We present and analyze Far Ultraviolet Spectroscopic Explorer ( FUSE ) observations of six solar analogs .
These are single , main-sequence G0–5 stars selected as proxies for the Sun at several stages of its main-sequence lifetime from \sim 130 Myr up to \sim 9 Gyr .
The emission features in the FUSE 920–1180 Å wavelength range allow for a critical probe of the hot plasma over three decades in temperature : \sim 10 ^ { 4 } K for the H i Lyman series to \sim 6 \cdot 10 ^ { 6 } K for the coronal Fe xviii \lambda 975 line .
Using the flux ratio C iii \lambda 1176/ \lambda 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 \sim 10 ^ { 5 } -K plasma decreases by a factor of \sim 70 between the young , fast-rotating ( P _ { rot } = 2.7 d ) magnetically active star and the old , slow-rotating ( P _ { rot } \sim 35 d ) inactive star .
Also , we 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 ( \sim 10 ^ { 4 } K ) to hotter coronal ( \sim 10 ^ { 7 } K ) plasma .
The relationship for the integrated ( 920–1180 Å ) 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 .