3.1.2.7 How are so many electrons accelerated on such short time scales to explain the observed hard X-ray fluxes?

Understand the transport of electrons from the coronal acceleration site down to the hard X-ray footpoints (Miller et al., 1997; Önel et al., 2007; Battaglia and Benz, 2007).
Understand effects of non-uniform plasma ionisation (Kontar et al., 2003), return current (Zharkova and Gordovskyy, 2006), and beam-plasma interaction via various plasma waves (Kontar, 2001).
Partially limb-occulted flare observations: provide unique information about the suprathermal electrons closest to the site in the corona where their acceleration is believed to occur (Krucker and Lin, 2008).
Study coronal phenomena in hard X-rays associated with CMEs:
- Highly occulted events associated with fast backside CMEs (Krucker et al., 2007), non-thermal bremsstrahlung.
  - Produced by flare-accelerated energetic electrons (>10 keV) trapped in magnetic structures related to the CME or
  - Accelerated in CME current sheets or other coronal magnetic restructuring related to the CME.
Explore the consequences of particle acceleration by Alfvén waves created by the magnetic reconfiguration during magnetic reconnection (Fletcher & Hudson, 2008).

SoloHI: Contribute (mode: shock+synoptic), no min. obs time, all distances, w/EUI-STIX-METIS.
EUI, EUI synoptic mode (S). EUI FD 174, 304, cadence 2 min. EUI HR 174 and Ly-alpha, cadence 1 min for 30 min before and during X-ray peak. Best when the solar limb from SO is connected to Earth, or other s/c.
EPD: All sensors: spectra, composition, fluxes, directional information, together with IS instruments. Solar source identification by RS instruments (full disk imaging). Also coordinated multi-s/c SEP observation campaigns with SPP & other missions.

METIS: Measurement of coronal outflow velocity and density in corona to identify the shock front. Measurement of shock passage timing relative to the flare occurrence.