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

The 28th ICRC (ICRC2003, Tsukuba, Japan)Oral session on Solar&Heliospheric Phenomena

(SH3.4; Long-term variations)

Solar modulation effect on the cosmic-ray proton spectra

measured by BESS

Yoshiaki Shikaze (Kobe University) for the BESS Collaboration

Spectrometer

Balloon

To high altitude

BESS spectrometer to be lunched

(Balloon-borne Experiment with Superconducting Magnet Spectrometer)

Motivation of the Observation

Climax Neutron monitor & sunspot number

97 　 98 　 99 　00Solar minimum Solar maximum

Half cycle of the solar modulation

Year

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 5g/cm2 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.

For tune of the calculation using transport equations on the correction of atmospheric secondary protons,

we have measured proton data during ascent (cutoff Rigidity~0.4GV) and descent (cutoff Rigidity~4.2GV).

The observed data below the cutoff is pure atmospheric secondary protons.

Solar Modulation

Atmospheric secondary protons

Observation

BESS Spectrometer and Balloon Observations

Features

1. Large Acceptance of 0.3m2Sr

2. Compact and Simple cylindrical structure

1, 2 ⇒ High statistics & low systematic error

3. Uniform magnetic field of 1T

Proton selection

β-band cut

4. PID by mass measurement

　 Mass = ReZ(β-2 - 1)1/2 　

Tracker (in B=1T) R = pc/Ze

50ps TOF counter 　 dE/dx, β

BESS Spectrometer and Balloon Observations

Fight Trajectory (1993-2000)

Map of Northern Canada

Lynn LakePeace River Ft Mcmurray

Flight to ~1000km west

(Cutoff Rigidity ~0.4GV)

120W 100W

54N

60N

Summary of BESS-2000

Pressure

Altitude

Live time~2.1h

Live time~30.5h

Correction of Atmospheric Secondary Protons

Papini et al. based on transport equations

Use dpmjet3 for proton energy after interaction

Re-tune generation function of recoil protons from air nuclei

modified

Comparison of the re-tuned calculation with observed proton data at different air depths

5.82g/cm2

11.9g/cm2

17.6g/cm2

Recoil proton generation function

Our detectable range

Papini et al.

re-tuned

tuned (Asaoka et al.)

Correction of Atmospheric Secondary ProtonsGrowth curve of proton flux below the kinetic energy of 1GeV

Blue line… recoil tune in Asaoka et el.

Light blue line… recoil re-tuneRed circles…observed proton data

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 10GeV

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.

Results of modulation parameter φ and χ2

Fitting range (GeV/n) : [ x, 20 ]

The data in 2000 is difficult to fit by Force Field approximation up to low energy.

Fitting line for 2000 is lower than data.

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.

The result on the He/p ratio indicates that solar modulation is mainly ruled by rigidity.

Proton flux at about 10GeV/n had good correlation with the Climax neutron intensity.

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.

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