Measurements of the coronal acceleration region of a solar flare

Säm Krucker, H. S. Hudson, L. Glesener, S. M. White, S. Masuda, J. P. Wuelser, R. P. Lin

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The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) and the Nobeyama Radioheliograph (NoRH) are used to investigate coronal hard X-ray and microwave emissions in the partially disk-occulted solar flare of 2007 December 31. The STEREO mission provides EUV images of the flare site at different viewing angles, establishing a two-ribbon flare geometry and occultation heights of the RHESSI and NoRH observations of ∼16 Mm and ∼25Mm, respectively. Despite the occultation, intense hard X-ray emission up to ∼80keV occurs during the impulsive phase from a coronal source that is also seen in microwaves. The hard X-ray and microwave source during the impulsive phase is located ∼6Mm above thermal flare loops seen later at the soft X-ray peak time, similar in location to the above-the-loop-top source in the Masuda flare. A single non-thermal electron population with a power-law distribution (with spectral index of ∼3.7 from ∼16keV up to the MeV range) radiating in both bremsstrahlung and gyrosynchrotron emission can explain the observed hard X-ray and microwave spectrum, respectively. This clearly establishes the non-thermal nature of the above-the-loop-top source. The large hard X-ray intensity requires a very large number (>5 × 1035 above 16keV for the derived upper limit of the ambient density of ∼8 × 109cm-3) of suprathermal electrons to be present in this above-the-loop-top source. This is of the same order of magnitude as the number of ambient thermal electrons. We show that collisional losses of these accelerated electrons would heat all ambient electrons to superhot temperatures (tens ofkeV) within seconds. Hence, the standard scenario, with hard X-rays produced by a beam comprising the tail of a dominant thermal core plasma, does not work. Instead, all electrons in the above-the-loop-top source seem to be accelerated, suggesting that the above-the-loop-top source is itself the electron acceleration region.

Original languageEnglish (US)
Pages (from-to)1108-1119
Number of pages12
JournalAstrophysical Journal
Issue number2
StatePublished - 2010


  • Sun: X-rays, gamma rays
  • Sun: flares
  • Sun: particle emission


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