Jupiter’s Aurora: Data analysis of Juno/JEDI data and modeling of auroral electron acceleration
Final Report Abstract
Jupiter possesses the most powerful auroral emission in the solar system. Until the arrival of NASA's Juno spacecraft at Jupiter in 2016, however, no in-situ measurements of the auroral particles and their energization mechanisms were available for a comprehensive understanding of its auroral system. Juno in its first polar passes of Jupiter detected two types of auroral electron distributions, i.e., energetically broadband and mono-energetic ones. But the relative occurrence of both populations was still not well constrained. The existence of the first population was in contrast to standard pre-Juno expectation. The objective of our DFG proposal was to systematically analyze the electrons measured by the Juno spacecraft and to shed light on the underlying mechanisms which produces these energetic electrons. Therefore, we performed a statistical analysis of data from the first 20 polar passes of Juno. We found that field-aligned electrons over the main auroral emission zone are in 93% of all cases energetically broadband and only in 7% of the time mono-energetic. Our finding thus strongly indicates that stochastic acceleration is the dominating auroral acceleration processes at Jupiter. In a second step we additionally analyzed magnetic field measurements and ultraviolet observations from Juno to understand their role in relationship to the auroral electrons. We found on magnetic field lines corresponding to Jupiter's diffuse aurora (equatorward of the main emission) large amplitude small-scale magnetic field fluctuations up 100 nT on time scales of seconds to 1 minute. On magnetic field lines linked to the main auroral emission, we find both large scale magnetic field perturbations associated with quasistatic field-aligned electric currents and small-scale field perturbations on the order of 10 nT consistent with a turbulent spectrum. Due to instrument limitations of the magnetic field measurements, the small-scale magnetic field fluctuations over the main emission zone can be detected only when Juno was further away than 4 Jupiter radii. Our analysis was accompanied by a theoretical study on the role of stochastic acceleration due to turbulent Alfvén waves. Since the moon Io generates well known Alfvén waves and auroral emission, we used this subsystem as a first analysis step and showed that the observed magnetic field fluctuations in Io's Alfvén wave system are Doppler-shifted spatial structures consistent with weak-MHD and sub-ion scale kinetic Alfvén wave turbulence. We also found that along magnetic field lines connecting to the main aurora dispersive Alfvén waves change their properties from temperature dominated kinetic to inertial dominated Alfvén waves. In summary, our analyses find observationally as well as theoretically that small-scale kinetic processes driven by turbulent waves play a key role in shaping Jupiter's auroral processes.
Publications
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Jovian Auroral Electron Precipitation Budget—A Statistical Analysis of Diffuse, Mono‐Energetic, and Broadband Auroral Electron Distributions. Journal of Geophysical Research: Space Physics, 127(8).
Salveter, A.; Saur, J.; Clark, G. & Mauk, B. H.
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Properties of Turbulent Alfvénic Fluctuations and Wave‐Particle Interaction Associated With Io's Footprint Tail. Journal of Geophysical Research: Space Physics, 127(12).
Janser, S.; Saur, J.; Clark, G.; Sulaiman, A. H. & Szalay, J. R.
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Relevance of Alfvénic turbulence for Jupiter’s auroral emissions Dissertation, Institut für Geophysik und Meteorologie der Universität zu Köln
Janser Sascha
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Statistical Classification of Jupiter’s Aurora - A joint analysis of complementary Juno observations, Dissertation, Institut für Geophysik und Meteorologie der Universität zu Köln
Salveter Annika
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Investigating Magnetic Field Fluctuations in Jovian Auroral Electron Beams. Journal of Geophysical Research: Space Physics, 130(7).
Salveter, A.; Saur, J.; Clark, G.; Sulaiman, A.; Mauk, B. H.; Connerney, J. E. P. & Bonfond, B.
