On the variability of Lyman-alpha with solar activity

After allowing for changes in instrument calibration and sensitivity, an expression for the relation between the Lyman-alpha irradiance and the solar flux at 10.7 cm is derived. The values deduced from this expression have an accuracy of +- 25%; the irradiance for a perfectly quiet Sun is (2.5 +- 0.5)x1011 photons cm-2 sec-1. The role of Lyman-alpha (121.567 nm) radiation is responsible for the formation of the D region of the ionosphere and is important in the mesospheric photodissociation processes involving O2, CO2, and especially H2O. The intensity of this radiation is of the same order as that of all the other solar radiation at wavelengths less than 150nm. Its strong dependence on the level of solar activity is examined. Since the numbers of observations of Lyman-alpha are much greater than those of other solar radiations, the data available can be subject to a more than usually critical analysis. (U.K.)