Characteristics of low-latitude Pc1 pulsations during geomagnetic storms

We use search- coil magnetometer data from a low- latitude station in Parkfield, California ( L = 1.77) to study the occurrence of Pc1 pulsations associated with geomagnetic storms. The Pc1 pulsations and storms are identified using automatic algorithms, and the statistical distributions are examined using a superposed epoch analysis technique, as a function of local time, time relative to storm main phase, and storm intensity. Results show that Pc1 pulsations are 2 - 3 times more likely ( than normal) to be observed in the 2 - 4 d following moderate storms and 4 - 5 times more likely in the 2 - 7 d following intense storms. The Pc1 frequencies are higher in moderate storms than they are in nonstorm times and become even higher and occupy a greater range of local times as the strength of the storms increase. These results are consistent with the idea that the source of EMIC waves extends to lower L values as storm intensity increases.