Loss Cone Offset Method for Evaluating the Effect of Magnetic Field Line Curvature Scattering (FLCS)

Abstract

Magnetic Field Line Curvature Scattering (FLCS) is one of the important loss mechanism for energetic particles, referring to the scattering phenomenon where charged particles experience changes in their pitch angles due to the curvature and non-uniformity of magnetic field. Previous methods evaluating FLCS were suitable for less stretched configurations like dipole magnetic fields, but under Ts05 model, they led to non-physical results, especially in regions where the adiabatic parameter $\epsilon$ exceeds 0.584. To address this, we developed a new method for evaluating FLCS, named the Loss Cone Offset method (LCOM). The method first anchors the offset of loss cone center due to Borovsky et al. (2022), https://doi.org/10.1029/2021ja030106 and works by constructing the pitch angle offset after one FLCS as a function of initial pitch angle and gyro-phase angle, and then correcting the function by parameters fitting using test-particle-tracing results. Our calculations can effectively evaluate particle scattering due to FLCS in the range of 0°–90° pitch angles and adiabatic parameter $\epsilon$ ranging from 0.1 to 0.96. Loss Cone Offset method has good compatibility with previous methods under dipole magnetic field or TS05 magnetic field with low adiabatic parameters. It can effectively avoid non-physical results under stretched magnetic field and high adiabatic parameters, and evaluate the FLCS influence. Comparison with theoretical calculations, empirical formulas, and test-particle results demonstrates that the LCOM serves as an easy-to-use and reliable model for predicting particle loss due to FLCS in the magnetospheric dynamics. Its application deepens understanding of FLCS mechanisms, providing robust methodological support for developing physical models.