Anomalous Cosmic Ray Hydrogen
Cosmic rays can be accelerated in different ways. Some cosmic rays (called anomalous cosmic rays or ACR) are accelerated in the Solar System, at the shock where the solar wind meets interstellar space. Others are accelerated outside the solar system, by various processes in the Milky Way Galaxy and are called GCRs. Eric's PhD thesis research involved separating these two components for cosmic ray hydrogen, and identifying which part of the observed hydrogen cosmic rays were ACRs.
The key idea was that ACRs and GCRs, because they are accelerated by different processes, have different energies. GCRs have higher energies than ACRs. Therefore, you should be able (theoretically) to identify the two components by looking at their spectra. This can be seen in the proton (hydrogen nuclei) energy spectra below, which show two separate humps. Comparing the hydrogen spectra with those from other elemental cosmic rays, Eric speculated that the lower energy hump was from the population of ACRs and the higher energy hump is from GCRs. The data he used in this work were from the Voyager 1 and 2 spacecraft, which approached the solar wind termination shock in the late 1980s.