Jupiter’s radiation belts as a target for NASA’s Heliophysics Division: Whitepaper #215 in the Decadal Survey for Solar and Space Physics (Heliophysics) 2024-2033.

NASA’s Heliophysics division studies “the Sun, the heliosphere, and Earth’s magnetosphere and... universal plasma phenomena”. We argue that Jupiter's radiation belts, magnetosphere, and near-space environment should be considered as relevant targets for NASA’s Heliophysics missions. All universal processes called out in the previous Decadal study can be found around Jupiter. Such physics is much more relevant and directly maps to the defined focus of NASA’s Heliophysics division than to the core sciences of NASA’s Planetary Sciences division. Jupiter’s magnetosphere is an environment of extremes: of all the planets in the solar system it is the largest natural particle accelerator, has the strongest magnetic field, the fastest spin, and the most geologically active moon that provides plasma to the magnetosphere. While the intensities of ultrarelativistic electrons with energies in excess of several MeV at Earth become significant only during the most rare and extreme events, electron acceleration to many tens of MeV occurs at Jupiter all the time. Even relativistic heavy ions such as oxygen and sulfur can be trapped up to tens of GeV. All this makes Jupiter ideal to particularly study the fundamentals of particle acceleration. At the same time, it also provides a unique opportunity to study the potential extremes of terrestrial space weather. While the waves driving acceleration processes at Earth are found throughout the L-shells of the radiation belts, at Jupiter they are limited to more discrete and narrow ranges, making it easier to disentangle local from non-local acceleration. While energetic particle dynamics at Earth’s magnetosphere are in part driven by magnetopause shadowing and substorms, Jupiter’s belts are embedded so deep within the magnetosphere that these effects are thought to be negligible: the outer magnetosphere is instead a huge reservoir for pre-accelerating radiation belt particles. The heavy ions released from the moons provide ample opportunity to 1 distinguish mass- and charge-dependent acceleration processes. Jupiter is therefore in many ways a very well controlled laboratory to study space physics processes, particularly extreme particle acceleration, that occur also at the Earth and in the rest of the universe. In addition, Jupiter is a stepping stone to extrasolar objects such as pulsar nebulae or magnetized stars. While the high energy components around such objects are otherwise only accessible through their X-ray and radio emissions, Jupiter is the only planet where we can observe the same emissions and at the same time establish a “ground truth” through in-situ measurements.