Small substorms: Solar wind input and magnetotail dynamics

Abstract

We investigated properties of 43 small magnetospheric substorms. Their general signatures were found to be consistent with the so‐called contracted oval or northern B z substorms. Small but clear pressure changes in the tail corresponding to growth and expansion phases detected in about a half of cases testify that these substorms follow the same loading‐unloading scheme as the larger ones. However, rate of the solar wind energy accumulation in the magnetosphere was low due to azimuthal IMF orientation with dominating IMF B y and small fluctuating IMF B z . Plasma sheet signatures could be very strong and likely were localized in their cross‐tail size. Negative bays in auroral X magnetograms were of order of 100–300 nT, with maxima at Bear Island station (71°geomagnetic latitude) and in few cases were delayed after magnetotail onsets by tens of minutes. Small substorms probably differ from their larger counterparts in a way that coherency of the magnetotail reconfiguration in the inner and middle‐tail regions and across the tail is lost in smaller substorms.