Browsing by Author "Sahraoui, F."
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- ItemDetection of Small-Scale Structures in the Dissipation Regime of Solar-Wind Turbulence(APS, 2012-11-08) Perri, S.; Goldstein, Melvyn; Dorelli, J. C.; Sahraoui, F.Recent observations of the solar wind have pointed out the existence of a cascade of magnetic energy from the scale of the proton Larmor radius ρₚ down to the electron Larmor radius ρₑ scale. In this Letter we study the spatial properties of magnetic field fluctuations in the solar wind and find that at small scales the magnetic field does not resemble a sea of homogeneous fluctuations, but rather a two-dimensional plane containing thin current sheets and discontinuities with spatial sizes ranging from l ≳ ρₚ down to ρₑ and below. These isolated structures may be manifestations of intermittency that localize sites of turbulent dissipation. Studying the relationship between turbulent dissipation, reconnection, and intermittency is crucial for understanding the dynamics of laboratory and astrophysical plasmas.
- ItemEvidence of a Cascade and Dissipation of Solar-Wind Turbulence at the Electron Gyroscale(APS, 2009-06-10) Sahraoui, F.; Goldstein, Melvyn; Robert, P.; Khotyaintsev, Yu. V.We report the first direct determination of the dissipation range of magnetofluid turbulence in the solar wind at the electron scales. Combining high resolution magnetic and electric field data of the Cluster spacecraft, we computed the spectrum of turbulence and found two distinct breakpoints in the magnetic spectrum at 0.4 and 35 Hz, which correspond, respectively, to the Doppler-shifted proton and electron gyroscales, fpp and fpe . Below fpp , the spectrum follows a Kolmogorov scaling f⁻¹.⁶², typical of spectra observed at 1 AU. Above fpp , a second inertial range is formed with a scaling f⁻².³ down to fpe . Above fpe , the spectrum has a steeper power law ∼f⁻⁴.¹ down to the noise level of the instrument. We interpret this as the dissipation range and show a remarkable agreement with theoretical predictions of a quasitwo-dimensional cascade into Kinetic Alfven Waves (KAW).
- ItemThe evolution of compressible solar wind turbulence in the inner heliosphere: PSP, THEMIS and MAVEN observationsAndrés, N.; Sahraoui, F.; Hadid, L. Z.; Huang, S. Y.; Romanelli, Norberto; Galtier, S.; DiBraccio, G.; Halekas, J.The first computation of the compressible energy transfer rate from ∼ 0.2 AU up to ∼ 1.7 AU is obtained using PSP, THEMIS and MAVEN observations. The compressible energy cascade rate εC is computed for hundred of events at different heliocentric distances, for time intervals when the spacecraft were in the pristine solar wind. The observational results show moderate increases of εC with respect to the incompressible cascade rate εI. Depending on the level of compressibility in the plasma, which reach up to 25 % in the PSP perihelion, the different terms in the compressible exact relation are shown to have different impact in the total cascade rate εC. Finally, the observational results are connected with the local ion temperature and the solar wind heating problem.
- ItemKelvin-Helmholtz waves under southward interplanetary magnetic field(AGU, 2011-08-05) Hwang, Kyoung-Joo; Kuznetsova, M. M.; Sahraoui, F.; Goldstein, Melvyn; Lee, E.; Parks, G. K.The Kelvin‐Helmholtz waves have been observed along the Earth’s low‐latitude magnetopause and have been suggested to play a certain role in the entry of solar wind plasma into Earth’s magnetosphere. In situ observations of the KH waves (KHW)and, in particular, a nonlinear stage of the KH instability, i.e., rolled‐up KH vortices(KHVs), have been reported to occur preferentially for northward interplanetary magnetic field (IMF). Using Cluster data, we present the first in situ observation of nonlinearly developed KHW during southward IMF. The analysis reveals that there is a mixture of less‐developed and more‐developed KHW that shows inconsistent variations in scale size and the magnetic perturbations in the context of the expected evolution of KH structures. A coherence analysis implies that the observed KHW under southward IMF appear to be irregular and intermittent. These irregular and turbulent characteristics are more noticeable than previously reported KHW events that have been detected preferentially during northward IMF. This suggests that under southward IMF KHVs become easily irregular and temporally intermittent, which might explain the preferential in situ detection of KHVs when the IMF is northward. MHD simulation of the present event shows that during southward IMF dynamically active subsolar environments can cause KHV that evolve with considerable intermittency. The MHD simulations appear to reproduce well the qualitative features of the Cluster observations.
