The midnight temperature maximum in the Earth's equatorial thermosphere

Abstract

Using the neutral atmosphere temperature instrument on AE‐E in circular orbit at 275 km and orbit inclination 19°, we have measured the kinetic temperature of N 2 throughout many orbits, providing for the first time direct in situ measurements of the diurnal temperature variation. In particular we have observed that near midnight the temperature increases substantially, sometimes to values greater than the afternoon maximum. Increases near midnight are observed about one‐half the time measurements are made showing the feature to be a persistent one. The meridional component of the neutral (N 2 ) wind is measured simultaneously by the instrument and correlates well with the temperature maxima showing equatorward motion before midnight and poleward afterward from which we infer a pressure increase and adiabatic heating. The cause of the temperature maximum is not well established but is probably a consequence of the convergence of atmospheric flows from the dayside influenced by momentum coupling of neutral winds and ions.