Intermittency in the Expanding Solar Wind: Observations from Parker Solar Probe (0.16au), Helios 1 (0.3-1au), and Voyager 1 (1-10au)

Abstract

We examine statistics of magnetic field vector components to explore how intermittency evolves from near Sun plasma to radial distances as large as 10 au. Statistics entering the analysis include auto-correlation, magnetic structure functions of order n (SF n ), and scale dependent kurtosis (SDK), each grouped in ranges of heliocentric distance. The Goddard Space Flight Center Space Physics Data Facility (SPDF) provides magnetic field measurements for resolutions of 6.8 ms for Parker Solar Probe, 6 s for Helios, and 1.92 s for Voyager 1. We compute SF 2 to determine the scales encompassing the inertial range and examine SDK to investigate degree of non-Gaussianity. Auto-correlations are used to resolve correlation scales. Correlation lengths and ion inertial lengths provide an estimate of effective Reynolds number (R e ). Variation in R e allows us to examine for the first time the relationship between SDK and R e in an interplanetary plasma. A conclusion from this observed relationship is that regions with lower R e at a fixed physical scale have on average lower kurtosis, implying less intermittent behavior. Kolmogorov refined similarity hypothesis is applied to magnetic SF n and kurtosis to calculate intermittency parameters and fractal scaling in the inertial range. A refined Voyager 1 magnetic field dataset is generated.

Magneto-inertial range dominated by magnetic helicity in space plasmas

“…1b of the Ref. [23]. Taylor's hypothesis was used to convert the time lags τ into the wavenumbers (in this figure k = 1/V 0 τ , where V 0 is the mean solar wind speed), and the wavenumber was taken in the units of the inverse ion inertial length 1/d i .…”

Reconciling Parker Solar Probe observations and magnetohydrodynamic theory

“…Going away from the Sun, it has been shown a radial evolution of the scaling properties of solar wind turbulence in the heliocentric range of distances 0.17-0.8 astronomical units (AU) (9). Although the near-Sun solar wind shares different properties with the near-Earth one (10,11), significant differences have been found in the variance of magnetic fluctuations (about two orders of magnitude), in the compressive component of inertial range turbulence, and in the imbalance between inward and outward Alfvénic fluctuations. (9) also reported a steepening of the power spectral density spectra, moving from -3/2 close to the Sun to -5/3 at distances larger than 0.4 AU.…”