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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 107, NO. A6, 1086, doi:10.1029/2001JA900153, 2002

Substorm energy budget during low and high solar activity: 1997 and 1999 compared

E. Tanskanen

Finnish Meteorological Institute, Helsinki, Finland


T. I. Pulkkinen

Finnish Meteorological Institute, Helsinki, Finland


H. E. J. Koskinen

Finnish Meteorological Institute, Helsinki, Finland


J. A. Slavin

NASA Goddard Space Flight Center, Greenbelt, Maryland, USA


Abstract

Substorms during the years 1997 and 1999 are analyzed to examine substorm energy budget just after solar minimum and prior to solar maximum. The energy input from the solar wind into the magnetosphere is estimated in terms of the time integral of Akasofu's epsilon parameter computed from Wind and ACE observations. The ionospheric Joule heating dissipation is estimated using the local electrojet index IL derived from the IMAGE magnetometer array of the MIRACLE ground-based network in the Scandinavian sector. In total, 839 substorms from the midnight sector have been investigated to show that on average the Northern Hemisphere Joule heating accounts for ∼30% of solar wind energy input during 1997 and 1999 substorms. We found that during the active year 1999, there were 26% more substorm events, they were 15% more intense, and they were located at lower latitudes than during 1997. Isolated and stormtime substorms were also examined separately. Mean intensity of isolated substorms was about −350 nT, whereas it was about −670 nT for stormtime events. This study confirms our previous results (for 1997 only) that the amount of Joule dissipation depends on the energy input during the substorm expansion phase. Furthermore, the correlation is best for substorms recorded in the postmidnight sector, indicating that the energy budget and substorms size are largely controlled by processes driven directly by the solar wind.

Published 26 June 2002.

Index Terms: 2788 Magnetospheric Physics: Storms and substorms; 2437 Ionosphere: Ionospheric dynamics; 2784 Magnetospheric Physics: Solar wind/magnetosphere interactions; 2736 Magnetospheric Physics: Magnetosphere/ionosphere interactions.

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Citation: Tanskanen, E., T. I. Pulkkinen, H. E. J. Koskinen, and J. A. Slavin (2002), Substorm energy budget during low and high solar activity: 1997 and 1999 compared, J. Geophys. Res., 107(A6), 1086, doi:10.1029/2001JA900153.