The 28 th ICRC ICRC 2003 Tsukuba Japan

The 28 th ICRC (ICRC 2003, Tsukuba, Japan) Oral session on Solar&Heliospheric Phenomena (SH 3. 4; Long-term variations) To high altitude Balloon Solar modulation effect on the cosmic-ray proton spectra measured by BESS spectrometer to be lunched Yoshiaki Shikaze (Kobe University) for the BESS Collaboration (Balloon-borne Experiment with Superconducting Magnet Spectrometer) Contents 1. Motivation of the Observation 2. BESS Spectrometer and Balloon Observations 3. Correction of Atmospheric Secondary Protons 4. Proton and Helium Spectra at the Top of Atmosphere from 1997 to 2000 5. Solar modulation effects on our obtained data 6. Summary Spectrometer

Motivation of the Observation Climax Neutron monitor & sunspot number 97 98 Year 99 00 Solar minimum Solar maximum Half cycle of the solar modulation

Motivation of the Observation For low energy protons, the solar modulation effect is large. To understand the solar modulation, it is important to know time variation of low energy proton flux precisely. Our proton data measured at air depth of 5 g/cm 2 consist of primary and secondary components. As the energy decrease, secondary protons increase. Increase of the secondary fraction in observed protons Difficulty in correction of atmospheric secondary protons at low energy region. Solar Modulation Atmospheric secondary protons Observation For tune of the calculation using transport equations on the correction of atmospheric secondary protons, we have measured proton data during The observed data below the cutoff is ascent (cutoff Rigidity~0. 4 GV) and pure atmospheric secondary protons. descent (cutoff Rigidity~4. 2 GV).

BESS Spectrometer and Balloon Observations 4. PID by mass measurement Mass = Re. Z(β-2 - 1)1/2 50 ps TOF counter d. E/dx, β Tracker (in B=1 T) R = pc/Ze Proton selection β-band cut Features 1. Large Acceptance of 0. 3 m 2 Sr 2. Compact and Simple cylindrical structure 1, 2 ⇒ High statistics & low systematic error 3. Uniform magnetic field of 1 T

BESS Spectrometer and Balloon Observations Map of Northern Canada Summary of BESS-2000 Pressure Fight Trajectory (1993 -2000) 60 N Peace River Lynn Lake Ft Mcmurray (Cutoff Rigidity ~0. 4 GV) Live time~2. 1 h 54 N Flight to ~1000 km west 120 W 100 W Live time~30. 5 h Altitude

Correction of Atmospheric Secondary Protons Papini et al. based on transport equations modified Recoil proton generation function Papini et al. Our detectable range Use dpmjet 3 for proton energy after interaction Re-tune generation function of recoil protons from air nuclei Comparison of the re-tuned calculation with observed proton data at different air depths 5. 82 g/cm 2 re-tuned 11. 9 g/cm 2 tuned (Asaoka et al. ) 17. 6 g/cm 2

Correction of Atmospheric Secondary Protons Growth curve of proton flux below the kinetic energy of 1 Ge. V Red circles…observed proton data Blue line… recoil tune in Asaoka et el. Light blue line… recoil re-tune

Proton and Helium Spectra at the Top of the Atmosphere from 1997 to 2000

Solar modulation effects on our obtained data Helium to Proton ratio Correlation of proton flux with Climax data Correlation at about 10 Ge. V Modulation of proton and helium in the same year is ruled by rigidity.

Solar modulation effects on our obtained data Force Field Approximation Interstellar proton flux is assumed by the simple function with 3 parameters. Fitting line for 2000 is lower than data. Results of modulation parameter φ and χ2 Fitting range (Ge. V/n) : [ x, 20 ] The data in 2000 is difficult to fit by Force Field approximation up to low energy.

Summary • Low energy proton and helium spectra from solar minimum to maximum were obtained. • by using the calculation of atmospheric protons revised to agree with the observed secondary protons. • From our spectra obtained precisely to low energy region in which cosmic rays are very sensitive to and largely affected by the solar modulation, the following modulation effects were observed. 1) The result on the He/p ratio indicates that solar modulation is mainly ruled by rigidity. 2) Proton flux at about 10 Ge. V/n had good correlation with the Climax neutron intensity. 3) The fitting result of our proton spectra by the Force Field approximation indicates the difficulty to describe precisely the large modulation effect at the solar maximum in 2000.