The Structure of Planetary Period Oscillations in Saturn's Equatorial Magnetosphere: Results From the Cassini Mission. Issue 11 (1st November 2019)

• Record Type: Journal Article
• Authors: Andrews, D. J.; Cowley, S. W. H.; Provan, G.; Hunt, G. J.; Hadid, L. Z.; Morooka, M. W.; Wahlund, J.‐E.
• Is Part Of: Journal of geophysical research. Volume 124:Issue 11(2019)
• Page Start: 8361
• Page End: 8395
• Publication Date: 2019-11-01
• Subjects: Saturn -- PPO -- magnetosphere -- currents -- magnetic fields -- plasmas; Magnetospheric physics -- Periodicals; Space environment -- Periodicals; Cosmic physics -- Periodicals; Planets -- Atmospheres -- Periodicals; Heliosphere (Astrophysics) -- Periodicals; Geophysics -- Periodicals; 523.01
• DOI: 10.1029/2019JA026804
• Languages: English
• ISSNs: 2169-9380

Abstract

Abstract: Saturn's magnetospheric magnetic field, planetary radio emissions, plasma populations, and magnetospheric structure are all known to be modulated at periods close to the assumed rotation period of the planetary interior. These oscillations are readily apparent despite the high degree of axisymmetry in the internally produced magnetic field of the planet and have different rotation periods in the northern and southern hemispheres. In this paper we study the spatial structure of (near‐)planetary period magnetic field oscillations in Saturn's equatorial magnetosphere. Extending previous analyses of these phenomena, we include all suitable data from the entire Cassini mission during its orbital tour of the planet so as to be able to quantify both the amplitude and phase of these field oscillations throughout Saturn's equatorial plane, to distances of 30 planetary radii. We study the structure of these field oscillations in view of both independently rotating northern and southern systems, finding spatial variations in both magnetic fields and inferred currents flowing north‐south that are common to both systems. With the greatly expanded coverage of the equatorial plane achieved during the latter years of the mission, we are able to present a complete survey of dawn‐dusk and day‐night asymmetries in the structure of the oscillating fields and currents. We show that the general structure of the rotating currents is simpler than previously reported and that the relatively enhanced nightside equatorial fields and currents are due in part to related periodic vertical motion of Saturn's magnetotail current sheet. Plain Language Summary: Saturn's magnetic field, produced in its interior by dynamo processes, is apparently perfectly symmetric with respect to the spin axis of the planet. Despite this, measurements of magnetic fields in Saturn's magnetosphere, radio emissions, and plasma populations all show oscillations with periods close to the (inferred) rotation rate of the interior of the planet. The origin of these oscillations is yet to be fully explained, but electrodynamic coupling between the upper atmosphere/ionosphere and magnetosphere plays a central role. In this study, we use magnetic field measurements from NASA's Cassini spacecraft to statistically study the spatial structure of the magnetic field oscillations and the electrical currents producing them. Expanding on previous studies, we find good overall agreement with existing theoretical models but with discrepancies suggestive of the influence of solar wind magnetosphere coupling on the system. Key Points: We conduct a complete analysis of the structure of Saturn's equatorial magnetic field oscillations Seasonal and beat‐phase variations are accounted for in an analysis of dual period oscillations Electrical currents flow vertically through the equatorial plane in two linked spirals