Production of the cosmogenic isotopes 3 H, 7 Be, 10 Be, and 36 Cl in the Earth s atmosphere by solar and galactic cosmic rays

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 112,, doi: /2007ja012499, 2007 Production of the cosmogenic isotopes 3 H, 7 Be, 10 Be, and 36 Cl in the Earth s atmosphere by solar and galactic cosmic rays W. R. Webber, 1 P. R. Higbie, 2 and K. G. McCracken 3 Received 26 April 2006; revised 7 June 2007; accepted 13 July 2007; published 19 October [1] In a follow-up study to the earlier work of Webber and Higbie (2003) on 10 Be production in the Earth s atmosphere by cosmic rays, we have calculated the atmospheric production of the cosmogenic isotopes 3 H, 7 Be, 10 Be, and 36 Cl using the FLUKA Monte Carlo code. This new calculation of atmospheric yields of these isotopes is based on 10 7 vertically incident protons at each of 24 logarithmically spaced energies from 10 MeV to 10 GeV, 10 2 times the number used in the earlier calculation, along with the latest cross sections. This permits a study of the production due to solar cosmic rays as well as galactic cosmic rays at lower energies where isotope production is a very sensitive function of energy. Solar cosmic ray spectra are reevaluated for all of the major events occurring since In terms of yearly production of 10 Be, only the February 1956 solar event makes a major contribution. For 36 Cl these yearly SCR production contributions are 2 5 times larger depending on the solar cosmic ray energy spectra. We have determined the yearly production of 10 Be, 36 Cl, and other cosmogenic isotopes above 65 geomagnetic latitude for the time period covering six solar 11-year (a) cycles. The average peak-to-peak 11-a amplitude of this yearly production is The effects of latitudinal mixing alter these direct polar production values considerably, giving an average peak-to-peak 11-a amplitude of 1.48 for the global average production. Citation: Webber, W. R., P. R. Higbie, and K. G. McCracken (2007), Production of the cosmogenic isotopes 3 H, 7 Be, 10 Be, and 36 Cl in the Earth s atmosphere by solar and galactic cosmic rays, J. Geophys. Res., 112,, doi: /2007ja Introduction [2] The interaction of cosmic ray protons and heavier nuclei with the Earth s atmosphere produces a cascade of secondary nucleons. These primaries as well as the secondary nucleons produced result in the production of several interesting cosmogenic radionuclides such as 3 H, 7 Be, 10 Be and 36 Cl. The development of accelerator mass spectrometry (AMS) has increased the detection sensitivity for these cosmogenic radionuclides by several orders of magnitude thus allowing the analysis with higher time resolution of the abundance of these nuclides in natural archives such as ice cores. The concentration level of these nuclides is the result of the combination of production and transport in the Earth s atmosphere and disposition on the Earth s surface. The production rate of the cosmogenic nuclides depends primarily on the cosmic ray particle flux at the top of the Earths atmosphere. Time-dependent changes in this production rate are mainly caused by the solar modulation of the galactic cosmic rays which is related 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. 3 Institute of Physical Science and Technology, University of Maryland, College Park, Maryland, USA. Copyright 2007 by the American Geophysical Union /07/2007JA to solar activity and also by variations in the geomagnetic field. [3] In a previous paper [Webber and Higbie, 2003] we considered the production of 10 Be in the Earth s atmosphere by galactic cosmic rays (GCR) including protons, helium and heavier nuclei. A more quantitative exploitation of cosmogenic records and recent solar activity, based on the observed temporal variations of 10 Be concentration particularly in the past few hundred years [e.g., Beer et al., 1990, 1998; Bard et al., 1997; McCracken and McDonald, 2001; McCracken et al., 2004; Usoskin et al., 2004], could benefit from such absolute production calculations. The experimental results over the last few hundred years appear to show a typical solar 11-year (a) cycle as expected from the 11-a modulation of galactic cosmic rays. Superimposed on this 11-a variability