2009 AGU Fall Meeting, San Francisco, CA, USA, 14-18 December 2009.

On the definition of geomagnetic activity at high latitudes: Case study at the Sodankylä station

D. Martini1, K. Mursula2, Th. Ulich3, J. Kultima3

1School of Space Research, Kyung Hee University, Yongin, Geonggi-Do, Korea,
2Dept of Physical Sciences, University of Oulu, Oulu, Finland,
3Sodankylä Geophysical Observatory, Sodankylä, Finland


Here we demonstrate that the traditional separation of regular (Sq) and irregular geomagnetic variations is not a straightforward process at high latitudes. We use various methods in estimating the local geomagnetic activity at the sub-auroral Sodankylä station, and compare their long-term properties. We show that at high-latitude the magnetic quiet daily curve cannot be classified purely as regular variation but, rather, an imprint of such current systems in the ionosphere and magnetosphere that are causing the irregular variations. That is, in this region the Sq variation actually is a part of geomagnetic activity itself. We also show that excluding certain part of the daily activity when defining a geomagnetic index, as in case of the IHV index, leads to an estimate that inherently differs in its response to solar wind drivers from the other three hourly K-based measures of geomagnetic activity.