In the Trenches of the Solar–Stellar Connection. I. Ultraviolet and X-Ray Flux–Flux Correlations across the Activity Cycles of the Sun and Alpha Centauri AB

Abstract

Abstract This study focuses on high-energy proxies of stellar magnetic activity over long-term starspot cycles of three low-activity Sun-like stars: α Centauri A (HD 128620: G2 V), α Cen B (HD 128621: K1 V), and the Sun itself. Data sets include: daily solar ultraviolet irradiance spectra and X-ray fluxes from the Solar Radiation and Climate Experiment, during recent sunspot Cycles 23 and 24; Interface Region Imaging Spectrograph long-slit stigmatic imaging of solar Mg ii h (2803 Å) and k (2796 Å) in quiet and active regions; and Hubble Space Telescope Imaging Spectrograph ultraviolet, and Chandra X-ray, campaigns on α Cen AB. Established stellar “flux–flux” relations, for example X-rays (T ∼ 1 MK) versus Mg ii (T ∼ 8000 K), showed increasing power-law slopes with increasing formation temperature, but these give way on the Sun to bent power laws, and surprising inversions in the activity hierarchy: Si iii is more “active” (steeper power laws) than N v, despite the latter’s four times higher formation temperature. The Sun’s flux–flux behavior, nevertheless, remarkably parallels its low-activity solar twin α Cen A. In contrast, the cooler, somewhat more active, K dwarf companion displays correlations more in line with the previous stellar paradigm. The new flux–flux relations offer a way to vet numerical spectral simulations and proxy-based irradiance models, and extrapolate solar global activity indices into regimes below or above the grasp of contemporary records, or to exoplanet hosts at the low end of the Sun-like activity ladder.