Electron trapping in shear Alfvén waves that power the aurora

Kinetic Alfvén waves and auroral particle acceleration: a review

Shear mode Alfvén waves are the carriers of field-aligned currents in the auroral zones of Earth and other planets. These waves travel along the magnetic field lines, coupling the outer magnetosphere with the ionosphere. However, in ideal magnetohydrodynamic (MHD) theory, the shear mode Alfvén wave does not carry a parallel electric field that could accelerate auroral particles. This can be modified by including kinetic effects, which lead to a parallel electric field when the perpendicular wavelength becomes comparable to the electron inertial length or the ion acoustic gyroradius. These small perpendicular wavelengths can be formed by phase mixing, ionospheric feedback, or nonlinear effects. Kinetic Alfvén waves are further constrained by their interaction with the ionosphere, which acts as a reflector for these waves. In addition, the strong plasma gradients in the topside ionosphere form an effective resonator that leads to fluctuations on time scales of seconds. These rapidly changing parallel electric fields can lead to broadband acceleration of auroral electrons, often called the Alfvénic aurora. Such interactions do not only take place in Earth's magnetosphere, but have also been observed in Jupiter's magnetosphere by the Juno satellite.

Small-Scale Dynamic Aurora

Small-scale dynamic auroras have spatial scales of a few km or less, and temporal scales of a few seconds or less, which visualize the complex interplay among charged particles, Alfvén waves, and plasma instabilities working in the magnetosphere-ionosphere coupled regions. We summarize the observed properties of flickering auroras, vortex motions, and filamentary structures. We also summarize the development of fundamental theories, such as dispersive Alfvén waves (DAWs), plasma instabilities in the auroral acceleration region, ionospheric feedback instabilities (IFI), and the ionospheric Alfvén resonator (IAR).

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s11214-021-00796-w.

A diagnosis of the plasma waves responsible for the explosive energy release of substorm onset

During geomagnetic substorms, stored magnetic and plasma thermal energies are explosively converted into plasma kinetic energy. This rapid reconfiguration of Earth's nightside magnetosphere is manifest in the ionosphere as an auroral display that fills the sky. Progress in understanding of how substorms are initiated is hindered by a lack of quantitative analysis of the single consistent feature of onset; the rapid brightening and structuring of the most equatorward arc in the ionosphere. Here, we exploit state-of-the-art auroral measurements to construct an observational dispersion relation of waves during substorm onset. Further, we use kinetic theory of high-beta plasma to demonstrate that the shear Alfven wave dispersion relation bears remarkable similarity to the auroral dispersion relation. In contrast to prevailing theories of substorm initiation, we demonstrate that auroral beads seen during the majority of substorm onsets are likely the signature of kinetic Alfven waves driven unstable in the high-beta magnetotail.

Probabilistic approach to nonlinear wave-particle resonant interaction

In this paper we provide a theoretical model describing the evolution of the charged-particle distribution function in a system with nonlinear wave-particle interactions. Considering a system with strong electrostatic waves propagating in an inhomogeneous magnetic field, we demonstrate that individual particle motion can be characterized by the probability of trapping into the resonance with the wave and by the efficiency of scattering at resonance. These characteristics, being derived for a particular plasma system, can be used to construct a kinetic equation (or generalized Fokker-Planck equation) modeling the long-term evolution of the particle distribution. In this equation, effects of charged-particle trapping and transport in phase space are simulated with a nonlocal operator. We demonstrate that solutions of the derived kinetic equations agree with results of test-particle tracing. The applicability of the proposed approach for the description of space and laboratory plasma systems is also discussed.