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Relation between Interplanetary Sector Structure and Geomagnetic Activity near the Minimum of Sunspot Cycle
Effects of interplanetary sector structure on geomagnetic storm have been investigated for the minimum of sunspot cycle.
The average structure of geomagnetic activity with respect to a sector boundary has been investigated by use of the superposed epoch technique.
In the present study, ΣK at Kakioka is increased significantly for a day after the Passage of the sector boundary and decreased very rapidly from the second day after reaching peak value for the period of 1964 to 1967. On the contrary, during the period of 1972 July to 1975 June, ΣK is increased for 3 days after the passage of the sector boundary and then decrease rapidly after reaching the maximum in away sector. On the other hand, in toward sector, activity of same degree continues until 7 days after the passage of a sector boundary.
It is noticeable fact that there is a great difference between 4 and 2 sectors in the beginning and ending of solar cycle, respectively.
The 27-day recurrence of geomagnetic storm is clearly seen not only gradually commencing storm but also sudden commencement for the period of sunspot minimum.
Especially，sudden commencement has occurred near the sector boundary and it has been observed to reach a maximum occurrence frequency after the passage of a sector boundary. This suggests that the sector boundary is accompanied by corotating shocks and magnetohydrodynamical turbulence. Therefore, a sudden commencement is not necessarily associated with a flare-induced shock wave, but may also be caused by a corotating sector boundary.
All these results lead to the conclusion that most of the geomagnetic disturbances can be explained by the interaction between corotating distortions in the solar wind connected with the sector structure and the magnetosphere.
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