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

Abstract

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.

Morphology of the Auroral Tail of Io, Europa, and Ganymede From JIRAM L‐Band Imager

“…In the case of the Galilean moons, the conductivity enhancement is triggered by the precipitation associated with each footprint (Allegrini et al, 2020;Szalay et al, 2020aSzalay et al, , 2020b. On the other hand, multiple factors can contribute to the background electric field.…”

Electron Partial Density and Temperature Over Jupiter's Main Auroral Emission Using Juno Observations

“…Figure 5 shows that JADE is able to measure electron partial density and temperature when connected to the brighter parts of the UV emissions equatorward of ZI. It is also true for the satellites' footprint tails, which are measured by both UVS and JADE (e.g., Allegrini, Gladstone, et al., 2020; Bonfond et al., 2017; Szalay et al., 2018, 2020), even though Io's footprint tail is not visible because it is too far away in longitude for this pass. Note that Waves makes very good measurements of the electron density in the region connected to the Io torus (Elliott et al., 2021).…”

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

“…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).…”