Browsing by Author "Belmont, G."
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- 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.
- 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.