Solar cycle and 27-day variations of the diurnal anisotropy of cosmic rays during the solar cycle 23

Abstract

The diurnal anisotropy of cosmic-ray intensity observed over the period 1997–2006, which coincides with the solar cycle 23, has been analyzed using cosmic ray data from Athens and Oulu neutron monitor stations. In this analysis it was observed that the time of the diurnal variation maximum shifted to earlier hours than the corotation direction from 1997 to 2000, where the polarity state of the magnetic field was positive ( $qA > 0$ ) and to later hours from 2001 to 2006 where the polarity state was negative ( $qA < 0$ ). Moreover the 27-day variation of the cosmic-ray diurnal anisotropy in connection with the 27-day variation of the interplanetary magnetic field has been studied. In this study six groups of ten Bartel rotations each one have been analyzed (2232–2241, 2249–2258, 2275–2284, 2286–2295, 2328–2337, 2356–2365). It is remarkable that the 27-day variation of cosmic- ray intensity is characterized by the well known sector structure and it is well correlated with the $B_{XY}$ component of the interplanetary magnetic field. These findings during the last solar cycle confirm once again the close relation of the diurnal variation of cosmic ray intensity and the interplanetary magnetic field.