Abstract The second harmonic of the cosmic ray daily variation has been explained in terms of a rising cosmic ray density gradient symmetric about the solar equatorial plane. The amplitude and rigidity dependence of the second harmonic is directly related to that of the solar cycle modulation. Hence studying the second harmonic of 100 GV primary particles provides information on the upper limit to the solar cycle modulation. Underground and surface counter telescope data from London, mainly for the years 1964–1967, is consistent with a modulation model in which the scattering mean free path is proportional to rigidity up to a limit in the range 70–100 GV and which is very large above this limit. Within the errors, observed properties of the interplanetary magnetic field close to 1 a.u. fit this mean free path model, which must apply on average over a region extending more than 1 a.u. outside the Earth's orbit.