Wave-Vector Dependence of Magnetic-Turbulence Spectra in the Solar Wind

Abstract

Using four-point measurements of the Cluster spacecraft, the energy distribution was determined for magnetic field fluctuations in the solar wind directly in the three-dimensional wave-vector domain in the range |k|<or=1.5x10{-3} rad/km. The energy distribution exhibits anisotropic features characterized by a prominently extended structure perpendicular to the mean field preferring the ecliptic north direction and also by a moderately extended structure parallel to the mean field. From the three-dimensional energy distribution wave vector anisotropy is estimated with respect to directions parallel and perpendicular to the mean magnetic field, and the result suggests the dominance of quasi-two-dimensional turbulence toward smaller spatial scales.

Anisotropy in solar wind plasma turbulence

“…Although this is consistent with linear Maxwell-Vlasov dispersion relations for very oblique KAWs (θ kB 0 = cos(k ·B 0 ) ≈ 88 • ), with such strong obliquities of the ks the results can also be interpreted as indicating the presence of relatively energetic quasi-two-dimensional turbulence. Indeed, Narita et al [71,72,76,77] use essentially the same multi-spacecraft technique, over similar wavenumber ranges, and state that they find no evidence of a linear dispersion relation. They favour an explanation in terms of well-developed strong quasi-two-dimensional turbulence.…”

Transport of solar wind fluctuations: A two-component model

“…On the other hand, observations also suggested that turbulence was present and may have contributed to heating [e.g., Coleman, 1968]. Subsequent observational studies indicated that at magnetohydrodynamic (MHD) scales there are at least two distinct types of fluctuation and that the wave-like energy may be a minor component Bieber et al, 1996;Milano et al, 2004;Dasso et al, 2005;Horbury et al, 2005Horbury et al, , 2008Podesta, 2009;Osman and Horbury, 2009;Narita et al, 2010]. This encouraged development of more complete transport theories for the energy-containing range quantities [e.g., Tu and Marsch, 1993;Matthaeus et al, 1994Matthaeus et al, , 1996Matthaeus et al, , 1999Matthaeus et al, , 2004Zank et al, 1996;Smith et al, 2001Smith et al, , 2006Isenberg et al, 2003Isenberg et al, , 2010aIsenberg, 2005;Breech et al, 2005Breech et al, , 2008Yokoi and Hamba, 2007;Usmanov and Goldstein, 2010;Ng et al, 2010], in which turbulence properties are built in, contrasting with WKB theory, in which the waves are noninteracting at leading order.…”

Nonaxisymmetric Anisotropy of Solar Wind Turbulence

“…The situation is different in plasma turbulence where the existence of a mean magnetic field sets a natural preferential direction for anisotropy. Anisotropy is thus a key topic in theoretical [3][4][5], numerical [6,7], and observational studies of plasma turbulence in the solar wind [8][9][10][11][12][13].The seminal study of Belcher and Davis [8] used Mariner 5 observations to investigate anisotropy of the solar wind magnetic fluctuations in the low frequency (energy containing) and inertial intervals. They found that the fluctuations on average have 5 : 4 : 1 power anisotropy in an orthogonal coordinate system whose axis are [e B × e R , e B × (e B × e R ), e B ], where e B is a unit vector in the average magnetic field direction and e R is a unit vector radially away from the sun.…”

“…The situation is different in plasma turbulence where the existence of a mean magnetic field sets a natural preferential direction for anisotropy. Anisotropy is thus a key topic in theoretical [3][4][5], numerical [6,7], and observational studies of plasma turbulence in the solar wind [8][9][10][11][12][13].…”