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Astron. Astrophys. 353, 729-740 (2000)

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6. Discussion

We have shown that substantial HXR signatures can arise from neutral beams by heating of the electrostatically dragged beam electrons in collision with a neutral background. This process may be important in a variety of astrophysical situations involving fast neutral streams. The process can also yield easily detectable HXR signatures from solar flares for neutral beams of sufficient power to produce flare heating, provided the neutral beam can survive propagation among the free electrons of the solar flare background plasma. We have corrected an error in the previous analysis by Brown et al. (1998a) of neutral beam propagation and shown that neutral beams can survive, once developed, in the flare corona and can survive from their outset in chromospheric conditions. However, both theoretical and observational considerations favour a coronal origin for accelerated beams. Theoretically reconnection and associated acceleration mechanisms are generally thought to operate best in coronal conditions. Observationally, the observed synchronism of HXR footpoints within 0.1s (Sakao 1994) cannot be explained by neutral beams originating at one chromospheric footpoint and travelling to the other, since 10 MeV protons take of order 1s to do so. These data rather favour symmetrical downward propagation of beams from a common central looptop source, as does the time of flight analysis by Aschwanden & Schwartz (1995) and by Brown et al. (1998b). The issue of whether magnetic and other conditions can exist whereby a neutral beam can be produced and survive under coronal conditions requires further investigation and is intimately connected with the discussion of neutral beam emergence from reconnection sites by Martens (1988) and by Litvinenko & Somov (1995).

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© European Southern Observatory (ESO) 2000

Online publication: December 17, 1999