Solar physics
The tremendous explosions occurring from time to time on the surface of our Sun are sources of major disturbances in the nearby interplanetary space. During the impulsive phase of large solar flares, as much energy as ~1032 ergs is released in the form of radiation, magnetic fields and high-energy particles. Fundamental questions concerning this energy release and the associated particle acceleration include the following:
•What triggers these solar energetic events? Is it possible to predict them with accuracy?
•What mechanisms accelerate so rapidly and efficiently electrons and ions to GeV energies?
•How do the energetic particles propagate from their acceleration site in solar flare magnetic loops to their interaction regions deeper in the solar atmosphere?
•What is the exact relationship of solar flare accelerated particles to solar energetic particles observed in the interplanetary medium
The importance of these questions transcends the field of solar physics, as the fundamental high-energy processes at work in solar flares also play a major role at various energetic sites throughout the Universe.
Gamma-ray astronomy is one of the best tools for studying the active Sun. Gamma-ray lines are emitted in large solar flares from positron annihilation, secondary neutron capture and de-excitation of nuclei excited by interactions of flare-accelerated ions with the solar atmosphere. Hard X-ray continuum emission is also produced from bremsstrahlung of accelerated electrons. This relatively intense, high-energy radiation carries a wealth of information on the composition, energy spectrum and angular distribution of the accelerated particles, as well as on the physical conditions prevailing in the flare magnetic loops (e.g. Murphy et al. 2007). Powerful X-class flares are often produced one after another with an interval of one to few days from the same active region moving across the solar disk. For DUAL to be able to perform detailed observations of prompt solar flare emissions, such a flaring active region will have to be rapidly declared a Target of Opportunity for the instrument. Three important γ-ray line features are emitted in the DUAL energy range: the “α line complex” at ~450 keV produced by interactions of accelerated α-particles with ambient 4He, the e+-e- annihilation line at 511 keV, and the 847 keV line from ambient 56Fe excited by collisions with energetic protons and α-particles. Thanks to the unprecedented sensitivity obtained with the Laue crystal lens, DUAL will be able to carry out for the first time detailed imaging spectroscopy of these three γ-ray lines, specially of the 847 keV line. Such measurements will provide unique information on the accelerated α/p abundance ratio, the directionality of the interacting α-particles, the density and temperature of the medium in which the secondary positrons annihilate, and will allow localizing the interaction sites of the accelerated protons and α-particles, as well as the annihilation region of the positrons. In addition, comparison of nuclear γ-ray line images with electron bremsstrahlung images obtained with the multilayer mirror will provide valuable information on the charge and mass dependences of the particle acceleration and transport processes.
The bombardment of the solar atmosphere by flare-accelerated ions can also synthesize radioactive nuclei, whose decay can produce delayed γ-ray lines in the aftermath of large flares (Tatischeff et al. 2006). The brightest delayed line for days after the flare is predicted to be the 511 keV line resulting from the decay of several beta+ radioisotopes (18F, 55Co, 57Ni…). After ~2 days however, the flux of the e+-e- annihilation line can become lower than that of the 847 keV line from the decay of 56Co (1/2=77.2 days). For very large flares, the 847 keV line flux could exceed 10-5 γ cm-2 s-1 during few months, i.e. few solar rotation periods. Another important delayed line is at 931 keV from the radioactivity of 55Co (1/2=17.5 hours), with a predicted flux exceeding 5x10-5 γ cm-2 s-1 for about one day. DUAL has the potential to observe for the first time this solar radioactivity. Such detection will provide a new insight into the acceleration of heavy ions in solar flares, because the radioisotopes are expected to be predominantly produced by interactions of fast heavy ions with ambient hydrogen and helium. Perhaps more importantly, the radioisotopes synthesized in flares can serve as tracers to study mixing processes in the solar atmosphere. The delayed lines should be strongly attenuated when the radioactive nuclei plunge deep in the solar convection zone. The spectro-imaging capabilities of DUAL will allow measurements of the size and development of the radioactive patch on the solar surface.
