Long-term stability of Swiss neutron monitors

Rolf Bütikofer and Erwin O. FlückigerUniversity of Bern, Switzerland

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

January 1960 – December 2007

Bieber et al., JGR, 2007

Bieber et al., JGR, 2007

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

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

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)

18-IGY Neutron Monitor Jungfraujoch

Roof of Sphinx building

IGY NM

3-NM64 Neutron Monitor Jungfraujoch

Roof of Research Station

Special Neutron Monitor BernRoof of Physics Institute, University of Bern

IGY

NM64

Jungfraujoch

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)

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%

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

ΔRc = 0.1 GV @ R

c ≈ 4.5 GV ---> ΔN/N ≈ 0.7%

ΔRc = 0.1 GV @ R

c ≈ 2.4 GV ---> ΔN/N ≈ 0.3%