Ion Plasma Properties in Jupiter's Plasma Sheet: Juno JADE-I Observations

This dissertation presents a survey of ion plasma properties in Jupiter’s magnetosphere from ~10-40 RJ (1 RJ=71,492 km), including a method to distinguish the two major ions O+ and S2+ (both having a mass-per-charge M/Q of ~16 amu/q), using in situ measurements from the Jovian Auroral Distributions Experiment Ion sensor (JADE-I) on NASA’s Juno Mission to Jupiter. JADE-I is a plasma instrument comprised of a top-hat electrostatic analyzer (ESA) and a time-of-flight (TOF) mass spectrometer that measures the energy-per-charge (E/Q) distribution of 0.01-46.2 keV/q ions over a M/Q range of 1-64 amu/q. Uniquely identifying O+ and S2+ in the observations provides an important constraint for understanding dynamic processes in the magnetosphere. However, because of their similar M/Q, it is a challenging task for plasma instruments that only measure E/Q or E/Q and M/Q (e.g., Juno/JADE-I, Voyager/PLS, and Galileo/PLS). Therefore, their density ratio is not well constrained beyond ~10 RJ, leading to questions on the characteristics of the neutral source from the Galilean satellite Io (the primary source of plasma in Jupiter’s magnetosphere) and the physical processes that lead to the distribution of these ions through the magnetosphere.

In Chapter 2, we present a ray-tracing simulation of the instrument that incorporates carbon-foil effects to demonstrate that O+ and S2+ can be distinguished in the JADE-I measurements due to two major constraints: (1) a significant fraction of O+ yields negative charge states after transiting the carbon foil while S2+ yields positive charge states, (2) a large nonlinear electrostatic potential (~8 kV ) in the TOF mass spectrometer accelerate/decelerate these ions creating a measurable difference in their TOF distributions. A comparison to the laboratory measurements of JADE-I Engineering Model (EM) shows a good agreement with the simulation results.

Using the simulation tool, combined with instrument response functions and omni-directional averaged convected kappa distributions for H+, O+, O2+, O3+, Na+, S+, S2+, and S3+, we forward model the JADE-I observations to obtain ion plasma parameters in Jupiter’s plasma sheet. The results are presented in Chapters 3 and 4. In Chapter 3, we present a case study for a plasma sheet crossing at ~36 RJ in the predawn sector of Jupiter’s magnetosphere. The O+ to S2+ density ratio was found to range between 0.2-0.7 with a mean value of 0.4+-0.1 for the selected interval. These observations were compared with estimates based on physical chemistry models at ~10 RJ. Chapter 4 presents a detailed survey of plasma sheet properties from ~10-40 RJ covering the dawn to midnight sector of Jupiter’s magnetosphere, including equatorial and off-equator regions. Radial profiles of ion flow speed, number density, temperature, and composition are derived and compared with previous observations. The O+ to S2+ density ratio is variable and shows a weak decreasing radial trend in the equatorial region. Latitudinal trends are observed in the ion composition. The physical implications of these survey results are discussed. Chapter 5 presents a summary of the dissertation results.