Magnetohydrodynamic turbulence in the solar wind

Low frequency fluctuations in the solar wind magnetic field and plasma velocity are often highly correlated, so much so that the fluctuations may be thought of as originating near the Sun as nearly perfect Alfven waves. Power spectra of these fluctuations from 10(-7) Hz to several Hz to suggest that the medium is turbulent. Near 1 AU, fluctuations below 10(-5) Hz have a relatively flat slope (similar to-1) and contain most of the energy in the fluctuating fields. From 10(-5) Hz to similar to 0.1 Hz, the spectra exhibit a power law inertial range similar to that seen in ordinary fluid turbulence. At the highest frequencies, the rapid fall-off of the power suggests that strong dissipation is occurring. From in situ measurements, it is clear that the fluctuations emanate from the solar corona. The turbulent cascade appears to evolve most rapidly in the vicinity of velocity shears and current sheets. Numerical solutions of both the compressible and incompressible equations of magnetohydrodynamics in both Cartesian and spherical geometry corroborate this interpretation. There are conflicting interpretations of observations suggesting that much of the power in magnetic field fluctuations resides in quasi-two-dimensional structures and simulations have helped to elucidate some of these issues. (C) 1999 American Institute of Physics. [S1070-664X(99)02811-6].