The Magnetic Future of the Sun

Abstract

We analyze space- and ground-based data for the old (7.0 ± 0.3 Gyr) solar analogs 16 Cyg A and B. The stars were observed with the Cosmic Origins UV Spectrographs on the Hubble Space Telescope (HST) on 2015 October 23 and 2016 February 3, respectively, and with the Chandra X-ray Observatory on 2016 February 7. Time-series data in Ca ii data are used to place the UV data in context. The UV spectra of 18 Sco (3.7 ± 0.5 Gyr), the Sun (4.6 ± 0.04 Gyr), and α Cen A () appear remarkably similar, pointing to a convergence of magnetic heating rates for G2 main-sequence stars older than ≈2–4 Gyr. But the B component's X-ray (0.3–2.5 keV) flux lies 20× below a well-known minimum level reported by Schmitt. As reported for α Cen A, the coronal temperature probably lies below that detectable in soft X-rays. No solar UV flux spectra of comparable resolution to those of stellar data exist, but they are badly needed for comparison with stellar data. Center-to-limb variations are reevaluated for lines such as Ca ii through X-rays, with important consequences for observing activity cycles in such features. We also call into question work that has mixed solar intensity–intensity statistics with flux–flux relations of stars.