Ratios of photoelectron to EUV ionization rates for aeronomic studies


  • P. G. Richards,

  • D. G. Torr


This study reveals that the ratios of the photoelectron to EUV ionization rates are not constant but depend on the degree of attenuation of the solar EUV flux and on the transport of photoelectrons. At high altitudes in the absence of photoelectron transport, the O+ and N2+ ionization rate ratios are about 0.35, but they increase with increasing optical depth to such an extent that in the vicinity of the ionization peak, photoelectron impact ionization is as important as photoionization for O+ and N2+. The O2+ ratio is about half that of O+ at high altitudes and also increases with increasing optical depth but reaches a peak of about 0.4. We present simple formulae which mimic the attenuation behavior of the ionization ratios. Transport effects become important above about 250 km where the ratios vary by a factor of 2 depending on the presence or absence of photoelectrons from the conjugate ionosphere. In addition to the photoelectron to EUV ionization ratios, we present photodissociative branching ratios for O2 and N2. These photodissociative ratios are also a function of the degree of attenuation of the EUV flux. In the region where attenuation is not important, the N+ to N2+ ratio is 0.14, and the O+ to O2+ ratio is 0.22. There is a factor of 2 uncertainty in our calculated ratios on account of uncertainties in the solar EUV flux spectrum and also uncertainties in the electron impact cross sections.