Abstract
AbstractDuring 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.
Highlights
On the final 22 proximal orbits of the Cassini mission at Saturn between late April and mid‐September 2017, corresponding to spacecraft revolutions (Revs) 271–292, the spacecraft passed through the equatorial plane near periapsis in the noon sector at variable altitudes between ~2,000 and ~4,000 km above the 1‐bar atmospheric layer, in the narrow gap between the inner boundary of Saturn's ring system and the denser layers of the planet's atmosphere
We present current and current density results as described above for four representative intra‐D ring passes
Following the initial reports of the presence of the intra‐D ring azimuthal field perturbations from the early proximal orbit magnetic field data by Dougherty et al (2018) and Khurana et al (2018) and the first survey of all the proximal data by Provan et al (2019), in this paper, we have examined the ionospheric meridional currents and related field‐aligned current densities associated with the observed field signatures, assuming approximate local axisymmetry
Summary
On the final 22 proximal orbits of the Cassini mission at Saturn between late April and mid‐September 2017, corresponding to spacecraft revolutions (Revs) 271–292, the spacecraft passed through the equatorial plane near periapsis in the noon sector at variable altitudes between ~2,000 and ~4,000 km above the 1‐bar atmospheric layer, in the narrow gap between the inner boundary of Saturn's ring system and the denser layers of the planet's atmosphere. The azimuthal field was usually positive in the sense of planetary rotation, a few tens of nanotesla in amplitude, and approximately symmetrical in form about the magnetic equator (Dougherty et al, 2018) Such field perturbations are suggestive of the presence of an interhemispheric field‐aligned current that usually flows north to south on and inside D ring field lines, which closes south to north in the region below the spacecraft, presumably in the Pedersen layer of the planetary ionosphere centered ~1,000 km above 1 bar (Galand et al, 2011). The field lines on which these intra‐D ring currents flow map to the ionosphere within ~30° of the equator in the north and within ~25° in the south, the north‐south asymmetry being due principally to the strong quadrupole planetary field component
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