Direct evidence of differences in magnetic rotation rate between Saturn's northern and southern polar regions

Southwood, D. J.

doi:10.1029/2010ja016070

Cited by 42 publications

(52 citation statements)

References 24 publications

(45 reference statements)

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“…It has also been found that the periods of the northern and southern SKR and auroral hiss modulations are distinct, with a shorter period of ∼10.6 h in the north compared with ∼10.8 h in the south during the Saturn southern summer interval investigated during the initial phase of the Cassini mission between 2004 and 2008 [ Kurth et al , 2008; Gurnett et al , 2009a, 2009b]. Correspondingly, the magnetic field perturbations in the high‐latitude polar magnetosphere and magnetospheric tail are also found to oscillate with distinct periods in the north and south that closely match the SKR periods [ Andrews et al , 2010b; Southwood , 2011; Provan et al , 2012]. The field oscillations within the central quasi‐dipolar magnetosphere to equatorial distances of ∼15 R S , referred to here as the ‘core’ region [e.g., Andrews et al , 2010a], are then found to be dominated by the southern period over the same southern summer interval, as are the modulations in the plasma properties cited above.…”

Section: Introductionmentioning

confidence: 99%

Planetary period oscillations in Saturn's magnetosphere: Evolution of magnetic oscillation properties from southern summer to post‐equinox

Andrews¹,

Cowley²,

Dougherty³

et al. 2012

J. Geophys. Res.

418

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[1] We investigate the evolution of the properties of planetary period magnetic field oscillations observed by the Cassini spacecraft in Saturn's magnetosphere over the interval from late 2004 to early 2011, spanning equinox in mid-2009. Oscillations within the inner quasi-dipolar region (L ≤ 12) consist of two components of close but distinct periods, corresponding essentially to the periods of the northern and southern Saturn kilometric radiation (SKR) modulations. These give rise to modulations of the combined amplitude and phase at the beat period of the two oscillations, from which the individual oscillation amplitudes and phases (and hence periods) can be determined. Phases are also determined from northern and southern polar oscillation data when available. Results indicate that the southern-period amplitude declines modestly over this interval, while the northern-period amplitude approximately doubles to become comparable with the southern-period oscillations during the equinox interval, producing clear effects in pass-to-pass oscillation properties. It is also shown that the periods of the two oscillations strongly converge over the equinox interval, such that the beat period increases significantly from $20 to more than 100 days, but that they do not coalesce or cross during the interval investigated, contrary to recent reports of the behavior of the SKR periods. Examination of polar oscillation data for similar beat phase effects yields a null result within a $10% upper limit on the relative amplitude of northern-period oscillations in the south and vice versa. This result strongly suggests a polar origin for the two oscillation periods.

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Section: Introductionmentioning

confidence: 99%

Planetary period oscillations in Saturn's magnetosphere: Evolution of magnetic oscillation properties from southern summer to post‐equinox

Andrews¹,

Cowley²,

Dougherty³

et al. 2012

J. Geophys. Res.

418

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“…It is also not plausible for high order magnetic anomalies to drift fast enough to account for such large shifts. Furthermore, the periods of electromagnetic properties differ in the northern and southern hemispheres and appear to vary with Saturn's seasons [ Gurnett et al , 2009b, 2010; Ye et al , 2010; Lamy , 2011; Southwood , 2011], an additional challenge to interpretation of their origin.…”

Section: Introductionmentioning

confidence: 99%

Driving Saturn's magnetospheric periodicities from the upper atmosphere/ionosphere

2012

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[1] Saturn's magnetospheric structure and the intensity of radio frequency emissions from its immediate surroundings are modulated at close to the planet's rotation period. Analogous rotation-modulated variations at Jupiter are readily interpreted as effects of the non-axisymmetric intrinsic magnetic field. At Saturn, to the contrary, the high level of axial symmetry in the intrinsic field suggests that the periodicity is not internally imposed. A number of mechanisms have been proposed to account for the observations. Each model explains a subset of the observations in a qualitative manner, but no quantitative models yet exist. Here, using a magnetohydrodynamic simulation, we investigate the magnetospheric perturbations that arise from a localized vortical flow structure in the ionosphere near 70 S-latitude that rotates at roughly the rate of planetary rotation. The model reproduces nearly quantitatively a host of observed magnetospheric periodicities associated with the period of the dominant (southern) radio frequency emissions during the Cassini epoch including rotating, quasi-uniform magnetic perturbations in the equatorial plane, rotating mass density perturbations, periodic plasmoid releases that we associate with observed bursts of energetic neutral atoms (ENAs), periodic oscillations of magnetospheric boundaries, current sheet flapping, and periodic modulation of the field-aligned currents linked to Saturn's kilometric radiation (SKR). The model is not unique but is representative of a class of models in which asymmetric flows in the (as yet unmeasured) upper atmosphere couple to the ionosphere and generate currents that flow into the magnetosphere. It can be extended to include the second periodicity that has been associated with SKR emissions in the northern hemisphere.Citation: Jia, X., M. G. Kivelson, and T. I. Gombosi (2012), Driving Saturn's magnetospheric periodicities from the upper atmosphere/ionosphere,

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“…Andrews et al [] also showed, just as Southwood [] found, that quite generally on Cassini's inclined orbits, the signals in respective polar caps were pure with less than 10% contamination by the signal associated with the opposite hemisphere. This result suggests that the two magnetic signals originate from their respective polar regions and that the SKR emissions are directly linked to these signals via field‐aligned currents associated with each perturbation field [ Southwood and Cowley , ].…”

Section: Introductionmentioning

confidence: 98%

“…Using the rotating magnetic phase as a coordinate as had been done by Southwood and Kivelson [], Southwood [] demonstrated directly the presence of separate northern and southern signals in the magnetometer date. Using data from a series of highly inclined orbits where the spacecraft spent most of the orbit on field lines above 75° magnetic invariant latitude, he not only demonstrated that separate periods were detected in northern and southern polar caps but also was able to show the conservation of both northern and southern phases.…”

Section: Introductionmentioning

confidence: 99%

Magnetic phase structure of Saturn's 10.7 h oscillations

2015

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Abstract.A source of Saturn's magnetic 10.7 hour period oscillations has yet to be identified. The oscillations are known to consist of signals with slightly different periods from separate northern and southern sources. Here we present a novel way of examining observations, focusing on signal phase. We show that although the signals are highly periodic they are usually not sinusoidal and that there are differences in both phase structure and polarization between the outer magnetosphere (on the nightside) and the inner dipolar region. Paying particular attention to the deep mid-tail passes of 2006, the contrasting behavior between the inner and outer regions is clear with approximate sinusoidal behavior in the dipolar region and a pulse like signal once per cycle in the tail. The latter structure seems to indicate that tail magnetic stress is released impulsively once per cycle in the tail. After equinox, in 2010-11, we find a different picture in the pre-midnight sector. The predetermined northern and southern frequencies are closer together and apparently show sudden shifts. Our signal reconstruction approach finds instances where it is likely that the narrow band filtering is not able to track completely the basic north and south periods as we find phase jumps indicating unpredicted beats.

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Direct evidence of differences in magnetic rotation rate between Saturn's northern and southern polar regions

Cited by 42 publications

References 24 publications

Planetary period oscillations in Saturn's magnetosphere: Evolution of magnetic oscillation properties from southern summer to post‐equinox

Planetary period oscillations in Saturn's magnetosphere: Evolution of magnetic oscillation properties from southern summer to post‐equinox

Driving Saturn's magnetospheric periodicities from the upper atmosphere/ionosphere

Magnetic phase structure of Saturn's 10.7 h oscillations

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