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

Current sheet formation and reconnection in collisionless turbulent plasmas
J. Büchner1,2, N. Jain2 and P. Muñoz 1
1
Max-Planck-Institute for Solar System Research, Göttingen, Germany
2
Berlin Institute of Technology, Center for Astronomy and Astrophysics, Berlin, Germany
e-mail (speaker): buechner@mps.mpg.deHot, collisionless plasmas are ubiquitous in astrophysicalenvironments as well as in laboratory (fusion) devices. In
the course of the turbulence evolution coherent structures
are formed like current sheets (CSs) and plasmoids.
Their formation influences the spectral properties of theturbulence and contribute to the dissipation of theturbulent energy, as it has been demonstrated
theoretically[1], observationally[2] and numerically[3]. The
evolution of these structures is, however, not wellunderstood, yet, in particular the energy dissipation atelectron scales.
Since all these processes are strongly nonlinear
numerical simulations are the method of choice. In thelast decade, mainly 2D hybrid-kinetic simulations
concentrated on the turbulence cascade at sub-ion
scales[1,3]. 2D fully kinetic turbulence simulations havecomplemented those findings by determining thestatistics of reconnection rates in current sheets anddistinguishing between “standard” reconnection eventsand the newly proposed electron-only reconnection
events (without ion coupling)[4,5,6]. In particular it was
shown that electron-scale reconnection events are more
likely in low rather than high-turbulence, although thereconnection rates were found to be in both regimes
close to the standard normalized reconnection value
0.1.[5]
In 3D turbulence the characterization of current sheetsand reconnection is much harder, so there has been lessprogress. A detection algorithm applied to hybrid-kinetic[7]
and fully-kinetic 3D simulations revealed a number ofphysical signatures of the decaying CSs such as thepresence of outflows and parallel electric field [8], in asimilar way as detected by spacecrafts in space. [9]Currently we obtained new results by using the fully
kinetic code ACRONYM and the hybrid-kinetic code
CHIEF. The latter allows a description of the electrons as
an inertial fluid, different from standard hybrid
approaches where the electrons are massless. [10,11]
A decaying turbulence is typicall initialized byrandomly-phased Alfvén-waves at large, injection scales.
In the course of the turbulence cascade CSs are formed
which thin down until they dissipate, e.g. via magneticreconnection, in plasmoids or structures at even smaller(electron-) scales. 2D hybrid kinetic simulations in thelimit of massless electrons had shown that current sheet
thinning is only limited by the grid resolution [12]. Forinertial electrons, however, their inertia determines thelimits of thinning and reconnection breakups which
proofs the necessity of the consideration of the electron
inertia in hybrid fluid-kinetic plasma models [13]3D hybrid-kinetic turbulence simulations with consider
inertial electrons have shown that the CS thinning islimited mainly by the electron inertia (see Fig. 1) [14]. andit was found that, unexpectedly, the electron inertia
influences the spectra of the parallel-current fluctuations
even well above the electron scales.
A comparison of the results obtained by fully kinetic
and by finite-electron-mass hybrid simulations revealed
that mainly the electron inertia limits the CS thinning andthen almost completely balances the reconnection
electric field either by the bulk electron inertia describedby an acceleration term in the generalized two-fluidOhm's law, to a lesser degree on by the thermal-electroninertia, described by a fully anisotropic pressure tensor.
2D fully kinetic investigations of the plasmadependence showed that in finite-guide-fieldreconnection the CS properties depend lesser on the offdiagonal elements of the pressure tensor for larger
=2.0, compared, e.g., to =0.1) compared to the role
played by the electron inertia for the reconnection
process. Also, for finite , the CS current densities staysmaller and the reconnection rates are reduced.[15]Fig. 1 Volume-rendered structures of the current density
obtained by 3D hybrid code simulations with the code
CHIEF (ions as particles and electrons as an inertial, i.e.
finite-mass fluid). Box size: (51,2 di)3 [14]References[1] Franci et al, ApJ 850, L16 (2017)[2] Loureiro, ApJ 890, 55 (2020)[3] Manzini et al, PRL, 130, 205201 (2023)[4] Haggerty et al, POP, 24, 102308 (2017)[5] Vega et al, ApJ, 893, L10 (2020)[6] Franci et al, ApJ, 936, 27 (2022)[7] Sisti et al, A&A, 655, A107 (2021)[8] Agudelo Rueda et al, JPP, 87, 905870228 (2022)[9] Khabarova et al, 2, Astroph. J. 933, 1, 97 (2022)[10] P. Muñoz et al, Comp. Phys. Comm., 224, 245 (2018)[11] J. Büchner (ed), Space and astrophysical plasma
simulation, 10.1007/978-3-031-11870-8, Springer (2023)[12] N. Jain et al, The Astrophys. J., ,919, 103 (2021)[13] N. Jain et al, Phys. Plasmas, 29, 053902 (2022)[14] P. Muñoz et al, submitted (2023). arXiv:2303.01322[15] P. Muñoz et al, COSPAR (2022) abst: D2.4-0013-22