results suggest that for more extreme events, these geomagnetic indices are not directly interchangeable, instead they contain different, complimentary information and should be used together in any detailed analysis. Such an analysis may improve the correlation between return levels and the impact of severe geomagnetic storms.