Coronal hard X-ray sources

1
Coronal hard X-ray sources

• The corona is tenuous and bremsstrahlung is
  inefficient gt it should be dark in hard X-rays
• Nevertheless several clear observations of
  coronal sources gt20 keV have been reported

2
The first clear evidence?
Coronal hard X-ray source from a limb-occulted
flare in 1969 (Frost Dennis, 1971). Note
Coronal height 30,000 km Impulsive phase
evident X-ray spectrum hn-2 in later
extended phase
Review by Cliver et al., ApJ 305, 920 (1986)
3
Coronal sources of hard X-rays?

• Late non-thermal sources (radio type IV?)
• Masuda above-the-loop-top hard X-rays
• Direct emission from escaping electrons
  (counterpart of Type III bursts)
• Coronal thick targets
• Early-phase hard spectrum sources (e.g. July 23
  2002)