MODEL RESULTS FOR THE IONOSPHERIC E-REGION - SOLAR AND SEASONAL-CHANGES

A new, empirical model for NO densities is developed, to include physi cally reasonable variations with local time, season, latitude and sola r cycle. Model calculations making full allowance for secondary produc tion, and ionising radiations at wavelengths down to 25 Angstrom, then give values for the peak density N(m)E that are only 6% below the emp irical IRI values for summer conditions at solar minimum. At solar max imum the difference increases to 16%. Solar-cycle changes in the EUVAC radiation model seem insufficient to explain the observed changes in N(m)E, with any reasonable modifications to current atmospheric consta nts. Hinteregger radiations give the correct change, with results that are just 2% below the IRI values throughout the solar cycle, but give too little ionisation in the E-F valley region. To match the observed solar increase in N(m)E, the high-flux reference spectrum in the EUVA C model needs an overall increase of about 20% (or 33% if the change i s confined to the less well defined radiations at lambda, < 150 Angstr om). Observed values of NmE show a seasonal anomaly, at mid-latitudes, with densities about 10% higher in winter than in summer (for a const ant solar zenith angle). Composition changes in the MSIS86 atmospheric model produce a summer-to-winter change in N(m)E of about -2% in the northern hemisphere, and +3% in the southern hemisphere. Seasonal chan ges in NO produce an additional increase of about 5% in winter, near s olar minimum, to give an overall seasonal anomaly of 8% in the souther n hemisphere. Near solar maximum, reported NO densities suggest a much smaller seasonal change that is insufficient to produce any winter in crease in N(m)E. Other mechanisms, such as the effects of winds or ele ctric fields, seem inadequate to explain the observed change in N(m)E. It therefore seems possible that current satellite data may underesti mate the mean seasonal variation in NO near solar maximum. A not unrea sonable change in the data, to give the same 2:1 variation as at solar minimum, can produce a seasonal anomaly in N(m)E that accounts for 35 -70% of the observed effect at all times.