Memoirs of the Kakioka Magnetic Observatory Vol.15 No.01， p.13, December, 1972
Increase of Horizontal Force caused before Commencement of Geomagnetic Storm
Dst of gradually commenced storm is studied using Sugiura's equatorial Dst during 1961−63. When the time of sharp decrease in horizontal force is taken as a reference point of storm time instead of the beginning vaguely determined, Dst of gradually commenced storm is clear. A gradual increase of horizontal force is found for one day or more before the reference point which is clearly the starting time of main phase. Many examples of SC-storm show also the same gradual increase before ssc. The gradual increase in the initial period may be a substantial character of storm phenomena, except severe isolated storms, without regard to occurrence of ssc.