Galactic Cosmic Ray Observations at Different Heliospheric Latitudes

Abstract

The local interstellar spectra (LIS) of galactic cosmic rays at low energies cannot be determined from measurements made near Earth, because of strong solar modulation processes from the heliospheric boundary to the observer. Computations done in the seventies based on Parker's cosmic ray modulation theory taking into account diffusion, convection, adiabatic deceleration, and drift effects in the global heliospheric magnetic field predicted that the cosmic ray proton spectrum, for instance, is only weakly modulated over the poles of the Sun. The data set from the high heliospheric latitude ESA/NASA Ulysses mission, launched in October 1990, is ideally suited to investigate galactic cosmic ray flux variations with heliospheric latitude. Observations from mid 1993 to the end of 1997 during moderate to low solar activity are presented and compared with model predictions. In contrast to what was expected Ulysses did not observe the LIS over the solar poles. However, these observations yield a new understanding of the importance of the processes involved in the transport of galactic cosmic rays, and that the LIS can only be determined by an interstellar mission going far beyond 100 AU