Ring Shadowing Effects on Saturn's Ionosphere: Implications for Ring Opacity and Plasma Transport

Hadid, Lina; Wahlund, Jan‐Erik; Moore, Luke; Cravens, T. E.; Hedman, Matthew M.; Edberg, Niklas J. T.; Vigren, Erik; Waite, J. H.; Perryman, R.; Kurth, W. S.; Farrell, W. M.; Eriksson, Anders

doi:10.1029/2018gl079150

Cited by 18 publications

(25 citation statements)

References 60 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…This includes the category B signatures with a reduction of B φ , these were observed at all altitude ranges. Furthermore, Hadid et al () showed the peak electron density to only be encountered on Cassini's final plunge (Rev 293) at an altitude of ~1,550 km, some 200‐km below the minimum altitude of Revs 288 and 292. Therefore, the peak of the ionospheric current layer was likely below the spacecraft on these Revs, too.…”

Section: Discussionmentioning

confidence: 99%

“…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%

See 1 more Smart Citation

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

et al. 2019

View full text Add to dashboard Cite

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.

show abstract

Section: Discussionmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

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

et al. 2019

View full text Add to dashboard Cite

show abstract

“…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

View full text Add to dashboard Cite

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.

show abstract

“…The electron number density varies from ∼60 cm −3 at the highest altitudes ( h > 3,900km) up to 2 × 10 4 cm −3 for the lowest altitudes ( h < 2,000km). To exclude the effect from the B‐ring shadow (Hadid et al, ), we limit our analysis to near‐equatorial latitudes −15 ∘ ≤[[spiphi]]≤15 ∘ (Figure b). Moreover, in order to study the electron density altitude profile of Saturn's topside ionosphere, we separate, classify, and label the proximal orbits, excluding the FP (Rev 293), into four different groups based on the c a altitude: GD, the closest orbits to the D‐ring, GH, the high altitude, GM, the midaltitude, and GL, the low altitude ones (Figure ).…”

Section: Observationsmentioning

confidence: 99%

Saturn's Ionosphere: Electron Density Altitude Profiles and D‐Ring Interaction From The Cassini Grand Finale

Hadid

Wahlund

Persoon

et al. 2019

Geophysical Research Letters

Self Cite

View full text Add to dashboard Cite

We present the electron density (ne) altitude profiles of Saturn's ionosphere at near‐equatorial latitudes from all 23 orbits of Cassini's Grand Finale. The data are collected by the Langmuir probe part of the Radio and Plasma Wave Science investigation. A high degree of variability in the electron density profiles is observed. However, organizing them by consecutive altitude ranges revealed clear differences between the southern and northern hemispheres. The ne profiles are shown to be more variable and connected to the D‐ring below 5,000 km in the southern hemisphere compared to the northern hemisphere. This observed variability is explained to be a consequence of an electrodynamic interaction with the D‐ring. Moreover, a density altitude profile is constructed for the northern hemisphere indicating the presence of three different ionospheric layers. Similar properties were observed during Cassini's final plunge, where the main ionospheric peak is crossed at ∼1,550‐km altitude.

show abstract

Ring Shadowing Effects on Saturn's Ionosphere: Implications for Ring Opacity and Plasma Transport

Cited by 18 publications

References 60 publications

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

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

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

Saturn's Ionosphere: Electron Density Altitude Profiles and D‐Ring Interaction From The Cassini Grand Finale

Contact Info

Product

Resources

About