Small-scale Gradients of Charged Particles in the Heliospheric Magnetic Field

TL;DR

This study uses numerical simulations to investigate small-scale intensity gradients of solar energetic particles in the heliospheric magnetic field, comparing turbulence models to spacecraft observations and identifying conditions for dropouts.

Contribution

It demonstrates that the foot-point random motion turbulence model can reproduce observed particle dropouts, unlike the two-component model, highlighting the importance of infrequent scattering.

Findings

Foot-point random motion model reproduces observed dropouts.

Two-component turbulence model does not produce dropouts.

Infrequent scattering is necessary for intensity dropouts.

Abstract

Using numerical simulations of charged-particles propagating in the heliospheric magnetic field, we study small-scale gradients, or "dropouts", in the intensity of solar energetic particles seen at 1 AU. We use two turbulence models, the foot-point random motion model (Jokipii & Parker 1969; Giacalone et al. 2006) and two-component model (Matthaeus et al. 1990), to generate fluctuating magnetic fields similar to spacecraft observations at 1 AU. The turbulence models include a Kolmogorov-like magnetic field power spectrum containing a broad range of spatial scales from those that lead to large-scale field-line random walk to small scales leading to resonant pitch-angle scattering of energetic particles. We release energetic protons (20 keV - 10 MeV) from a spatially compact and instantaneous source. The trajectories of energetic charged particles in turbulent magnetic fields are numerically integrated. Spacecraft observations are mimicked by collecting particles in small windows when they pass the windows at a distance of 1 AU. We show that small-scale gradients in the intensity of energetic particles and velocity dispersions observed by spacecraft can be reproduced using the foot-point random motion model. However, no dropouts is seen in simulations using the two-component magnetic turbulence model. We also show that particle scattering in the solar wind magnetic field needs to be infrequent for intensity dropouts to form.