Superthin current sheets supported by anisotropic electrons

Abstract

Current sheets with strong transverse (cross field) currents are commonly observed in planetary magnetospheres and serve as a natural energy source for magnetic reconnection. As the most investigated current sheet, the current sheet in the Earth's magnetotail forms in a high-β plasma, with hot ions dominantly contributing to the diamagnetic currents. Spacecraft observations have shown, however, that a superthin electron dominated current sheet can be embedded in the Earth's magnetotail current sheet. In this paper, we develop a model of such superthin current sheets with strong currents produced by anisotropic electrons. We also compare the model with spacecraft observations, which shows reasonable agreement in spatial profiles and magnitudes of the current density. The spatial scale (thickness) of the superthin current sheet is controlled by the equatorial magnetic field component, whereas the current density magnitude is controlled by the electron fire-hose parameter at the equator. Although the current density peak within the superthin current sheet can significantly exceed the background (embedding) current density, the magnetic field magnitude at the superthin current sheet boundary does not exceed 10% of the total magnetic field magnitude. These superthin current sheets are sub-ion (or even electron-scale) structures, which are not sufficiently large/intense to perturb ion dynamics. We discuss applications of the proposed model for the analysis of plasma instabilities in superthin electron-dominated current sheets.

Force‐Free Current Sheets in the Jovian Magnetodisk: The Key Role of Electron Field‐Aligned Anisotropy

“…These streams contribute to the field‐aligned electron anisotropy, A e > 1, and fire‐hose parameter Λ e > 0. In typical thick current sheets such anisotropy supports the cross‐field electron currents (see Equation ) and may create a thin, sub‐ion scale current sheet embedded into a thick, ion scale current sheet (e.g., Kamaletdinov et al., 2020; Mingalev et al., 2018; Zelenyi et al., 2004, 2022). Indeed, the magnetic field profiles in Figure 5 exhibit stronger gradients around B l ∼ 0.…”

Configuration of magnetotail current sheet prior to magnetic reconnection onset

“…If the current density J y exceeds c(∂P/∂x)/B z , the magnetic and thermal stress balance in the magnetotail current sheet requires a contribution from pressure anisotropy (e.g., electron anisotropy, see Artemyev, Vasko, et al, 2016) or nongyrotropy (e.g., P xz = 0 due to ion demagnetized motion around the equatorial plane; see Burkhart et al, 1992;Ashour-Abdalla et al, 1994;Mingalev et al, 2007). Electron anisotropy can be quite strong for some of the observed current sheets (see examples in Lu, Artemyev, et al, 2019;Kamaletdinov et al, 2020), but cannot balance more than 30% of J y for the majority of thin current sheets (Artemyev et al, 2020). The ion nongyrotropy has not been well measured (see discussion in Aunai et al, 2011;Artemyev et al, 2019), but theoretically it could potentially be sufficiently strong to balance the observed current density magnitudes as J y ∼ c(∂P xz /∂z)/B z (see discussion in Zhou et al, 2009;Sitnov & Merkin, 2016).…”

On the Role of Whistler‐Mode Waves in Electron Interaction With Dipolarizing Flux Bundles

“…The thermal (0.1–5 keV) and energetic (5–300 keV) electron populations in the Earth's magnetotail arise by the energization of initially cold ≤100 eV electrons from the shocked solar wind or the ionosphere. An investigation of the relative contributions of different energization mechanisms is important, because electrons play a crucial role in magnetosphere‐ionosphere coupling (Khazanov et al., 2018; Ni et al., 2016; Nishimura et al., 2020) and are responsible for magnetotail current sheet currents (Artemyev, Petrukovich, et al., 2011; Kamaletdinov et al., 2020; Lu et al., 2019; Runov et al., 2006). The two main electron acceleration mechanisms are adiabatic heating and wave‐particle interactions.…”