Solar flares , which are the basis of our models of stellar flares, are a complex phenomenon and there is as yet no detailed description of the physical processes involved with which everybody will agree. Nevertheless, certain details are becoming increasingly clear. Many flares occur within complexes of magnetic fields called active regions. Most will agree that a flare occurs when an instability develops in one or more magnetic loops which requires the magnetic configuration to rearrange itself. As a by-product of this rearrangement, magnetic energy is released. This energy may either produce local heating in the vicinity of the instability or, as seems more likely on the basis of recent evidence, it may either accelerate a beam of non-thermal electrons or generate a conduction front. The electrons in such circumstances would be constrained to move parallel to the magnetic lines of force i.e. along the axis of the magnetic loop. The density of matter in the corona, where the bulk of the loop exists, is sufficiently low for the electrons to travel relatively unimpeded. On reaching the chromosphere at the base of the loop the density rises steeply and the electrons undergoes collisional braking. In the process a substantial part of their energy may be dissipated as heat. As a result, the formerly chromospheric material is heated to coronal temperatures and expands rapidly to fill the entire loop. Here it cools on a timescale of tens of minutes by radiating in the soft X-ray part of the spectrum.
In order to study the flare process in detail requires multiwavelength observations, which allows the extracting of maximum physics (e.g. that obtained from the EUVE satellite, and telescopes such as the WHT , or those available at SAAO , etc). However, such data is only available for a limited number of events. Valuable information can, however, be obtained from spectroscopic observations alone.
The active dwarf M stars are characterized by their Balmer line emission which is direct evidence for the existence of an active chromosphere , although it should be noted that the less active dM stars also have chromospheres. During flare activity both continuum and emission features become enhanced and observations have been made of very large asymmetric broadenings in the bases of chromospheric Balmer lines which are usually interpreted as mass flow events. The first evidence of flare activity is best observed in the ultra-violet continuum. The rise time is usually much sharper in the late M dwarfs, perhaps a few seconds ranging to a few minutes in the K or early M dwarfs. On-the-other-hand, flare activity in the RS CVn binaries tends to be more gradual in nature, sometimes lasting several hours. Staff involved in activity on M dwarfs and RS CVn stars, include Gerry Doyle , John Butler , David Garcia Alvarez , Watch this space!