Ion and electron energization by Alfvén waves in magnetic shears

In order to efficiently convert AW energy into particle energy, the original fluctuation must decay from the initial macroscopic (fluid) scales to smaller (kinetic) scales. This decay can be promoted by the interaction of counter-propagating AWs or by the interaction between AWs and an inhomogeneous background. It has been shown that AWs interacting with an inhomogeneous background can cascade to smaller scales via the phase-mixing process [1]. When the cascade reaches scales comparable with the ion Larmor radius, AWs can efficiently be converted into ”kinetic” Alfvén  waves (KAWs), which represent the natural extension of AWs in the kinetic branch of the wave dispersion relation for wavevectors nearly perpendicular to the background magnetic field [2]. In this work we present the results of a numerical experiment in which the decay of AWs into KAWs is studied self-consistently in a range that goes from fluid to kinetic electron scales. We show how the AW-to-KAW transition, promoted by an inhomogeneous background, leads to the heating of both ions and electrons via two different physical mechanisms. Both mechanisms are illustrated via a simple argument on how the two species can access the kinetic and magnetic energy carried by AWs. Our findings are supported by in-situ observations of KAWs in the Earth’s magnetosphere [3].

[1]  J. Heyvaerts  and  E.  Priest,  Coronal  heating  by  phase-mixed shear Alfv ́en waves, Astronomy and Astrophysics117, 220 (1983).
[2]  J. Hollweg, Kinetic Alfv ́en wave revisited, Journal of Geo-physical Research:  Space Physics104, 14811 (1999)
[3] D. Gershman, F. Adolfo, J. Dorelli, S. Boardsen, L. Avanov, P. Bellan, S. Schwartz, B. Lavraud, V. Cof-fey, M. Chandler,et  al., Wave-particle energy exchangedirectly observed in a kinetic Alfv ́en-branch wave, Naturecommunications8, 1 (2017).