Temperature of the Source Plasma in Gradual Solar Energetic Particle Events

TL;DR

This study introduces a new method to determine the source plasma temperature in gradual solar energetic particle events by analyzing element abundance enhancements and their dependence on the mass-to-charge ratio, revealing typical coronal plasma temperatures and variations linked to different event types.

Contribution

It presents a novel approach to estimate source plasma temperature using element enhancement patterns, providing insights into the plasma conditions during SEP events and their relation to CME-driven shocks.

Findings

Most events have source temperatures of 0.8-1.6 MK.

Some events show higher temperatures of 2.5-3.2 MK, similar to impulsive SEP events.

Ground-level events have similar temperature distributions to other gradual events.

Abstract

Scattering, during interplanetary transport in large, "gradual" solar energetic-particle (SEP) events, can cause element abundance enhancements or suppressions that depend upon the mass-to-charge ratio A/Q of the ions as an increasing function early in events and a decreasing function of the residual scattered ions later. Since the Q values for the ions depend upon the source plasma temperature T, best fits, assuming a primarily power-law dependence of enhancements vs. A/Q, provide a fundamentally new method to determine the most probable value of T for these events in the region of 3-10 MeV/amu. Complicated variations in the grouping of element enhancements or suppressions match similar variations in A/Q at the best-fit temperature. We find that fits to the times of increasing and decreasing powers give similar values of T, most commonly in the range of 0.8-1.6 MK for 69% of the events, consistent with the acceleration of ambient coronal plasma by shock waves driven out from the Sun by coronal mass ejections (CMEs). However, 24% of the SEP events studied showed plasma of 2.5-3.2 MK, typical of that previously determined for the smaller impulsive SEP events; these particles may be reaccelerated preferentially by quasi-perpendicular shock waves that require a high injection threshold that the impulsive-event ions exceed or simply by high intensities of impulsive suprathermal ions at the shock. The source-temperature distribution of ten higher-energy ground-level events (GLEs) in the sample is similar to that of the other gradual events. Some events show evidence that a portion of the ions have been further stripped of electrons; such events are smaller and tend to cluster late in the solar cycle.