7th Asia-Pacific Conference on Plasma Physics, 12-17 Nov, 2023 at Port Messe Nagoya

Current and vorticity sheets disruption in collisionless plasma turbulence
Daniela Grasso1, Dario Borgogno11Istituto dei Sistemi Complessi—CNR and Dipartimento di Energia, Politecnico di Torino. Italye-mail (speaker): daniela.grasso@polito.it

Magnetic reconnection is a fundamental process inmagnetized plasmas, associated with significant magnetic
energy release, and is ubiquitous in several astrophysicalenvironments characterized by high Reynolds numbers
and turbulence[1]. In the presence of strong turbulence inthe medium, magnetic field line reconnection iscontinuously met along the flow. This makes magnetic
reconnection an intrinsic element of the turbulent
cascade and vice versa[2].
Magnetic reconnection typically occurs where intense
current sheets form, giving rise to plasmoid formation,
while the turbulent cascade explains how the energy
provided at large scales is dissipated at small scales.
Significant efforts have been devoted to understanding
how the reconnection process, via plasmoid instability,
influences the turbulent cascade in resistive
magnetohydrodynamics (MHD) [3-6]. However, space
and astrophysical plasmas where reconnection andturbulence have mutual influence are most of the timecollisionless suggesting that the electron inertia may playa crucial role as a driving mechanism for reconnection. In
the framework of collisionless magnetic reconnection
described by two-fluid equations, several studies [7,8]have also been devoted to the evolution of current andvorticity sheets that undergo secondary instabilities ofthe fluid type, especially the Kelvin–Helmholtz (KH)
instability.
The evolution of current and vorticity sheets is studied
here in the framework of collissionless turbulent
plasmas, where magnetic and fluid instabilities coexist
and play a role in determining their evolution.
A detailed linear analysis of the stability of a magnetized
Bickley jet [9], that effectively model the shearedmagnetic and velocity layers observed in ideal plasmas,
will guide us in the interpretation of the numericalsimulations. Thin current sheets may be prone toplasmoid or KH instabilities depending on the local
values of the magnetic and velocity fields [10]. We findthat the coexistence of these two instabilities prevent thecolllisionelss plasma to achieve the fully mediatedplasmoid turbulent regime. This behavior not only
influence how the current and vorticity sheets break, butalso affect the energy cascade, explaining thediscrepancy between the magnetic energy spectrum
evaluated from the numerical simulations and the one wederived analytically under the assumption that the fully
plasmoid-mediated turbulent regime has been achieved.
We will also see that, unlike the resistive
magnetohydrodynamic context, in collisionless plasmas
the magnetic and kinetic spectra decouple.
Preliminary results on the effect of electron temperatureon the described scenario will also be shown. As an
example fig. 1 shows a comparison of the current density
distribution and magnetic surfaces for two simulations
run with and without electron temperature effects.
Starting from the effect on the linear growth rate of thereconnecting and KH modes we interpret the new results.
References[1] Lazarian, A. et al., 2015, RSPTA, 373, 20140144[2] Servidio, S. et al., 2009, PhRvL, 102, 115003[3] Biskamp, D., & Welter, H. 1989, PhFlB, 1, 1964
[4] Politano, H. et al., 1995, PhPl, 2, 2931
[5] Boldyrev, S., & Loureiro, N. F. 2017, ApJ, 844, 125[6] Dong, C. et al., 2022, Sci.Adv., 8, eabn7627[7] Del Sarto, D. et al., 2003, PhRvL, 91, 235001[8] Grasso, D. et al., 2007, PhPl, 14, 055703[9] Biskamp, D. 2000, Magnetic Reconnection in
Plasmas (Cambridge: Cambridge Univ. Press)[10] Borgogno, D. et al., 2022, ApJ 929:62

Figure 1: Current density distribution with superimposed magnetic field lines for a case with electron temperatureeffects (right panel) and without (left panel)