In a recent paper by Kudsk et al. in Quaternary Science Reviews, researchers from Denmark including Associate Professors Jesper Olsen and Christoffer Karoff from the Department of Physics and Astronomy, and Mads F. Knudsen from the Department of Geoscience at Aarhus University studied the solar activity between 650 CE and 1900 from Danish oak tree rings. They have uncovered that between 656–707 CE, the activity in the Sun’s 11-year cycle was at its minimum, and proposed to name this unnamed minimum the Horrebow Minimum after Christian Horrebow, an early Danish astronomer who studied the occurrence of sunspots and was the first to propose that they follow a cyclic behaviour. 

There’s a lot going on in the Sun that may affect our weather and climate on scales of centuries or longer, especially the Sun's 11-year cycle. In this cycle, the sun’s radiation level and the number of sunspots (i.e. regions of reduced surface temperature that appear as dark spots) change over 11 years where it reaches a maximum and minimum. This cycle is periodic and has been observed for centuries by changes in the Sun's appearance and by phenomena on Earth such as aurora. Understanding and predicting the solar cycle remains one of the exciting areas of research in astrophysics with major consequences for space science. Moreover, solar activity impacts space- and ground-based technologies as well as the Earth's atmosphere. Therefore, understanding solar activity is an important and relevant area of research. 

So, how do we study solar activity? It turns out that trees are a good source of information to study solar activity! Let’s dive deeper and understand how. 

In the Earth's atmosphere, nuclear reactions with galactic cosmic rays—the majority of which are protons, produce radiocarbon ¹⁴C. The geomagnetic field and solar magnetic activity both influence the flux of cosmic rays. Radiocarbon quickly oxidizes in the atmosphere to generate ¹⁴CO₂ and is assimilated by trees whereby radiocarbon enters the global carbon cycle. The ¹⁴C content of tree rings serves as a record of cosmic ray intensity and solar activity over a few tens of millennia because ¹⁴C is a radioisotope with a half-life of 5,730 years. New high-resolution ¹⁴C records spanning different time intervals exist. However, it is difficult to obtain an overview of what they can tell us about past solar activity, in part because a combined analysis of the records has not been carried out. In this paper, researchers analyse solar variability between 650 CE and 1900 using all available high-resolution ¹⁴C records, including new ¹⁴C data from the Danish oak tree rings covering the time period 1058–1250 CE. During this period, the Sun went through several grand solar minima separated by periods with moderate solar activity. They investigate these grand solar minima as well as changes in the amplitude and cycle length of the Sun's 11-year cycle based on the compilation of high-resolution ¹⁴C records. They also conduct a systematic search for abrupt ¹⁴C changes which may possibly be linked to near-Earth supernova explosions. Their findings provide novel insights and an overview of solar variability over the past 1300 years.

Read the paper.

Other references:

1.https://www.nature.com/articles/ncomms2783