The Sun is active and its activity may take different forms. Most energetic are sporadic solar eruptive events, such as flares or coronal mass ejections, that lead to bulk acceleration of solar plasma to high energies. In some conditions it may produce strong solar energetic particle events, known also as solar particle storms. About 70 such solar particle storms (called ground-level enhancements, GLEs) have been recorded by ground-based detectors during the last 75 years, with the strongest observed event being GLE #5 on 23-Feb-1956 with the peak intensity of 5000% above the background caused by galactic cosmic rays. However, the data from the space era does not allow one to assess the possibility of such or stronger events to occur. In other words, the worst-case scenario or the strongest possible solar particle storm and the probability of its occurrence is unclear, but it is needed for many purposes. Such assessments can be only made with the help of indirect proxies, viz. cosmogenic radio-isotopes in terrestrial or lunar natural archives, that cover centuries, millennia, up to millions of years. Here we summarize the present state of the art and suggest that the historical solar particle storm of 775 AD can conservatively serve as a worst-case storm on the time scale of up to a million years. We also discuss possible terrestrial effects of such an extreme event.
Mode of presentation: oral (Need to be confirmed by the SOC)