Extreme ultraviolet and X-ray photons from the Sun generate a partially ionized region in the atmosphere of a planet called an ionosphere. Solar activity, including solar flares, coronal mass ejections, and changes in the ionizing irradiance affect planetary ionospheres on short and long timescales. These effects are significant because the ionosphere is a reservoir from which atmospheric species are lost to space, and changes to a planet’s ionosphere have implications for atmospheric loss and planetary habitability over the course of solar system history.
This dissertation aims to fill a gap in our understanding of how the ionospheres of Venus and Mars react to solar activity; namely, how the electron density and ion composition respond to changing solar irradiance in the highly variable region above the ionospheric peak. Using remote sensing observations from Venus Express and Mars Global Surveyor, I find that the observed increases in electron density during periods of high solar irradiance exceed the predictions of photochemical equilibrium theory at high altitudes at both Venus and Mars. I use one-dimensional photochemical equilibrium models to show that the overall expansion of the neutral atmosphere due to increasing neutral temperature is the dominant factor in determining how electron densities in the dayside ionosphere above the ionospheric peak react to changing solar irradiance.
Using in situ observations from Pioneer Venus and MAVEN, I show that the expansion of the neutral atmosphere can also alter the ion composition at both planets. Assessing the observations in a fixed neutral number density frame rather than a fixed altitude frame reveals differences between the planets. At Mars, changes in the ion composition are driven by increasing solar flux. At Venus, changes in the ion composition are driven by changes to the composition of the neutral atmosphere as well as increasing solar flux. Thus, higher solar irradiance increases the electron density and alters the makeup of the ions available to be lost to space. Because the neutral atmosphere is so important to ionospheric behavior, I find that for the best interpretation of ionospheric data, simultaneous in situ observations of the neutral atmosphere should be analyzed wherever possible.
| Identifer | oai:union.ndltd.org:bu.edu/oai:open.bu.edu:2144/43242 |
| Date | 28 October 2021 |
| Creators | Hensley, Kerrin G. |
| Contributors | Withers, Paul |
| Source Sets | Boston University |
| Language | en_US |
| Detected Language | English |
| Type | Thesis/Dissertation |
| Rights | Attribution-NonCommercial 4.0 International, http://creativecommons.org/licenses/by-nc/4.0/ |
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