are a number of interesting shorterterm effects as well as longer-term effects, such as a high 10 Be concentration during the Maunder minimum and a general decrease in 10 Be concentration by a factor of nearly 2 over the last 100 a or so, from a high level around 1900 to much lower levels at the present time. If these changed concentration levels are interpreted in terms of solar modulation effects on galactic cosmic rays, for example, the highest 10 Be concentration levels observed during the Maunder minimum imply very little solar modulation, in effect the estimated interstellar (IS) cosmic ray spectrum has almost complete access to the Earth around 1900 and during the Maunder minimum in contrast to the situation over the 1of7

2 Table 1. Production of Cosmogenic Isotopes in the Earth s Atmosphere in Particles per Incident Proton Energy, GeV 3 H 7 Be 10 Be 36 Cl E-01 a 2.56E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E-06 a Read 4.2E-01 as last 50 a when even at the 11-a modulation minima, a large reduction of the IS intensity is observed. [4] The effect of 10 Be production by solar cosmic rays (SCR), which have an irregular production cycle nearly out of sync with 11-a modulation cycle of galactic cosmic rays, has also been recently discussed (see summary by Usoskin et al. [2006]). These solar cosmic rays are generally of low rigidity and thus have access to only the polar regions, the same locations at which the 10 Be ice core studies have been made. [5] In this paper we extend our earlier work on 10 Be production by GCR protons and heavier nuclei in the atmosphere to lower energies and include also the production of other cosmogenic isotopes such as 3 H, 7 Be and 36 Cl, all calculated using the same FLUKA Monte Carlo program. In addition we have now included the production of these cosmogenic isotopes by SCR. These results show interesting patterns for SCR production at energies below 1 GeV. the SCR spectra, here assumed to be mainly protons, as an input. [7] We have calculated the production of the various cosmogenic nuclei in the atmosphere using the 2002 version of the FLUKA code [Fasso et al., 2001a, 2001b]. This is a Monte Carlo calculation of the atmospheric production of cosmogenic and other nuclides using production cross sections for these nuclides which are included in the code. Webber and Higbie [2003] discuss the estimated accuracy of the cross sections in the FLUKA program on the basis of (1) neutron latitude surveys at different altitudes and (2) a comparison of the production cross sections in the FLUKA code for 14 N, 16 O and 40 Ar targets with a new cross section program by Webber et al., 2003, which is designed to give a best fit to all available measurements for these targets above 150 MeV/nucleon energies; (3) various newer cross section measurements at lower energies as summarized by Masarik and Beer [1999]. We believe the FLUKA cross sections for production from protons used in this program are accurate to better than ±20% over the entire energy range for the production of the isotopes 7 Be, 10 Be and 36 Cl. The neutron cross sections are more uncertain. Our calculations using vertically incident particles reproduce the observed latitude effects of nucleon production from the equator to the polar plateau measured in neutron monitor surveys at various altitudes to within a few percent [see Webber and Higbie, 2003, Figure 4]. Since our calculations agree well with these observations which are global (omnidirectional) in nature we believe that this confirms our use of vertical cutoff particles to describe the latitude dependence of production. [8] The calculated total atmospheric yields, S(P), of the different cosmogenic isotopes are obtained by injecting 10 7 protons from the vertical (10 2 times the number used in the earlier Webber and Higbie [2003] calculation for 10 Be production by galactic cosmic rays) at each of 24 logarithmic spaced energy intervals from 10 MeV to 10 GeV. These yields per incident proton are shown in Table 1 and also in 2. Calculation of the Cosmogenic Isotope Production Rates [6] The production rate, P j,of 10 Be and other cosmogenic nuclides at energy E and depth x in the atmosphere is described by the equation P j