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On the triggering of auroral substorms by northward turnings of the interplanetary magnetic field
J. A. Wild1, E. E. Woodfield1, and S. K. Morley2,*
1Department of Communication Systems, Lancaster University, Lancaster, LA1 4WA, UK
2School of Mathematical and Physical Sciences, University of Newcastle, Callaghan, NSW 2308, Australia
*now at: Space Science and Applications, Los Alamos National Laboratory, Los Alamos, NM 87545, USA
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Abstract. Some studies over the last decade have indicated that the instability
responsible for substorm expansion phase onset may require an external
trigger such as a northward turning of the interplanetary magnetic field
(IMF). Statistical investigations have lead to contrasting interpretations
regarding the relationship between proposed solar wind triggers and substorm
onsets identified from geomagnetic data. We therefore present the results of
a study into the possible triggering of 260 substorms between 2001–2005,
exploiting data from the Cluster and IMAGE satellite missions. We find that
only a small fraction (<25%) of the substorms studied are associated with
northward turnings of the IMF. However, the majority of the observed onsets
are associated with a growth phase characterised using a subset of the
criteria employed to define northward-turning IMF triggers. Based upon a
case-by-case investigation and the results of an analysis using the
statistics of point processes, we conclude that northward-turning structures
in the IMF, while sometimes coinciding with the initial phase of individual
substorms, are not required to trigger the magnetospheric instability
associated with substorm expansion phase onset.
Full Article (PDF, 1340 KB) Special Issue
Citation: Wild, J. A., Woodfield, E. E., and Morley, S. K.: On the triggering of auroral substorms by northward turnings of the interplanetary magnetic field, Ann. Geophys., 27, 3559-3570, doi:10.5194/angeo-27-3559-2009, 2009. Bibtex EndNote Reference Manager XML
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