What Voyager cosmic ray data in the outer heliosphere tells us about 10 Be production in the Earth s polar atmosphere in the recent past

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1 Click Here for Full Article JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 115,, doi: /2009ja014532, 2010 What Voyager cosmic ray data in the outer heliosphere tells us about 10 Be production in the Earth s polar atmosphere in the recent past W. R. Webber 1 and P. R. Higbie 2 Received 8 June 2009; revised 27 October 2009; accepted 17 December 2009; published 11 May [1] Voyager measurements of the galactic proton and Helium nuclei spectra beyond the heliospheric termination shock and out to 110 AU seem to imply lower interstellar cosmic ray intensities of these nuclei than previously estimated. Using new interstellar spectra that are in much better agreement with these Voyager measurements we have calculated the production of 10 Be in the Earth s polar atmosphere. This maximum possible 10 Be production is only 1.47 ± 0.05 times the production occurring at recent times of minimum solar 11 year modulation between 1954 and This implies that the 10 Be concentrations measured in polar ice cores at the times of the recent Spoerer and Maunder minima, which were between times those measured recently at the times of minimum solar modulation, are most likely not solely related to changes in solar heliospheric modulation between these time periods, but other effects such as local and regional climate near the measuring sites may play a significant role in the differences in the relative 10 Be concentration measurements at the two times. Citation: Webber, W. R., and P. R. Higbie (2010), What Voyager cosmic ray data in the outer heliosphere tells us about 10 Be production in the Earth s polar atmosphere in the recent past, J. Geophys. Res., 115,, doi: /2009ja Introduction [2] The concentration changes of the radioactive isotope 10 Be measured in polar ice cores in the historical past have been widely used to determine changes in the 10 Be production rates caused by changes in the solar heliospheric environment changes [e.g., McCracken et al., 2004, and references therein]. One of the first uses of this 10 Be variability was to trace the 11 year cycle of solar activity covering several hundred years [Beer et al., 1990]. These studies have all assumed that the production rate of 10 Be in the Earth s polar atmosphere is directly (linearly) related to the measured concentration [e.g., McCracken et al., 2004] and this assumption is sometimes made even though many studies have indicated the importance of climatic effects on both a short term and long term scale [e.g., Raisbeck et al., 1981; Lal, 1987; Pedro et al., 2006; Field et al., 2006; Heikkila et al., 2008; Field et al., 2009]. [3] 10 Be is principally produced by galactic cosmic rays incident on the Earth s atmosphere. This means that the production of 10 Be, including its latitude dependence, will closely follow the changes in the input function, in this case the galactic cosmic ray intensity. The actual temporal concentration of the 10 Be that is observed at any location may 1 Department of Astronomy, New Mexico State University, Las Cruces, New Mexico, USA. 2 Physics Department, New Mexico State University, Las Cruces, New Mexico, USA. Copyright 2010 by the American Geophysical Union /10/2009JA be different, however, than the temporal production. 10 Be is believed to have a response time of 1 2 years before its eventual precipitation [e.g., Beer et al., 1990], which involves the atmosphere and its circulation in addition to possible local effects depending on how and where the 10 Be is measured. [4] Detailed studies of cosmic rays incident on the Earth s atmosphere have only been possible for the last years and spacecraft observations have been made during the last 40 years. These studies have indeed revealed a solar 11 year cycle of intensity in anticorrelation with solar activity as well as a more subtle 22 year intensity cycle related to the solar magnetic polarity but no long term changes of the 11 year maximum intensity 1 2% have been observed over this time period. We understand that these 11 and 22 year cycles are due to the solar modulation of galactic cosmic rays throughout the heliosphere, a region whose effective modulation extends to at least 120 AU as revealed by Voyager measurements [Stone et al., 2008]. The essential details of this 11 year modulation are that, at times of minimum solar activity, the cosmic ray intensity is a maximum and at times of maximum solar activity it is a minimum, with perhaps a delay 1 year as the solar wind travels out to the heliospheric boundary. These modulation effects are observed throughout the heliosphere, even at the current locations of the Voyager spacecraft now well beyond the heliospheric termination shock located at 90 AU. What we are now beginning to understand from these Voyager measurements is how this maximum intensity, observed at times of minimum modulation, varies with distance from the 1of5

