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Register a new userNonlinear and Linear Timescales near Kinetic Scales in Solar Wind 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.
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The nature of the plasma wave modes around the ion kinetic scales in highly Alfvenic slow solar wind turbulence is investigated using data from the NASAs Parker Solar Probe taken in the inner heliosphere, at 0.18 Astronomical Unit (AU) from the sun. The joint distribution of the normalized reduced magnetic helicity ${sigma}_m ({theta}_{RB}, {tau})$ is obtained, where ${theta}_{RB}$ is the angle between the local mean magnetic field and the radial direction and ${tau}$ is the temporal scale. Two populations around ion scales are identified: the first population has ${sigma}_m ({theta}_{RB}, {tau}) < 0$ for frequencies (in the spacecraft frame) ranging from 2.1 to 26 Hz for $60^{circ} < {theta}_{RB} < 130^{circ}$, corresponding to kinetic Alfven waves (KAWs), and the second population has ${sigma}_m ({theta}_{RB}, {tau}) > 0$ in the frequency range [1.4, 4.9] Hz for ${theta}_{RB} > 150^{circ}$, corresponding to Alfven ion Cyclotron Waves (ACWs). This demonstrates for the first time the co-existence of KAWs and ACWs in the slow solar wind in the inner heliosphere, which contrasts with previous observations in the slow solar wind at 1 AU. This discrepancy between 0.18 and 1 AU could be explained, either by i) a dissipation of ACWs via cyclotron resonance during their outward journey, or by ii) the high Alfvenicity of the slow solar wind at 0.18 AU that may be favorable for the excitation of ACWs.
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.
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.
We use fluctuating magnetic helicity to investigate the polarisation properties of Alfvenic fluctuations at ion-kinetic scales in the solar wind as a function of $beta_p$, the ratio of proton thermal pressure to magnetic pressure, and $theta_{vB}$, the angle between the proton flow and local mean magnetic field, $mathbf{B}_0$. Using almost 15 years of textit{Wind} observations, we separate the contributions to helicity from fluctuations with wave-vectors, $textbf{k}$, quasi-parallel and oblique to $mathbf{B}_0$, finding that the helicity of Alfvenic fluctuations is consistent with predictions from linear Vlasov theory. This result suggests that the non-linear turbulent fluctuations at these scales share at least some polarisation properties with Alfven waves. We also investigate the dependence of proton temperature in the $beta_p$-$theta_{vB}$ plane to probe for possible signatures of turbulent dissipation, finding that it correlates with $theta_{vB}$. The proton temperature parallel to $mathbf{B}_0$ is higher in the parameter space where we measure the helicity of right-handed Alfvenic fluctuations, and the temperature perpendicular to $mathbf{B}_0$ is higher where we measure left-handed fluctuations. This finding is inconsistent with the general assumption that by sampling different $theta_{vB}$ in the solar wind we can analyse the dependence of the turbulence distribution on $theta_{kB}$, the angle between $textbf{k}$ and $mathbf{B}_0$. After ruling out both instrumental and expansion effects, we conclude that our results provide new evidence for the importance of local kinetic processes that depend on $theta_{vB}$ in determining proton temperature in the solar wind.
Turbulent spectra of magnetic fluctuations in the free solar wind are studied from MHD to electron scales using Cluster observations. We discuss the problem of the instrumental noise and its influence on the measurements at the electron scales. We confirm the presence of a curvature of the spectrum $sim exp{sqrt{krho_e}}$ over the broad frequency range $sim[10,100]$ Hz, indicating the presence of a dissipation. Analysis of seven spectra under different plasma conditions show clearly the presence of a quasi-universal power-law spectrum at MHD and ion scales. However, the transition from the inertial range $sim k^{-1.7}$ to the spectrum at ion scales $sim k^{-2.7}$ is not universal. Finally, we discuss the role of different kinetic plasma scales on the spectral shape, considering normalized dimensionless spectra.
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