Properties of the energetic particle distributions during the October 28, 2003 solar flare from INTEGRAL/SPI observations

Abstract

Analysis of spectra obtained with the gamma-ray spectrometer SPI onboard INTEGRAL of the GOES X17-class flare on October 28, 2003 is presented. In the energy range 600 keV-8 MeV three prominent narrow lines at 2.223, 4.4 and 6.1 MeV, resulting from nuclear interactions of accelerated ions within the solar atmosphere could be observed. Time profiles of the three lines and the underlying continuum indicate distinct phases with several emission peaks and varying continuum-to-line ratio for several minutes before a smoother decay phase sets in. Due to the high-resolution Ge detectors of SPI and the exceptional intensity of the flare, detailed studies of the 4.4 and 6.1 MeV line shapes was possible for the first time. Comparison with calculated line shapes using a thick target interaction model and several energetic particle angular distributions indicates that the nuclear interactions were induced by downward-directed particle beams with alpha-to-proton ratios of the order of 0.1. There are also indications that the 4.4 MeV to 6.1 MeV line fluence ratio changed between the beginning and the decay phase of the flare, possibly due to a temporal evolution of the energetic particle alpha-to-proton ratio.

Solar flares and energetic particles

“…The comparison of the results of detailed calculations of line shapes with the observations provides additional constraints on the ratio of accelerated helium with respect to accelerated protons (a/p) in the flare [43]. The line shapes depend, however, on many parameters: angular distribution of interacting ions, spectral index of the energetic ions and a/p so that only the combination of line shapes and line fluences can provide information on regions of allowed parameters; in the case shown in the figure, this leads to the determination of the ion spectral index between −3 and −4, a relatively low value of a/p around 0.1 and a relatively wide angular distribution of emitting ions [43]. The number of flares for which g-ray line spectra at high resolution have been obtained is still very small (around five combining RHESSI and INTEGRAL/SPI observations; for a review, see [13]), but this is a very promising field to deduce more constraints on the parameters of accelerated ions in flares.…”

Cosmic Gamma-Ray Spectroscopy

“…A bright flare occurred on 28 Oct 2003, early in both those missions, and resulted in bright gamma-ray line signals extending over a period of 15 min [64]. The gamma-ray flare started off with intense continuum emission, but within a minute, line emission set in, with nuclear de-excitation lines preceding the neutron capture line at 2.223 MeV, as expected from the required slowingdown processes.…”

“…These were interpreted as a beam geometry of accelerated particles being downward-directed and rather These arise from pitch-angle scattering of solar-flare particles, as they hit the solar atmosphere from above. See [64] for details.…”

Nuclear interaction gamma-ray lines from the Galactic center region

“…These lines may be a good tracer for subrelativistic cosmic rays, because the line brightness can give us information about the amount of subrelativistic particles. Thus, prominent nuclear de-excitation lines are observed in emission spectra of solar flares, which allows derivation of valuable information on solar ambient abundances, density and temperature, as well as on accelerated particle composition, spectra, and transport in the solar atmosphere (see e.g., Smith et al 2003;and Kiener et al 2006, for recent solar observations with RHESSI and INTEGRAL, respectively). Dogiel (2001) predicted that galaxy clusters could emit a detectable flux of de-excitation gamma-ray lines.…”