A string of seven optical detectors deployed from a ship was used to detect the Cherenkov light from muons at ocean depths ranging from 2000 to 4000 m in intervals of -500 m. The flux and angular distributions of cosmic-ray muons were measured. An effective area for fivefold coincidences of 420 mZ for downward-going muons was achieved. The results are consistent with those derived from underground observations and theoretical calculations. The measured vertical intensity ranges from (9.8416.5 ) X lo-' cm-2 s-' srp' at 2090 m of water equivalent (mwe) to (4.57kl. 37)X l o p 9 cm-*s-' sr-' at 4157 mwe. String Bollom Conlroller FIG. 1. Mechanical configuration of the short prototype string. Optical module spacing is 5.18 m.

The 800-ton cosmic-ray spectrograph (MUTRON) has been used to measure the sea-level energy spectrum of cosmic-ray muons arriving from 86" to 90" zenith angles in the momentum region of 100-20000 GeV/c. The measured muon energy spectrum can be interpreted by using a cosmic-ray primary spectrum of (1.80 ~m -~s -' s r -' G e~-' )~-~~~~d~ (E in GeV) and a scaling model incorporating an increasing interaction cross section for meson production in hadron-hadron interaction. The muon charge ratio at energies up to 15 TeV in the same zenith-angle range has been measured. It shows a small enhancement with increasing energy. By combining both results we may conclude that the cosmic-ray primary particle composition stays the same up to about 100 TeV as that obtained by direct measurements in the energy range below 1 TeV.

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The electric fields associated with thunderclouds change the intensity of secondary cosmic rays observed on the ground. This effect has been investigated using several detectors located at the Mount Norikura Cosmic Ray Observatory where excesses of 1% and more of the average counting rate may be observed when the Observatory is covered by thunderclouds. A frequency analysis of the time series of days with such excesses for the period 26 October 1990 to 15 January 2002 shows the expected summer maximum in the rate of occurrence and, more surprisingly, a 26-day variation. An electric field mill was installed to help determine the relationship between the intensity variations and the strength and direction of the field near the detector system: the excess is usually observed when a negative electric field ͑accelerating negative charges downward͒ greater than 10 kV/m is present in the atmosphere above the observatory. Based on Monte Carlo simulations we predict that excess counting rates measured without charge discrimination will be expected as a consequence of the excess of positive muons among the secondary cosmic rays.

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.