Solar Energetic Particle drifts in the Parker spiral

TL;DR

This paper investigates the significance of particle drifts in the Parker spiral magnetic field for Solar Energetic Particles, revealing that drift effects are substantial at high energies and latitudes, impacting SEP propagation models.

Contribution

It demonstrates that drift velocities in the Parker spiral are significant for high-energy SEPs, especially with scattering, challenging previous assumptions of their negligible role.

Findings

Drift velocities increase with heliolatitude and radial distance.

High-energy SEPs experience significant gradient and curvature drifts.

Heavy ions exhibit stronger drift effects than protons.

Abstract

Drifts in the Parker spiral interplanetary magnetic field are known to be an important component in the propagation of galactic cosmic rays, while they are thought to be negligible for Solar Energetic Particles (SEPs). As a result they have so far been ignored in SEP propagation modelling and data analysis. We examine drift velocities in the Parker spiral within single particle first-order adiabatic theory, in a local coordinate system with an axis parallel to the magnetic field. We show that, in the presence of scattering in interplanetary space, protons at the high end of the SEP energy range experience significant gradient and curvature drift. In the scatter-free case, drift due to magnetic field curvature is present. The magnitude of drift velocity increases by more than an order of magnitude at high heliographic latitudes compared to near the ecliptic; it has a strong dependence on radial distance r from the Sun, reaching a maximum at r~1 AU at low heliolatitudes and r~10 AU at high heliolatitudes. Due to the mass over charge dependence of drift velocities, the effect of drift for partially ionised SEP heavy ions is stronger than for protons. Drift is therefore likely to be a considerable source of cross field transport for high energy SEPs.