A solar cycle variation of the interplanetary magnetic field

Measurements of the north-south (B_z component of the interplanetary field as compiled by King (1975) when organized into yearly histograms of the values of ¦B_z¦ reveal the following. (1) The histograms decrease exponentially from a maximum occurrence frequency at the value ¦B_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 ¦B_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 B_z and the ordinary indicators of solar activity are roughly correlated. (5) The solar cycle variation of B_zis distinctly different than that of the solar wind speed and that of the geomagnetic Ap disturbance index.Now at the Aerospace Corporation, El Segundo, Calif. 90245, U.S.A.