Abstract
AbstractWe 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.
Highlights
During the Grand Finale phase of the Cassini mission, the spacecraft made 22 highly inclined periapsis passes through the near-noon equatorial plane between the inner edge of Saturn’s ring system and the upper atmosphere/ionosphere of the planet, before being destroyed in the atmosphere on spacecraft revolution (Rev) 293 on 15 September 2017
Summary and Conclusions Following initial reports by Dougherty et al (2018) and Khurana et al (2018) describing the magnetic field measurements during the early proximal orbits, in this paper we have made a first overview of the azimuthal field perturbations observed on intra-D ring field lines on all 23 proximal passes, including the partial pass made on final Rev 293
We have shown that the profiles are considerably variable in form and amplitude, though usually approximately symmetric about the central point of the pass where the trajectory lies parallel to a flux shell, consistent with the effect of interhemispheric field-aligned current flow
Summary
During the Grand Finale phase of the Cassini mission, the spacecraft made 22 highly inclined periapsis passes through the near-noon equatorial plane between the inner edge of Saturn’s ring system and the upper atmosphere/ionosphere of the planet, before being destroyed in the atmosphere on spacecraft revolution (Rev) 293 on 15 September 2017. These perturbations begin and end on D ring field lines between the blue solid and dashed vertical lines, nearer to the inner than the outer boundaries in this case, located approximately symmetrically on either side. Examination confirms that similar conclusions apply to the proximal data set as a whole (see Dougherty et al (2018) concerning the initial set of proximal orbits), these findings supporting the conclusions of Dougherty et al (2018) and Khurana et al (2018) that the intra-D ring azimuthal field represents a locally quasiaxisymmetric structure that spans the noon sector between at least ~10.3 h (Rev 292) and ~13.7 h (Rev 271) LT
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