Comprehensive Model of Jupiter's Polar Aurora
Houston, Stephen Jeffrey
University of Kansas
Physics & Astronomy
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The source of auroral X-ray emission from the Jovian polar caps, whether from electron bremsstrahlung or heavy ion precipitation, has been a topic of debate for the past 40 years, beginning with the Einstein Observatory's first measurement of X-ray emission in 1979. Since then the Roentgen satellite, Chandra X-ray Observatory, and XMM-Newton have distinguished heavy ion (oxygen and sulfur) line emission in the X-ray spectrum and measure a total power of about 1 GW. There have been many attempts to model both bremsstrahlung and ion precipitation with the goal of reproducing what is being seen; however, both have encountered push back. Electron bremsstrahlung modeling has fallen short of producing the total overall power output being observed by our earth-orbiting X-ray observatories. Whereas heavy ion precipitation has been able to reproduce strong X-ray fluxes, but the proposed incident ion energies seemed to likely be much higher (1 MeV/nucleon) than what was thought to be present above Jupiter's polar caps. Now with the National Aeronautics and Space Administration's (NASA's) Juno spacecraft arriving at Jupiter, there have been many measurements of heavy ion populations above the polar cap with energies up to 300-400 keV/nucleon (keV/u), well below predictions the of previous models. Meanwhile, Schultz et al. (2019) have provided a new outlook on how ion-neutral collisions in the Jovian atmosphere are occurring, providing an entirely new set of impact cross-sections and a total of 35 collision processes (prior models only account for 9). A model is described for the transport of magnetospheric oxygen and sulfur ions with low charge state and energies up to several MeV/nucleon (MeV/u) as they precipitate into Jupiter’s polar atmosphere. A revised and updated hybrid Monte Carlo model originally developed by Ozak et al. (2010) is used to model the Jovian X-ray aurora. The current model uses a wide range of incident oxygen ion energies (10 keV/u - 5 MeV/u) and the most up-to-date collision cross-sections. In addition, the effects of the secondary electrons generated from the heavy ion precipitation are included using a two-stream transport model that computes the secondary electron fluxes and their escape from the atmosphere. The model also determines H2 Lyman-Werner band emission intensities, including a predicted spectrum and the associated color ratio. I predict X-ray fluxes, efficiencies, and synthetic spectra for various initial ion energies considering opacity effects from two different atmospheres. The data is made available for quick X-ray calculations given an input ion flux. A calculation is given that demonstrates an in situ measured heavy ion flux above Jupiter's polar cap is capable of producing over 1 GW of X-ray emission. Implications of the new model results for the interpretation of data from NASA’s Juno mission are discussed.
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