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Annales Geophysicae An interactive open-access journal of the European Geosciences Union
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Volume 29, issue 3
Ann. Geophys., 29, 591–611, 2011
https://doi.org/10.5194/angeo-29-591-2011
© Author(s) 2011. This work is distributed under
the Creative Commons Attribution 3.0 License.
Ann. Geophys., 29, 591–611, 2011
https://doi.org/10.5194/angeo-29-591-2011
© Author(s) 2011. This work is distributed under
the Creative Commons Attribution 3.0 License.

  28 Mar 2011

28 Mar 2011

THEMIS observations of double-onset substorms and their association with IMF variations

C.-C. Cheng1, C. T. Russell2, V. Angelopoulos2, I. R. Mann3, K.-H. Glassmeier4,5, and W. Baumjohann6 C.-C. Cheng et al.
  • 1Department of Electronic Engineering, National Formosa University, Hu-Wei 63201, Taiwan
  • 2Department of Earth and Space Sciences and Institute of Geophysics and Planetary Physics, University of California, Los Angeles, CA 90095, USA
  • 3Department of Physics, University of Alberta, Edmonton, Alberta T6G 2J1, Canada
  • 4Technical University of Braunschweig, Germany
  • 5Max-Planck Institute for Solar System Research, Katlenburg-Lindau, Germany
  • 6Space Research Institute, Austrian Academy of Sciences, Graz 8042, Austria

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.

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