Seasonal-longitudinal variation of substorm occurrence frequency: Evidence for ionospheric control
Article first published online: 7 APR 2007
Copyright 2007 by the American Geophysical Union.
Geophysical Research Letters
Volume 34, Issue 7, April 2007
How to Cite
2007), Seasonal-longitudinal variation of substorm occurrence frequency: Evidence for ionospheric control, Geophys. Res. Lett., 34, L07104, doi:10.1029/2007GL029423., and (
- Issue published online: 7 APR 2007
- Article first published online: 7 APR 2007
- Manuscript Accepted: 9 MAR 2007
- Manuscript Revised: 1 MAR 2007
- Manuscript Received: 22 JAN 2007
- aurorla ionosphere;
- merging electric field
 Based on 2760 well-defined substorm onsets in the northern hemisphere and 1432 in the southern hemisphere observed by the FUV (Far Ultraviolet) Imager on board IMAGE (Imager for Magnetosphere-to-Aurora Global Exploration) spacecraft, a statistical study is performed for both hemispheres. The main emphasis is put on a possible dependence of the substorm occurrence frequency on season and longitude (S/L). It was found that around December solstice UT noon-time and around June solstice UT nighttimes are more favorable for substorms to occur. The occurrence frequency varies by a factor of 2. The sum of ionospheric Pedersen conductances of both hemispheres caused by solar illumination in the nightside auroral regions can account for the S/L dependence. Lower total conductivity seems to reduce the trigger level. We find that the IMF threshold for initiating a substorm is on average lower during favorable times, thus, substorms can occur more frequently.