Abstract
We have analyzed short-period geomagnetic pulsations measured by the north-south chain of pulsation magnetometers in Finland in association with the storm sudden commencement (SSC) recorded on September 22, 1999 at 12:22 UT. We observe that structured Pc 1 pulsations with amplitude modulation (modulation period is about 1000 seconds) were excited before the SSC whereas intense wave elements with rising frequency appear soon after SSC. It is concluded that the first effect is due to the interaction of the Earth's magnetosphere with the interplanetary foreshock region. The second one is likely the result of modification of Pc 1 waves due to interaction between the magnetosphere and the shock front structure leading to compression of the magnetosphere during the initial phase of the magnetic storm.
