The enhancement of solar wind fluctuations at the proton thermal gyroradius

Average power spectra of solar wind fluctuations at frequencies up to 0.87 Hz are calculated from Ogo 5 measurements of positive ion flux. Although the general spectral trend follows the power law spectrum observed at lower frequencies, a small but statistically significant power enhancement is observed at the frequency υ/2πR, where υ is the solar wind velocity and R = (2mkT)1/2/eB is the gyroradius of proton thermal motions in the solar wind. The measured power spectrum is in rough agreement with that deduced from radio scintillation observations.