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Hard X-ray imaging observations show most prominent emissions from footpoints of flare loops in the chromosphere where the ambient density is high enough to stop flare-accelerated electrons by collisions (e.g. Hoyng et al. 1981). However, fainter, co-temporal hard X-ray sources are also seen in the corona (e.g. Frost & Dennis 1971, Masuda et al. 1994, Veronig & Brown 2004, Battaglia & Benz 2006, Krucker et al. 2007) consistent with electron acceleration in the corona. In particular, RHESSI observations of partially-occulted flares show that at least 90% of all flares have coronal hard X-ray sources (Krucker & Lin 2008). Further evidence for a coronal acceleration region comes from radio observations (e.g. Benz 1985, Aurass et al. 2004, Mann et al. 2006). The details of the transport of electrons from the coronal acceleration site down to the hard X-ray footpoints are still unclear (e.g. Miller et al. 1997, Önel et al. 2007, Battaglia & Benz 2007). 

 

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It is becoming increasingly clear that propagation of energetic electrons does not follow a simple collisional thick-target scenario, so more sophisticated models of electron transport are required including effects of non-

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uniform plasma ionization (e.g. Kontar et al.

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2003), return current (e.g. Zharkova

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& Gordovskyy

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2006), and beam-plasma interaction via various plasma waves (e.g. Kontar

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2001). The deposited energy of the non- thermal electrons heats the chromospheric plasma and the resulting overpressure drives the hot plasma up the legs of the magnetic loops (e.g. Brown 1973) in the process termed chromospheric evaporation. Hard X-ray observations provide thermal diagnostics of the heated flare loops. 

Observing plans for this objective should include in particular partially limb-occulted flare observations

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and observations of hard X-ray emissions associated with CMEs:

 

  • 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).

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