Field‐aligned currents in Saturn's northern nightside magnetosphere: Evidence for interhemispheric current flow associated with planetary period oscillations

Hunt, G. J.; Cowley, S. W. H.; Provan, G.; Bunce, E. J.; Alexeev, I. I.; Belenkaya, E. S.; Калегаев, В. В.; Dougherty, M. K.; Coates, A. J.

doi:10.1002/2015ja021454

Cited by 73 publications

(324 citation statements)

References 55 publications

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“…6), in a multi-instrument study of Cassini data in the dusk to midnight sector, compatible with these results. They also found that the poleward boundary of the main upward field-aligned current region, presumed associated with the main auroral region, lay on average ∼ 1.8 • equatorward of this in the Northern Hemisphere, thus typically at ∼ 15.1 • in this LT sector, compatible with the results of Hunt et al (2015). Given a similar small equatorward displacement of the auroral oval from the open-closed boundary in the near-noon sector, we would thus expect its poleward boundary in the Northern Hemisphere to lie just equatorward ∼ 10 • on the basis of most of the results in Fig.…”

Section: Linear Approximation Paraboloid Model Parameterssupporting

confidence: 77%

“…We note that the model does not include a representation of the quasisteady internal field-aligned current system associated with subcorotation of the magnetospheric plasma (Hunt et al, 2014(Hunt et al, , 2015, which in general will cause a small deviation in LT of the field mapping between the equatorial plane and the ionosphere. Inclusion of related effects associated with the rotating "planetary period oscillation" (PPO) current system is also beyond the scope of the model.…”

Section: Saturn Paraboloid Modelmentioning

confidence: 99%

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Optimization of Saturn paraboloid magnetospheric field model parameters using Cassini equatorial magnetic field data

et al. 2016

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Abstract. The paraboloid model of Saturn's magnetosphere describes the magnetic field as being due to the sum of contributions from the internal field of the planet, the ring current, and the tail current, all contained by surface currents inside a magnetopause boundary which is taken to be a paraboloid of revolution about the planet-Sun line. The parameters of the model have previously been determined by comparison with data from a few passes through Saturn's magnetosphere in compressed and expanded states, depending on the prevailing dynamic pressure of the solar wind. Here we significantly expand such comparisons through examination of Cassini magnetic field data from 18 near-equatorial passes that span wide ranges of local time, focusing on modelling the co-latitudinal field component that defines the magnetic flux passing through the equatorial plane. For 12 of these passes, spanning pre-dawn, via noon, to post-midnight, the spacecraft crossed the magnetopause during the pass, thus allowing an estimate of the concurrent subsolar radial distance of the magnetopause R 1 to be made, considered to be the primary parameter defining the scale size of the system. The best-fit model parameters from these passes are then employed to determine how the parameters vary with R 1 , using least-squares linear fits, thus providing predictive model parameters for any value of R 1 within the range. We show that the fits obtained using the linear approximation parameters are of the same order as those for the individually selected parameters. We also show that the magnetic flux mapping to the tail lobes in these models is generally in good accord with observations of the location of the open-closed field line boundary in Saturn's ionosphere, and the related position of the auroral oval. We then investigate the field data on six passes through the nightside magnetosphere, for which the spacecraft did not cross the magnetopause, such that in this case we compare the observations with three linear approximation models representative of compressed, intermediate, and expanded states. Reasonable agreement is found in these cases for models representing intermediate or expanded states.

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Section: Linear Approximation Paraboloid Model Parameterssupporting

confidence: 77%

Section: Saturn Paraboloid Modelmentioning

confidence: 99%

Optimization of Saturn paraboloid magnetospheric field model parameters using Cassini equatorial magnetic field data

et al. 2016

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“…Hunt et al [2014] have reported PPO modulations of the southern hemisphere field-aligned current system. Further work by Hunt et al [2015] provided the first evidence of interhemispheric PPO-related field-aligned currents, as previously suggested by Southwood and Kivelson [2007]. Badman et al [2015] reported a solar wind compression impacting on Saturn's magnetosphere, describing an enhancement in the northern hemisphere field-aligned currents at the time of this solar wind compression.…”

