Electron and ion particle acceleration regimes observed by Juno over Jupiter's main aurora

Over Jupiter's most intense main aurora, the Juno spacecraft has identified up to four different particle acceleration regimes at energies above 30 keV. Some of these regimes are very different than any of the particle acceleration regimes observed over Earth's auoras. We explore here the relationships between these different regimes and their similarities and differences to those at Earth. <P /

Implications of Juno energetic particle observations over Jupiter’s polar regions for understanding magnetosphere-ionosphere coupling at strongly magnetized planets

Juno obtained low altitude space environment measurements over Jupiter’s poles on 27 August 2016 and then again on 11 December 2016. Particle distributions were observed over the poles within the downward loss cones sufficient to power nominally observed auroral emissions and with the characteristic energies anticipated from remote spectroscopic ultra-violet auroral imaging. However, the character of the particle distributions apparently causing the most intense auroral emissions were very different from those that cause the most intense aurora at Earth and from those anticipated from prevailing models of magnetosphere-ionosphere coupling at Jupiter. The observations are highly suggestive of a predominance of a magnetic field-aligned stochastic acceleration of energetic auroral electrons rather than the more coherent acceleration processes anticipated. The Juno observations have similarities to observations observed at higher altitudes at Saturn by the Cassini mission suggesting that there may be some commonality between the magnetosphere-ionosphere couplings at these two giant planets. Here we present the Juno energetic particle observations, discuss their similarities and differences with published observations from Earth and Saturn, and deliberate on the implications of these finding for general understanding of magnetosphere-ionosphere coupling processes

Particle energization and structuring of Jupiter’s main auroral oval as diagnosed with Juno measurements of (>30 keV) energetic particles

Juno polar low-altitude energetic particle observations indicate that the most intense emissions from Jupiter’s main auroral oval are caused by the impingement onto the atmosphere of relatively flat, energy-monotonic electron distributions, often extending to energies >1 MeV. They can be associated with bi-directional angular beaming with upward fluxes greater than the downward fluxes. Downward fluxes of >800 mW/m^2 have been observed. However, when viewed in high time resolution ( 1.0s) these distributions are sometimes (3 of 8)) intermixed with >50keV downward accelerated electron distributions with the classic inverted-V configuration, indicative of steady magnetic field-aligned electric fields. The highest downward energy peak observed so far is 400 keV. The inverted-V energy distributions lack the high energy tails observed in adjacent regions, and thus, contrary to what is observed at Earth, the associated downward energy fluxes are generally lower than the downward energy fluxes associated with the more intense energy-monotonic distributions. The relationship between these two modes of auroral particle energization is unclear. Do the classic auroral processes that create inverted-V distributions become so powerful that instabilities are stimulated that cause stochastic energization to turn on and dominate, or do these two different forms of auroral acceleration represent distinctly different processes? These and other questions are explored

Particle energization and structuring of Jupiter’s main auroral oval as diagnosed with Juno measurements of (>30 keV) energetic particles

Juno polar low-altitude energetic particle observations indicate that the most intense emissions from Jupiter’s main auroral oval are caused by the impingement onto the atmosphere of relatively flat, energy-monotonic electron distributions, often extending to energies >1 MeV. They can be associated with bi-directional angular beaming with upward fluxes greater than the downward fluxes. Downward fluxes of >800 mW/m^2 have been observed. However, when viewed in high time resolution ( 1.0s) these distributions are sometimes (3 of 8)) intermixed with >50keV downward accelerated electron distributions with the classic inverted-V configuration, indicative of steady magnetic field-aligned electric fields. The highest downward energy peak observed so far is 400 keV. The inverted-V energy distributions lack the high energy tails observed in adjacent regions, and thus, contrary to what is observed at Earth, the associated downward energy fluxes are generally lower than the downward energy fluxes associated with the more intense energy-monotonic distributions. The relationship between these two modes of auroral particle energization is unclear. Do the classic auroral processes that create inverted-V distributions become so powerful that instabilities are stimulated that cause stochastic energization to turn on and dominate, or do these two different forms of auroral acceleration represent distinctly different processes? These and other questions are explored

A comparative examination of auroral acceleration processes at Jupiter and Earth as enabled by the Juno mission to Jupiter

Particle distributions observed by Juno’s Energetic Particle Detector Investigation (JEDI) at low altitudes over Jupiter’s polar regions are exceedingly diverse in directionality and in the shapes of their 3-dimensional energy distributions. Asymmetric, bi-directional angular beams with broad energy distributions are often observed near Jupiter’s main auroral oval with considerable variability as to whether upward or downward intensities are the strongest. Signatures of upward and downward magnetic field-aligned potentials, with inferred potentials up to 100’s of kV are sometimes observed, but unlike at Earth, these potentials do not seem to be associated with the strongest discrete-like auroral emission intensities. Particle distributions have similarities to those observed at Earth over the various phenomenological auroral emission regions, but they are observed in unexpected places with respect to the strongest auroral emission regions, and the jovian distributions are much more energetic. We present a comparative examination of auroral acceleration processes observed at Earth and Jupiter in relation to the respective auroral emission regions

Electron Pitch Angle Distributions Along Field Lines Connected to the Auroral Region from ~25 to ~1.2 RJ Measured by the Jovian Auroral Distributions Experiment-Electrons (JADE-E) on Juno

The Jovian Auroral Distributions Experiment (JADE) on Juno provides critical in situ measurements of electrons and ions needed to understand the plasma distributions and processes that fill the Jovian magnetosphere and ultimately produce Jupiter’s bright and dynamic aurora. JADE is an instrument suite that includes two essentially identical electron sensors (JADE-Es) and a single ion sensor (JADE-I). JADE-E measures electron energy distributions from ~0.1 to 100 keV and provides detailed electron pitch angle distributions (PAD) at ~7.5° resolution. Juno’s trajectories in the northern hemisphere have allowed JADE to sample electron energy and pitch angle distributions on field lines connected to the auroral regions from as close as ~1.2 RJ all the way to distances greater than 25 RJ. Here, we report on the evolution of these distributions. Specifically, the PADs change from mostly uniform at distances greater than ~20 RJ, to butterfly from ~18 to ~12 RJ, to field aligned or pancake, depending on the energy, closer to Jupiter. Below ~1.5 RJ, electron beams and loss cones are observed

An overview of the first year of observations of Jupiter’s auroras by Juno-UVS with multi-wavelength comparisons

Juno’s Ultraviolet Spectrograph (Juno-UVS) has observed the Jovian aurora during eight perijove passes. UVS typically observes Jupiter for 10 hours centered on closest approach in a series of swaths, with one swath per Juno spin (~30s). During this period the spacecraft range to Jupiter’s aurora decreases from ~6 RJ to ~0.3 RJ (or less) in the north, and then reverses this in the south, so that spatial resolution changes dramatically. A scan mirror is used to target different features or raster across the entire auroral region. Juno-UVS observes a particular location for roughly 17 ms/swath, so the series of swaths provide snapshots of ultraviolet auroral brightness and color. A variety of forms and activity levels are represented in the Juno-UVS data–some have been described before with HST observations, but others are new. One interesting result is that the color ratio, often used as a proxy for energetic particle precipitation, may instead (in certain regions) indicate excitation of H2 by low-energy ionospheric electrons. Additional results from comparisons with simultaneous observations at x-ray, visible, and near-IR wavelengths will also be presented