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Register a new userGyrofluid description of Alfvenic turbulence and its parallel electric field
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Anisotropic Alfv{e}nic fluctuations with $k_{parallel}/k_{perp}ll 1$ remain at frequencies much smaller than the ion cyclotron frequency in the presence of a strong background magnetic field. Based on the simplest truncation of the electromagnetic gyrofluid equations in a homogeneous plasma, a model for the energy cascade produced by Alfv{e}nic turbulence is constructed, which smoothly connect the large magnetohydrodynamics (MHD) scales and the small kinetic scales. Scaling relations are obtained for the electromagnetic fluctuations, as a function of $k_{perp}$ and $k_{parallel}$. Moreover, a particular attention is paid to the spectral structure of the parallel electric field which is produced by Alfv{e}nic turbulence. The reason is the potential implication of this parallel electric field in turbulent acceleration and transport of particles. For electromagnetic turbulence, this issue was raised some time ago in [A. Hasegawa, K. Mima, J. Geophys. Res. {bf 83} 1117 (1978)].
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{This work aims to investigate the spectral structure of the parallel electric field generated by strong anisotropic and balanced Alfvenic turbulence in relation with the problem of electron acceleration from the thermal population in solar flare plasma conditions.} {We consider anisotropic Alfvenic fluctuations in the presence of a strong background magnetic field. Exploiting this anisotropy, a set of reduced equations governing non-linear, two-fluid plasma dynamics is derived. The low-$beta$ limit of this model is used to follow the turbulent cascade of the energy resulting from the non-linear interaction between kinetic Alfven waves, from the large magnetohydrodynamics (MHD) scales with $k_{perp}rho_{s}ll 1$ down to the small kinetic scales with $k_{perp}rho_{s} gg 1$, $rho_{s}$ being the ion sound gyroradius.} {Scaling relations are obtained for the magnitude of the turbulent electromagnetic fluctuations, as a function of $k_{perp}$ and $k_{parallel}$, showing that the electric field develops a component parallel to the magnetic field at large MHD scales.} {The spectrum we derive for the parallel electric field fluctuations can be effectively used to model stochastic resonant acceleration and heating of electrons by Alfven waves in solar flare plasma conditions}
We present numerical evidence that in strong Alfvenic turbulence, the critical balance principle---equality of the nonlinear decorrelation and linear propagation times---is scale invariant, in the sense that the probability distribution of the ratio of these times is independent of scale. This result only holds if the local alignment of the Elsasser fields is taken into account in calculating the nonlinear time. At any given scale, the degree of alignment is found to increase with fluctuation amplitude, supporting the idea that the cause of alignment is mutual dynamical shearing of Elsasser fields. The scale-invariance of critical balance (while all other quantities of interest are strongly intermittent, i.e., have scale-dependent distributions) suggests that it is the most robust of the scaling principles used to describe Alfvenic turbulence. The quality afforded by situ fluctuation measurements in the solar wind allows for direct verification of this fundamental principle.
Previous numerical studies have identified phase mixing of low-frequency Alfven waves as a mean of parallel electric field amplification and acceleration of electrons in a collisionless plasma. Theoretical explanations are given of how this produces an amplification of the parallel electric field, and as a consequence, also leads to enhanced collisionless damping of the wave by energy transfer to the electrons. Our results are based on the properties of the Alfven waves in a warm plasma which are obtained from drift-kinetic theory, in particular, the rate of their electron Landau damping. Phase mixing in a collisionless low-$beta$ plasma proceeds in a manner very similar to the visco-resistive case, except for the fact that electron Landau damping is the primary energy dissipation channel. The time and length scales involved are evaluated. We also focus on the evolution of the parallel electric field and calculate its maximum value in the course of its amplification.
We make use of the Parker Solar Probe (PSP) data to explore the nature of solar wind turbulence focusing on the Alfvenic character and power spectra of the fluctuations and their dependence on distance and context (i.e. large scale solar wind properties), aiming to understand the role that different effects such as source properties, solar wind expansion, stream interaction might play in determining the turbulent state. We carry out a statistical survey of the data from the first five orbits of PSP with a focus on how the fluctuation properties at the large, MHD scales, vary with different solar wind streams and distance from the Sun. A more in-depth analysis from several selected periods is also presented. Our results show that as fluctuations are transported outward by the solar wind, the magnetic field spectrum steepens while the shape of the velocity spectrum remains unchanged. The steepening process is controlled by the age of the turbulence, determined by the wind speed together with the radial distance. Statistically, faster solar wind has higher Alfvenicity, with more dominant outward propagating wave component and more balanced magnetic/kinetic energies. The outward wave dominance gradually weakens with radial distance, while the excess of magnetic energy is found to be stronger as we move closer toward the Sun. We show that the turbulence properties can vary significantly stream to stream even if these streams are of similar speed, indicating very different origins of these streams. Especially, the slow wind that originates near the polar coronal holes has much lower Alfvenicity compared with the slow wind that originates from the active regions/pseudostreamers. We show that structures such as heliospheric current sheets and velocity shears can play an important role in modifying the properties of the turbulence.
A Lie-Poisson bracket is presented for a five-field gyrofluid model, thereby showing the model to be Hamiltonian. The model includes the effects of magnetic field curvature and describes the evolution of the electron and ion gyro-center densities, the parallel component of the ion and electron velocities, and the ion temperature. The quasineutrality property and Amperes law determine respectively the electrostatic potential and magnetic flux. The Casimir invariants are presented, and shown to be associated to five Lagrangian invariants advected by distinct velocity fields. A linear, local study of the model is conducted both with and without Landau and diamagnetic resonant damping terms. Stability criteria and dispersion relations for the electrostatic and the electromagnetic cases are derived and compared with their analogs for fluid and kinetic models.
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