SECOND HARMONIC OF COSMIC RAY DAILY VARIATION

Since on the Parker model for the quiet interplanetary magnetic field the spirals are less tightly wound at high solar latitudes, cosmic rays arriving along the Sun's polar field lines may suffer less modulation than those arriving in the ecliptic plane. Hence a rising cosmic ray density gradient is expected away from the solar equatorial plane which gives rise to a second harmonic variation with maxima at right angles to the spiral field direction. A model for the density gradient is considered in which particle diffusion is important only along the field direction and the resulting second harmonic varies as the first positive power of magnetic rigidity in the range 1 → 15 GV. This is shown to be in reasonable accord with experimental results over the solar cycle from 1957 to 1964. © 1968.