Wave‐Particle Interaction of Alfvén Waves in Jupiter's Magnetosphere: Auroral and Magnetospheric Particle Acceleration

Abstract

We investigate spatial and temporal scales at which wave-particle interaction of Alfvén waves occurs in Jupiter's magnetosphere. We consider electrons, protons, and oxygen ions and study the regions along magnetic flux tubes where the plasma is the densest, that is, the equatorial plasma sheet, and where the plasma is the most dilute, that is, above the ionosphere, where auroral particle acceleration is expected to occur. We find that within a dipole L-shell of roughly 30, the electron inertial length scale in the auroral region is the dominating scale, suggesting that electron Landau damping of kinetic Alfvén waves can play an important role in converting field energy into auroral particle acceleration. This mechanism is consistent with the broadband bidirectional electron distributions frequently observed by Juno. Due to interchange-driven mass transport in Jupiter's magnetosphere, its magnetosphere-ionosphere coupling is expected to be mostly not in local force balance. This might be a key reason for the dominant role of Alfvénically driven stochastic acceleration compared to the less frequently occurring, locally forced-balanced, and thus static mono-energetic unidirectional acceleration. Outside of approximately L = 30, the ion gyroperiod is the dominating scale suggesting that ion cyclotron damping of heavy ions plays a major role in heating magnetospheric plasma. We also present properties of the dispersion relationship and the polarization relationships of kinetic Alfvén waves including the important effects from the relativistic correction due to the displacement current in Ampère's law.

Energetic Particle Signatures Above Saturn's Aurorae

“…However, the first passes through Jupiter's auroral region did not reveal a powerful acceleration region and strong field-aligned currents (FACs) as expected (Cowley et al, 2017;Ray et al, 2010; but weaker FACs of a filamentary nature and signatures of aurorae powered by stochastic/broadband acceleration processes substantially different from those at Earth Mauk et al, 2017bMauk et al, , 2017a. Juno magnetometer measurements as well as recent theoretical work further suggest that Alfvén waves may play an important role for auroral particle acceleration (Gershman et al, 2019;Saur et al, 2018). Contrary to the initial lack of such observations, later spacecraft passes indicated the presence of strong parallel electric fields, coherent particle acceleration, and inverted-V structures Mauk et al, 2018;Paranicas et al, 2018).…”

Proton Acceleration by Io's Alfvénic Interaction

“…Thus, not only electrons as in the case of the main auroral oval (Saur et al, 2018) but also protons might be subject to wave-particle interaction with kinetic Alfvén waves in the dilute plasma region at M-shell distances of Io. In contrast to the generally assumed larger electron temperatures in the acceleration region of the Jovian main auroral oval (M~20-30), lower electron temperatures at M = 6 might provide a possible reason for significant damping of kinetic Alfvén waves at perpendicular scales smaller than the electron inertial length.…”

“…Earth-based observations have indicated the presence of parallel potential structures capable of accelerating electrons on the order a few hundred electron volts (Hess et al, 2009). Even beyond the IFPT, the predominance of Alfvénic acceleration at Jupiter compared to the more parallel potential-driven terrestrial aurora has been a major finding of the Juno mission (Mauk et al, 2017;Saur et al, 2018). Even beyond the IFPT, the predominance of Alfvénic acceleration at Jupiter compared to the more parallel potential-driven terrestrial aurora has been a major finding of the Juno mission (Mauk et al, 2017;Saur et al, 2018).…”

Kinetic Simulations of Electron Acceleration by Dispersive Scale Alfvén Waves in Jupiter's Magnetosphere

“…While dispersive scale Alfvén waves have not been directly observed at Jupiter, Alfvénic acceleration has long been invoked for Io's auroral footprint and the associated wake (e.g., Bonfond et al, ; Chust et al, ; Crary, ; Hess et al, ; Su et al, ; Szalay et al, ) and it is suggestive that Chust et al () noted small‐scale magnetic field fluctuations near Io. Theoretical studies have predicted that E || in inertial Alfvén waves would lead to generation of electron beams (e.g., Crary, ; Saur et al, ). Su et al () also considered the propagation of dispersive scale Alfvén waves in the Io‐Jupiter interaction using a gyrofluid model, but without coupling to kinetic electrons.…”