The Effect of Field Line Curvature Scattering on Ring Current Dynamics: A Comparative Study of Loss Cone Offset Method Versus Young's Method

Abstract

Magnetic field line curvature scattering (FLCS) is one of the key mechanisms responsible for the loss of energetic particles in the magnetosphere. Young's method, an empirical parameterization derived from extensive test particle simulations (Young et al., 2002, https://doi.org/10.1029/2000JA000294 ; 2008, https://doi.org/10.1029/2006ja012133 ), has been the most widely used approach for quantifying FLCS. However, this method is constrained by the adiabatic parameter condition (, the ratio of particle gyroradius to magnetic field curvature radius), which often leads to nonphysical results in stretched magnetic configurations. In contrast, the recently proposed Loss Cone Offset Method (LCOM) (Wei et al., 2025, https://doi.org/10.1029/2024ja033422 ), which evaluates the degree of pitch angle scattering by anchoring the offset loss cone center overcomes this limitation and remains valid for , extending its applicability to highly stretched geometries. In this study, we employed the Storm Time Ring Current Model (STRIM) to simulate the geomagnetic storm of 17 March 2013, and evaluated the FLCS effects on the ring current dynamics by comparing the two methods. The simulations reveal that FLCS contributes to the ion loss by approximately and under the Young's method and LCOM, respectively, corresponding to Dst index reductions of 11 and 4 nT. The results further indicate that the precipitating energy in association with FLCS is primarily contributed by oxygen ions. Furthermore, the LCOM calculation reveals a decrease in proton precipitation within a magnetic dip region, that is absent in the simulation using Young's method. Moreover, FLCS not only modifies the pitch angle distribution of ring current ions on the nightside, but also alters it in nonlocal regions, through drift motion.