Wind Turbulence

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Linghua Wang - One of the best experts on this subject based on the ideXlab platform.

  • Dependence of 3D Self-correlation Level Contours on the Scales in the Inertial Range of Solar Wind Turbulence
    The Astrophysical Journal, 2019
    Co-Authors: Xin Wang, Linghua Wang
    Abstract:

    The self-correlation level contours at the 1010 cm scale reveal a 3D isotropic feature in the slow solar Wind and a quasi-anisotropic feature in the fast solar Wind. However, the 1010 cm scale is approximately near the lowfrequency break (outer scale of Turbulence cascade), especially in the fast Wind. How the self-correlation level contours behave with dependence on the scales in the inertial range of solar Wind Turbulence remains unknown. Here we present the 3D self-correlation function level contours and their dependence on the scales in the inertial range for the first time. We use data at 1 au from instruments on the Wind spacecraft in the period 2005-2018. We show the 3D isotropic self-correlation level contours of the magnetic field in the inertial range of both slow and fast solar Wind Turbulence. We also find that the self-correlation level contours of the velocity in the inertial range present 2D anisotropy with an elongation in the perpendicular direction and 2D isotropy in the plane perpendicular to the mean magnetic field. These results indicate differences between the magnetic field and the velocity, providing new clues to interpret the solar Wind Turbulence on the inertial scale.

  • On the Full-range β Dependence of Ion-scale Spectral Break in the Solar Wind Turbulence
    The Astrophysical Journal, 2018
    Co-Authors: Xin Wang, Linghua Wang
    Abstract:

    The power spectrum of magnetic fluctuations has a break at the high-frequency end of the inertial range. Beyond this break, the spectrum becomes steeper than the Kolmogorov law f −5/3. The break frequency was found to be associated with plasma beta (β). However, the full-range β dependence of the ion-scale spectral break has not been presented before in observational studies. Here we show the continuous variation of the break frequency on full-range β in the solar Wind Turbulence. By using measurements from the Wind and Ulysses spacecraft, we show the break frequency (f b ) normalized, respectively, by the frequencies corresponding to ion inertial length (f di ), ion gyroradius (), and cyclotron resonance scale (f ri ) as a function of β for 1306 intervals. Their β values spread from 0.005 to 20, which nearly covers the full β range of the observed solar Wind Turbulence. It is found that () generally decreases (increases) with β, while is nearly a constant. We perform a linear fit on the statistical result, and obtain the empirical formulas , , and to describe the relation between f b and β. We also compare our observations with a numerical simulation and the prediction by ion cyclotron resonance theory. Our result favors the idea that the cyclotron resonance is an important mechanism for energy dissipation at the spectral break. When β 1 and β 1, the break at f di and may also be associated with other processes.

  • Two cases of convecting structure in the slow solar Wind Turbulence
    2016
    Co-Authors: Xin Wang, Eckart Marsch, Linghua Wang
    Abstract:

    The slow solar Wind Turbulence has recently been considered as fully evolved Turbulence which can be described by critical balance theory. However, here we present two cases of convecting structure that support a different understanding. By using the measurements from Wind spacecraft in the slow solar Wind, we find that Elsasser variables Z± of these two cases do not represent inward and outward Alfven waves, but are determined mainly by the magnetic variations, including tangentially varying structures. We then propose that the slow Wind Turbulence may be composed of convecting magnetic-field tangential and directional turnings, as well as current sheets, which may be considered as left-over fossils from Kolmogorov fluid Turbulence. The fluid kinetic energy has been damped out, and the remaining magnetic fluctuations thus tend to become force-free structures.

