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Particle Dynamics in the Earth's Radiation Belts: Review of Current INTRODUCTION TO Particle Dynamics in the Earth's Radiation Belts: Review A SPECIAL SECTION of Current Research and Open Questions 10.1029/2019JA026735 J.‐F. Ripoll1, S. G. Claudepierre2,3, A. Y. Ukhorskiy4, C. Colpitts5,X.Li6, J. F. Fennell2, and 7 Special Section: C. Crabtree Particle Dynamics in the Earth's Radiation Belts 1CEA, DAM, DIF, Arpajon, France, 2The Aerospace Corporation, El Segundo, CA, USA, 3Department of Atmospheric and Oceanic Sciences, University of California, Los Angeles, CA, USA, 4The Johns Hopkins University Applied Physics Laboratory, Laurel, MD, USA, 5School of Physics and Astronomy, University of Minnesota, Minneapolis, MN, USA, Key Points: 6 7 • We review and discuss current LASP, University of Colorado Boulder, Boulder, CO, USA, Naval Research Laboratory, Washington, DC, USA research and open questions relative to Earth's radiation belts • Aspects of modern radiation belt Abstract The past decade transformed our observational understanding of energetic particle processes in research concern particle near‐Earth space. An unprecedented suite of observational systems was in operation including the Van acceleration and transport, particle loss, and the role of nonlinear Allen Probes, Arase, Magnetospheric Multiscale, Time History of Events and Macroscale Interactions during processes Substorms, Cluster, GPS, GOES, and Los Alamos National Laboratory‐GEO magnetospheric missions. • We also discuss new radiation belt They were supported by conjugate low‐altitude measurements on spacecraft, balloons, and ground‐based modeling capabilities, the fi quantification of model arrays. Together, these signi cantly improved our ability to determine and quantify the mechanisms that uncertainties, and laboratory plasma control the buildup and subsequent variability of energetic particle intensities in the inner magnetosphere. experiments The high‐quality data from National Aeronautics and Space Administration's Van Allen Probes are the most comprehensive in situ measurements ever taken in the near‐Earth space radiation environment. These observations, coupled with recent advances in radiation belt theory and modeling, including dramatic Correspondence to: increases in computational power, have ushered in a new era, perhaps a “golden era,” in radiation belt ‐ J. F. Ripoll, research. We have edited a Journal of Geophysical Research: Space Science Special Collection dedicated to jean‐[email protected] Particle Dynamics in the Earth's Radiation Belts in which we gather the most recent scientific findings and understanding of this important region of geospace. This collection includes the results presented at the Citation: American Geophysical Union Chapman International Conference in Cascais, Portugal (March 2018) and Ripoll, J.‐F., Claudepierre, S. G., Ukhorskiy, A. Y., Colpitts, C., Li, X., many other recent and relevant contributions. The present article introduces and review the context, current Fennell, J., & Crabtree, C. (2020). research, and main questions that motivate modern radiation belt research divided into the following Particle Dynamics in the Earth's topics: (1) particle acceleration and transport, (2) particle loss, (3) the role of nonlinear processes, (4) Radiation Belts: Review of Current fi Research and Open Questions. Journal new radiation belt modeling capabilities and the quanti cation of model uncertainties, and (5) laboratory of Geophysical Research: Space Physics, plasma experiments. 125, e2019JA026735. https://doi.org/ 10.1029/2019JA026735 1. Introduction Received 27 MAR 2019 Accepted 20 NOV 2019 Earth's radiation belts consist of two toroidal belts of energetic charged particles (electrons and ions) sur- Accepted article online 26 DEC 2019 rounding Earth. The outer belt typically lies at geocentric radial distances between 3 and 7 Earth radii (1 Earth radius = 6,370 km) in the equatorial plane and consists primarily of highly energetic (0.1–10 MeV) electrons and high‐energy protons (1–100 keV), though other ion species and lower‐energy particles are also present. The inner belt sits between 1 and 3 Earth radii and contains primarily hundreds of kiloelectron volts electrons along with extremely energetic (e.g., hundreds of megaelectron volts) protons. This description is, however, an idealized representation of a simplified structure. This representation can be valid during quiet geomagnetic times but dynamic/disturbed conditions bring complex dynamic monobelt or multibelt struc- tures (e.g., Baker, Kanekal, Hoxie, Henderson, et al., 2013) forming within the inner magnetosphere below ~7–8 Earth radii. Earth's radiation belt location is also energy dependent. Many competing processes contri- bute to the dynamic formation and depletion of the belts, including