Global features of energetic proton pancake pitch angle distributions in the Earth's radiation belt

The temporal and spatial evolution of the energetic protons in the Earth's radiation belt is highly correlated with substorm activities. Understanding the variations of the pitch angle distributions (PADs) would be an important and convenient approach to better distinguish the dominant mechanisms. Based on Van Allen Probe observations from 2012 to 2019, the present study surveyed the energetic protons by fitting them as a function of sin (n)partial derivative (where partial derivative is the pitch angle and n is the fitted power-low index). The global distributions show that "typical" (n > 2) pancake PADs occur more frequently on the dayside (>95%) than on the nightside (<20%), showing a distinct day-night asymmetry. This asymmetry becomes much more pronounced for protons at higher energies, at higher L-shells, and during geomagnetic active times. Further investigations demonstrated that the commencement and disappearance of typical pancake PADs are mainly modulated by the plasmapause location. Moreover, "pronounced" (n > 5) and "extreme" (n > 10) pancake PADs can also be found at dayside at L > 5, with occurrence rates of up to 35% and 5%, respectively, during active times. The radial diffusion and magnetopause shadowing effects act as essential processes for increasing and decreasing the occurrence of pancake PADs. In addition, alternative mechanisms, such as wave-particle interactions led by plasma waves, are also important candidates. The present study provides a comprehensive analysis of the dependence of proton PADs on energy and geomagnetic storms, which would be useful for advanced radiation belt modeling.