D. Aaron Roberts
Lots of Things We Don't Know About Solar Wind Turbulence
Some aspects of solar wind turbulence continue to defy explanation. While it seems likely that the evolution of some correlations and power spectra are largely explained by the sheering of an initial population of solar-generated Alfvenic fluctuations, the evolution of the anisotropies of the turbulence does not fit into the model so far. A two-component model, consisting of slab waves and "quasi-two- dimensional" fluctuations, offers some ideas, but does not account for the turning of both wave-vector-space power anisotropies and minimum variance directions in the fluctuating vectors as the Parker spiral turns. We will show observations that indicate that the minimum variance evolution is likely not due to traditional turbulence mechanisms, as has often been suggested, and offer arguments that the idea of two-component turbulence is at best a local approximation that is of little help in explaining the evolution of the fluctuations. Conventional linear theories do not provide answers either. Finally, time-permitting, we will discuss some observations that suggest that the low Alfvenicity of many regions of the solar wind in the inner heliosphere is not due to turbulent evolution, but rather to the existence of convected structures, including mini-clouds and other twisted flux tubes, that were formed with low Alfvenicity. There is still a role for turbuence in the above picture, but it is highly modified from the traditional views.