Solar System Collisions with Dense Interstellar Gas Clouds and Radiocarbon Traces of Periods with “Abnormally Low” Solar Modulation of Cosmic Rays

Abstract

The causes of a sharp increase in the radiocarbon content in the Earth’s atmosphere for periods with abnormally low solar modulation of galactic cosmic rays (GCRs) are considered. It is shown that such periods around 660 BC and 5480 BC can also be caused (in addition to the low activity of the Sun itself) by heliospheric compression as a result of the passage of the Solar System through a compact interstellar cloud with a density of n_H ~ 100 cm^–3 and characteristic dimensions of 10–100 AU. In this case, GCR modulation is suppressed, while solar cyclic processes are preserved. Wavelet analysis of radiocarbon data shows the presence of the 11-year solar-activity cycle in the period around 5480 BC. Based on the model of the interaction of cosmic rays with the Earth’s atmosphere, the ¹⁴C production rates are calculated for various GCR model spectra typical for the local interstellar medium.