The Two Sources of Solar Energetic Particles

TL;DR

This paper reviews the two distinct physical mechanisms behind solar energetic particle acceleration: impulsive events from magnetic reconnection and gradual events driven by CME shocks, highlighting their differences and recent theoretical insights.

Contribution

It provides a comprehensive comparison of impulsive and gradual SEP events, integrating recent models and observations to clarify their distinct acceleration processes and spatial characteristics.

Findings

Impulsive SEP events are linked to magnetic reconnection and produce specific isotope enhancements.

Gradual SEP events are driven by CME shock waves, sampling coronal abundances.

Reservoirs of SEPs explain slow intensity decay in gradual events.

Abstract

Evidence for two different physical mechanisms for acceleration of solar energetic particles (SEPs) arose 50 years ago with radio observations of type III bursts, produced by outward streaming electrons, and type II bursts from coronal and interplanetary shock waves. Since that time we have found that the former are related to "impulsive" SEP events from impulsive flares or jets. Here, resonant stochastic acceleration, related to magnetic reconnection involving open field lines, produces not only electrons but 1000-fold enhancements of 3He/4He and of (Z>50)/O. Alternatively, in "gradual" SEP events, shock waves, driven out from the Sun by coronal mass ejections (CMEs), more democratically sample ion abundances that are even used to measure the coronal abundances of the elements. Sometimes residual impulsive suprathermal ions contribute to the seed population for shock acceleration, complicating the abundance picture, but this process has now been modeled theoretically. Initially, impulsive events define a point source on the Sun, selectively filling few magnetic flux tubes, while gradual events show broad acceleration that can fill half of the inner heliosphere, beginning when the shock reaches ~2 solar radii. Shock acceleration occurs as ions are scattered back and forth across the shock by resonant Alfven waves amplified by the accelerated protons themselves. These waves also can produce a streaming-limited maximum SEP intensity and plateau region upstream of the shock. Behind the shock lies the large expanse of the "reservoir", a spatially extensive trapped volume of uniform SEP intensities with invariant energy-spectral shapes where overall intensities decrease with time as the enclosing "magnetic bottle" expands adiabatically. These reservoirs now explain the slow intensity decrease that defines gradual events, once erroneously attributed solely to slow outward diffusion.