Statistical results of correlation between energetic electron butterfly distributions and plasma waves in the radiation belt slot region

Abstract:

Butterfly pitch angle distributions (PADs) of energetic (hundreds of keV) electrons frequently occur in the radiation belt slot region. Resonant interactions with magnetosonic (MS) and whistler-mode waves are two plausible physical mechanisms for the formation of these peculiar distributions. Here we perform a statistical study (over 11600 events) on Van Allen Probes data of energetic electron PADs in the slot region during a period from May 1,2013 to May 31,2016. Our results show that electron butterfly PADs are closely related to MS waves rather than to whistler-mode waves. Both electron butterfly PADs and MS waves occur more frequently at the geomagnetically active times than at the quiet times. In a statistical sense, more distinct butterfly PADs usually correspond to MS waves with larger amplitudes and vice versa. The averaged MS wave amplitude is less than 5 pT in the case of normal and flat-top PADs with a butterfly index BI=1 but reaches about 35-95 pT in the case of distinct butterfly PADs with BI>1.3. For MS waves with amplitudes 50 pT, the occurrence rate of butterfly distribution is above 80%. The current study suggests that energetic electron butterfly distributions in the slot region are primarily caused by MS waves.