A statistical study on the whistler waves behind dipolarization fronts

Abstract

We present a statistical study of whistler waves behind dipolarization fronts (DFs) based on the Cluster satellites measurements during the years 2001-2007. We find 732 DFs during the 7 year tail periods (X_GSM ≤ -8 R_E and |Y_GSM| ≤ 10 R_E) in the plasma sheet. By constraining the whistler waves in a 1 min interval behind the DFs (the maximum B_z), we find that 381 DFs (about 50%) are followed by whistler waves. We study the occurrence rate of whistler waves, the wave characteristic parameters, and the corresponding electron distribution, not only in a global view but also in the local DF coordinate. In a global view, behind the DFs, the whistler waves mostly occur in the radial distance between 17 and 18 R_E. They have a higher occurrence rate on the dawnside than the duskside. On the other hand, in the local DF coordinate, whistler waves have a higher occurrence rate around the meridian of DF. In addition, the average wave amplitudes increase toward the dawnside of DF. Associated with the whistler waves, electron distributions have a dominant perpendicular anisotropy for electrons with energy higher than 5 keV. Lower energy electron distributions do not have such perpendicular anisotropy dominance. Moreover, the perpendicular anisotropy for electrons >5 keV increases toward the dawnside of DF, which may be caused by the drift-betatron acceleration. We suggest that the free energy source for whistler waves behind the DFs is probably the perpendicular anisotropy of >5 keV electrons caused by the betatron acceleration.