The Jovian auroral oval

Hill, T. W.

doi:10.1029/2000ja000302

Cited by 282 publications

(389 citation statements)

References 32 publications

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“…No detailed self-consistent dynamical models of these processes have yet been proposed for Saturn as they have for Jupiter (e.g. Hill, 1979Hill, , 2001Pontius, 1997;Cowley et al, 2002;Nichols and Cowley, 2003). However, Voyager plasma observations in the dayside equatorial magnetosphere show that the plasma angular velocity falls from near-rigid corotation close to the planet, within ∼5 R S , to values of ∼50% of rigid corotation at distances of ∼15 R S , within a few R S of the dayside magnetopause (Richardson, 1986).…”

Section: Equatorial Flowsmentioning

confidence: 99%

“…The first possibility is that they are associated with the current system that transfers angular momentum from the atmosphere of the rapidly-spinning planet to the sub-corotating equatorial magnetospheric plasma (Hill, 1979;Vasyliunas, 1983). It has recently been proposed that Jupiter's main auroral oval is formed in this way Cowley and Bunce, 2001;Hill, 2001;Southwood and Kivelson, 2001), the angular momentum transfer being required by the equatorial radial outflow of sulphur and oxygen plasma from the Io plasma torus. At Saturn, the magnetospheric plasma source, originating from icy moons and ring grains, is believed to be on a lesser scale than at Jupiter by more than an order of magnitude (e.g.…”

Section: Introductionmentioning

confidence: 99%

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

2004

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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 byCorrespondence to: S. W. H. Cowley (swhc1@ion.le.ac.uk) 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 o...

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Section: Equatorial Flowsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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

2004

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“…At Jupiter, however, the equivalent middle magnetosphere "corotation breakdown" currents are more intense by at least an order of magnitude (e.g. Cowley et al, 2005b), and are believed to be associated with accelerated electron precipitation that produces the bright "main oval" auroral emissions in that case Cowley and Bunce, 2001;Hill, 2001;Southwood and Kivelson, 2001;Grodent et al, 2003).…”

Section: Physical Picturementioning

confidence: 99%

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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“…As in those works, here we took the colatitude of the open-closed field line boundary to be the colatitude of the auroral oval ring (first-order approximation), but we recall that these two colatitudes may not match exactly (e.g. Siscoe & Chen 1975;Hill 2001). The aperture of the auroral ring can be related to the size of the planet magnetosphere r M as…”

Section: Active M-dwarf Planet Hostsmentioning

confidence: 99%

Effects of M dwarf magnetic fields on potentially habitable planets

Vidotto

Jardine

Morin

et al. 2013

A&A

152

161

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We investigate the effect of the magnetic fields of M dwarf (dM) stars on potentially habitable Earth-like planets. These fields can reduce the size of planetary magnetospheres to such an extent that a significant fraction of the planet's atmosphere may be exposed to erosion by the stellar wind. We used a sample of 15 active dM stars, for which surface magnetic-field maps were reconstructed, to determine the magnetic pressure at the planet orbit and hence the largest size of its magnetosphere, which would only be decreased by considering the stellar wind. Our method provides a fast means to assess which planets are most affected by the stellar magnetic field, which can be used as a first study to be followed by more sophisticated models. We show that hypothetical Earth-like planets with similar terrestrial magnetisation (∼1 G) orbiting at the inner (outer) edge of the habitable zone of these stars would present magnetospheres that extend at most up to 6 (11.7) planetary radii. To be able to sustain an Earth-sized magnetosphere, with the exception of only a few cases, the terrestrial planet would either (1) need to orbit significantly farther out than the traditional limits of the habitable zone; or else, (2) if it were orbiting within the habitable zone, it would require at least a magnetic field ranging from a few G to up to a few thousand G. By assuming a magnetospheric size that is more appropriate for the young-Earth (3.4 Gyr ago), the required planetary magnetic fields are one order of magnitude weaker. However, in this case, the polar-cap area of the planet, which is unprotected from transport of particles to/from interplanetary space, is twice as large. At present, we do not know how small the smallest area of the planetary surface is that could be exposed and would still not affect the potential for formation and development of life in a planet. As the star becomes older and, therefore, its rotation rate and magnetic field reduce, the interplanetary magnetic pressure decreases and the magnetosphere of planets probably expands. Using an empirically derived rotation-activity/magnetism relation, we provide an analytical expression for estimating the shortest stellar rotation period for which an Earth-analogue in the habitable zone could sustain an Earth-sized magnetosphere. We find that the required rotation rate of the early-and mid-dM stars (with periods 37-202 days) is slower than the solar one, and even slower for the late-dM stars ( 63-263 days). Planets orbiting in the habitable zone of dM stars that rotate faster than this have smaller magnetospheric sizes than that of the Earth magnetosphere. Because many late-dM stars are fast rotators, conditions for terrestrial planets to harbour Earth-sized magnetospheres are more easily achieved for planets orbiting slowly rotating early-and mid-dM stars.

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The Jovian auroral oval

Cited by 282 publications

References 32 publications

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

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

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

Effects of M dwarf magnetic fields on potentially habitable planets

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