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Juno Observations of Large‐Scale Azimuthal Fields in Jupiter's Nightside Magnetosphere: Extended Analysis and Physical Implications.

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Title: Juno Observations of Large‐Scale Azimuthal Fields in Jupiter's Nightside Magnetosphere: Extended Analysis and Physical Implications.
Authors: Cowley, S. W. H.1 (AUTHOR) swhc1@leicester.ac.uk, Provan, G.1 (AUTHOR), Nichols, J. D.1 (AUTHOR)
Source: Journal of Geophysical Research. Space Physics. Aug2025, Vol. 130 Issue 8, p1-25. 25p.
Subject Terms: Jupiter (Planet), Magnetosphere, Plasma flow, Magnetic coupling, Time, Juno (Space probe), Magnetic fields, Torque
Abstract: We study Jupiter's nightside magnetospheric azimuthal magnetic field using measurements from the first 58 data‐taking periapsides of the Juno spacecraft, spanning post‐dawn to pre‐dusk local times (LTs). The methodology follows Provan et al. (2024, https://doi.org/10.1029/2024JA032677) but extends the analysis to 40 Jupiter radii distances, allowing better discrimination of field variations at dusk. The field has two components, (a) a field varying along field lines inversely with perpendicular distance from the magnetic axis, taken to be associated with magnetosphere‐ionosphere coupling, and (b) a quasi‐uniform field consistent with a field ∼4 nT pointing sunward/antisunward in the southern/northern hemisphere. These components are separated by subtracting the quasi‐uniform field, yielding colatitude profiles of the ionospheric current versus LT from which the atmospheric torque on the magnetospheric plasma is calculated. Currents peak on outer magnetosphere field lines at ∼12.5 MA per radian of azimuth at dusk, falling to ∼6.5 MA rad−1 at midnight but remain at ∼2 MA rad−1 polar values at dawn. Corresponding outer magnetosphere torques are ∼1.1 × 1018 N m in the dusk‐midnight quadrant, falling to ∼0.2 × 1018 N m in the midnight‐dawn quadrant. The quasi‐uniform field also provides a torque on the equatorial plasma, corotational in the midnight‐dawn sector ∼0.6–0.8 × 1018 N m augmenting the atmospheric torque, but anti‐corotational in the dusk sector opposing the atmospheric torque. We propose that the quasi‐uniform fields and related ionospheric LT asymmetries are associated with the confinement of iogenic plasma outflow in the down‐tail direction, possibly augmented by more direct interaction with the solar wind at the magnetopause. Key Points: Jovian nightside azimuthal fields are due to magnetosphere‐ionosphere coupling plus a quasi‐uniform field sunward/tailward in south/northPeak ionospheric currents and integrated torques depend on local time with values at dusk/dawn ∼12.5/2 MA rad−1 and ∼1.1 × 1018/0.2 × 1018 N mComparable equatorial torques due to quasi‐uniform field oppose/augment corotation at dusk/dawn due to plasma outflow confinement down‐tail [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: Juno Observations of Large‐Scale Azimuthal Fields in Jupiter's Nightside Magnetosphere: Extended Analysis and Physical Implications.
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  Data: <searchLink fieldCode="AR" term="%22Cowley%2C+S%2E+W%2E+H%2E%22">Cowley, S. W. H.</searchLink><relatesTo>1</relatesTo> (AUTHOR)<i> swhc1@leicester.ac.uk</i><br /><searchLink fieldCode="AR" term="%22Provan%2C+G%2E%22">Provan, G.</searchLink><relatesTo>1</relatesTo> (AUTHOR)<br /><searchLink fieldCode="AR" term="%22Nichols%2C+J%2E+D%2E%22">Nichols, J. D.</searchLink><relatesTo>1</relatesTo> (AUTHOR)
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  Data: <searchLink fieldCode="JN" term="%22Journal+of+Geophysical+Research%2E+Space+Physics%22">Journal of Geophysical Research. Space Physics</searchLink>. Aug2025, Vol. 130 Issue 8, p1-25. 25p.
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  Label: Abstract
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  Data: We study Jupiter's nightside magnetospheric azimuthal magnetic field using measurements from the first 58 data‐taking periapsides of the Juno spacecraft, spanning post‐dawn to pre‐dusk local times (LTs). The methodology follows Provan et al. (2024, https://doi.org/10.1029/2024JA032677) but extends the analysis to 40 Jupiter radii distances, allowing better discrimination of field variations at dusk. The field has two components, (a) a field varying along field lines inversely with perpendicular distance from the magnetic axis, taken to be associated with magnetosphere‐ionosphere coupling, and (b) a quasi‐uniform field consistent with a field ∼4 nT pointing sunward/antisunward in the southern/northern hemisphere. These components are separated by subtracting the quasi‐uniform field, yielding colatitude profiles of the ionospheric current versus LT from which the atmospheric torque on the magnetospheric plasma is calculated. Currents peak on outer magnetosphere field lines at ∼12.5 MA per radian of azimuth at dusk, falling to ∼6.5 MA rad−1 at midnight but remain at ∼2 MA rad−1 polar values at dawn. Corresponding outer magnetosphere torques are ∼1.1 × 1018 N m in the dusk‐midnight quadrant, falling to ∼0.2 × 1018 N m in the midnight‐dawn quadrant. The quasi‐uniform field also provides a torque on the equatorial plasma, corotational in the midnight‐dawn sector ∼0.6–0.8 × 1018 N m augmenting the atmospheric torque, but anti‐corotational in the dusk sector opposing the atmospheric torque. We propose that the quasi‐uniform fields and related ionospheric LT asymmetries are associated with the confinement of iogenic plasma outflow in the down‐tail direction, possibly augmented by more direct interaction with the solar wind at the magnetopause. Key Points: Jovian nightside azimuthal fields are due to magnetosphere‐ionosphere coupling plus a quasi‐uniform field sunward/tailward in south/northPeak ionospheric currents and integrated torques depend on local time with values at dusk/dawn ∼12.5/2 MA rad−1 and ∼1.1 × 1018/0.2 × 1018 N mComparable equatorial torques due to quasi‐uniform field oppose/augment corotation at dusk/dawn due to plasma outflow confinement down‐tail [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: 10.1029/2025JA033993
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      – Code: eng
        Text: English
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        PageCount: 25
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    Subjects:
      – SubjectFull: Jupiter (Planet)
        Type: general
      – SubjectFull: Magnetosphere
        Type: general
      – SubjectFull: Plasma flow
        Type: general
      – SubjectFull: Magnetic coupling
        Type: general
      – SubjectFull: Time
        Type: general
      – SubjectFull: Juno (Space probe)
        Type: general
      – SubjectFull: Magnetic fields
        Type: general
      – SubjectFull: Torque
        Type: general
    Titles:
      – TitleFull: Juno Observations of Large‐Scale Azimuthal Fields in Jupiter's Nightside Magnetosphere: Extended Analysis and Physical Implications.
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              M: 08
              Text: Aug2025
              Type: published
              Y: 2025
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