- ItemLimitations of multispacecraft data techniques in measuring wave number spectra of space plasma turbulence(AGU, 2010-04-10) Sahraoui, F.; Belmont, G.; Goldstein, Melvyn; Rezeau, L.Unambiguous determination of spatial properties of space plasma turbulence from temporal measurements has been one of the major goals of the Cluster mission. For that purpose, techniques, such as the k filtering, have been developed. Such multipoint measurement techniques combine several time series recorded simultaneously at different points in space to estimate the corresponding energy density in wave number space. Here we present results of such an analysis, including a detailed discussion of the errors and limitations that arise due to the separation of the spacecraft and the quality of the tetrahedral configuration. Specifically, we answer the following questions: (1) What are the minimum and maximum scales that can be accurately measured given a specific distance between the satellites? (2) How important is the geometry of the tetrahedron, and what is the relationship of that geometry to spatial aliasing? (3) How should one perform a proper integration of the angular frequencies to infer wave number spectra, and what role does the Doppler shift play when the magnetofluid is rapidly convecting past the spacecraft? We illustrate the results with analyses with both simulated and Cluster magnetometer data recorded in the solar wind. We also discuss the potential impact on future multispacecraft missions, such as Magnetospheric MultiScale and Cross-Scale.
- ItemMagnetic energy distribution in the four-dimensional frequency and wave vector domain in the solar wind(AGU, 2010-04-01) Narita, Y.; Sahraoui, F.; Goldstein,Melvyn; Glassmeier, K.-H.We present a measurement of the energy distribution in the four-dimensional (4-D) frequency and wave vector domain of magnetic field fluctuations in the solar wind. The measurement makes use of the wave telescope technique that has been developed particularly for multispacecraft data analysis. We review briefly the theoretical background and then present a numerical test using synthetic data; the technique is then applied to magnetic field data obtained while the Cluster spacecraft was in the solar wind. The energy distribution is determined in the flow rest frame in the frequency range below 0.2 rad/s and the wave number range below 0.0015 rad/km, corrected for the Doppler shift. We find the following properties in the energy distribution in the rest frame: (1) a double anisotropy in the wave vector domain associated with the mean magnetic field and the flow directions, (2) a symmetric distribution with respect to the sign of wave vector, and (3) no evidence for a linear dispersion relation in the frequency and wave number domain. Since the flow direction in the analyzed time interval is close to the normal direction to the bow shock, the anisotropy may well be associated with the bow shock. These results suggest that the solar wind is in a state of well-developed strong turbulence and justifies the theoretical picture of quasi-two-dimensional turbulence that obtains in the presence of a (relatively) strong DC magnetic field. However, the fluctuations are not axisymmetric around the mean field and the energy distribution is extended in the perpendicular direction to the flow or shock normal. Anisotropy associated with the boundary is reminiscent of previously reported magnetosheath turbulence. This study opens a way to investigate solar wind turbulence in the full 4-D frequency and wave vector space.
- ItemNew Insight into Short-Wavelength Solar Wind Fluctuations from Vlasov Theory(IOP, 2012-03-13) Sahraoui, F.; Belmont, G.; Goldstein, MelvynThe nature of solar wind (SW) turbulence below the proton gyroscale is a topic that is being investigated extensively nowadays, both theoretically and observationally. Although recent observations gave evidence of the dominance of kinetic Alfvén waves (KAWs) at sub-ion scales with ω < ωcᵢ, other studies suggest that the KAW mode cannot carry the turbulence cascade down to electron scales and that the whistler mode (i.e., ω > ωcᵢ) is more relevant. Here, we study key properties of the short-wavelength plasma modes under limited, but realistic, SW conditions, typically βᵢ ≳ βₑ ∼ 1 and for high oblique angles of propagation 80° ⩽ ΘkB < 90° as observed from the Cluster spacecraft data. The linear properties of the plasma modes under these conditions are poorly known, which contrasts with the well-documented cold plasma limit and/or moderate oblique angles of propagation (ΘkB < 80°). Based on linear solutions of the Vlasov kinetic theory, we discuss the relevance of each plasma mode (fast, Bernstein, KAW, whistler) in carrying the energy cascade down to electron scales. We show, in particular, that the shear Alfvén mode (known in the magnetohydrodynamic limit) extends at scales kρᵢ ≳ 1 to frequencies either larger or smaller than ωcᵢ, depending on the anisotropy k∥/k⊥. This extension into small scales is more readily called whistler (ω > ωcᵢ) or KAW (ω < ωcᵢ), although the mode is essentially the same. This contrasts with the well-accepted idea that the whistler branch always develops as a continuation at high frequencies of the fast magnetosonic mode. We show, furthermore, that the whistler branch is more damped than the KAW one, which makes the latter the more relevant candidate to carry the energy cascade down to electron scales. We discuss how these new findings may facilitate resolution of the controversy concerning the nature of the small-scale turbulence, and we discuss the implications for present and future spacecraft wave measurements in the SW.
