The plasma environment of Titan: The magnetodisk of Saturn near the encounters as derived from ion densities measured by the Cassini/CAPS plasma spectrometer

Abstract

[1] We analyze ion densities derived from the data of the Cassini Plasma Spectrometer for the time period of the prime mission till the end of May 2008, in the low latitude outer magnetosphere near Titan encounters. We have found that the central line of the magnetodisk is surrounded by a structured plasma sheet, a smooth, broad ion layer composed of light ions, and a heavy ion layer displaying narrow substructures. The heavy ion densities are spiky; the co-location of the observed enhanced ion plasma densities with the change of sign of the radial component of the magnetic field is demonstrated. At these locations the heavy ion density is the highest. The plasma sheet is denser and wider on the dayside of Saturn than on the nightside; in the lobes the protons were dominant. Based on a statistical analysis for proton densities measured between radial distances from 10 R S to 22 R S we project densities to Titan's orbital distance. We show that the projected proton density in the magnetodisk in organized by SLS3 longitude and, therefore, is modulated by SKR. In the lobes, the proton density is nearly constant. High heavy ion density in the sheet is accompanied by low heavy ion temperature. The magnetospheric interaction with Titan is primarily defined by the SLS3 phase of the encounter and the distance of the moon from the magnetic equator. Accordingly, the incoming plasma flow impinging on Titan cannot be stable for a few hours before the encounter.

The latitudinal structure of the nightside outer magnetosphere of Saturn as revealed by velocity moments of thermal ions

“…The period of the modulation is near the ro-tational period generally observed by magnetic and plasma measurements Andrews et al, 2012;Szego et al, 2011Szego et al, , 2013. The observed periodicity is compatible with the magnetic phases of Provan et al (2011) however the data coverage is not good enough to allow a stronger statement.…”

“…Persoon et al (2009) show that the density depends strongly on the distance from the central sheet of the magnetodisk, the density being highest in the central sheet and steadily decreasing towards the lobes. This dependence together with the vertical flapping motion of the magnetodisk leads to the periodic modulation of the measured density along the orbit of the spacecraft, which was recently found in Cassini ion data Szego et al, 2011Szego et al, , 2013. This suggests that the observed periodic modulation of the azimuthal velocity also correlates with the motion of the magnetodisk.…”

“…Periodic variations were found in the density of thermal ions as well Szego et al, 2011Szego et al, , 2012Szego et al, , 2013Arridge et al, 2011b). The thermal plasma sheet properties are shown to be different for different ion species.…”

Sustained lobe reconnection in Saturn's magnetotail

“…By selecting both examples in Figure from near current sheet crossings, we avoid the possibility that the composition differences we see are attributable to the strong latitude dependence of heavy ions mentioned above [e.g., Szego et al ., , ]. To demonstrate that the differences between the two plasma sheet crossings of Figure were in fact characteristic of the plasma sheet prior to and after periapsis, Figure shows a concatenation of 2 h CAPS spectrogram segments from all the current sheet crossings by Cassini between day 240 and day 263 for which there were detectable ions in CAPS IMS.…”

Ion Composition in Titan's Exosphere via the Cassini Plasma Spectrometer

“…This is generally done in one of two ways. The first involves direct integration of total ion counts over a partially sampled velocity distribution that is filled in by assuming symmetry in the reference frame of the flowing plasma [Sittler et al, 2005;Thomsen and Delapp, 2005;Sittler et al, 2006;Thomsen et al, 2010;Szego et al, 2011;and Szego et al, 2012]. The second method fits either isotropic or anisotropic Maxwell distributions to energy-and angle-discriminated ion counts in order to extract moments from the fit parameters [Tokar et al, 2005 andHill et al, 2008;Wilson et al, 2008;McAndrews et al, 2009;and Elrod et al, 2012].…”