Universität zu Köln

Roth, Lorenz ORCID: 0000-0003-0554-4691, Saur, Joachim ORCID: 0000-0003-1413-1231, Retherford, Kurt D., Strobel, Darrell F., Feldman, Paul D., McGrath, Melissa A., Spencer, John R., Bloecker, Aljona and Ivchenko, Nickolay ORCID: 0000-0003-2422-5426 (2016). Europa's far ultraviolet oxygen aurora from a comprehensive set of HST observations. J. Geophys. Res-Space Phys., 121 (3). S. 2143 - 2171. WASHINGTON: AMER GEOPHYSICAL UNION. ISSN 2169-9402

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Abstract

We analyze a large set of far ultraviolet oxygen aurora images of Europa's atmosphere taken by Hubble's Space Telescope Imaging Spectrograph (HST/STIS) in 1999 and on 19 occasions between 2012 and 2015. We find that both brightness and aurora morphology undergo systematic variations correlated to the periodically changing plasma environment. The time variable morphology seems to be strongly affected by Europa's interaction with the magnetospheric plasma. The brightest emissions are often found in the polar region where the ambient Jovian magnetic field line is normal to Europa's disk. Near the equator, where bright spots are found at Io, Europa's aurora is faint suggesting a general difference in how the plasma interaction shapes the aurora at Io and Europa. The dusk side is consistently brighter than the dawnside with only few exceptions, which cannot be readily explained by obvious plasma physical or known atmospheric effects. Brightness ratios of the near-surface OI] 1356 angstrom to OI 1304 angstrom emissions between 1.5 and 2.8 with a mean ratio of 2.0 are measured, confirming that Europa's bound atmosphere is dominated by O-2. The 1356/1304 ratio decreases with increasing altitude in agreement with a more extended atomic O corona, but O-2 prevails at least up to altitudes of approximate to 900 km. Differing 1356/1304 line ratios on the plasma upstream and downstream hemispheres are explained by a differing O mixing ratio in the near-surface O-2 atmosphere of approximate to 5% (upstream) and less than or similar to 1% (downstream), respectively. During several eclipse observations, the aurora does not reveal any signs of systematic changes compared to the sunlit images suggesting no or only weak influence of sunlight on the aurora and an optically thin atmosphere.

Item Type:	Journal Article
Creators:	Creators Email ORCID ORCID Put Code Roth, Lorenz UNSPECIFIED orcid.org/0000-0003-0554-4691 UNSPECIFIED Saur, Joachim UNSPECIFIED orcid.org/0000-0003-1413-1231 UNSPECIFIED Retherford, Kurt D. UNSPECIFIED UNSPECIFIED UNSPECIFIED Strobel, Darrell F. UNSPECIFIED UNSPECIFIED UNSPECIFIED Feldman, Paul D. UNSPECIFIED UNSPECIFIED UNSPECIFIED McGrath, Melissa A. UNSPECIFIED UNSPECIFIED UNSPECIFIED Spencer, John R. UNSPECIFIED UNSPECIFIED UNSPECIFIED Bloecker, Aljona UNSPECIFIED UNSPECIFIED UNSPECIFIED Ivchenko, Nickolay UNSPECIFIED orcid.org/0000-0003-2422-5426 UNSPECIFIED
URN:	urn:nbn:de:hbz:38-282080
DOI:	10.1002/2015JA022073
Journal or Publication Title:	J. Geophys. Res-Space Phys.
Volume:	121
Number:	3
Page Range:	S. 2143 - 2171
Date:	2016
Publisher:	AMER GEOPHYSICAL UNION
Place of Publication:	WASHINGTON
ISSN:	2169-9402
Language:	English
Faculty:	Unspecified
Divisions:	Unspecified
Subjects:	no entry
Uncontrolled Keywords:	Keywords Language EXCITATION CROSS-SECTIONS; IO PLASMA TORUS; ATOMIC OXYGEN; JOVIAN MAGNETOSPHERE; GALILEAN SATELLITES; SUBSURFACE OCEANS; WATER-VAPOR; GAS TORUS; ATMOSPHERE; CONSTRAINTS Multiple languages Astronomy & Astrophysics Multiple languages
Refereed:	Yes
URI:	http://kups.ub.uni-koeln.de/id/eprint/28208