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 x-ray bursts is discussed. Beam electrons lag behin d the protons, due to differential drag in collisions with the background, but their longitudinal velocities are closely tied to the protons by the el ectric field generated. However, collisions with the background also scatte r the beam electrons resulting in rms (quasi-thermal) transverse velocities well in excess of the proton speed. We demonstrate the initial development of this effect using an electrostatic particle simulation with scaled coll ision rate and then study its full development using an approximate analyti c treatment. In particular, the heating of the beam electrons under the bom bardment effect of the background is limited by the warm target effect but mean electron energies ('temperatures') of up to E-e similar or equal to 0. 02E(p0) result during the propagation of a neutral beam of initial proton e nergy E-p0. Thus, for example, HXR bremsstrahlung in the range 20-200 keV c an be generated by protons in the range 1MeV-10MeV. The energy efficiency o f the bremsstrahlung production is also limited by the warm target effect b ut, depending on the HXR spectrum, can exceed similar or equal to 0.2 of th e efficiency of the standard thick target electron beam model. This suggest s that the MeV neutral beam model is, in terms of power requirements, unlik ely 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 bu rst signatures. Also, while the neutral beam model needs more power (simila r or equal to 5x) than an electron beam to yield a given HXR burst flare, i t requires a much smaller beam number flux (similar or equal to 0.07x). The issue of the HXR spectral distribution expected from the neutral beam mode l is also discussed.