The sun is an important energy source for the earth. Although human observation of solar activities (sunspots) has only a history of 400 years, cosmogenic nuclides produced by cosmic rays will be recorded in tree rings or ice cores, which can be used as an important clue to trace the changes of solar activities thousands of years ago. 14 C is a radioactive isotope of carbon, which is formed by cosmic rays bombarding nitrogen in the upper atmosphere. These 14 C were oxidized in the atmosphere to form CO 14 2, which was absorbed by trees through photosynthesis and recorded in the annual rings formed that year, and was no longer exchanged with the outside world. Therefore, the abundance change of 14 C in tree rings can restore the change of 14 C in ancient atmosphere, and then record the cosmic ray intensity and solar activity. Tree rings have the characteristics of accurate dating, high resolution and good continuity, and have the potential to reconstruct the interannual and even seasonal solar activity changes (Uusitalo et al., 20 18). In addition, high-resolution historical documents, such as sunspot activity history and ancient Korean aurora chronology (SILSO WDC, 2019; ; , Wan et al., 2020), has the characteristics of accurate dating and high time resolution, and can also be used to study solar activities.

By measuring the annual resolution of tree rings, Japanese scholar Miyake once found a significant increase in the abundance of 14 C in 774-775 and 992-993, revealing two strong cosmic ray events (Miyake et al., 20 12, 20 13). The occurrence of these two events is most likely related to solar high-energy particle events (Mekhaldi et al., 2015; See the frontier report "Strong Solar Activity in 774-775 AD"). However, the above research only uses the tree ring 14 C record of the fragment. At present, there is still a lack of records of tree ring 14 C with annual resolution and continuous Millennium.

Brehm team of Swiss Federal Institute of Technology Zurich recently published the longest, continuous and annual resolution tree ring 14 C record in Nature Geoscience. Tree-ring 14 C recorded three rapid rises of 14 C since 2000 (figure 1a), and the short-term rapid rise of 14 C in the atmosphere was related to solar high-energy particle events. The first time happened in 993 AD. This solar high-energy particle event caused the output of 14 C to increase nearly three times in one year. The second significant increase of 14 C occurred in AD 1052, which can be verified by the model in the previously published data set of 14 C (Figure 2a). However, the model did not detect the third significant increase in the abundance of 14 C that occurred in 1279, which may be related to the accuracy of the selected data set (Figure 2b). Even so, the discovery of these two suspected events shows that solar high-energy particle events may be far more frequent than we thought. It is worth noting that three solar high-energy particle events occurred before 990- 1290, but not in the last 700 years. The events of 1052 and 1279 all occurred in the minimum period of solar activity.

Fig. 1 969- 1933 14 C and solar regulation (Brehm etal ., 202 1).

Fig. 2 Two new suspected 14 C abundance increase events discovered in the past Millennium (Brehm et al., 202 1).

The solar activity has a period of 1 1 year, and the change of14 C caused by the above period will be recorded in the tree ring, but this study is very challenging, because the change of114 C caused by the periodic solar activity is only 2‰,/kloc-0. Brehm et al. conducted band-pass filtering analysis on 14 C and sunspot records and calculated the distance between peaks, and set the signal with amplitude less than 1.2‰ as noise (Figure 3). The results show that the period with small amplitude may be caused by noise. These noises lead to smaller amplitude and shorter period of solar activity in the minimum period. This means that the possible influence of noise should be considered when observing solar activities with the smallest period or amplitude in the future.

Fig. 3 spectrum analysis of band-pass filtering 14 C records (Brehm et al., 202 1). The blue and orange crosses in Figure B indicate the amplitude >; Peak-to-peak distance 1.2‰, and dark blue histogram indicates amplitude >; 1.2‰ period length, and black represents the distribution of the period length of sunspot number.

In order to further evaluate the change of period and amplitude caused by noise in 14 C records, the author compares the period and amplitude of 14 C after band-pass filtering with the randomly generated test data (Figure 4). The results show that when a sinusoidal signal with an amplitude of 0.8‰ and a period of 10.4 years is selected as the simulation data, the distribution of its period and amplitude matches the measured records best. Therefore, the solar cycle of 1 1 year is recorded in 14 C record, and its average amplitude is 0.8‰, which is lower than the previously reported amplitude (2‰). This amplitude is not constant, but depends on the intensity of solar activity. In the minimum period of solar activity, the amplitude is 0.6‰ and in the maximum period of solar activity, the amplitude is about 0.9‰.

The significance of this study is to discover new solar high-energy particle events by using high-resolution tree-ring 14 C records, reconstruct the long-term trend and interdecadal changes of solar changes, and reveal the laws of solar activities and the 14 C changes caused by them. At present, more and more studies reveal the changing law of solar activity by using high-resolution tree ring 14 C records.

Fig. 4 amplitude and period distribution (Brehmet et al., 202 1). Orange represents band-pass filtered 14 C records, and blue represents 10.4yr periodic signals and analog data with different amplitudes.

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