Quantifying the effect of magnetic field line curvature scattering on the Rapid Loss of ring current ions

During geomagnetic storms the ring current ions sometimes exhibit rapid loss, as indicated by the fast recovery of Dst index on timescale of a few hours. Even though various physical mechanisms have been proposed to explain the rapid loss of ring current ions their relative importance is still a subject of active debate. Recent studies have shown that magnetic field line curvature (FLC) scattering can play a major role in the rapid loss of ring current ions. It occurs when the gyroradius of the ion is large compared to the radius of curvature of the magnetic field line, leading to non-adiabatic motion of ions. Quantifying the effect of FLC scattering on the loss of ring current ions is critical to self-consistently modeling the ring current dynamics. Here we studied the effects of FLC scattering on ring current decay by performing direct test particle simulations of ring current ions in realistic magnetic field models (e.g., the Tsyganenko T89 model). We find that during intense geomagnetic storms (e.g., Kp=6), FLC scattering can lead to significant loss of ring current ions at in the nightside of the magnetosphere for L ⪆ 4. The loss effects also depend on ion pitch angle, energy, L shell and species, with faster loss for higher energy, lower pitch angle, heavier ions or at higher L shell. The e-folding time ( 𝛕 ) for the FLC scattering loss of ring current ions to the atmosphere has also been calculated for ions with different initial energies, equatorial pitch angles, L shells and under various geomagnetic conditions. We demonstrate that 𝛕 can be on the order of 1-10 hrs for ~100 keV proton and O+ at L ⪆ 4 for Kp=6 of the T89 model. This loss timescale due to FLC scattering is on the same order as the observed Dst recovery timescale during the storm recovery phase. The parameterized ion lifetime 𝛕 due to FLC scattering is then self-consistently incorporated into the ring current-atmosphere interaction model with self-consistent magnetic field (RAM-SCB). By comparing the RAM-SCB model results with and without the FLC scattering and against real-time ring current and field observations, we will quantify the effects of FLC scattering and determine its relative contribution to the observed ring current losses.