Magnetohydrodynamic turbulence in the solar wind

Low frequency fluctuations in the solar wind magnetic field and plasma velo city are often highly correlated, so much so that the fluctuations may be t hought 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 re latively flat slope (similar to-1) and contain most of the energy in the fl uctuating 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 turbulenc e. At the highest frequencies, the rapid fall-off of the power suggests tha t strong dissipation is occurring. From in situ measurements, it is clear t hat the fluctuations emanate from the solar corona. The turbulent cascade a ppears to evolve most rapidly in the vicinity of velocity shears and curren t sheets. Numerical solutions of both the compressible and incompressible e quations of magnetohydrodynamics in both Cartesian and spherical geometry c orroborate this interpretation. There are conflicting interpretations of ob servations 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. [S 1070-664X(99)02811-6].