Gamma Rays and the Evolving, Compact Structures of the 2003 October 28 X17 Flare

The X17 flare on 2003 October 28 was observed by high-resolution imaging or spectroscopic instruments on CORONAS, GOES, INTEGRAL, RHESSI, SOHO, and TRACE. These spacecraft observed the temporal evolution of the γ-ray positron-annihilation and nuclear de-excitation line spectra, imaged the hard X-ray bremsstrahlung and EUV and UV emission, and measured the surface magnetic field and subphotospheric pressure perturbations. In the usual pattern, the onset of the flare is dominated by particle acceleration and interaction, and by the filling of coronal magnetic structures with hot plasma. The associated positron-annihilation signatures early in the impulsive phase from 11:06 to 11:16 UT have a line-broadening temperature characteristic of a few hundred thousand kelvins. The most intense precipitation sites within the extended flare ribbons are very compact, with diameters of less than 1400 km, and a 195 Å TRACE intensity that can exceed 7500 times the quiescent active-region value. These regions appear to move at speeds of up to 60 km s^-1. The associated rapidly evolving, compact perturbations of the photosphere below these sites excite acoustic pulses that propagate into the solar interior. Less intense precipitation sites typically persist for several minutes behind the advancing flare ribbons. After ~1 ks, the flare enters a second phase, dominated by coronal plasma cooling and downflows and by annihilation-line radiation characteristic of a photospheric environment. We point out (1) that these detailed observations underscore that flare models need to explicitly incorporate the multitude of successively excited environments whose evolving signals differ at least in their temporal offsets and energy budgets, if not also in the exciting particle populations and penetration depths, and (2) that the spectral signatures of the positron annihilation do not fit conventional model assumptions.