Lyman-alpha line intensity as a solar activity index in the far ultraviolet range

The relations between variations of far UV (FUV) emission in 115–210 nm waveband and L 121.6 nm and F_10.7 are studied. The changes of FUV flux are found to lag changes of F_10.7 - as a rule for 1 day. It is shown that such a difference may be caused by two factors: 1) differences between the rates of decrease of local sources' (active regions) brightness in FUV and 10.7 cm; 2) differences between limb-darkening curves for different wavelengths. One may expect the fluxes at different wavelengths to exhibit phase shifts of one relative to another. Cross-correlation analysis reveals no time-delay between emission fluxes within the FUV waveband, in spite of different laws for limb-brightening (darkening) for different spectral intervals. The absence of a phase delay can be caused by relatively small contribution of active regions to the flux of the whole Sun at these wavelengths. Thus the Lyman-alpha line intensity variation reflects variations of Solar FUV emission more precisely than F_10.7. Therefore, using the L intensity for flux intensity calculations of other FUV wavelengths is preferable to using the F_10.7 index.