Plasma fluctuations in the solar wind

Ogo 5 plasma and magnetic field data are used to compute power spectra of solar wind fluctuations over the frequency interval 10 −3 ‐10 −1 Hz. We confirm the validity of the assumption made in earlier papers that the power spectra calculated from total flux measurements are approximately equal to the power spectra of density fluctuations times the square of the average solar wind speed. The relative density power spectrum P n /n o ² is usually of the same order of magnitude as the power spectrum of speed fluctuations relative to the Alfvén speed, P v /v A ². All cases studied show evidence of the presence of Alfvén waves in this frequency range. In some data sets the density and field fluctuations are consistent with magnetosonic waves. In other sets the ratio of the power in field magnitude fluctuations to that in density fluctuations is inconsistent with magnetosonic waves; for these cases we postulate static inhomogeneities with a balance between electron thermal and magnetic pressures. Finally, we suggest that the power enhancements near 1 Hz reported in earlier papers may be caused by a resonant proton cyclotron instability driven by the proton thermal anisotropy in the solar wind.