Combining UV Spectra and Physical Chemistry to Constrain the Hot Electron Fraction in the Io Plasma Torus

Abstract

We have developed a spectral emission model that is a function of the plasma composition, electron temperature, and density in the Io plasma torus. The lines are excited by electron collisions and spontaneously decay resulting in UV emission that is diagnostic of the plasma conditions. In a previous study we used a single Maxwellian distribution to model the UV spectra obtained by the Cassini UVIS instrument in January 2001. We now try to determine the fraction of hot electrons using a double Maxwellian distribution where the core, thermal electron distribution is combined with a hot electron distribution, also assumed to be a Maxwellian. This spectral emission model does not well constrain the fraction of hot electrons, which can be seen by the χ 2 output. By using physical chemistry modeling of equilibrium conditions, we determine the fraction of hot electrons. Our physical chemistry model shows that in order to match the plasma composition and temperatures near the orbit of Io during the Cassini flyby of Jupiter we need the hot electrons to comprise <0.3% of the total electron density.

Water‐Group Pickup Ions From Europa‐Genic Neutrals Orbiting Jupiter

“…This is solved using explicit finite differencing via $${F}_{i+1}={F}_{i}+{P}_{i}{\rho }_{i}{\Delta}\rho $$, where $$F=\,n\rho {v}_{\rho }$$, and a small value of $${\Delta}\rho =0.025$$ R J is chosen such that the results are insensitive to the exact value of radial step size. We use a radial transport speed profile (Bagenal & Delamere, 2011) corresponding to a total Jovian plasma source of 0.7 ton/s, which represents the typical magnetospheric configuration (Delamere et al., 2004; Nerney & Bagenal, 2020). For the pickup ion input source, we approximate the modeled H 2 + pickup ion input given in Figure 10 of Smyth and Marconi (2006) as $${P}_{E}(R)={P}_{0E}{e}^{-\frac{{\left(R-{R}_{E}\right)}^{2}}{2{\sigma }_{E}}}$$where P 0E is the production rate of H 2 + and R E = 9.4 R J , and σ E = 0.4 R J .…”

“…This is solved using explicit finite differencing via 𝐴𝐴 𝐴𝐴𝑖𝑖+1 = 𝐴𝐴𝑖𝑖 + 𝑃𝑃𝑖𝑖𝜌𝜌𝑖𝑖Δ𝜌𝜌 , where 𝐴𝐴 𝐴𝐴 = 𝑛𝑛𝑛𝑛𝑛𝑛𝑛𝑛 , and a small value of 𝐴𝐴 Δ𝜌𝜌 = 0.025 R J is chosen such that the results are insensitive to the exact value of radial step size. We use a radial transport speed profile (Bagenal & Delamere, 2011) corresponding to a total Jovian plasma source of 0.7 ton/s, which represents the typical magnetospheric configuration (Delamere et al, 2004;Nerney & Bagenal, 2020). For the pickup ion input source, we approximate the modeled H 2 + pickup ion input given in Figure 10 of Smyth and Marconi (2006) as…”

The Space Environment of Io and Europa

“…The in situ plasma measurements provide detailed velocity distributions but suffer from limited spatial and temporal coverage as well as poor determination of parameters for individual ionic species in the warm region of the torus where the spectral peaks for different species overlap. Since these two data sets are complementary, they can be combined to construct a description of the plasma conditions in the torus, that is, an empirical model, that can be compared to theoretical models based on the physical chemistry of the torus plasma (e.g., Bagenal, 1994; Herbert & Hall, 1998; Herbert et al, 2008, 2003; Nerney & Bagenal, 2020; Smyth & Marconi, 2005; Smyth et al, 2011). A summary of observations and theoretical understanding at end of Galileo mission is provided by Thomas et al (2004).…”

“…Figure 16 shows the flow of mass and energy through the torus system from Nerney and Bagenal (2020) taking the “cubic centimeter” physical chemistry model at 6 R J (that matches conditions derived from the Cassini UVIS spectrum) as typical of the main torus. The model is not very sensitive to the temperature of the hot electrons (40–400 eV).…”

“…Most of the energy is radiated away via UV emissions. Input of energy from hot electrons is key for maintaining the torus (based on Nerney & Bagenal, 2020).…”