Alfvénic Acceleration Sustains Ganymede's Footprint Tail Aurora

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114

The electrodynamic coupling between Io and Jupiter gives rise to wave-particle interactions across multiple spatial scales. Here we report observations during Juno's 12th perijove (PJ) high-latitude northern crossing of the flux tube connected to Io's auroral footprint. We focus on plasma wave measurements, clearly differentiating between magnetohydrodynamic (MHD), ion, and electron scales. We find (i) evidence of Alfvén waves undergoing a turbulent cascade, suggesting Alfvénic acceleration processes together with observations of bi-directional, broadband electrons; (ii) intense ion cyclotron waves with an estimated heating rate that is consistent with the generation of ion conics reported by Clark et al. (2020,

Section: Interpretation and Discussionsupporting

confidence: 52%

Wave‐Particle Interactions Associated With Io's Auroral Footprint: Evidence of Alfvén, Ion Cyclotron, and Whistler Modes

Sulaiman

Hospodarsky

Elliott

et al. 2020

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114

“…Therefore, the error on the source size and on the ensuing results (electron energy and beaming angle ) should be very small. Szalay et al (2020a) showed that in this present case, Ganymede's footprint tail is sustained by Alfvénic acceleration processes. Thus, as suggested by Louarn et al (2018), all the radio emissions seem to be triggered by Alfvénic acceleration (as at Earth, Su et al, 2007).…”

Section: 1029/2020gl090021supporting

confidence: 51%

“…In conclusion, we analyzed the first example of in situ particle measurements of a Ganymede-induced radio emission, during the Ganymede's tail flux tube crossing (Szalay et al, 2020a) connected to FUV emission. Looking at the distribution functions before, during, and after the source crossing, we showed that the loss cone-driven CMI can explain the generation of decametric radio emission.…”

Section: Conclusion and Discussionmentioning

confidence: 99%

“…In this study, we focus on Juno's crossing of field lines connected to Ganymede's footprint tail, which took place on 29 May 2019, where simultaneous FUV (Szalay et al, 2020a) and radio emission were observed (see section 2). In section 3 we present the instability leading to radio emission and the calculation to determine the different parameters of the radio emission (growth rate, electron distribution function and energy, and opening of the emission cone).…”

mentioning

confidence: 99%

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

Louis

Allegrini

Kurth

et al. 2020

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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 (

“…Ions are also accelerated, most likely from Alfvénic interactions, closer to Io (Szalay et al, 2020). And finally, the first report of in situ electron measurements during a Ganymede FPT crossing suggests that Alfvén waves accelerate the electrons that create the tail (Szalay et al, 2020a). This recent body of evidence supports the argument that Aflvénic acceleration sustains satellite FPTs.…”

Section: Introductionmentioning

confidence: 99%

First Report of Electron Measurements During a Europa Footprint Tail Crossing by Juno

Allegrini

Gladstone

Hue

et al. 2020

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We report the first in situ observations of electron measurements at a Europa footprint tail (FPT) crossing in the auroral region. During its 12th science perijove pass, Juno crossed magnetic field lines connected to Europa's FPT. We find that electrons in the range~0.4 to~25 keV, with a characteristic energy of 3.6 ± 0.5 keV, precipitate into Jupiter's atmosphere to create the footprint aurora. The energy flux peaks at~36 mW/m 2 , while the peak ultraviolet (UV) brightness is estimated at 37 kR. We estimate the peak electron density and temperature to be 17.3 cm −3 and 1.8 ± 0.1 keV, respectively. Using magnetic flux shell mapping, we estimate that the radial width of the interaction at Europa's orbit spans roughly 3.6 ± 1.0 Europa radii. In contrast to typical Io FPT crossings, the instrument background caused by penetrating energetic radiation (>~5-10 MeV electrons) increased during the Europa FPT crossing. Plain Language Summary Jupiter's moons interact with Jupiter's space environment, or magnetosphere, and create auroral spots and tails in Jupiter's ionosphere. Io's aurora footprint on Jupiter is the strongest and most persistent of all moons, but Ganymede, Callisto, and Europa's auroral footprints are also routinely observed by remote platforms. NASA's Juno mission and its instrument suite occasionally fly through regions that are connected to the moon-magnetosphere interactions. During these crossings, Juno samples the electrons and ions that create the aurora. This paper is the first report of electron measurements taken during a Juno crossing of Europa's tail. These measurements confirm previous results based on remote observations. Most importantly, they provide a sample of the conditions in the regions associated with Europa's footprint aurora in Jupiter's magnetosphere.