Mapping Saturn's Nightside Plasma Sheet Using Cassini's Proximal Orbits

Abstract

Between April and the end of its mission on 15 September, Cassini executed a series of 22 very similar 6.5-day-period proximal orbits, covering the mid-latitude region of the nightside magnetosphere. These passes provided us with the opportunity to examine the variability of the nightside plasma sheet within this time scale for the first time. We use Cassini particle and magnetic field data to quantify the magnetospheric dynamics along these orbits, as reflected in the variability of certain relevant plasma parameters, including the energetic ion pressure and partial (hot) plasma beta. We use the University College London/Achilleos-Guio-Arridge magnetodisk model to map these quantities to the conjugate magnetospheric equator, thus providing an equivalent equatorial radial profile for these parameters. By quantifying the variation in the plasma parameters, we further identify the different states of the nightside ring current (quiescent and disturbed) in order to confirm and add to the context previously established by analogous studies based on long-term, near-equatorial measurements.Plain Language Summary Highly energized charge particles are trapped in the rapidly rotating magnetic cavity (the magnetosphere) of Saturn. Subject to the strong centrifugal potential, these particles, with energies in the order of few keV to few MeV, deform the dipole magnetic field of the planet and form an equatorial disk shaped structure, the magnetodisk. In this study we present how a theoretical magnetodisk model can be combined with Cassini particle data and accurately describe the field and plasma properties of the Saturnian magnetodisk. We further demonstrate how the previously observed variability of the electric current encircling the planet, the ring current, is also clearly identified in the measurements obtained during the 22 Cassini proximal orbits of nearly identical geometry. We find that this short-time temporal variability is comparable to the documented local time asymmetry of the ring current.

The Periodic Flapping and Breathing of Saturn's Magnetodisk During Equinox

“…A limitation of using a single mean ion mass along each individual field line is that the model cannot account for fine structural variation in magnetodisk thickness, caused by the concentration of heavier water ions towards the equatorial plane (e.g., Németh et al, ; Persoon et al, ) other limitations are discussed in detail in Achilleos, Guio and Arridge (). However, as demonstrated in that study, and Achilleos, Guio, Arridge, Sergis, et al () and Sergis et al (), the model can accurately reproduce global average trends observed in Saturn's magnetodisk. This model is therefore demonstrably adequate for reproducing the relatively large‐scale amplitude oscillations in the magnetic field data that are analyzed in this study.…”

Convection in the Magnetosphere of Saturn During the Cassini Mission Derived From MIMI INCA and CHEMS Measurements

“…The hot plasma (>3 keV) is well sampled by the Magnetospheric Imaging Instrument (MIMI, Krimigis et al, 2004) cluster of instruments. MIMI instrument measurements have been used to characterize the hot plasma and its moments in Saturn's magnetosphere (e.g., Dialynas et al, 2017;Dialynas et al, 2018;Kane et al, 2014Kane et al, , 2008Sergis et al, 2018Sergis et al, , 2017Sergis et al, , 2010Sergis et al, , 2009Sergis et al, , 2007. Two of the three MIMI instruments, the Ion and Neutral Camera (INCA) and the Charge Energy Mass Spectrometer (CHEMS) provided the data analyzed here.…”

Local Time Variation in the Large‐Scale Structure of Saturn's Magnetosphere

“…This mitigates our sensitivity to the high‐latitude loci of the field lines predicted by our models. In addition, similar mappings of UCL/AGA model calculations have been used in Sergis et al () to confirm that hot plasma pressure is approximately uniform along magnetic field lines, using high‐latitude proximal Cassini orbits.…”