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We analyze the precursor of a Forbush Decrease (FD) observed with the GlobalMuon Detector Network on December 14, 2006. An intense geomagnetic storm isalso recorded during this FD with the peak Kp index of 8+. By using the"two-dimensional map" of the cosmic ray intensity produced after removing thecontribution from the diurnal anisotropy, we succeed in extracting clearsignatures of the precursor. A striking feature of this event is that a weakloss-cone signature is first recorded more than a day prior to the Storm SuddenCommencement (SSC) onset. This suggests that the loss-cone precursor appearedonly 7 hours after the Coronal Mass Ejection (CME) eruption from the Sun, whenthe Interplanetary (IP) shock driven by the Interplanetary Coronal MassEjection (ICME) located at 0.4 AU from the Sun. We find the precursor beingsuccessively observed with multiple detectors in the network according to theEarth's spin and confirmed that the precursor continuously exists in space. Thelong lead time (15.6 hours) of this precursor which is almost twice the typicalvalue indicates that the IMF was more quiet in this event than a typical powerspectrum assumed for the IMF turbulence. The amplitude (-6.45 %) of theloss-cone anisotropy at the SSC onset is more than twice the FD size,indicating that the maximum intensity depression behind the IP shock is muchlarger than the FD size recorded at the Earth in this event. We also find theexcess intensity from the sunward IMF direction clearly observed during ~10hours preceding the SSC onset. It is shown that this excess intensity isconsistent with the measurement of the particles accelerated by the head-oncollisions with the approaching shock. This is the first detailed observationof the precursor due to the shock reflected particles with muon detectors.

Precursors of the Forbush Decrease on December 14, 2006 observed with the Global Muon Detector Network (GMDN)