ANGEO Annales Geophysicae ANGEO Ann. Geophys. 1432-0576 Copernicus Publications Göttingen, Germany 10.5194/angeo-27-1113-2009 Azimuthal expansion of the dipolarization at geosynchronous orbits associated with substorms Watson C. J. 1 Jayachandran P. T. 1 Physics Dept., University of New Brunswick, P.O. Box 4400, 8 Bailey Drive, Fredericton, New Brunswick, E3B 5A3, Canada 05 03 2009 27 3 1113 1118 Copyright: © 2009 C. J. Watson 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/27/1113/2009/angeo-27-1113-2009.html The full text article is available as a PDF file from https://angeo.copernicus.org/articles/27/1113/2009/angeo-27-1113-2009.pdf

Dipolarization, one of the main indicators of substorm expansion onset, represents topological changes in the magnetotail. It is believed that dipolarization is initiated at a longitudinally confined region in the tail, followed by the azimuthal expansion. There are very limited studies in the literature regarding the azimuthal propagation of the dipolarization front in the magnetotail. In this study we have used ten years of GOES data and POLAR and IMAGE data to study the characteristics of the propagation of the dipolarization fronts at the geosynchronous orbit. We have identified a number of dipolarization events in the GOES magnetic field data and substorm onsets from POLAR UVI and IMAGE-FUV measurements. Using the delay of dipolarization signatures at the two GOES satellites and onset times from POLAR and IMAGE measurements we have estimated the propagation speed of the dipolarization fronts. The calculated speeds vary between 10 km/s and 420 km/s and show a power law distribution.

 References Bak, P., Tang, C., and Wiesenfeld, K.: Self-organized criticality: An explanation of 1/f noise, Phys. Rev. Lett., 59, 381–384, 1987. Belehaki, A., Tsagouri, I., and Mavromichalaki, H.: Study of the longitudinal expansion velocity of the substorm current wedge, Ann. Geophys., 16, 1423–1433, 1998. Cheng, C. Z.: Physics of substorm growth phase, onset and dipolarization, Space Sci. Rev., 113, 207–270, 2004. Frank, L. A., Paterson, W. R., Sigwarth, J. B., and Kokubun, S.: Observations of magnetic field dipolarization during auroral substorm onset, J. Geophys. Res., 105, 15897–15912, 2000. Frey, H. U., Mende, S. B., Angelopoulus, V., and Donovan, E. F.: Substorm onset observations by IMAGE-FUV, J. Geophys. Res., 109, 10304.1–10304.6, 2004. Jayachandran, P. T., MacDougall, J. W., Hamza, A. M., and Henderson, M. G.: Observations of dipolarization at geosynchronous orbits and its response in the polar cap convection during extreme southward interplanetary magnetic field conditions, J. Geophys. Res., 112, 10216.1–10216.5, 2007. Kokubun, S. and McPherron, R. L.: Substorm signatures at synchronous altitude, J. Geophys. Res., 86, 11265–11277, 1981. Liou, K., Meng, C. I., Lui, A. T. Y., Newell, P. T., and Wing, S.: Magnetic dipolarization with substorm expansion onset, J. Geophys. Res., 107, 1131, https://doi.org/10.1029/2001JA000179, 2002. Liou, K., Newell, P. T., Sibeck, D. G., Meng, C. I., Brittnacher, M., and Parks, G.: Observation of IMF and seasonal effects in the location of auroral substorm onset, J. Geophys. Res., 106, 5799–5810, 2001. Lopez, R. E. and Lui, A. T. Y.: A multisatellite case study of the expansion of a substorm current wedge in the near-earth magnetotail, J. Geophys. Res., 95, 8009–8017, 1990. Lui, A. T. Y., Chapman, S. C., Liou, K., Newell, P. T., Meng, C. I., Brittnacher, M., and Parks, G.K.: Is the dynamic magnetosphere an avalanching system?, Geophys. Res. Lett., 27, 911–914, 2000. Lui, A. T. Y.: A synthesis of magnetospheric substorm models, J. Geophys. Res., 96, 1849–1856, 1991. McPherron, R. L., Russell, C. T., and Aubry, M. P.: Satellite studies of magnetospheric substorms on August 15, 1968, Phenomenological model for substorms, J. Geophys. Res., 78, 3131–3149, 1973. Nagai, T.: An empirical model of substorm-related magnetic field variations at synchronous orbit, Geophys. Monogr. Ser., 64, 91–95, 1991. Ohtani, S.: Substorm trigger processes in the magnetotail: recent observations and outstanding issues, Space Sci. Rev., 95, 347–359, 2001.