Abstract

Galactic cosmic rays (GCRs) arriving at Earth, interact with nuclei of atmospheric gases leading to the production of cosmogenic radionuclides. The latter, following a series of processes, are eventually stored in terrestrial archives such as ice cores and tree rings. The information that can be collected, traces back millions of years. Since the solar activity has a large impact on GCRs, via modulation processes in the heliosphere, one can gain knowledge about the solar activity in the past using the cosmogenic radionuclide records. The primary goal of our study is to reconstruct the solar activity in the past combining all the available information from different cosmogenic radionuclides and databases obtained from different locations on Earth. The principle idea is that, using terrestrial archives and existing models of radionuclide production, the GCR flux can be reconstructed over the past millennia. Thereafter, the solar activity, after applying a model correlating the GCR flux to different solar parameters, can be reconstructed. To achieve this it was necessary to develop an empirical model reconstructing the tilt angle of the Heliospheric Current Sheet (HCS), one of the important heliospheric factors influencing cosmic ray propagation inside the heliosphere. The HCS tilt has been traced with observations since 1976. Following, a semi-empirical model of GCR modulation in the heliosphere was developed using measurements and reconstructions of the heliospheric parameters that play a major role in the modulation processes. In particular these are the open solar magnetic flux, the solar magnetic polarity and the HCS tilt angle. With this model the cosmic ray variability on Earth in millennial time scales was reconstructed and used as an input parameter in existing models, calculating the production and deposition of the cosmogenic radionuclides ¹⁴C and ¹⁰Be. The results were then compared with the data aiming to achieve the best fit that will lead to best reconstruction of the solar activity.