A longitudinal distribution of
ssc rise time at middle and low latitudes has been obtained by using rapid-run magnetograms at Koror, Honolulu, Tucson, and Fredericksburg which cover 150° extent in longitude. The result shows that the
ssc rise time on the daylight side of the earth is shorter than that on the night side. This may be interpreted by transit-time differences of hydromagnetic waves, that are generated by the impact of the solar plasma flow on the geomagnetic field, from the geomagnetic cavity boundary down to the earth in various propagation paths. It is estimated from the
ssc rise times in the nighttime that an effective size of the geomagnetic cavity in the antisolar direction at the time of
ssc may be of the order of 17-26 earth radii.
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