1. Long-term stability of Swiss neutron monitors
Rolf Bütikofer and Erwin O. Flückiger University of Bern, Switzerland

2. 1997 peak rate of South Pole NM
Motivation
J.W. Bieber, J. Clem, D. Desilets, P. Evenson, D. Lal, and R. Pyle, “Long-term decline of South Pole neutron rates”, JGR, Vol. 112, A12102, 2007
1997 peak rate of South Pole NM
was ~8% lower than
1965 peak rate

5. Is South Pole NM Count Rate Decrease Instrumental?
Barometer drifts
Changing relative height of NM and Barometer
Detector tube aging
Operating temperature
Electronic and physical background
Environmental change

6. South Pole Neutron Monitor
3-NM64 neutron monitor
Location: 90°S, 0°E
Altitude: 2820 m asl
Cutoff rigidity, Rc: ~0.05 GV
Count rate (1997 epoch): ~106 counts per second per tube

7. Swiss Neutron Monitors
University of Bern, Switzerland operates 3 NMs:
18-IGY NM at Jungfraujoch, 3570 m asl, since Count rate: ~9 cts per seond per tube (1997)
3-NM64 NM at Jungfraujoch, 3475 m asl, since 1986
(Nov – Dec. 1977, Pic du Midi, 2860 m asl) Count rate: ~124 cts per seond per tube (1997)
Special Neutron Monitor at Bern, 570 m asl, since Count rate: ~1 cts per seond per tube (1997)

8. 18-IGY Neutron Monitor Jungfraujoch
Roof of Sphinx building
IGY NM

9. 3-NM64 Neutron Monitor Jungfraujoch
Roof of Research Station

10. Special Neutron Monitor Bern
Roof of Physics Institute, University of Bern

13. Jungfraujoch
IGY
NM64

19. NMs Kiel and Oulu
Kiel 18-NM64, 54 m asl Rc ~2.4 GV Count rate: 6 cts per sec per tube (1997)
Oulu 9-NM64, 0 m asl Rc ~0.85 GV Count rate: 12 cts per sec per tube (1997)

20. Change in peak rate 1965 to 1997 1965 to 2008:
Jungfraujoch IGY: % %
Kiel: % %
Oulu: % %

24. Conclusions
1997 peak rate of South Pole NM was ~8% lower than 1965 peak rate
Peak rate decrease : JJ IGY: ~6%, Kiel: ~3%, Oulu: ~2%
Peak rate decrease : JJ IGY: ~4%, Kiel: ~3%, Oulu (increase): ~1% Corresponding to JJ IGY: -0.08%/a, Kiel: -0.07%/a, Oulu: +0.02%/a
Changes in cutoff rigidities during time period cause only a small change in NM count rates
JJ NM64: -0.15%/a (from comparison with Bern NM)
An observable aging of counter tubes can not be excluded over periods of decades

26. ΔRc = 0.1 GV @ Rc ≈ 4.5 GV ---> ΔN/N ≈ 0.7%

27. ΔRc = 0.1 GV @ Rc ≈ 2.4 GV ---> ΔN/N ≈ 0.3%