Simulation of 14C production rates for the troposphere and stratosphere in weak geomagnetic intensity at 26,000 yr BP 1 Graduate School of Science and Engineering, Yamagata University 2 Departments of Physics, Yamagata University TAIICHI SATO1, HIROHISA SAKURAI 2, YUI TAKAHASHI1 SAORI NAMAI1

Introduction Motivation: In order to investigate periodic variations in 14C concentrations relating to an 11-yr solar cycle at 26,000 yr BP, we have been measuring the 14C concentrations of the single-yr tree rings using AMS. Atmospheric Δ14C variation in the duration of 53000 yr BP Variation of the earth dipole moment in the duration of 80000 yr BP 40% weaker 40% higher The geomagnetic dipole moment is roughly in anti-phase to 14C concentrations. In the vicinity of 26,000 yr BP, the geomagnetic dipole moment was approximately 40% of the present days. For the weak geomagnetic state of the earth, we show simulated results of the 14C production rates.

Process to 14C from cosmic rays Solar modulation of cosmic rays 11-yr cyclic variation~28% The geomagnetic intensity at 26,000 yr BP was 40% Weaker than at the present. Geomagnetic field 14C Atmosphere 14C 14CO2 Global carbon cycle Variation of 14C concentrations ~ 3‰ 14C concentrations ~ ?% Data What is the solar moderation of 14C concentration at weak geomagnetic fields?

The Measurements of the 14C concentrations in single-yr tree rings KAMINOYAMA Tree Rings Profile for 14C concentration of single-yr tree rings at 26000 cal BP approximately 26000 cal BP (Calibration curve : IntCal09) Preliminary data Ring number: 100 Diameter: 30cm Height: 30cm Ring width:1.0mm N38◦ 07’ E140◦ 17’ 14C concentrations are measured by AMS facilities.

Simulation of 14C production rates EXPACS (Excel-based Program for calculation of Atmospheric Cosmic-ray Spectrum) (Sato T et. al., 2008) Process of the simulation Cosmic rays Expacs calculates neutron spectra for anywhere in the atmosphere at the altitudes. Solar moderation (Solar moderation parameter : Φ) Neutron spectrum F (E,h,λ) EXPACS Effect of geomagnetic field (cutoff rigidity : RC) Atmosphere (Altitude : h) Neutron spectrum: F(E,h,λ)[atoms/cm2/s/MeV] P(h): 14C production rates F(E,h,λ) : Neutron energy spectrum at an altitude and an magnetic latitude N(h): Number density of nitrogen atoms /cm3 s(E): 14N(n,p)14C Cross Section Calculating 14C production rates

Geomagnetic cut-off rigidity (N. Lifton, D.F. Smart and M.A. Shea,2008) M*：Geomagnetic dipole moment ratio M＊＝　M/M1950 （ 26000 yr BP : M*=0.4 ） λ ：Dipolar geomagnetic latitude (λ<|±60°|) Geomagnetic latitude (λ) Solar modulation parameter (Ilya G, Usoskin et. al., 2011) We applied for the simulation the solar modulation parameters from1987 to 2009. 11-yr solar modulation The maximum is at 1991 The minimum is at 1997

Comparison between EXPACS and Geant 3 Geomagnetic dipole moment ratio M*=1 Solar modulation parameter Φ = 550 J. Masarik and J. Beer（1999） Average at EXPACS : 2.19 Average at Geant 3 : 2.21 Geomagnetic latitude (λ) The 14C production rate by EXPACS was comparable to that by Geant 3.

The altitude distributions of the 14C production rates 14C kg/yr/km 14C kg/yr/km The peak altitude of the 14C production rates is approximately 12 km. The maxima of the 14C production rates are near the tropopause. (The tropopuse is approximately 13 km.)

The troposphere components of the production rate as a function of geomagnetic intensity are gradually decreasing to the weak M*. The decreasing rate is approximately 2.5% at both the Solar maximum and minimum of the 11-yr cycle. The low energy component of primary cosmic rays increases. The production rate at the stratosphere is relatively abundant compared with at the troposphere.

If the variation at the present is 3‰ The variation of 14C production rates related to the 11-yr solar cycle as function of geomagnetic intensity The S/SM*=1correspond to the M* The M* in vicinity of 26,000 yr BP is approximately 0.4. S/SM*=1 In 26,000 yr BP, the variation of production rates is 1.6 times at the present. It is estimated that the variation of 14C concentrations related to the 11-yr solar cycle in the vicinity of 26,000 yr BP is 4.8 ‰. If the variation at the present is 3‰

Summary We have measured 14C concentrations of the single-yr tree rings using AMS, in order to investigate periodic variations of 14C concentrations relating to an 11-yr solar cycle at 26,000 yr BP. We have globally simulated the 14C production rates as functions of latitude, longitude, and altitude using EXPACS to investigate the affection of weak geomagnetic intensity to the 14C production. Results of simulation The peak altitude of the 14C production rates was approximately 12 km, indicating the maxima at near the tropopause. The decreasing rate of troposphere components is approximately 2.5% at both the Solar maximum and minimum of the 11-yr cycle. It indices the low energy component of primary cosmic rays increases at stratosphere relatively. For the geomagnetic intensity in the vicinity of 26,000 yr BP, the variations of 14C production rates increase to 1.6 times of the variations of present. It is estimated that the variation of 14C concentrations related to the 11-yr solar cycle in the vicinity of 26,000 yr BP was 4.8 ‰ If the variation at the present was 3‰.

EXPACS was developed for the estimation of the atmospheric cosmicray spectrum based on the “PHITS (Particle and Heavy Ion Transport code System) based Analytical Radiation Model in the Atmosphere,” PARMA. It calculates neutron, proton, He nucleus, muon, electron, positron and photon spectra for anywhere in the atmosphere at the altitudes, when we set geomagnetic cut-off rigidity and solar modulation parameter. RADIATION RESEARCH 166, 544–555 (2006), Sato Tatuhko, JAEA RADIATION RESEARCH 170, 244–259 (2008)