CEDAR: Comprehensive Use of Incoherent Scatter Radar Data to Study the Equatorial Midnight Plasma and Neutral Temperature Maxima (MTM)

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This project will investigate the midnight temperature maximum (MTM), an enhancement of the neutral temperature (Tn) of roughly 50-200 K in the nighttime equatorial thermosphere. The phenomenon is created by the convergence of air near midnight at low latitudes. Current physics-based numerical models are not able to reproduce the MTM phenomenon with the amplitudes commonly observed. The implication of this inability indicates that the ion-neutral coupling between the low-latitude F-region plasma and the thermosphere and the influence of tidal forcing from below are not well understood. This project will investigate the MTM feature using ion temperature data obtained with Incoherent Scatter Radars (ISRs) from the CEDAR, Madrigal, and local databases. Specifically, since the MTM is an equatorial process, emphasis is placed on MTM characterization using data from the low-latitude Arecibo and Jicamarca ISRs. The ISR MTM will be characterized in terms of amplitude, time of occurrence, and temporal duration. A preliminary count indicates that there are ~400 days of Arecibo and Jicamarca data available for the study, distributed in a way that will allow the determination of seasonal and solar activity variations of the MTM. A ground-based climatology of MTM occurrence has been obtained with Fabry-Perot interferometers (FPIs) at Arequipa, but this climatology does not include the austral summer months (November to early March) due to cloudy conditions. The current project fills this gap with ISR ion temperature data from Jicamarca and expands the climatology to Arecibo's latitude. FPIs provide height-integrated optical information and so provide no information on the height dependence of the MTM. Radars, on the other hand, do provide results as a function of altitude. A systematic study of the height dependence of the MTM is one of the main goals of this project. Knowledge of this height dependence is crucial to any attempt to model, for example, the height penetration of tidal modes, so, as an additional goal, model comparisons of MTM characteristics will be carried out. An additional objective is the comparison of the radar data with optical (FPI and all-sky imager) and digisonde data available at Arecibo and near Jicamarca. This effort provides the first multi-diagnostic study of the MTM. All-sky imager data at El Leoncito (31.8 S) and Millstone Hill ISR (42.6 N) data will be used to study the latitudinal extent of the equatorial MTM phenomenon. The scientific question addressed in this project is: can a multi-diagnostic approach established around the temporal, altitudinal and latitudinal information provided by ISRs improve our knowledge of the MTM?