Far‐Ultraviolet Emissions of the Sun in Time: Probing Solar Magnetic Activity and Effects on Evolution of Paleoplanetary Atmospheres

Abstract

We present and analyze Far Ultraviolet Spectroscopic Explorer (FUSE) observations of six solar analogs. These are single main-sequence G0-G5 stars selected as proxies for the Sun at several stages of its main-sequence lifetime from ~130 Myr to ~9 Gyr. The emission features in the FUSE 920-1180 ? wavelength range allow for a critical probe of the hot plasma over three decades in temperature: from ~104 K for the H I Lyman series to ~6 ? 106 K for the coronal Fe XVIII ?975 line. Using the flux ratio C III ?1176/?977 as diagnostics, we investigate the dependence of the electron pressure of the transition region as a function of the rotation period, age, and magnetic activity. The results from these solar proxies indicate that the electron pressure of the stellar ~105 K plasma decreases by a factor of ~70 between the young fast-rotating (Prot = 2.7 days) magnetically active star and the old, slow-rotating (Prot ~ 35 days) inactive star. We also study the variations in the total surface flux for specific emission features that trace the hot gas in the stellar chromosphere (C II), transition region (C III, O VI), and corona (Fe XVIII). The observations indicate that the average surface fluxes of the analyzed emission features strongly decrease with increasing stellar age and longer rotation period. The emission flux evolution with age or rotation period is well fitted by power laws, which become steeper from cooler chromospheric (~104 K) to hotter coronal (~107 K) plasma. The relationship for the integrated (920-1180 ?) FUSE flux indicates that the solar far-ultraviolet (FUV) emissions were about twice the present value 2.5 Gyr ago and about 4 times the present value 3.5 Gyr ago. Note also that the FUSE/FUV flux of the zero-age main-sequence Sun could have been higher by as much as 50 times. Our analysis suggests that the strong FUV emissions of the young Sun may have played a crucial role in the developing planetary system, in particular, through the photoionization, photochemical evolution, and possible erosion of the planetary atmospheres. Some examples of the effects of the early Sun's enhanced FUV irradiance on the atmospheres of Earth and Mars are also discussed.