Solar modulation affe
cts the se
condary
cosmi
c rays responsible for in situ
cosmogeni
c nu
clide (CN) produ
ction the most at the high geomagneti
c latitudes to whi
ch CN produ
ction rates are traditionally referen
ced. While this has long been re
cognized (e.g., D. Lal, B.
Peters, Cosmi
c ray produ
ced radioa
ctivity on the Earth, in: K. Sitte (Ed.), Handbu
ch Der Physik XLVI/2, Springer-Verlag, Berlin, 1967, pp. 551–612 and D. Lal, Theoreti
cally expe
cted variations in the terrestrial
cosmi
c ray produ
ction rates of isotopes, in: G.C. Castagnoli (Ed.), Pro
ceedings of the Enri
co Fermi International S
chool of Physi
cs 95, Italian Physi
cal So
ciety, Varenna 1988, pp. 216–233), these variations
can lead to potentially signifi
cant s
caling model un
certainties that have not been addressed in detail. These un
certainties in
clude the long-term (millennial-s
cale) average solar modulation level to whi
ch se
condary
cosmi
c rays should be referen
ced, and short-term flu
ctuations in
cosmi
c ray intensity measurements used to derive published se
condary
cosmi
c ray s
caling models. We have developed new s
caling models for spallogeni
c nu
cleons, slow-muon
capture and fast-muon intera
ctions that spe
cifi
cally address these un
certainties. Our spallogeni
c nu
cleon s
caling model, whi
ch in
cludes data from portions of 5 solar
cy
cles, expli
citly in
corporates a measure of solar modulation (
S), and our fast- and slow-muon s
caling models (based on more limited data) a
ccount for solar modulation effe
cts through in
creased un
certainties. These models improve on previously published models by better sampling the observed variability in measured
cosmi
c ray intensities as a fun
ction of geomagneti
c latitude, altitude, and solar a
ctivity. Furthermore, pla
cing the spallogeni
c nu
cleon data in a
consistent time-spa
ce framework allows for a more realisti
c assessment of un
certainties in our model than in earlier ones.
We demonstrate here that our models reasonably account for the effects of solar modulation on measured secondary cosmic ray intensities, within the uncertainties of our combined source datasets. We also estimate solar modulation variations over the last 11.4 ka from a recent physics-based sunspot number reconstruction derived from tree-ring 14C data. This approximation suggests that spallogenic nucleon scaling factors in our model for sea level and high geomagnetic latitudes can differ by up to
c=""http://www.sciencedirect.com/scidirimg/entities/223c.gif"" alt=""not, vert, similar"" border=0> 10 % , depending on the time step over which the model sunspot numbers are averaged. The potential magnitude of this difference supports our contention that incorporating long-term solar modulation into secondary cosmic ray scaling is important. Although millennial-scale solar modulation clearly requires further study, we believe it is reasonable at present to use our S value record for scaling spallogenic nucleons during the last 11.4 ka, and the weighted mean S value for that period of 0.950 for longer exposure times. By accounting for solar modulation effects on the global variations in nucleon and muon fluxes, these models thus provide a useful framework on which to base CN production rate scaling functions.