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Long‐term trends in thermospheric neutral temperature and density above Millstone Hill

Incoherent scatter radar measurements of ionospheric temperature and density collected above Millstone Hill over the years 1976–2013 are analyzed to show the long‐term trends in noontime neutral temperature and neutral O density over the height region 120–500 km. Exospheric temperature cooled by 69.3 ± 6.4 K over the period, an order of magnitude greater than that expected from greenhouse gas action. The O density dropped 0.081 ± 5.6% at 400 km altitude but rose by 36.9 ± 5.0% at 120 km over this period. This trend in density at 400 km agrees with that determined from satellite drag. The increase in density at 120 km counteracts the thermal contraction of the thermosphere expected to be associated with the cooling, resulting in only a small density response at 400 km. The long‐term O density increase at 120 km may be caused by a long‐term descent of the turbopause height of 4.2 km. Such a descent has been documented by a series of rocket mass spectrometer measurements.