Analysis of narrowband emission observed in the Saturn magnetosphere

Abstract

[1] Narrowband emission is observed at Saturn centered near 5 kHz and 20 kHz and harmonics of 20 kHz. This emission appears to be in many ways similar to Jovian narrowband emission observed at higher frequencies. We analyze one example of this emission near a possible source region. In situ electron distributions suggest narrowband emission has a source region associated with electrostatic cyclotron harmonic and upper hybrid emission. Linear growth rate calculations indicate that the observed plasma distributions are unstable to the growth of electrostatic harmonic emissions. In addition, it is found that when the local upper hybrid frequency is close to 2 f ce or 3 f ce (f ce is the electron cyclotron frequency), electromagnetic Z mode and weak ordinary (O mode) emission can be directly generated by the cyclotron maser instability. In the presence of density gradients, Z mode emission can mode-convert into O mode emission, and this might explain the narrowband emission observed by the Cassini spacecraft.

Source mechanism of Saturn narrowband emission

“…We have been fortunate to obtain electron phase space densities with a sufficient pitch angle distribution for analysis. The results reported here support and extend the work of Menietti et al (2009).…”

“…Ye et al (2009) In this paper we examine in some detail new examples of NB observed by Cassini at Saturn with center frequency near 15 kHz. The examples are associated with electrostatic cyclotron harmonic (ECH) emission and probable upper hybrid waves, similar to the case examined by Menietti et al (2009), but in this new case, the source is in the Northern Hemisphere and the ratio of f p /f ce appears to be somewhat higher. We conduct a similar analysis of the in-situ wave and particle data, including a spatial growth calculation, to demonstrate that the cyclotron maser instability is likely responsible for the direct generation of Z-mode emission which may mode convert to O mode or NB emission as observed.…”

“…In addition, it is found that when the local hybrid frequency is close to 3 f ce (f ce is the electron cyclotron frequency), electromagnetic Z-mode and weak ordinary (O-mode) emission are directly generated by the cyclotron maser instability. In the presence of density gradients observed near the purported source region, Z-mode emission can mode-convert into Omode emission, and Menietti et al (2009) suggested that this might explain the NB emission observed by the Cassini spacecraft. These authors noted that it will be important to test this scenario for other encounters with source regions of NB, especially for conditions of varying ratios of f p/ f ce .…”

“…Wang et al (2009) summarized the observations of narrowband radio emissions by Cassini and examined the possible association of narrowband radio emissions with a revolving plasma cloud detected by the MIMI/INCA (Magnetospheric Imaging/Ion and Neutral Camera) instrument onboard Cassini. Menietti et al (2009) analyzed an example of NB emission near a possible source region in the mid-latitude Southern Hemisphere of Saturn. In situ electron distributions suggest NB has a source region associated with electrostatic cyclotron and upper hybrid emission.…”

Rotational modulation of Saturn Kilometric Radiation, narrrowband emission and auroral hiss

“…Ye et al [2010b] showed that the L-O mode narrowband emissions are mode converted from Z mode narrowband emissions observed near Saturn. Less intense narrowband emissions at higher frequencies (∼ 10-70 kHz) are found to originate from upper hybrid resonance emissions on the boundary of the plasma torus Menietti et al, 2009;. A review of the studies on Saturn narrowband emissions can be found in Ye et al [2011].…”