Energy Balance in Planetary Thermospheres: A Focus on Earth and Venus

Abstract

The goal of this dissertation is to help understand how the energy balance at Venus, Earth and Mars create the different planetary atmospheres that are observed today. This is started by using an existing ionosphere-thermosphere model developed for Earth, the ac{GITM} and branching this model off to create a version for Venus. Throughout the use of GITM and development of the ac{V-GITM}, there has been an improved understanding of the physics that go into each atmosphere.

At the beginning of this thesis, it is understood that physics-based models attempt to predict densities at Earth to assist in orbit propagation. These models have a great deal of uncertainty, including model biases and model misrepresentation of the atmospheric response to energy input. These may stem from inaccurate approximations of terms in the Navier-Stokes equations, unmodeled physics, incorrect boundary conditions, or incorrect parameterizations. This work shows the effectiveness of using the retrospective cost model refinement (RCMR) technique at removing model bias caused by various sources within ac{GITM}. Numerical experiments, Challenging Minisatellite Payload (CHAMP) and Gravity Recovery and Climate Experiment (GRACE) data during real events are used to show that RCMR can compensate for model bias caused by both inaccurate parameterizations and drivers. RCMR is used to show that eliminating model bias before a storm allows for more accurate predictions throughout the storm.

Secondly, Venus Global Ionosphere Thermosphere Model (V-GITM) is introduced which incorporates the terrestrial GITM framework with Venus-specific parameters, ion-neutral chemistry, and radiative processes in order to simulate some of the observable features regarding the temperatures, composition, and dynamical structure of the Venus atmosphere from 70 km to 170 km. Atmospheric processes are included based upon formulations used in previous Venus GCMs, several augmentations exist, such as improved horizontal and vertical momentum equations and tracking exothermic chemistry. Explicitly solving the momentum equations allows for the exploration of its dynamical effects on the day-night structure. In addition, V-GITM's use of exothermic chemistry instead of a strong heating efficiency accounts for the heating due to the solar EUV while producing comparable temperatures to empirical models. V-GITM neutral temperatures and neutral-ion densities are compared to upper atmosphere measurements obtained from Pioneer Venus and Venus Express. V-GITM demonstrates asymmetric horizontal wind velocities through the cloud tops to the middle thermosphere and explains the mechanisms for sustaining the wind structure. In addition, V-GITM produces reasonable dayside ion densities and shows that the neutral winds can carry the ions to the nightside via an experiment advecting O$_2^+$.

However, the results produced by V-GITM contained uncertainties stemming from the treatment of the internal physics and parameterizations. Recognizing that the model drivers have imperfections gives an opportunity to vary these terms and evaluate the response. In this study, the Venus Global Ionosphere–Thermosphere Model (V-GITM) modifies the implementations of the solar EUV, solar near IR, eddy diffusion, radiative cooling in the lower thermosphere, and thermal conduction to determine the impact on globally averaged temperatures. It is found that among these, uncertainty in the eddy diffusion coefficient and solar EUV most strongly translate to uncertainty in the temperature and density results. In addition, variations in the eddy diffusion coefficient are shown to result in significant uncertainty in the thermospheric composition and height of Venus' transition zone.