Long‐term drift of the coronal source magnetic flux and the total solar irradiance

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 Δ I = 1.65±0.23 Wm −2 .