Field-aligned currents observed by CHAMP during the intense 2003 geomagnetic storm events

Abstract. This study concentrates on the characteristics of field-aligned currents (FACs) in both hemispheres during the extreme storms in October and November 2003. High-resolution CHAMP magnetic data reflect the dynamics of FACs during these geomagnetic storms, which are different from normal periods. The peak intensity and most equatorward location of FACs in response to the storm phases are examined separately for both hemispheres, as well as for the dayside and nightside. The corresponding large-scale FAC peak densities are, on average, enhanced by about a factor of 5 compared to the quiet-time FACs' strengths. And the FAC densities on the dayside are, on average, 2.5 times larger in the Southern (summer) than in the Northern (winter) Hemisphere, while the observed intensities on the nightside are comparable between the two hemispheres. Solar wind dynamic pressure is correlated with the FACs strength on the dayside. However, the latitudinal variations of the FACs are compared with the variations in D_st and the interplanetary magnetic field component B_z, in order to determine how these parameters control the large-scale FACs' configuration in the polar region. We have determined that (1) the equatorward shift of FACs on the dayside is directly controlled by the southward IMF B_z and there is a saturation of the latitudinal displacement for large value of negative B_z. In the winter hemisphere this saturation occurs at higher latitudes than in the summer hemisphere. (2) The equatorward expansion of the nightside FACs is delayed with respect to the solar wind input. The poleward recovery of FACs on the nightside is slower than on the dayside. The latitudinal variations on the nightside are better described by the variations of the D_st index. (3) The latitudinal width of the FAC region on the nightside spreads over a wide range of about 25° in latitude.