The Flare-Ona of 31 Comae

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 Angstroms 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*E(6) K is from the more evolved (post helium flash) G8 III primary. Our work was supported by EUVE Guest Observer grants from NASA.