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By using data from the Technical University of Braunschweig flux-gate and search-coil magnetometer experiments on board of Helios 2 we study the spectral properties of the interplanetary magnetic field over a frequency range from 2.4 x 10-5 Hz up to 470 Hz. Examples of power spectral density estimates at different heliocentric distances are shown as well as the change of the spectra during the progress of a high speed stream. A general feature of the spectra is that in a log-log spectral representation the steepness of the power spectral density estimates varies as a function of frequency. If we relate the spectral densities by a power law P ∼ f –α, the spectral index α increases with increasing frequency. At 1 AU α varies on average from 1.6 to 3.4 and at 0.3 AU from 1.0 to 3.4, the major changes in the spectral index occurring at low frequencies. In addition, just within the frequency gap between the two experiments, between 2Hz and 4.7 Hz, an inflexion point is inferred from the spectrum above and below this frequency range. This spectral feature can at least partly be attributed to the damping of the Alfven-mode waves near the proton and also α-particle cyclotron frequencies. The observed power spectra are compared with models of MHD turbulence and it is found that at least some of the properties of MHD turbulence fit the observations remarkably well.
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