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31 Comae (HD111812: G0~III) is a rapidly rotating moderate mass giant in the middle of the Hertzsprung gap. It is a luminous source of soft X-rays and C~IV~$\lambda$1549, although it is a member of the ``X-ray deficient'' class of F/G giants (and early-F dwarfs) identified by Simon \& Drake (1989, ApJ, 346, 303).
We discuss the nature of the corona of 31~Com based on an 80~ks pointing by the Extreme Ultraviolet Explorer (EUVE), and historical FUV and X-ray exposures from IUE and ROSAT, respectively. We provide a context for the 31~Com energy distribution by applying an identical analysis to archival EUVE, IUE, and ROSAT/PSPC spectra of the evolved stars $\alpha$~CMi (F5~IV), $\alpha$~Aur (G8~III + G0~III), and HR1099 (K0~IV [+ G5~V]).
31~Com shows a remarkably hot coronal energy distribution that rivals that of the hyperactive RS~CVn system HR1099 (and the dMe flare star AU~Mic). It is decidedly different from soft coronal sources like the Sun and $\alpha$~CMi. The hot coronal structure is reminiscent of a continuously flaring plasma. However, the overt signatures of impulsive outbursts -- common on RS~CVn's and dMe flare stars -- are absent. We propose that the G0~III giants at the red edge of the Simon \& Drake X-ray deficiency boundary are undergoing a transient evolutionary episode of intense magnetic activity. We believe that the surface manifestations include widespread compact flaring ``bright points'' commensurate with the shallow unstable envelopes of these newly convective stars. The ``flare-ona'' of 31~Com suggested by the EUVE allows us to interpret the peculiar 80--300~\AA\ spectrum of $\alpha$~Aur reported by Dupree and collaborators (1993, ApJL, 418, L41). The enhanced emission measure at and above $10^7$~K very likely is from the active G0~III secondary, while the pronounced peak in EM near $5\times10^6$~K is from the more evolved (post helium flash) G8~III primary.
Our work was supported by EUVE Guest Observer grants from NASA.
