"Danjon Effect", Solar-Triggered Volcanic Activity, and Relation to Climate Change

The “Danjon effect” is a phenomenon that presents a tendency to concentrate the so-called “dark” total lunar eclipses (DTLE) near solar sunspot cycle minimum phases. It was a starting point for the present study, whose main subject is a statistical analysis of relationship between solar and volcanic activity for the maximum long time. To this end, the Smithsonian National Museum of Natural History's volcanic activity catalog was used. On its basis, a time series of the total annual volcanic eruptions for the period 1551–2020 AD has been built and explored for cycles of possible solar origin. Cycles with duration of 10–11, 19–25, ∼60, and ∼240 years (all with possible solar origin) has been established. It has also been found that there are two certain peaks of volcanic activity during the sunspot activity cycle: the first one is close to or after the sunspot minimum (sunspot cycle phase 0.9 ≤ Φ ≤1.0 and 0.1 ≤ Φ ≤ 0.2), and the second is wider – close to the sunspot cycle maximum (0.3 ≤ Φ ≤ 0.5). A third maximum is detected about 3–4 years after the sunspot cycle maximum (0.7 ≤ Φ ≤ 0.8) for the “moderate strong” volcanic eruptions with volcanic eruptive index VEI = 5. It corresponds to the geomagnetic activity secondary maximum, which usually occurs 3–4 years after the sunspot maximum. Φ is calculated separately on the basis of each sunspot cycle length. Finally, without any exclusions, all most powerful volcanic eruptions for which VEI ≥ 6 are centered near the ∼11-year Schwabe-Wolf cycle extremes. Trigger mechanisms of solar and geomagnetic activity over volcanic events, as well as their relation to climate change (in interaction with galactic cosmic rays (GCR) and/or solar energetic particles (SEP)), are discussed. The Pinatubo eruption in 1991 as an example of a “pure” strong solar–volcanism relationship has been analyzed in detail.