Danjon Effect, Solar Activity, and Volcanism

— Volcanic eruption data available in the Internet archive of the Global Volcanism Program of the National Museum of Natural History of the Smithsonian Institution have been used to obtain a time of series annual numbers of volcanic eruptions occurred from 1551 to 2020. Statistically significant cycles of 10–11, 19–25, ~60, and 200–250 years have been established. For powerful eruptions (VEI ≥ 4), a 33-year cycle has also been revealed. A trend of increased volcanic activity has been found predominantly in the phases of minima and maxima of 11-year solar cycles. Variations in galactic cosmic rays (GCRs), solar flares with coronal mass ejections, and geomagnetic activity appear to be the primary sources of the trigger effect of the heliogeophysical situation on volcanism. These phenomena can affect the climate through volcanic activity. An increase in the rate of aerosol and cloud formation due to GCRs in combination with an increase in volcanic activity leads to a decrease in the transparency of the Earth’s atmosphere in the epochs of solar minima and, as a result, to an increased frequency of “dark” total lunar eclipses (DTLEs) (the Danjon effect).