Extremely long rise time of the geomagnetic sudden commencement of December 15, 1995

*Tomohiko Takeuchi[1] ,Tohru Araki [1]
Department of Earth and Planetary Science, Kyoto University[1]

The rise time of SC in low-latitude generally lies within a range from 2 to 10 min. An SC observed on December 15, 1995 was remarkable because it had extremely long rise time of about 30 min although the interplanetary shock responsible for the SC was sharp as usual. The geomagnetic response in high-latitude and the in-situ measurement of the magnetotail region by GEOTAIL exhibited that the compression was unusually gradual compared with general SCs. These observations can not be explained by mechanisms proposed for the SC rise time in the past. We first suggest that the orientation of the interplanetary shock is one of important factors determining the SC rise time.

The rise time of SC in low-latitude generally lies within a range from 2 to 10 min. An SC observed on December 15, 1995 was remarkable because it had extremely long rise time of about 30 min although the interplanetary shock responsible for the SC was sharp as usual. The amplitude of the SC was consistent with the estimated value experimentally deduced from the increase in the solar wind dynamic pressure, implying that the whole compression of the magnetosphere occurred. However, the geomagnetic response in high-latitude and the in-situ measurement of the magnetotail region by GEOTAIL exhibited that the compression was unusually gradual compared with general SCs. These observations can not be explained by mechanisms proposed for the SC rise time in the past. We first suggest that the orientation of the interplanetary shock is one of important factors determining the SC rise time.