The midnight temperature maximum from Arecibo incoherent scatter radar ion temperature measurements

Abstract

The midnight temperature maximum (MTM) is studied using ion temperature data from the incoherent scatter radar at the Arecibo Observatory (18.3°N, 66.2°W). The MTM is characterized by fitting the radar data with a function that takes into account diurnal, semidiurnal and terdiurnal components. Under the hypothesis that the MTM is related to the amplification of the terdiurnal wave, a Gaussian amplification window is included in the fitting to automatically determine the time of occurrence, temporal duration, and amplitude of the MTM observed. This study focused initially on altitudes near 300km, the typical height of MTM observations from Fabry Perot Interferometers (FPIs). Out of the 491 days available between 1967 and 2010 only 82 showed reliable temperature determination throughout the night at this typically bottomside altitude of often low density and sharp density gradient. The analysis was expanded to include 229 usable nights at heights close to 330km and 367km, where better conditions for temperature determination exist. Most of these nights showed an MTM with amplitudes between 20 and 150K and peak occurrence times during local summer months. The seasonal dependence of MTM parameters is also investigated and compared with previous experimental and modeling studies.