Survey of Voyager plasma science ions at Jupiter: 2. Heavy ions

Abstract

We take the plasma parameters derived by Bagenal et al. (2017) from Voyager plasma science data in the Jovian magnetosphere and examine the radial profiles of density, temperature, composition, and azimuthal flow. The plasma sheet shows a relatively uniform structure of decreasing electron density (Ne) and increasing temperature out to about 20 RJ, but beyond about 15 RJ there is increasing disorder with sporadic blobs of cold plasma. These small (~0.5 RJ) blobs of cold (~20 eV) plasma make a minor contribution to the net outward flux of iogenic plasma. The ion composition in the cold blobs is consistent with the ion abundances derived from physical chemistry models extending from 6 to ~9 RJ, whereupon the collisional reactions slow down and radial transport speeds up, effectively freezing in the ion composition to the following abundances: O+/Ne = 15–22%, S++/Ne = 10–19%, O++/Ne = 4–8%, S+++/Ne = 4–6%, and S+/Ne = 1–5%. Beyond about 7 RJ the component of hot (suprathermal, approximately hundreds of eV) ions becomes a significant fraction of the total density. The radial profile of the plasma's azimuthal flow speed shows that corotation begins to breakdown at about 9 RJ, dipping down to about 20% below corotation before increasing back up to corotation briefly (~17–20 RJ), reaching an asymptotic value of about 225 km/s (corresponding to rigid corotation at ~18 RJ). We present a 2‐D model of the plasma sheet beyond 6 RJ based on simple functions for the equatorial profiles of plasma properties and steady state diffusive equilibrium along magnetic flux tubes. Cold plasma blobs in the Jovian plasma sheet show ion composition consistent with physical chemistry models. Azimuthal flow speeds dip below corotation 9–15 Jovian radii. Radial profiles of plasma properties are combined to make a 2‐D model of plasma sheet.

Method to Derive Ion Properties From Juno JADE Including Abundance Estimates for O+ and S2+

“…The flow speed obtained in this study ranges from 71–95% of the rigid corotation speed. These results are higher than previously reported values in this region (e.g., Dougherty et al, ; Kane et al, ; McNutt et al, ). The highest flow speed is observed in interval L which coincides with when Juno is at a magnetic latitude (MLat) of near zero and the magnitude of B r is smallest (see Figure ).…”

“…The number density estimates of jovian magnetospheric ions using Voyager Plasma Science (PLS) (Bridge et al, ) have been presented in several studies (Bagenal & Sullivan, ; Bagenal et al, ; Bodisch et al, ; Dougherty et al, ; McNutt et al, ). They selected intervals where the peaks in the count‐ E / Q distributions were clearly resolvable (i.e., cold plasma).…”

“…However, the physical chemistry model only can cover radial distances up to ∼10 R J and the 1.02 value is obtained assuming freezing‐in of the charge state. This assumption is used in previous studies (e.g., Bagenal et al, , ; Bodisch et al, ; Dougherty et al, ) to obtain ion properties from Voyager PLS and Galileo PLS.…”

“…For the distribution functions, we assume that all heavy ions share common temperature based on the fact that the Coulomb equilibration time is much smaller than the transport time from the Io plasma torus to the plasma sheet (Delamere et al, ). This assumption was also used in analysis of Voyager PLS (Bagenal et al, ; Dougherty et al, ) and Galileo PLS data Bagenal et al (). Also, we had to fix the proton kappa value due to the complexity of their distributions.…”

Alfvén Wave Propagation in the Io Plasma Torus

“…The equations of this diffusive equilibrium are straightforward to solve, particularly if we assume the gravity of Jupiter can be neglected and that the ions are isotropic (e.g., see the appendix of Dougherty et al, 2017). This means that the sulfur and oxygen ions experience a considerable centrifugal force that tends to confine them to the farthest point from the planet's spin axis-the centrifugal equator.…”