Institution(s): 1. University of Colorado
A particularly impressive far-ultraviolet flare (corresponding to a GOES X25,000-class event: 50 is largest ever recorded on the Sun) was captured serendipitously by Hubble's Cosmic Origins Spectrograph, during a campaign on young (~50 Myr) solar analog EK Draconis. The large outburst was like a classical solar 2-ribbon flare: it lasted several hours and all the atmospheric temperature regimes -- from C II (30,000 K) through C IV (100,000 K), and up to Fe XXI (10 MK) -- were affected simultaneously. Scaling laws suggest that the EK Dra event was in the upper echelons of the historical sample of stellar high-energy outbursts, but the first to be captured in the FUV with the unprecedented sensitivity, spectral resolution, and high time cadence of an instrument like COS. Remarkably, time-resolved line profiles of hot species like Si IV and C IV displayed strong, highly redshifted components during the event; contrary to the blueshifts one might naively anticipate from an isotropic stellar explosion. Instead, the conspicuous redshifts probably are signatures of a post-flare cooling process, analogous to "coronal rain" or "super-arcade downflows" associated with large solar events. Flares of this magnitude probably are common on young hyperactive sunlike stars, occurring perhaps twice a day on EK Dra itself. The transient doses of FUV radiation probably are not a significant impediment to planetary habitability, compared with the "quiescent" FUV output of the star (if you can call an object with 1000 times the X-ray luminosity of the Sun "quiet"), but the associated very hard X-ray and γ-ray fluences could have a much larger impact on the ionization of a primitive planetary atmosphere, boosting its exposure to gas-stripping by stellar winds or coronal mass ejections.