Precursor signatures of the storm sudden commencement in 2008

Plasma structures ejected from solar eruptions travel in interplanetary space with speed of hundreds of kilometers per second and may hit the Earth. Signatures preceding the arrival of such plasma structures can be detected in the high-energy cosmic ray intensity observed with four multidirectional muon telescopes in the Global Muon Detector Network (GMDN) on the Earth. A typical signature is a "loss cone" effect which is observed as a systematic intensity decrease of cosmic rays with small pitch angles measured from the sunward interplanetary magnetic field direction. Corrections for the atmospheric pressure and temperature effects are applied to the muon data before examining the pitch angle distributions in the two-days-period preceding a moderate geomagnetic storms observed in November 24th 2008. The contribution of the first order anisotropy, which is not of primary interest when searching for the "loss cone" effect, is also subtracted from the observed intensity.