Characteristics of pitch angle distributions of 100 s keV electrons in the slot region and inner radiation belt

The pitch angle distribution (PAD) of energetic electrons in the slot region and inner radiation belt received little attention in the past decades due to the lack of quality measurements. Using the state-of-art pitch-angle-resolved data from the Magnetic Electron Ion Spectrometer (MagEIS) instrument onboard the Van Allen Probes, a detailed analysis of 100 s keV electron PADs below L = 4 is performed, in which the PADs is categorized into three types: normal (flux peaking at 90o), cap (exceedingly peaking narrowly around 90o) and 90o-minimum (lower flux at 90o) PADs. By examining the characteristics of the PADs of ~460 keV electrons for over a year, we find that the 90o-minimum PADs are generally present in the inner belt (L < 2), while normal PADs dominate at .L ~3.5 - 4. In the region between, 90o-minimum PADs dominate during injection times and normal PADs dominate during quiet times. Cap PADs appear mostly at the decay phase of storms in the slot region and are likely caused by the pitch angle scattering of hiss waves. Fitting the normal PADs into sinnα form, the parameter n is much higher below L = 3 than that in the outer belt and relatively constant in the inner belt but changes significantly in the slot region (2 < L < 3) during injection times. As for the 90o-minimum PADs, by performing a detailed case study, we find in the slot region this type of PAD is likely caused by chorus wave heating, butthis mechanism can hardly explain the formation of 90o-minimum PADs at the center of inner belt.