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, it is concluded that the line weakenings are due to global variations of surface properties. Model atmosphere analysis suggests that the observed response pattern 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 100 km of the convection zone.