3-DIMENSIONAL NONSTEADY COMPRESSIBLE MAGNETOHYDRODYNAMIC FLUCTUATIONS IN THE SOLAR-WIND

We investigate some basic properties of three-dimensional nonsteady compressible magnetohydrodynamic (MHD) perturbations in a polytropic radial MHD wind with spherical symmetry. At a large radial distance r, we derive analytical solutions for MHD slow- and fast-type perturbations for several rational values of the polytropic index gamma. The propagation characteristics of MHD slow-type perturbations, which are more magnetic at large r, are very similar to those of Alfvenic perturbations; namely, there exists the same characteristic frequency f(c) for both types of perturbations, and the transverse magnetic field perturbation associated with MHD slow-type perturbations also tends to dominate the background radial magnetic field at large r except in the zero frequency limit. The propagation characteristics of MHD fast-type perturbations, which are more acoustic at large r, depend upon the value of gamma because the sound speed C(S) in the wind scales as approximately r-(gamma-1) at large r. For the typical case of gamma < 2, MHD fast-type perturbations always propagate relative to the wind. For the atypical case of gamma > 2, acoustic perturbations actually become MHD slow perturbations at large r because CS is now slower than the Alfven speed C(A), which scales as approximately r-1 at large r, and these acoustic perturbations do not propagate relative to the wind, yet the wind advects them radially outward. For the special case of gamma = 2, there exists another characteristic frequency f(a). For perturbation frequency f > f(a), acoustic perturbations propagate relative to the wind, whereas for f < f(a), they appear standing relative to the wind. We discuss the relevance of these results, both MHD fast- and slow-type perturbations with gamma < 2, to interplanetary fluctuations in the inner solar wind. In particular, we interpret the increasing trends with r of the relative density and magnetic field fluctuations observed in high-speed solar winds within 0.3 to 1 AU as manifestation of MHD fast- and slow-type perturbations.