Flux time variation measurements of cosmic-ray helium isotopes with BESS- Polar I

The Balloon-borne Experiment with Superconducting magnet Spectrometer (BESS) instrument in- cludes various particle detectors, such as plastic scintillator counters and a central jet-type drift chamber sur- rounded by a superconducting solenoid magnet. Measurements provided by these detectors allow precise identi- fication of cosmic-ray helium isotopes. The long-duration flight of the BESS-Polar I instrument, about 8.5 days over Antarctica in 2004, allows for the first time to measure time variations of isotope fluxes. The time variation of the helium isotope flux for rigidities from 1.2 GV to 2.5 GV is presented, and the results are discussed and compared with previously reported proton data and neutron monitor data.