SOLAR WIND DENSITY SPECTRA AROUND THE ION SPECTRAL BREAK

This paper presents a large statistical analysis of approximate to 5800 frequency spectra of the solar wind density fluctuations in the range 0.001-5 Hz (corresponding to spatial scales of 100-5 x 10(5) km). The analysis confirms that the spectrum consists of three segments divided by two breakpoints and that each of the segments can be described by a power-law function with a spectral index alpha. The first segment corresponds to MHD scales and is followed by a plateau, and the third segment can be associated with the kinetic range. The statistics show that the values of the spectral slopes depend on the density fluctuations; their increasing amplitude leads to a steepening of each segment. The index of -1.8 can typically be found at MHD scales and averaging of the spectra in the frequency domain leads to an index of -8/3 at kinetic scales, whereas averaging in frequencies normalized to the ion gyrostructure frequency, f(g), defined as the ratio of the solar wind bulk speed and thermal ion gyroradius, provides a value of -7/3. Both breakpoint locations are controlled by the gyrostructure frequency.