A statistical study of proton pitch angle distributions measured by the Radiation Belt Storm Probes Ion Composition Experiment

A statistical study of ring current-energy proton pitch angle distributions (PADs) in Earth's inner magnetosphere is reported here. The data are from the Radiation Belt Storm Probes Ion Composition Experiment (RBSPICE) on board the Van Allen Probe B spacecraft from 1 January 2013 to 15 April 2015. By fitting the data to the functional form sin(n)alpha, where alpha is the proton pitch angle, we examine proton PADs at the energies 50, 100, 180, 328, and 488 keV in the L shell range from L = 2.5 to L = 6. Three PAD types are classified: trapped (90 degrees peaked), butterfly, and isotropic. The proton PAD dependence on the particle energy, magnetic local time (MLT), L shell, and geomagnetic activity are analyzed in detail. The results show a strong dependence of the proton PADs on MLT. On the nightside, the n values outside the plasmapause are clearly lower than those inside the plasmapause. At higher energies and during intense magnetic activity, nightside butterfly PADs can be observed at L shells down to the vicinity of the plasmapause. The averaged n values on the dayside are larger than on the nightside. A maximum of the averaged n values occurs around L = 4.5 in the postnoon sector (12-16 MLT). The averaged n values show a dawn-dusk asymmetry with lower values on the dawnside at high L shells, which is consistent with previous studies of butterfly PADs. The MLT dependence of the proton PADs becomes more distinct with increasing particle energy. These features suggest that drift shell splitting coupled with a radial flux gradient play an important role in the formation of PADs, particularly at L > similar to 4.5.