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Jupiter's Radiation Belts As a Target for NASA's Heliophysics Division

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Jupiter's Radiation Belts As a Target for NASA's Heliophysics Division Heliophysics 2050 White Papers (2021) 4026.pdf Jupiter’s radiation belts as a target for NASA’s Heliophysics Division P. Kollmann1, D. L. Turner1, E. Roussos2, Q. Nénon3, G. Clark1, I. Cohen1, W. Li4, A. Sulaiman5 1 JHU/APL, Laurel MD, USA; 2 MPS, Göttingen, Germany; 3 SSL, Berkeley CA, USA; 4 Uni Boston, CO, USA; 5 Uni Iowa, IA, USA Fundamental processes NASA’s Heliophysics division covers not strictly the heliosphere but also a large range of space physics topics. One of the key science goals of its 2013 Decadal Strategy for Solar and Space Physics is to “Discover and characterize fundamental processes that occur both within the heliosphere and throughout the universe” by using “the Sun, the heliosphere, and Earth’s magnetosphere and ionosphere [to] serve as cosmic laboratories for studying universal plasma phenomena”. Here we argue that planetary magnetospheres, particularly Jupiter’s radiation belts, are also such cosmic laboratories that enable studies of space physics, are of broad relevance to astrophysics, and should as such be treated as valid targets that deserve focused investigations from NASA’s Heliophysics Division. Jupiter’s magnetosphere covers all universal processes called out in the 2013 Decadal: Jupiter has an internal dynamo producing its magnetic field (Moore+19 Nat.). The Jupiter system sheds mass that is mostly released by its moons (Bagenal+11 JGR), which can be described as a planetary wind. Magnetic reconnection occurs on both the magnetopause (Ebert+17 GRL) and the magnetotail (Vogt+11 JGR). A collisionless shock separates it from the solar wind (Hospodarsky+17 GRL). Turbulence plays a role in particle acceleration (Saur+18 JGR). Plasma‐ neutral interactions are not just limited to the thermosphere but occur through large parts of the magnetosphere thanks to the material liberated from moons (Kollmann+16 GRL). By several metrics, Jupiter’s magnetosphere is the most efficient particle accelerator in the Solar System. Earth’s magnetosphere Another key science goal of the Strategy for Solar and Space Physics is to “Determine the dynamics and coupling of Earth’s magnetosphere” with a “priority” to “understanding charged‐ particle acceleration, scattering, and loss”. Fig. 1: Jupiter’s electron energy spectra have a clear cutoff. The blue line shows this cutoff energy, estimated from combining integral measurements. The orange curve compares to adiabatic heating (Kollmann+18 JGR). At small distances to Jupiter, both wave intensities (green, Menietti+16 JGR) and electron intensities rise, suggesting that the dominant physics may switch to local acceleration. Originally, acceleration in Earth’s radiation belts was thought to be mostly driven by adiabatic transport (Schulz+74 Springer). The potential of local acceleration was recognized much later (Horne+1998 GRL). Acceleration is a multifaceted, stepwise process. Because of the complex interplay between the competing processes, each processes’ relative importance at Earth is still under active investigation, both in the big picture (W. Li+16 JGR vs. Q. Ma+18 JGR) as well as for single events (Shprits+13 Nat.Phys. vs. Mann+16 Nat.Phys.). 1 Heliophysics 2050 White Papers (2021) 4026.pdf Jupiter’s radiation belts offer a new perspective to particle acceleration in a planetary magnetic field. For example, there are indications that MeV electrons in Jupiter’s outer magnetosphere are accelerated by adiabatic transport (Fig. 1), while acceleration in the inner radiation belts may occur through local acceleration (Woodfield+14 JGR). It is conceivable that Jupiter may provide a unique opportunity to study the driving mechanisms behind these two processes with less ambiguity because a giant magnetosphere may be a simpler system in this aspect compared to Earth’s relatively small system. Closer analysis is needed to support or reject this hypothesis. Jupiter’s high energies cannot be only the result of adiabatic transport and local acceleration, otherwise we would not find MeV electrons at the edge of the magnetosphere (Kollmann+18 JGR). Other processes are needed, like the scattering of MeV particles that are produced by auroral processes (Mauk+17 Nat.) into the equatorial plane. Also, what was long thought to be a detail, the opposite direction of Jupiter’s magnetic field relative to the Earth, appears as a game changer and allows for very efficient electron acceleration (Roussos+18 Icarus). Overall, Jupiter therefore offers the opportunity to discover the importance of processes that are insignificant or obfuscated at Earth but may play a role in other parts of the universe, as discussed below. Why Jupiter? All sufficiently magnetized planets form radiation belts. Jupiter sets itself apart by having the strongest