Recent progress in nonlinear kinetic Alfvén waves

Abstract

Abstract. This paper presents a review of recent progress in nonlinear kinetic Alfvén wave (KAW hereafter). We start with the two-fluid theory of KAWs and show how the difference between the motions of electrons and ions in smallscale fields of KAWs modifies the Alfvén wave properties. Then, we focus on nonlinear solitary structures of KAWs. A general criterion of the existence for solitary KAW (SKAW hereafter) and its exact analytical solution in a low-β plasma (β<m e /m i ) are presented, where the electron drift velocity along the background magnetic field is larger than the thermal speed within a SKAW, and hence can excite, for instance, ion acoustic turbulence as showed by in situ observations of satellites in space plasmas. In consequence, the turbulence results in kinetic dissipation of the SKAW and dynamical evolution in its structure. We further discuss the structure of the dissipated SKAW (DSKAW hereafter) that evolves from the SKAW due to the dissipation. The result shows that the DSKAW has a local shock-like structure in its density profile and a net electric potential drop over the shock-like structure. In particular, the electric potential drop of the DSKAW can be expected to accelerate electrons efficiently to the order of the local Alfvén speed. The application of the DSKAW acceleration mechanism to the auroral electron acceleration is also discussed. Finally, a few perspectives of KAW studies in future are presented.

Exact solutions of dispersion equation for MHD waves with short-wavelength modification

“…In particular, it has been widely accepted that the dispersive small-scale MHD waves could be responsible for the dissipation of wave energies, as well as particle acceleration and plasma heating, which occur continually in various eruptive phenomena from terrestrial aurora and solar flares, to cosmic rays. The dispersive small-scale Alfvén wave, called KAW, has been extensively investigated and applied to plasma energization processes [1][2][3][4]. In analogous works, however, little attention has been paid to the fast and slow modes.…”

“…In the case of the Alfvén wave, the dispersive wave is called the kinetic Alfvén wave (KAW). In the KAW, the perturbed electric fields have a non-zero component parallel to the background magnetic field that may cause efficient heating and acceleration of the plasma particles [1][2][3][4].…”

“…In the case of the Alfvén wave, the dispersive wave is called the kinetic Alfvén wave (KAW). In the KAW, the perturbed electric fields have a non-zero component parallel to the background magnetic field that may cause efficient heating and acceleration of the plasma particles [1][2][3][4].KAWs have been extensively discussed because of their potential importance in the particle energization of plasmas.*Corresponding author (email: djwu@pmo.ac.cn) Also, they have been applied to laboratory, space and astrophysical plasmas, such as tokamak plasma heating [5][6][7], auroral electron acceleration [8][9][10][11], solar coronal plasma heating [12][13][14], and abnormal heating of heavy ions in the extended corona [15,16]. On the other hand, the other two modes, the fast and slow waves, have been given little attention.…”

Field-aligned particle acceleration on auroral field lines by interaction with transient density cavities stimulated by kinetic Alfvén waves

“…These studies have shown that the kinetic Alfvén waves are accompanied by spikes in the electric field directed both parallel and perpendicular to the ambient magnetic field and associated decreases in plasma density with n/n∼50% and transverse scales of the order of ∼c/ω pe (Louarn et al, 1994;Chaston et al, 1999;Genot et al, 1999;Wu and Chao, 2004).…”

“…waves are associated with instabilities in regions where fieldaligned currents vary strongly across the field lines (Kozlovsky and Lyatsky, 1997; Wu and Seyler, 2003), and have been studied intensively, both experimentally and theoretically (Louarn et al, 1994;Carlson et al, 1998;Chaston et al, 1999Chaston et al, , 2003Chaston et al, , 2004Stasiewicz et al, 2000;Wygant et al, 2000;Paschmann et al, 2002;Wu and Chao, 2004). These studies have shown that the kinetic Alfvén waves are accompanied by spikes in the electric field directed both parallel and perpendicular to the ambient magnetic field and associated decreases in plasma density with n/n∼50% and transverse scales of the order of ∼c/ω pe (Louarn et al, 1994;Chaston et al, 1999;Genot et al, 1999;Wu and Chao, 2004).…”