Morphological differences between Saturn's ultraviolet aurorae and those of Earth and Jupiter

Clarke, J. T.; Gérard, Jean‐Claude; Grodent, Denis; Wannawichian, S.; Gustin, Jacques; Connerney, J. E. P.; Crary, F. J.; Dougherty, M. K.; Kurth, W. S.; Cowley, S. W. H.; Bunce, E. J.; Hill, T. W.; Kim, J.

doi:10.1038/nature03331

Cited by 155 publications

(177 citation statements)

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“…In overall terms, observations show that the auroral brightness typically lies in the range ∼1-100 kR, while the total auroral output power generally varies from a few GW to a few tens of GW (Gérard et al, 1995Trauger et al, 1998;Cowley et al, 2004a;Clarke et al, 2005;Grodent et al, 2005;Badman et al, 2005). The values derived from our model lie in similar ranges, and are thus in basic accord with the observations.…”

Section: Auroral Parameterssupporting

confidence: 77%

A model of the plasma flow and current in Saturn's polar ionosphere under conditions of strong Dungey cycle driving

Jackman

Cowley²

2006

Ann. Geophys.

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Abstract.We propose a simple model of the flow and currents in Saturn's polar ionosphere. This model is motivated by theoretical reasoning, and guided quantitatively by in situ field and flow data from space missions, ground-based IR Doppler measurements, and Hubble Space Telescope images. The flow pattern consists of components which represent (1) plasma sub-corotation in the middle magnetosphere region resulting from plasma pick-up and radial transport from internal sources; (2) the Vasyliunas-cycle of internal plasma mass-loss down the magnetospheric tail at higher latitudes; and (3) the polar Dungey-cycle flow driven by the solar wind interaction. Upstream measurements of the interplanetary magnetic field (IMF) indicate the occurrence of both extended low-field rarefaction intervals with essentially negligible Dungey-cycle flow, and few-day high-field compression regions in which the Dungey-cycle voltage peaks at a few hundred kV. Here we model the latter conditions when the Dungey-cycle is active, advancing on previous axisymmetric models which may be more directly applicable to quiet conditions. For theoretical convenience the overall flow pattern is constructed by adding together two components -a purely rotational flow similar to previous axisymmetric models, and a sun-aligned twin vortex representing the dawn-dusk asymmetry effects associated with the Vasyliunas-and Dungey-cycle flows. We calculate the horizontal ionospheric current associated with the flow and the field-aligned current from its divergence. These calculations show that a sheet of upward-directed field-aligned current flows at the boundary of open field lines which is strongly modulated in local-time by the Dungey-cycle flows. We then consider implications of the field-aligned current for magnetospheric electron acceleration and aurorae using two plasma source populations (hot outer magnetospheric electrons and cool dense magnetosheath electrons). Both sources displayCorrespondence to: C. M. Jackman (cj47@ion.le.ac.uk) a strong dawn-dusk asymmetry in the accelerating voltages required and the energy fluxes produced, resulting from the corresponding asymmetry in the current. The auroral intensities for the outer magnetosphere source are typically ∼50 kR at dawn and ∼5 kR at dusk, in conformity with recent auroral observations under appropriate conditions. However, those for the magnetosheath source are much smaller. When the calculated precipitating electron energy flux values are integrated across the current layer and around the open closed field line boundary, this yields total UV output powers of ∼10 GW for the hot outer magnetosphere source, which also agrees with observations.

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Section: Auroral Parameterssupporting

confidence: 77%

A model of the plasma flow and current in Saturn's polar ionosphere under conditions of strong Dungey cycle driving

Jackman

Cowley²

2006

Ann. Geophys.

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“…5a (7 April at 06:26:11 UT) a latitudinally broad band of auroras spans from beyond the dawn meridian towards noon, extending poleward close to the magnetic/spin pole of the planet itself. Previous studies have shown that such "disturbed" auroral morphologies occur in response to rapid compressions of the magnetosphere by the solar wind Clarke et al, 2005Clarke et al, , 2009, possibly resulting in rapid reconnection and open-flux closure in the tail .…”

