Waves, structures, and the appearance of two-component turbulence in the solar wind

Spacecraft observations of magnetic field fluctuations in the solar wind reveals a "Maltese Cross" pattern in the two-dimensional correlation function measurements of solar wind fluctuations [Matthaeus et al., 1990]. This pattern suggests the presence of two components: fluctuations with their (Fourier) wave vector approximately parallel to the ambient magnetic field (e.g., slab turbulence) and fluctuations with their (Fourier) wave vector approximately perpendicular to the ambient magnetic field (e.g,, quasi two-dimensional turbulence). To date, the appearance of such a pattern has never been reproduced from numerical simulation studies. Here we present results of several MHD simulations that address this issue using both two-and-one-half dimensional and three-dimensional compressible models and a wide variety of initial states and plasma parameters. Slab turbulence and quasi two-dimensional turbulence appear in various runs, however, their simultaneous appearance is difficult to achieve and seems to rely upon their separate existence in the initial data. In contrast, the presence of transverse pressure-balanced magnetic structures causes slab turbulence to evolve in such a manner that a two-component correlation function emerges through time averaging. We suggest that the Maltese Cross and similar observations may be a consequence of either the initial data or of averaging over different parcels of solar wind.