Response of the Magnetosphere -ionosphere- earth system to solm Wind Dynamic Prssure Variation

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1. A geomagnetic sudden commencement (SC) occurred on March 24,1991 was analyzed in detail. It was found that (1) A large (202nT at Kakioka) positive H-component pulse of short duration (about 1 min) appeared in the initial stage of SC.Such a sharp pulse has never been observed before, (2) This pulse was caused by propagation of the effect of a sudden compression of the whole magnetosphere from dayside to nightside, (3) A sharp pulse of the dynamic pressure should exist in the solar wind in order to produce the pulse, (4) As for the propagation of the pulse near the earth, the peak of the pulse propagated with HM wave velocity in the ionosphere but its onset propagated nearly instantaneously in the space between the ionosphere and the earth, (5) A coherent Pc-5 geomagnetic pulsation was observed world-widely after about 6 hours.2. We analyzed past large SCs observed at Kakioka and found that (1) The largest SC occurred on March 24,1940 and its amplitude was 273nT,(2)Twelve SCs with amplitude larger than 100nT were observed in the period between 1924 and 1994, (3)large amplitude SCs tend to occur in the declining phase of the sunspot number.3. A scatter plot of the Dst index versus the square root of the solar wind dynamic pressure (Pd) shows a upper envelope increasing linearly. We interpret that this envelope shows Pd dependence of Dst.4. We also analyzed (1) successive negative geomagnetic impulses observed on the ground, (2) geosynchronous magnetopause crossings, and (3) SC on February 21,1994.