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Register a new userProton Kinetic Effects in Vlasov and Solar Wind Turbulence
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Kinetic plasma processes have been investigated in the framework of solar wind turbulence, employing Hybrid Vlasov-Maxwell (HVM) simulations. The dependency of proton temperature anisotropy T_{perp}/T_{parallel} on the parallel plasma beta beta_{parallel}, commonly observed in spacecraft data, has been recovered using an ensemble of HVM simulations. By varying plasma parameters, such as plasma beta and fluctuation level, the simulations explore distinct regions of the parameter space given by T_{perp}/T_{parallel} and beta_{parallel}, similar to solar wind sub-datasets. Moreover, both simulation and solar wind data suggest that temperature anisotropy is not only associated with magnetic intermittent events, but also with gradient-type structures in the flow and in the density. This connection between non-Maxwellian kinetic effects and various types of intermittency may be a key point for understanding the complex nature of plasma turbulence.
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The application of linear kinetic treatments to plasma waves, damping, and instability requires favorable inequalities between the associated linear timescales and timescales for nonlinear (e.g., turbulence) evolution. In the solar wind these two types of timescales may be directly compared using standard Kolmogorov-style analysis and observational data. The estimated local nonlinear magnetohydrodynamic cascade times, evaluated as relevant kinetic scales are approached, remain slower than the cyclotron period, but comparable to, or faster than, the typical timescales of instabilities, anisotropic waves, and wave damping. The variation with length scale of the turbulence timescales is supported by observations and simulations. On this basis the use of linear theory - which assumes constant parameters to calculate the associated kinetic rates - may be questioned. It is suggested that the product of proton gyrofrequency and nonlinear time at the ion gyroscales provides a simple measure of turbulence influence on proton kinetic behavior.
Based on in-situ measurements by Wind spacecraft from 2005 to 2015, this letter reports for the first time a clearly scale-dependent connection between proton temperatures and the turbulence in the solar wind. A statistical analysis of proton-scale turbulence shows that increasing helicity magnitudes correspond to steeper magnetic energy spectra. In particular, there exists a positive power-law correlation (with a slope $sim 0.4$) between the proton perpendicular temperature and the turbulent magnetic energy at scales $0.3 lesssim krho_p lesssim 1$, with $k$ being the wavenumber and $rho_p$ being the proton gyroradius. These findings present evidence of solar wind heating by the proton-scale turbulence. They also provide insight and observational constraint on the physics of turbulent dissipation in the solar wind.
Plasma kinetic waves and alpha-proton differential flow are two important subjects on the topic of evolution of the solar wind. Based on the Wind data during 2005-2015, this paper reports that the occurrence of electromagnetic cyclotron waves (ECWs) near the proton cyclotron frequency significantly depends on the direction of alpha-proton differential flow Vd. As Vd rotates from anti-Sunward direction to Sunward direction, the occurrence rate of ECWs as well as the percentage of left-handed (LH) polarized ECWs decreases considerably. In particular, it is shown that the dominant polarization changes from LH polarization to right-handed polarization during the rotation. The investigation on proton and alpha particle parameters ordered by the direction of Vd further illustrates that large kinetic energies of alpha-proton differential flow correspond to high occurrence rates of ECWs. These results are well consistent with theoretical predictions for effects of alpha-proton differential flow on proton temperature anisotropy instabilities.
The anisotropy of solar wind turbulence is a critical issue in understanding the physics of energy transfer between scales and energy conversion between fields and particles in the heliosphere. Using the measurement of emph{Parker Solar Probe} (emph{PSP}), we present an observation of the anisotropy at kinetic scales in the slow, Alfvenic, solar wind in the inner heliosphere. textbf{The magnetic compressibility behaves as expected for kinetic Alfvenic turbulence below the ion scale.} A steepened transition range is found between the inertial and kinetic ranges in all directions with respect to the local background magnetic field direction. The anisotropy of $k_perp gg k_parallel$ is found evident in both transition and kinetic ranges, with the power anisotropy $P_perp/P_parallel > 10$ in the kinetic range leading over that in the transition range and being stronger than that at 1 au. The spectral index varies from $alpha_{tparallel}=-5.7pm 1.0$ to $alpha_{tperp}=-3.7pm 0.3$ in the transition range and $alpha_{kparallel}=-3.12pm 0.22$ to $alpha_{kperp}=-2.57pm 0.09$ in the kinetic range. The corresponding wavevector anisotropy has the scaling of $k_parallel sim k_perp^{0.71pm 0.17}$ in the transition range, and changes to $k_parallel sim k_perp^{0.38pm 0.09}$ in the kinetic range, consistent with the kinetic Alfvenic turbulence at sub-ion scales.
Turbulence is a chaotic flow regime filled by irregular flows. The dissipation of turbulence is a fundamental problem in the realm of physics. Theoretically, dissipation cannot be ultimately achieved without collisions, and so how turbulent kinetic energy is dissipated in the nearly collisionless solar wind is a challenging problem. Wave particle interactions and magnetic reconnection are two possible dissipation mechanisms, but which mechanism dominates is still a controversial topic. Here we analyze the dissipation region scaling around a solar wind magnetic reconnection region. We find that the magnetic reconnection region shows a unique multifractal scaling in the dissipation range, while the ambient solar wind turbulence reveals a monofractal dissipation process for most of the time. These results provide the first observational evidences for the intermittent multifractal dissipation region scaling around a magnetic reconnection site, and they also have significant implications for the fundamental energy dissipation process.
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