The solar wind is a sparse and hot plasma, flowing from the surface of the Sun to the edge of the heliosphere. Our best knowledge of the solar wind close to the Sun comes from the two Helios spacecraft launched in the 1970s, but this will soon be joined by data from Parker Solar Probe and Solar Orbiter. This thesis re-visits in situ measurements of the solar wind taken by Helios in preparation for these new missions.

Measurements of solar wind protons and alpha particles taken by Helios are re-analysed to extract the thermal anisotropy of the particle distribution functions. The new dataset is used to investigate three areas of solar wind formation and evolution.

The properties of number density structures inside 1 AU located away from the heliospheric current sheet are investigated and shown to account for < 1% of the solar wind. Their use as tracers of the ambient solar wind speed is validated, and 90% of the structures detected are shown quantitively to be too small to be detected in existing remote sensing images.

A new solar wind source identification scheme is constructed for in situ measurements made inside 1 AU, with distinct solar wind sources classified into three categories based on proton temperature anisotropy and Alfvénicity. This categorisation scheme is used to demonstrate that the traditional categories of slow and fast solar wind do not map well to distinct solar sources.

The radial evolution of alpha particle temperatures inside 1 AU is presented. Alpha particles are found to undergo heating in the magnetic field parallel direction, and cooling in the magnetic field parallel direction during propagation. This is shown to be at least due to the influence of temperature anisotropy instabilities. These results also imply that alpha particles are heated in the perpendicular direction below 0.3 AU.