RHESSI e⁺-e^- Annihilation Radiation Observations: Implications for Conditions in the Flaring Solar Chromosphere

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 above 10⁵ K. The measured continuum from positronium and from Compton scattering was unobservable, with the exception of the first 4 minutes of the October 28 flare observation; this indicates that the density at which most annihilations occurred was greater than ~10¹⁴ H cm^-3. The width of the line narrowed in 2 minutes to ~1 keV late in the October 28 flare, consistent with annihilation in ionized H<10⁴ K and >=10¹⁵ cm^-3. There is evidence for a similar decrease in line width late in the November 2 flare. These observations suggest a highly dynamic flaring atmosphere at chromospheric densities that can reach transition-region temperatures, then cool to less than 10⁴ K in minutes while remaining 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, and this raises questions about the energy source.