Estimation of the solar Lyman alpha flux from ground based measurements of the Ca II K line

Measurements of the solar Lyman alpha and CaII K from October 1981 to April 1989 show a strong correlation (r=.95) that allows estimation of the Lyman alpha flux at 1 AU from 1975 to December 1989. Our estimated Lyman alpha strength of 3.9 × 10 11 ±.15 × 10 11 photons/s cm 2 on 7 December 1989 is at the same maximum levels seen in Cycle 21. Relative to other UV surrogates (sunspot number, 10.7 cm radio flux, and HeI 10830 line strength), Lyman alpha estimates computed from the K line track the SME measurements well from solar maximum, through solar minimum, and into Cycle 22.