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Register a new userApplicability of Taylors Hypothesis during Parker Solar Probe perihelia
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We investigate the validity of Taylors Hypothesis (TH) in the analysis of Alfvenic fluctuations of velocity and magnetic fields in solar wind streams measured by Parker Solar Probe (PSP)~during the first four encounters. 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 $epsilon=delta u_0/sqrt{2}V_perp$, which quantifies the ratio between the typical speed of large-scale fluctuations, $delta u_0$, and the local perpendicular PSP speed in the solar wind frame, $V_perp$. TH is expected to be applicable for $epsilonlesssim0.5$ when PSP is moving nearly perpendicular to the local magnetic field in the plasma frame, irrespective of the Alfven Mach number $M_{rm A}=V_{rm SW}/V_{rm A}$, where $V_{rm SW}$ and $V_{rm A}$ are the local solar wind and Alfven speed, respectively. For the four selected solar wind intervals we find that between 10% to 60% of the time the parameter $epsilon$ is below 0.2 when the sampling angle (between the spacecraft velocity in the plasma frame and the local magnetic field) is greater than $30^circ$. For angles above $30^circ$, the sampling direction is sufficiently oblique to allow one to reconstruct the reduced energy spectrum $E(k_perp)$ of magnetic fluctuations from its measured frequency spectra. The spectral indices determined from power-law 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 large-scale sweeping that requires careful consideration, the spatial spectrum can be recovered to obtain the distribution of fluctuations energy among scales in the plasma frame.
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