Saturn's polar ionospheric flows and their relation to the main auroral oval

By S. Cowley, E. Bunce, R. Prangé. 2003

170 citations recorded by the source corpus.

Abstract. We consider the flows and currents in Saturn's polar ionosphere which are implied by a three-component picture of large-scale magnetospheric flow driven both by planetary rotation and the solar wind interaction. With increasing radial distance in the equatorial plane, these components consist of a region dominated by planetary rotation where planetary plasma sub-corotates on closed field lines, a surrounding region where planetary plasma is lost down the dusk tail by the stretching out of closed field lines followed by plasmoid formation and pinch-off, as first described for Jupiter by Vasyliunas, and an outer region driven by the interaction with the solar wind, specifically by reconnection at the dayside magnetopause and in the dawn tail, first discussed for Earth by Dungey. The sub-corotating flow on closed field lines in the dayside magnetosphere is constrained by Voyager plasma observations, showing that the plasma angular velocity falls to around half of rigid corotation in the outer magnetosphere, possibly increasing somewhat near the dayside magnetopause, while here we provide theoretical arguments which indicate that the flow should drop to considerably smaller values on open field lines in the polar cap. The implied ionospheric current system requires a four-ring pattern of field-aligned currents, with distributed downward currents on open field lines in the polar cap, a narrow ring of upward current near the boundary of open and closed field lines, and regions of distributed downward and upward current on closed field lines at lower latitudes associated with the transfer of angular momentum from the planetary atmosphere to the sub-corotating planetary magnetospheric plasma. Recent work has shown that the upward current associated with sub-corotation is not sufficiently intense to produce significant auroral acceleration and emission. Here we suggest that the observed auroral oval at Saturn instead corresponds to the ring of upward current bounding the region of open and closed field lines. Estimates indicate that auroras of brightness from a few kR to a few tens of kR can be produced by precipitating accelerated magnetospheric electrons of a few keV to a few tens of keV energy, if the current flows in a region which is sufficiently narrow, of the order of or less than ~1000 km (~1° latitude) wide. Arguments are also given which indicate that the auroras should typically be significantly brighter on the dawn side of the oval than at dusk, by roughly an order of magnitude, and should be displaced somewhat towards dawn by the down-tail outflow at dusk associated with the Vasyliunas cycle. Model estimates are found to be in good agreement with data derived from high quality images newly obtained using the Space Telescope Imaging Spectrograph on the Hubble Space Telescope, both in regard to physical parameters, as well as local time effects. The implication of this picture is that the form, position, and brightness of Saturn's main auroral oval provide remote diagnostics of the magnetospheric interaction with the solar wind, including dynamics associated with magnetopause and tail plasma interaction processes. Key words. Magnetospheric physics (auroral phenomena, magnetosphere-ionosphere interactions, solar windmagnetosphere interactions)

Subfields: Astronomy and Astrophysics, Molecular Biology

View Paper: https://www.semanticscholar.org/paper/f40666c2586678204c579b11b38b656555d78e96

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In Astronomy and Astrophysics

1. Philippe Zarka
2. S. Cowley
3. E. Bunce
4. D. Grodent
5. John O'Rourke
6. J. Clarke
7. J. Duval
8. W. Kurth

In Molecular Biology

1. Philippe Zarka
2. E. Bunce
3. S. Cowley
4. John O'Rourke
5. J. Clarke
6. W. Kurth
7. D. Grodent

Citations in the corpus

• Reconnection in a rotation-dominated magnetosphere and its relation to Saturn's auroral dynamics (2005, background): “…by the Hubble Space Telescope (HST) over past years have indeed shown a highly variable auroral morphology [Gérard et al., 1995, 2004; Trauger et al., 1998; Cowley et al., 2004a], but in the absence of upstream data it has not been possible to relate these to concurrent interplanetary conditions.”
• Plasma interactions of exoplanets with their parent star and associated radio emissions (2006, methodology): “These include (i) magnetic reconnection between planetary and SW magnetic field lines at the magnetopause nose (see Cowley et al., 2004, for Saturn), (ii) magnetic reconnection following magnetotail field pinching (Dungey, 1961), (iii) magnetospheric plasma corotation breakdown (see Cowley and…”
• A simple quantitative model of plasma flows and currents in Saturn's polar ionosphere (2004, methodology): “…and Ajello, 1983], and in data from the IUE spacecraft [Clarke et al., 1981; McGrath and Clarke, 1992], images of increasing resolution and sensitivity have been obtained by a series instruments on the Hubble Space Telescope (HST) [Gérard et al., 1995; Trauger et al., 1998; Cowley et al., 2004].”
• Response of Jupiter's and Saturn's auroral activity to the solar wind (2009, background): “There has been a theoretical prediction that the corotation enforcement currents would be insufficient at Saturn to produce bright auroral emission, and that the main auroral oval must then map close to the solar wind boundary [Cowley and Bunce, 2003; Cowley et al., 2003].”