Drift-induced deceleration of Solar Energetic Particles

TL;DR

This study uses relativistic simulations to show that Solar Energetic Particles experience significant deceleration during their journey through space, mainly due to particle drift effects alongside adiabatic deceleration.

Contribution

The paper demonstrates the importance of drift-induced deceleration in SEP energy loss, highlighting effects previously underrepresented in transport models.

Findings

Protons lose 35-90% of energy over four days depending on initial energy.

Drift-induced deceleration has a stronger impact on SEPs than on galactic cosmic rays.

Current models only approximate drift effects, requiring further validation.

Abstract

We investigate the deceleration of Solar Energetic Particles (SEPs) during their propagation from the Sun through interplanetary space, in the presence of weak to strong scattering in a Parker spiral configuration, using relativistic full orbit test particle simulations. The calculations retain all three spatial variables describing particles' trajectories, allowing to model any transport across the magnetic field. Large energy change is shown to occur for protons, due to the combined effect of standard adiabatic deceleration and a significant contribution from particle drift in the direction opposite to that of the solar wind electric field. The latter drift-induced deceleration is found to have a stronger effect for SEP energies than for galactic cosmic rays. The kinetic energy of protons injected at 1 MeV is found to be reduced by between 35 and 90% after four days, and for protons injected at 100 MeV by between 20 and 55%. The overall degree of deceleration is a weak function of the scattering mean free path, showing that, although adiabatic deceleration plays a role, a large contribution is due to particle drift. Current SEP transport models are found to account for drift-induced deceleration in an approximate way and their accuracy will need to be assessed in future work.