Density fluctuations driven by Alfvén waves

The equations for a linearly polarized Alfven wave, propagating parallel to the direction of the average magnetic field in a perfectly conducting fluid, are solved to second order in the wave quanities for cases where the fluid obeys single adiabatic or double adiabatic equations of state. To this order, we find no change in the wave magnetic field or transverse wave velocity, but longitudinal wave velocity and density fluctuations appear, driven by gradients in the wave magnetic-field pressure. This is in contrast to the common belief that even large-amplitude Alfven waves remain purely transverse. The density fluctuations can become quite large when the Alfven speed is close to the ion sound speed in the fluid; this condition may at times exist in the solar wind at 1 AU. We suggest that part of the density fluctuations observed in the solar wind by satellites and interplanetary scintillation may be associated with large-amplitude Alfven waves. Heating of the solar wind might result if the ion sound waves, which are driven by the Alfven waves, are appreciably damped.