If you are trying to advance your knowledge about how space works, you may want to put your hiking boots on and head to the woods. Tree rings can reveal the quantity of a certain type of carbon that was present in the atmosphere during a particular year, which, in turn, indicates how active or inactive the Sun was then.
Historical materials, in the meantime, can provide vital clues about what’s been happening on Earth. For example, by digging through the archival record of when cherry flowers began to blossom in Kyoto in each of the past 1,200 years, you can determine the region’s warming and cooling trends over those centuries.
When you put all these resources together, there emerges a clearer picture of how solar activity affects Earth and the lives of people living on it. In this installment of the Science Report, one of the scientists who have made successful use of this holistic research approach will discuss an important discovery she and her team have made thus far, as well as the next goal they are aiming to achieve.

Tracing Sun’s Activity in 2,000-year-old Yakusugi Cedar Stump

Yakushima, a small Japanese island off the coast of the country’s southernmost region, is known for a pristine beach where sea turtles come ashore to lay eggs. Designed as a UNESCO World Heritage site, the island is also home to the “yakusugi,” Japanese cedar trees specifically named as such to indicate they are all over 1,000 years old and grow naturally on Yakushima, typically at elevations higher than 500 meters above sea level. It was in this deepest part of Yakushima’s subtropical forest where Prof. Miyahara and her team began their new research project about two decades ago to identify patterns in solar activity.

The amount of carbon-14 in the atmosphere changes, depending on solar activity. As the presence of carbon-14 in the atmosphere increases, so does the quantity of the radiocarbon in a tree ring from that year. Thus, the yakusugi stump demonstrated to Prof. Miyahara’s group how intense or weak solar activity was in the corresponding periods.

“Tree rings can reveal how much influence solar activity has on the temperature and the amount of rainfall, as well,” Prof. Miyahara said.

In 2008, the group successfully reconstructed the solar activity from about 1,100 years earlier. Based on that, Prof. Miyahara said, they determined the Sun was very active then, which may have contributed to the so-called  Medieval Climate Optimum, also known as the Medieval Warm Period, a period of warmer than usual climate, occurring from the 10th through 14th Century.

As the next step in their research, Prof. Miyahara and her team decided to look at the 27-day solar cycle. They began compiling lightning events recorded in the “Hirosaki Hancho Nikki,” a diary kept by the Hirosaki feudal clan over generations, and “Ishikawa Nikki,” another Edo Period diary. In 2018, Team Miyahara announced their findings that the more active the Sun became, the more cyclical summer lightning became in Japan and that the 27-day solar cycle affects Earth in many more ways than expected, including the weather.

The Ishikawa Diary was kept by a farming family. It contains a lot of information about the family’s farming activities, crops and weather, though the entries vary in how extensive they are, according to Prof. Miyahara.

“Masters from some generations liked to make elaborate entries, while others kept things brief and would only say something like, ‘It rained today,’” Prof. Miyahara said, laughing. “Sometimes, you don’t see any record of lightning over a stretch of several years.”

On the other hand, all entries in the Hirosaki Hancho Nikki, which was a public document, are comprehensive.

“It has 200 years worth of data showing on what date and at what time of day lightning occurred. This was a very valuable document because it enables you to analyze weather events hour by hour. Hirosaki is in a cold climate region, and that might have made them attuned to weather changes,” she said.

Space as vantage point to understand Earth

The 27-day solar activity cycle holds a key in understanding many other solar-related phenomena, including auroras, Prof. Miyahara said.

“It’s a known fact that auroras that happen in the upper atmosphere have a 27-day cycle. Our findings in this research show that the influence of the solar activity cycle extends to even those deep layers of the atmosphere,” she said.

Researchers still struggle to understand precisely how solar activity impacts Earth, though, according to Prof. Miyahara. Conventional wisdom among those who study the subject can change in a major way at times.

For example, she said, a 2001 research paper that claimed a strong correlation between solar activity and Earth's climate steered the science community away from the previous common belief to the contrary, Prof. Miyahara said.

Until the 2001 paper came out, an increase in solar activity was believed to have little effect on the atmospheric temperatures. This belief was primarily due to a satellite research project from the end of the 1970s that looked at how solar radiation changed in intensity and quantity and concluded that it changed so little that it could not influence Earth’s temperatures,” Prof. Miyahara said.

“It was therefore natural to think that climate is determined by the factors within the climate system,” she said. “But the findings in the 2001 paper gave us reason to suspect that some variations in climate can be traced to what’s happening in space.”

Identifying causal paths of solar impact on climate

“Solar-related parameters,” such as solar radiations, solar ultraviolet (UV), solar wind and cosmic rays, change by varying degrees as the Sun rotates on its axis.

“Interestingly, each parameter changes in its own distinctive way during the 27-day cycle, because how it originates in the Sun is different for all the parameters.  This means, by comparing the parameters’ changes and weather changes on Earth -- such as temperature and precipitation fluctuations – one could identify the path through which the Sun affects Earth,” Prof. Miyahara said. “In the long term, though, parameters changes all look very similar, and so it becomes difficult to identify which parameter is the link between the Sun and Earth.”

In fact, researchers have been studying these parameter connections. Solar UV, for example, is known to heat the ozone layer at 10-50 kilometers from Earth’s surface, and it is assumed that these ozone layer temperature changes affect the troposphere, the lowest layer of the atmosphere. Cosmic rays are another subject of intensive research, as there have been reports about a possible correlation between cosmic ray intensity and the amount of clouds. It has been suggested that ions that cosmic rays produce in the atmosphere may change the lifetime of cloud.

Forecasting solar activity and its impact on Earth

“Space weather forecasting” refers to the prediction of solar activity and associated changes that occur near Earth. Today, this type of forecasting to some extent is already being conducted.

Research teams from around the world, including those in Japan, are making forecasts to help prevent problems for satellites and communication systems. Prof. Miyahara’s goal is to develop a way to combine those forecasts of space weather with the ones for Earth’s weather.

“If we can identify the paths through which the Sun’s energy affects Earth’s climate, that would enable us to pair an atmospheric forecast model with the right space weather forecasting system. Because of the repetitive nature of solar activity, it may be even possible to forecast the weather for the next few months, which is much longer than the current conventional forecasts for up to 10 days,” Prof. Miyahara said.

Interviewer: Rue Ikeya
Photographs: Yuji Iijima unless noted otherwise
Released on: Oct. 10, 2019 (The Japanese version released on Oct. 10, 2018)