Interplanetary cosmic ray latitudinal gradient in 1984 to 1987 using IMP 8 and Voyager data

Abstract

We have used the cosmic ray particle measurements for E > 70 MeV from the Voyager 1 and 2 and IMP 8 detectors together with observations of the tilt of the heliospheric current sheet to determine the latitudinal gradient in 1984–1987 by a new method. We find that the cosmic ray intensity decreases with heliographic latitude and the changing latitudinal gradient observed during this time period can be directly attributed to the changes in the tilt of current sheet. For a radial gradient of 2.0%/AU between IMP 8 and Voyager 2 near the heliographic equator, the latitude gradient varies from ∼ −1.5%/deg to ∼ −0.9%/deg as the current sheet tilt decreases, and the latitudinal gradient that would be measured for a tilt of 0° is −1.5%/deg or −3.3%/AU. This is larger in magnitude than the radial gradient of 2.0%/AU existing at this time. The different methods used to determine the latitudinal gradient are discussed. The measured latitudinal gradients are then compared with the predictions of the drift models for cosmic ray propagation and the effects of the latitudinal variation of the solar wind upon cosmic rays. The measured gradients are generally much larger than predicted, especially in the case of the drift models. In all cases there is an important distinction between the measured average 26‐day gradient and the instantaneous or daily average gradient. The 26‐day variations of intensity seen at Voyager 1 during the period 1984–1987 are much smaller and more irregular than the predicted variations due to the wavy current sheet. It appears that neither the effects of drifts nor of the solar wind speed variations as a function of distance from the current sheet can fully explain the features of the latitudinal gradient reported here.