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

 

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). 

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-uniform plasma ionization (e.g. Kontar et al. 2003), return current (e.g. Zharkova & Gordovskyy 2006), and beam-plasma interaction via various plasma waves (e.g. Kontar 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 and observations of hard X-ray emissions associated with CMEs.


Partially limb-occulted flare observations:


Partial limb-occultation will frequently provide view angles from which purely coronal emission (e.g. Hudson 1978) can be readily seen because the footpoint sources are occulted. This will allow us to study faint coronal sources that otherwise would have been lost in the limited dynamic range (≤20) of indirect imaging instruments. Coronal hard X-ray emission in the absence of hard X-ray footpoint emissions, but with the thermal flare loop still partially visible, are best studied for flares occurring up to ~20° behind the limb. This suggests that for a single spacecraft about 20% of the observed flares have occulted footpoints with the main flare loop still partially visible above the limb. The following figure shows RHESSI observations of partially occulted-limb flares. In at least 90% of all such events, non-thermal coronal emission is observed, most prominently during the rise of the thermal emission (Krucker & Lin 2008). Most often the non-thermal emission is seen close to the thermal loop (Figure A1), but occasionally from above the thermal flare loops (Figure A2) similar to what was reported for the Masuda flare (Masuda et al. 1994). As electron acceleration is thought to occur in the corona, hard X-ray imaging spectroscopy provides crucial information on the location and spectrum of energetic electrons before they precipitate to the chromosphere. STIX will provide frequent partially limb-occulted observations at very high sensitivity and will be able to see up to 15 weaker coronal emissions than RHESSI. This will allow us to image hard X-ray emission from the corona at the highest-every sensitivity, free from the intense footpoint sources, thus providing unique information about the supra-thermal electrons closest to the site in the corona where their acceleration is believed to occur. 

 

 

  • 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.
    • Products:
      • CME velocity maps 
      • CME density maps 
      • CME directionality
      • CME flag
    • Modes:
      • GLOBAL (before the event, if possible), min. obs time 2 hr, data volume ≤ 300 Mb.
      • CMEOBS, starts after CME flag rise, min. obs time 1 hr (high cadence, 1 min), data volume ~ 2.137 Gb.
      • GLOBAL (after the event), min. obs time 2 hr, data volume ≤ 300 Mb.
    • Other instruments: PHI, EUI, STIX, SoloHI, MAG, SWA, EPD, RPW.

 

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