A ''loss cone'' precursor of an approaching shock observed by a cosmic ray muon hodoscope on October 28, 2003

[1] We analyze a loss cone anisotropy observed by a ground-based muon hodoscope at Mt. Norikura in Japan for 7 hours preceding the arrival of an interplanetary shock at Earth on October 28, 2003. Best fitting a model to the observed anisotropy suggests that the loss cone in this event has a rather broad pitch-angle distribution with a half-width about 50° from the IMF. According to numerical simulations of high-energy particle transport across the shock, this implies that the shock is a “quasi-parallel” shock in which the angle between the magnetic field and the shock normal is only 6°. It is also suggested that the lead-time of this precursor is almost independent of the rigidity and about 4 hour at both 30 GV for muon detectors and 10 GV for neutron monitors.