On Aspects Pertaining to the Perpendicular Diffusion of Solar Energetic Particles

TL;DR

This paper investigates how perpendicular diffusion affects the spread and intensity of solar energetic particles, revealing the importance of pitch-angle dependence and asymmetries in their longitudinal distribution.

Contribution

It introduces a numerical model incorporating perpendicular diffusion with pitch-angle dependence to better understand SEP transport and distribution.

Findings

SEP intensities are asymmetric in longitude, favoring the west.

Maximum SEP intensity can shift away from optimal magnetic connection.

The pitch-angle dependence of diffusion significantly influences SEP propagation.

Abstract

The multitude of recent multi-point spacecraft observations of solar energetic particle (SEP) events have made it possible to study the longitudinal distribution of SEPs in great detail. SEPs, even those accelerated during impulsive events, show a much wider than expected longitudinal extent, bringing into question the processes responsible for their transport perpendicular to the local magnetic field. In this paper we examine some aspects of perpendicular transport by including perpendicular diffusion into a numerical SEP transport model that simulates the propagation of impulsively accelerated SEP electrons in the ecliptic plane. We find that: (i) The pitch-angle dependence of the perpendicular diffusion coefficient is an important, and currently mainly overlooked, transport parameter. (ii) SEP intensities are generally asymmetric in longitude, being enhanced towards the west of optimal magnetic connection to the acceleration region. (iii) The maximum SEP intensity may also be shifted (parameter dependently) away from the longitude of best magnetic connectivity at 1 AU. We also calculate the maximum intensity, the time of maximum intensity, the onset time and the maximum anisotropy as a function of longitude at Earth's orbit and compare the results, in a qualitative fashion, to recent spacecraft observations.