Alfvén to kinetic Alfvén wave conversion in inhomogeneous heliospheric plasmas

Abstract

<p>We present a mechanism through which an Alfvén wave (AW) propagating in a magnetic shear is converted into kinetic Alfvén waves (KAWs), producing ion and electron heating [1]. It is well known that an AW propagating in a medium in which the Alfvén speed is inhomogeneous undergoes phase-mixing, which produces a variation of the AW phase in the direction perpendicular to the field along which the AW is propagating. When such a perpendicular scale of variation eventually becomes comparable to the ion Larmor scale, the AW is converted into KAWs in the region where the Aflvén speed is inhomogeneous [2]. Through fully kinetic Particle-in-Cell simulations, we show that this mechanism produces a differential heating of ions and electrons. Using a heuristic argument, we claim that such differential heating is justified because ions and electrons can access a different amount of the kinetic and magnetic energy transported by the AW. Our simulations also recover the electron Velocity Distribution Functions (VDF) that have been detected in the presence of KAW activity in previous observations of the Earth’s magnetospheric plasmas [3]. The mechanism we propose may explain the origin of those electron VDF features. Moreover, it may contribute to heating other heliospheric plasmas (e. g., the solar corona and the solar wind), where both Alfvén waves and magnetic shears are present. </p><p>[1] F. Bacchini, F. Pucci, F. Malara, and G. Lapenta, <em>Kinetic Heating by Alfvén Waves in Magnetic Shears,</em> Phys. Rev. Lett. 128, 025101, 2022</p><p>[2] C. L. Vásconez, F. Pucci, F. Valentini, S. Servidio, W. H. Matthaeus,  & F. Malara, <em>Kinetic Alfvén wave generation by large-scale phase mixing</em>, The Astrophysical Journal, <em>815</em>(1), 7, 2015</p><p>[3] D. J. Gershman, et al., <em>Wave-particle energy exchange directly observed in a kinetic Alfvén-branch wave</em>, Nature communications, <em>8</em>(1), 1-10,  2017</p><p> </p>