  • THE INFLUENCE OF INTERMITTENCY ON THE SPECTRAL ANISOTROPY OF SOLAR Wind Turbulence
    The Astrophysical Journal, 2014
    Co-Authors: Xin Wang, Eckart Marsch, Linghua Wang
    Abstract:

    The relation between the intermittency and the anisotropy of the power spectrum in the solar Wind Turbulence is studied by applying the wavelet technique to the magnetic field and flow velocity data measured by the Wind spacecraft. It is found that when the intermittency is removed from the Turbulence, the spectral indices of the power spectra of the field and velocity turn out to be independent of the angle θRB between the direction of the local scale-dependent background magnetic field and the heliocentric direction. The spectral index becomes –1.63 ± 0.02 for magnetic field fluctuations and –1.56 ± 0.02 for velocity fluctuations. These results may suggest that the recently found spectral anisotropy of solar Wind power spectra in the inertial range could result from Turbulence intermittency. As a consequence, a new concept is here proposed of an intermittency-associated sub-range of the inertial domain adjacent to the dissipation range. Since spectral anisotropy was previously explained as evidence for the presence of a "critical balance" type turbulent cascade, and also for the existence of kinetic Alfven waves, this new finding may stimulate fresh thoughts on how to analyze and interpret solar Wind Turbulence and the associated heating.

Xin Wang - One of the best experts on this subject based on the ideXlab platform.

  • Nature of Elsässer Variables in the slow solar Wind Turbulence
    2020
    Co-Authors: Xin Wang
    Abstract:

    <p>Elsässer Variables z± are widely considered as outward and inward propagating Alfvén waves in the solar Wind Turbulence study. It is believed that they can interact nonlinearly with each other to generate energy cascade. However, z− variations sometimes show a feature of convective structures or a combination of white noise and pseudo-structures. Here we present the amplitude of z± in σc (normalized cross helicity) - σr (normalized residual energy) plane in order to get some information on the nature of z±. Measurements from the Wind spacecraft in the slow solar Wind during 2007-2009 are used for analysis. In each interval with length of 20 min, we calculate σc, σr, and consider the variance of z± as the amplitude of them for the given interval. We find that in the σc-σr plane, the level contours of the average z- amplitude present a feature of nearly horizontal stratification, which means that the amplitude of z- is independent of the value of σc, and is just related to σr. The horizontal-stratification feature suggests that z- could be convective structures. While the level contours of the average amplitude of z+ are approximately concentric semicircles, and the circle with larger radius corresponds to larger z+ amplitude. It indicates that z+ represents Alfvén waves. The nature of z± in the slow Wind here will help us to understand more about the cascade process in the solar Wind Turbulence.</p>

  • Dependence of 3D Self-correlation Level Contours on the Scales in the Inertial Range of Solar Wind Turbulence
    The Astrophysical Journal, 2019
    Co-Authors: Xin Wang, Linghua Wang
    Abstract:

    The self-correlation level contours at the 1010 cm scale reveal a 3D isotropic feature in the slow solar Wind and a quasi-anisotropic feature in the fast solar Wind. However, the 1010 cm scale is approximately near the lowfrequency break (outer scale of Turbulence cascade), especially in the fast Wind. How the self-correlation level contours behave with dependence on the scales in the inertial range of solar Wind Turbulence remains unknown. Here we present the 3D self-correlation function level contours and their dependence on the scales in the inertial range for the first time. We use data at 1 au from instruments on the Wind spacecraft in the period 2005-2018. We show the 3D isotropic self-correlation level contours of the magnetic field in the inertial range of both slow and fast solar Wind Turbulence. We also find that the self-correlation level contours of the velocity in the inertial range present 2D anisotropy with an elongation in the perpendicular direction and 2D isotropy in the plane perpendicular to the mean magnetic field. These results indicate differences between the magnetic field and the velocity, providing new clues to interpret the solar Wind Turbulence on the inertial scale.

  • On the Full-range β Dependence of Ion-scale Spectral Break in the Solar Wind Turbulence
    The Astrophysical Journal, 2018
    Co-Authors: Xin Wang, Linghua Wang
    Abstract:

    The power spectrum of magnetic fluctuations has a break at the high-frequency end of the inertial range. Beyond this break, the spectrum becomes steeper than the Kolmogorov law f −5/3. The break frequency was found to be associated with plasma beta (β). However, the full-range β dependence of the ion-scale spectral break has not been presented before in observational studies. Here we show the continuous variation of the break frequency on full-range β in the solar Wind Turbulence. By using measurements from the Wind and Ulysses spacecraft, we show the break frequency (f b ) normalized, respectively, by the frequencies corresponding to ion inertial length (f di ), ion gyroradius (), and cyclotron resonance scale (f ri ) as a function of β for 1306 intervals. Their β values spread from 0.005 to 20, which nearly covers the full β range of the observed solar Wind Turbulence. It is found that () generally decreases (increases) with β, while is nearly a constant. We perform a linear fit on the statistical result, and obtain the empirical formulas , , and to describe the relation between f b and β. We also compare our observations with a numerical simulation and the prediction by ion cyclotron resonance theory. Our result favors the idea that the cyclotron resonance is an important mechanism for energy dissipation at the spectral break. When β 1 and β 1, the break at f di and may also be associated with other processes.

