Relation of the magnetic field spectral indices with plasma properties within the heliosphere

The power spectral densities (PSDs) of solar wind ion moments and magnetic field turbulence in the MHD range of frequencies can be fitted by a power law with the index of -5/3 and with the power index ranging from 2 to 4 at frequencies exceeding the proton gyroscale. However, this general statement has many exceptions. As examples, (i) the density spectra exhibit a clear flattening at the high-frequency part in the MHD range but a similar effect was not reported for any other quantity, (ii) the -5/3 index is a good approximation for the magnetic field at the Earth orbit but -3/2 fits the velocity spectra better, (iii) the magnetic field spectral index evolves trough the inner heliosphere, reaching -5/3 value at 0.3 AU.  

 

For this reason, the paper analyzes the power spectra of solar wind and magnetic field fluctuations computed in the frequency range around the break between MHD and kinetic scales. We use Spektr-R proton moments and Wind magnetic field at 1 AU, combine them with Parker Solar Probe and Solar Orbiter observations in the inner heliosphere and concentrate on the overall PSD profiles of the density, thermal speed, parallel and perpendicular components of magnetic field and velocity fluctuations and investigate statistically the role of parameters like the fluctuation amplitude, collisional age, temperature anisotropy, ion and/or electron beta and cross-helicity.