Comparative planetary X-ray emissions

Lead Research Organisation: University of Leicester
Department Name: Physics and Astronomy

Abstract

This programme of research includes data analysis and theoretical modelling of X-ray emissions from diverse planetary space environments. We will use space-based telescopes to observe Jupiter and Saturn in the X-ray (using the XMM/Newton and Chandra X-ray Observatory), and compare them with emissions measured in the ultraviolet (using the Hubble Space Telescope). These data will increase our understanding of the processes which produce both 'auroral' and 'non-auroral' planetary X-ray emissions in general, and will allow direct investigation of the relationship between the X-ray and ultraviolet emissions at Jupiter and Saturn. Studies of this nature allow an advanced understanding of the physical mechanisms which produce the emissions, and hence lead to a greater understanding of astrophysical plasmas in general. Planetary X-ray (and UV) signatures are an intriguing manifestation of the interaction between the body in question with its surrounding plasma environment, and as such offer important clues as to the nature of the interaction between the solar wind, the magnetosphere, and the upper atmosphere and/or the surface of the planet. We will develop existing theoretical models for Jupiter and Saturn, and compare the results of these models with the X-ray and ultraviolet observations. This comparative planetary science will enable the development of a new research programme on the subject of Mercury in preparation for the BepiColombo mission (the Space Research Centre will build the X-ray spectrometer - MIXS). We will begin our studies of Mercury by re-analysing the Mariner-10 data, develop theoretical models of the current systems which exist, and estimate the X-ray flux associated with the solar wind interaction with Mercury's magnetosphere. We will analyse the new data from the NASA Messenger mission to Mercury, which will become available during the final year of this grant. Studies of this nature are essential to allow full understanding and exploitation of the data from the MIXS experiment at Mercury.

Publications

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Arridge C (2008) Saturn's magnetodisc current sheet in Journal of Geophysical Research: Space Physics

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Bunce E (2007) Cassini observations of the variation of Saturn's ring current parameters with system size in Journal of Geophysical Research: Space Physics

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Bunce E (2008) Origin of Saturn's aurora: Simultaneous observations by Cassini and the Hubble Space Telescope in Journal of Geophysical Research: Space Physics

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Hubert B (2008) Open magnetic flux and magnetic flux closure during sawtooth events in Geophysical Research Letters

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Jackman C (2008) A multi-instrument view of tail reconnection at Saturn in Journal of Geophysical Research: Space Physics

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Kellett S (2009) Thickness of Saturn's ring current determined from north-south Cassini passes through the current layer in Journal of Geophysical Research: Space Physics

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Kellett S (2010) Nature of the ring current in Saturn's dayside magnetosphere in Journal of Geophysical Research: Space Physics

 
Description We are interested in comparing the similarities and differences associated with planetary auroral processes which are exhibited throughout the solar system. We have made a significant contribution to the progress made by the Cassini mission, notably through our involvement in the magnetometer team and through successful collaboration with other instrument teams. We have examined the residual magnetic field vectors due to the ring current in Saturn's magnetosphere and fit them to a simple model [Bunce et al., 2007]. The magnetic effect of the ring current acts to distend field lines away from Saturn, and thus alters the magnetic mapping of spacecraft position to the ionosphere. Examining the variation of the fitted ring current parameters and secondary parameters such as total current and magnetic moment with changing size of the magnetosphere shows that the derived parameters increase significantly with system size, due principally to the increasing radius of the outer edge of the ring. Bunce et al.(2008a) have also shown the middle magnetosphere field to be quasi-dipolar in form when the magnetosphere is strongly compressed, but extends into a magnetodisc when it is strongly expanded (contrary to previous understanding). We also find that the region occupied by the modelled ring current corresponds to an essentially fixed shell of field lines that expands and contracts with the size of the system, thus mapping to an almost fixed co-latitude range in Saturn's ionosphere, between ~16° and ~22° in the southern hemisphere. The median dayside UV auroral oval at 14-16° is therefore found to map from near the poleward edge of the modelled ring current towards the boundary of open field lines at smaller co-latitudes.
During the high-latitude phase of the Prime Mission, the Cassini orbits were tilted off the equatorial plane affording the first opportunity to view Saturn's polar magnetosphere (Nov 2006-Mar 2007). During this interval, in January 2007, we observed the UV auroral emissions using the Hubble Space Telescope. This is the first time that such measurements have been made in a non-terrestrial magnetosphere. With this exclusive combination of auroral images and Cassini data, we discovered that the noon aurora indeed lies in the boundary between open and closed field lines, where a substantial layer of upward-directed field-aligned current flows. The density of the upward-directed field-aligned currents requires downward acceleration of magnetospheric electrons sufficient to produce the aurora observed by the HST. Therefore, these observations confirm that the quasi-continuous main oval is produced by the magnetosphere-solar wind interaction through the shear in rotational flow across the open-closed field line boundary [Bunce et al.,2008b]. Following on from this important result, we have completed the first high-latitude surveys of in situ perturbations associated with the auroral current system in Saturn's magnetosphere using magnetic field and electron spectrograms. Similar studies at the Earth have fundamentally changed our understanding of the directions and spatial distributions of field-aligned current systems associated with the coupling between the interplanetary medium and the magnetosphere. Talboys et al., 2009a&b have confirmed the basic picture in our theoretical work (mentioned above), but also exposes additional complexity in the small-scale structure of the high-latitude currents and evidence of important dynamics, thus requiring further careful investigation.
At Jupiter we have extended our existing theoretical model of the auroral current systems to include mapping of the ionospheric model parameters out to ~30 RJ in the magnetosphere [Cowley, Bunce et al., 2008]. We have compared the results with the NASA Juno mission orbits.
We are beginning to investigate Mercury's magnetosphere in anticipation of the Leicester lead BepiColombo experiment designed to map X-ray emissions emanating from the surface.
Exploitation Route As with all research whose main output are journal publications, this work has led to the ongoing study and understanding of the topics described above both here at Leicester and beyond in the international community.
Sectors Aerospace, Defence and Marine,Education

 
Description For education and public outreach purposes, through media interviews, invited seminars and talks at a variety of public and education related events.
First Year Of Impact 2006
Sector Education
Impact Types Societal