Description
(Text)
This thesis focuses on the investigation of the prevalence of magnetic fields in the ionosphere of Venus, and on their implications and dependence on external factors. Strong ultraviolet radiation from the Sun ionizes the upper atmosphere of Venus, creating a dense ionosphere on the dayside of the planet. In contrast to Earth, the ionosphere of Venus is not protected against the solar wind by a magnetic field. However, the interaction between the ionospheric plasma and the solar wind dynamic and magnetic pressure creates a pseudomagnetosphere which deflects the solar wind flow around the planet. The combination of changing solar radiation and solar wind intensities leads to a highly variable structure and plasma composition of the ionosphere. The instrumentation of the Venus Express spacecraft allows to measure the magnetic field as well as the electron energy spectrum and the ion composition of the upper ionosphere. These observations have been analyzed to determine the positions of the photoelectron boundary (PEB), the induced magnetopause and the bow shock together with their dependence on the solar zenith angle. It is of particular interest to explore the different magnetic states of the ionosphere, since these influence the local plasma conductivity, currents and also the escape of electrons and ions. The penetration of magnetic fields into the ionosphere depends on external conditions as well as on ionospheric properties. The analysis of a large number of orbits allowed for the definition of criteria to distinguish between the so-called magnetized and unmagnetized ionospheric states.
(Extract)
In the visible wavelength regime, Venus shows only an impenetrable cloud layer with very little structure. Many more details in these clouds can be observed in UV light - nowadays not only by professional Earth-bound telescopes and spacecraft, but even by amateur astronomers who use special UV filters in combination with digital sensors. On the exploration and composition of the atmosphere, including the clouds, see Section 1.2.2. Even though Venus is much closer to the Sun, it actually absorbs less radiation than the Earth due to its high albedo (Taylor (2006)). In the past, this led many people to expect Earth-like conditions on its surface. This notion began to change as early as 1922, when John and Nicholson (1922) reported detecting no trace of oxygen or water vapour in the spectrum of Venus. This was confirmed by measurements of Adams and Dunham (1932), who furthermore found strong indications for the presence of carbon dioxide.
