Universität zu Köln

Hartkorn, Oliver (2017). Modeling Callisto's Ionosphere, Airglow and Magnetic Field Environment. PhD thesis, Universität zu Köln.

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Abstract

Previous observations of the Galileo spacecraft and the Hubble Space Telescope indicate that Callisto possesses a neutral atmosphere which is mostly composed of O2 and additionally contains H2O and CO2. The first aim of our study is to constrain density and structure of the atmospheric O2. Based on existent observations and findings, we construct a phenomenological model of Callisto's atmosphere. Then, we use this atmosphere model as input information for an ionosphere model, which is based on physical principles and has been developed by us specifically for Callisto. Using this coupled description of Callisto's atmosphere-ionosphere system, we calculate the spatial distribution of ionospheric electron densities and the atmospheric ultraviolet emission, i.e., airglow. By varying the prescribed O2 atmosphere and comparing calculated electron densities with Galileo radio occultation measurements and calculated UV emission intensities with Hubble Space Telescope observations, we are able to constrain density and structure of Callisto's O2 atmosphere. We find an average O2 column density of 2.1 (+/-1.1) x 10^{19} 1/m^2 and a likely day-night asymmetry of the O2 atmosphere. In the framework of our ionosphere model we calculate the electron energy distribution function at each point in the ionosphere by solving a coupled set of equations consisting of the Boltzmann equation for suprathermal electrons and the continuity and energy equation for thermal electrons. Since we can neglect electron transport for our purposes, we assume a stationary balance between local sources and sinks of electrons and electron energy. Photoionization is expected to be the major source of ionospheric electrons at Callisto. Therefore, our model includes photoionization and secondary ionization processes from collisions of photoelectrons with neutrals. Using our ionosphere model, we also investigate the formation process of Callisto's O2 atmosphere. Atmospheric O2 is most likely generated by surface sputtering and sublimation. Assuming that surface sputtering is the main source and causes an orbital phase dependent atmospheric O2 density, we predict atmospheric UV emission intensities for different orbital phases of Callisto. These predictions can be used by other scientists to interpret telescope observations of Callisto regarding the question about the origin of Callisto's atmosphere. Further, we wonder whether electromagnetic induction within Callisto’s ionosphere can explain observed magnetic field disturbances that have been interpreted as evidence for a subsurface ocean. The rotation of Jupiter’s magnetic field causes a periodically time varying magnetic field in the rest frame of Callisto, which induces currents within Callisto’s ionosphere. We derive the conductivity structure of Callisto’s ionosphere from our ionosphere model and simulate this induction process. From analytic considerations, we expect a nearly perfect Cowling channel in Callisto’s ionosphere and, hence, only a weak continuation of ionospheric currents in the surrounding magnetospheric plasma. Based on these findings, we construct a detailed numerical model to calculate the induced currents and according secondary magnetic fields quantitatively. We compare our results with the magnetic field measurements from the Galileo flybys C-3 and C-9, during which magnetic field disturbances have been observed that are diagnostic for induction in a conductive spherical shell. Our model results show that induction within Callisto’s ionosphere is an important and non-negligible process that is responsible for a major part of the observed magnetic field disturbances. Due to the present model-uncertainties regarding Callisto's ionosphere, we can not rule out the existence of an conductive subsurface layer like a subsurface ocean. However, if properties of such a subsurface layer are derived from future observations, for example, observations of the JUpiter ICy moon Explorer (JUICE) spacecraft, a consideration of induction in the ionosphere is mandatory.

Item Type:	Thesis (PhD thesis)
Creators:	Creators Email ORCID ORCID Put Code Hartkorn, Oliver oliverhartkorn@gmail.com UNSPECIFIED UNSPECIFIED
URN:	urn:nbn:de:hbz:38-76836
Date:	10 July 2017
Language:	English
Faculty:	Faculty of Mathematics and Natural Sciences
Divisions:	Faculty of Mathematics and Natural Sciences > Department of Geosciences > Institute for Geophysics and Meteorology
Subjects:	Natural sciences and mathematics Physics Earth sciences
Uncontrolled Keywords:	Keywords Language Callisto English Subsurface ocean English Planetary atmospheres English
Date of oral exam:	26 June 2017
Referee:	Name Academic Title Saur, Joachim Prof. Dr. Tezkan, Bülent Prof. Dr.
Refereed:	Yes
URI:	http://kups.ub.uni-koeln.de/id/eprint/7683