Azimuthal modulation of the event rate of cosmic ray extensive air showers by the geomagnet.pdf

1Azimuthal modulation of the event rate of cosmic ray extensive air showers by the geomagnetic field A.A.Ivanov, V.P.Egorova, V.A.Kolosov, A.D.Krasilnikov, M.I.Pravdin, I.Ye.Sleptsov Insitute of Cosmophysical Research and Aeronomy, Yakutsk 677891, Russia a.a.ivanov@sci.yakutia.ru Abstract The Earth’s magnetic field effect on the azimuthal distribution of extensive air showers (EAS) of cosmic rays has been evaluated using a bulk of the Yakutsk array data. The uniform azimuthal distribution of the EAS event rate is rejected at the significance level 10 -14 . Amplitude of the first harmonics of observed distribution depends on zenith angle as A1 ≈ 0.2×sin 2 θ and is almost independent of the primary energy; the phase coincides with the magnetic meridian. Basing upon the value of measured effect, the correction factor has been derived for the particle density depending on a geomagnetic parameter of a shower. The charged particle trajectories of the EAS developing in the atmosphere are distorted in the geomagnetic field. It results in stretched lateral distribution of particles in the plane perpendicular to the shower axis along the direction of the Lorentz force. The observation of this effect at the Yakutsk array in very inclined showers at zenith angles θ 60 o has revealed the significant asymmetry [1] of the lateral distribution function (LDF) of particles due to the deviation of muons proportional to the geomagnetic parameter, g, determined by the angle between the shower axis and the field vector: g = sinχ / cos 2 θ, where χ = arccos(cosθ cosθH + sinθ sinθH cosα), θH=14 0 , α - azimuth. The geomagnetic effect on the lateral distribution of particles was confirmed analyzing the showers in zenith angle range 20 o θ 60 o [2]. The calculation results of the expected effect in the QGS model were used to analyze a shower of the highest energy Eo10 20 eV observed at the Yakutsk array [3]. In [4 a, b] it was shown that the showers initiated by extremely high energy ph