Statistical properties of low‐frequency magnetic field fluctuations in the solar wind from 0.29 to 1.0 AU during solar minimum conditions: HELIOS 1 and HELIOS 2

Abstract

To obtain information on the temporal and spatial evolution of MHD waves and discontinuities in the solar wind, we studied by means of statistical methods magnetic field fluctuations measured by the two HELIOS spacecraft in the frequency range between 2.4×10−5 Hz and 1.3×10−2 Hz at distances from the sun between 0.29 AU and 1.0 AU. Some statistical properties as magnetic field variances show the fluctuations remaining very similar on their way out from the sun. A different picture of the fluctuations emerges from analyzing the spectral properties of the magnetic field: The slope of the power spectral densities (both the vector components and the magnitude of the magnetic field) as a function of frequency becomes steeper with increasing heliographic distance—the solar wind seems to act as a low‐pass filter. The major changes occur within a heliographic distance up to about 0.4 AU. Field magnitude fluctuations fall off less rapidly with increasing radius than do fluctuations in the vector components. Cross spectral analysis between magnetic field components reveal the fluctuations as generally being field magnitude conserving.