Effects of Coronal Hole Morphology and High Speed Solar Wind Streams on Diurnal Variations in Galactic Cosmic Rays

Abstract

Data from the Princess Sirindhorn Neutron Monitor at Doi Inthanon, Thailand, with a vertical cutoff rigidity of 16.8 GV, have been utilized to determine the diurnal anisotropy of Galactic cosmic rays (GCRs) near Earth during solar minimum conditions between November 2007 and November 2010. By investigating the synoptic maps of solar coronal holes and solar wind parameters observed by spacecraft, we found that the intensity and anisotropy of cosmic rays are directly associated with the structure and evolution of the equatorial and higher-latitude coronal holes. For example, a strong diurnal anisotropy was observed after the passage of some, but not all, corotating interaction regions. In some cases, a high-speed solar wind stream (HSSWS) from an equatorial coronal hole can merge with that from a trailing mid- latitude extension of a polar coronal hole. Such a slanted HSSWS structure in space, within which the cosmic ray density is depressed, leads to a local latitudinal gradient, which can account for noticeable events of temporary enhancement or suppression (depending on the magnetic field direction) of the diurnal anisotropy at Doi Inthanon. We conclude that the B £ rn anisotropy associated with coronal hole morphology is a primary source of temporary changes in the GCR diurnal anisotropy under solar minimum conditions.