Abstract
Turbulent cascade transferring the free energy contained within the large scale fluctuations of the magnetic field, velocity and density into the smaller ones is probably one of the most important mechanisms responsible for heating of the solar corona and solar wind, thus the turbulent behavior of these quantities is intensively studied. The temperature is also highly fluctuating quantity but its variations are studied only rarely. There are probably two reasons, first the temperature is tensor and, second, an experimental determination of temperature variations requires knowledge of the full velocity distribution with an appropriate time resolution but such measurements are scarce. To overcome this problem, the Bright Monitor of the Solar Wind (BMSW) on board Spektr-R used the Maxwellian approximation and provided the thermal velocity with a 32 ms resolution, investigating factors influencing the temperature power spectral density shape. We discuss the question whether the temperature spectra determined from Faraday cups are real or apparent and analyze mutual relations of power spectral densities of parameters like the density, parallel and perpendicular components of the velocity and magnetic field fluctuations. Finally, we compare their spectral slopes with the slopes of the thermal velocity in both inertial and kinetic ranges and their evolution in course of solar wind expansion.
Highlights
As a super-Alfvénic solar wind expands into space, a particle motion in the magnetic field and their collisions as well as plasma instabilities change the shape of a velocity distribution
Our discussion of measuring methods used for a determination of solar wind parameters illustrates effects that are often neglected and emphasizes the fact that measurement methods and spatial, energetic and temporal resolutions of a particular instrument should be taken into account in processing and presentation of the experimental data as discussed by many authors
The density or velocity can be related to the proton core, the proton core plus proton beam or it can include somehow weighted contribution of heavier ions
Summary
As a super-Alfvénic solar wind expands into space, a particle motion in the magnetic field and their collisions as well as plasma instabilities change the shape of a velocity distribution. Their results suggest that they see stronger anisotropic heating as PSP moves closer to the Sun, but they note that a careful treatment of the shape of a VDF is needed to correctly describe the temperature This short and incomplete survey reveals possible instrumental effects on the solar wind proton temperature determination and on clear interpretation of measured data. In a large statistical study, we address spectral slopes in relation to the properties of the solar wind represented by proton β, collisional age, temperature anisotropy, and a standard deviation of fluctuations of the magnetic field perpendicular component.
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