Variance Anisotropy of Solar Wind Velocity and Magnetic Field Fluctuations

Abstract:

At MHD scales in the solar wind, velocity and magnetic field
fluctuations are typically observed to have much more energy in the
components transverse to the mean magnetic field, relative to the
parallel components [eg, 1,2]. This is often referred to as
variance anisotropy. Various explanations for it have been suggested,
including that the fluctuations are predominantly shear Alfv\'en waves
[1] and that turbulent dynamics leads to such states [eg, 3].
Here we investigate the origin and strength of such variance
anisotropies, using spectral method simulations of the
compressible (polytropic) 3D MHD equations.

We report on results from runs with several different classes of
initial conditions. These classes include
(i) fluctuations polarized only in the same sense as shear Alfv\'en
waves (aka toroidal polarization),
(ii) randomly polarized fluctuations, and
(iii) fluctuations restricted so that most of the energy is in
modes which have their wavevectors perpendicular, or nearly so, to the
background magnetic field: quasi-2D modes.

The plasma beta and Mach number dependence [4] of quantities like the variance anisotropy, Alfven ratio, and fraction of the energy in the toroidal fluctuations will be examined, along with the timescales for the development of any systematic features.
Implications for solar wind fluctuations will be discussed.

References:
[1] Belcher & Davis 1971, J. Geophys. Res, 76, 3534.
[2] Oughton et al 2015, Phil Trans Roy Soc A, 373, 20140152.
[3] Matthaeus et al 1996, J. Geophys. Res, 101, 7619.
[4] Smith et al 2006, J. Geophys. Res, 111, A09111.