Temperature of the Source Plasma for Impulsive Solar Energetic Particles

TL;DR

This study investigates the source plasma temperature of impulsive solar energetic particles, finding they originate from active-region plasma around 2-4 MK rather than hotter flare-heated plasma, challenging previous assumptions.

Contribution

It identifies the specific temperature range of source plasma for impulsive SEPs, showing they are not from flare-heated plasma but from active-region plasma near the flare.

Findings

Impulsive SEP source plasma is typically 2-4 MK.

No SEP events were found from plasma outside 2.5-3.2 MK.

The abundance enhancements follow a power-law dependence on A/Q.

Abstract

The steep power-law dependence of element abundance enhancements on the mass-to-charge ratios [A/Q] of the ions in impulsive solar energetic-particle (SEP) events causes these enhancements to reflect the temperature-dependent pattern of Q of the ions in the source plasma. We searched for SEP events from coronal plasma that is hotter or cooler than the limited region of 2.5 - 3.2 MK previously found to dominate 111 impulsive SEP events. Fifteen new events were found, four (three) originated in 2-MK (4-MK) plasma, but none from outside this temperature range. Although the impulsive SEP events are strongly associated with flares, this result indicates that these ions are not accelerated from flare-heated plasma, which can often exceed 10 MK. Evidently the ions of 2 - 20 MeV/amu that we observe in space are accelerated from active-region plasma on open magnetic-field lines near the flare, but not from the closed loops of the flare. The power-law dependence of the abundance enhancements on A/Q of the ions is expected from theoretical models of acceleration from regions of magnetic reconnection.