The relationship between Pc5 ULF waves and the asymmetric ring current during the magnetic storm on July 15, 2000 is studied using ground-based magnetometer data and particle observations by the LANL satellite. A frequency decreases of Pc5 is noted during the main phase, which can be attributed to injections of heavier ions (O+ and He+) into the ring current. In addition, it is observed that during the main phase of the storm, the frequency drop of the Pc5 in the IMAGE chain around the MLT midnight is larger than that in the ALASKA chain at MLT post noon. Meanwhile, the longitudinal variation of Δ H with the maximum around midnight implies a highly asymmetric distribution of the ring current. These results suggested that the frequency variation of Pc5 pulsation during storms is related to the partial distribution in the ring current.
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