Juno Plasma Wave Observations at Ganymede

Abstract

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.

Juno Magnetometer Observations at Ganymede: Comparisons With a Global Hybrid Simulation and Indications of Magnetopause Reconnection

“…In both cases, the predicted inbound magnetopause location is very close to the simulated boundary separating Jovian and open magnetic field lines from LatHyS (Figure 2e), in agreement with Allegrini et al (2022), Kurth et al (2022), andDuling et al (2022). This is also the case for Ganymede's magnetopause location along Juno's outbound leg.…”

Ganymede MHD Model: Magnetospheric Context for Juno's PJ34 Flyby

“…The various instruments onboard Juno detected the outbound magnetopause crossing more clearly than the inbound, matching expectations of a more dynamic magnetotail without field rotations through the magnetopause. Our model predicts that Juno left Ganymede's magnetosphere at 17:00:16, 14 s earlier than JEDI (Clark et al, 2022), 23 s earlier than JADE (Allegrini et al, 2022) and about 40 s earlier than MAG (Romanelli et al, 2022) and the Waves instrument (Kurth et al, 2022) identified the outbound crossing. Uncertain model parameters could not explain this deviation, leaving an open question for possible further required physics.…”

Energetic Charged Particle Observations During Juno's Close Flyby of Ganymede

“…Asymmetric PADs and electron energy distributions presented in Figure 4 depict higher intensities in the downward LC (inferred from JEDI observations only) than in the upward LC. The mechanism responsible for precipitation is not addressed here, but and Kurth et al (2022) provide evidence that it may be due to the pitch angle scattering via plasma waves. Note, Figure 4b shows particle fluxes increase toward zero degrees in pitch angle, which is not consistent with quasi-linear diffusion theory through cyclotron resonances with plasma waves (e.g., Li et al, 2013;Li et al, 2021).…”

“…The color bar scales are different between the two panels, but intensity ratios between ∼90 𝐴𝐴 • and ∼0 𝐴𝐴 • reveal that 500 keV electrons are further decimated between 5% and 70% as compared to their 50 keV counterparts. The reason for this energy dependence is unclear, but one likely candidate is energy dependent pitch angle scattering that is addressed in Kurth et al (2022).…”