Abstract
It has been already reported by the present authors that ion cyclotron waves in the frequency range from 0.1Hz to 1.0Hz are generated at the time of storm sudden commencements (ssc's) and the following initial phase of the magnetic storms. In the present paper, the ion cyclotron waves are also shown to be associated with sudden impulses (si's) which are not followed by storm-type disturbances. Our conclusion is that the ion cyclotron waves are excited by gyroresonant instabilities which are caused by a compression of the magnetosphere at the times of both ssc and si.
