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 pre-vious 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.

BACKGROUND

NASA 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.

Time in years

1970 1980 1990 2000 2010

Neu

tron

Mon

itor

Cou

nt

Rat

e [M

cMurd

o]

7500

8000

8500

9000

9500

10000

10500

Time in years

1970 1980 1990 2000 2010N

eutr

on M

onitor

Cou

nt

Rat

e [T

hule

]

3400

3600

3800

4000

4200

4400

4600

4800

5000

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

Time in years

1960 1970 1980 1990 2000 2010

Ref

eren

ce (

%)

70

80

90

100

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.

Time in years (zero=Feb 14, 1987)

-20 -10 0 10 20

Rat

io:

McM

urd

o/Ref

eren

ce

0.98

0.99

1.00

1.01

1.02

1.03

McMurdoy = 6.26753e-4*x+1.00233

Time in years (zero=Feb 14, 1987)

-20 -10 0 10 20

Rat

io:

Thule

/Ref

eren

ce

0.97

0.98

0.99

1.00

1.01

1.02

1.03

1.04

Thuley = -8.26273e-4*x+1.00666

[Examples]

Time in years (zero=Feb 14, 1987)

-20 -10 0 10 20

Det

rende

d Rat

io*R

efer

ence

70

75

80

85

90

95

100

105

110Apatity100 %

Time in years (zero=Feb 14, 1987)

-20 -10 0 10 20

Det

rende

d Rat

io*R

efer

ence

70

75

80

85

90

95

100

105

110Inuvik100 %

Time in years (zero=Feb 14, 1987)

-20 -10 0 10 20

Det

rende

d Rat

io*R

efer

ence

70

75

80

85

90

95

100

105

110Oulu100 %

Time in years (zero=Feb 14, 1987)

-20 -10 0 10 20

Det

rende

d Rat

io*R

efer

ence

70

75

80

85

90

95

100

105

110Thule100 %

After detrending count rates, a consistent picture emerges.

Time in years (zero=Feb 14, 1987)

-20 -10 0 10 20

Det

rende

d Rat

io*R

efer

ence

70

75

80

85

90

95

100

105

110Kerguelen100 %

Time in years (zero=Feb 14, 1987)

-20 -10 0 10 20

Det

rende

d Rat

io*R

efer

ence

70

75

80

85

90

95

100

105

110McMurdo100 %

Time in years (zero=Feb 14, 1987)

-20 -10 0 10 20

Det

rende

d Rat

io*R

efer

ence

70

75

80

85

90

95

100

105

110Sanae100 %

Time in years (zero=Feb 14, 1987)

-20 -10 0 10 20

Det

rende

d Rat

io*R

efer

ence

70

75

80

85

90

95

100

105

110Terre Adelie100 %

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 0.65

Inuvik 3.47 0.18

Oulu 2.90 0.81

Thule 3.94 0.00

Kerguelen 2.78 1.14

McMurdo 3.13 0.01

Sanae 2.56 1.06

Terre 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.