Evolution of the Solar Activity over Time and Effects on Planetary Atmospheres: I. High-energy Irradiances (1–1700 A)

We report on the results of the Sun in Time multi-wavelength program (X-rays to the UV) of solar analogs with ages covering ∼0.1–7 Gyr. The chief science goals are to study the solar magnetic dynamo and to determine the radiative and magnetic properties of the Sun during its evolution across the main sequence. The present paper focuses on the latter goal, which has the ultimate purpose of providing the spectral irradiance evolution of solar-type stars to be used in the study and modeling of planetary atmospheres. The results from the Sun in Time program suggest that the coronal X-ray–EUV emissions of the young main-sequence Sun were ∼100– 1000 times stronger than those of the present Sun. Similarly, the transition region and chromospheric FUV–UV emissions of the young Sun are expected to be 20–60 and 10–20 times stronger, respectively, than at present. When considering the integrated high-energy emission from 1 to 1200 A the resulting relationship indicates that the solar high-energy flux was about 2.5 times the present value 2.5 Gyr ago and about 6 times the present value about 3.5 Gyr ago (when life supposedly arose on Earth). The strong radiation emissions inferred should have had major influences on the thermal structure, photochemistry, and photoionization of planetary atmospheres and also played an important role in the development of primitive life in the Solar System. Some examples of the application of the Sun in Time results on exoplanets and on early Solar System planets are discussed. Subject headings: stars: late-type — stars: chromospheres — stars: coronae — stars: activity — Sun: evolution — solar-terrestrial relations