Solar and interplanetary causes of superstorms from 1957 to present

Abstract:

We present a comprehensive study of all superstorms (Dst ≤ -250 nT) occurring from 1957 to the present to determine their interplanetary causes and their associated solar disturbances. We found that 87% of the superstorms exhibit complex main phases. Almost all superstorms are caused by sheath only or a combination of sheath and magnetic cloud (MC). Only one superstorm is caused by the MC only. There were no superstorms induced by co-rotating interaction regions associated with high speed streams. For the interplanetary shocks anti-sunward of the interplanetary coronal mass ejections, the highest magnetosonic Mach number was 5.1. The shock normal angles were almost all quasi-perpendicular (~> 40 degrees). Larger shock normal angles statistically correspond to greater intensity superstorms. 90% of the superstorms occurred either near a solar maximum or during the declining phase of a solar cycle, 8% of the superstorms occurred during later ascending phase of a solar cycle, and 2% of the superstorms occurred during a solar minimum. 54% of superstorms were associated with X-class solar flares, 36% were associated with M-class solar flares, 5% with C-class flares. Most associated flares were located in the central meridian or slightly west of this as expected. There is little relationship between flare intensity and storm intensity, while a more intense flare generally leads to a shorter time delay from the flare occurrence to the sudden impulse.