Narrowband Z‐mode emissions interior to Saturn's plasma torus

Abstract

[1] During Cassini's close approach to Saturn, on 1 July 2004, a set of narrow bandwidth plasma emissions were detected by the Radio and Plasma Wave Science (RPWS) instrument in the inner magnetosphere. These discrete tones were detected between 3 and 70 kHz, with individual tone bandwidths as low as a few hundred Hertz. The tones persisted for long times ($1 hour) as Cassini flew in planetocentric radial distances of less than 2.5 R s . During this time at lower radial distances, the spacecraft passed inside the inner edge of a clear and distinct plasma torus; this torus is located between 2.2 and $10 R s . We describe the emissions, and demonstrate that the mode of propagation is the Z-mode. The emissions are found to originate at locations where electron plasma oscillations along the plasma torus edge are relatively intense. We describe a mechanism for f p electrostatic-to-electromagnetic wave conversion to explain the origin of the narrowband Z-mode tones. The tones allow remote-sensing of the plasma torus and indicate that the torus is dynamic, with changes in density in the tens of percent over the course of an hour.

Observation of similar radio signatures at Saturn and Jupiter: Implications for the magnetospheric dynamics

“… Gurnett et al [2005] have reported on an intense wave activity in the region of transition from super to sub‐corotation of neutrals (r ∼ 1.8 Rs). The density is there smaller than 0.1 cm −3 [ Wahlund et al , 2005] and electromagnetic waves, at n‐SMR frequencies, are locally observed [ Farrell et al , 2005]. As sketched in Figure 4, these waves could access free space at large polar angles and explain the regular n‐SMR detection from high latitudes.…”

“…To progress, we will assume that the n‐SMR is generated in the inner magnetosphere, on density gradients. Here, it is interesting to note that radio tones, with spectra similar to those of n‐SMR, have been observed at the inner edge of Saturn's plasma disk [ Farrell et al , 2005; Gurnett et al , 2005]. It is of interest to investigate whether the modulation of the radiation is synchronous with corotation.…”

Mass unloading along the inner edge of the Enceladus plasma torus

“…In examining plasma density profiles alone, it is relatively difficult to determine where the Enceladus torus ends and the ring‐ionosphere plasma dominates. Fortunately, this inner edge is active and radiates remotely‐propagating z‐mode from active regions with intensified f p emission [ Farrell et al , 2005]. The emission, in essence, “tags” the active plasma sources at various locations and allows a method to remote‐sense the source plasma evolution.…”

“…In reference to Figure 1, between 03:15 and 04:15 SCET, Cassini was passing over the nightside faces of the B and A rings. We note that between 03:00 and 03:43 SCET, the electron plasma frequency is identified by the upper cutoff of an auroral hiss/whistler mode emission [ Gurnett et al , 2005; Xin et al , 2006] while thereafter an obvious emission at the electron plasma frequency, f pe , can be uniquely identified, this appearing as a steeply up‐drifting narrowband tone that is labeled as “f p” on the figure [ Gurnett et al , 2005; Farrell et al , 2005]. The electron density derived from this f pe emission is consistent with independent RPWS/Langmuir Probe measurements [ Wahlund et al , 2005].…”

“…Besides emissions at the plasma frequency, there is the presence of narrowband drifting tones in the spectrogram. A previous study [ Farrell et al , 2005] identified these as remotely‐propagating z‐mode emissions which are found to easily penetrate into regions where f < f p . The detailed arguments for z‐mode emission are presented therein.…”