- ItemScaling of the Electron Dissipation Range of Solar Wind Turbulence(IOP, 2013-10-09) Sahraoui, F.; Huang, S. Y.; Belmont, G.; Goldstein, Melvyn; Rétino, A.; Robert, P.; De Patoul, J.Electron scale solar wind (SW) turbulence has attracted great interest in recent years. Considerable evidence exists that the turbulence is not fully dissipated near the proton scale, but continues cascading down to electron scales. However, the scaling of the magnetic energy spectra as well as the nature of the plasma modes involved at those small scales are still not fully determined. Here we survey 10 yr of the Cluster STAFF search-coil magnetometer waveforms measured in the SW and perform a statistical study of the magnetic energy spectra in the frequency range [1, 180] Hz. We found that 75% of the analyzed spectra exhibit breakpoints near the electron gyroscale ρe, followed by steeper power-law-like spectra. We show that the scaling below the electron breakpoint cannot be determined unambiguously due to instrumental limitations that we discuss in detail. We compare our results to those reported in other studies and discuss their implications for the physical mechanisms involved and for theoretical modeling of energy dissipation in the SW.
- ItemThree Dimensional Anisotropic k Spectra of Turbulence at Subproton Scales in the Solar Wind(APS, 2010-09-23) Sahraoui, F.; Goldstein, Melvyn; Belmont, G.; Canu, P.; Rezeau, L.We show the first three dimensional (3D) dispersion relations and k spectra of magnetic turbulence in the solar wind at subproton scales. We used the Cluster data with short separations and applied the k-filtering technique to the frequency range where the transition to subproton scales occurs. We show that the cascade is carried by highly oblique kinetic Alfvén waves with ωₚₗₐₛ≤ 0.1 ωcᵢ down to k⊥ρᵢ∼ 2. Each k spectrum in the direction perpendicular to B₀ shows two scaling ranges separated by a breakpoint (in the interval [0.4,1]k⊥ρᵢ): a Kolmogorov scaling k⊥⁻¹.⁷ followed by a steeper scaling ∼k⊥⁻⁴.⁵. We conjecture that the turbulence undergoes a transition range, where part of the energy is dissipated into proton heating via Landau damping and the remaining energy cascades down to electron scales where electron Landau damping may predominate.
- ItemThree-dimensional spatial structures of solar wind turbulence from 10 000-km to 100-km scales(EGU, 2011-10-05) Narita, Y.; Glassmeier, K.-H.; Goldstein, Melvyn; Motschmann, U.; Sahraoui, F.Using the four Cluster spacecraft, we have determined the three-dimensional wave-vector spectra of fluctuating magnetic fields in the solar wind. Three different solar wind intervals of Cluster data are investigated for this purpose, representing three different spatial scales: 10 000 km, 1000 km, and 100 km. The spectra are determined using the wave telescope technique (k-filtering technique) without assuming the validity of Taylor's frozen-in-flow hypothesis nor are any assumptions made as to the symmetry properties of the fluctuations. We find that the spectra are anisotropic on all the three scales and the power is extended primarily in the directions perpendicular to the mean magnetic field, as might be expected of two-dimensional turbulence, however, the analyzed fluctuations are not axisymmetric. The lack of axisymmetry invalidates some earlier techniques using single spacecraft observations that were used to estimate the percentage of magnetic energy residing in quasi-two-dimensional power. However, the dominance of two-dimensional turbulence is consistent with the relatively long mean free paths of cosmic rays in observed in the heliosphere. On the other hand, the spectra also exhibit secondary extended structures oblique from the mean magnetic field direction. We discuss possible origins of anisotropy and asymmetry of solar wind turbulence spectra.
- ItemWave-Vector Dependence of Magnetic-Turbulence Spectra in the Solar Wind(APS, 2010-04-28) Narita, Y.; Glassmeier, K.-H.; Sahraoui, F.; Goldstein, MelvynUsing four-point measurements of the Cluster spacecraft, the energy distribution was determined for magnetic field fluctuations in the solar wind directly in the three-dimensional wave-vector domain in the range |k|≤1.5×10 ⁻³ rad/km. The energy distribution exhibits anisotropic features characterized by a prominently extended structure perpendicular to the mean field preferring the ecliptic north direction and also by a moderately extended structure parallel to the mean field. From the three-dimensional energy distribution wave vector anisotropy is estimated with respect to directions parallel and perpendicular to the mean magnetic field, and the result suggests the dominance of quasi-two-dimensional turbulence toward smaller spatial scales.