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Turbulence in the solar wind: 3D measurements of wavenumber spectra using the k-filtering technique

Abstract:

Magnetic turbulence in the solar wind has been studied for many years. Most of the observational work have been focused on the large (MHD) scales, i.e. the so-called inertial range with a Kolmogorov scaling k-5/3 [e.g.,1,2]. The inertial range is widely believed to form by strong nonlinear interaction of Alfvén waves. However, the anisotropy of the turbulence (in wavenumber space) is still hotly debated [e.g., 3]. From single spacecraft data, inferring the wavenumber spectra from the temporal measured ones onboard the satellite can be achieved only by using the Taylor frozen-in assumption ωsat∼k.Vsw. This means that all the phase speeds of the waves need to be smaller than the solar wind speed Vsw. While this assumption is generally valid at MHD scales (because the Alfvén speed VA ≪Vsw), it breaks down at the sub-ion (and electron) scales where whistler modes may exit. Moreover, even when the Taylor assumption is justified it can yield only one component of the wavenumber spectra: along the flow Vsw [4-6]. The two other directions perpendicular to Vsw are thus missing unless additional assumptions, such as isotropy, are used. Therefore multispacraft data and appropriate space-correlations methods are necessary in order to fully determine the 3D wavenumber spectra of space turbulence. This can be achieved by applying the k-filtering technique on the four Cluster spacecraft data. © 2011 IEEE.