Laboratory study of Kelvin–Helmholtz instability at ion kinetic scales

Abstract

Kelvin–Helmholtz instability (KHI) is considered important in transporting energy and mass at the magnetopause of Earth and other planets. However, the ion kinetic effect influences the generation and evolution of KHI, as the spatial length of the magnetopause may be smaller than the Larmor radius of the ion; this influence is not yet fully understood. In this investigation, laboratory experiments were designed to study the excitation of KHI at the ion kinetic scale. The ion kinetic scale was modeled by controlling the ratio of the Larmor radius and the electric scale length ρ i / L E > 1, and the KHI was excited at the spatial scale of LE by a controllable sheared E × B flow. It was found that the ion kinetic effect on KHI growth manifests as the ion Larmor radius reaches the shear length scale, and the KHI is suppressed as the ion Larmor radius increases. Incorporating a theoretical analysis by substituting our experimental parameters, the suppression of the KHI was attributed to the fact that the KHI linear growth rate decreases with the ratio change of the ion Larmor radius because the relative orientations of the ion diamagnetic drift velocity ( V d) and the shear flow velocity ( V 0) are opposite. Our experimental conditions ( V d / V 0 < 0) are similar to the dusk-side conditions of the magnetospheres of Earth and Mercury under northward interplanetary magnetic fields; therefore, this result can be extended to understand the evolution of KHI in the planetary boundary layer.

Existence of Electrostatic Ion Cyclotron Waves in a Laboratory Created E Region Ionospheric‐Like Plasma

“…EIC waves can be excited by a field-aligned electron current; in particular, the excitation threshold decreases when the field-aligned current is inhomogeneous (Agrimson et al, 2002). Therefore, a radial nonuniform fieldaligned electron current I z was produced to excite EIC waves using a coplanar, concentric segment electrode (Zhang, Liu, Lei, Huang, Jin, & Dang, 2023). Figure 2 shows the radial profiles of the field-aligned current I z (a), electron density n e (b), field-aligned electron velocity v z (c), and gradient of the velocity ∇v z (d) with increasing ν in /f ci .…”

Experimental Study of the Response of Sheared E × B Flow to Varying Ion–Neutral Collisions

“…For example, the transverse sheared E × B flow has been observed to trigger instabilities in a broad band frequency range, such as Kelvin-Helmholtz instability (Peñano & Ganguli 2000;Liu et al 2018a), inhomogeneous energy-density driven instability (Ganguli & Palmadesso 1988;Koepke et al 1994;Liu et al 2018b), and electron-ion hybrid (EIH) instability (Amatucci 1999;DuBois et al 2013;Liu et al 2014). In the laboratory, the sheared E × B flow and electron density gradient were usually generated using a biased concentric ring electrode, and plasma instabilities such as KHI were excited in different collision regimes (Zhang et al 2023). These instabilities are frequently observed in solar plasma and have been mainly investigated using numerical methods, such as analytical approaches to either single-or multiple-fluid treatment (Keskinen et al 1988;Hysell & Kudeki 2004;Khomenko 2016).…”