3D Emission & Physical Chemistry Simulations of the Io Plasma Torus

Nerney, Edward Gregory

Graduate Thesis Or Dissertation

3D Emission & Physical Chemistry Simulations of the Io Plasma Torus Public Deposited

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Citeable URL: https://scholar.colorado.edu/concern/graduate_thesis_or_dissertations/tt44pp64k

Abstract

- The Galilean moon Io emits volcanic gases into space at a rate of about a ton per second. The gases become ionized and trapped in Jupiter’s strong magnetic field, forming a torus of plasma that emits 2 terawatts of UV emissions. After reanalyzing UV emissions observed by Voyager, Galileo, & Cassini, this work found that the Voyager plasma conditions were consistent with a physical chemistry model with a neutral source of dissociated sulfur dioxide from Io. The Voyager analysis of Shemansky (1988) [121] found an O/S ratio of the neutral source of 4 required to match UV observations whereas we find it to be 2 consistent with dissociation of SO₂. There are plenty of ways I could see it being less than 2 when also including sources from SO, S₂, and other sulfur compounds but it is much harder to explain it being larger than 2.
- By using a double Maxwellian distribution where both the core thermal electrons as well as supra-thermal “hot” electron population are assumed to be Maxwellians I have modeled the emission in the UV using the CHIANTI atomic database. This double Maxwellian model of UV emission spectra when compared with a spectrum from CASSIN UVIS at 6 R_J does not well constrain the fraction of hot electrons. Additional physics from energy constraints from physical chemistry modeling allows me to determine that for nominal warm torus plasma parameters the fraction of hot electrons is about 0.25% at 6 R_J. This research determined the mass and energy budget and dominant chemical pathways in the Io plasma torus. This result is particularly important due to the abundance of recent spectral analyses of UV data from JAXA's Hisaki satellite. Spectral analysis of the Hisaki observations has found fractions of hot electrons on the order of a few percent (Yoshioka et al. (2014); Tsuchiya et al. (2015)) [163] [150] inconsistent with our model and previous results.
- ESA's JUICE mission and NASA's Europa Clipper are sending UVS instruments to the Jupiter system that will view the Io plasma torus. In anticipation of these missions, I have built a 3D Io plasma torus emission model in order to simulate what we would expect to see from both UVS instruments looking at the Io plasma torus. In addition, our model allows for observation planning to predict if particular torus stare scenarios will produce sufficient signal to determine plasma conditions. The Colorado Io Torus Emission Package 2 (CITEP 2) calculates the line of sight given the position and pointing of the spacecraft and produces a synthetic spectrum given plasma densities and temperatures along the line of sight using the CHIANTI atomic database version 9 to compute volume emission rates.
- I have adapted and built upon a 3D model of the physical chemistry (Copper et al. (2016)) [33] while varying the neutral source rate and diffusion coefficient in order to model the warm torus, ribbon, and cold torus self-consistently. I have corroborated Copper's results and adapted the model for my own purposes. I have moved the model in from the warm torus to simulate the cold torus, gap region, ribbon, and warm torus. I am able to produce the ribbon and a peak in flux-tube content at L=5.7 by applying a discontinuity in the diffusion coefficient in that region consistent with a change in flux-tube interchange processes. By applying the “notched" D_LL profile that Taylor (1996) [142] used for a few model runs I was able to produce a cold torus peak and gap region by fixing the neutral density profiles to the Koga et al. (2018b) [79] scaled up by a factor of 1.5 but with a fast power law fall of +20 and cutting it off inside 5.65 R_J. I found that if I didn't have the neutral densities fall off much steeper than the Koga et al. (2018b) [79] implied power law of +12 inside 5.7 than my electron and ion temperatures would stay far too high due to pickup energy. This implies that inside the peak in neutral density at 5.7 R_J Koga et al. (2018b) [79] was overestimating densities due to line of sight projection effects.
- I investigate the tipping point of inflow versus outflow of mass and energy and quantify the transport timescales given the diffusion coefficient profile and flux tube content radial profiles. I found radial transport timescales ranging between tens of days to hundreds of days in the warm torus depending on the method used and many hundreds of days to a thousand days in cold torus. I found a separable transient solution to the radial Fokker-Planck equation I have never seen applied to Jupiter for flux-tube interchange motion. I found an e-folding timescale for the transient separable solution exactly the same as what is used as a radial transport timescale in the literature and found similar values for this e-folding timescale to match the torus profiles as is found using the integrated transport timescale formulation. I performed a numerical experiment to determine the time for a perturbation to move through the warm torus. By taking our nominal steady-state output in the warm torus and perturbing the solution at L=6 we find shorter timescales for the perturbation to reach L=10 of about 30 days as opposed to around 100 days for the integrated transport timescale.
- The output of our 3D physical chemistry model produces a 3D model of densities and temperatures which can be used in conjunction with CITEP 2 to simulate corresponding emission profiles for a given viewing geometry.

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