Observation of solar-terrestrial variation of the cosmic muon flux

A high statistics measurement of the cosmic muon flux has been made in search for the solar-terrestrial phenomena during a shutdown period of our search for the rare neutral kaon decay at KEK-PS (E391a). A barrel-shape photon detector composed of the stacked lead-plastic scintillator layers was utilized. In the period of our observation, a large coronal mass ejection event has occurred several times, and a disturbance of terrestrial magnetic field resulted in a measurable deficit of the muon flux, i.e. the Forbush decrease. A change in the counting rate of cosmic rays is proportionally related to the change in atmospheric pressure. After correcting for the observed ratio of -0.08%/hPa, and applying a normalization with the trailing average of the preceding counting rates, the muon deficit was detected almost in coincidence with the geomagnetic disturbance. Its information, the time and the strength of magnetic storm expressed in Kp index, was obtained from the GeoForschungs Zentrum.