Azimuthal modulation of the event rate of cosmic-ray extensive air showers by the geomagnetic field

The effect of the Earth’s magnetic field on the azimuthal distribution of the number of extensive air showers is measured using data obtained from many years of observations at the Yakutsk array. A uniform azimuthal distribution of the event rates of these showers is rejected at a 10−14 significance level. The amplitude of the harmonic of the distribution depends on the zenith angle as A1≈0.2 sin2θ and is virtually independent of the primary-particle energy, while the phase coincides with the magnetic meridian. From the magnitude of the measured effect a correction factor for the particle density is determined which depends on the geomagnetic parameter of the shower.