Evolution of magnetic field fluctuations and their spectral properties within the heliosphere: Statistical approach

The contribution presents the first comprehensive statistical study of the evolution of both compressive and non-compressive magnetic field fluctuations in the inner heliosphere. Based on Parker Solar Probe and Solar Orbiter data in various distances from the Sun, we address the general trends and compare them with Wind observations near 1 AU. We analyze solar wind power spectra of magnetic field fluctuations in the inertial and kinetic ranges of frequencies. We found a systematic steepening of the spectrum in the inertial range with the spectral index of around –3/2 at closest approach to the Sun toward –5/3 at larger distances (above 0.4 AU), the spectrum of the magnetic field component perpendicular to the background field being steeper at all distances. In the kinetic range, spectral indices increase from –4.5 at closest PSP approach to –3 at ≈0.4 AU and this value remains constant toward 1 AU. We show that the radial profiles of spectral slopes, fluctuation amplitudes, spectral breaks and their mutual relation rapidly change near 0.4 AU.