We review the mechanisms that shape the spatial distributions of energetic electrons trapped in the magnetospheres of Jupiter, Saturn, Uranus and Neptune. To determine what controls the energy and spatial distributions throughout the different magnetospheres, we compute the time evolution of particle distributions with the help of a diffusion theory particle transport code that solves the governing 3-D Fokker-Planck equation. We discuss the processes already accounted for in our physics-based models of the outer planet electron radiation belts but also those suspected to be missing to improve our model results. Our theoretical modeling is guided by the analysis of particle, field and wave data collected by Pioneer 10&11 and Galileo at Jupiter, Cassini at Saturn, and during Voyager 2’s flyby of Uranus in January 1986 and at Neptune in August 1989.

We present our latest understanding of the processes that shape the spatial distributions of energetic electrons trapped in the magnetospheres of Uranus (L < 15) and Neptune (L < 25). To determine what controls the energy and spatial distributions throughout the different magnetospheres, we compute the time evolution of particle distributions with the help of a diffusion theory particle transport code that solves the governing 3-D Fokker-Planck equation. Different mechanisms of particle loss, source and transport are numerically examined. Our theoretical modeling is guided by the analysis of particle, field and wave data collected during Voyager 2’s flyby of Uranus in January 1986 and at Neptune in August 1989. Our preliminary data-model comparison results at Uranus show that adiabatic transport cannot explain the radial and angular features of warm to ultra-relativistic electron populations within the ~1-15 L region. Our simulation results also suggest that, with absence of loss mechanisms inside L = 15, energetic and radiation-belt electron populations would be higher by 1-3 orders of magnitude in intensity close to the planet (L ~ 1-8). Particularly, our results confirm that moon sweeping effect is a significant loss mechanism at Uranus. Nonetheless, other radial, energy and pitch-angle dependent mechanisms seem to be missing to explain the in-situ data. We will thus present our ongoing effort to examine the role of - for instance, Uranus’ rings system, atomic hydrogen corona and wave activity inward of L ~ 8-10 to improve our modeling of Uranus’ electron populations between L values of 1 and 15. Our first physics-based model of energetic electrons at Neptune will be presented, emphasizing first the role of radial transport and moon sweeping effect for the 1-25 L region before investigating new processes. Our models developed for Uranus and Neptune are based on the theoretical modeling of electron distributions at Saturn, which included the modeling of radial transport and interactions of electrons with Saturn’s dust/neutral/plasma environments and waves, as well as particle sources from high-latitudes, interchange injections, and outer magnetospheric region. Comparisons between the distributions of electron populations at Gas and Ice Giant systems will be discussed. Data analysis, theoretical modeling, and numerical computations for Uranus and Neptune are carried out by adapting the Kronian modeling tools developed at Southwest Research Institute to the Ice Giants environment. Key data analysis, theoretical modeling, and numerical computational tasks for Saturn were carried out at Southwest Research Institute under NASA GSFC grant 80NSSC18K1100.

The in-situ magnetospheric exploration of the four large planets of our solar system had started with Pioneer 10’s flyby of Jupiter in Dec. 1973. The second collection of field, particle and radio data of the gas giant was carried out by Pioneer 11 in Dec. 1974, before this spacecraft made its closest approach to Saturn in Sep. 1979. Around the same period, Voyager 1 (2) flew by Jupiter in Mar. (Jul.) 1979 then Saturn in Nov. (Aug.) 1980 (1981). As of today, only Voyager 2 visited the magnetospheres of Uranus (Jan. 1986) and Neptune (Aug. 1989). Galileo had remained the only spacecraft to orbit an outer planet for several years (1995 - 2003) until the arrival of Juno at Jupiter in 2016. Between 2004 and 2017, the Cassini mission had provided a wealth of in-situ data pertinent to the study of magnetospheric particles at Saturn. In this paper, we present our current understanding of the processes that shape the spatial distributions of energetic electrons trapped in the magnetospheres of Jupiter (L < 6), Saturn (L < 15) and Uranus (L < 15) obtained by combining multi-instrument analyses of data from past missions (Pioneer, Voyager, Galileo, Cassini) and computational models of charged particle fluxes. To determine what controls the energy and spatial distributions throughout the different magnetospheres, we compute the time evolution of particle distributions with the help of a diffusion theory particle transport code that solves the governing 3-D Fokker-Planck equation. Particle, field and wave datasets are either used to provide model constraints, assist in modeling physical processes, or validate our simulation results. We first emphasize our latest results regarding the relative (or coupled) role of mechanisms at Saturn, including the radial transport and interactions of electrons with Saturn’s dust/neutral/plasma environments and waves, as well as particle sources from high-latitudes, interchange injections, and outer magnetospheric region. The lessons learned from our modeling of electron distributions at Saturn are used to identify the processes that may be missing in our modeling of Jupiter’s energetic electron environment or those in need to be implemented using new modeling concepts. Our first physics-based modeling of electron populations at Uranus is also assessed with our data-model comparison approach.