ANGEO Annales Geophysicae ANGEO Ann. Geophys. 1432-0576 Copernicus Publications Göttingen, Germany 10.5194/angeo-27-3559-2009 On the triggering of auroral substorms by northward turnings of the interplanetary magnetic field Wild J. A. 1 Woodfield E. E. 1 Morley S. K. 2 3 Department of Communication Systems, Lancaster University, Lancaster, LA1 4WA, UK School 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 25 09 2009 27 9 3559 3570 Copyright: © 2009 J. A. Wild et al. 2009 This work is licensed under the Creative Commons Attribution 3.0 Unported License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/3.0/ This article is available from https://angeo.copernicus.org/articles/27/3559/2009/angeo-27-3559-2009.html The full text article is available as a PDF file from https://angeo.copernicus.org/articles/27/3559/2009/angeo-27-3559-2009.pdf

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.

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