Similarity of the Jovian satellite footprints: Spots multiplicity and dynamics

Abstract

In the magnetospheres of Jupiter and Saturn, the intense interaction of the satellites Io, Europa, Ganymede and Enceladus with their surrounding plasma environment leaves a signature in the aurora of the planet. Called satellite footprints, these auroral features appear either as a single spot (Europa and Enceladus) or as multiple spots (Io and Ganymede). Moreover, they can be followed by extended trailing tails in the case of Io and Europa, while no tail has been reported for Ganymede and Enceladus, yet. Here we show that all Jovian footprints can be made of several spots. Furthermore, the footprints all experience brightness variations on timescale of 2-3 min. We also demonstrate that the satellite location relative to the plasma sheet is not the only driver for the footprint brightness, but that the plasma environment and the magnetic field strength also play a role. These new findings demonstrate that the Europa and Ganymede footprints are very similar to the Io footprint. As a consequence, the processes expected to take place at Io, such as the bi-directional electron acceleration by Alfvén waves or the partial reflection of these waves on plasma density gradients, can most likely be extended to the other footprints, suggesting that they are indeed universal processes.

Infrared observations of Jovian aurora from Juno's first orbits: Main oval and satellite footprints

“…Similar inferences apply to Europa's footprint; it has a similar intensity as Io's, but it is shorter and, being closer to the main oval, it is observable only for few tens of degrees in longitude. The footprint of Ganymede is visible, and it has two main spots [Bonfond et al, 2013a[Bonfond et al, , 2013b[Bonfond et al, , 2017~10°westward of the predicted locations (i.e., above, in the pictures, see the blue arrow in Figures 3c and 3d). It is visible particularly in Figure 3; the two spots are visible in Figures 3a and 3d; in Figure 3c they form a segment and they are undistinguishable.…”

Juno‐UVS Observation of the Io Footprint During Solar Eclipse

“…Previous HST observations suggest that the EP of the different footprint spots are variable over short timescales (Bonfond et al, , ). Fluctuations on the order of ∼30% are frequently observed for the different Io spots, as well as other footprint (e.g., Bonfond et al, ; Grodent et al, ). The fluctuations of the different spots are correlated for some λ III sectors, increasing the overall fluctuation of the entire footprint, when the spots are spatially unresolved.…”

The Space Environment of Io and Europa

“…Gurnett and Goertz (1981) suggested that multiple ionospheric reflections of such Alfvénic disturbances between north and south hemispheres of Jupiter (Figure 5d) could produce the pattern of arcs in the frequency‐time spectrograms of the Voyager Planetary Radio Astronomy instrument (Warwick et al, 1979). As Earth‐based telescopes improved, auroral emissions in Jupiter's atmosphere—IR, UV—revealed features at first associated with Io (Clarke et al, 1996; Connerney et al, 1993; Prange et al, 1996), and then not just Io but also Ganymede, Europa (Bonfond et al, 2017, Bonfond et al, 2017; Clarke et al, 2002; Grodent et al, 2006), and, recently, Callisto (Bhattacharyya et al, 2018). These auroral emissions (Figure 5c) indicate that the plasma‐satellite interactions all involve electrodynamic perturbations, which generate Alfvén waves propagating from the moon, carrying electric currents parallel to the magnetic field, accelerating electrons that bombard Jupiter's atmosphere and generate auroral emissions.…”

“… Note . (1) Saur et al (1999); (2) Saur et al (1998) calculated for a small upstream density ~40 cm −3 ; (3) Kivelson, Khurana, Walker, Linker, et al (1996); (4) Kivelson et al (1997); (5) Dols et al (2008); (6) Dols et al (2016); (7) Saur et al (2002); (8) Bagenal (1997); (9) Frank et al (1996); (10) Kurth et al (2001); (11) Retherford et al (2003, 2007); (12) Wolven et al (2001) and Ballester et al (1987); (13) Bouchez et al (2000) and Geissler et al (2004); (14) Roth et al (2016); (15) Roth et al (2011); (16) Hansen et al (2005); (17) Williams et al (1996); Williams & Thorne (2003) and Frankand Paterson (1999); (18) Mauk et al (2001); (19) Gerard et al (2006) and Bonfond et al (2008); (20) Bonfond, Grodent, et al (2017) and Grodent et al (2006); (21) Khurana et al (2011); (22) Roth, Saur, et al (2017); (23) Blöcker et al (2018); (24) Zimmer et al (2000) and Schilling et al (2004). …”