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.