ðe; x Þ ¼ S i N i S k Z w 0 s ijk ðe k Þ J k E k ; x de ð1þ where N i is the number of atoms of the target element i per Kg of the atmosphere, s ijk is the cross section for the production of nuclide j from the target element i by particles of type k with energy E k and J k (E k, x) is the total flux of particles of type k with energy E k at depth x inside the atmosphere. The J k (E, x) are calculated starting with the GCR, assumed to be protons, helium and heavier nuclei and Figure 1. Total production of various cosmogenic isotopes in the atmosphere per incident vertical cosmic ray proton as a function of energy as calculated using the FLUKA Monte Carlo program, 10 7 protons per incident energy (see Table 1). 2of7

3 Figure 2. Differential production of cosmogenic 7 Be (also 36 Cl) and 10 Be nuclei as a function of energy during the October November 2003 series of events. SCR fluence spectra are from Mewaldt et al. [2005b]. Figure 1. The production by helium and heavier nuclei is taken into account as described by Webber and Higbie [2003]. [9] The rapid fall off of the yields below a few hundred MeV means that the total isotope production by SCR is very sensitive to the shape of the SCR energy spectrum. Note that the energy dependence of the yields for 3 H, 7 Be, 10 Be and 36 Cl isotopes above 1 GeV are very similar from the equator to the poles, differing mainly by a multiplicative constant. The average values of this constant above 1 GeV are 5.2 ± 0.3 for the 3 H/ 10 Be ratio and ± for the 36 Cl/ 10 Be ratio. This means that the production calculations of these isotopes in the Earths atmosphere by GCR will show very similar latitude curves. (For example, the latitude effect for 10 Be as shown later in Figure 3, which is a factor of 6.07 from the equator to the pole for a modulation parameter f = 400 MV will be essentially the same for the other cosmogenic isotopes.) [10] The yields for 7 Be and 36 Cl in Figure 1 show an excess around 30 MeV due to resonances for 7 Be and 36 Cl production for proton interactions with 14 N and 40 Ar in the atmosphere. Thus the production of these isotopes will also show similar latitude curves for higher-energy GCR, but will have excess production at low energies due to SCR. [11] The absolute values for our production cross sections generally agree with earlier calculations as summarized by Masarik and Beer [1999] to an accuracy ±10 15% for 3 H, 7 Be and 10 Be. For 36 Cl our atmospheric production is 65% of that summarized by Masarik and Beer [1999]. We believe this difference in 36 Cl production is accounted for by the newer cross sections at higher energies used in this work. Our calculated latitude effects agree within a few percent with those of Masarik and Beer [1999], who find total factors of between the equator and the polar plateau for the production of the different cosmogenic isotopes for f = 400 MV, although their approach to the calculation is quite different. [12] To illustrate the sensitivity of the total isotope production to the shape of the SCR energy spectrum we take as an example a SCR spectrum which fits the time averaged integrated (fluence) spectrum for the October November 2003 events, one of the largest in the present solar activity cycle (sunspot cycle 23 SS23), using data from Mewaldt et al. [2005a]. These SCR intensities are multiplied by the yield functions for 7 Be (similar to 36 Cl) and 10 Be as given in Figure 1. These differential isotopic production functions for 7 Be, 36 Cl and 10 Be are shown as a function of energy in Figure 2. It is seen that the production of 10 Be peaks at 100 MeV whereas the 7 Be and 36 Cl production peaks at 25 MeV for this particular event. The use of steeper SCR spectra than that of the October November 2003 series of events will move these production peaks to lower energies, whereas flatter spectra, such as the recent large 20 January 2005 solar event, will move the peak to higher energies. Note that the total integrated production of both 7 Be and 36 Cl by SCR is enhanced by a factor 3 relative to 10 Be for this event because of the resonance with atmospheric 14 N(or 40 Ar). These aspects will be discussed more fully in the following section where the various SCR events and their spectra