  • Two cases of convecting structure in the slow solar Wind Turbulence
    2016
    Co-Authors: Xin Wang, Eckart Marsch, Linghua Wang
    Abstract:

    The slow solar Wind Turbulence has recently been considered as fully evolved Turbulence which can be described by critical balance theory. However, here we present two cases of convecting structure that support a different understanding. By using the measurements from Wind spacecraft in the slow solar Wind, we find that Elsasser variables Z± of these two cases do not represent inward and outward Alfven waves, but are determined mainly by the magnetic variations, including tangentially varying structures. We then propose that the slow Wind Turbulence may be composed of convecting magnetic-field tangential and directional turnings, as well as current sheets, which may be considered as left-over fossils from Kolmogorov fluid Turbulence. The fluid kinetic energy has been damped out, and the remaining magnetic fluctuations thus tend to become force-free structures.

  • THE INFLUENCE OF INTERMITTENCY ON THE SPECTRAL ANISOTROPY OF SOLAR Wind Turbulence
    The Astrophysical Journal, 2014
    Co-Authors: Xin Wang, Eckart Marsch, Linghua Wang
    Abstract:

    The relation between the intermittency and the anisotropy of the power spectrum in the solar Wind Turbulence is studied by applying the wavelet technique to the magnetic field and flow velocity data measured by the Wind spacecraft. It is found that when the intermittency is removed from the Turbulence, the spectral indices of the power spectra of the field and velocity turn out to be independent of the angle θRB between the direction of the local scale-dependent background magnetic field and the heliocentric direction. The spectral index becomes –1.63 ± 0.02 for magnetic field fluctuations and –1.56 ± 0.02 for velocity fluctuations. These results may suggest that the recently found spectral anisotropy of solar Wind power spectra in the inertial range could result from Turbulence intermittency. As a consequence, a new concept is here proposed of an intermittency-associated sub-range of the inertial domain adjacent to the dissipation range. Since spectral anisotropy was previously explained as evidence for the presence of a "critical balance" type turbulent cascade, and also for the existence of kinetic Alfven waves, this new finding may stimulate fresh thoughts on how to analyze and interpret solar Wind Turbulence and the associated heating.

Stuart D. Bale - One of the best experts on this subject based on the ideXlab platform.

  • Solar Wind Turbulence and the Role of Ion Instabilities
    Space Science Reviews, 2013
    Co-Authors: Olga Alexandrova, Christopher H. K. Chen, Luca Sorriso-valvo, Timothy S. Horbury, Stuart D. Bale
    Abstract:

    Solar Wind is probably the best laboratory to study Turbulence in astrophysical plasmas. In addition to the presence of magnetic field, the differences with neutral fluid isotropic Turbulence are: (i) weakness of collisional dissipation and (ii) presence of several characteristic space and time scales. In this paper we discuss observational properties of solar Wind Turbulence in a large range from the MHD to the electron scales. At MHD scales, within the inertial range, Turbulence cascade of magnetic fluctuations develops mostly in the plane perpendicular to the mean field, with the Kolmogorov scaling $k_{\perp}^{-5/3}$ for the perpendicular cascade and $k_{\|}^{-2}$ for the parallel one. Solar Wind Turbulence is compressible in nature: density fluctuations at MHD scales have the Kolmogorov spectrum. Velocity fluctuations do not follow magnetic field ones: their spectrum is a power-law with a −3/2 spectral index. Probability distribution functions of different plasma parameters are not Gaussian, indicating presence of intermittency. At the moment there is no global model taking into account all these observed properties of the inertial range. At ion scales, turbulent spectra have a break, compressibility increases and the density fluctuation spectrum has a local flattening. Around ion scales, magnetic spectra are variable and ion instabilities occur as a function of the local plasma parameters. Between ion and electron scales, a small scale turbulent cascade seems to be established. It is characterized by a well defined power-law spectrum in magnetic and density fluctuations with a spectral index close to −2.8. Approaching electron scales, the fluctuations are no more self-similar: an exponential cut-off is usually observed (for time intervals without quasi-parallel whistlers) indicating an onset of dissipation. The small scale inertial range between ion and electron scales and the electron dissipation range can be together described by $\sim k_{\perp}^{-\alpha}\exp(-k_{\perp}\ell_{d})$ , with α ≃8/3 and the dissipation scale ℓ _ d close to the electron Larmor radius ℓ _ d ≃ ρ _ e . The nature of this small scale cascade and a possible dissipation mechanism are still under debate.

