Modeling the loss of inner belt protons by magnetic field line curvature scattering

Abstract

The sudden loss of energetic protons in the inner radiation belt has been observed during geomagnetic storms. It is hypothesized that this sudden loss occurs because of changes in the geomagnetic field configuration which lead to a breakdown of the first adiabatic invariant, μ, in a process called magnetic field line curvature scattering or μ scattering. Comparison of observations to various analytic model predictions for μ scattering induced loss has, however, yielded discrepancies. To better understand how well the analytic models predict the proton loss, test particle simulations are carried out for various magnetic field configurations. Although our simulation results agree well with the analytic models for single μ-scattering events, the results after cumulative μ scattering can show significant disagreement with the theoretical predictions based on analytic models. In particular, we find the assumption that protons with predicted initial δμ/μ > 0.01 or ε > 0.1 are ultimately lost overestimates the proton loss. Based on the test particle simulation results, we develop a new empirical model, called the "ε-onset" model, to predict the minimum value of ε at which all protons of a given pitch angle and energy can be assumed to be lost due to μ scattering. By applying our ε-onset model as the variable cutoff condition between trapping and detrapping, we obtain very good agreement between theoretical predictions and the simulation results for a range of Kp, suggesting that the ε-onset model can potentially serve as an easy-to-use and more reliable predictor of inner belt proton loss due to μ scattering than the previously used fixed-valued cutoff conditions.

Particle Dynamics in the Earth's Radiation Belts: Review of Current Research and Open Questions

“…However, we do not discuss the proton radiation belt since we do not have contributions on this subject in this Special Collection. More information on the proton belt can be found in, for example, Spjeldvik (), Beutier et al (), Albert et al (), Looper et al (), Selesnick, Looper, and Mewaldt (), Ginet et al (), Selesnick, Hudson, and Kress (), Selesnick et al (, , ), Mazur et al (, ), Tu, Cowee, and Liu (), and Borovsky et al (). In addition, we do not discuss any kind of trapped particles that originate from the nuclear reaction of ultrahigh energy proton (e.g., Gusev, Kohno et al, ; Gusev, Martin et al, ; Pugacheva et al, ; Selesnick, Looper, Mewaldt, & Labrador, ), suprathermal ionospheric heavy ions (e.g., Spjeldvik, ) such as iron ions (Christon et al, ; Spjeldvik et al, ) or carbon ions (Spjeldvik, ), high‐energy solar protons (e.g., O'Brien et al, ), or cosmic rays (e.g., Amato & Blasi, ; Blake et al, ; Smart et al, ; Shea et al, ; Smart & Shea, ).…”

Comparison of Van Allen Probes radiation belt proton data with test particle simulation for the 17 March 2015 storm

“…Previous work [ Hudson et al , ; Young et al , ; Tu et al , ] has suggested that proton loss from the outer edge of the inner radiation belt is due to field line curvature scattering (FCS). For this reason we have run all of our simulations using the Lorentz force equation, so that the gyromotion of the particles is accurately modeled.…”

Simulations of inner radiation belt proton loss during geomagnetic storms

“…In this paper we focus on loss events. Several mechanisms for inner ion belt losses during storms have been theorized including the following: hydromagnetic waves [Dragt, 1961], low-frequency magnetic fluctuations [McIlwain, 1965], perturbation of the boundary of the inner zone [Gussenhoven et al, 1994], and breakdown of adiabatic motion of protons due to magnetic field disturbances [Anderson et al, 1997;Young et al, 2002;Tu et al, 2014]. Because the loss event studied in this paper is correlated to the minimum Dst of an accompanying geomagnetic storm, we attribute it to the breakdown of adiabatic motion and subsequent field line curvature scattering (FCS).…”