and intensities are discussed. [13] Also note that these peak energies of response for SCR cosmogenic isotope production are at 100 MeV and lower which is generally well below those of the cross sections used in earlier calculations of 10 Be production by GCR by Webber and Higbie [2003]. Usoskin et al. [2006] have recently studied the production of 10 Be by SCR using the specific yields given by Webber and Higbie [2003]. These yields only went down to 80 MeV and reference to Figure 2 shows that they may have incurred significant errors particularly in the case of SCR with steep spectra. [14] This SCR production will lie atop the galactic production in the polar regions. In Figure 3 we show a typical geomagnetic latitude dependence of the expected Figure 3. The 10 Be production rate as a function of latitude from GCR for different solar modulation levels. Additional 10 Be production due to SCR for the October November 2003 series of events is shown as a shaded region above 65 for f = 1000 MV. 3of7

4 precipitation could be important in determining the 10 Be concentration that is measured [see Beer et al., 1991] and more comprehensive models of mixing such as described by Field et al. [2006] become important. Figure 4. Yearly average 10 Be production above 65 latitude by GCR from 1940 to 2006 assuming no latitudinal mixing (top curve). Additional production due to SCR from 1956 onward is shown as shaded regions. The estimated production from four SCR events between 1942 and 1949 is shown as banded regions. The bottom curve is the same as the top curve except for yearly global average 10 Be production for complete latitudinal mixing. production rate for 10 Be from GCR for a solar modulation parameter, f = 0 MV (no modulation local interstellar cosmic ray spectrum), f = 400 MV (typical modulation near sunspot minimum) and f = 1000 MV (typical modulation near sunspot maximum in ). Also shown in Figure 3 is the geomagnetic cutoff rigidity as a function of latitude. Most of the SCR contribution will occur below 0.5 GV, at latitudes >65. Thus we show the contribution from the October November 2003 events as a shaded region on top of the GCR contribution for a modulation level = 1000 MV appropriate to that time. Note that the SCR contribution will increase the total production rate of 10 Be in the polar regions for the year 2003 by about 5%. For 7 Be and 36 Cl this SCR contribution is relatively 3 times greater. [15] This simple picture for the production is modified by interlatitudinal mixing so that the actual concentration of 10 Be measured at the surface in the polar caps could be a weighted average of the production rates over a range of geomagnetic latitudes. This is because, following production, a cosmogenic 10 Be atom attaches itself to an aerosol particle. This combination is believed to have a mean residence time in the atmosphere 1 2 a before precipitation in some form to the surface. The exact amount of interlatitudinal mixing is uncertain and the subject of much debate as discussed by McCracken [2004]. For example, if there were complete latitudinal mixing between polar and equatorial latitudes, the latitudinal production values we show in Figure 3 after latitudinal mixing would result in a latitude-independent distribution of 10 Be with the global average reduced by a factor from that calculated for latitudes >65 (see also Masarik and Beer [1999] and Figures 4 and 5, to be discussed later). In addition to this general mixing, local effects as well as the method of 3. Galactic and Solar Cosmic Ray History of 10 Be and 36 Cl Production on the Polar Plateau From 1940 to 2006 [16] To determine the time history of 10 Be or other cosmogenic nuclei production by GCR we have first determined the production rate of this nuclide on the polar plateau as a function solar modulation. This production rate is derived from the work of Webber and Higbie [2003, Figure 7] and also again for 10 Be in Figure 3 of this paper for selected levels of solar modulation. For the yearly average solar modulation level f, we have used the work of Usoskin et al. [2005], which covers the time period from 1951 to 2005 with an extension to times between 1940 and 1950 using the work of McCracken and Heikkila [2003]. The production history of 10 Be from GCR that we calculate at latitudes >65 from 1940 to 2005 is shown in Figure 4. The production history for 36 Cl is shown in Figure 5. [17] For the production by SCR, because the 10 Be and other cosmogenic isotope production is so sensitive to the spectra and also because the individual event spectra themselves are complex and differ greatly from event to event, we have compiled new sets of both integral and differential fluence spectra for all years from 1940 onward and for all major SCR events during this 65-a time period. This recompilation depends heavily on a large number of earlier compilations and references including McDonald [1963], Goswami et al. [1988], Sauer [1993], Shea and Smart [1990, 1991], and Mewaldt et al. [2005b, and references Figure 5. The 36 Cl production above 65 latitude by GCR from 1940 to 2006 assuming no latitudinal mixing (top curve). Additional production due to SCR from 1956 onward is shown as shaded regions. The estimated production from four SCR events between 1942 and 1949 is shown as banded regions. The bottom curve is the same as the top curve except for yearly Global average 36 Cl production for complete latitudinal mixing. 4of7

5 Figure 6. Integral fluence spectra for 13 large SCR events between 1956 and Approximate peak response energies for SCR production by nitrates, 7 Be, 10 Be, and neutrons at sea level (neutron monitor) are shown as shaded regions. Events are as numbered in Table 2. therein] as well as data from GEOS 6 11 in the last two 11-a solar cycles available at In Figure 6 we show the fluences for 13 interesting large events since These fluence spectra are needed to calculate the production of the cosmogenic isotopes. This compilation of fluences does not include all of the largest events as determined by the integral fluence above, e.g., 10 MeV as shown in Figure 7, but we believe it includes all events since 1956 that could produce significant 10 Be or 36 Cl. This new set of yearly average integral fluences above 10, 30 and 100 MeV is shown in Figure 7 along with the GCR yearly fluences >100 MeV. [18] Some significant parameters, including the integral fluence intensities of SCR for these 13 events are also shown in Table 2. Table 2 also includes the estimated peak intensities of the associated ground level events as observed by neutron monitors and the estimated SCR production of 10 Be or 36 Cl in the other 12 events relative to the production in the reference event of October November This SCR production is then added to the production above 65 by GCR on a yearly basis as shown in Figures 4 and 5. This time history represents our best estimate of the total 10 Be and 36 Cl production above 65 latitude from 1940 to the present time. 4. Discussion [19] We will now consider the total 10 Be or 36 Cl production from 1940 to 2005 by both GCR and SCR in the polar regions first without significant latitudinal or local mixing effects (as shown in the top curves of Figures 4 and 5). [20] It is seen that this polar production for GCR ranges from a maximum = 0.052/cm 2 s in 1944 to a minimum of 0.025/cm 2 s in 1990 for 10 Be in Figure 4. For 36 Cl in Figure 5 these maximum and minimum values are cm 2 s and cm 2 s respectively. For 10 Be the maximum production by SCR = 0.017/cm 2 s which occurs in 1956 will produce a secondary maximum = 0.059/cm 2 s in the total production in solar cycle 19 in 1956 and the SCR events in and 1989 will alter the shape of the minima in cycles 19 and 22 but not change the actual minimum year production. The remaining SCR events in our compilation will not greatly alter the GCR production profiles for 10 Be. [21] For 36 Cl production by SCR the relative production is larger and, because of the different SCR spectra, different SCR events will be emphasized. The largest events are in 1959, 1960, 1972 and 1989 where the SCR and GCR yearly contributions are now comparable. These events should be observable in 36 Cl records of sufficient time resolution provided there is little latitudinal mixing and may provide an insight into the degree of latitudinal mixing itself. [22] The range of maximum/minimum yearly 10 Be production by GCR above 65 (no latitudinal mixing) for