The aeronomic dissociation of water vapor by solar H Lyman α radiation

Abstract

The photoabsorption cross sections of molecular oxygen and water vapor have been measured as a function of temperature at 0.1 Å intervals between 1214 and 1219 Å. Over the width of the solar Lyman α line there is no fine structure apparent in the cross sections which were measured with 0.05 Å resolution. A large wavelength and temperature dependence is seen in the molecular oxygen cross sections, but the variations are much smaller in the case of water vapor. The measured cross sections are used in calculations of the rate of photodissociation of atmospheric water vapor by solar Lyman α radiation, taking full account of both the temperature and wavelength dependences. It is found that the photodissociation rates may be accurately reproduced at altitudes above those where Lyman α ceases to dominate the dissociation by using molecular oxygen and water vapor cross sections corresponding to fixed temperatures near 200 K. Analytical models for these cross sections are presented for use in photochemical calculations.