Ganymede‐Induced Decametric Radio Emission: In Situ Observations and Measurements by Juno

Abstract

At Jupiter, part of the auroral radio emissions are induced by the Galilean moons Io, Europa, and Ganymede. Until now, they have been remotely detected, using ground-based radio telescopes or electric antennas aboard spacecraft. The polar trajectory of the Juno orbiter allows the spacecraft to cross the magnetic flux tubes connected to these moons, or their tail, and gives a direct measure of the characteristics of these decametric moon-induced radio emissions. In this study, we focus on the detection of a radio emission during the crossing of magnetic field lines connected to Ganymede's tail. Using electromagnetic waves (Juno/Waves) and in situ electron measurements (Juno/JADE-E), we estimate the radio source size of ∼250 km, a radio emission growth rate >3 × 10 −4 , a resonant electron population of energy E = 4-15 keV and an emission beaming angle of = 76-83 • , at a frequency ∼1.005-1.021 × f ce. We also confirmed that radio emission is associated with Ganymede's downtail far ultraviolet emission. Plain Language Summary The Juno spacecraft crossed magnetic field lines connected to Ganymede's auroral signature in Jupiter's atmosphere. At the same time, Juno also crossed a decametric radio source. By measuring the electrons during this radio source crossing, we determine that this emission is produced by the cyclotron maser instability driven by upgoing electrons, at a frequency 0.5% to 2.1% above the cyclotron electronic frequency with electrons of energy 4-15 keV. 1. Introduction With the arrival of Juno at Jupiter in July 2016, the probe passes above the poles once per orbit, every 53 days and acquires high-resolution measurements in the auroral regions (Bagenal et al., 2017). In particular, the instruments Waves (radio and plasma waves, Kurth et al., 2017), JADE-E (Jovian Auroral Distributions Experiment-Electrons, McComas et al., 2017), and MAG (Magnetometer, Connerney et al., 2017) offer a unique opportunity to investigate in situ the Jovian radio emissions produced in these polar regions. These instruments enable constraints on the position of the sources (

Reply to Comment by Lamy et al. on “Locating the source field lines of Jovian decametric radio emissions”

“…in which α is the emission angle, v is the speed of the energetic electrons, c is the speed of light, f ce is the electron's cyclotron frequency, and f ce,max is the maximum cyclotron frequency that the electrons can reach along the active field line from which the DAM is emitted. This formula is based on the loss-cone distribution of electrons, which has been supported by Juno in situ observations (Louarn et al, 2017(Louarn et al, , 2018Louis et al, 2020). Another constraint in the method is the hemisphere from which the DAM emits.…”

A Comprehensive Set of Juno In Situ and Remote Sensing Observations of the Ganymede Auroral Footprint

“…At higher frequencies (see Figure S3 in Supporting Information S1), Waves measured strong intensification at frequencies near the local electron cyclotron frequency f ce around the time of crossing, suggesting Juno may have flown through or near the radio source. From several radio source crossings recorded by Juno, the cyclotron maser instability (CMI) driven by a loss-cone distribution function has been established as a major process at Jupiter to generate hectometric and decametric emissions, induced or not by the Galilean moons (Louarn et al, 2017(Louarn et al, , 2018Louis et al, 2020). Following these studies and assuming a loss-cone-driven CMI emission in a weakly relativistic case, the electron energy can be estimated.…”

“…Unlike most Io crossings, measurements connected to the Europa footprint tail showed signs of an electron distribution resulting at least in part from electrostatic acceleration processes, with enhanced precipitating electrons in the 0.38-25 keV range . Evidence for Alfvénic acceleration was observed during a Ganymede tail crossing (Szalay et al, 2020a), showing (a) broadband electrons with precipitating fluxes of ∼11 mW/m 2 and enhanced flux in the 0.5-40 keV range, (b) a strong magnetic Alfvénic perturbation with associated Poynting flux of ∼100 mW/m 2 , that is, ∼10 times the precipitating electron energy flux (EF) measured by Juno's in situ instruments, and (c) strong associated decametric emissions (Louis et al, 2020).…”

Energetic electron lensing caused by Ganymede's magnetic field

“…2, right panel). When Juno crosses an M-shell that intersects the orbit of one of Jupiter's moons, variations in the charged particle environment (Szalay et al, 2020), plasma waves (Sulaiman et al, 2020), radio emissions (Louis et al, 2020), and magnetic field (Gershman et al, 2019;Connerney et al, 2020;Allegrini et al, 2020) are observed associated with the moon's interaction with the Jovian magnetosphere. Juno's 11th orbit presented a particularly auspicious opportunity to observe Ganymede's interaction with the magnetosphere, with numerous traversals of Ganymede's M-shell occurring at small phase angle separation.…”