Measurements of the ion-neutral collision frequencies with the EISCAT UHF incoherent scatter radar in the high-latitude, lower E region (90–105 km) show that for a time period of 2 weeks (at the beginning of February 1984) under different geophysical conditions the collision frequencies change little and compare well with a model calculated from the CIRA (1972) neutral atmosphere. When the ion and electron temperatures are approximately equal, it is confirmed that the measured collision frequency is consistent with the induced dipole collision frequency and an atmosphere in static equilibrium. One event at another time where collision frequencies increase by a factor of 1.5–2 over a period of 1½ hr could be due to neutral density enhancements from the passage of a large-amplitude, long-period gravity wave.
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