Abstract:Radiocarbon 14 C is a cosmogenic isotope, which is most extensively used by scientists from a wide variety of fields. Its rate of generation in the atmosphere depends on solar modulation and thus, studying 14 C concentration in natural archives, one can reconstruct solar activity level in the past. The paper shows results of box-model calculations of generation of the 14 C isotope in the atmosphere and its relative abundance during the time interval 1389-1800 AD, taking into account influence of changing climate. This interval includes the deep minimum of solar activity and period of significant change in atmospheric concentration of CO 2 and global temperature. The performed analysis showed that concentration of 14 C in the atmosphere reflects not only variations of the galactic cosmic rays intensity but as well changes of temperature and atmospheric CO 2 concentration. It is shown that the decrease in CO 2 concentration in the atmosphere during 1550-1600 can be connected with absorption of CO 2 by the ocean surface layer. Thus, taking into account the climatic changes is an important condition for the reconstruction of solar activity in the past using data based on cosmogenic isotopes.

Sub-second time structure of hard X-rays observed during solar flares by spectrometer "IRIS" on a board of CORONAS satellite are discussed. The burst duration of 80ms and FWHM about 30ms are recorded in powerful flare of

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We analyse the long lived radionuclide data (14C, 10Be and 36Cl) in tree rings and Greenland ice cores referred to the 660 BC event. The hypotheses of solar superflare impact on the atmosphere and Solar system collision with small sized dense interstellar cloud are considered. Decisive role in clarifying the situation is the experimental data on other isotope content available in ice for the periods under discussion. The data on 10Be and 36Cl (GRIP and NGRIP stations in Greenland) during the 660 BC event favour the second hypothesis. Various assumptions on the relationship between isotope production and deposition rates in the atmosphere are considered.