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The Sun constantly emits a flow of energetic particles, some of which reach the Earth. The density and energy of this flow forms the basis of space weather, which can interfere with the operation of satellites and other spacecraft. A key unresolved question in the field is how often the Sun emits bursts of energetic particles powerful enough to deactivate or destroy space electronics.
Dendrochronological recording is a promising avenue for determining the rate of such events. This approach is based on the process by which a solar energy particle (SEP) hits the atmosphere, causing a chain reaction that results in the production of a carbon-14 atom. This atom can then be incorporated into the structure of a tree; thus, the concentration of carbon 14 atoms in a tree ring can indicate the impact rate of SEPs in a given year.
To date, three events of extreme MS production are well described in the literature, occurring approximately in the years 660 BCE, 774-775 CE, and 992-993 CE. Each event was roughly an order of magnitude stronger than anything that had been measured in the age of space exploration. Miyake et al. describe such an event, which occurred between 5411 AEC and 5410 AEC. As a result of this explosion, atmospheric carbon-14 increased 0.6% year-over-year in the northern hemisphere and held steady for several years before dropping to typical levels.
The authors deduced the presence of this event using samples taken from trees in three widely dispersed localities: a Bristlecone pine in California, a Scots pine in Finland and a European larch in Switzerland. Each sample had its separate tree rings, and the material in each ring was subjected to accelerator mass spectrometry to determine its carbon-14 content.
Using statistical methods, the researchers identified a pattern of small fluctuations in carbon-14 consistent with the Sun’s 11-year solar cycle; the event recorded in the tree ring occurred during a period of solar maximum. Notably, other evidence suggests that the Sun was also undergoing a period of increasing activity of several decades.
If an extreme MS burst is indeed the cause of the extra carbon-14, then these observations could help predict future events. However, measurements of tree rings cannot rule out other extraterrestrial causes, such as a nearby supernova explosion. Confirmation will require isotopic measurements of beryllium and chlorine from ice cores, according to the authors. (Geophysical research letters, https://doi.org/10.1029/2021GL093419, 2021)
âMorgan Rehnberg, science writer
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