2 Lal [1987], McCracken and McDonald [2001], and Webber and Higbie [2003], among others). [6] Essentially the Voyager spacecraft are now measuring intensities of GCR protons and helium nuclei beyond 110 AU that are well below those to be expected from some of the earlier estimates of the LIS. This has provided the impetus for a new calculation of the LIS of cosmic ray nuclei [Webber and Higbie, 2009]. These new LIS spectra for cosmic ray H and He nuclei are used in this paper along with an updated calculation of 10 Be production in the Earth s atmosphere [Webber and Higbie, 2003] with particular emphasis on this production at times of minimum solar modulation in the Space Age when both accurate production calculations can be made and coincident 10 Be concentration measurements are available. With this comparison as a reference we then determine the maximum 10 Be production resulting from the full LIS spectrum. This limits the maximum measured 10 Be concentration in the recent past that can be directly attributable to production variations without concurrent changes in other conditions, e.g., climatic, Earth s magnetic field, etc. Figure 1. Measurements and calculations of the GCR proton spectra. IS W and L is the local interstellar spectrum of Webber and Lockwood [2001]. LIS is the local interstellar spectrum from Webber and Higbie [2009]. The curves labeled HTS ( heliospheric termination shock) and Earth are the calculated spectrum at the HTS (at 90 AU) and at the Earth (modulation = 300 MV). Measurements at the Earth and at V1 at 65 AU (1998.5) and at 109 AU (2009.0) (well beyond the HTS at 90 AU) are shown (the shaded regions indicate a possible background from lower energy anomalous cosmic rays). The V1 measurements are discussed by Webber et al. [2008]; for the measurements at the Earth see Sanuki et al. [2000]. Sun and approaches the Local Interstellar Spectrum (LIS). The LIS will provide the largest possible production rates of the cosmogenic isotopes such as 10 Be in the Earth s atmosphere. It therefore provides, in a sense, an upper limit to the concentration of these isotopes that would be observed, resulting simply from production changes only. If larger concentrations are observed they must involve other changes of conditions, compared with those existing at times of minimum modulation in the Space Age, where the production parameters can be accurately measured. [5] Previous estimates of the LIS for cosmic rays have varied significantly, so that this upper limit has been uncertain by perhaps ±50% [e.g., Strong and Moskalenko, 1998; Webber and Lockwood, 2001]. This uncertainty has allowed wide speculation in the magnitudes of the changes in 10 Be concentration in the past that can be ascribed to direct solar modulation effects as opposed, for example, to climatic variations (see earlier discussions by 2. A Review of 10 Be Production in the Earth s Atmosphere [7] The production rate P j of 10 Be and other cosmogenic nuclides (including 14 C) at energy E and depth x in the Earth s atmosphere is described by the equation: P j ðe; xþ ¼ X N X i i k Z 1 E k ijk ðeþj k ðe; xþde Where N i is the number of atoms of the target element i per Kg of the atmosphere, s ijk are the cross sections for the production of nuclide j from the target element i by particles of type k with energy E k and J k (E, x) is the total intensity of particles of type k with energy E k at depth x in the atmosphere. The J k (E, x) are calculated starting with the galactic cosmic ray spectra assumed to be protons, helium and heavier nuclei. In this paper we use earlier calculations of P j based on a Monte Carlo production code which includes the latest production cross sections for the various cosmogenic nuclei [Webber and Higbie, 2003; Webber et al., 2007]. The only differences are the new LIS spectra for protons and helium nuclei as follows [Webber and Higbie, 2009]: Protons FLIS ¼ð18:9=T 2:79 Þ=ð1 þ 6:75=T 1:22 þ 1:30=T 2:80 þ 0:0087=T 4:32 Þ Helium FLIS ¼ð0:99=T 2:77 Þ=ð1 þ 4:14=T 1:09 þ 0:65=T 2:79 þ 0:0094=T 4:20 Þ These new LIS spectra for protons and helium nuclei are shown in Figures 1 and 2 along with a IS spectrum used in earlier calculations, as well as various Voyager (V1) measurements at different distances from the Sun and also Earth based measurements which define a reference intensity incident on the top of the Earth s atmosphere in the polar regions at a recent time of minimum modulation. These new LIS spectra have absolute uncertainties estimated at <±5% at 2of5

