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Register a new userSolar wind turbulent spectrum from MHD to electron scales
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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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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.
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.
Various remote sensing observations have been used so far to probe the turbulent properties of the solar wind. Using the recently reported density modulation indices that are derived using angular broadening observations of Crab Nebula during 1952 - 2013, we measured the solar wind proton heating using the kinetic $rm Alfvacute{e}n$ wave dispersion equation. The estimated heating rates vary from $approx 1.58 times 10^{-14}$ to $1.01 times 10^{-8} ~rm erg~ cm^{-3}~ s^{-1}$ in the heliocentric distance range 5 - 45 $rm R_{odot}$. Further, we found that heating rates vary with the solar cycle in correlation with density modulation indices. The models derived using in-situ measurements (for example, electron/proton density, temperature, and magnetic field) that the recently launched Parker Solar Probe observes (planned closest perihelia $rm 9.86~ R_{odot}$ from the center of the Sun) are useful in the estimation of the turbulent heating rate precisely. Further, we compared our heating rate estimates with the one derived using previously reported remote sensing and in-situ observations.
The solar wind is highly structured in fast and slow flows. These two dynamical regimes remarkably differ not only for the average values of magnetic field and plasma parameters but also for the type of fluctuations they transport. Fast wind is characterized by large amplitude, incompressible fluctuations, mainly Alfv{e}nic, slow wind is generally populated by smaller amplitude and less Alfv{e}nic fluctuations, mainly compressive. The typical corotating fast stream is characterized by a stream interface, a fast wind region and a slower rarefaction region formed by the trailing expansion edge of the stream. Moving {between these two regions}, from faster to slower wind, we observe the following behavior: a) the power level of magnetic fluctuations within the inertial range largely decreases, keeping the typical Kolmogorov scaling; b) at proton scales, for about one decade right beyond the high frequency break, the spectral index becomes flatter and flatter towards a value around -2.7; c) at higher frequencies, before the electron scales, the spectral index remains around -2.7 and, {based on suitable observations available for $4$ corotating streams}, the power level does not change, irrespective of the flow speed. All these spectral features, characteristic of high speed streams, suggest the existence of a sort of magnetic field background spectrum. This spectrum would be common to both faster and slower wind but, any time the observer would cross the inner part of a fluxtube channeling the faster wind into the interplanetary space, a turbulent and large amplitude Alfv{e}nic spectrum would be superposed to it.
Motivated by prior remote observations of a transition from striated solar coronal structures to more isotropic ``flocculated fluctuations, we propose that the dynamics of the inner solar wind just outside the Alfven critical zone, and in the vicinity of the first $beta=1$ surface, is powered by the relative velocities of adjacent coronal magnetic flux tubes. We suggest that large amplitude flow contrasts are magnetically constrained at lower altitude but shear-driven dynamics are triggered as such constraints are released above the Alfven critical zone, as suggested by global magnetohydrodynamic (MHD) simulations that include self-consistent turbulence transport. We argue that this dynamical evolution accounts for features observed by {it Parker Solar Probe} ({it PSP}) near initial perihelia, including magnetic ``switchbacks, and large transverse velocities that are partially corotational and saturate near the local Alfven speed. Large-scale magnetic increments are more longitudinal than latitudinal, a state unlikely to originate in or below the lower corona. We attribute this to preferentially longitudinal velocity shear from varying degrees of corotation. Supporting evidence includes comparison with a high Mach number three-dimensional compressible MHD simulation of nonlinear shear-driven turbulence, reproducing several observed diagnostics, including characteristic distributions of fluctuations that are qualitatively similar to {it PSP} observations near the first perihelion. The concurrence of evidence from remote sensing observations, {it in situ} measurements, and both global and local simulations supports the idea that the dynamics just above the Alfven critical zone boost low-frequency plasma turbulence to the level routinely observed throughout the explored solar system.
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