Extreme Geomagnetic Storms – 1868–2010

We present the first large statistical study of extreme geomagnetic storms basedon historical data from the time period 1868 – 2010. This article is the first of two companionpapers. Here we describe how the storms were selected and focus on their near-Earth characteristics.The second article presents our investigation of the corresponding solar eventsand their characteristics. The storms were selected based on their intensity in the aa index,which constitutes the longest existing continuous series of geomagnetic activity. They areanalyzed statistically in the context of more well-known geomagnetic indices, such as theKp and Dcx/Dst index. This reveals that neither Kp nor Dcx/Dst provide a comprehensivegeomagnetic measure of the extreme storms. We rank the storms by including long seriesof single magnetic observatory data. The top storms on the rank list are the New York Railroadstorm occurring in May 1921 and the Quebec storm from March 1989. We identifykey characteristics of the storms by combining several different available data sources, listsof storm sudden commencements (SSCs) signifying occurrence of interplanetary shocks,solar wind in-situ measurements, neutron monitor data, and associated identifications ofForbush decreases as well as satellite measurements of energetic proton fluxes in the near-Earth space environment. From this we find, among other results, that the extreme storms arevery strongly correlated with the occurrence of interplanetary shocks (91 – 100 %), Forbushdecreases (100 %), and energetic solar proton events (70 %). A quantitative comparison ofthese associations relative to less intense storms is also presented. Most notably, we findthat most often the extreme storms are characterized by a complexity that is associated with multiple, often interacting, solar wind disturbances and that they frequently occur when thegeomagnetic activity is already elevated. We also investigate the semiannual variation instorm occurrence and confirm previous findings that geomagnetic storms tend to occur lessfrequently near solstices and that this tendency increases with storm intensity. However, wefind that the semiannual variation depends on both the solar wind source and the storm level.Storms associated with weak SSC do not show any semiannual variation, in contrast to weakstorms without SSC.