From Sun to interplanetary space: What is the pathlength of Solar Energetic Particles?

TL;DR

This paper investigates the actual pathlength of Solar Energetic Particles (SEPs) traveling through turbulent solar wind, introducing a new model that accounts for magnetic fieldline meandering and provides more accurate estimates of their travel distances.

Contribution

The authors develop a fieldline random walk model that incorporates magnetic island scales, improving SEP pathlength estimation by addressing limitations of previous diffusion models.

Findings

SEP pathlengths can be shorter than the Parker spiral length for certain solar longitudes.

Fieldline meandering can cause SEP pathlengths to exceed 2 au for western and behind-the-limb events.

The new model corrects unrealistic short pathlengths from previous diffusion-based approaches.

Abstract

Solar energetic particles (SEPs), accelerated during solar eruptions, propagate in turbulent solar wind before being observed with in situ instruments. In order to interpret their origin through comparison with remote-sensing observations of the solar eruption, we thus must deconvolve the transport effects due to the turbulent magnetic fields from the SEP observations. Recent research suggests that the SEP propagation is guided by the turbulent meandering of the magnetic fieldlines across the mean magnetic field. However, the lengthening of the distance the SEPs travel, due to the fieldline meandering, has so far not been included in SEP event analysis. This omission can cause significant errors in estimation of the release times of SEPs at the Sun. We investigate the distance travelled by the SEPs by considering them to propagate along fieldlines that meander around closed magnetic islands that are inherent in turbulent plasma. We introduce a fieldline randow walk model which takes into account the physical scales associated to the magnetic islands. Our method remedies the problem of the diffusion equation resulting in unrealistically short pathlengths, and the fractal dependence of the pathlength of random walk on the length of the random-walk step. We find that the pathlength from the Sun to 1 au can be below the nominal Parker spiral length for SEP events taking place at solar longitudes 45E to 60W, whereas the western and behind-the-limb particles can experience pathlengths longer than 2 au due to fieldline meandering.