Fully Kinetic Simulation of 3D Kinetic Alfvén Turbulence

Abstract

We present results from a three-dimensional particle-in-cell simulation of plasma turbulence, resembling the plasma conditions found at kinetic scales of the solar wind. The spectral properties of the turbulence in the subion range are consistent with theoretical expectations for kinetic Alfvén waves. Furthermore, we calculate the local anisotropy, defined by the relation k_{∥}(k_{⊥}), where k_{∥} is a characteristic wave number along the local mean magnetic field at perpendicular scale l_{⊥}∼1/k_{⊥}. The subion range anisotropy is scale dependent with k_{∥}<k_{⊥} and the ratio of linear to nonlinear time scales is of order unity, suggesting that the kinetic cascade is close to a state of critical balance. Our results compare favorably against a number of in situ solar wind observations and demonstrate-from first principles-the feasibility of plasma turbulence models based on a critically balanced cascade of kinetic Alfvén waves.

Kinetic Plasma Turbulence: Recent Insights and Open Questions From 3D3V Simulations

“…However, solar wind observations typically exhibit much steeper magnetic spectra, namely ∼ k −2.8 ⊥ (e.g., Alexandrova et al, 2009Alexandrova et al, , 2013Sahraoui et al, 2010;Chen, 2016;Kobayashi et al, 2017;Sorriso-Valvo et al, 2018). Similar spectral exponents were also reported in recent 3D kinetic simulations (Told et al, 2015;Cerri et al, 2017bCerri et al, , 2018Franci et al, 2018a,b;Grošelj et al, 2018;Arzamasskiy et al, 2019;Grošelj et al, 2019). Recently, refined predictions were proposed to explain steeper spectra.…”

“…This is essentially the result of KAWs developing a certain degree of magnetic compressibility at sub-ion scales, which sets the relation between δB ⊥ and δB , and requiring that compressive magnetic fluctuations are pressure balanced, which in turn provides a relation between δB and δn (see, e.g., Schekochihin et al, 2009;Boldyrev et al, 2013). As found in previous studies (e.g., Salem et al, 2012;TenBarge et al, 2012;Chen et al, 2013;Cerri et al, 2017b;Franci et al, 2018b;Grošelj et al, 2018), the spectral field ratios are overall consistent with KAW-like turbulence at sub-ion scales. This is not completely surprising, as both in CAMELIA and OSIRIS simulations, Alfvénic fluctuations are injected.…”

Modeling Imbalanced Collisionless Alfvén Wave Turbulence with Nonlinear Diffusion Equations

“…Spectra, obtained with Landau damping and electron inertia, are also displayed in this figure: for χ = 0.6 with E + (k 0 )/E − (k 0 ) = 1 or E + (k 0 )/E − (k 0 ) = 100, showing that the spectra get steeper as the imbalance increases, and for χ = 1 with E + (k 0 )/E − (k 0 ) = 100, showing that the steepening is more pronounced as χ is decreased. Such a steepening (although more moderate) is also observed in 3D fully-kinetic simulations (Grošelj et al 2018).…”

Role of reconnection in inertial kinetic-Alfvén turbulence

“…The character of turbulence then changes qualitatively. Numerical and analytical studies suggest that the energy spectrum becomes relatively steep in the sub-proton range, with the spectral index between −7/3 and −8/3, and fluctuations become compressible, with density and magnetic field fluctuations comparable to each other [e.g., [22][23][24][25][26][27]. Available solar wind measurements broadly agree with these predictions, with the measured energy spectral slope scattered around a slightly steeper value −2.8 [28][29][30][31][32].…”