Abstract
In the multi-component, turbulent and weakly collisional solar wind, kinetic processes predominate and significant deviations from a local thermodynamic equilibrium may arise. Non-thermal features such as a second population of any ion component (ion beam) are often observed.
However, the underlying processes which drive development of such features are still not fully explained. We focus on properties of the proton beam, the secondary proton component generally flowing faster than the proton core by about local Alfv'en speed.
With motivation to study proton beam development during solar wind expansion, we reprocessed 3D ion VDFs measured by the Helios spacecraft between 0.28 AU and 1 AU from the Sun. We found that the relative content of the proton beam to proton core in the fast solar wind increases with increasing distance from the Sun.
On the other hand, it almost does not change in the slow solar wind. To explain these observations, we studied the proton beam properties for various solar wind conditions.
We found that the proton beam and α particle contents are coupled together. Moreover, we showed that a behavior of the proton beam in the high-speed streams corresponds well to the model based on interaction between protons and oblique Alfven wave.
In the slow solar wind, the proton beam could be formed by the parametric instability.