Wavelet analysis of scaling in plasma fluctuations in the magnetohydrodynamic range of solar wind turbulence seen by Parker Solar Probe

The high Reynolds number solar wind flow provides a natural laboratory for the study of turbulence in-situ. Parker Solar Probe has to date executed nine sampling distances between 0.2 AU to 1 AU, providing an opportunity to study how turbulence evolves in the expanding solar wind. We focus on data from the PSP/FIELDS[1] and the PSP/SWEAP[2] experiments which provide magnetic field and plasma observations respectively at sub-second cadence. We have identified multiple intervals of uniform solar wind turbulence, selected to exclude coherent structures such as pressure pulses and current sheets, and in which the primary proton population velocity varies by less than 20% of its mean value. We focus on events of multiple-hour duration, which span the spectral scales from the approximately 1/f range at low frequencies, through the magnetohydrodynamic (MHD) inertial range of turbulence and into the kinetic range, below the ion gyrofrequency. We perform a Haar wavelet decomposition[3] which provides accurate estimations of the exponents of these power-law ranges of the spectra, and of higher-order moments. This allows us to study how the spectral exponents may vary with distance from the sun and with solar wind conditions such as the plasma beta. We perform this analysis for both the magnetic field components and magnitude, which track Alfvenic and compressive turbulent fluctuations, respectively. At 1 AU, compressive fluctuations are known to exhibit scaling properties distinct from that of the individual magnetic field components.[4] Here we will investigate this behaviour at different distances from the Sun, plasma beta, and proton density.

We acknowledge the NASA Parker Solar Probe Mission and the SWEAP team led by J. Kasper and the FIELDS team led by S. D. Bale for use of data.

[1] Bale, S.D., Goetz, K., Harvey, P.R. et al. The FIELDS Instrument Suite for Solar Probe Plus.Space Sci Rev 204, 49–82 (2016). https://doi.org/10.1007/s11214-016-0244-5

[2] Kasper, J.C., Abiad, R., Austin, G. et al. Solar Wind Electrons Alphas and Protons (SWEAP) Investigation: Design of the Solar Wind and Coronal Plasma Instrument Suite for Solar Probe Plus. Space Sci Rev 204, 131–186 (2016). https://doi.org/10.1007/s11214-015-0206-3

[3] Kiyani, K.H., Chapman, S.C., Sahraoui, F., Hnat, B., Fauvarque, O., Khotyaintsev, Y.V.: Enhanced Magnetic Compressibility and Isotropic Scale Invariance at Sub-ion Larmor Scales in Solar Wind Turbulence. The Astrophysical Journal 763(1), 10 (2012). https://doi.org/10.1088/0004-637x/763/1/10

[4] Hnat, B., Chapman, S., Gogoberidze, G., Wicks, R.: Scale-free texture of the fast solar wind. Physical review. E, Statistical, nonlinear, and soft matter physics 84, 065401 (2011). https://doi.org/10.1103/PhysRevE.84.065401