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

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

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 1977 Count 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 Bern Roof of Physics Institute, University of Bern

Jungfraujoch IGY NM64

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)

Change in peak rate 1965 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 @ Rc ≈ 4.5 GV ---> ΔN/N ≈ 0.7%

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