Propagation and modulation of cosmic rays in the heliosphere

Abstract

Two of the fundamental problems in modern cosmic ray astrophysics are 1) the determination of the local interstellar spectra of galactic cosmic rays at low energies (E∼1 GeV/N), and 2) the need for a correct quantitative treatment of particle propagation in turbulent magnetic fields. Both problems are linked to each other since the energy spectra measured in the vicinity of the Earth are altered by solar modulation, described by Parker’s transport equation. Computations done in the seventies predicted that the cosmic ray nuclei spectra are only weakly modulated over the poles of the Sun. The high heliospheric latitude ESA/NASA Ulysses mission, however, has measured strongly modulated energy spectra over the poles of the Sun around solar minimum, leading to a new understanding of the importance of the transport processes, especially the diffusion and drift coefficients, involved. Several attempts have been made to derive these parameters directly from the particle interaction with the turbulent magnetized plasma. Another approach is to utilize a cosmic ray propagation model and adjust the diffusion tensor so that the model “fits” in‐situ observations.