Relativistic Electron Acceleration in Planetary Radiation Belts: Paradigm Shifts from the Van Allen Probes Mission at Earth and a Call for a Dedicated Study at Jupiter

Abstract:

Prior to the launch of NASA’s Van Allen Probes in 2012, it was widely thought that Earth’s electron radiation belts consisted of two distinct zones, a stable inner belt and highly time-variable outer belt, separated by a distinct slot region largely devoid of relativistic electrons, and it was still unknown (yet hotly debated) whether local acceleration or inward radial diffusion was the dominant acceleration mechanism of relativistic electrons. After 7 years of continuous operations and observations of Earth’s radiation belts, the paradigm has been drastically shifted due to new insight and understanding enabled by Van Allen Probes. We know now that outer radiation belt electrons up to ~8 MeV are produced by a combination of local acceleration of an injected seed population of 10s to ~300 keV electrons by whistler-mode chorus waves and inward radial diffusion from the heart of the outer belt (L ~4.5) to L~3 after enhancement events. We know now that the inner radiation belt electrons only exist up to ~1 MeV, and most surprisingly, there are no observable levels of electrons with energies higher than that. We also know that the two-belt structure for those ≤ 1 MeV electrons is not uniform or stable, but that the slot region is highly energy dependent and sporadically flooded with 100s of keV electrons in sudden, ≤ 2 hour-long injection processes for which we have only hypothesized the cause. All of these new revelations concerning Earth’s relativistic electron radiation belts were enabled by the state-of-the-art instrumentation and necessary orbital coverage of the Van Allen Probes mission. The Jovian system has the strongest radiation belts in the solar system, including high-intensity populations of several 10s of MeV electrons, and also many of the same plasma wave modes known to be critical for shaping Earth’s radiation belts. Discovering the nature of such extremely efficient acceleration of relativistic electrons will enlighten us concerning processes that drive particle acceleration at the other Giant planets and exoplanetary magnetospheres and cosmic ray acceleration throughout the universe. To enable such discoveries, a specifically designed and operated, dedicated mission to study Jupiter’s radiation belts is needed. In this talk, we’ll review some of the most impactful and revelatory discoveries and results from the Van Allen Probes mission before outlining the motivation for and some aspects of a hypothetical mission designed specifically to study the ion and electron radiation belts at Jupiter.