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Currents Associated With Saturn's Intra‐D Ring Azimuthal Field Perturbations.

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Title: Currents Associated With Saturn's Intra‐D Ring Azimuthal Field Perturbations.
Authors: Hunt, G.J.1 g.hunt@imperial.ac.uk, Cowley, S.W.H.2, Provan, G.2, Cao, H.1,3,4, Bunce, E.J.2, Dougherty, M.K.1, Southwood, D.J.1
Source: Journal of Geophysical Research. Space Physics. Jul2019, Vol. 124 Issue 7, p5675-5691. 17p.
Subject Terms: *Ionosphere, Rings of Saturn, Perturbation theory, Planetary atmospheres, Azimuth
Abstract: During the final 22 full revolutions of the Cassini mission in 2017, the spacecraft passed at periapsis near the noon meridian through the gap between the inner edge of Saturn's D ring and the denser layers of the planet's atmosphere, revealing the presence of an unanticipated low‐latitude current system via the associated azimuthal perturbation field peaking typically at ~10–30 nT. Assuming approximate axisymmetry, here we use the field data to calculate the associated horizontal meridional currents flowing in the ionosphere at the feet of the field lines traversed, together with the exterior field‐aligned currents required by current continuity. We show that the ionospheric currents are typically~0.5–1.5 MA per radian of azimuth, similar to auroral region currents, while the field‐aligned current densities above the ionosphere are typically ~5–10 nA/m2, more than an order less than auroral values. The principal factor involved in this difference is the ionospheric areas into which the currents map. While around a third of passes exhibit unidirectional currents flowing northward in the ionosphere closing southward along exterior field lines, many passes also display layers of reversed northward field‐aligned current of comparable or larger magnitude in the region interior to the D ring, which may reverse sign again on the innermost field lines traversed. Overall, however, the currents generally show a high degree of north‐south conjugacy indicative of an interhemispheric system, certainly on the larger overall spatial scales involved, if less so for the smaller‐scale structures, possibly due to rapid temporal or local time variations. Key Points: We calculate ionospheric and field‐aligned currents associated with the intra‐D ring azimuthal fields observed on Cassini's proximal passesIonospheric currents are 0.5‐1.5 MA/rad similar to auroral values, while current densities are 5‐10 nA/m2 more than an order smallerComplex current patterns often show close north‐south conjugacy on larger spatial scales consistent with a variable interhemispheric system [ABSTRACT FROM AUTHOR]
Copyright of Journal of Geophysical Research. Space Physics is the property of Wiley-Blackwell and its content may not be copied or emailed to multiple sites without the copyright holder's express written permission. Additionally, content may not be used with any artificial intelligence tools or machine learning technologies. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)
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  Data: Currents Associated With Saturn's Intra‐D Ring Azimuthal Field Perturbations.
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  Data: <searchLink fieldCode="AR" term="%22Hunt%2C+G%2EJ%2E%22">Hunt, G.J.</searchLink><relatesTo>1</relatesTo><i> g.hunt@imperial.ac.uk</i><br /><searchLink fieldCode="AR" term="%22Cowley%2C+S%2EW%2EH%2E%22">Cowley, S.W.H.</searchLink><relatesTo>2</relatesTo><br /><searchLink fieldCode="AR" term="%22Provan%2C+G%2E%22">Provan, G.</searchLink><relatesTo>2</relatesTo><br /><searchLink fieldCode="AR" term="%22Cao%2C+H%2E%22">Cao, H.</searchLink><relatesTo>1,3,4</relatesTo><br /><searchLink fieldCode="AR" term="%22Bunce%2C+E%2EJ%2E%22">Bunce, E.J.</searchLink><relatesTo>2</relatesTo><br /><searchLink fieldCode="AR" term="%22Dougherty%2C+M%2EK%2E%22">Dougherty, M.K.</searchLink><relatesTo>1</relatesTo><br /><searchLink fieldCode="AR" term="%22Southwood%2C+D%2EJ%2E%22">Southwood, D.J.</searchLink><relatesTo>1</relatesTo>
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  Data: <searchLink fieldCode="JN" term="%22Journal+of+Geophysical+Research%2E+Space+Physics%22">Journal of Geophysical Research. Space Physics</searchLink>. Jul2019, Vol. 124 Issue 7, p5675-5691. 17p.
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  Data: *<searchLink fieldCode="DE" term="%22Ionosphere%22">Ionosphere</searchLink><br /><searchLink fieldCode="DE" term="%22Rings+of+Saturn%22">Rings of Saturn</searchLink><br /><searchLink fieldCode="DE" term="%22Perturbation+theory%22">Perturbation theory</searchLink><br /><searchLink fieldCode="DE" term="%22Planetary+atmospheres%22">Planetary atmospheres</searchLink><br /><searchLink fieldCode="DE" term="%22Azimuth%22">Azimuth</searchLink>
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  Data: During the final 22 full revolutions of the Cassini mission in 2017, the spacecraft passed at periapsis near the noon meridian through the gap between the inner edge of Saturn's D ring and the denser layers of the planet's atmosphere, revealing the presence of an unanticipated low‐latitude current system via the associated azimuthal perturbation field peaking typically at ~10–30 nT. Assuming approximate axisymmetry, here we use the field data to calculate the associated horizontal meridional currents flowing in the ionosphere at the feet of the field lines traversed, together with the exterior field‐aligned currents required by current continuity. We show that the ionospheric currents are typically~0.5–1.5 MA per radian of azimuth, similar to auroral region currents, while the field‐aligned current densities above the ionosphere are typically ~5–10 nA/m2, more than an order less than auroral values. The principal factor involved in this difference is the ionospheric areas into which the currents map. While around a third of passes exhibit unidirectional currents flowing northward in the ionosphere closing southward along exterior field lines, many passes also display layers of reversed northward field‐aligned current of comparable or larger magnitude in the region interior to the D ring, which may reverse sign again on the innermost field lines traversed. Overall, however, the currents generally show a high degree of north‐south conjugacy indicative of an interhemispheric system, certainly on the larger overall spatial scales involved, if less so for the smaller‐scale structures, possibly due to rapid temporal or local time variations. Key Points: We calculate ionospheric and field‐aligned currents associated with the intra‐D ring azimuthal fields observed on Cassini's proximal passesIonospheric currents are 0.5‐1.5 MA/rad similar to auroral values, while current densities are 5‐10 nA/m2 more than an order smallerComplex current patterns often show close north‐south conjugacy on larger spatial scales consistent with a variable interhemispheric system [ABSTRACT FROM AUTHOR]
– Name: AbstractSuppliedCopyright
  Label:
  Group: Ab
  Data: <i>Copyright of Journal of Geophysical Research. Space Physics is the property of Wiley-Blackwell and its content may not be copied or emailed to multiple sites without the copyright holder's express written permission. Additionally, content may not be used with any artificial intelligence tools or machine learning technologies. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract.</i> (Copyright applies to all Abstracts.)
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        Value: 10.1029/2019JA026588
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      – Code: eng
        Text: English
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        PageCount: 17
        StartPage: 5675
    Subjects:
      – SubjectFull: Ionosphere
        Type: general
      – SubjectFull: Rings of Saturn
        Type: general
      – SubjectFull: Perturbation theory
        Type: general
      – SubjectFull: Planetary atmospheres
        Type: general
      – SubjectFull: Azimuth
        Type: general
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      – TitleFull: Currents Associated With Saturn's Intra‐D Ring Azimuthal Field Perturbations.
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              Text: Jul2019
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              Y: 2019
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