We test the method of Lockwood et al. [1999> for deriving the coronal source flux from the geomagnetic aa index and show it to be accurate to within 12% for annual means and 4.5% for averages over a sunspot cycle. Using data from four solar constant monitors during 1981--1995, we find a linear relationship between this magnetic flux and the total solar irradiance. From this correlation, we show that the 131% rise in the mean coronal source field over the interval 1901--1995 corresponds to a rise in the average total solar irradiance of Δl=1.65¿0.23 W m-2. ¿ 1999 American Geophysical Union