Statistical analysis of substorms with different time durations

Geomagnetic substorms release plasma and energy from the magnetotail to the inner magnetosphere and high latitude ionosphere. In this study, we investigate substorms of different time duration, observed during the years 1982 to 2012. We measure substorm duration by a method based on the SME index. Our results demonstrate that longer-duration substorms occur more frequently during solar maximum years, and more frequently between the months of May and July, likely due to the higher polar ionospheric conductivity and more sunward geomagnetic pole directions during summer in the northern hemisphere. Additionally, longer substorms occur more frequently under stronger, longer-lasting southward IMF, and are accompanied by slower solar winds, suggesting that extended magnetic reconnection is required to drive longer substorms. Substorm durations are not significantly related to the minimum SML index, but longer substorms usually have higher SMU and continuously rising PC indices. The SME indices of long-duration substorms show a bimodal MLT distribution located near midnight and dusk after substorm onset, with the peak originally at midnight moving eastward to the morning side during the late recovery phase. Longer duration substorms have a stronger effect on the overall ring current at all MLT sectors except dawnside. Our results provide new insights into the development process and influencing factors of substorms, from the expansion phase to the recovery phase.