Cassini observations of planetary‐period magnetic field oscillations in Saturn's magnetosphere: Doppler shifts and phase motion

Cowley, S. W. H.; Wright, D. M.; Bunce, E. J.; Carter, A. C.; Dougherty, M. K.; Giampieri, G.; Nichols, J. D.; Robinson, T. R.

doi:10.1029/2005gl025522

Cited by 71 publications

(102 citation statements)

References 13 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…Gurnett et al, 2005), which varies over time, differs from periodicities in magnetic field data (e.g. Cowley et al, 2006). Since our primary aim is to investigate the physics of the upper atmosphere, rather than to make any precise numerical predictions, our choice of S should not have a significant qualitative effect on our results.…”

Section: Model Descriptionmentioning

confidence: 99%

See 1 more Smart Citation

Coupled rotational dynamics of Saturn's thermosphere and magnetosphere: a thermospheric modelling study

Smith

Aylward

2008

Ann. Geophys.

View full text Add to dashboard Cite

Abstract.We use a numerical model of Saturn's thermosphere to investigate the flow of angular momentum from the atmosphere to the magnetosphere. The thermosphere model is driven by Joule heating and ion drag calculated from a simple model of the magnetospheric plasma flows and a fixed model of the ionospheric conductivity. We describe an initial study in which our plasma flow model is fixed and find that this leads to several inconsistencies in our results. We thus describe an improved model in which the plasma flows are allowed to vary in response to the structure of the thermospheric winds. Using this improved model we are able to analyse in detail the mechanism by which angular momentum extracted from the thermosphere by the magnetosphere is replaced by transport from the lower atmosphere. Previously, this transport was believed to be dominated by vertical transport due to eddy viscosity. Our results suggest that transport within the upper atmosphere by meridional winds is a much more important mechanism. As a consequence of this, we find that the rotational structures of the thermosphere and magnetosphere are related in a more complex way than the eddy viscosity model implies. Rather than the thermosphere behaving as a passive component of the system, the thermosphere-magnetosphere interaction is shown to be a two-way process in which rotational structures develop mutually. As an example of this, we are able to show that thermospheric dynamics offer an explanation of the small degree of super-corotation that has been observed in the inner magnetosphere. These results call into question the usefulness of the effective Pedersen conductivity as a parameterisation of the neutral atmosphere. We suggest that a two-parameter model employing the true Pedersen conductivity and the true thermospheric rotation velocity may be a more accurate representation of the thermospheric behaviour.

show abstract

Section: Model Descriptionmentioning

confidence: 99%

“…Gurnett et al, 2005); and in magnetic field measurements (e.g. Cowley et al, 2006). This paper is concerned with the first of these phenomena -the actual physical rotation of the plasma -and its relationship to the rotational dynamics of the planet's neutral upper atmosphere.…”

Section: Introductionmentioning

confidence: 99%

Coupled rotational dynamics of Saturn's thermosphere and magnetosphere: a thermospheric modelling study

Smith

Aylward

2008

Ann. Geophys.

View full text Add to dashboard Cite

show abstract

“…2 correspond to a single current layer that oscillates across the spacecraft, probably related to the planetary period oscillations in field and plasma that are ubiquitous in Saturn's magnetosphere (e.g. Cowley et al, 2006;Gurnett et al, 2007;Southwood and Kivelson, 2007;Andrews et al, 2008;Nichols et al, 2008). This is the hypothesis that will be adopted here, while remaining a topic for future investigation.…”

Section: S W H Cowley Et Almentioning

confidence: 99%

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

et al. 2008

Self Cite

View full text Add to dashboard Cite

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.

show abstract

“…Despite this, however, oscillations with a period near thẽ 11 h planetary period are observed throughout Saturn's magnetosphere. Such modulations are observed in the magnetic field, Saturnian kilometric radiation (SKR), plasma parameters, energetic particle fluxes, and associated neutral atom emissions, as well as auroral ultraviolet, infrared, and radio emissions [e.g., Warwick et al, 1981Warwick et al, , 1982Desch and Kaiser, 1981;Gurnett et al, 1981Gurnett et al, , 2007Gurnett et al, , 2010aSandel and Broadfoot, 1981;Sandel et al, 1982;Carbary and Krimigis, 1982;Espinosa and Dougherty, 2000;Krupp et al, 2005;Cowley et al, 2006;Kurth et al, 2007;Southwood and Kivelson, 2007;Carbary et al, 2007Carbary et al, , 2008aCarbary et al, , 2008bZarka et al, 2007;Andrews et al, 2008;Nichols et al, 2008Nichols et al, , 2010aNichols et al, , 2010bBurch et al, 2009;Provan et al, 2009aProvan et al, , 2009bClarke et al, 2006;Badman et al, 2012;Lamy et al, 2013]. Those oscillations were first observed in Voyager measurements of the SKR emissions, around the time of Saturn's northern hemisphere spring equinox [Kaiser et al, 1980].…”

Section: Introductionmentioning

confidence: 99%

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

Self Cite

View full text Add to dashboard Cite

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.

show abstract

Cassini observations of planetary‐period magnetic field oscillations in Saturn's magnetosphere: Doppler shifts and phase motion

Cited by 71 publications

References 13 publications

Coupled rotational dynamics of Saturn's thermosphere and magnetosphere: a thermospheric modelling study

Coupled rotational dynamics of Saturn's thermosphere and magnetosphere: a thermospheric modelling study

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

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

Contact Info

Product

Resources

About