A solar cycle variation of the interplanetary magnetic field

Measurements of the north-south (Bsub(z)) component of the interplanetary field as compiled by King (1975) when organized into yearly histograms of the values of mod(Bsub(z)) reveal the following. (1) The histograms decrease exponentially from a maximum occurrence frequency at the value mod(Bsub(z))=0. (2) The slope of the exponential on a semi-log plot varies systematically roughly in phase with the sunspot number in such a way that the probability of large values of mod(Bsub(z)) is much greater in the years near sunspot maximum than in the years near sunspot minimum. (3) There is a sparsely populated high-value tail, for which the data are too meager to discern any solar cycle variation. The high-value tail is perhaps associated with travelling interplanetary disturbances. (4) The solar cycle variations of Bsub(z) and the ordinary indicators of solar activity are roughly correlated. (5) The solar cycle variation of Bsub(z) is distinctly different than that of the solar wind speed and that of the geomagnetic Ap disturbance index. (Auth.)