each solar cycle is: solar cycle 18 = 1.89, 19 = 1.89, 20 = 1.49, 21 = 1.69, 22 = 1.94 and 23 = 1.58 for an average range = These GCR ratios will be the same for 36 Cl. Also of interest are several time periods when the GCR production decreases rapidly over a period of 1 2 a near the beginning of 11-a modulation cycles. These times include , and when the production decreases by 40 50%. These periods should be observable as markers in the concentration level of 10 Be and 36 Cl if they are not obscured by atmospheric effects. [23] In the case of complete latitudinal mixing within a 1 2 a time frame we have calculated the corresponding global average production rates starting from the latitudinal rates as illustrated in Figure 3 for 10 Be [e.g., see also Masarik and Beer, 1999]. These yearly average global production rates are shown as the bottom curves in Figures 4 and 5 for 10 Be and 36 Cl respectively. First of all one observes that these global average rates are 40 45% of the average production rates above 65. However, the 11-a range of maximum/minimum production still is significant, for cycle 18 = 1.55, 19 = 1.55, 20 = 1.32, 21 = 1.42, 22 = 1.58 and 23 = 1.37 for an average range = 1.48 (compare with the ratios in the previous paragraph). Thus the 11-a modulation cycle will still be observable at a reduced amplitude for both 10 Be and 36 Cl. The effects of Figure 7. Yearly estimated fluences of GCR > 100 MeV and of SCR > 10, 30, and 100 MeV from 1956 to 2006 (see also Table 2). 5of7

6 Table 2. Total Fluences and 10 Be Production for 13 Major SCR Events Between 1956 and 2005 Event Solar Cycle Date GLE, a Peak% Relative Production b F > 360 MeV c F > 100MeV c F > 30MeV c F >10MeV c 10 Be 36 Cl 1 23 Feb E + 07 d 3.0E E E Jul ,-,? E E E E Nov , E E E E Aug E E E E Sep E E E E Oct ,25, E E E E May ,10 neg neg 1.0E E E E Jun neg neg 2.0E E E E Jul E E E E Apr neg neg 6.0E E E E Nov E E E E Oct ,35, E E E E Jan E E E E + 08 a GLE is ground level event, peak intensity increase of neutron monitor at sea level on the polar plateau. b Here the 29 October 2003 event 1.0 = 5% of 0.05 cm 2 s 1 for 10 Be, 15% of cm 2 s 1 for 36 Cl. c Fluences are in particles cm 2. d Read 6.0E + 07 as SCR will be greatly diminished by latitudinal mixing, however. For example, for 10 Be the largest event in 1956 produces a 40% increase in production above 65 latitude, but this increases the global average production for that year by 12%. Similarly for 36 Cl the largest SCR event in 1960 produces a 110% increase above 65 latitude, but the increase in the global average is 30%. Thus, in effect, SCR events will be more difficult to detect in the 10 Be and 36 Cl time records if significant latitudinal mixing occurs. [24] Finally we should note that there were four large GLE between 1942 and 1949 for which there are very limited SCR fluence data. For these events there is, however, ion chamber, muon, and neutron monitor data at higher energies in work by Smart and Shea [1991], and for energies 10 MeV there is total nitrate fluence data in work by McCracken et al. [2001]. Both of these studies also include both particle and nitrate data from the February 1956 event and the October 1989 events that can be used as a reference. On the basis of these comparisons we conclude that 10 Be and 36 Cl production in the 19 November 1949 event is the largest of the four events and amounts to 1.5 times the October 1989 series of events. The 25 July 1946 is the next largest = 0.7 times the 19 November 1949 event and the two 1942 events combine to give a 10 Be or 36 Cl production 0.3 times that of the 19 November 1949 event. This ordering in magnitude of these events relative to the 23 February 1956 event is similar to that of Smart and Shea [1991], who use only particle data at higher energies. These events, with a larger estimated uncertainty of ±50%, are shown in Figure 4, but not in Table Summary and Conclusions [25] Using the FLUKA Monte Carlo program [Fasso et al., 2001a, 2001b] we have determined the production at lower energies for a number of cosmogenic isotopes such as 3 H, 7 Be, 10 Be and 36 Cl in the Earths atmosphere as a follow up to our earlier work on 10 Be [Webber and Higbie, 2003]. This production is now obtained