INTERCOMPARISON OF PHYSICAL MODELS AND OBSERVATIONS OF THE IONOSPHERE

Five physical models of the ionosphere were compared with each other a nd with data obtained at the Millstone Hill Observatory. Two of the mo dels were self-consistent ionosphere-thermosphere models, while for th e other ionospheric models the thermospheric parameters were provided by empirical inputs. The comparisons were restricted to midlatitudes a nd low geomagnetic activity, but four geophysical cases were considere d that covered both the summer and winter solstices at solar maximum a nd minimum. The original motivation of the study was to determine why several physical models consistently underestimated the F region peak electron density, by up to a factor of 2, in the midlatitude, daytime ionosphere at solar maximum. This problem was resolved, but the resolu tion did not identify alack of physics in any of the models. Instead, various chemical reaction rates, photoionization processes, and diffus ion coefficients had to be adjusted, with the main one being the adopt ion of the Burnside factor of 1.7 for the diffusion coefficients. The subsequent comparisons of the models and data were for ''standard'' si mulations in which uncertain inputs or processes were not adjusted to get better agreement with the data. For these comparisons, the five mo dels displayed diurnal variations that, in general, agreed with the me asurements. However, each one of the five models exhibited a clear def iciency in at least one of the four geophysical cases that was not com mon to the other models. Therefore, contrary to expectations, the coup led ionosphere-thermosphere models were not found to be superior to th e uncoupled ionospheric models for the cases considered. The spread in NmF2 calculated by the five models was typically less than a factor o f 2 during the day but was as large as a factor of 10 at certain local times during the night. The latter problem was traced to insufficient nocturnal maintenance processes in two of the uncoupled ionospheric m odels. The general findings of this study have important implications for the National Space Weather Program.