The variation of the flux ratios is close to those of Ribas et al. who obtained power-law fits of 3.10t~°'72 and 6.40t~1'23 for the scaled Lya and XUV (1-1200 A) ratios, respectively, at 1 AU. Similarly, Wood et al. (2005) found for G V and F V stars a power-law fit of Lya flux at the stellar surface x P_1-°9 ±° °8, which when combined with the age-rotation relationship P <x t° 6 gives Lya flux oct°-65 ±°-12.
5.5.5 Solar XUV and Ly-a emission flux evolution
The method can be applied to calculate the evolution of the solar XUV and Ly-a emission flux with age. In Fig. 5.10 is given the evolution with age of the solar B-V, radius and effective temperature, based on the model for a 1 solar mass star and metallicity (Z© = 0.019) of the sun. The evolution of stellar B-V with age can be used with eqn (5.24) and eqn (5.28) to calculate the evolution of the rotation period and Rossby number with age, respectively. The variation with stellar age of the XUV and Ly-a emission flux at a planetary distance of r measured in AU from a star, relative to present-day mean solar values at 1 AU, can be calculated from
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