Section: Similarly Inmentioning

confidence: 99%

Magnetospheric magnetic field modelling for the 2011 and 2012 HST Saturn aurora campaigns – implications for auroral source regions

et al. 2014

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Abstract.A unique set of images of Saturn's northern polar UV aurora was obtained by the Hubble Space Telescope in 2011 and 2012 at times when the Cassini spacecraft was located in the solar wind just upstream of Saturn's bow shock. This rare situation provides an opportunity to use the Kronian paraboloid magnetic field model to examine source locations of the bright auroral features by mapping them along field lines into the magnetosphere, taking account of the interplanetary magnetic field (IMF) measured near simultaneously by Cassini. It is found that the persistent dawn arc maps to closed field lines in the dawn to noon sector, with an equatorward edge generally located in the inner part of the ring current, typically at ∼ 7 Saturn radii (R S ) near dawn, and a poleward edge that maps variously between the centre of the ring current and beyond its outer edge at ∼ 15 R S , depending on the latitudinal width of the arc. This location, together with a lack of response in properties to the concurrent IMF, suggests a principal connection with ring-current and nightside processes. The higher-latitude patchy auroras observed intermittently near to noon and at later local times extending towards dusk are instead found to straddle the model open-closed field boundary, thus mapping along field lines to the dayside outer magnetosphere and magnetopause. These emissions, which occur preferentially for northward IMF directions, are thus likely associated with reconnection and open-flux production at the magnetopause. One image for southward IMF also exhibits a prominent patch of very high latitude emissions extending poleward of patchy dawn arc emissions in the pre-noon sector. This is found to lie centrally within the region of open model field lines, suggesting an origin in the current system associated with lobe reconnection, similar to that observed in the terrestrial magnetosphere for northward IMF.

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“…The XMM-Newton (3) observation (28 Oct. 2005, 06:34 to 29 Oct. 2005 was contemporaneus with three exposures of the Saturnian aurora taken with the HST Advanced Camera for Surveys, during a coordinated campaign with in-situ measurements by the Cassini spacecraft. This was a time of quiet solar wind conditions, and yet the UV aurora was seen to be very variable; the measured UV power at the time of the XMM-Newton observation varied between 3.3 and 8.0 GW Clarke et al (2005). (d) Bhardwaj et al (2007).…”

Section: The Origin Of the Rings X-ray Emissionmentioning

confidence: 99%

X-rays from Saturn: a study withXMM-NewtonandChandraover the years 2002–05

Branduardi‐Raymont¹,

Bhardwaj²,

Elsner³

et al. 2010

A&A

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Aims. We approach the study of Saturn and its environment in a novel way using X-ray data, by making a systematic and uniform spectral analysis of all the X-ray observations of the planet to date. Methods. We present the results of the two most recent (2005) XMM-Newton observations of Saturn together with the re-analysis of an earlier (2002) observation from the XMM-Newton archive and of three Chandra observations in 2003 and 2004. While the XMM-Newton telescope resolution does not enable us to resolve spatially the contributions of the planet's disk and rings to the X-ray flux, we can estimate their strengths and their evolution over the years from spectral analysis, and compare them with those observed with Chandra. Results. The spectrum of the X-ray emission is well fitted by an optically thin coronal model with an average temperature of 0.5 keV. The addition of a fluorescent oxygen emission line at ∼0.53 keV improves the fits significantly. In accordance with earlier reports, we interpret the coronal component as emission from the planetary disk, produced by the scattering of solar X-rays in Saturn's upper atmosphere, and the line as originating from the Saturnian rings. The strength of the disk X-ray emission is seen to decrease over the period [2002][2003][2004][2005], following the decay of solar activity towards the current minimum in the solar cycle. By comparing the relative fluxes of the disk X-ray emission and the oxygen line, we suggest that the line strength does not vary over the years in the same fashion as the disk flux. We consider possible alternatives for the origin of the line. The connection between solar activity and the strength of Saturn's disk X-ray emission is investigated and compared with that of Jupiter. We also discuss the apparent lack of X-ray aurorae on Saturn; by comparing the planet's parameters relevant to aurora production with those of Jupiter we conclude that Saturnian X-ray aurorae are likely to have gone undetected because they are below the sensitivity threshold of current Earth-bound observatories. A similar comparison for Uranus and Neptune leads to the same disappointing conclusion, which is likely to hold true also with the planned next generation International X-ray Observatory. The next step in advancing this research can only be realised with in-situ X-ray observations at the planets.

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Morphological differences between Saturn's ultraviolet aurorae and those of Earth and Jupiter

Cited by 155 publications

References 17 publications

A model of the plasma flow and current in Saturn's polar ionosphere under conditions of strong Dungey cycle driving

A model of the plasma flow and current in Saturn's polar ionosphere under conditions of strong Dungey cycle driving

Magnetospheric magnetic field modelling for the 2011 and 2012 HST Saturn aurora campaigns – implications for auroral source regions

X-rays from Saturn: a study withXMM-NewtonandChandraover the years 2002–05

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