Abstract
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.
Highlights
As reviewed by Baker et al (1996), substorm disturbances in the near-Earth magnetotail commonly include energetic particle injections, the formation of a current wedge, auroral breakups, high latitude magnetic bays and Pi2 pulsations
We examine if the magnetic variations affected by the substorm current wedge (SCW) formation occurred at geostationary orbit at the onset time of ground Pi2 bursts
INTERMAGNET magnetograms at auroral latitudes showed magnetic variations affected by two recurrent electrojets for each pair
Summary
As reviewed by Baker et al (1996), substorm disturbances in the near-Earth magnetotail commonly include energetic particle injections, the formation of a current wedge, auroral breakups, high latitude magnetic bays and Pi2 pulsations. The interplay between the two neutral points in the magnetotail creates two onsets, one when reconnection at the near-Earth neutral point begins on closed field lines within the plasma sheet, and the second when the nearEarth neutral point reaches the open flux of the tail lobes, magnetically linking to the outer neutral point and releasing the plasmoid (Russell, 1974) In this model, the timing of the second substorm onset is controlled by the distant neutral point that in turn is controlled by the northward turning of the IMF. The CLUSTER 4 probe was located in the south lobe of the nightside magnetosphere These observations together with complementary upstream and ground measurements provided us an opportunity to study the link between consecutive Pi2 bursts, the impulsive onsets in the near-Earth magnetotail and the IMF variations during double-onset substorms
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