The Characteristic Pitch Angle Distributions of 1 eV to 600 keV Protons Near the Equator Based On Van Allen Probes Observations

Understanding the source and loss processes of various plasma populations is greatly aided by having accurate knowledge of their pitch angle distributions (PADs). Here we statistically analyze ~1 eV to 600 keV hydrogen (H⁺) PADs near the geomagnetic equator in the inner magnetosphere based on Van Allen Probes measurements, to comprehensively investigate how the H⁺ PADs vary with different energies, magnetic local times (MLTs), L shells, and geomagnetic conditions. Our survey clearly indicates four distinct populations with different PADs: (1) a pancake distribution of the plasmaspheric H⁺ at low L shells except for dawn sector; (2) a bidirectional field-aligned distribution of the warm plasma cloak; (3) pancake or isotropic distributions of ring current H⁺; (4) radiation belt particles show pancake, butterfly, and isotropic distributions depending on their energy, MLT, and L shell. Meanwhile, the pancake distribution of ring current H⁺ moves to lower energies as L shell increases, which is primarily caused by adiabatic transport. Furthermore, energetic H⁺ (>10 keV) PADs become more isotropic following the substorm injections, indicating wave-particle interactions. The radiation belt H⁺ butterfly distributions are identified in a narrow energy range of 100 < E < 400 keV at large L (L > 5), which are less significant during quiet times and extend from dusk to dawn sector through midnight during substorms. The different PADs near the equator provide clues of the underlying physical processes that produce the dynamics of these different populations.