Variable Ion Compositions of Solar Energetic Particle Events in the Inner Heliosphere: A Field-line Braiding Model with Compound Injections

TL;DR

This paper models how turbulent magnetic fields and spatial offsets in source regions cause significant variability in ion composition ratios of solar energetic particles observed near the Sun, explaining recent PSP data.

Contribution

It introduces a field-line braiding model with compound injections to interpret ion composition variability in SEP events, supported by numerical simulations.

Findings

Ion composition ratios vary strongly due to source offset and magnetic turbulence.

Variability depends on radial distance, testable by multi-spacecraft observations.

Model explains observed He/H and He isotope ratio fluctuations.

Abstract

We propose a model for interpreting highly variable ion composition ratios in solar energetic particles (SEP) events recently observed by Parker Solar Probe (PSP) at $0.3 - 0.45$ astronomical unit. We use numerical simulations to calculate SEP propagation in a turbulent interplanetary magnetic field with a Kolmogorov power spectrum from large scale down to the gyration scale of energetic particles. We show that when the source regions of different species are offset by a distance comparable to the size of the source regions, the observed energetic particle composition He/H can be strongly variable over more than two orders of magnitude, even if the source ratio is at the nominal value. Assuming a $^3$He/$^4$He source ratio of $10 \%$ in impulsive $^3$He-rich events and the same spatial offset of the source regions, the $^3$He/$^4$He ratio at observation sites also vary considerably. The variability of the ion composition ratios depends on the radial distance, which can be tested by observations made at different radial locations. We discuss the implication of these results on the variability of ion composition of impulsive events and on further PSP and Solar Orbiter observations close to the Sun.