Discrete and broadband electron acceleration in Jupiter’s powerful aurora

Abstract

The most intense auroral emissions from Earth's polar regions, called discrete for their sharply defined spatial configurations, are generated by a process involving coherent acceleration of electrons by slowly evolving, powerful electric fields directed along the magnetic field lines that connect Earth's space environment to its polar regions. In contrast, Earth's less intense auroras are generally caused by wave scattering of magnetically trapped populations of hot electrons (in the case of diffuse aurora) or by the turbulent or stochastic downward acceleration of electrons along magnetic field lines by waves during transitory periods (in the case of broadband or Alfvénic aurora). Jupiter's relatively steady main aurora has a power density that is so much larger than Earth's that it has been taken for granted that it must be generated primarily by the discrete auroral process. However, preliminary in situ measurements of Jupiter's auroral regions yielded no evidence of such a process. Here we report observations of distinct, high-energy, downward, discrete electron acceleration in Jupiter's auroral polar regions. We also infer upward magnetic-field-aligned electric potentials of up to 400 kiloelectronvolts, an order of magnitude larger than the largest potentials observed at Earth. Despite the magnitude of these upward electric potentials and the expectations from observations at Earth, the downward energy flux from discrete acceleration is less at Jupiter than that caused by broadband or stochastic processes, with broadband and stochastic characteristics that are substantially different from those at Earth.

Energetic Electron Pitch Angle Distributions During the Cassini Final Orbits

“…These PADs of Saturn's energetic electrons also resemble the unidirectional and bidirectional distributions recently detected at similar energies by the JEDI instrument on the Juno spacecraft at Jupiter (Mauk et al, 2017b(Mauk et al, , 2017a. In some instances, the Jovian spectra exhibit a peak and/or cutoff in their energy spectra that indicate a field-aligned potential.…”

Energetic Particle Signatures Above Saturn's Aurorae

“…The accelerated ion beam and conic fluxes were shown to vary with solar illumination (Peterson et al, 2006). 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. Prior to the arrival of NASA's Juno spacecraft at Jupiter, the wave-particle interaction processes responsible for auroral acceleration at the giant planets were nevertheless assumed to be similar to what is observed in the terrestrial magnetosphere.…”

Proton Acceleration by Io's Alfvénic Interaction

“…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).…”