Because of the role of Z‐mode emission in the diffusive scattering and resonant acceleration of electrons, we conduct a survey of intensity in the Saturn inner magnetosphere. Z mode is primarily observed as “5 kHz” narrowband emission in the lower density regions where the ratio of cyclotron to plasma frequency, fc/fp > 1 to which we limit this study. This occurs at Saturn along the inner edge of the Enceladus torus near the equator and at higher latitudes. We present profiles and parametric fits of intensity as a function of frequency, radius, latitude, and local time. The magnetic field intensity levels are lower than chorus, but the electric field intensities are comparable. We conclude that Z‐mode wave‐particle interactions may make a significant contribution to electron acceleration in the inner magnetosphere of Saturn, supplementing acceleration produced by chorus emission.

In order to conduct theoretical studies or modeling of pitch angle scattering of electrons by whistler mode chorus emission at Saturn, a knowledge of chorus occurrence and magnetic intensity levels, P B , as well as the distribution of P B relative to frequency and spatial parameters is essential. In this paper an extensive survey of whistler mode magnetic intensity levels at Saturn is carried out, and Gaussian fits of P B are performed. We fit the spectrum of wave magnetic intensity between the lower hybrid frequency and f ceq /2 and for frequencies in the interval f ceq /2 < f < 0.9 f ceq , where f ceq is the cyclotron frequency mapped to the equator. Saturn chorus is observed over most local times, but is dominant on the nightside in the range of 4.5 < L <7.5, with minimum power at the equator and peak power in the range of 5°< λ < 10°. Saturn wave magnetic intensity averaged in frequency bins peaks in the range of 10 À5 < P B < 10 À4 nT 2 for 0.4 < β < 0.5 (β = f/f ceq ). Gaussian fits of P B with frequency and latitude are obtained for lower band chorus. Plasma injection regions are occasionally encountered with significant chorus power levels. Upper band chorus is seen almost exclusively within plasma injection regions, and the number of events is very limited, but when present, the average levels of P B can be higher than the lower band chorus. The overall magnetic intensity contribution of the upper band, however, is insignificant relative to the lower band.

[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.

Just prior to the end of its prime mission, Juno flew by Ganymede (Hansen et al., 2022). The flyby takes advantage of Juno's advanced instrument complement to study details of the plasma, energetic particles and fields involved in the interaction between Ganymede's and Jupiter's magnetospheres. This paper focuses on plasma waves in Ganymede's magnetosphere.Galileo plasma wave and magnetic field measurements revealed the existence of Ganymede's magnetosphere during its first flyby of the moon (Gurnett et al., 1996;Kivelson et al., 1996). Additional Galileo studies included six close flybys (Shprits et al., 2018). The plasma wave observations showed a variety of emissions commonly associated with planetary magnetospheres, including whistler-mode emissions, electron cyclotron harmonics, a band at the upper hybrid frequency, broadband noise bursts at the magnetopause, and even radio emissions emanating from the moon's magnetosphere (Gurnett et al., 1996;Kurth et al., 1997).The Juno spacecraft executed a close flyby of Ganymede at 16:56 on 7 June, day 158, 2021 with a closest approach altitude of 1046 km. The trajectory approached Ganymede over its leading hemisphere or downstream from the moon relative to the co-rotational flow of Jupiter's magnetospheric plasma. The trajectory projected into the z-x and y-x planes is shown in Figure 1 using Ganymede-centered co-rotational coordinates (sometimes referred to as G PhiO ). The +z axis is parallel to Jupiter's rotational axis and the +x axis is parallel to the nominal co-rotational plasma flow. The +y axis is in the direction of Jupiter. The radius of Ganymede (R G ) is 2,631.2 km. The blue, green, and red bars denoted with the numbers 1, 2, and 3 identify regions observed in Ganymede's magnetosphere and will be used to organize the discussion of the Waves observations.

[1] The Cassini spacecraft flew very near a source region of Saturn kilometric radiation (SKR) on day 73 of 2008, the second known encounter with a source region at high latitude. The radio and plasma wave instrument, Radio and Plasma Wave Science, observed intense kilometric emission in the extraordinary X mode, ordinary O mode, and Z mode. The electron low-energy spectrometer obtained a phase space distribution of sufficient energy and pitch angle resolution to allow growth rate calculations. There is evidence of a shell-like electron plasma distribution that is unstable to the growth of SKR via the cyclotron maser instability. The growth rates calculated are adequate to explain the observed X and Z mode emission, but nonlinear effects are required to explain the large O mode gain (as is true for terrestrial observations). Narrowband emission, also present at the time, could also explain both the Z mode and the O mode. We present the results for comparison with a previously reported source region encounter and with similar observations at Earth auroral kilometric source regions.

We have conducted an extensive survey of whistler mode hiss emission at Saturn using nearly the complete data set from the Cassini Radio and Plasma Wave Science investigation. We distinguish the hiss emission by frequency and intensity threshold criteria. The survey includes all longitudes and latitudes >25°for r < 10 R S , limited to the availability of plasma density data. The results indicate that hiss intensity is highest at high latitude, and the level of intensity is larger than chorus by at least an order of magnitude. After a rapid increase in intensity starting near L = 13, the hiss intensity remains relatively constant extending to L > 100. The intensity of the hiss as a function of frequency has a shallow peak near 35 Hz but remains always in the range~5 × 10 −5 nT 2 < I <~10 −3 nT 2. These survey results will be important in modeling the scattering interactions of whistler mode emissions with electrons in the Saturn magnetosphere. Based on recent particle and field observations, sources of Saturn hiss occur on and near the footprints of field-aligned currents, plasma injections, and reconnected field lines.

The Juno Waves instrument measured plasma waves associated with Ganymede's magnetosphere during its flyby on 7 June, day 158, 2021. Three distinct regions were identified including a wake, and nightside and dayside regions in the magnetosphere distinguished by their electron densities and associated variability. The magnetosphere includes electron cyclotron harmonic emissions including a band at the upper hybrid frequency, as well as whistler-mode chorus and hiss. These waves likely interact with energetic electrons in Ganymede's magnetosphere by pitch angle scattering and/or accelerating the electrons. The wake is accentuated by low-frequency turbulence and electrostatic solitary waves. Radio emissions observed before and after the flyby likely have their source in Ganymede's magnetosphere.

Twenty high‐inclination ring‐grazing orbits occurred in the final period of the Cassini mission. These orbits intercepted a region of intense Z‐mode and narrowband (NB) emission (Ye et al., 2010, https://doi.org/10.1029/2009JA015167) along with isolated, intense upper hybrid resonance (UHR) emissions that are often associated with NB source regions. We have singled out such UHR emission seen on earlier Cassini orbits that also lie near the region crossed by the ring‐grazing orbits. These previous orbits are important because Cassini electron phase‐space distributions are available and dispersion analysis can be performed to better understand the free energy source and instability of the UHR emission. We present an example of UHR emission on a previous orbit that is similar to that observed during the ring‐grazing orbits. Analysis of the observed plasma distribution of the previous orbit leads us to conclude that episodes of UHR emission and NB radiation observed during the ring‐grazing orbits are likely due to plasma distributions containing loss cones, temperature anisotropies, and strong density gradients near the ring plane. Z‐mode emissions associated with UHR and NB emission can be in Landau resonance with electrons to produce scattering or acceleration (Woodfield et al., 2018, https://doi.org/10.1038/s41467-018-07549-4).