The daily-variation second harmonic and a cosmic-ray intensity gradient perpendicular to the ecliptic

Abstract

Since the spiral interplanetary magnetic field is expected to be much less tightly wound at high solar latitudes, galactic particles arriving along the sun's polar field lines may suffer much less modulation than those arriving in the ecliptic plane. Hence a rising cosmic-ray density gradient is expected away from the ecliptic which will give rise to a second harmonic in the cosmic-ray daily variation with maxima at right angles to the spiral field direction. Two extreme models for the gradient are considered: one for predominantly particle diffusion along the field lines and the other when the diffusion is all perpendicular to the lines. Both models give a first positive power of rigidity dependence to the second harmonic in the 1 to 15 GV range with a double amplitude of about 0.05% at 10 GV. This is in rough accord with experimental results, which also can be shown to favor the parallel diffusion model at higher rigidities.