Statistical analysis of SC-associated geosynchronous magnetic field perturbations

Kokubun (1983) reported the local time variation of normalized amplitude of sudden commencement (SC) with a strong day-night asymmetry (i.e., maximum amplitude near noon and minimum amplitude near midnight) at geosynchronous orbit with 81 SC events. Further careful inspection of Kokubun's local time distribution reveals that the normalized SC amplitudes in the prenoon sector (MLT = 9-12) are larger than those in the postnoon sector (MLT = 12-15). That is, there is a morning-afternoon asymmetry in the normalized SC amplitudes. Until now, however, there are no studies on this SC-associated morning-afternoon asymmetry at geosynchronous orbit. Motivated by this previous observation, we investigate a large data set (422 SC events in total) of geosynchronous SC observations and confirm that the geosynchronous SC amplitudes normalized to SYM-H are larger in the morning sector than in the afternoon sector. This morning-asymmetry is probably caused by the enhancement of partial ring current, which is located in the premidnight sector, due to solar wind dynamic pressure increase. We also examine seasonal variations of the normalized SC amplitude and find that the SC-associated geosynchronous magnetic field perturbations are dependent on seasons of the year. This may be due to the location of the magnetopause current and cross-tail current enhanced during the SC interval with respect to geosynchronous spacecraft position.