Ganymede--induced decametric radio emission: in-situ observations and measurements by Juno

Abstract

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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 ), 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, 2018; Louis 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 (Allegrini, Gladstone, et al., 2020). 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).…”

Latitudinal Beaming of Jupiter's Radio Emissions From Juno/Waves Flux Density Measurements

“…The emission mechanism of high‐latitude components has been identified as the electron cyclotron maser instability and well studied (Louarn et al., 2017, 2018; Louis, Louarn, et al., 2020; Treumann, 2006; Zarka, 1998), to the point where the time‐frequency morphology of these emissions can be modeled quite accurately (Hess et al., 2008; Louis, Lamy, Zarka, Cecconi, Imai, et al., 2017). By contrast, low‐latitude emissions are attributed to mode conversion mechanisms producing emissions at or near the local plasma frequency, possibly on density gradients (e.g., plasma torus or magnetopause), but those are less quantitatively documented (Barbosa, 1982; Gurnett & Scarf, 1983; Jones, 1988; Ronnmark, 1992).…”