Working Group 5: The Release of Energetic Particles in the Low Corona: Shock or Reconnection?

Group Leaders: Alexis Rouillard, Rami Vainio

Particle acceleration and transport during the extended phases of large gradual SEP events as well as particle acceleration during impulsive events can be understood relatively well in terms of the origin of the particles. The former are produced in CME-driven shocks and the latter in impulsive flares. The energy reserve for the accelerator is, thus, either the coronal shock wave or some flare-reconnection-related process, respectively.

The situation is not as clear when the early phase of large gradual events is considered alone during which the highest energy particles are produced. Relativistic proton release often occurs in close connection with the maximum energy release of the flare, but can be significantly delayed relative to the onset of metric type II burst. Recent imaging observations show that, during the flare impulsive phase, CME cavities often undergo strong lateral expansions, probably driving a shock on their flanks. Other imaging work reveals that the significant delays observed between some particle onsets relative to the flare peaks or metric type IIs can be associated with the time required for CME pressure waves (and their shocks) to impinge on the S/C-connected lines. Charge states in large gradual energetic particle events show distributions with high-charge components, indicating either flare origin or stripping in the dense coronal plasma. Elemental abundances at high energies in many large gradual events show heavy-ion enhancements at high energies, but also these have been interpreted differently by different authors: do the enrichments imply quasi-perpendicular shocks or flares?

The objective is to form a coherent picture of particle acceleration and release in the low corona and to gain understanding on the respective roles of CME-driven shocks and flares in the acceleration processes during the early phases of the eruptions. Both observational and modeling contributions are welcome.