Recent near-sun solar-wind observations from Parker Solar Probe have found a highly dynamic magnetic environment, permeated by abrupt radial-field reversals, or "switchbacks." We show that many features of the observed turbulence are reproduced by a spectrum of Alfvénic fluctuations advected by a radially expanding flow. Starting from simple superpositions of low-amplitude outward-propagating waves, our expanding-box compressible MHD simulations naturally develop switchbacks because (i) the normalized amplitude of waves grows due to expansion and (ii) fluctuations evolve towards spherical polarization (i.e., nearly constant field strength). These results suggest that switchbacks form in-situ in the expanding solar wind and are not indicative of impulsive processes in the chromosphere or corona.

Hall magnetohydrodynamics (MHD) is investigated through three-dimensional direct numerical simulations. We show that the Hall effect induces a spontaneous chiral symmetry breaking of the turbulent dynamics. The normalized magnetic polarization is introduced to separate the right- (R) and left-handed (L) fluctuations. A classical k(-7/3) spectrum is found at small scales for R magnetic fluctuations which corresponds to the electron MHD prediction. A spectrum compatible with k(-11/3) is obtained at large-scales for the L magnetic fluctuations; we call this regime the ion MHD. These results are explained heuristically by rewriting the Hall MHD equations in a succinct vortex dynamical form. Applications to solar wind turbulence are discussed.

One of the most important predictions in magnetohydrodynamics (MHD) is that in the presence of a uniform magnetic field b0ê a transition from weak to strong wave turbulence should occur when going from large to small perpendicular scales. This transition is believed to be a universal property of several anisotropic turbulent systems. We present for the first time direct evidence of such a transition using a decaying three-dimensional direct numerical simulation of incompressible balanced MHD turbulence with a grid resolution of 3072 2 × 256. From large to small-scales, the change of regime is characterized by i) a change of slope in the energy spectrum going from approximately −2 to −3/2; ii) an increase of the ratio between the wave and nonlinear times, with a critical ratio of χc ∼ 1/3; iii) a modification of the iso-contours of energy revealing a transition from a purely perpendicular cascade to a cascade compatible with the critical balance type phenomenology, and iv) an absence followed by a dramatic increase of the communication between Alfvén modes. The changes happen at approximately the same transition scale and can be seen as manifest signatures of the transition from weak to strong wave turbulence. Furthermore, we observe a significant non-local three-wave coupling between strongly and weakly nonlinear modes resulting in an inverse transfer of energy from small to large-scales.