Abstract
Abstract. Narrowband emission (NB) is observed at Saturn centered near 5 kHz and 20 kHz and harmonics. This emission appears similar in many ways to Jovian kilometric narrowband emission observed at higher frequencies, and therefore may have a similar source mechanism. Source regions of NB near 20 kHz are believed to be located near density gradients in the inner magnetosphere and the emission appears to be correlated with the occurrence of large neutral plasma clouds observed in the Saturn magnetotail. In this work we present the results of a growth rate analysis of NB emission (~20 kHz) near or within a probable source region. This is made possible by the sampling of in-situ wave and particle data. The results indicate waves are likely to be generated by the mode-conversion of directly generated Z-mode emission to O-mode near a density gradient. When the local hybrid frequency is close n fce (n is an integer and fce is the electron cyclotron frequency) with n=4, 5 or 6 in our case, electromagnetic Z-mode and weak ordinary (O-mode) emission can be directly generated by the cyclotron maser instability.
Highlights
Earth orbiting satellites have revealed two general types of non-thermal emission: directly generated emission and mode-converted emission
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 fp/fce appears to be somewhat higher
To Saturn kilometric radiation, Saturn Narrowband emission (NB) emission is perhaps among the more common and intense of the radio emissions observed at Saturn
Summary
Earth orbiting satellites have revealed two general types of non-thermal emission: directly generated emission and mode-converted (indirect) emission. In explaining the generation of terrestrial auroral roar, Yoon et al (1996, 1998) have shown that the growth rates of Z-mode are greatly enhanced when fU2H=fc2e + fp2=(nfce), where n=2 and 3 (fp is the electron plasma frequency) This Z-mode can escape into free space by a linear mode conversion into ordinary (O) or whistler mode (e.g. Ellis, 1956). The emission frequencies vary from about 10 kHz to about 35 kHz. 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 fp/fce 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
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