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dc.contributor.authorGirazian, Zacharyen_US
dc.date.accessioned2016-02-19T15:58:08Z
dc.date.available2016-02-19T15:58:08Z
dc.date.issued2016
dc.identifier.urihttps://hdl.handle.net/2144/14518
dc.description.abstractThe ionospheres of Venus and Mars are important components of the planet-space boundary that play a major role in atmospheric escape processes. Characterization of these regions reveals the physical processes that control them and provides a foundation for more detailed studies of chemistry, dynamics, and energetics. At both planets the ionospheres contain two layers: the main layer, which is formed by photoionization from extreme ultraviolet radiation (EUV, λ<120 nm), and the lower layer, which is formed by photoionization from soft X-rays (SXRs, λ<10 nm) and subsequent electron impact ionization. In this dissertation I investigate how the solar EUV and SXR irradiance controls these layers at Venus and Mars. First, I develop an empirical model of the ultraviolet (UV, λ<190 nm) solar spectrum as a function of F10.7, which is a commonly used proxy of the UV irradiance. I derive power-law relationships between F10.7 and the ionizing irradiance for five neutral species and show that the relationships are nonlinear. These relationships can be used to estimate the EUV irradiance when no solar spectrum measurements are available. Second, I show that the peak electron densities in the ionospheres of Venus and Mars are proportional to the square-root of the ionizing irradiance, which is in contrast to previous studies that have used F10.7 as their representation of the UV irradiance. This finding ameliorates a discrepancy between theory and observations and is in agreement with the prediction that dissociative recombination is the main ion loss mechanism near the ionospheric peaks at Venus and Mars. Third, using a numerical model and electron density profiles from Venus Express, I examine the behavior of the peak altitude, peak density, and morphology of the lower layer at Venus. I show that the peak altitudes and densities in the lower and main layers vary similarly with solar zenith angle (SZA). This implies that neutral and electron thermal gradients at these altitudes vary little with SZA. I also show that, compared to the main layer, the lower layer morphology and peak density varies more over the solar cycle due to the hardening of the solar spectrum.en_US
dc.language.isoen_US
dc.rightsAttribution-NonCommercial-NoDerivatives 4.0 Internationalen_US
dc.rights.urihttp://creativecommons.org/licenses/by-nc-nd/4.0/
dc.subjectAeronomyen_US
dc.subjectEUVen_US
dc.subjectIonosphereen_US
dc.subjectMarsen_US
dc.subjectThermosphereen_US
dc.subjectVenusen_US
dc.titleThe Sun's Influence on the vertical structure of the ionospheres of Venus and Marsen_US
dc.typeThesis/Dissertationen_US
dc.date.updated2016-02-13T02:21:24Z
etd.degree.nameDoctor of Philosophyen_US
etd.degree.leveldoctoralen_US
etd.degree.disciplineAstronomyen_US
etd.degree.grantorBoston Universityen_US


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Attribution-NonCommercial-NoDerivatives 4.0 International
Except where otherwise noted, this item's license is described as Attribution-NonCommercial-NoDerivatives 4.0 International