Solar wind on the path from the Sun to Earth

Solar corona fills the whole solar system with the stream of ionized particles – solar wind. Its basic parameters and their evolution with the distance from the Sun were predicted by the Eugene Parker’s hydrodynamic theory in the middle of the last century but the latest observations covering the range from 0.09 to 100 AU give us the possibility to check and modify this crude simplification.

Two essential features distinguish the solar wind from a classical hydrodynamic flow - its weakly collisional nature and the presence of a magnetic field. The absence of frequent collisions allows a motion of different ion populations with various velocities and thus one should ask what a “real” solar wind velocity is. The magnetic field is not just passively frozen in the solar wind plasma as is often assumed, but its force action plays an important role in the release of the solar wind from the corona. Moreover, the magnetic field facilitates excitation and propagation of a variety of waves. The wave interactions lead to turbulence and form interplanetary shocks but their role in the solar-wind acceleration and heating is still not fully understood.

The lecture synthesizes multi-decade observations from numerous spacecraft to address these issues and to discuss their implications for solar-wind formation and evolution through heliosphere. Recent studies have revealed significant changes in the radial trends of plasma and magnetic-field parameters, including ion velocity (Nemecek et al. 2020), plasma beta (Safrankova et al. 2023), interplanetary shock properties and occurrence rates (Kruparova et al. 2025; Park et al. 2023), velocity-temperature relations (Durovcova et al. 2026), and the cross helicity of fluctuations (Park et al. 2025) in the region near Mercury’s orbit. We focus on the physical processes shaping this region and discuss possible interpretations of the observed phenomena. While solar-wind formation and evolution are currently the subject of intense investigation enabled by new observational capabilities, this lecture emphasizes our group's contributions to present knowledge.