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Published November 2019 | Published
Journal Article Open

Magnetic Field Observations on Cassini's Proximal Periapsis Passes: Planetary Period Oscillations and Mean Residual Fields

Abstract

We analyze periapsis pass magnetic field data from the final 23 orbits of the Cassini spacecraft at Saturn, uniquely encompassing auroral, subauroral, ring region, and intra‐ring field lines, to determine the planetary period oscillations (PPOs) and mean residual fields in these regions. Dual modulation by northern and southern PPO systems is found almost continuously, demonstrating for the first time the presence of PPOs on and inside ring region field lines. The azimuthal component displays the largest ~10–15nT PPO amplitudes on auroral field lines, falling across the subauroral region to ~3–5 nT on main ring field lines in the northern hemisphere, less in the southern hemisphere, while increasing to ~5–8 nT on D ring and intra‐D ring field lines. Auroral and subauroral amplitudes mapped along field lines are in good agreement with previous analyses in regions of overlap. Colatitudinal and radial field oscillations generally have a half and a quarter the amplitude of the azimuthal component, respectively. Inner region oscillation phases are typically several tens of degrees "earlier" than those of outer subauroral and auroral regions. Mean residual poloidal fields (internal and ring current fields subtracted) show quasi‐sinusoidal latitude variations of ~2.5nT amplitude, with radial and colatitudinal fields approximately in quadrature. Mean azimuthal fields peaking at ~15 nT are approximately symmetrical about the equator on and inside D ring field lines as previously reported, but are unexpectedly superposed on ~3–5nT "lagging" fields which extend continuously through the ring region onto subauroral field lines north and south.

Additional Information

© 2019 The Authors. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. Received 3 APR 2019; Accepted 25 SEP 2019; Accepted article online 10 OCT 2019; Published online 19 NOV 2019. Work at the University of Leicester and Imperial College London was supported by STFC consolidated grants ST/N000749/1 and ST/N000692/1, respectively. E.J.B. was supported by a Royal Society Wolfson Research Merit Award and M.K.D. by a Royal Society Research Professorship. T.J.B. was supported by STFC Quota Studentship ST/N504117/1. We thank Steve Kellock and the Cassini magnetometer team at Imperial College for the access to processed magnetic field data. 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/).

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Additional details

Created:
August 22, 2023
Modified:
October 19, 2023