On Stellar Coronae and Solar Active Regions

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Abstract
Based on Yohkoh Soft X-Ray Telescope (SXT) observations of the Sun near peak activity level obtained on 1992 January 6, we search for coronal structures that have emission measure distributions EM(T) that match the observed stellar coronal emission measure distributions derived for the intermediate-activity stars Eri (K2 V) and ξ Boo A (G8 V) from Extreme Ultraviolet Explorer spectroscopic observations. We find that the temperatures of the peaks of the observed stellar distributions EM(T), as well as their slopes in the temperature range 6.0 log T 6.5, are very similar to those obtained for the brightest of the solar active regions in the 1992 January 6 SXT images. The observed slopes correspond approximately to EM ∝ Tβ with β ~ 4, which is much steeper than predicted by static, uniformly heated loop models. Plasma densities in the coronae of Eri and ξ Boo A are also observed to be essentially the same as the plasma densities typical of solar active regions. These data provide the best observational support yet obtained for the hypothesis that solar-like stars up to the activity levels of Eri (K2 V) and ξ Boo A are dominated by active regions similar to, though possibly considerably larger than, those observed on the Sun. The surface filling factor of bright active regions needed to explain the observed stellar emission measures is approximately unity. We speculate on the scenario in which small-scale "nanoflares" dominate the heating of active regions up to activity levels similar to those of Eri (K2 V) and ξ Boo A. At higher activity levels still, the interactions of the active regions themselves may lead to increasing flaring on larger scales that is responsible for heating plasma to the observed coronal temperatures of T 107 K on very active stars. Observations of X-ray and EUV light curves using more sensitive instruments than are currently available, together with determinations of plasma densities over the full range of coronal temperatures (106-107 K and higher), will be important to confirm flare heating hypotheses and to elicit further details concerning coronal structures at solar-like active region temperatures (T 5 × 106 K) and the temperatures that characterize the most active stars (T 107 K).