A Review of the Experimental and Modeling Studies of the Midnight Temperature Maximum Phenomenon

Abstract:

The midnight temperature maximum (MTM) phenomenon is a prominent feature of low-latitude thermospheric dynamics but appears oftentimes at mid-latitudes as well. From the experimental standpoint based upon Fabry-Perot interferometer temperatures or incoherent scatter radar ion temperatures, the primary MTM peak features a nighttime enhancement as much as 150 K that is observed in the post-midnight hours. Also observed generally within the same night at mid-latitude locations by the FPI instrument is a secondary MTM peak that is seen one or two hours after evening twilight. Its amplitude is generally of order of 20 to 40 K. The local time separation of both peaks is about 5 to 6 hrs. The times of both peaks are seen to occur later within the night for higher latitude locations. Thus, the secondary peak may exist in the low-latitude thermosphere dynamics but is not seen experimentally by the FPI because the time of appearance lies within the daytime or twilight illuminated thermosphere, which is when the background continuum is too bright for the peak detection. The FPI measurements of the thermosphere horizontal winds show an abatement of the meridional wind or even a reversal of direction from equatorwards to poleward coincident with the MTM peak. Also coincident with the MTM peak is a significant brightening of the 630-nm nightglow emission that is associated with the downward descent of the F-layer by 50 to 100 km. High resolution WAM and TIEGCM modeling have both been successful in developing a simulation of the primary MTM peak but only if a tidal blend model is adopted that includes the terdiurnal and other higher order modes of tidal wave forcing. Several questions remain concerning the climatology and phenomenology of the MTM phenomenon, and the advent of the ICON satellite observations of nighttime thermospheric dynamics above 210 km should be quite helpful in extending our understanding of this phenomenon. Because the tidal wave forcing is inherently variable in amplitude and occurrence, considerable variability in the nighttime low latitude thermospheric dynamics can be expected to occur affecting in turn other thermospheric quantities such as composition as well as possibly modifying the coupling of these tidal winds to high frequency neutral wave disturbances (i.e. gravity waves).