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

Abstract

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.

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

“…It can be seen that the current begins to increase to positive values on D ring field lines both north and south and continues to increase near linearly with colatitude in the region inside the D ring, reaching peak values of ~1.5 MA/rad at the largest colatitudes reached, corresponding to ~71° in the northern hemisphere and ~98° in the southern hemisphere. This value is similar to those presented by Dougherty et al (), Khurana et al (), and Provan et al (). It seems clear from this profile, however, that the current is still rising strongly with colatitude in the innermost part of the region sampled, with the actual peak northward current flowing across the equatorial region remaining undetermined.…”

“…We now present current and current density results as described above for four representative intra-D ring passes. Provan et al (2019) have recently suggested a broad division of passes into four categories, though noting that the data generally display more a continuum of behaviors rather than completely distinct types. In essentially all cases, the field profiles show approximate symmetry about the magnetic equator, though differing in the number, form, and sign of the peaks in field observed.…”

“…If from Figure b, we take the latter dimension to be, for example, ~0.25 R S , then we require the azimuthal length scale to be at least, for example, ~0.5 R S , corresponding to ~2 hr of LT (see further in Appendix ). The data discussed by Provan et al () demonstrate directly from the LT coverage on each pass (~1.5‐hr LT) and over the proximal orbit data set as a whole (~3.5‐hr LT) that the corresponding structures are at least as azimuthally extended as this.…”

“…Initial analysis of the data follows the methodology adopted by Provan et al (). To isolate the main intra‐D ring B φ feature, we first fit a fourth order polynomial to the data over the interval shown but excluding the intra‐D ring data between the middle pairs of blue dashed lines in the figure.…”

A Persistent, Large‐Scale, and Ordered Electrodynamic Connection Between Saturn and Its Main Rings

“…As we do not observe field perturbations of this magnitude, we conclude the upper limit of the current density to be 1 nA/m 2 . In addition, the azimuthal magnetic field could be influenced by other sources as shown by the fits performed by Provan et al (). Indeed, Hunt et al () showed the presence of the planetary period oscillations in the azimuthal field component during the closest approach of the ring‐grazing orbits prior to the Grand Finale.…”