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

Jackman, C. M.; Cowley, S. W. H.

doi:10.5194/angeo-24-1029-2006

Cited by 21 publications

(23 citation statements)

References 53 publications

(107 reference statements)

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“…Very narrow arcs at this boundary, such as that shown in Fig. 1c, may then correspond simply to a region of upwarddirected field-aligned current, as, for example, in the theoretical model of Jackman and Cowley (2006). However, it seems reasonable to suppose that broader features, such as those seen e.g.…”

Section: Summary and Discussionmentioning

confidence: 77%

“…In this case we expect the auroras at Saturn to be associated with Dungey-cycle and possibly Vasyliunas-cycle dynamics, thus also explaining the persistent dawn-dusk local time asymmetries which are observed, as can be seen in Fig. 1 (Cowley et al, 2004aJackman and Cowley, 2006).…”

Section: Summary and Discussionmentioning

confidence: 77%

“…These auroral components are strongly modulated by prevailing solar wind conditions, and will generally also present persistent asymmetries in local time (e.g. Jackman and Cowley, 2006).…”

Section: Introductionmentioning

confidence: 99%

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A statistical analysis of the location and width of Saturn's southern auroras

Badman

Cowley

Gérard³

et al. 2006

Ann. Geophys.

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104

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Abstract.A selection of twenty-two Hubble Space Telescope images of Saturn's ultraviolet auroras obtained during [1997][1998][1999][2000][2001][2002][2003][2004] has been analysed to determine the median location and width of the auroral oval, and their variability. Limitations of coverage restrict the analysis to the southern hemisphere, and to local times from the post-midnight sector to just past dusk, via dawn and noon. It is found that the overall median location of the poleward and equatorward boundaries of the oval with respect to the southern pole are at ∼14 • and ∼16 • co-latitude, respectively, with a median latitudinal width of ∼2 • . These median values vary only modestly with local time around the oval, though the poleward boundary moves closer to the pole near noon (∼12.5 • ) such that the oval is wider in that sector (median width ∼3.5 • ) than it is at both dawn and dusk (∼1.5 • ). It is also shown that the position of the auroral boundaries at Saturn are extremely variable, the poleward boundary being located between 2 • and 20 • co-latitude, and the equatorward boundary between 6 • and 23 • , this variability contrasting sharply with the essentially fixed location of the main oval at Jupiter. Comparison with Voyager plasma angular velocity data mapped magnetically from the equatorial magnetosphere into the southern ionosphere indicates that the dayside aurora lie poleward of the main upward-directed field-aligned current region associated with corotation enforcement, which maps to ∼20 • -24 • co-latitude, while agreeing reasonably with the position of the open-closed field line boundary based on estimates of the open flux in Saturn's tail, located between ∼11 • and ∼15 • . In this case, the variability in location can be understood in terms of changes in the open flux present in the system, the changes implied by the Saturn data then matching those observed at Earth as fractions of the total planetary flux. We infer that the broad (few degrees) diffuse auroral emissions and sub-corotating auroral patches observed in the dayside Correspondence to: S. V. Badman (svb4@ion.le.ac.uk) sector at Saturn result from precipitation from hot plasma sub-corotating in the outer magnetosphere in a layer a few Saturn radii wide adjacent to the magnetopause, probably having been injected either by Dungey-cycle or Vasyliunascycle dynamics on the nightside.

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Section: Summary and Discussionmentioning

confidence: 77%

Section: Summary and Discussionmentioning

confidence: 77%

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A statistical analysis of the location and width of Saturn's southern auroras

Badman

Cowley

Gérard³

et al. 2006

Ann. Geophys.

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104

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“…The second possibility is the flow shear expected to occur at the boundary between closed field lines that moderately sub-corotate, and open field lines in the polar region that strongly sub-corotate according to both theory and observation (Isbell et al, 1984;Stallard et al, 2003Stallard et al, , 2004. Cowley et al (2004a) thus suggested that Saturn's main oval maps to this upward current layer at the boundary between open and closed field lines, a suggestion shown in subsequent modelling to be plausible in terms of accelerated electron and auroral parameters (Cowley et al, 2004b;Jackman and Cowley, 2006). Similar processes may also occur at Jupiter, forming a component of the auroras lying poleward of the main oval .…”

