edu.ucar.opensky::articles:15203 UCAR/NCAR - Library OpenSky Support PO Box 3000 Boulder CO 80307-3000 opensky@ucar.edu http://opensky.ucar.edu/ homepage pointOfContact 2025-12-26T14:59:16.346010 ISO 19115-2 Geographic Information - Metadata - Part 2: Extensions for Imagery and Gridded Data ISO 19115-2:2009(E) https://n2t.org/ark:/85065/d75q4x4d Unusual declining phase of solar cycle 23: Weak semi-annual variations of auroral hemispheric power and geomagnetic activity 2009-11-20T00:00:00Z publication https://n2t.org/ark:/85065/d75q4x4d https://doi.org/10.1029/2009GL040825 Luan, Xiaoli author Wang, Wenbin author Burns, Alan author Solomon, Stanley C. author Zhang, Y. author Paxton, L. author UCAR/NCAR - Library publisher Seasonal variations of geomagnetic activity during the declining phase of solar cycle 23 (SC23-D, 2002-2007) have been studied using auroral hemispheric power (HP), solar wind and interplanetary magnetic field (IMF) data and the Kp index. The well known semi-annual variations of geomagnetic activity, with peaks occurring during equinoxes, were virtually absent during SC23-D. This makes SC23-D markedly different from previous solar cycles which had clear semi-annual variations. In SC23-D, both Kp and HP had unusual peaks around the December solstice (in years 2003, 2004 and 2006) and August (in years 2004 and 2005), instead of at the equinoxes. These peaks appeared to be associated with solar wind/IMF and the consequent merging electric field peaks in the same periods. Furthermore, the absolute values and relative changes of the Kp index were much smaller in SC23-D than in other solar cycles. The very weak dynamic pressure and southward IMF in SC23-D might also limit the regular modulation effects that contribute to the occurrence of peaks in equinoxes. UCAR/NCAR - Library OpenSky Support PO Box 3000 Boulder CO 80307-3000 opensky@ucar.edu http://opensky.ucar.edu/ homepage pointOfContact PDF Text theme Resource Type 2016-01-01T00:00:00Z publication An edited version of this paper was published by AGU. Copyright 2009 American Geophysical Union. None eng geoscientificInformation