Flare hard X-rays from neutral beams

A new mechanism is presented for the production of bremsstrahlung radiation from neutral beams (p+,e−) and its possible relevance to flare heating and production of hard xray bursts is discussed. Beam electrons lag behind the protons, due to differential drag in collisions with the background, but their longitudinal velocities are closely tied to the protons by the electric field generated. However, collisions with the background also scatter the beam electrons resulting in rms (quasithermal) transverse velocities well in excess of the proton speed. We demonstrate the initial development of this effect using an electrostatic particle simulation with scaled collision rate and then study its full development using an approximate analytic treatment. In particular, the heating of the beam electrons under the bombardment effect of the background is limited by the warm target effect but mean electron energies (‘temperatures’) of up to Ee ' 0.02Ep0 result during the propagation of a neutral beam of initial proton energy Ep0. Thus, for example, HXR bremsstrahlung in the range 20–200 keV can be generated by protons in the range 1MeV-10MeV. The energy efficiency of the bremsstrahlung production is also limited by the warm target effect but, depending on the HXR spectrum, can exceed ' 0.2 of the efficiency of the standard thick target electron beam model. This suggests that the MeV neutral beam model is, in terms of power requirements, unlikely to be the source of ‘HXR-rich’ flare bursts but that neutral beams able to provide the impulsive flare heating will yield easily detectable HXR burst signatures. Also, while the neutral beam model needs more power (' 5×) than an electron beam to yield a given HXR burst flare, it requires a much smaller beam number flux (' 0.07×). The issue of the HXR spectral distribution expected from the neutral beam model is also discussed.