posted on 2019-01-14, 10:26authored byG. Provan, S. W. H. Cowley, E. Bunce, T. J. Bradley, G. J. Hunt, H. Cao, M. K. Dougherty
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.
Funding
Work at the University of Leicester was supported by STFC grant
ST/N000749/1. Work at Imperial College was supported by STFC grant ST/N000692/1.
EJB was supported by a Royal Society Wolfson Research Merit Award. MKD was
supported by Royal Society Research Professorship RP140004. TJB was supported by STFC
Quota Studentship ST/N504117/1. We thank Steve Kellock and the Cassini magnetometer
team at Imperial College for access to processed magnetic field data. We also thank the
reviewers for useful comments. Calibrated magnetic field data from the Cassini mission are
available from the NASA Planetary Data System at the Jet Propulsion Laboratory
(https://pds.jpl.nasa.gov/).
History
Citation
Journal of Geophysical Research: Space Physics, 2019
Author affiliation
/Organisation/COLLEGE OF SCIENCE AND ENGINEERING/Department of Physics and Astronomy