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- Title
Extreme Event Statistics in Dst, SYM‐H, and SMR Geomagnetic Indices.
- Authors
Bergin, A.; Chapman, S. C.; Watkins, N. W.; Moloney, N. R.; Gjerloev, J. W.
- Abstract
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: SYM‐H (derived from 6 stations) and SMR (derived from up to 120 stations). EVT analysis reveals a divergence between the return level found for Dst, and those for SYM‐H and SMR, that increases non‐linearly with return period. For return periods below 10 years, hourly averaged SYM‐H and SMR 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 SYM‐H and SMR 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 indicate that consideration should be given to the differences between the indices if selecting one to use as a bench mark in model validation or resilience planning for the wide range of space weather sensitive systems that underpin our society. Plain Language Summary: Extreme space weather events, which disturb Earth's near‐Earth plasma and magnetic environment, have the potential to cause significant disruption to our infrastructure. Extreme events are rare, and quantifying their likelihood relies upon long‐term continuous observations. High‐quality ground‐based magnetometer observations underpin geomagnetic indices that monitor space weather. The Dst index has become the de‐facto measure for space weather storms. Space weather storms cause magnetic perturbation that can be localized in space and time. We now have the SYM‐H and SuperMAG SMR indices, constructed to be 1‐min versions of Dst, with SMR using a larger set of magnetometers. We perform the first extreme value theory analysis of SYM‐H and SMR. We find that space weather storms are more intense when measured by the 1‐min SYM‐H or SMR indices but the relationship between extreme values identified in the Dst timeseries and those in SYM‐H or SMR is not linear. Our results indicate that consideration should be given to the differences between the indices if selecting one to use as a benchmark in model validation or resilience planning for the wide range of space weather sensitive systems that underpin our society. Key Points: These indices are not totally interchangeable, consideration should be given to index choice in model validation or cross‐study comparisonHourly averaged SMR and SYM‐H return levels track Dst for return periods below 10 years. Above that they exceed Dst; at 100 years by >10%One minute cadence SMR and SYM‐H 5, 10, 50, and 100 year return levels exceed that of Dst by about 10%, 12%, 20%, and 25% respectively
- Subjects
EXTREME value theory; SPACE environment; EXTREME weather; MAGNETIC storms; SOLAR cycle; GEOMAGNETISM
- Publication
Space Weather: The International Journal of Research & Applications, 2023, Vol 21, Issue 3, p1
- ISSN
1539-4956
- Publication type
Article
- DOI
10.1029/2022SW003304