Cosmogenic nuclides in the atmosphere

The atmosphere and the chemical and physical processes, which occur in it, have profound effect on the terrestrial environment and on human population. In spite of the importance of atmospheric processes there is still a great deal which is unknown about how the atmosphere function. The cosmogenic radionuclides 10Be and 36Cl are unique among tracer species measured in ice core in two important ways:

  1. the production rates of these nuclides are related directly to the galactic cosmic ray (GCR) intensity which is strongly influenced both by solar activity and by the geomagnetic field strength, and
  2.  these nuclides are produced in the stratosphere and upper troposphere. In contrast, other atmospheric trace chemical species, are predominantly produced and influenced by boundary layer processes at their source regions such as continental or ocean surfaces. The concentration of such trace species, in ice cores and in the atmosphere is therefore strongly influenced by variations of wind speed and direction at the source region and by changes in production path. Cosmogenic nuclides, on the other hand, are introduced into the atmosphere in a uniform and predictable way.

We have been involved for a number of years in an effort to simulate cosmogenic nuclide production rates in the extraterrestrial and later terrestrial samples, including those from polar ice cores, to investigate long-term variations in their concentration and to relate these variations to the fundamental processes at work. The concentration of the cosmogenic nuclides 10Be and 36Cl are influenced both by factors external to the atmosphere, such as changes in solar activity or in the galactic cosmic ray flux, and by internal factors, such as changes in the atmospheric circulation, in stratosphere-troposphere exchange processes, in snow accumulation rat, and in the geomagnetic field. Our previous work provided already basic information for the interpretation of experimental results. Questions, however, remain:

We propose here to directly attack these questions and to examine the fundamental geophysics and geochemistry, which influence the cosmogenic radionuclide record in polar ice. The project is intended to provide a detailed model description of cosmogenic nuclides production in the atmosphere and earth's surface. We are proposing to calculate worldwide distribution of 3H, 7Be, 10Be, 14C, and 36Cl. In order to study processes which affect the atmospheric production of cosmogenic nuclides and subsequent global distribution of these nuclides. The comparison with the results of measurements carried out in ice cores at the Lawrence Livermore National Laboratory and ETH accelerator mass spectrometry facilities will provide information invaluable in understanding the production and mixing of cosmogenic radionuclides in the atmosphere, will aid in the interpretation of the very detailed GRIP and GISP-II records and will provide useful information about the special atmospheric circulation processes and stratosphere-troposphere exchange mechanisms.

To simulate in detail the development of the cascade and to calculate the corresponding production rates of cosmogenic isotopes in the atmosphere the GEANT  and MCNP  code systems were applied. These codes use only basic physical quantities and parameters, without including any free parameters, to numerically simulate all processes relevant in particle production and transport. This enables to trace the fate of each individual particle and in doing so to study in detail the effects of various parameters on the production rate such as geomagnetic and solar modulation for a wide range of possible conditions.

Particular contribution expected