Section: Introductionmentioning

confidence: 88%

Auroral current systems in Saturn's magnetosphere: comparison of theoretical models with Cassini and HST observations

et al. 2008

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Abstract. The first simultaneous observations of fields and plasmas in Saturn's high-latitude magnetosphere and UV images of the conjugate auroral oval were obtained by the Cassini spacecraft and the Hubble Space Telescope (HST) in January 2007. These data have shown that the southern auroral oval near noon maps to the dayside cusp boundary between open and closed field lines, associated with a major layer of upward-directed field-aligned current (Bunce et al., 2008). The results thus support earlier theoretical discussion and quantitative modelling of magnetosphereionosphere coupling at Saturn , that suggests the oval is produced by electron acceleration in the field-aligned current layer required by rotational flow shear between strongly sub-corotating flow on open field lines and near-corotating flow on closed field lines. Here we quantitatively compare these modelling results (the "CBO" model) with the Cassini-HST data set. The comparison shows good qualitative agreement between model and data, the principal difference being that the model currents are too small by factors of about five, as determined from the magnetic perturbations observed by Cassini. This is suggested to be principally indicative of a more highly conducting summer southern ionosphere than was assumed in the CBO model. A revised model is therefore proposed in which the heightintegrated ionospheric Pedersen conductivity is increased by a factor of four from 1 to 4 mho, together with more minor adjustments to the co-latitude of the boundary, the flow shear across it, the width of the current layer, and the properties of the source electrons. It is shown that the revised model agrees well with the combined Cassini-HST data, reCorrespondence to: S. W. H. Cowley (swhc1@ion.le.ac.uk) quiring downward acceleration of outer magnetosphere electrons through a ∼10 kV potential in the current layer at the open-closed field line boundary to produce an auroral oval of ∼1 • width with UV emission intensities of a few tens of kR.

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“…Gérard et al, 2004;Clarke et al, 2005;Grodent et al, 2005;Badman et al, 2005). The observed auroral emissions relate both to precipitation from hot plasma populations produced within the magnetosphere, and to intense structured field-aligned current systems that are associated with magnetosphere-ionosphere momentum transfer (Cowley et al, 2004a(Cowley et al, , b, 2005aGustin et al, 2004Gustin et al, , 2006Nichols and Cowley, 2004;Jackman and Cowley, 2006).…”

Section: Introductionmentioning

confidence: 99%

Significance of Dungey-cycle flows in Jupiter's and Saturn's magnetospheres, and their identification on closed equatorial field lines

2007

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Abstract. We consider the contribution of the solar winddriven Dungey-cycle to flux transport in Jupiter's and Saturn's magnetospheres, the associated voltages being based on estimates of the magnetopause reconnection rates recently derived from observations of the interplanetary medium in the vicinity of the corresponding planetary orbits. At Jupiter, the reconnection voltages are estimated to be ∼150 kV during several-day weak-field rarefaction regions, increasing to ∼1 MV during few-day strong-field compression regions.The corresponding values at Saturn are ∼25 kV for rarefaction regions, increasing to ∼150 kV for compressions. These values are compared with the voltages associated with the flows driven by planetary rotation. Estimates of the rotational flux transport in the "middle" and "outer" magnetosphere regions are shown to yield voltages of several MV and several hundred kV at Jupiter and Saturn respectively, thus being of the same order as the estimated peak Dungeycycle voltages. We conclude that under such circumstances the Dungey-cycle "return" flow will make a significant contribution to the flux transport in the outer magnetospheric regions. The "return" Dungey-cycle flows are then expected to form layers which are a few planetary radii wide inside the dawn and morning magnetopause. In the absence of significant cross-field plasma diffusion, these layers will be characterized by the presence of hot light ions originating from either the planetary ionosphere or the solar wind, while the inner layers associated with the Vasyliunas-cycle and middle magnetosphere transport will be dominated by hot heavy ions originating from internal moon/ring plasma sources. The temperature of these ions is estimated to be of the order of a few keV at Saturn and a few tens of keV at Jupiter, in both layers.

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

Cited by 21 publications

References 53 publications

A statistical analysis of the location and width of Saturn's southern auroras

A statistical analysis of the location and width of Saturn's southern auroras

Auroral current systems in Saturn's magnetosphere: comparison of theoretical models with Cassini and HST observations

Significance of Dungey-cycle flows in Jupiter's and Saturn's magnetospheres, and their identification on closed equatorial field lines

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