Abstract

Solar activity was intense in September 2017 and its effects were observed in different detectorsplaced at the Earth’s surface. Three halo Coronal Mass Ejections (CME) hit the planet and caused magnetic storms. The effects of the CMEs on the flux of galactic cosmic rays at ground levelwere observed by the Tanca detector, which is one of the water-Cherenkov detectors (WCD) thatmake up the Latin American Giant Observatory (LAGO). In this paper we present the detection of Forbush events observed by Tanca during the month of September 2017. This WCD is installedon the campus of the University of Campinas, in Brazil, having three photomultiplier tubes thatdetect Cherenkov photons produced by cosmic radiation in 11400 liters of ultra pure water. Wepresent the description and performance of the experimental apparatus and the observation ondays 6th, 8th and 13th of the Forbush events originated by the CMEs. A decrease in the cosmicrays flux due to a stream inTeraction region was also observed on 14th September. These resultswere compared with observations made by neutron monitors and indices of the Earth’s magneticactivity.