Proton Acceleration by Io's Alfvénic Interaction

Abstract

The Jovian Auroral Distributions Experiment aboard Juno observed accelerated proton populations connected to Io's footprint tail aurora. While accelerated electron populations have been previously linked with Io's auroral footprint tail aurora, we present new evidence for proton acceleration due to Io's Alfvénic interaction with Jupiter's magnetosphere. Separate populations were accelerated above the Io torus and at high latitudes near Jupiter. The timing suggests the acceleration is due to Alfvén waves associated with Io's Main Alfvén Wing. The inferred high-latitude proton acceleration region spans 0.9-2.5 Jovian radii in altitude, comparable to the expected location for electron acceleration, and suggests the associated Alfvén waves are able to accelerate electrons and protons in similar locations. The proton populations magnetically connected to Io's orbit are recently perturbed, equilibrating with the nominal torus plasma population on a timescale smaller than Io's System III orbital period of~13 h, likely due to wave-particle interactions. The tail populations are split into a wake-like structure with distinct inner and outer regions, where the inner region maps to an equatorial width nearly identical to the diameter of Io. The approximately symmetric surrounding outer regions are each slightly smaller than the central region and may be related to Io's atmospheric extent. The nominal, corotational torus proton population exhibits energization throughout all regions, peaking at the anti-Jovian flank of the inner core region mapping to Io's diameter. These proton observations suggest Alfvén waves are capable of accelerating protons in multiple locations and provide further evidence that Io's Alfvénic interaction is bifurcated. Plain Language SummaryThe interaction between Jupiter's moon Io and Jupiter's rapidly rotating magnetic field produces a persistent aurora in Jupiter's upper atmosphere. The Juno spacecraft's trajectory crossed magnetic field lines connected to this aurora. We found that protons are accelerated in multiple places between Jupiter and Io. The interaction is evidenced in two distinct regions, with the central core region mapping to almost exactly the size of Io in the equatorial plane. We also find the protons that comprise the nominal "background" population are hotter in this central region.

A New Framework to Explain Changes in Io's Footprint Tail Electron Fluxes

“…Without comparing to coincident magnetic field data, we cannot conclusively determine whether these signatures are Alfvénically accelerated versus another broadband acceleration mechanism. Yet, given the success of the Alfvén wave back‐tracing and that the distributions are not consistent with static downward inverted‐V acceleration, such distributions fit within the body of increasing evidence for Alfvénic acceleration sustaining tail emissions in the Juno era (Bonfond, Saur, et al, 2017; Damiano et al, 2019; Gershman et al, 2019; Saur et al, 2018; Sulaiman et al, 2020; Szalay, Bagenal, et al, 2020; Szalay et al, 2018). We mention that during the PJ12N transit, Poynting fluxes associated with the turbulent cascade of low‐frequency transverse fluctuations in the magnetic field indicative of Alfvénic fluctuations were estimated to be ~3,000 mW/m 2 (Gershman et al, 2019).…”

The Space Environment of Io and Europa

“…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. The Juno mission is revealing new details about these auroral emissions and flying through the polar end of the fluxtubes that couple to the moons (Hue et al, 2019; Mura et al, 2018; Paranicas et al, 2019; Szalay et al, 2018, 2019).…”

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

“…Analyses of the Io UV footprint morphology indicated that the electron acceleration leading to the aurora should be both bidirectional and broadband (Bonfond et al, 2008, 2009), and subsequent theoretical modeling by Hess et al (2010) demonstrated that such acceleration occurs at high latitudes by a turbulent cascade of Alfvén waves. Moreover, broadband energy distributions found in the Io footprint tail (Szalay et al, 2018, 2019) suggested continued Alfvénic interaction downstream of Io (Bonfond et al, 2017; Jacobsen et al, 2010; Mura et al, 2018). Similar distributions in Ganymede's footprint tail further reinforce the conclusion that Alfvénic interactions sustain satellite‐magnetosphere coupling well into the downstream region (Szalay, Allegrini, et al, 2020).…”