Abstract
Recent estimates of the O+–O collision cross section QO+–O have suggested that the commonly used value of Banks [1966b] is too low by a factor of 1.2–1.9. On the other hand, past deductions of the F region neutral atomic oxygen density [O] from incoherent scatter (IS) radar studies have been quite compatible with the original Banks value, and any serious adjustment of that value would cause serious disagreement between these radar [O] results and those of the mass spectrometer incoherent scatter (MSIS) model. We have derived the mean daytime value for the product of QO+–O and [O] for 153 days during the period 1970–1975 from Millstone Hill IS radar measurements of ionospheric temperature and density. If we assume that MSIS model [O] is correct on average over this period, we find that QO+–O is only 75% of the Banks value and 40 to 60% lower than other values recently proposed. We show that all previous derivations of [O] from IS radar energy‐budget studies are compatible with our value, and thus a discrepancy exists in QO+–O determination by different methods. We could make our QO+–O results compatible with the larger values noted above by allowing a neutral temperature gradient to exist at high altitudes, where none is currently expected, or by increasing the heat input to the ion gas. We note that both of these effects could be produced by a sufficient reservoir of hot oxygen in the topside of the ionosphere. Independent of the way in which we choose a value of QO+–O to make the radar and MSIS [O] values agree on average over our 6‐year span of data, the radar record shows [O] to have episodic departures from MSIS predictions reaching amplitudes of up to 50% and lasting for periods of several months. This argues for caution in analyses which depend crucially upon the validity of MSIS densities during campaign‐like experimental studies.
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