Precipitating Electron Energy Flux and Characteristic Energies in Jupiter's Main Auroral Region as Measured by Juno/JEDI

Abstract

The relationship between electron energy flux and the characteristic energy of electron distributions in the main auroral loss cone bridges the gap between predictions made by theory and measurements just recently available from Juno. For decades such relationships have been inferred from remote sensing observations of the Jovian aurora, primarily from the Hubble Space Telescope, and also more recently from Hisaki. However, to infer these quantities, remote sensing techniques had to assume properties of the Jovian atmospheric structure—leading to uncertainties in their profile. Juno's arrival and subsequent auroral passes have allowed us to obtain these relationships unambiguously for the first time, when the spacecraft passes through the auroral acceleration region. Using Juno/Jupiter Energetic particle Detector Instrument (JEDI), an energetic particle instrument, we present these relationships for the 30‐keV to 1‐MeV electron population. Observations presented here show that the electron energy flux in the loss cone is a nonlinear function of the characteristic or mean electron energy and supports both the predictions from Knight (1973, https://doi.org/10.1016/0032-0633(73)90093-7) and magnetohydrodynamic turbulence acceleration theories (e.g., Saur et al., 2003, https://doi.org/10.1029/2002GL015761). Finally, we compare the in situ analyses of Juno with remote Hisaki observations and use them to help constrain Jupiter's atmospheric profile. We find a possible solution that provides the best agreement between these data sets is an atmospheric profile that more efficiently transports the hydrocarbons to higher altitudes. If this is correct, it supports the previously published idea (e.g., Parkinson et al., 2006, https://doi.org/10.1029/2005JE002539) that precipitating electrons increase the hydrocarbon eddy diffusion coefficients in the auroral regions.

Alfvénic Fluctuations Associated With Jupiter's Auroral Emissions

“…Broadband Alfvénic turbulence was observed at all System III longitudes but was confined to within polar latitudes immediately equatorward of the statistical auroral oval. The broadband fluctuations observed in PJ1 aligned with the broad‐energy electron signatures reported by the energetic particle instruments (Allegrini et al, ; Clark et al, ; Mauk et al, , , ). No corresponding fluctuations were observed in concert with the discrete‐energy electron signatures described by Mauk et al (, , ).…”

Energy Flux and Characteristic Energy of Electrons Over Jupiter's Main Auroral Emission

“…The black symbols correspond to the 3‐ to 30‐keV energy flux peak intervals, and the orange (blue) symbols are equatorward (polarward) of those intervals. Clark et al (2018) showed similar results for >30‐keV electrons. This study includes lower‐energy electrons (down to ~50 eV).…”

“…We calculate the characteristic energy, E char , and the energy flux, E Flux , using equations (1) and (2) in Clark et al (2018) (see also Mauk, Haggerty, Paranicas, Clark, Kollmann, Rymer, Mitchell et al (2017)): and where I is the particle intensity ((cm 2 ·s·sr·keV) −1 ), E is the electron energy (keV), E min is the lowest energy channel from JADE, and E max is geometric mean of the highest energy channel for JEDI, and the factor π represents the area‐projected‐weighted loss cone (see Mauk, Haggerty, Paranicas, Clark, Kollmann, Rymer, Mitchell et al (2017)). We estimate the width of the loss cone by using the simple relationship Loss Cone Angle ~asin(1/ R 3 ) 1/2 (see also Mauk, Haggerty, Paranicas, Clark, Kollmann, Rymer, Mitchell et al (2017)), where R is the Jovicentric distance in Jovian radius.…”

“…This ratio is also a measure of the attenuation of the H 2 emission by hydrocarbons (typically assumed to be methane); therefore, the assumed distribution of hydrocarbons can influence the interpretations of the color ratio. Previous studies have shown that the hydrocarbons in Jupiter's auroral region are enhanced due to much more effective eddy mixing because of the intense auroral energy input from precipitating electrons in the upper atmosphere (Clark et al, 2018; Parkinson et al, 2006). The only region that shows a clear relationship between the UV brightness and the intensity of the methane absorption (a proxy for the precipitating electron energy) is the main emission (Gérard et al, 2016).…”

“…In contrast, these distributions have been shown to drive the brightest aurora at Earth (Carlson et al, 1998 and references therein). It appears that the broad energy distributions (distributions that lack sharp peaks in energy) are the more common and dominant particle energy signature in the main auroral region (Allegrini et al, 2017; Clark et al, 2018; Mauk, Haggerty, Paranicas, Clark, Kollmann, Rymer, Mitchell et al (2017); Szalay et al, 2017, 2018). Several authors have suggested that these distributions are indicative of wave‐particle accelerations via, say, Alfvén or whistler mode waves (e.g., Elliott et al, 2018; Kurth et al, 2018; Saur et al, 2018).…”