The inner magnetosphere of Saturn: Cassini RPWS cold plasma results from the first encounter

Abstract

We present new results from the inner magnetosphere of Saturn obtained by the Radio and Plasma Wave Science (RPWS) investigation onboard Cassini around the period of the Saturn orbit injection (July 1, 2004). Plasma wave electric field emissions, voltage sweeps by the Langmuir probe (LP) and radio sounder data were used to infer the cold plasma (<100 eV) characteristics within 20 RS of Saturn. A dense (<150 cm−3) and cold (<7 eV) plasma torus was found just outside the visible F‐ring. This torus of partly dusty plasma does not perfectly co‐rotate with Saturn, which suggests the cold plasma is electro‐dynamically coupled to the charged ring‐dust particles. The spacecraft potential was a few volts negative above the E‐ and G‐rings, indicating the dust‐particles were likewise negatively charged. The cold ion characteristics changed near the magnetically conjugate position of Dione, indicating release of volatile material from this icy moon.

Characteristics of charged dust inferred from the Cassini RPWS measurements in the vicinity of Enceladus

“…Early measurements by the Cassini plasma instruments (Sittler et al, 2005) in some places deviated significantly from the predictions of the Richardson model. Rather high negative dust equilibrium potential inferred from the dust charge measurements by CDA (at least for distances ro5R S , see Kempf et al, 2006) as well as from measurements of the spacecraft potential by the RPWS Langmuir probe (LP) (Wahlund et al, 2005 were also not consistent with the Richardson model.…”

Electron acoustic waves in double-kappa plasmas: Application to Saturn's magnetosphere

“…The outer region of the magnetosphere, which extends beyond (12-14) R S up to the magnetopause boundary (∼20 R S ), is characterized by a low plasma density and is strongly influenced by the solar wind. It also consists mainly of lighter ions with masses m i ] 10 amu (including protons), a tenuous (n e ∼ 0.1-0.5 cm −3 ) hot plasma, and a quiet magnetic field [Wahlund et al, 2005;André et al, 2008].…”

The electron density of Saturn's magnetosphere

“…from how the current depends on the bias potential. Although the LP is well designed to characterize the plasma by direct analysis of the collected plasma particle current in a dense and cold plasma regime (Wahlund et al, 2005), the direct measurement from the probe current becomes difficult in the tenuous plasma. Instead, the photoelectrons generated from the sunlit spacecraft dominate in the space region, and the probe current is balanced by the photoelectrons escaping from the probe and the ambient electrons (the ion contribution is small).…”

“…Recent observations have made it possible to study the magnetospheric electron densities in more detail (Moncuquet et al, 2005;Persoon et al, 2005Persoon et al, , 2006Wahlund et al, 2005. Persoon et al (2005) used upper hybrid resonance emissions to measure the equatorial electron density, and found that the electron density varies significantly inside 5 R S , but consistently drops off exponentially with increasing distance from Saturn beyond 5 R S .…”

“…This is because the spacecraft must maintain a current balance between the photoelectrons it emits and the plasma electrons it collects. Using the floating potential of the probe (U float ) (Wahlund et al, 2005), which depends directly on the spacecraft potential (Cully et al, 2007), we can obtain the electron number density in Saturn's magnetosphere, as described in Sect. 2 below.…”