The past decade transformed our observational understanding of energetic particle processes in near‐Earth space. An unprecedented suite of observational systems was in operation including the Van Allen Probes, Arase, Magnetospheric Multiscale, Time History of Events and Macroscale Interactions during Substorms, Cluster, GPS, GOES, and Los Alamos National Laboratory‐GEO magnetospheric missions. They were supported by conjugate low‐altitude measurements on spacecraft, balloons, and ground‐based arrays. Together, these significantly improved our ability to determine and quantify the mechanisms that control the buildup and subsequent variability of energetic particle intensities in the inner magnetosphere. 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 increases in computational power, have ushered in a new era, perhaps a “golden era,” in radiation belt research. We have edited a Journal of Geophysical Research: Space Science Special Collection dedicated to 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 American Geophysical Union Chapman International Conference in Cascais, Portugal (March 2018) and many other recent and relevant contributions. The present article introduces and review the context, current research, and main questions that motivate modern radiation belt research divided into the following topics: (1) particle acceleration and transport, (2) particle loss, (3) the role of nonlinear processes, (4) new radiation belt modeling capabilities and the quantification of model uncertainties, and (5) laboratory plasma experiments.
Kinetic Alfven wave turbulence in solar wind is considered and it is shown that nonMaxwellian electron distribution function has a significant effect on the dynamics of the solar wind plasmas. Linear Landau damping leads to the formation of a plateau in the parallel electron distribution function which diminishes the Landau damping rate significantly. Nonlinear scattering of waves by plasma particles is generalized to short wavelengths and it is found that for the solar wind parameters this scattering is the dominant process as compared to three wave decay and coalescence in the wave vector. Incorporation of these effects lead to the steepening of the wave spectrum between the inertial and the dissipation ranges with a spectral index between 2 and 3. This region can be labeled as the scattering range. Such steepening has been observed in the solar wind plasmas.2
[1] The stability of low-frequency drift magnetosonic waves in the nightside magnetosphere between the outer ring current and the distant neutral line is investigated. As low-frequency Pi2 oscillations are seen before, during, and after substorm onset, these compressional waves are important for understanding the connection between the two substorm onset mechanisms of (1) the near geosynchronous orbit (NGO) mechanism and (2) the near-Earth neutral line (NENL) mechanism. It is found that there are two different regions of parameter space where drift-compressional modes can be unstable: (1) when pressure gradients become sufficiently steep to reverse the magnetic-guiding center drift and (2) when the temperature gradient is in the opposite direction to the density gradient. Nonlocal bounce-averaged eigenmode equations are solved with a simple analytic model of the magnetospheric magnetic field and with the Tsyganenko and Stern [1996] magnetic field model. Resulting growth rates, frequencies, and eigenmode structures are reported.
[1] A dispersion relation for ultralow frequency drift compressional modes in high pressure plasmas with finite gyroradius is derived from the linearized gyrokineticMaxwell equations with cold electrons and narrow eigenmode localization width along the field line. The dispersion relation demonstrates instability under two different conditions: 1) when the density gradient and proton temperature gradient are in opposite directions, or 2) when the magnetic guiding center drift is reversed with respect to the proton diamagnetic drift, i.e., drift reversal, which could occur during periods of strong magnetospheric disturbance. Furthermore, it is found that the most unstable modes have short azimuthal wavelengths comparable to the proton gyroradius.
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