over an extended energy range from 10 MeV to >10 GeV thus permitting an evaluation of SCR production at low energies as well as GCR production. For GCR the production of all of the cosmogenic nuclei are very similar above 1 GeV so that the latitude curves for the production of these isotopes are essentially indistinguishable, differing only by multiplicative constants = 5.2 ± 0.3 for the 3 H/ 10 Be ratio and ± for the 36 Cl/ 10 Be ratio. The 10 Be production above 65 = atoms/cm 2 s at a time of minimum solar modulation (f = 400 MV) and would increase to atoms/cm 2 s if the full interstellar cosmic ray spectrum were incident on the Earth. For 36 Cl these values are /cm 2 s and /cm 2 s respectively. The 11-a solar modulation of GCR dominates over SCR production as a source of changes in the production rate for both 10 Be and 36 Cl for the time period from 1939 to 2005 covered in this study although the SCR contribution to 36 Cl production is relatively 2 5 times greater than 10 Be. The average amplitude of the 11-a periodicity in the yearly averaged production rate above 65 for the six 11-a solar cycles for both 10 Be and 36 Cl = 1.77 if latitudinal mixing is small, and = 1.48 if there is complete latitudinal mixing. At least three time periods, , and , have rapid decreases in cosmogenic nuclei production 40 50% in a time period 1 2 a and therefore should be directly observable in the concentration levels of the cosmogenic nuclei if their residence time is 1 a or less. [26] For the SCR production of 10 Be only the event of 23 February 1956 makes a significant contribution to the total 10 Be production on a yearly basis, contributing about 40% of the total production above 65 latitude in For 36 Cl the SCR contribution is generally larger because of a resonance near 25 MeV for production from 40 Ar and the production above 65 latitude is largest in 1960 where the SCR production is 110% of the yearly production by GCR. For complete latitudinal mixing the effects of SCR are almost completely washed out for 10 Be. [27] The different energy sensitivities of the various methods of cosmogenic isotope production along with the SCR energy spectrum itself suggests a hierarchy of responses to SCR events. For events detected by their nitrate production, for example [e.g., McCracken et al., 2001], the peak response appears to be at 10 MeV, for 6of7

7 SCR detection by 7 Be or 36 Cl production in the atmosphere the peak response is MeV and for detection by 10 Be production in the atmosphere the peak response is MeV. Thus SCR events detected by one method may not be seen using another and vice versa. In fact, since the only cases of direct detection of a SCR event to date appear to be as a result of nitrate production [McCracken et al., 2001, and references therein] this appears to be by far the most sensitive indicator of the lower-energy SCR fluences. [28] Acknowledgments. Zuyin Pu thanks the reviewers for their assistance in evaluating this paper. References Bard, E., G. M. Raisbeck, F. Yiou, and J. Jouzel (1997), Solar modulation of cosmogenic nuclide production over the last millennium: Comparison between 14 C and 10 Be records, Earth Planet. Sci. Lett., 150, Beer, J., et al. (1990), Use of 10 Be in the polar ice to trace the 11-year cycle of solar activity, Nature, 347, Beer, J., et al. 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Mursula (2005), Heliospheric modulation of cosmic rays: Monthly reconstruction for , J. Geophys. Res., 110, A12108, doi: / 2005JA Usoskin, I. G., S. K. Solanki, G. A. Kovaltsov, J. Beer, and B. Kromer (2006), Solar proton events in cosmogenic isotope data, Geophys. Res. Lett., 33, L08107, doi: /2006gl Webber, W. R., and P. R. Higbie (2003), Production of cosmogenic Be nuclei in the Earth s atmosphere by cosmic rays: Its dependence on solar modulation and the interstellar cosmic ray spectrum, J. Geophys. Res., 108(A9), 1355, doi: /2003ja P. R. Higbie, Physics Department, New Mexico State University, Las Cruces, NM 88003, USA. K. G. McCracken, Institute of Physical Science and Technology, University of Maryland, College Park, MD 20742, USA. W. R. Webber, Department of Astronomy, New Mexico State University, Las Cruces, NM 88003, USA. (bwebber@nmsu.edu) 7of7