3 associated with an earlier LIS spectrum and the differential production rate associated with the 1997 solar minimum spectrum at the Earth. [11] In an earlier paper [Webber and Higbie, 2003] we compared the integrated production rates of 10 Be at times of minimum solar modulation (for an assumed = 400 MV) and the 10 Be production rate to be expected from the full LIS spectra (in this case the W and L spectrum) and obtained a factor = 1.72 appropriate to polar latitudes [see Webber and Higbie, 2003, Figure 7 and Table 2]. Now if we compare the integrated production rates obtained from Figure 3 for the new W and H LIS spectra and the minimum modulation spectrum in 1997 we obtain a factor = 1.47 ± This includes the estimated errors in the new primary spectra relative to the spectra measured at the Earth at times of minimum modulation. Other uncertainties in this factor, difficult to quantify, are estimated at no more than ±5%. Figure 2. nuclei. The same as Figure 1 except for GCR helium energies 1 GeV increasing to 10% at energies of a few hundred MeV as noted by Webber and Higbie [2009]. The precision spectra measured at the Earth at the time of minimum modulation in the Space Age [Sanuki et al., 2000] have an absolute uncertainty ±3%. [8] The curves labeled HTS (90 AU) and Earth (300 MV) are calculated from the LIS using a simple spherically symmetric transport model with the diffusion coefficients adjusted to give a modulation potential = 90 MV at the HTS and 300 MV at the Earth [Caballero Lopez and Moraal, 2004]. A correction of the V1 data in 1998 and 2009 that accounts for a lower energy anomalous H and He nuclei contribution is shown as the shaded regions in Figures 1 and 2. [9] We note that the spectrum at the Earth at sunspot min has not changed in these new calculations, just the LIS. But the interpretation of this spectrum at the Earth in terms of the modulation level required to reproduce the LIS spectrum does change; from earlier values which correspond to 400 MV and above, to new values 300 MV, thus indicating a smaller overall modulation in the heliosphere. This results in a smaller increase in the total 10 Be production as the solar modulation decreases from its values 300 MV at the Earth at times of minimum solar modulation in 1954, 1965, 1976, 1987 and 1997, to its value = 0 for the LIS. This results in less 10 Be production at polar latitudes when the full LIS is incident on the Earth than in previous calculations. [10] This new differential production rate of 10 Be is shown in Figure 3 along with the differential production rate 3. Discussion of Results and Implications for 10 Be Production in the Last Few Hundred Years [12] It is well determined that the average measured 10 Be concentration in polar ice cores was larger by a factor at times of the Spoerer and Maunder minima between than it was at the times of the recent solar modulation minima in 1954, 1965, 1976, 1987 and 1997 (when it was observed to be the same within a few percent) [e.g., Beer et al., 1990; Raisbeck et al., 1981; McCracken et al., 2004; Webber et al., 2007]. Over this time period of just a few hundred years, the changes in other parameters, such as the Earth s magnetic field were small and well constrained so that the main cause of these concentration changes would be expected to be changes in the 10 Be production function along with possible changes related to local or larger scale climatic effects on the deposition of 10 Be. [13] A simple indication of the relative importance of production changes and climatic effects is to compare the increase in 10 Be concentration measured at times of minimum modulation during the current space age to the concentration measured at times of the Maunder and Spoerer minima with the concentration increase predicted when the maximum possible 10 Be production resulting from the full LIS cosmic ray spectrum occurs. [14] A recent analysis using this approach and using an earlier LIS cosmic ray spectrum and 10 Be production function concludes that the increase in 10 Be concentration at the times of the Spoerer and Maunder minima can be fully explained within the limits of the LIS cosmic ray spectrum without the need of other effects [see McCracken et al., 2004]. But the overall level of solar modulation required to provide the increased 10 Be production necessary to produce the higher concentration levels observed at the times of the Spoerer and Maunder minimum is only about 10% of the modulation observed at the times of the 11 year sunspot minima in 1954, 1965, 1976, 1987 and This suggests the possibility of a drastically altered heliospheric modulation environment. [15] The results of our new estimate of 10 Be production due to a modified (lower) LIS can be summarized as follows. Using essentially the same calculations and param- 3of5

