The magnetic future of the Sun

Abstract

We analyze space- and ground-based data for the old ($7.0\pm0.3$~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 23 October 2015 and 3 February 2016 respectively, and with the Chandra X-ray Observatory on 7 February 2016. Time-series data in \ion{Ca}{2} data are used to place the UV data in context. The UV spectra of 18 Sco (3.7$\pm0.5$ Gyr), the Sun (4.6$\pm0.04$ Gyr) and $\alpha$ Cen A ($5.4_{-0.2}^{+1.2}$ Gyr), appear remarkably similar, pointing to a convergence of magnetic heating rates for G2 main-sequence stars older than $\approx 2-4$ Gyr. But the B component's X-ray (0.3-2.5 keV) flux lies 20$\times$ below a well-known minimum level reported by Schmitt. As reported for $\alpha$~Cen~A, the coronal temperature probably lies below that detectable in soft X-rays. No solar UV flux spectra of comparable resolution to stellar data exist, but they are badly needed for comparison with stellar data. Center-to-limb (C-L) variations are re-evaluated for lines such as \ion{Ca}{2} through to 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.