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 n m waveband and L(alpha) 121.6 nm and F10.7 are studied. The changes of FUV flux are found to lag changes of F10.7 - as a rule for 1 day. It is shown that such a difference may be caused by two factors: 1) diffe rences 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 differ ent wavelengths to exhibit phase shifts of one relative to another. Cr oss-correlation analysis reveals no time-delay between emission fluxes within the FUV waveband, in spite of different laws for limb-brighten ing (darkening) for different spectral intervals. The absence of a pha se delay can be caused by relatively small contribution of active regi ons to the flux of the whole Sun at these wavelengths. Thus the Lyman- alpha line intensity variation reflects variations of Solar FUV emissi on more precisely than F10.7. Therefore, using the L(alpha) intensity for flux intensity calculations of other FUV wavelengths is preferable to using the F10.7 index.