10 Be concentration in the ice shelf of Queen Maud Land , Antarctica

Introduction Cosmic rays come from the galaxy and beyond. Their intensity has been measured continuously during the past 50 to 60 years, both on Earth and in space. At energies <10 GeV per nucleon (magnetic rigidity P ≈ 10 GV), the cosmic ray intensity varies in response to solar activity. As an example, Fig. 1 shows the counting rate of the SANAE neutron monitor (71°40’S, 2°51’W, lower cutoff rigidity 0.8 GV) from its inception in 1965. The variation, with a dominant 11-year periodicity, is called solar modulation of the galactic cosmic ray intensity. To first order, this variation is in anti-phase with solar activity as measured by the sunspot number, also shown in Fig. 1. Physically, the modulation is due to the heliospheric magnetic field (HMF) and its irregularities that are embedded in the solar wind. The charged cosmic ray particles scatter off these field irregularities, which causes an inward diffusive flux into the heliosphere. The irregularities are carried outward by the solar wind (speed ~ 400 km/s), which also leads to an effective outward cosmic ray convection. When the sun is active, the HMF is more disturbed and stronger, the scattering and outward convection are more effective, and fewer cosmic rays can penetrate into the inner heliosphere, leading to a lower intensity. Cosmic ray modulation therefore provides an indirect means to study aspects of solar activity such as the solar wind, and solar and heliospheric magnetic fields. Solar activity has been studied systematically much further back into the past than the cosmic ray era, by observations of the number of sunspots, as shown in Fig. 2. This firmly establishes the 11-year cycle as a semi-permanent feature, as well as the so-called Grand Minima of solar activity, such as the Maunder (AD 1645–1715), Dalton (AD 1800–1830), and Gleissberg (AD 1880–1910) minima. Sunspots are, however, not directly related to the heliomagnetic processes that cause this modulation, and are therefore not the best indicator to infer past levels of the cosmic ray intensity and the parameters that influence it. When cosmic rays strike the atmosphere, they fragment the nuclei of air molecules, one of the fragmentation products being the isotope Be. This Be settles in polar ice, and its concentration in cores drilled in Antarctica and Greenland during the past 20 years has produced an indirect cosmic ray record that extends over the past 100 000 years. Several aspects of experimental and theoretical Be/cosmic ray studies can be found in CaballeroLopez et al. and McCracken et al. The relationship between the Be concentration in polar ice and the cosmic ray intensity is an inferred one, based on calculations of the Be yield by the nuclear processes in the atmosphere, and its transport to polar regions. This relationship is difficult to measure because in vertically drilled boreholes, the uppermost layers of firn (partially consolidated snow) from the last 50 years or so are not sufficiently compacted to come out as a solid core. This upper part of the borehole must often be supported by metal casing. Percolation of melt-water in the firn causes additional uncertainties. A further problem in the Be/cosmic ray relationship is the uncertainty about how quickly the Be settles. Owing to the spectrometric effects of the geomagnetic field on the charged cosmic ray particles, the Be Physics in South Africa South African Journal of Science 101, May/June 2005 299