In situ measurements of Saturn’s ionosphere show that it is dynamic and interacts with the rings

Wahlund, Jan‐Erik; Morooka, M.; Hadid, Lina; Persoon, A. M.; Farrell, W. M.; Gurnett, D. A.; Hospodarsky, G. B.; Kurth, W. S.; Ye, Shengyi; Andrews, David; Edberg, Niklas J. T.; Eriksson, Anders; Vigren, Erik

doi:10.1126/science.aao4134

Cited by 47 publications

(104 citation statements)

References 22 publications

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“…The physical reason for the variability thus remains to be elucidated. However, the evidence points toward a significant role for variability in thermospheric winds, likely in combination with ionospheric conductivities (Hadid, Morooka, Wahlund, Moore, et al, ,Hadid et al, ; Wahlund et al, ), and possibly influenced by the variable inflow of ~10 4 kg/s of volatiles and dust into equatorial atmosphere (Hsu et al, ; Mitchell et al, ; Perry et al, ; Waite et al, ). Significant nonconjugacy in the field and current profiles is also often present on smaller spatial scales.…”

Section: Discussionmentioning

confidence: 99%

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

et al. 2019

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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.

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Section: Discussionmentioning

confidence: 99%

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

et al. 2019

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“…The low and positive potential was possibly the consequence of the presence of negative ions or secondary electron emission from the SC induced by ions or fast neutrals (Morooka et al, 2019;Wahlund et al, 2017). The low and positive potential was possibly the consequence of the presence of negative ions or secondary electron emission from the SC induced by ions or fast neutrals (Morooka et al, 2019;Wahlund et al, 2017).…”

Section: Effects Of the Magnetic Field (B ≠ 0)mentioning

confidence: 99%

Magnetic Field Effect on Antenna Signals Induced by Dust Particle Impacts

et al. 2020

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The Radio and Plasma Wave Science instrument on Cassini has observed fewer than expected dust particle impacts during the mission's Grand Finale orbits. The relatively strong magnetic field in the close vicinity of the planet has been suggested to affect the intensity of the dust impact generated signals. A laboratory investigation is performed using dust particles accelerated to ≥20 km/s speed impacting onto a previously developed model of the spacecraft and the Radio and Plasma Wave Science antennas. The external magnetic field is generated by two sets of magnetic coils. The recorded antenna waveforms are decomposed into contributions from the electrons and ions of the dust impact generated plasma cloud. A good qualitative understanding of the waveforms is achieved by dividing the electron and ion population into two portions: one that is escaping from the spacecraft and another that is collected by the spacecraft. The experimental results show that the part of the signal corresponding to escaping electrons is affected by the magnetic field and that dust impact signals can be significantly reduced for spacecraft floating potentials close to zero.

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“…The perturbations on category A passes imply the growth or maintenance of such conditions into the innermost field lines sampled, with superposed shorter‐period fluctuations perhaps being associated with waves generated by the shears in flow. In situ Cassini measurements on the proximal passes indeed show the presence of strong variability in the upper equatorial ionospheric plasma from pass to pass, indicative of significant dynamical interactions with D ring material as well as ring shadow effects (Hadid et al, ; Wahlund et al, ). Observations on these passes also demonstrate the influx of ~10 4 kg s −1 of volatiles and dust into the equatorial atmosphere, captured from the inner D ring by atmospheric drag (Hsu et al, ; Mitchell et al, ; Perry et al, ; Waite et al, ).…”

Section: Possible Origins Of Intra–d Ring Azimuthal Field Variabilitymentioning

confidence: 99%

Variability of Intra–D Ring Azimuthal Magnetic Field Profiles Observed on Cassini's Proximal Periapsis Passes

et al. 2019

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We overview the properties of the azimuthal magnetic fields observed during the periapsis passes of the final 23 orbits of the Cassini spacecraft, including the partial orbit at end of mission, on near equatorial field lines passing inside of Saturn's D ring. The signatures are variable in form and amplitude, though generally approximately symmetric about the point where the spacecraft trajectory lies tangent to a flux shell, corresponding to where the ionospheric field line feet map closest to the equator, consistent with the effect of interhemispheric field‐aligned currents. The perturbations usually begin and end near symmetrically at some point on field lines threading the D ring and extend into the interior region, but in no case do they clearly extend outward onto field lines passing through the C ring. About 35% of cases display a ~20‐40 nT single positive central field peak indicative of southward field‐aligned current flow, while a further ~30% display two or three weaker ~10‐20 nT positive peaks indicative of multiple sheets of northward and southward current. Significant smaller‐scale >5 nT peak‐to‐peak field fluctuations are commonly superposed. A further ~20% of cases exhibit unique profiles within the data set, including two with ~20‐30 nT negative fields and two with only <10 nT fluctuating fields. The variable nature of the signatures is not connected with the pass altitude, local time, planetary period oscillation phase, or D68 ringlet phase but may relate to variable structured thermospheric winds and/or ionospheric conductivities that suggest a significant dynamical role for D ring‐atmosphere interactions.

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In situ measurements of Saturn’s ionosphere show that it is dynamic and interacts with the rings

Cited by 47 publications

References 22 publications

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

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

Magnetic Field Effect on Antenna Signals Induced by Dust Particle Impacts

Variability of Intra–D Ring Azimuthal Magnetic Field Profiles Observed on Cassini's Proximal Periapsis Passes

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