Evidence for the presence of quasi‐two‐dimensional nearly incompressible fluctuations in the solar wind

TLDR

Solar wind fluctuations are usually modeled as MHD waves with slab or isotropic symmetry, yet these assumptions have long‑standing problems. The authors propose a third viewpoint that solar wind fluctuations include a subpopulation with wave vectors nearly transverse to both the mean magnetic field and the fluctuations about the mean. They base this proposal on laboratory plasma experiments, two‑dimensional MHD simulations, recent theory, and extensive solar wind data analyses. The identified quasi‑two‑dimensional component aligns minimum variance with the mean magnetic field, exhibits small anticorrelated density fluctuations, nearly constant total pressure at small scales, and suppresses pitch‑angle scattering, thereby addressing the long‑standing discrepancies in cosmic‑ray mean free paths and WKB predictions.

Abstract

Solar wind fluctuations are commonly regarded as a superposition of MHD waves, primarily in the Alfvén mode. These MHD fluctuations are frequently assumed to possess “slab” or isotropic symmetry, particularly in the development of models of the propagation of cosmic rays throughout the heliosphere. There are, however, several long‐standing problems with either of these choices. One problem is that the mean free path for pitch angle scattering of cosmic rays in the heliosphere is apparently longer than can be accounted for by using either assumption about the statistical symmetry of the fluctuations. Another problem is the prediction of WKB theory that the direction of minimum variance should tend to lie along the radial direction rather than along the mean magnetic field as is observed. Motivated by laboratory plasma experiments, a series of two‐dimensional MHD simulations, recent theoretical work, and extensive analyses of solar wind data, we suggest that there is a third possible viewpoint with potentially important implications for solar wind studies. From this perspective we suggest that solar wind fluctuations contain a subpopulation that have wave vectors nearly transverse to both the mean magnetic field and the fluctuations about the mean. For this quasi‐two‐dimensional component the direction of minimum variance lies along the mean magnetic field, density fluctuations are small and anticorrelated with | B |, the total pressure at small scales is nearly constant, and pitch angle scattering by resonant wave‐particle interactions is suppressed.