Solar luminosity variation. IV. The photospheric lines, 1976--1980

Kitt Peak full disk spectrophotometric records covering the period 1976--1980 have been analyzed to study the behavior of seven spectrum lines sensitive to photospheric parameters. A secular decrease of equivalent widths ranging from 0 to 2.3% is observed. From the lack of correlation with localized, short-lived features like sunspots and plages we conclude that the line weakenings are due to global variations of surface properties. Model atmosphere analysis suggests that the observed response patterns reflects a slight flattening of the lower photospheric temperature gradient, corresponding to a 15% increase in mixing length, at constant, effective temperature. The associated increase in the efficiency of convection can be reconciled with a constant luminosity if the change is assumed to occur only in the outer approx.100 km of the convection zone