Effects of the Heliospheric Current Sheet on Trains of Enhanced Diurnal Variations in Galactic Cosmic Rays

Abstract

We present recurrent trains of the enhanced diurnal anisotropy (DA) of Galactic cosmic rays (GCRs) during 2010 as observed by the Princess Sirindhorn Neutron Monitor at Doi Inthanon with a vertical cutoff rigidity of 16.8 GV. By investigating synoptic charts of the heliospheric current sheet (HCS) and the trains of enhanced DA, we found some consistency between them. During most time periods when recurrent trains of enhanced DA occurred, the DAs were noticeably enhanced more during the away sector than during the toward sector of the interplanetary magnetic field $\mathbf{B}$ . We interpret these temporal features in terms of a $\mathbf{B}\times\nabla n$ gradient anisotropy, which is associated with latitudinal gradients of the GCR density ( $n$ ) derived from the perpendicular component of DA in the solar wind frame. The gradual changes of DA in the trains in turn are associated with the gradual change in the latitudinal gradient of the GCR density along the HCS. We found that the north–south asymmetry (offset) of the HCS is responsible for the asymmetric recurrent modulation of the GCR intensity and suggest that particle drift along the wavy HCS and the adiabatic loss can cause the recurrent modulation, which is related to a local latitudinal gradient in the GCR density. The causes of the latitudinal gradient are discussed in light of current theories and observations.