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Applicability of Taylor s hypothesis during Parker Solar Probe perihelia
Author  Perez, Jean; Bourouaine, Sofiane; Chen, Christopher; Raouafi, Nour; 
Keywords  Solar wind; Sun: heliosphere; turbulence; magnetohydrodynamics (MHD); plasmas; Astrophysics  Solar and Stellar Astrophysics; Physics  Plasma Physics; Physics  Space Physics; Parker Data Used 
Abstract  We investigate the validity of Taylor s hypothesis (TH) in the analysis of velocity and magnetic field fluctuations in Alfvénic solar wind streams measured by Parker Solar Probe (PSP) during the first four encounters. The analysis is based on a recent model of the spacetime correlation of magnetohydrodynamic (MHD) turbulence, which has been validated in highresolution numerical simulations of strong reduced MHD turbulence. We use PSP velocity and magnetic field measurements from 24 h intervals selected from each of the first four encounters. The applicability of TH is investigated by measuring the parameter ϵ = δu_{0}/√2V_{⊥}, which quantifies the ratio between the typical speed of largescale fluctuations, δu_{0}, and the local perpendicular PSP speed in the solar wind frame, V_{⊥}. TH is expected to be applicable for ϵ ≲ 0.5 when PSP is moving nearly perpendicular to the local magnetic field in the plasma frame, irrespective of the Alfvén Mach number M_{A} = V_{SW}∕V_{A}, where V_{SW} and V_{A} are the local solar wind and Alfvén speed, respectively. For the four selected solar wind intervals, we find that between 10 and 60% of the time, the parameter ϵ is below 0.2 and the sampling angle (between the spacecraft velocity in the plasma frame and the local magnetic field) is greater than 30°. For angles above 30°, the sampling direction is sufficiently oblique to allow one to reconstruct the reduced energy spectrum E(k_{⊥}) of magnetic fluctuations from its measured frequency spectra. The spectral indices determined from powerlaw fits of the measured frequency spectrum accurately represent the spectral indices associated with the underlying spatial spectrum of turbulent fluctuations in the plasma frame. Aside from a frequency broadening due to largescale sweeping that requires careful consideration, the spatial spectrum can be recovered to obtain the distribution of fluctuation s energy across scales in the plasma frame. 
Year of Publication  2021 
Journal  Astronomy and Astrophysics 
Volume  650 
Number of Pages  A22 
Section  
Date Published  06/2021 
ISBN  
URL  https://ui.adsabs.harvard.edu/abs/2021A&A...650A..22P 
DOI  10.1051/00046361/202039879 