Anisotropy of magnetosonic turbulence in the solar wind between 0.1 and 0.4 AU

The anisotropy of the density fluctuations in the solar wind is studied wit h the assumption that density fluctuations are connected to magnetosonic tu rbulence. It, is shown that the anisotropy of density fluctuations does not coincide with that of the turbulence energy in the case of the fast magnet osonic waves while they are equal for the slow magnetosonic waves. The radi al evolution of the two-dimensional (2-D) anisotropy ratio in the interplan etary scintillation (IPS) pattern plane due to the change of the ambient ma gnetic field direction is investigated. In the case of the Parker's spiral magnetic field the radial dependence of the 2-D anisotropy ratio is shown t o be considerably stronger for the fast magnetosonic waves than for the slo w magnetosonic waves. The comparison between theoretical estimations and th e results of the Kashima high-frequency IFS measurements shows that the geo metric effects are not sufficient to account for the observed radial depend ence of the axial ratio, and that the angular distribution of turbulent ene rgy evolves in the region of heliocentric distances between 0.1 and 0.4 AU toward the state enriched by the magnetosonic waves propagating at smaller angles relative to the ambient, magnetic field. The nonlinear wave-wave int eractions seem to be responsible for this evolution of the density turbulen ce spatial power spectrum.