tokens: that auroral emission excited by energetic particles is likely to dominate over airglow excited by EUV and X-rays. Nevertheless, if high-energy photons are incident at ∼3 × 10 30 erg s −1 (10 34 erg over 1 hr), the incident energy is ∼135 TW for a planet of Earth’s radius at 0.05 au. The fraction converted to airglow depends completely on the photochemistry of the upper atmosphere. If there is significant ozone present, essentially all the light shortward of 320 nm will be absorbed in the ozone Hartley band, and will potentially be available for atmospheric chemistry and excitation processes, such as excitation of the rotational–vibrational Meinel series of hydroxyl (OH) emission lines in the near infrared. Leinert et al. ( 1998 ) tabulated the strongest terrestrial airglow lines, and the OH line series is by far the strongest, at 4.5 MR (mega-Rayleighs, where R is the Rayleigh unit of surface brightness), compared to, e.g., the green oxygen line at 558 nm of 250 R, or 10 kR auroral 558 nm emission for typical zenith airglow brightnesses. With the high-energy photon flux, and the timescales for the stellar wind and CMEs being comparable, high-resolution spectroscopic searches for optical/NIR line emission are likely to benefit from filtering according to this timescale—the optimal times to seek an atmospheric response are during and for the few hours following strong flares. An instrument such as the CRyogenic high-resolution InfraRed Echelle Spectrograph (CRIRES) on the European Southern Observatory’s Very Large Telescope (VLT) would seem well suited to a search of this nature, or the High Accuracy Radial velocity Planet Searcher (HARPS) spectrograph analyzed in detail by Luger et al. ( 2017 ). Lovis et al. ( 2017 ) discuss coupling the high-resolution spectrograph ESPRESSO to the adaptive optics instrument SPHERE at the VLT in order to measure both a spatial offset and a Doppler shift for Proxima Cen b, and conclude that such a measurement is feasible. If additional filtering on the flares were introduced, the signal-to-noise ratio could potentially be improved further. 5.4. Complexities and Opportunities The geometric distribution of flares on the surface of the star is unknown. Hunt-Walker et al. ( 2012 ) found no correlation with rotational phase for AD Leo, similar to Hawley et al. ( 2014 ), indicating a distribution of many flare sources at different longitudes and a relatively uniform flare distribution, stochastic in both outburst frequency and location. A non-uniform flare distribution could influence the illumination pattern of the space local to the star, and could work either in favor if flares are concentrated to the equatorial regions, or against if to the poles (presuming the planets to share the same equatorial plane). The planet may be illuminated by flares that we do not see, because they are both “around the back” of the star, and conversely, we may see flares that the planets do not. The degree to which flares are
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