Section: Discussionmentioning

confidence: 51%

Planetary period oscillations in Saturn's magnetosphere: Examining the relationship between abrupt changes in behavior and solar wind‐induced magnetospheric compressions and expansions

et al. 2015

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We examine planetary period oscillations (PPOs) observed in Saturn's magnetospheric magnetic field data from the time of Saturn's equinox in 2009. In particular, we focus on the time period commencing February 2011, when the oscillations started to display sudden and unexpected changes in behavior at ~100–200 day intervals. These were characterized by large simultaneous changes in the amplitude of the northern and southern PPO systems, together with small changes in period and jumps in phase. Nine significant abrupt changes have been observed in the postequinox interval to date, commencing as the Sun started to emerge from a long extended solar minimum. We perform a statistical study to determine whether these modulations in PPO behavior were associated with changes in the solar and/or upstream solar wind conditions. We report that the upstream solar wind conditions show elevated values of solar wind dynamic pressure and density around the time of PPO behavioral transitions, as opposed to before and after these times. We suggest that abrupt changes in PPO behavior may be related to significant changes in the size of the Saturnian magnetosphere in response to varying solar wind conditions.

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“…Cowley et al (2004a) suggested that strong upward field-aligned currents associated with the auroral emissions are generated due to the azimuthal velocity shear between closed and open field lines. Closed field lines corotate with modest lag from rigid corotation, while on the open field lines corotation partially breaks dawn (with ∼ 30 % of rigid corotation; Stallard et al, 2004;Hunt et al, 2015), as they are dragged tailward by the solar wind. The strong upward field-aligned currents flowing at the boundary between the two different types of field lines are carried by downwardaccelerated magnetospheric electrons, which excite auroral emissions at ionospheric height in the auroral oval.…”

Section: Comparison Of Open and Partially Closed Saturn Magnetospherimentioning

confidence: 99%

“…In this paper the term "polar cap" is taken to be the area bounded by the auroral oval. Since the strong upward currents associated with the auroral oval are located on closed field lines just equatorward of the open-closed boundary (Jinks et al, 2014;Hunt et al, 2015), this polar cap region will consist almost wholly of open field lines. However, strong field-aligned currents at the boundary between topologically different field regions could arise not only in the case of an open magnetosphere.…”

Section: Comparison Of Open and Partially Closed Saturn Magnetospherimentioning

confidence: 99%

Open and partially closed models of the solar wind interaction with outer planet magnetospheres: the case of Saturn

et al. 2017

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Abstract. A wide variety of interactions take place between the magnetized solar wind plasma outflow from the Sun and celestial bodies within the solar system. Magnetized planets form magnetospheres in the solar wind, with the planetary field creating an obstacle in the flow. The reconnection efficiency of the solar-wind-magnetized planet interaction depends on the conditions in the magnetized plasma flow passing the planet. When the reconnection efficiency is very low, the interplanetary magnetic field (IMF) does not penetrate the magnetosphere, a condition that has been widely discussed in the recent literature for the case of Saturn. In the present paper, we study this issue for Saturn using Cassini magnetometer data, images of Saturn's ultraviolet aurora obtained by the HST, and the paraboloid model of Saturn's magnetospheric magnetic field. Two models are considered: first, an open model in which the IMF penetrates the magnetosphere, and second, a partially closed model in which field lines from the ionosphere go to the distant tail and interact with the solar wind at its end. We conclude that the open model is preferable, which is more obvious for southward IMF. For northward IMF, the model calculations do not allow us to reach definite conclusions. However, analysis of the observations available in the literature provides evidence in favor of the open model in this case too. The difference in magnetospheric structure for these two IMF orientations is due to the fact that the reconnection topology and location depend on the relative orientation of the IMF vector and the planetary dipole magnetic moment. When these vectors are parallel, two-dimensional reconnection occurs at the low-latitude neutral line. When they are antiparallel, three-dimensional reconnection takes place in the cusp regions. Different magnetospheric topologies determine different mapping of the open-closed boundary in the ionosphere, which can be considered as a proxy for the poleward edge of the auroral oval.

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Field‐aligned currents in Saturn's northern nightside magnetosphere: Evidence for interhemispheric current flow associated with planetary period oscillations

Cited by 73 publications

References 55 publications

Optimization of Saturn paraboloid magnetospheric field model parameters using Cassini equatorial magnetic field data

Optimization of Saturn paraboloid magnetospheric field model parameters using Cassini equatorial magnetic field data

Planetary period oscillations in Saturn's magnetosphere: Examining the relationship between abrupt changes in behavior and solar wind‐induced magnetospheric compressions and expansions

Open and partially closed models of the solar wind interaction with outer planet magnetospheres: the case of Saturn

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