F2 PEAK ELECTRON-DENSITY AT MILLSTONE HILL AND HOBART - COMPARISON OF THEORY AND MEASUREMENT AT SOLAR MAXIMUM

This paper compares the observed behavior of the F2 layer of the ionosphere at Millstone Hill and Hobart with calculations from the field line interhemispheric plasma (FLIP) model for solar maximum, solstice conditions in 1990. During the study period the daily F10.7 index varied by more than a factor of 2 (123 to 280), but the 81-day mean F10.7 (F10.7A) was almost constant near 190. Calculations were performed with and without the effects of vibrationally excited N2 (N2*) which affects the loss rate of atomic oxygen ions. In the case without N2* there is generally good agreement between the model and measurement for the daytime, peak density of the F region (NmF2). Both the model and the measurement show a strong seasonal anomaly with the winter noon densities a factor of 3 to 4 greater than the summer noon densities at Millstone Hill and a factor of 2 greater at Hobart. The seasonal anomaly in the model is caused by changes in the neutral composition as given by the mass spectrometer and incoherent scatter (MSIS) 86 neutral density model. There is generally little or no increase in the observed noon NmF2 as a function of daily F10.7 except at Millstone Hill in winter. In May-July, where the measured NmF2 shows least dependence on daily F10.7, there is excellent agreement between the model and data. The modeled NmF2 is about 30% less than the measured values at Millstone Hill at the December solstice, but both model and data increase with increasing daily F10.7 index. At Hobart, on the other hand, the model densities are greater than or comparable to the measured densities for the December solstice. This suggests that the differences between model and data are not due to the incorrect solar EUV flux. The effect of including N2* is to worsen the agreement between model and data at Millstone Hill by reducing the summer densities from good agreement to 40% below the data. In winter the N2* effects are much smaller, and the densities are reduced by only 10%. While N2* worsens the model-data comparison at Millstone Hill, it does bring the model seasonal density ratio into better agreement with the data and also improves the agreement at Hobart. Although the 1990 daytime ionosphere can be well modeled without N2*, it may still be important for high levels of solar and magnetic activity. There is a very close relationship between the height at which peak density occurs hmF2 variation and the NmF2 variation with F10.7 in summer at Millstone Hill. In contrast to the generally good agreement between model and data at noon, the model badly underestimates the density at night at Millstone Hill at all seasons. At Hobart the model reproduces the nighttime density variations well in both winter and summer. The international reference ionosphere (IRI) model generally provides a good representation of the average behavior of noon NmF2 and hmF2 but because the data show a lot of day-to-day variability, there are often large differences. The FLIP model is able to reproduce this variability when hmF2 is specified. The IRI model peak densities are better than the FLIP densities at night, but the IRI model does not represent the Millstone Hill summer data very well at night in 1990.