Wave and Particle Analysis of Z‐Mode and O‐Mode Emission in the Jovian Inner Magnetosphere

Abstract

We report some of the most intense Z-mode and O-mode observations obtained by the Juno spacecraft while in orbit about Jupiter in a low to mid-latitude region near the inner edge of the Io torus. We have been able to estimate the density of the plasma in this region based on the lower frequency cutoff of the observed Z-mode emission. The results are compatible with the electron density measurements of the Jovian Auroral Distributions Experiment (JADE), on board the Juno spacecraft, if we account for unmeasured cold plasma. Direction-finding measurements indicate that the Z-and O-mode emission have distinct source regions. We have also used the measured phase space density of the JADE and the Jupiter energetic particle detector instruments to calculate estimated local growth rates of the observed O-mode and Z-mode emission assuming a loss cone instability and quasilinear analysis. The results suggest the emissions were observed near, but not within, a source region, and the free energy source is consistent with a loss cone. We have thus carried out the quasilinear wave analysis of the assumed remote Z-and O-mode wave growths. It is shown that the remotely generated waves, propagated through an inhomogeneous medium to the satellite location, may account for the observed wave characteristics. The importance of Z-mode in accelerating electrons in the inner Jovian magnetosphere makes these new wave mode confirmations at Jupiter of particular interest.

Survey and Analysis of Whistler‐ and Z‐Mode Emission in the Juno Extended Mission

“…The Z-mode observations span the frequency range from ∼3 kHz to 85 kHz, peaking at ∼10 −6 nT 2 at 15 kHz then falling at middle frequencies until peaking again at 75 kHz, and falling off sharply at higher frequencies. The variation of intensity with magnetic latitude and M-shell is consistent with a source region in the outer edges of the inner Io torus (Menietti et al, 2023), but does not rule out sources at higher latitudes, and at lower radial distances outside the Io plasma torus. Figure 5a shows a dawn-dusk asymmetry in the Z-mode intensity similar to that of the chorus in Figure 1a, and the same comments regarding the nature of the asymmetry discussed for chorus may also apply to Z-mode.…”

“…These waves are typically one to two orders of magnitude less intense than whistler-mode chorus, but are about two orders of magnitude larger than typical intensities of Z-mode observed at Saturn Menietti, Averkamp, Ye, Sulaiman, et al, 2018). Menietti et al (2023) have identified both Z-mode and O-mode propagating in the same region with different intensity, pointing out the difficulties in always distinguishing Z-mode without polarization measurements. The Z-mode observations span the frequency range from ∼3 kHz to 85 kHz, peaking at ∼10 −6 nT 2 at 15 kHz then falling at middle frequencies until peaking again at 75 kHz, and falling off sharply at higher frequencies.…”

“…Menietti et al. (2023) have identified both Z‐mode and O‐mode propagating in the same region with different intensity, pointing out the difficulties in always distinguishing Z‐mode without polarization measurements.…”