Record Neutron Monitor Ratesfrom Galactic Cosmic Rays

2010 SHINE Workshop

Su Yeon Oh1, John Bieber1, Paul Evenson1, John Clem1, Yu Yi2(syoh@bartol.udel.edu)

1 Bartol Research Institute, Dept. of Physics & Astronomy, University of Delaware2 Dept. of Astronomy & Space Science, Chungnam National University, Korea

ABSTRACT The recent prolonged, deep minimum in solar activity suggests that modulated Galactic cosmic rays might have attained new space-age highs. However reported neutron monitor rates are ambiguous; some record new highs, and others do not. This work examines the record of eight long-running, high-latitude neutron monitors, four in the northern hemisphere (Apatity, Inuvik, Oulu, Thule) and four in the southern hemisphere (Keguelen, McMurdo, Sanae, Terre Adelie).

Most stations display changes in sensitivity, which we characterize by a simple linear trend. After correcting for the change in sensitivity, a consistent picture emerges: All stations considered display new highs at the recent solar minimum, approximately 3% above the previous record high. This work was supported by the BAERI Nuclear R & D Program of MEST/KOSEF, NSF grant OPP-0739620, and by NASA EPSCoR grant NNX09AB05A.

Energetic iron nuclei counted by the CRIS on NASA's ACE spacecraft reveal that cosmic ray levels have jumped 19% above the previous Space Age high.

BACKGROUNDNASA SCIENCE NEWS on Sept. 29, 2009: Cosmic Rays Hit Space Age High

According to sensors on NASA's ACE (Advanced Composition Explorer) spacecraft, galactic cosmic rays have just hit a Space Age high.

Neutron Monitors seem to disagree aboutwhether there was a space age record.

We created a reference data set by averaging8 high-latitude neutron monitors.

The reference suggests there was a new recordAbout 3.2 % above the old record.

Comparing individual neutron monitor with the reference revealsthat many neutron monitors have long-term changes in sensitivity.

Currently we do not understand the source of thesechanges in sensitivity. They are mysteries.

[Examples]

After detrending count rates, a consistent picture emerges.

Conclusion: At the recent solar minimum, Galactic cosmic rays at neutron monitor energies reached a new space age high approximately 3 % above the previous high.

Neutron Monitor StationJump (%) at Bartels Rotation 2406

(2009.11.21)Rc (GV)

Apatity 3.62 0.65Inuvik 3.47 0.18Oulu 2.90 0.81Thule 3.94 0.00

Kerguelen 2.78 1.14McMurdo 3.13 0.01

Sanae 2.56 1.06Terre Adelie Missing data 0.01

This work was supported by the BAERI Nuclear R&D Program of MEST/KOSEF, NSF grant OPP-0739620, and by NASA EPSCoR grant NNX09AB05A.