Statistical survey of pitch angle anisotropy of relativistic electrons in the outer radiation belt and its variation with solar wind/geomagnetic activity

In this study, we use 7 years (2012 -2019) of pitch angle resolved electron flux measurements from Van Allen Probe-B spacecraft to study the variation of near-equatorial pitch angle distributions (PADs) of outer radiation belt (L ≥ 3) relativistic electrons (E ≥ 0.5 MeV) with different levels of geomagnetic activity. We calculate a pitch angle anisotropy index (PAI) to categorize the PADs into three types: pancake, PAI ≥ 1.05; butterfly, PAI ≤ 0.95; and flattop, 0.95 < PAI < 1.05. Our statistical results show that L shells ≥ 5 are dominated by pancake PADs on the dayside (9 < MLT < 15), butterfly PADs on the nightside (21 < MLT < 3), and flattop PADs in the dawn (3 < MLT < 9) and dusk (15 < MLT < 21) sectors, across almost all relativistic energies. In the inner L shells, the pancake and flattop PADs exhibit dependence on both L-shell and energy, with the occurrence rate increasing with decreasing L and increasing energy. For the butterfly PADs, we discovered a second population of low-L butterflies that are present at almost all local times. When the variation of electron PAI is compared with solar wind dynamic pressure P dyn and geomagnetic indices SYM-H and AL, P dyn is found to be the dominant parameter in driving the outer radiation belt pitch angle anisotropy. During periods of enhanced P dyn , pancake PADs on the dayside become more 90 • -peaked, butterfly PADs on the nightside exhibit enhanced flux dips around 90 • pitch angle along with an enhanced azimuthal and radial extent, and flattop PADs turn into either pancake or butterfly PADs.