radial transport, local wave acceleration, particle loss to the magnetopause, particle precipitation into the atmosphere, and others. These competing energization, loss, and transport mechanisms greatly contribute to generating complex structures far beyond the ideal two‐belt structure. These competing mechanisms typically occur simultaneously (e.g., Baker et al., 2019 in this collection) and are energy dependent; an accurate description of the radiation belts must account for their combined effects. The relative importance of each process is the most fundamental, unan- ©2019. American Geophysical Union. swered question in radiation belt physics. This question cannot be answered fully without the combined All Rights Reserved. effort of, and collaboration between, experimentalists, theorists, and modelers. RIPOLL ET AL. 1of48 Journal of Geophysical Research: Space Physics 10.1029/2019JA026735 The motion of a charged particle in the Earth's magnetic field was first formulated by Störmer (2018) and was subsequently studied by him and several others in connection with auroral phenomena and cosmic rays (Störmer, 2017). The motion and the stability of charged and trapped particles in Earth's magnetic field was then well established by 1960 (e.g., Northrop & Teller, 1960; Dragt, 1965; Fälthammar, 1965) and has provided the theoretical basis for the presence of Earth's radiation belts discovered by pioneering space missions (Van Allen, 1959; Vernov et al., 1959). It was shown that in the approximately dipolar magnetic field of the inner magnetosphere including the Earth's Van Allen radiation belts, charged par- ticles undergo quasiperiodic motion composed of gyro, bounce, and gradient‐curvature drift motions, each associated with an adiabatic invariant. This set of three invariants defines a stable drift shell encir- cling Earth. Subsequent experiments revealed that particle intensities across the belts can vary signifi- cantly with time, which requires violation of one or more of the adiabatic invariants. The theoretical interpretation of the variability of radiation belt intensities was largely inspired by the experiments in particle acceleration by random‐phased electrostatic waves in synchrocyclotron devices and by the subse- quent development of the theory of weak plasma turbulence. It was thus suggested that the adiabatic invariants of trapped particles can be violated by small‐amplitude waves, which resonantly interact with the quasiperiodic particle motion (Balescu, 1960; Lenard, 1960; Vedenov et al., 1961). Since both the den- sity and energy density of radiation belt particles are negligible compared to other plasma populations, their motion does not affect the fields that govern them (with some exceptions, e.g., chorus waves). Thus, it was suggested that the evolution of radiation belt intensities can be described kinetically and sta- tistically as a quasilinear diffusion in the three adiabatic invariants (Northrop & Teller, 1960) under the action of prescribed wave fields, with the diffusion coefficients determined by resonant wave‐particle interactions (e.g., Hess, 1968; Walt, 1970; Schulz & Lanzerotti, 1974). The theoretical framework of quasi- linear diffusion of radiation belt particles, developed within the first decade following the discovery of the belts, has been the backbone of most of the modeling of global variability of radiation belt intensities (see recent reviews, e.g., Hudson et al., 2008, Shprits et al., 2008a, 2008b, Thorne, 2010, and see discus- sion in section 5). We will see many aspects of this approach treated in this JGR Special Collection. In addition, it is now clear that nonlinear effects must also be considered in radiation belt dynamics and this will also be addressed specifically (e.g. section 4). Understanding the variability of the Van Allen radiation belts, to the point of predictability, is one of the great outstanding questions in heliophysics research. In the coupled Sun‐Earth system, solar wind energy is transferred into the radiation belts, leading to charged particle dynamics over a broad range of timescales (e.g., seconds to years). Radiation belt enhancements have wide‐ranging implications for the man‐made technologies that operate in this region of geospace, such as radiation hazards that can affect astronauts, or charged particle spacecraft interactions that can damage satellites (e.g., Lanzerotti, 2017). Therefore, a more complete understanding of the highly variable dynamics of radiation belt particles is an international priority, which has led to many recent missions devoted to exploring the belts. The main current mission is National Aeronautics and Space Administration's (NASA) Van Allen Probes launched in 2012, a two‐space- craft mission devoted to unraveling the mysteries of the dynamics of the
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