magnetic field, the most active moons that act as plasma sources, the fastest rotation, the most powerful aurora, and intensities of high energy particles that are unlike any other planet in our solar system (Mauk+14 JGR). While electrons at the Earth only reach into the several (< 10) MeV range, high electron fluxes at Jupiter are common at least at ten times higher Fig. 2. Very energetic heavy ions in Jupiter’s energies (Nenon+17 JGR). While at Earth we have radiation belts. Orange: helium ions to wait for extreme events to study acceleration (Fischer+96 Sci.), red: Z≥6 40MeV/nuc to high energies, acceleration to even higher (Roussos+19 ESA2050). energies is the norm at Jupiter. Oxygen and sulfur are amongst Jupiter’s major ion species. They originate from Jupiter’s geologically active moons and are subsequently ionized and accelerated to at least hundreds of MeV (Fig. 2). The wealth of particle masses and charge states makes Jupiter ideal to distinguish fundamental processes like acceleration and loss that are mass‐ and charge‐dependent. Link to astrophysics Plasma is the dominant state of matter in the visible Universe. The Heliophysics Decadal Survey points out the importance of studying universal processes because space plasma physics has important implications and “applications to laboratory plasma physics, fusion research, and plasma astrophysics.” Jupiter covers such an immense parameter range in plasma, magnetic field, energetic particles, and waves that it has relevance to several astrophysical systems including exoplanets. Because of the high energies in Jupiter’s magnetosphere, it can be considered as a missing link to even more energetic extrasolar objects that we can only observe indirectly 2 Heliophysics 2050 White Papers (2021) 4026.pdf through their emissions. For example, the upper end of Jupiter’s electron distribution overlaps in energy with the lower end of the distribution in the Crab Nebula (Fig. 3), suggesting that Jupiter may allow us to understand the seed population of that famous nebula. Similarly, bow shocks in the outer solar system, which have relatively high Mach numbers compared to Earth’s bow shock, are the missing links between supernova shocks and Earth’s bow shock and have been demonstrated to be useful to understand the acceleration of cosmic rays and shocks (including the termination shock) in general (Sulaiman+15 Phys.Rev.Lett.). Fig. 3. Electron intensities from MeV to EeV energies (Mauk+12 Geophys. Monog). Black: Jupiter from in‐situ measurements, red and blue: two inversions of remote synchrotron measurements of the Crab nebula Jupiter is 100 times more intense in the overlapping energy range. The need for cross‐divisional studies Past, ongoing, and planned missions to Jupiter have made tremendous progress in studying Jupiter. However, Van Allen Probes demonstrated the value of a mission with dedicated and specifically‐designed instrumentation and orbits to study radiation belt physics and can be considered as a gold standard on how to explore comprehensively the radiation environment of a planet. Studies at other planets should follow the same standard instead of being based on extrapolations based on the Earth. One example is the unexpected discovery that all previous observations of MeV “electrons” in the inner radiation belt were actually contamination from very energetic protons (Fennell+14 GRL); such a discovery was impossible without specifically and carefully designed instrumentation. Future observations at Jupiter may force us to change a paradigm and consider solar wind drivers of its electron belt (Han+18 JGR). Currently planetary magnetospheres are exclusively studied through NASA’s Planetary division. After the initial survey of Jupiter and Saturn by flagship missions with a broad scientific focus, future missions will mostly be focused on specific questions that will derive from the Planetary Decadal Survey, which does not highlight space plasma physics. The Vision and Voyages 2013‐ 2022 largely focused on solar system formation, planetary habitats, and dangers to Earth. Space physics only tangentially addresses these planetary topics and needs to rely on the most generic subquestion “How have the myriad chemical and physical processes that shaped the solar system operated, interacted, and evolved over time?” to demonstrate relevance. Space plasma physics at planetary systems is much more relevant to the defined focus of NASA’s Heliophysics division, and Heliophysics should support missions to study magnetospheric physics at other planets in the solar system, like the Jovian radiation belts. We therefore propose that NASA’s Heliophysics division should include planetary magnetospheres and radiation belts as relevant targets for Heliophysics missions and support cross‐divisional opportunities where a future Planetary mission receives augmentation from the Heliophysics division (Cohen+20 this issue). 3 .
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