Observations from the Voyager spacecraft showing negative latitude gradients of a few percent per degree for galactic cosmic rays and anomalous components after 1985 at radii beyond 15 AU have prompted a reexamination of the measurements made by Pioneer 11, which reached a heliographic latitude of 16¿ near a radius of 4 AU in 1975--1976. Based on the Pioneer data, Bastian et al. (1979) and McKibben et al. (1979) reported a positive latitude gradient of ~2%/degree for anomalous helium, although with some uncertainty as to whether time variations in the modulation might also account for the result. Since the Sun's magnetic polarity changed sign in 1980, a change in the sign of the latitude gradient from 1976 to 1986 would offer support to theories of solar modulation that predict sensitivity of the cosmic ray modulated intensity to the sign of the solar magnetic dipole. Thus it is important to reduce the uncertainty in the interpretation of the Pioneer measurements. Based on a complete reanalysis of the data from Pioneer 11, and using a variety of techniques to examine the latitude dependence, we confirm the earlier results both as to magnitude and sign of the latitude gradients, and are able to reduce considerably possible confusion from time-dependent effects. However, whereas the earlier analysis has assumed an intensity distribution symmetrical about the heliographic equator, we now find evidence that the intensity increased steadily from south to north over the entire range of heliographic latitude (-7¿ to +16¿) sampled by spacecraft in 1975--1976. ¿ American Geophysical Union 1989 |