Evolution of power spectral density of magnetic field fluctuations in the inner heliosphere

The solar wind is a unique laboratory to study the turbulent processes occurring in a collisionless plasma with high Reynolds numbers. The paper analyzes power spectra of magnetic field fluctuations that are computed in the frequency range around the break between inertial and kinetic scales. We use observations made during first nine Parker Solar Probe encounters and compare them with observations of the spacecraft moving in other distances from the Sun (e.g., Solar Orbiter) and closer to 1 AU (Wind). A preliminary analysis of magnetic field fluctuations based on PSP and Wind measurements from the MHD to kinetic scales has shown that a relative level of compressive fluctuations increases until 0.25 AU and remains constant till 1 AU whereas a relative level of perpendicular fluctuations does not change with the distance from the Sun. We can conclude that the B slope is controlled by different process(es) close to the Sun against 1 AU and that, in spite of expectations, the critical distance for turbulence evolution is as large as 0.25 AU. We also discuss the role of important physical parameters (e.g., ion beta, temperature anisotropy, collisional age, magnetic field fluctuation amplitude) determining the properties of the turbulent cascade in different heliospheric locations.