EAGER: What is the Cause of Strong Carbon Dioxide 15 Micron Emission in the Mesosphere and Lower Thermosphere? Resolving the Puzzle

This award supports a 2-year investigation into various infrared (IR) emission mechanisms in the Mesosphere and Lower Thermosphere (MLT) region that may be responsible for largely unexplained enhanced IR emission in the CO2 15-μm band above ~90 km. This 15-μm emission is the main cooling mechanism of MLTs of Earth, Venus and Mars. This IR emission is also used to retrieve MLT temperatures by a number of satellite instruments such as SABER/TIMED, MIPAS/ENVUSAT-1, CRISTA/ASTRO-SPAS, and others.

In the MLT region the formation of this radiation is dominated by excitation of CO2 molecules by inelastic collisions with the O(3P) atoms and subsequent emission of the 15-μm quanta. However, this process remains still poorly understood despite many studies over the past several decades. Laboratory measurements at different laboratories applying various techniques have found low values of the CO2+O(3P) reaction rate coefficient.

The use of this low rate in the model calculations does not result in the CO2(ν2) excitation needed to reproduce the strong CO2 15-μm emission observed. In order to fit these observations, the laboratory rate must be increased by a factor of 4. This award is aimed at a search for one or more significant mechanisms of the CO2 excitation that is not accounted for in the current non-local thermodynamic equilibrium (non-LTE) models of the MLT IR emissions.

This award would feature the inclusion of a number of radically different and complex processes of energy transfer to molecular vibrational and rotational states which may be excited in the MLT region.

The new investigation is expected to lead to a more accurate understanding of how this 15-μm IR emission serve as the main IR cooling pathway for the MLT region. The new results would potentially have high impact on the modeling of atmospheric dynamics, chemistry and climate for the MLT region. Additionally, this study will provide a more robust analysis of IR radiances that can be later expanded to other space missions, including those designed to observe Mars, Venus and terrestrial exo-planets.

The award would also examine whether there is a need for significant alteration of the calculations of the IR radiative cooling/heating of MLT in GCMs.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.