CEDAR: Evaluating the Molecular Viscosity Assumption for Gravity Waves in the Upper Thermosphere Using Ground-Based Airglow Observations

This award would determine experimentally what the proper expression should be for the quantity of dynamic molecular viscosity that is used in thermospheric gravity-wave models. The approach is to analyze multiple types of ground-based observations of optical emissions and gravity-wave and upper-atmospheric emission model results. The optical instruments include four all-sky imagers (ASI), three Fabry-Perot interferometers (FPI), and three narrow field-of-view photometers, all located at Arecibo Observatory (AO) and the Culebra Remote Optical Facility (ROF) in Puerto Rico.

Ionospheric observations made by an ionosonde located in Puerto Rico are also to be included. Observations would be made of OI 6300Å and OI 8446Å emissions, both of which are produced in the upper thermosphere, with the peak of OI 8446Å typically 20-70 km above that of OI 6300Å during twilight and nighttime hours. Peak heights for the two emission lines would be determined by applying triangulation to the ASI images from the two ground sites.

The FPIs would provide perturbation vertical wind velocities above the ground sites. The ASIs would provide horizontal gravity-wave phase structure. Gravity-wave periods will be obtained from all three types of optical emission observations.

Background temperature and horizontal wind, required for gravity-wave modeling, would be obtained from FPI observations. The amplitudes of the perturbation vertical velocity wave fields would be determined with the help of a full-wave, fully compressible gravity-wave model including molecular viscosity and thermal diffusion and a photoelectron/photon transport and optical emissions model.

The changes in phase and amplitude of the vertical velocity wave fields going from the 6300Å peak altitude to the 8446Å peak altitude would be compared with gravity-wave model predictions for a range of different dynamic molecular viscosity parameter values to determine the value that best fits the observations. The expected outcome of this research is to resolve an open question raised by Vadas and Crowley as to whether the standard formula for dynamic molecular viscosity gives values that are too large in the upper thermosphere.

The 8446Å FPI at AO would be improved by detector replacement and two ASIs would be added to the labs at AO and the RFO. This research would be integrated into the Science Technical Engineering and Math (STEM) outreach and educational programs at Arecibo Observatory. A large poster describing the work in layman’s terms would be created to describe the research for visitors to the Angel Ramos Foundation Visitor Center at AO.

A public lecture would be provided at that facility during each observing travel visit by Dr. Kerr. The Visitor Center at AO has had more than 100,000 visitors annually, mostly Hispanic K-12 students in formal and informal education programs.

Improved modeling of gravity wave propagation in the low to mid-latitude F-region thermosphere will lead to an improved understanding of ionospheric irregularity triggering. These irregularities cause radio frequency scintillations and disruptions of communication and navigation system

The state of knowledge in the areas of gravity-wave modeling and upper atmospheric physics would be advanced through the application of a novel combination of observations and modeling to an open question about viscosity in the upper thermosphere. Background wind and temperature, needed as inputs to gravity-wave models, will be provided by FPI observations.

Observed changes in both phase and amplitude as gravity waves propagate and dissipate going from one altitude to another in the upper thermosphere will be compared with model predictions. The combination of observations will largely eliminate the need for simplifying assumptions typically present in observational gravity-wave studies, making it possible to answer the above-mentioned open question about viscosity in the upper thermosphere.

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.