Paper Title:	What the Annihilation Line Tells Us About the Flaring
Chromosphere

Author List:	Share, G., Murphy, R.

Abstract:	RHESSI has measured the positron-electron
annihilation line and continuum in three solar flares: 2002 July 23,
2003 October 28 and 2003 November 2. The 511 keV line was broad (~4 - 8 keV) 
in all three flares, consistent with annihilations in an
ambient ionized medium at temperatures > 10^5 K. The measured
continuum from positronium and from Compton scattering was unobservable
with the exception of the first 4 min of the October 28 flare
observation; this indicates that the density where most annihilations
occurred was > 10^{14} H cm^{-3}. The width of the line narrowed in
2 min to ~ 1 keV late in the October 28 flare consistent with
annihilation in ionized H  <10^4 K and >= 10^{15} cm^{-3}. There
is evidence for a similar decrease in line width late in the Nov. 2
flare. These observations suggest a highly dynamic flaring atmosphere at
chromospheric densities that can reach transition-region temperatures,
cool to <10^4 K in minutes, and still remains highly ionized. Although
the energy contained in high-energy accelerated particles may have been
enough to heat the plasma, the rate of deposition is not correlated with
the temperature determined by the 511-keV line width; this raises
questions about the energy source.

Poster:	No