Why is it difficult to identify the sources of galactic cosmic rays from their arrival directions?

It is difficult to identify the sources of galactic cosmic rays (GCRs) from their arrival directions because these particles are electrically charged atomic nuclei. As they travel through the Milky Way Galaxy, they are deflected by magnetic fields.1

These magnetic fields cause the cosmic rays to follow convoluted, spiral paths rather than traveling in straight lines from their points of origin. Consequently, by the time these particles reach the top of Earth’s atmosphere, they arrive nearly uniformly from all directions, effectively masking the location of their sources.12

Because of this deflection, astronomers cannot use arrival directions to pinpoint specific cosmic ray sources. Instead, they must infer the origins of GCRs by analyzing the elemental and isotopic abundances of the particles and comparing them to spectroscopic data from stars and interstellar regions. In contrast, cosmic gamma rays are not affected by magnetic fields, allowing their arrival directions to be used to identify energetic objects like supernova remnants.132