A ‘Ulysses’ cosmic-ray latitude gradient observation and modulation models

The ‘EPAC’ instrument on Ulysses is sensitive to relativistic cosmic rays when far from Jupiter and in the absence of energetic solar particles. Measurement of the latitude gradient of these particles, after correction for time variations, has been made for the 1993–1994 south polar pass. The average magnitude of the gradient is about the same or smaller than predicted by a model which includes full gradient drift. However, the latitude dependence of the solar plasma output into interplanetary space (including fast-stream and magnetic turbulence effects) seems to be important in determining the magnitude of modulation.