Solar wind turbulence: anisotropy, anisotropy, anisotropy!

Turbulence heats the solar wind as it expands away from the Sun, but where and how does heating of ions and electrons occur? In order to understand this we must first look at the fluctuations making up the cascade, the properties and anisotropies of which will determine whether ions or electrons are heated and whether field-parallel or -perpendicular heating will occur, all of which amounts to a lot of different anisotropies! With this in mind, we present a review of recent advances in the observation of plasma turbulence in the solar wind and comparison with simulations; which features of solar wind turbulence are well reproduced and which need to be captured better?

The first anisotropy is that of the fluctuations making up the turbulent cascade itself, fluctuations are known to be highly transverse, meaning that the perpendicular magnetic field components are dominant over the field-parallel component. The second anisotropy is that of the scaling of amplitude towards smaller scales with steeper spectra parallel to the local magnetic field direction. Observations of the anisotropy of the full power spectral tensor will be discussed, in particular with reference to Alfvenic and pseudo-Alfvenic fluctuations (effectively two different polarizations of Alfven waves), the next step beyond the traditional "slab + 2D" approach to incompressible MHD turbulence. The third anisotropy is that of the ion and electron distributions. Both sets of charged particles frequently show non-Maxwellian distributions with higher temperatures found either perpendicular to or parallel to the magnetic field direction. Proton distributions often show beams and the heavier alpha particles are often hotter than the protons. Localized structures such as current sheets and magnetic discontinuities are shown to be sites of intense and anisotropic heating. Small scale fluctuations filling the space between such discontinuities may also dissipate energy into ions and electrons, either through electric fields intrinsic to the modes generated by the turbulence or through resonant or stochastic processes. Observations show that kinetic Alfven waves are the dominant mode.