Speaker
Description
This study reports the detection of a major Forbush Decrease recorded on 20 January 2026 by the Tanca detector. Tanca is a ground-level water-Cherenkov detector located at the University of Campinas and operates as part of the Latin American Giant Observatory. The instrument consists of a polyethylene cylinder containing 11,400 litres of ultra-pure water, equipped with three photomultiplier tubes that record Cherenkov radiation produced by secondary cosmic-ray particles—primarily muons and electromagnetic components. The detector is installed on the university campus in Campinas (latitude 22.82° S, longitude 47.07° W, altitude 650 m above sea level), within the region of the South Atlantic Magnetic Anomaly, where the Earth's magnetic field intensity is significantly reduced. Following a fast Interplanetary Coronal Mass Ejection that triggered a severe G4-class geomagnetic storm, Tanca recorded a peak suppression of 8.88% in the galactic cosmic-ray flux relative to the pre-shock baseline. The decrease was driven by the ICME’s sheath region and magnetic cloud, which efficiently scattered and excluded galactic cosmic rays from the near-Earth environment. The results were compared with data from the Global Neutron Monitor Network. Although both detector types clearly registered the FD, the suppression observed by Tanca was smaller in magnitude than that measured by neutron monitors (NMs). This discrepancy is fundamentally explained by the different energy responses of the two detection techniques: whereas NMs are primarily sensitive to the hadronic component at lower effective primary energies, Tanca’s response is dominated by the muonic component. Consequently, Tanca’s effective primary energy threshold lies in the 30–40 GeV range, higher than that of NMs at comparable geomagnetic cut-off rigidities. These findings highlight the distinctive role of water-Cherenkov detectors in providing complementary observations, thereby broadening the energy coverage of ground-based cosmic-ray monitoring networks.