Test Particle Simulations of μ-Scattering in the Inner Proton Belt

Observations have shown that energetic protons in the inner belt (L < 4) can suddenly become de-trapped during geomagnetic storms. It is hypothesized that this sudden loss occurs because of changes in geomagnetic field which lead to a breakdown of the first adiabatic invariant, in a process called μ-scattering (or magnetic field line curvature scattering). To investigate this problem we carry out sensitivity tests to determine how μ-scattering of energetic inner belt protons depends on the proton L, energy, pitch angle, and gyrophase angle and on the quiet versus storm-time geomagnetic field (e.g. T89c magnetic field model). We find that characterizing the change in μ during a bounce period is more complicated inside L = 4 than at higher L or in the tail where it has been traditionally assumed that the changes in μ only occur as the particles cross the magnetic equator (e.g. travel through the region of minimum radius of curvature of the field). At low L where the field is more dipolar, a proton may be continuously undergoing small changes in μ along the full length of the field line. Additionally, our results confirm previous studies that the commonly used parameter, ɛ (ratio of the gyroradius of the particle to the radius of curvature of the magnetic field at the equator), cannot, by itself, predict if an energetic proton will be trapped or not.