Juno Plasma Wave Observations at Ganymede

Abstract

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.

Keywords: Ganymede; Juno; plasma waves.

Figure 1

A summary of the geometry of the Juno flyby in co‐rotational coordinates (a) projected into the z‐x plane and (b) projected into the y‐x plane. The red ‘whiskers’ indicate the component of the magnetic field as measured by the Juno Magnetometer in that plane. The colors indicated on the trajectory are the log of the electron density inferred from f _uh . The blue, green, and red bars are regions identified by differing characteristics in the plasma wave spectrum.

Figure 2

A summary of electron observations and radio and plasma wave observations from the Juno Ganymede flyby. (a) Energetic electron energy‐time spectrogram from the JEDI instrument. (b) Plasma electron energy‐time spectrogram from the JADE instrument. (c) Electron densities inferred from the f _uh band shown in (d). (d) Electric field spectrogram from 10 to 70 kHz. (e) Electric field spectrogram from 40 Hz to 12 kHz. (f) Magnetic field spectrogram from 40 Hz to 12 kHz. The white trace superposed in panels e and f is f _ce .

Figure 3

A detail of the electron cyclotron harmonic emissions from ∼10 to 55 kHz. The red traces are harmonics of f _ce . The emission identified as f _uh is highlighted in blue.

Figure 4

Details of the electric and magnetic spectra observed in Regions 1, 2, and 3 using a logarithmic frequency scale. (a) Electric field spectrogram. (b) Magnetic spectrogram. (c) E/cB versus frequency and time; values near one indicate electromagnetic emissions. Ratios >> 1 indicate electrostatic or quasi‐electrostatic emissions.