Universität zu Köln

Hartkorn, Oliver, Saur, Joachim ORCID: 0000-0003-1413-1231 and Strobel, Darrell F. (2017). Structure and density of Callisto's atmosphere from a fluid-kinetic model of its ionosphere: Comparison with Hubble Space Telescope and Galileo observations. Icarus, 282. S. 237 - 260. SAN DIEGO: ACADEMIC PRESS INC ELSEVIER SCIENCE. ISSN 1090-2643

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Abstract

We develop a model of the ionospheric electron population of Jupiter's moon Callisto using a prescribed neutral atmosphere composed of O-2, CO2 and H2O. A kinetic description of ionospheric suprathermal electrons coupled with a fluid description of ionospheric thermal electrons is well suited to jointly analyze and interpret observations of electron density and atmospheric UV emission. Accordingly, 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. We assume a stationary balance between local sources and sinks of electrons and electron energy. Electron transport within the ionosphere is neglected, since collision time scales are shorter than transport time scales in the region of Callisto's ionosphere where the major concentrations of electrons is located and the major part of the atmospheric UV emission is generated. We consider photoionization, which is the dominant ionospheric electron source, and secondary ionization from collisions of photoelectrons with neutrals. Our calculations yield electron densities and electron impact generated UV emissions from Callisto's atmosphere. Comparing our modeled UV emission intensities with the Hubble Space Telescope observation of Cunningham et al. (2015), we find that Callisto's atmosphere has a mean O-2 column density of 2.1(-1.1)(+1.1) x 10(19) m(-2). A joint comparison with this HST observation and radio occultation observations of Kliore et al. (2002) shows that Callisto's atmosphere possesses a day night asymmetry. We derive terminator 02 column densities of similar to 0.4 x 10(19) m(-2), for which we find subsolar O-2 column densities in the range of 2.4 - 9.8 x 10(19) m(-2). Our calculations also show that the electron density is very sensitive to the relative abundance of H2O due to the thermal electron cooling by rotational state excitation of H2O. For the efficiency of Callisto's atmospheric UV emission we find that on average one photon is emitted at OI lambda 135.6 nm per every 170 electron ion pairs generated and per every 60 electron ion pairs produced by secondary electron impact ionization. (C) 2016 The Authors. Published by Elsevier Inc.

Item Type:	Journal Article
Creators:	Creators Email ORCID ORCID Put Code Hartkorn, Oliver UNSPECIFIED UNSPECIFIED UNSPECIFIED Saur, Joachim UNSPECIFIED orcid.org/0000-0003-1413-1231 UNSPECIFIED Strobel, Darrell F. UNSPECIFIED UNSPECIFIED UNSPECIFIED
URN:	urn:nbn:de:hbz:38-242502
DOI:	10.1016/j.icarus.2016.09.020
Journal or Publication Title:	Icarus
Volume:	282
Page Range:	S. 237 - 260
Date:	2017
Publisher:	ACADEMIC PRESS INC ELSEVIER SCIENCE
Place of Publication:	SAN DIEGO
ISSN:	1090-2643
Language:	English
Faculty:	Unspecified
Divisions:	Unspecified
Subjects:	no entry
Uncontrolled Keywords:	Keywords Language ELECTRON-DISTRIBUTION FUNCTION; CROSS-SECTIONS; DISSOCIATIVE RECOMBINATION; OXYGEN ATMOSPHERE; IMPACT IONIZATION; ENERGY; WATER; ION; COLLISIONS; EXCITATION Multiple languages Astronomy & Astrophysics Multiple languages
Refereed:	Yes
URI:	http://kups.ub.uni-koeln.de/id/eprint/24250