2D Isotropic Feature of Solar Wind Turbulence as Shown by Self-correlation Level Contours at Hour Timescales

The slow wind anisotropy is observed by Dasso et al. as elongation along the magnetic field direction in the magnetic self-correlation contours calculated from data sets of two-day-long data and averaged for five years in 1998-2002, which is consistent with prediction by the "critical balance cascade theory." More pronounced elongation at smaller scales than at larger scales has also been predicted by this theory. However, this prediction has not yet been checked by observations. Here, we present a check of the variation trend of the anisotropy with scales by presenting level contours of magnetic field and velocity self-correlations using intervals with durations varying from two days to one hour as observed by the Advanced Composition Explorer during 1998-2002 in the slow wind. We find that the level contours elongate along the magnetic field direction at durations of two days and one day. But they become isotropic for shorter intervals from about 10 hours to 1 hour. We also find that in the fast wind, the variation of the anisotropy with the scale has the same trend as in the slow wind. The 2D isotropic feature of the solar wind fluctuations shown by these statistical results is not consistent with the existing theory and will open a new avenue for studying solar wind turbulence.