Applicability of Taylor's hypothesis during Parker Solar Probe perihelia

We investigate the validity of Taylor's hypothesis (TH) in the analysis of velocity and magnetic field fluctuations in Alfvenic 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 high-resolution 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 epsilon = delta u(0)/ root 2V(perpendicular to), 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-perpendicular to. TH is expected to be applicable for epsilon <= 0.5 when PSP is moving nearly perpendicular to the local magnetic field in the plasma frame, irrespective of the Alfven Mach number M-A = V-SW/V-A, where V-SW and V-A are the local solar wind and Alfven speed, respectively. For the four selected solar wind intervals, we find that between 10 and 60% of the time, the parameter epsilon 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 degrees For angles above 30 degrees, the sampling direction is sufficiently oblique to allow one to reconstruct the reduced energy spectrum E(k(perpendicular to)) 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 fluctuation's energy across scales in the plasma frame.

Automated summary

In this study, the validity of Taylor's hypothesis in analyzing velocity and magnetic field fluctuations in Alfvenic solar wind streams is investigated using data from PSP. The results show that the hypothesis is applicable when PSP is moving nearly perpendicular to the local magnetic field.