Low‐Latitude Whistler‐Mode and Higher‐Latitude Z‐Mode Emission at Jupiter Observed by Juno

Abstract

Whistler‐mode chorus emission is important in the scattering and acceleration of electrons and filling of the radiation belts at Jupiter. In this work whistler mode magnetic intensity levels at Jupiter are comprehensively binned and parameterized. The frequency range of whistler mode under study extends from the proton cyclotron frequency, fcH, to fceq/2, where fceq is the cyclotron frequency mapped to the magnetic equator. Parametric dependence of magnetic plasma wave intensity is obtained versus frequency, latitude, and M‐shell, as determined using a current magnetic field model based on Juno data. The results extend similar analyses of Jupiter whistler‐mode emission obtained by the Galileo spacecraft, particularly on the nightside, and provide better coverage in latitude. Peaks in whistler‐mode emission occur near M ∼ 8–9, similar to previous studies, with average peak intensities approaching 10−2 nT2, as also found by Galileo on the dayside. Auroral hiss and probably Z‐mode are observed at higher latitudes. Jovian chorus emissions near an equatorial source region are more broad‐banded than terrestrial chorus, and are coincident with a broad‐banded electron distribution of free energy and strong electron scattering to large pitch angles. Intense whistler mode within a young plasma injection region is also observed, similar to injections in Saturn's magnetosphere. Future study of wave particle interactions within the chorus source region will be important. Possible Z‐mode emission at significant intensity levels is observed in Jupiter's inner magnetosphere, more intense, but not unlike Z‐mode observed at Saturn.

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

“…Pitch angle scattering of the chorus emission is a gyroresonant interaction and is dependent on the parallel energy resonant condition. However, Menietti et al (2020) point out that Juno observations of whistler mode emission at magnetic latitudes   E 30° are likely to be auroral hiss emission from the Jovian polar regions with a larger range of wave normal angles than chorus emission, which has a source region near the magnetic equator, and a different free energy source. Reference to Figure 3 of Menietti et al (2020) demonstrates that Jovian auroral hiss can also be bursty in morphology.…”

“…In this study, we will focus on electron distributions that are measured over an energy range of about 25 eV->800 keV. Sampling time for the electrons is 0.5 s. Menietti et al (2020) ("previous survey") reported an initial low-latitude survey of whistler-mode emission observed by the Juno spacecraft during orbits 1-25, which orbited at a high inclination with a period of about 53 days. Juno's line of apsides precesses about one degree per orbit, moving the perijove latitude north from its initial location near the equator at the beginning of the mission.…”

“…Hence, later in the extended mission, the trajectory crosses the equator progressively closer to Jupiter on the inbound leg, allowing the traversal of the near-equatorial plasma sheet and the generation region for chorus. The methodology of the survey is the same as presented in Menietti et al (2020) and will be discussed only briefly here.…”

“…Except when noted, the plasma density is obtained using interpolated values of analytic density models as discussed in Imai et al (2015) and Menietti et al (2020). The magnetic field model is the JRM09 + CAN model (Connerney et al, 1981(Connerney et al, , 2018, which is used to map field lines from the satellite position to the magnetic equator.…”

“…Menietti et al (2015Menietti et al ( , 2018 and Ye et al (2010) reported significant intensities of Z-mode emission at Saturn in the low plasma density region near the inner edge of the Enceladus plasma torus ranging from just above the equator to higher latitudes. Menietti et al (2020) first reported Juno observations of possible Z-mode emission at Jupiter confined between 20° and 50° magnetic latitude and ∼7 Jovian radii. Woodfield et al (2018) presented results of stochastic modeling of electron scattering of chorus and Z-mode emissions in the Saturn magnetosphere.…”