Analysis of plasma waves observed in the inner Saturn magnetosphere

Abstract

Abstract. Plasma waves observed in the Saturn magnetosphere provide an indication of the plasma population present in the rotationally dominated inner magnetosphere. Electrostatic cyclotron emissions often with harmonics and whistler mode emission are a common feature of Saturn's inner magnetosphere. The electron observations for a region near 5 R S outside and near a plasma injection region indicate a cooler low-energy (<100 eV), nearly isotropic plasma, and a much warmer (E>1000 eV) more pancake or butterfly distribution. We model the electron plasma distributions to conduct a linear dispersion analysis of the wave modes. The results suggest that the electrostatic electron cyclotron emissions can be generated by phase space density gradients associated with a loss cone that may be up to 20 • wide. This loss cone is sometimes, but not always, observed because the field of view of the electron detectors does not include the magnetic field line at the time of the observations. The whistler mode emission can be generated by the pancake-like distribution and temperature anisotropy (T ⊥ /T || >1) of the warmer plasma population.

Chorus, ECH, and Z mode emissions observed at Jupiter and Saturn and possible electron acceleration

“…For lower values of f p /f c (at higher latitudes) the whistler resonance energy increases, and we would expect interaction of the waves with higher‐energy electrons which are available in the distribution. The electron distributions observed by Menietti et al [2008c] with electron populations in the range of 100 eV to 10 4 eV are typical of this region of the Saturn magnetosphere [cf. Schippers et al , 2008].…”

“…It is interesting, however, to note the electron distributions associated with chorus emissions. Menietti et al [2008c] studied chorus growth rate based on in situ electron distributions obtained by the Cassini electron spectrometer (ELS) near the magnetic equator during an equatorial orbit where moderately strong chorus Figure 4a near the region of suspected Z mode emission. Electric (top) and magnetic field spectral density are shown for a reduced and linear frequency range from Figure 4a.…”

“…Centrifugal force from the rapid rotation of Jupiter and Saturn drives flux interchange instabilities whereby outward transport of cold dense plasma is replaced by inward transport of higher energy particles that develop temperature anisotropy and excite whistler mode waves (Jupiter: Bolton et al [1997] and Thorne et al [1997]; Saturn: Rymer et al [2008] and Menietti et al , 2008a, 2008c]). This concept is discussed in some detail by Hill et al [1983; 2005] (see also Thorne [1983, Figure 12.12], which depicts the flow of centrifugally driven plasma in the Jovian magnetosphere).…”

The Generation of Saturn's Aurora at Lower Latitudes by Electrostatic Waves

“…ECH emissions are observed near the equator having wave intensity levels in the range 10 −13 < I < 10 −10 V 2 · m −2 · Hz −1 (Menietti et al, ; Tao et al, ). Menietti et al (, ) have performed a linear dispersion analysis of the expected wave modes using the observed electron distribution function. These authors find that the observed ECH wave characteristics can be obtained by a loss cone of about 3° in the cold electron population in the injection event.…”

“…The observed ECH wave spectral intensity and the observed electron distribution function are used. The calculations are performed at two L‐shells (L = 5.35 and L = 7.0) for which observations of ECH wave and magnetospheric electron data have been reported (Menietti et al, , ). In section 2 we present the calculation details of precipitation flux, electron differential flux, and UV intensities.…”