Diurnal anisotropy enhancement due to non local effects of coronal mass ejections

Abstract

In addition to solar modulation according to the ~11-year sunspot cycle and ~22-year solar magnetic cycle, the time profile of the Galactic cosmic ray flux can also exhibit short-term (~2-week) modulation events. These are distinct from Forbush decreases in that they are more symmetric in time and evolve over a time scale much longer than the transit of an interplanetary shock and/or coronal mass ejection (CME). Using data from the Princess Sirindhorn Neutron Monitor at the summit of Doi Inthanon, Thailand, with the world’s highest effective vertical geomagnetic cutoff rigidity for a fixed station (16.7 GV), this present study examined the solar diurnal anisotropy. The results exhibited strong peaks during two short-term modulation events in 2012, which were indeed stronger than the long-term diurnal anisotropy variation from sunspot minimum to maximum. This study attributes these short-term modulation events to non-local effects of CME shocks (possibly single, multiple, or merged) as they propagated beyond Earth and inhibited the access of cosmic rays for ~2 weeks. In addition, the general behavior of cosmic ray anisotropy observed at this high cutoff rigidity is discussed. IMPLICATION OF THE THESIS: This research improves the understanding of the interactions between interplanetary shocks and galactic cosmic rays monitored on Earth. The study also revealed that data from neutron monitors with high cutoff rigidity are particularly interesting since they are less affected by heliospheric disturbances. Further research is needed to investigate similar phenomena in data from other sources in the previous and present solar cycles.