Inertial-range Turbulence Anisotropy of the Young Solar Wind from Different Source Regions

Abstract

We investigate the wavevector and variance anisotropies in the inertial range of the young solar wind observed by the Parker Solar Probe (PSP). Using the first 19 encounters of PSP measurements, we identify the young solar wind from different source regions: coronal hole (CH) interiors, streamers, and low Mach-number boundary layers (LMBLs), i.e., the peripheral region inside CHs. We assess the wavevector anisotropy with the 2D and slab turbulence model for the CH wind and the streamer wind, and the nearly incompressible (NI) MHD turbulence model for the LMBL wind where Taylor's hypothesis becomes questionable. Unlike the $\sim80\%$ 2D contribution typically reported at 1 au, our results show that only $26\%$ of the inertial range energy is associated with 2D fluctuations in the CH wind, and this fraction increases to $45\%$ in the streamer wind. As a representation of the LMBL wind, similarly, the oblique sub-Alfv\'enic intervals and the near-subsonic intervals are characterized by the dominance of slab fluctuations. All the results suggest that slab fluctuations are more abundant in the young solar wind below 0.3 au than at 1 au. Furthermore, we find a dependence of the variance anisotropy in the inertial range on proton plasma beta $\beta_p$. The variance anisotropy is the strongest in the LMBL wind with the lowest $\beta_p$, and the weakest in the streamer wind with the highest $\beta_p$. This contrast can be interpreted as the remnant of fluctuations from the coronal sources.