Abstract
Abstract: Using two long data sets analyzed on equal footing, the properties of Alfvenic fluctuations in the fast (coronal-hole-origin) solar wind and Navier-stokes turbulence are compared. A 26.4-s-long interval of hot-wire measurements in the ONERA wind tunnel is used and a 71-hr-long interval of unperturbed coronal-hole plasma measured by the WIND spacecraft at 1 AU is used. Similarities and differences between a Navier-Stokes fluid and the collisionless magnetized solar-wind plasma are discussed, as are differences between the physical natures of the advecting evolving turbulent fluctuations and the propagating non-evolving Alfvenic fluctuations. The details of the power spectral densities of the turbulence and the Alfvenic fluctuations are compared. Statistics of first and second time derivatives are examined for the wind-tunnel and solar-wind time series and the statistics are compared with the statistics of time derivatives of phase-randomized time series. Using running medians, the statistics of flat spots in the time series of Alfvenic fluctuations is examined, which is evidence of a cellular structure to the magnetic field and velocity field. A call for a campaign of expanded coordinated future work is made.
Highlights
A side-by-side comparison is made between Navier–Stokes turbulence measured in a wind tunnel and Alfvénic fluctuations measured in the fast solar wind
An outstanding question is why would the inertial-range spectral properties of the outward-propagating Alfvénic fluctuations in the solar wind be similar to the properties of Navier–Stokes turbulence? A related question is how the outward-propagating structures obtained their properties? Three possibilities are suggested here
(1) Maybe the outwardpropagating fluctuations are fossils of turbulence at the Sun in the sense that the structure seen in the inner heliosphere is the relaxation of an MHD turbulence near the Sun to an Alfvénic state (e.g., Dobrowolny et al, 1980; Matthaeus et al, 2008; Telloni et al, 2016)
Summary
A side-by-side comparison is made between Navier–Stokes turbulence measured in a wind tunnel and Alfvénic fluctuations measured in the fast solar wind. Navier–Stokes fluid turbulence comprised rapidly evolving advecting structures (eddies) that strongly interact with each other, whereas the Alfvénic fluctuations in the solar wind propagate en masse through the plasma away from the Sun and are largely non-evolving. As will be pointed out, the Navier–Stokes Kolmogorov turbulence measurements in the wind tunnel and the MHD Alfvénic structure propagation measurements in the fast solar wind are observations of two completely different processes. Similarities and Differences in the Statistics of Derivatives examines the statistics of time derivatives in the wind-tunnel Navier–Stokes turbulence measurements vs the measurements of the Alfvénic fluctuations of the solar wind. E. (2020a) and Nemecek et al (2020) have developed methodologies to find the moving reference frame of the solar-wind magnetic structure relative to the solarwind plasma; at 1 AU, they find that the magnetic structure propagates at about 0.7 vA along the Parker-spiral direction
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