Decrease inD-region O₂(¹Δg) Photoionization rates resulting from CO₂absorption

Photoionization of the metastable O2 (1Δg) molecule has been proposed as an important source of ionization in the D region. The wavelength region of interest is from 1027 to 1118 A, where ground state O2 is the primary absorber of the incident solar flux but is not ionized itself. This paper presents improved ion production rates based on new laboratory data, including new O2 (1Δg) photoionization cross sections and more detailed curves in several ground state O2 windows. Recent solar flux measurements reduce the continum intensity by about a factor of 5. This reduction is partly compensated by including in the calculations the Si III multiplet, which is at the deepest O2 window (1108.2 A, minimum cross section =4.4×10−21 cm2) and by the new ionization cross sections, which are generally larger than the previously assumed values. An important factor not previously considered is absorption by carbon dioxide, which has a much larger absorption cross section than ground state O2 throughout this region. The ground level mixing ratio has been assumed. The ion production rates are somewhat less than Hunten and McElroy's [1968] curves if CO2 is not included. With CO2, a production rate of 1 ion cm−3 sec−1 is reached at approximately 80, 84, and 86 km for zenith angles of 0°, 45°, and 60°, respectively. Hunten and McElroy find this rate to be 10 to 15 km lower. Concentrations of O2+ are calculated for a zenith angle of 45° for a variety of conditions, including a dry atmosphere and an atmosphere with 5 ppm H2O.