Time Evolution of Elemental Ratios in Solar Energetic Particle events

TL;DR

This study analyzes the time evolution of heavy ion ratios in Solar Energetic Particle events, revealing a monotonic relationship between decay rates and ionic mass-to-charge ratios, with notable anomalies at specific ratios like He/H.

Contribution

The paper introduces a comprehensive analysis of ionic ratio decay behaviors in SEP events, highlighting the ordered dependence on the mass-to-charge ratio and identifying anomalies at specific ratios.

Findings

Ratios with S>1 decrease over time, S<1 increase over time.

Decay rate B correlates monotonically with S.

Anomalous behavior observed at S=2.0 (He/H ratio).

Abstract

Heavy ion ratio abundances in Solar Energetic Particle (SEP) events, e.g.~Fe/O, often exhibit decreases over time. Using particle instruments on the ACE, SOHO and STEREO spacecraft, we analysed heavy ion data from 4 SEP events taking place between December 2006 and December 2014. We constructed 36 different ionic pairs and studied their time evolution in each event. We quantified the temporal behaviour of abundant SEP ratios by fitting the data to derive a decay time constant $B$. We also considered the ratio of ionic mass--to--charge for each pair, the $S$ value given e.g.~for Fe/O by $S_{\rm Fe/O} = (M/Q)_{\rm Fe}\big/(M/Q)_{\rm O}$. We found that the temporal behaviour of SEP ratios is ordered by the value of $S$: ratios with $S>1$ showed decreases over time (i.e.~$B<0$) and those with $S<1$ showed increases ($B>0$). We plotted $B$ as a function of $S$ and observed a clear monotonic dependence: ratios with a large $S$ decayed at a higher rate. A prominent discontinuity at $S=2.0$ (corresponding to He/H) was found in 3 of the 4 events, suggesting anomalous behaviour of protons. The X/H ratios often show an initial increase followed by a decrease, and decay at a slower rate. We discuss possible causes of the observed $B$ versus $S$ trends within current understanding of SEP propagation.