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Consecutive bursts of Pi2 pulsations are examined with magnetic field data obtained by the SMALL array in 1999. With reference to the H -component magnetic bays in the high-latitude magnetograms at CPMN, 10 events consisting of two consecutive Pi2 bursts simultaneously observed by the Beijing (BJI, L = 1.46) and Wuhan (WHN, L = 1.20) stations are identified as associated with substorm onsets. Owing to the same waveform seen by the CPMN and IGPP/LANL arrays, they are the global phenomena. Their occurrences are mostly in the 2100-2300 LT (local time) sector in which the dominant frequencies at WHN are higher than the mean frequency, but those at BJI are lower and close to the frequency of the surface wave at the plasmapause. Moreover, the LT dependence of azimuth and polarization of two consecutive Pi2 bursts at BJI and WHN are analyzed and consistent with the ULF waves theory by Itonaga and Yumoto (1998). GOES 8 and GOES 10 confirm the formation of the substorm current wedge after the onsets of two Pi2 bursts. Thus during substorm onsets, Pi2 pulsations at low latitudes may result from hydromagnetic waves driven by an impulsive source in the magnetotail which could commence in the longitude of 2230 LT and later propagate westward and eastward as well. Low-latitude Pi2 waves near the source site may be affected by several factors as they propagate by the stimulation of a surface wave at the plasmapause, by a localized field line oscillation inside the plasmapause, and by the magnetospheric/plasmaspheric cavity (resonance) mode.

Characteristics of consecutive bursts of Pi2 pulsations observed at the SMALL array: A new implication