Pi 2 pulsations at low latitudes are examined with magnetic fields data at the 210• magnetic meridian (MM) stations in 1991. Due to high degree of coherence over most latitudes, 68 low-latitude Pi 2 events at the 210• MM stations are identified with reference to same waveforms on the magnetogram at Lunping (189.5• MM, L = 1.06). With spectral power analysis, the ratio of the first four harmonic frequencies of low-latitude Pi 2 pulsations is about 1 : (1.7 ± 0.1) : (2.3 ± 0.1) : (2.9 ± 0.1). By using a box model for the inner magnetosphere, the cold linearized MHD wave equations are studied with realistic Alfven speed profile for nonuniform ambient magnetic fields. With appropriate parameters to depict the magnetospheric environments during the aforementioned period, numerical results are acquired with the fourth order Runge-Kutta method. It is found that the ratio of the first four harmonic frequencies of plasmaspheric cavity resonances is about 1 : 1.7 : 2.4 : 3.1 that is consistent with data analysis. This suggests that low-latitude Pi 2 pulsations at the 210• MM stations may be plasmaspheric cavity resonances driven by fast compressional waves owing to the impulsive source at the magnetotail.

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[1] The relationships between double-onset substorm, pseudobreakup, and IMF variation were investigated with magnetic, auroral, and particle observations from space to the ground during 0200-0600 UT on 4 September 1999. There were five consecutive bursts of Pi2 pulsations on the ground during the time of interest. The onset time of ground Pi2s maps to the same variation sequence in the IMF structure seen propagating to the Earth in multiple satellite observations in the upstream region. The comparison of auroral and energetic particle data with IMF observations shows that a sequence of two doubleonset substorms intervened by a pseudobreakup appears in two distinct cycles of southward IMF followed by a northward interval. For the first substorm, the first onset begins when the B y magnitude declines after the IMF turns southward for about 90 min, and the second onset occurs after northward turning of the IMF accompanied by an increasing B y magnitude. The pseudobreakup appears while the IMF turns southward and the B y magnitude slightly decreases. For the second substorm, the first onset commences while the IMF remains southward with a steady B y magnitude, and the second onset occurs after the IMF becomes strongly northward and the B y magnitude decreases instead. These observations can be explained with the two-neutral-point model. The first onset occurs when the IMF turns southward. Reconnection at the near-Earth neutral point first begins on closed field lines within the plasma sheet, and the second onset occurs when the IMF turns northward and reconnection at the distant neutral point ceases and reconnection at the near-Earth neutral point may reach the open flux of the tail lobes. In addition, a decrease in the B y magnitude may help reduce magnetotail convection and release all the built-up flux to allow the onset to commence after northward turning of the IMF. If the IMF remains southward, the reduction of magnetotail convection due to a decreasing B y would lead to a pseudobreakup instead. In contrast, an increasing B y magnitude would increase magnetotail convection and weaken magnetospheric substorm after the IMF turns northward. Consequently, for the occurrence of double-onset substorms and pseudobreakups, not only the first onset begins spontaneously during steady southward IMF and the second onset appears after northward turning of the IMF but the B y change also affects magnetotail convection which may evoke (or abate) the substorm-related activation while the IMF turns southward (or northward).

Abstract. On 16 July 2008, two pairs of consecutive bursts of Pi2 pulsations were recorded simultaneously across the THEMIS ground-based observatory system. Wavelet transformation reveals that for each high-latitude pair, the dominant frequency of the first burst is higher than that of the second. But at low latitudes, the dominant frequency does not change. It is suggested that both pairs result from fast magnetospheric cavity waves with the second burst also containing shear Alfvén waves. INTERMAGNET magnetograms at auroral latitudes showed magnetic variations affected by two recurrent electrojets for each pair. The ground-based magnetometers and those at geostationary orbit sensed magnetic perturbations consistent with the formation of the substorm current wedge. Four consecutive enhancements of energetic electron and ion fluxes detected by the THEMIS probes in the dayside magnetosphere appeared in the later afternoon and then in the early afternoon. The horizontal magnetic variation vectors had vortex patterns similar to those induced by the upward and downward field-aligned currents during substorm times. The hodogram at mid-L stations had a polarization pattern similar to the one induced by the substorm current wedge for each Pi2 burst. The mapping of ground Pi2 onset timing to the interplanetary magnetic field (IMF) observations shows that they appear under two cycles of north-to-south and then north variation. CLUSTER 4 in the south lobe observed wave-like magnetic fluctuations, probably driven by near-Earth reconnection, similar to those on the ground. These two observations are consistent with the link of double-onset substorms to magnetotail reconnection externally triggered by IMF variations.

The relationship between a series of Pi2 pulsations and their associated substorm onsets with the IMF is examined on 5 May 1999. The magnetograms from the CANOPUS array show the occurrence of three substorm onsets, accompanied by Pi2 bursts. Analysis of the auroral electrojet location and current intensity as deduced from the Churchill line in the CANOPUS array shows that the following onsets move poleward. ACE and Wind magnetic field data show that the first onset occurs during a period of weak southward interplanetary magnetic field (IMF) and that the second onset occurs about an hour later when the IMF has become slightly northward. The third onset occurs during strongly northward IMF. Magnetic disturbances at GOES 8 and GOES 10 confirm that each onset is accompanied by disturbances in the field like those expected from the substorm current wedge. Meanwhile, from the IMP 8 observations in the magnetotail, the decreases of the total and X components of the magnetic field imply that significant removal of magnetic flux from the tail has occurred after the third onset. Thus a single cycle of dayside reconnection transport of open flux and reconnection of that open flux may contain within it multiple onsets of tail reconnection and dipolarization of the night magnetosphere, some possibly not involving the open flux in the lobes.