Gerry Doyle's homepage Flare Dynamics

Flare Dynamics

Overview

The solar cycle is vivid evidence that the sun is not quiet, steady and changeless. Its most violent outbursts are flare related events, which comprise of super-hot plasma which can cover several million square kilometers of the star's surface. In general, solar flares are associated with sunspots (i.e. the dark regions that occasionally appear on the solar disc where strong magnetic fields emerge and where the normal rise of hot gas from the solar interior is suppressed). The sun being our nearest star, is the only one which astronomers can examine at sufficiently close range to resolve spatial features. However, the complex processes of particle acceleration and energy release happen even more dramatically elsewhere. It therefore follows that the better we understand the sun, the more insight we have of activity in other stars. One of the big issues in astrophysics is the role of stellar magnetic activity, (e.g. what role do magnetic structures play in the storage and release of energy). In particular, the sudden heating of material to extremely high temperatures is relevant to theories of stellar flares and the loss of stellar matter (areas of interest in which the astronomers at Armagh play a prominent role).

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 ,

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