J. B. Faden

Electron densities inferred from plasma wave spectra obtained by the Waves instrument on Van Allen Probes

The twin Van Allen Probe spacecraft, launched in August 2012, carry identical scientific payloads. The Electric and Magnetic Field Instrument Suite and Integrated Science suite includes a plasma wave instrument (Waves) that measures three magnetic and three electric components of plasma waves in the frequency range of 10 Hz to 12 kHz using triaxial search coils and the Electric Fields and Waves triaxial electric field sensors. The Waves instrument also measures a single electric field component of waves in the frequency range of 10 to 500 kHz. A primary objective of the higher-frequency measurements is the determination of the electron density ne at the spacecraft, primarily inferred from the upper hybrid resonance frequency fuh. Considerable work has gone into developing a process and tools for identifying and digitizing the upper hybrid resonance frequency in order to infer the electron density as an essential parameter for interpreting not only the plasma wave data from the mission but also as input to various magnetospheric models. Good progress has been made in developing algorithms to identify fuh and create a data set of electron densities. However, it is often difficult to interpret the plasma wave spectra during active times to identify fuh and accurately determine ne. In some cases, there is no clear signature of the upper hybrid band, and the low-frequency cutoff of the continuum radiation is used. We describe the expected accuracy of ne and issues in the interpretation of the electrostatic wave spectrum.

Evidence for a seasonally dependent ring plasma in the region between Saturn's A Ring and Enceladus' orbit

Equatorial electron density measurements from the Cassini Radio and Plasma Wave Science experiment are derived from the upper hybrid resonance frequency from Saturn Orbit Insertion (SOI) on 1 July 2004 through 21 May 2013. These densities are used to determine the characteristics of the plasma in the inner magnetosphere of Saturn between the outer edge of the A Ring and the orbit of Enceladus. Electron densities obtained when Cassini first arrived at Saturn on 1 July 2004 showed a plasma distribution decreasing radially outward from Saturn in the direction of Enceladus, the expected distribution of a centrifugally driven plasma expanding radially outward from a source in the main rings. We examine equatorial electron densities in the region between 2.4 and 4.0 Rs and show that the density measurements in this region exhibit a strong seasonal dependence resulting from photon-induced decomposition of icy particles on the ring surfaces, a decomposition process which is controlled by the solar incidence angle. This seasonal dependence will have plasma density implications for Cassini when the spacecraft returns to the region just beyond the A Ring in 2016.