Late-type dwarf stars also exhibit a range of phenomena collectively called chromospheric activity. The chromosphere being a narrow part of the atmosphere above the photosphere not normally visible without special observing techniques. Its main characteristic being a rise in temperature (from a few thousand degrees to over 20 thousand degrees) with height and a complex time-changing structure. These include evidence of large-scale spots analogous to sunspots, frequent optical flares analogous to solar flares and strong emission lines in their optical spectra indicative of the presence of heated outer atmospheres. On a solar model for these phenomena, they would indicate the presence on the surfaces of these stars of concentrations of strong magnetic fields.
In the absence of a comprehensive physical theory for the heating of the chromosphere and corona, modelling work to date has mainly concentrated on studying their structure as a means of placing constraints on heating theories . In the optically thin conditions in the corona and most of the transition region, emission measure analysis may be used. This derives the column depth in a given species of excited atom from the total flux observed in the line. By observing a sequence of suitable lines of species of different excitation the run of volume emission measure may be derived. Combining this with a number of density diagnostic lines, a reasonably complete model of the run of gas density versus temperature can be derived.
In the optically thick chromosphere, however, the situation is much more complex. In the bulk of the chromosphere, photons in the cores of the lines of many species undergo repeated scattering and only those generated in a thin layer near the top escape to infinity. Scattering in the deep chromosphere, however, can result in photons from those layers being shifted to the line wings where the optical depth is low and such photons may escape. Thus the profile of a very optically thick line can yield information on a range of depths in the chromosphere. The derivation of this information requires detailed modelling of the process of radiative transfer within the optically thick medium for comparison with observation. At Armagh Observatory we have been involved in a programme of such computations and the resulting comparison with data for the last decade.
The careful modelling of high resolution profiles of the Balmer lines has been developed and has allowed us to produce the first complete chromospheric structure of plages on a dMe star. The modelling approach and subsequent models have provided several unexpected results of crucial importance in understanding the formation and heating mechanisms of chromospheres, such as: (i) the transition region is extremely thin and at a very high column density, (ii) for some objects, the chromospheric temperature gradient is as high as those required to explain solar flares which highlights the very large amount of non-thermal energy that is deposited at any time in the chromosphere and (iii) for the very high activity objects, hydrogen continuum losses is perhaps the most important source of radiative losses. However, if one desires to understand the mechanisms that generate a chromosphere, we need to extend the modelling to other spectral lines. We are now in a position to do this having complied atomic datasets for several elements, in particular Na I & Ca I.