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The subject of our study is the possible relationships between the state of the lower ionosphere and tectonic events on Earth during episodes of high solar activity. There is evidence that during such episodes, electrical interactions between the lower ionosphere (layers D and E), the Earth's surface, and the lithosphere can play an important role in triggering seismic or volcanic events, if the geological objects involved are close to the near-critical state. As examples, we consider three strong earthquakes that occurred during solar cycle 24 (SC24 according to the Zurich classification) and the possibility of their initiation by strong solar flares and related events (sudden ionospheric disturbances, SIDs): 1. The Mexican earthquake (M = 8.1) on September 7–8, 2017; 2. The Chilean earthquake (M = 8.1) of April 1, 2014, and 3. The Japanese earthquake of November 3, 2011. The possibility of using indirect data to study solar flare–ionospheric triggers of large earthquakes that occurred after 1874 A.D. is discussed. This indirect information refers to the areas of sunspot groups, their morphology types, and apparent locations on the solar disk for the corresponding dates. The Chirpan earthquake of April 14, 1928 is given as an example. The inverse piezoelectric effect is one of the most possible physical mechanisms to explain the identified patterns. The possibility of using SID monitor (VLF) data to study these phenomena is briefly discussed. [ABSTRACT FROM AUTHOR]

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). [ABSTRACT FROM AUTHOR]

Abstract—The main subject of the study is an analysis of time series of the width of annual rings of spruce and pine (D) that were obtained during the implementation of a scientific project supported by the Institute of Astronomy of the Bulgarian Academy of Sciences and the Ministry of Agriculture and Food in 2012–2014. The project is a continuation of our previous study (Komitov and Kaftan, 2019). Of the 45 wood samples collected in the project, only four are Pinopsidae: one is spruce (Picea), and the other three are pine (Pinus). The age of the samples is in the range of 65–129 years. We used two methods to obtain statistically significant cycles of fluctuations in the width of annual rings: T-R periodogram algorithm and wavelet analysis. Statistically significant cycles lasting 10–11, 20–22, 25, 38, and 64–68 years were confidently identified. There are solar and geophysical analogs for all of these fluctuations. Statistically significant mutual correlation relationships that take into account the time shift in the range of 0–5 years between D pine samples, temperatures, and precipitation at neighboring meteorological stations were established. There are also secondary extrema of the correlation coefficients after the mutual displacement of time series by 20–25 years. This result seems to be additional evidence of the solar–geophysical origin of the obtained dependencies. [ABSTRACT FROM AUTHOR]

Abstract—This work analyzes a time series of annual growth ring widths for beech trees (Fagus sylvatica) in the western and central Balkans (Stara Planina, Bulgaria) to identify solar-related climate cycles. The effects of solar activity on beech growth during the Dalton Grand Solar Minimum (1793/98–1833) are studied based on samples ≥200 years of age. Two different methods (T–R periodogram analysis and wavelet analysis) are used to identify statistically significant variations with periods of 20–22, ~40, 60–70, and 210–220 years in most of the samples. Two samples are found to have a cycle of about 110 years. Statistically significant correlations between the ring widths and precipitation amounts and the temperatures from data of the nearest weather stations for 1899–2012 confidently manifest themselves in all four central Balkan samples and one western Balkan sample. Based on the results, it is concluded that a period of cooling and increased precipitation has likely begun in the climate of Bulgaria. The process is proceeding slowly, having begun approximately in 2007–2010, but it may noticeably accelerate after the end of solar cycle 25 and during cycles 26 and 27. It is concluded that cycle 26 is likely to be delayed in development. [ABSTRACT FROM AUTHOR]