Ion densities and velocities in the inner plasma torus of Saturn

Abstract

We present plasma data from the Cassini Radio and Plasma Wave Science (RPWS) Langmuir probe (LP), mapping the ion density and velocity of Saturn's inner plasma torus. Data from 129 orbits, recorded during the period from the 1st of February 2005 to the 27th of June 2010, are used to map the extension of the inner plasma torus. The dominant part of the plasma torus is shown to be located in between 2.5 and 8 Saturn radii (1RS=60,268km) from the planet, with a north-southward extension of ±2RS. The plasma disk ion density shows a broad maximum in between the orbits of Enceladus and Tethys. Ion density values vary between 20 and 125cm−3 at the location of the density maximum, indicating considerable dynamics of the plasma disk. The equatorial density structure, |z|<0.5RS, shows a slower decrease away from the planet than towards. The outward decrease, from 5RS, is well described by the relation neq=2.2×104(1/R)3.63. The plume of the moon Enceladus is clearly visible as an ion density maximum of 105cm−3, only present at the south side of the ring plane. A less prominent density peak, of 115cm−3, is also detected at the orbit of Tethys, at ∼4.9RS. No density peaks are recorded at the orbits of the moons Mimas, Dione, and Rhea. The presented ion velocity vi,θ shows a clear general trend in the region between 3 and 7RS, described by vi,θ=1.5R2−8.7R+39. The average vi,θ starts to deviate from corotation at around 3RS, reaching ∼68% of corotation close to 5RS.