  • Solar Wind Turbulence and the Role of Ion Instabilities
    Space Science Reviews, 2013
    Co-Authors: Olga Alexandrova, Christopher H. K. Chen, Luca Sorriso-valvo, Timothy S. Horbury, Stuart D. Bale
    Abstract:

    Solar Wind is probably the best laboratory to study Turbulence in astrophysical plasmas. In addition to the presence of magnetic field, the differences with neutral fluid isotropic Turbulence are: weakness of collisional dissipation and presence of several characteristic space and time scales. In this paper we discuss observational properties of solar Wind Turbulence in a large range from the MHD to the electron scales. At MHD scales, within the inertial range, Turbulence cascade of magnetic fluctuations develops mostly in the plane perpendicular to the mean field. Solar Wind Turbulence is compressible in nature. The spectrum of velocity fluctuations do not follow magnetic field one. Probability distribution functions of different plasma parameters are not Gaussian, indicating presence of intermittency. At the moment there is no global model taking into account all these observed properties of the inertial range. At ion scales, turbulent spectra have a break, compressibility increases and the density fluctuation spectrum has a local flattening. Around ion scales, magnetic spectra are variable and ion instabilities occur as a function of the local plasma parameters. Between ion and electron scales, a small scale turbulent cascade seems to be established. Approaching electron scales, the fluctuations are no more self-similar: an exponential cut-off is usually observed indicating an onset of dissipation. The nature of the small scale cascade and a possible dissipation mechanism are still under debate.

  • Three-Dimensional Structure of Solar Wind Turbulence
    The Astrophysical Journal, 2012
    Co-Authors: Christopher H. K. Chen, Timothy S. Horbury, Alfred Mallet, Alexander Schekochihin, Robert T. Wicks, Stuart D. Bale
    Abstract:

    We present a measurement of the scale-dependent, three-dimensional structure of the magnetic field fluctuations in inertial range solar Wind Turbulence with respect to a local, physically motivated coordinate system. The Alfvenic fluctuations are three-dimensionally anisotropic, with the sense of this anisotropy varying from large to small scales. At the outer scale, the magnetic field correlations are longest in the local fluctuation direction, consistent with Alfven waves. At the proton gyroscale, they are longest along the local mean field direction and shortest in the direction perpendicular to the local mean field and the local field fluctuation. The compressive fluctuations are highly elongated along the local mean field direction, although axially symmetric perpendicular to it. Their large anisotropy may explain why they are not heavily damped in the solar Wind.

  • density fluctuation spectrum of solar Wind Turbulence between ion and electron scales
    Physical Review Letters, 2012
    Co-Authors: Christopher H. K. Chen, C Salem, J W Bonnell, F S Mozer, Stuart D. Bale
    Abstract:

    We present a measurement of the spectral index of density fluctuations between ion and electron scales in solar Wind Turbulence using the EFI instrument on the ARTEMIS spacecraft. The mean spectral index at 1 AU was found to be -2.75 +/- 0.06, steeper than predictions for pure whistler or kinetic Alfven wave Turbulence, but consistent with previous magnetic field measurements. The steep spectra are also consistent with expectations of increased intermittency or damping of some of the turbulent energy over this range of scales. Neither the spectral index nor the flattening of the density spectra before ion scales were found to depend on the proximity to the pressure anisotropy instability thresholds, suggesting that they are features inherent to the turbulent cascade.

