Details of the processes involved in cosmic-ray production and acceleration remain unclear, but it appears that particle acceleration can be accomplished by expanding shock waves from supernovas. Energetic particles emerge from solar flares where they have been accelerated by strong magnetic fields. Most of these particles are protons, with a decreasing number of helium and heavier nuclei. Observations of the helium-oxygen ratio among energetic solar particles have contributed significantly to solar studies, because the Sun's helium abundance is difficult to estimate by means of conventional spectroscopy. Pioneering studies were conducted atop mountains where only secondary particles were detectable.
Rockets can reach greater heights but with smaller payloads and for only a few minutes.
From the early 1930s to the 1950s, cosmic rays played a critical role in the scientific study of the atomic nucleus and its components, for they were the only source of high-energy particles. The field of particle physics was in fact established as a result of such discoveries, beginning with those of the positron and the muon. Even with the advent of powerful particle accelerators in the 1950s, investigators in the field have continued to study cosmic rays, albeit on a more limited scale, because they contain particles with energies far beyond those attainable under laboratory conditions.