Solar and stellar flares

Abstract

Short-lived increases in the brightness of many red dwarfs have been observed for the last 30 yr, and a variety of more or less exotic models have been proposed to account for such flares.Information about flares in the Sun has progressed greatly in recent years as a result of spacecraft experiments, and properties of coronal flare plasma are becoming increasingly better known. In this paper, after briefly reviewing optical, radio and X-ray observations of stellar flares, we show how a simplified model which describes conductive plus radiative cooling of the coronal flare plasma in solar flares has been modified to apply to optical and X-ray stellar flare phenomena. This model reproduces many characteristic features of stellar flares, including the mean UBV colors of flare light, the direction of flare decay in the two-color diagram, precursors, stillstands, secondary maxima, lack of sensitivity of flare color to flare amplitude, low flux of flare X-rays, distinction between so-called spike flares and slow flares, Balmer jumps of as much as 6-8, and emission line redshifts up to 3000 km s -1. In all probability, therefore, stellar flares involve physical processes which are no more exotic (and no less!) than those in solar flares. Advantages of observing stellar flares include the possibilities of (i) applying optical diagnostics to coronal flare plasma, whereas this is almost impossible in the Sun, and (ii) testing solar flare models in environments which are not generally accessible in the solar atmosphere.

X-ray-heated models of stellar flare atmospheres - Theory and comparison with observations

“…In the past, the continuum in stellar flares has been fitted to various emission models, including hydrogen recombination radiation (e.g., Kunkel 1970), optically thin free-free emission from a hot (T ~ 10 7 K) coronal source (e.g., Mullan 1977;Giampapa 1983), and blackbody radiation from a plasma with a temperature near 10 4 K (e.g., Giampapa 1983;Kahler et al 1982;Mochnacki & Zirin 1980). In Figure 11 we compare the observed flare continuum with a fit to the T20 atmospheric model of this paper, a free-free fit, and a blackbody fit, at four different phases of the flare.…”

Review of the theoretical models of flares of the UV Ceti-type stars

“…, Kaler et al (1982),Chugainov (1987),de Jager et al (1989).Such approach permitted us to draw a direct analogy between stellar flares and solar white flares, whose optical continuum arises also in transition region between photosphere and chromosphere(Neidig, 1989). We estimated also the area of continuum formation region of strong stellar flares (at the maxima of light): S co " <u 1 -2% of stellar disk area.Summarizing the data on the optical spectra of the flares let us note briefly some properties of the emission lines profiles.The observations show that near the maxima of strong flares the hydrogen lines are broadening by several angstroms as a rule.…”