The current state of investigations regarding the thermospheric midnight temperature maximum (MTM)

Abstract

The thermospheric midnight temperature maximum (MTM) is an upper atmospheric effect found at low latitudes. It is accompanied by an increase in pressure and a signature poleward abatement or reversal in the meridional neutral winds. The MTM exhibits a poleward propagation away from the geographic equator with two secondary maxima developing at approximately ±15° latitude. In this paper, we review early works and recent efforts regarding the MTM. Outstanding questions dealing with seasonal and longitudinal dependencies of the MTM's basic characteristics are discussed. All-sky imaging systems at Arequipa and El Leoncito observed the propagation of 6300 A ̊ airglow enhancements related to the MTM past 35°S latitude. This provides useful information on the upper latitude limit of the MTM. TIEGCM modeling efforts simulate the MTM through upward propagating semi-diurnal tides but have difficulty reproducing accurately its amplitude and occurrence time. It is suggested that the role of the terdiurnal tidal mode may be more important than previously thought. Recent comparative observation and modeling studies of MTM related 6300 A ̊ emission proved unsuccessful. We report that the amplitude of the modeled MTM was not strong enough to instigate the ‘midnight collapse’ of the F-region needed to produce the airglow signature.