4 Figure Be production in the polar atmosphere of the Earth as a function of energy for (1) The LIS spectra of Webber and Lockwood [2001]; (2) The LIS of Webber and Higbie [2009]; (3) The modulated spectrum incident on the Earth s polar atmosphere at a time of minimum modulation in eters discussed by McCracken et al. [2004, Tables 1 and 2]. The reference level for the measured 10 Be concentration at the Dye 3 site in the N hemisphere is atoms/gm for the 11 year sunspot minimum periods of 1965 and 1975 [see also Beer et al., 1990]. Using the earlier LIS of Webber and Lockwood [2001] the factor necessary to account for the full LIS incident on the polar atmosphere was 1.72 as noted earlier, which gives a total maximum 10 Be concentration of atoms/gm. The 22 year average maximum measured 10 Be concentration for the Spoerer and Maunder minima time periods at this same site is atoms/gm or 6% less than that calculated for the full LIS as noted by McCracken et al. [2004]. This would imply an average solar modulation of only 68 MV ± 37 MV at those time periods compared with a value 450 MV required to account for the 10 Be concentration observed in 1965 and 1975 according to McCracken et al. [2004]. [16] However, using the factor = 1.47 ± 0.05 from the new LIS reported here leads to a total maximum 10 Be concentration = (1.25 ± 0.03) 10 4 atoms/gm. This concentration is between 6% and 15% less (at a 90% confidence level) than the average concentration maxima observed at the times of the Spoerer or Maunder minima [see McCracken et al., 2004, Figure 3]. This implies, at the confidence levels quoted above, that the concentration levels measured at those times cannot be explained by a simple linear relationship between production and concentration and that other effects (such as regional or local climate) need to be involved to produce the high concentration levels measured at those times. [17] Indeed the role of climatic effects has been increasingly recognized following earlier suggestions by Raisbeck et al. [1981] and Lal [1987]. Pedro et al. [2006], Field et al. [2006], and more recently Heikkila et al. [2008] and Field et al. [2009], have evaluated these climatic effects in increasing detail with Field et al. [2009], particularly noting that the modulation estimates of McCracken et al. [2004], near the Maunder minimum would be significantly modified by climatic effects, in line with our own conclusions above. 4. Summary and Conclusions [18] Using a model for interstellar propagation of cosmic ray protons and helium nuclei that introduces an increasingly larger diffusion coefficient at rigidities <2 3 GVwe have determined new LIS for these nuclei [Webber and Higbie, 2009]. These new diffusion coefficients are based on those determined from a comparison of the polar galactic radio spectrum and the propagated galactic electron spectra [Webber and Higbie, 2008] and so have a basis in direct galactic propagation studies. These new spectra are only slightly larger (and as a result are more consistent) than the proton and Helium spectra now being measured well beyond the HTS by Voyager 1. The new spectra are now also more consistent with the LIS for Carbon and heavier nuclei being used by ACE researchers [Davis et al., 2000; Wiedenbeck et al., 2005] to explain their detailed spectra of heavier nuclei with Z 6 in the sense that both the ACE based LIS and the newer proton and Helium based LIS 4of5

5 imply that the solar modulation at recent times of minimum modulation is only 300 MV, not the value of MV that has been widely used in the past. [19] These lower LIS result in an overall reduction of 10 Be production from the full interstellar proton and Helium spectra of between 15 and 20%. As result the 10 Be concentrations in polar ice cores measured at the times of the recent Spoerer and Maunder minima now exceed those concentrations predicted for the full LIS incident on the polar regions of the Earth by 6 15% assuming a simple linear relationship (1:1 correspondence) between production and concentration as has been used in many earlier publications [e.g., McCracken et al., 2004]. [20] The role of other, as yet unidentified long term effects, but possibly including local and regional climate effects, need to continue to be more precisely evaluated before extended historical 10 Be records can be used as a definitive indicator of changes in the heliospheric modulation of cosmic rays extending to the Maunder minimum and earlier. [21] Acknowledgments. The authors appreciate the support, data availability and discussions with other members of the Voyager CRS team, Ed Stone, P.I., Frank McDonald, Alan Cummings and Bryant Heikkila. [22] Wolfgang Baumjohann thanks the reviewers for their assistance in evaluating this paper. References Beer, J., et al. (1990), Use of 10 Be in polar ice to trace the 11 year cycle of solar activity, Nature, 347, , doi: /347164a0. Caballero Lopez, R. A., and H. Moraal (2004), Limitations of the force field equation to describe cosmic ray modulation, J. Geophys. Res., 109, A01101, doi: /2003ja Davis, A. J., et al. 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