Kinetic Alfvén waves and auroral particle acceleration: a review

Abstract

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.

Rapid Acceleration Bursts in the Van Allen Radiation Belt

“…Similarly, during a recovery phase of a storm, the newly trapped electrons can be transported inwards and thus accelerated by the ULF waves (e.g., Ozeke et al., 2020; Silva et al., 2022) or accelerated locally due to the interaction with hiss (e.g., J. Li et al., 2019) or chorus waves (e.g., Horne et al., 2005; Hua et al., 2022). Lower energy electrons (1–10s keV) are also often impacted by the presence of kinetic Alfven waves in the auroral regions of the magnetosphere (see, e.g., review by Lysak (2023), and references within). These processes are often referred to as radiation belt “acceleration” or “enhancement” (see, e.g., review by Millan and Baker (2012)).…”

“…Li et al, 2019) or chorus waves (e.g., Horne et al, 2005;Hua et al, 2022). Lower energy electrons (1-10s keV) are also often impacted by the presence of kinetic Alfven waves in the auroral regions of the magnetosphere (see, e.g., review by Lysak (2023), and references within). These processes are often referred to as radiation belt "acceleration" or "enhancement" (see, e.g., review by Millan and Baker (2012)).…”

Kinetic and Shear Alfvén Waves in a Large Guide-field Reconnection at Earth's Magnetopause

“…KAWs have been considered as an energy source for auroral particle acceleration (e.g., Lysak 2023). The electrons that generate discrete aurora in the upper ionosphere are accelerated by the parallel electric field, which is induced by KAWs from a reconnection site in the magnetotail.…”

The effect of heavy ions on the dispersion properties of kinetic Alfvén waves in astrophysical plasmas

“…Kinetic Alfvén waves (KAWs) are Alfvénic waves that propagate at large wave-normal angles, when the mode turns compressive and develops a large parallel electric field due to kinetic effects, which give this mode its name. These characteristic kinetic effects become relevant under two conditions in typical electron-proton plasma: 1) in the hot electron case (β > m e /m p ) as the perpendicular wavelength reaches the order of the scale of the protons' gyroradius (Hasegawa 1977;Gary 1986;Hollweg 1999) and 2) in the cold electron case (β < m e /m p ) as it reaches the electron's inertial length (Goertz & Boswell 1979;Lysak & Lotko 1996;Tamrakar et al 2019;Lysak 2023). In this latter case, the waves can also be referred to as inertial Alfvén waves (de Assis et al 2020).…”

“…The large-amplitude parallel electric perturbations enable KAWs to accelerate charged particles along geomagnetic field lines, which allows for the strong acceleration and subsequent energization of electrons, in particular (Lysak & Lotko 1996;Tian et al 2022;Zhang et al 2022;Lysak 2023). Among other observations, satellite measurements have provided direct evidence of electron acceleration by KAWs in the plasma sheet boundary layer (Wygant et al 2002;Chaston et al 2012;Zhang et al 2022) and the equatorial inner magnetosphere (Angelopoulos et al 2002;Tian et al 2022).…”