Radial Evolution and Kinetics of Ion Species: Helios Observations

Despite being measured over 40 years ago, the Helios data still provide a fantastic opportunity to study the solar wind. For ion studies, previous work has mainly focused on moments of the three-dimensional distribution functions or fits to the proton core and alpha-particle populations. However, moments do not offer the resolution needed for many investigations and proton beam populations, which can be significant, are not captured by the existing fitting methods. Here we adapt recently developed code to include the proton beam as a third fitted population. With three populations, the ion distribution functions can often be well characterised, supporting more in-depth kinetic studies. We present statistics of the proton beam and alpha-particle populations in various solar wind streams, focusing on parameters such as drift speeds and temperature anisotropies. We also investigate the radial evolution of each ion population.