Long-term stability of Swiss neutron monitors Rolf Bütikofer and Erwin O. Flückiger University of...
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Long-term stability of Swiss neutron monitors
Rolf Bütikofer and Erwin O. FlückigerUniversity of Bern, Switzerland
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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
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January 1960 – December 2007
Bieber et al., JGR, 2007
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Bieber et al., JGR, 2007
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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
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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
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Swiss Neutron Monitors
University of Bern, Switzerland operates 3 NMs:• 18-IGY NM at Jungfraujoch, 3570 m asl, since 1958.
Count rate: ~9 cts per seond per tube (1997)
• 3-NM64 NM at Jungfraujoch, 3475 m asl, since 1986 (Nov. 1964 – 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 1977Count rate: ~1 cts per seond per tube (1997)
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18-IGY Neutron Monitor Jungfraujoch
Roof of Sphinx building
IGY NM
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3-NM64 Neutron Monitor Jungfraujoch
Roof of Research Station
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Special Neutron Monitor BernRoof of Physics Institute, University of Bern
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IGY
NM64
Jungfraujoch
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NMs Kiel and Oulu
• Kiel 18-NM64, 54 m aslRc ~2.4 GVCount rate: 6 cts per sec per tube (1997)
• Oulu 9-NM64, 0 m aslRc ~0.85 GVCount rate: 12 cts per sec per tube (1997)
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Change in peak rate1965 to 1997 1965 to 2008:
Jungfraujoch IGY: -5.9% -3.6%Kiel: -2.9% -2.8%Oulu: -1.5% +0.9%
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Conclusions• 1997 peak rate of South Pole NM was ~8% lower
than 1965 peak rate• Peak rate decrease 1965-1997:
JJ IGY: ~6%, Kiel: ~3%, Oulu: ~2%• Peak rate decrease1965-2008:
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 1958-2008 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
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ΔRc = 0.1 GV @ R
c ≈ 4.5 GV ---> ΔN/N ≈ 0.7%
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ΔRc = 0.1 GV @ R
c ≈ 2.4 GV ---> ΔN/N ≈ 0.3%
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