  • density fluctuation spectrum of solar Wind Turbulence between ion and electron scales
    Physical Review Letters, 2012
    Co-Authors: Christopher H. K. Chen, J W Bonnell, F S Mozer, C S Salem, Stuart D. Bale
    Abstract:

    We present a measurement of the spectral index of density fluctuations between ion and electron scales in solar Wind Turbulence using the EFI instrument on the ARTEMIS spacecraft. The mean spectral index at 1 AU was found to be $\ensuremath{-}2.75\ifmmode\pm\else\textpm\fi{}0.06$, steeper than predictions for pure whistler or kinetic Alfv\'en wave Turbulence but consistent with previous magnetic field measurements. The steep spectra are also consistent with expectations of increased intermittency or damping of some of the turbulent energy over this range of scales. Neither the spectral index nor the flattening of the density spectra before ion scales were found to depend on the proximity to the pressure anisotropy instability thresholds, suggesting that they are features inherent to the turbulent cascade.

Sébastien Galtier - One of the best experts on this subject based on the ideXlab platform.

  • energy cascade rate in compressible fast and slow solar Wind Turbulence
    The Astrophysical Journal, 2017
    Co-Authors: Lina Hadid, Fouad Sahraoui, Sébastien Galtier
    Abstract:

    Estimation of the energy cascade rate in the inertial range of solar Wind Turbulence has been done so far mostly within the incompressible magnetohydrodynamics (MHD) theory. Here, we go beyond that approximation to include plasma compressibility using a reduced form of a recently derived exact law for compressible, isothermal MHD Turbulence. Using in-situ data from the THEMIS/ARTEMIS spacecraft in the fast and slow solar Wind, we investigate in detail the role of the compressible fluctuations in modifying the energy cascade rate with respect to the prediction of the incompressible MHD model. In particular, we found that the energy cascade rate: i) is amplified particularly in the slow solar Wind; ii) exhibits weaker fluctuations in spatial scales, which leads to a broader inertial range than the previous reported ones; iii) has a power law scaling with the turbulent Mach number; iv) has a lower level of spatial anisotropy. Other features of solar Wind Turbulence are discussed along with their comparison with previous studies that used incompressible or heuristic (non exact) compressible MHD models.

  • Energy Cascade Rate in Compressible Fast and Slow Solar Wind Turbulence
    The Astrophysical Journal, 2017
    Co-Authors: Lina Hadid, Fouad Sahraoui, Sébastien Galtier
    Abstract:

    Estimation of the energy cascade rate in the inertial range of solar Wind Turbulence has been done so far mostly within incompressible magnetohydrodynamics (MHD) theory. Here, we go beyond that approximation to include plasma compressibility using a reduced form of a recently derived exact law for compressible, isothermal MHD Turbulence. Using in situ data from the THEMIS/ARTEMIS spacecraft in the fast and slow solar Wind, we investigate in detail the role of the compressible fluctuations in modifying the energy cascade rate with respect to the prediction of the incompressible MHD model. In particular, we found that the energy cascade rate (1) is amplified particularly in the slow solar Wind; (2) exhibits weaker fluctuations in spatial scales, which leads to a broader inertial range than the previous reported ones; (3) has a power-law scaling with the turbulent Mach number; (4) has a lower level of spatial anisotropy. Other features of solar Wind Turbulence are discussed along with their comparison with previous studies that used incompressible or heuristic (nonexact) compressible MHD models.

Smalikho - One of the best experts on this subject based on the ideXlab platform.

  • Lidar Estimates of the Anisotropy of Wind Turbulence in a Stable Atmospheric Boundary Layer
    Remote Sensing, 2019
    Co-Authors: Banakh, Smalikho
    Abstract:

    In this paper, a method is proposed to estimate Wind Turbulence parameters using measurements recorded by a conically scanning coherent Doppler lidar with two different elevation angles. This methodology helps determine the anisotropy of the spatial correlation of Wind velocity turbulent fluctuations. The proposed method was tested in a field experiment with a Stream Line lidar (Halo Photonics, Brockamin, Worcester, United Kingdom) under stable temperature stratification conditions in the atmospheric boundary layer. The results show that the studied anisotropy coefficient in a stable boundary layer may be up to three or larger.

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