Universität zu Köln

Peter, Kerstin (2018). Small scale disturbances in the lower dayside ionosphere of Mars as seen by the MaRS radio science experiment on Mars Express. PhD thesis, Universität zu Köln.

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Abstract

The lower dayside ionosphere of Mars consists of two major features, the main ionospheric layer M2 and the lower and weaker M1 region. In 2005, Pätzold et al. (2005} discovered a local and sporadic third layer below the established two layered structure in the observations of the Mars Express MaRS radio science experiment. Radio science experiments can only sense the electron density distribution but not the ions, therefore the origin and composition of the newly discovered features remained unknown. Until today, no in-situ observations of the atmospheric and ionospheric composition have been conducted in the altitude region between 70 and 110 km. Models predict a possible meteoric origin of these layers due to the ablation and ionization of meteoric magnesium. This work focuses on the origin of the small scale sporadic electron density structures merged with the main ionospheric body (Mm). Potential sources for the excess electron densities are i.) the interaction of short solar X-ray radiation with the planetary upper mesosphere and thermosphere, ii.) the influx and ionization of meteoric material, iii.) internal gravity waves in the neutral atmosphere at mesospheric / thermospheric heights and iv.) the influx of solar energetic particles. The focus of this work is on a potential correlation between the merged excess electron densities and the short solar X-ray interaction with the planetary neutral atmosphere. More than 10 years of Mars Express MaRS radio science observations (2004 - 2014) provide a profound data base for this exploration. Statistical correlations between the occurrence rate/characteristics of the identified Mm and the observational and environmental parameters of the observation provide information about potential formation processes. Approximately 44 % of the investigated observations contain pronounced merged excess electron densities. They are found for all solar zenith angles of the ionospheric dayside accessible with the MaRS data set (50° to 90°) in approximately 70 to 110 km altitude. The bulk of Mm has been identified over regions with low crustal magnetic fields, which makes the wind-shear theory (sporadic E layer formation on Earth) an unlikely origin. The merged excess electron densities show a positive correlation with the Sun's activity and their base is on average found deeper in the atmosphere, than the base of the averaged undisturbed MaRS electron density profiles. This indicates a dependence of the formation process on energy sources which penetrate deep into the atmosphere. IonA-2 is a 1-D time-marching photochemical transport model with an implemented diurnal cycle. It has been developed to investigate the potential Mm formation by solar X-ray and explore the effect of a changing neutral atmosphere on the Mm composition. The characteristics of the potential Mm derived from the IonA-2 model electron density depend directly on the solar radiation below 1.5 nm. They show good agreement with the general V-shape structure of the observed Mm, and with regard to altitude range, maximum electron density and total vertical electron content. An increase/decrease in solar flux < 1.5 nm yields a prompt (in minutes) increase/decrease of the potential Mm, which might partially explain the sporadic appearances of the Mm structure in the MaRS observations. The higher the NO+/O2+ ratio of a potential merged excess electron density, the longer is its lifetime during a decrease in short solar X-ray. IonA-2 shows, that short solar X-ray which ionizes the local neutral atmosphere provides a satisfying explanation for the identified V-shaped merged excess electron densities in the MaRS observations. This result is in contrast to previous model predictions who assumed a meteoric origin of the sporadic layers. However, other potential sources might provide an additional electron density contribution (meteoric Mg+), additional substructures (gravity waves) or strong Mm enhancements with large disturbances in the ionosphere (SEPs). The sporadic occurrence of the merged excess electron densities in the MaRS observations is assumed to be a combination of observational (increased observation noise level, shift of the lower baseline by ionospheric deviations from radial symmetry) and environmental (solar flux variation, changes in the neutral atmospheric composition) factors.

Item Type:	Thesis (PhD thesis)
Creators:	Creators Email ORCID ORCID Put Code Peter, Kerstin kerstin.peter@uni-koeln.de UNSPECIFIED UNSPECIFIED
URN:	urn:nbn:de:hbz:38-81108
Date:	8 January 2018
Language:	English
Faculty:	Faculty of Mathematics and Natural Sciences
Divisions:	Ehemalige Fakultäten, Institute, Seminare > Faculty of Mathematics and Natural Sciences > no entry
Subjects:	Physics Earth sciences
Uncontrolled Keywords:	Keywords Language Mars; ionosphere; lower ionosphere; meteor layers; small scale disturbances English
Date of oral exam:	26 February 2018
Referee:	Name Academic Title Pätzold, Martin Priv. Doz. Dr. Eckart, Andreas Prof. Dr.
Refereed:	Yes
URI:	http://kups.ub.uni-koeln.de/id/eprint/8110