Hard X-ray (HXR) observations frequently exhibit fast temporal variations during the impulsive phase of solar flares and this is usually ascribed to the propagation of beams of accelerated particles and to the dissipation of their energy in lower layers of the solar atmosphere. As a result of fast heating and non-thermal processes, several chromospheric lines show significant impulsive brightenings. We first review observational attempts of detecting such fast (sub-second) variations of the line intensities, namely in the Halpha line, and discuss the problems associated with such observations. Second, we describe new radiation-hydrodynamical (RHD) simulations of the pulse-beam heating and show how they predict both HXR and optical-line intensity variations on very short time scales. We also discuss the effect of the return current on the energy deposit in the atmosphere. Using new spatially-resolved HXR observations (RHESSI) made simultaneously with a high-cadence detection of selected optical lines, one should be able to diagnose the properties of particle beams, provided that the response of the lower atmospheric layers to beam pulses is strong enough.
Energy Conversion and Particle Acceleration in the Solar Corona
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