[SFDXA] Solar activity reconstructed over a millennium

Sat Jan 23 15:47:12 EST 2021

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NEWS RELEASE 19-JAN-2021

  Solar activity reconstructed over a millennium

ETH ZURICH

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IMAGE

IMAGE: SOLAR ACTIVITY OVER THE LAST 1000 YEARS (BLUE, WITH ERROR 
INTERVAL IN WHITE), SUNSPOT RECORDS (RED CURVE) GOING BACK LESS THAN 400 
YEARS. THE BACKGROUND SHOWS A TYPICAL ELEVEN-?YEAR CYCLE... view more 
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CREDIT: ETH ZÜRICH

What goes on in the sun can only be observed indirectly. Sunspots, for 
instance, reveal the degree of solar activity - the more sunspots are 
visible on the surface of the sun, the more active is our central star 
deep inside. Even though sunspots have been known since antiquity, they 
have only been documented in detail since the invention of the telescope 
around 400 years ago. Thanks to that, we now know that the number of 
spots varies in regular eleven-year cycles and that, moreover, there are 
long-lasting periods of strong and weak solar activity, which is also 
reflected in the climate on Earth.

However, how solar activity developed before the start of systematic 
records has so far been difficult to reconstruct. An international 
research team led by Hans-Arno Synal and Lukas Wacker of the Laboratory 
of Ion Beam Physics at ETH, which included the Max Planck Institute for 
Solar System Research in Göttingen and Lund University in Sweden, has 
now traced back the sun's eleven-year cycle all the way to the year 969 
using measurements of the concentration of radioactive carbon in tree 
rings. At the same time, the researchers have thus created an important 
database for more precise age determination using the C14 method. Their 
results were recently published in the scientific journal /Nature 
Geoscience/.

Solar activity from tree rings

To reconstruct solar activity over a millennium with an extremely good 
time resolution of just one year, the researchers used tree-ring 
archives from England and Switzerland. In those tree rings, whose ages 
can be precisely determined by counting the rings, there is a tiny 
fraction of radioactive carbon C14, with only one out of every 1000 
billion atoms being radioactive. From the known half-life of the C14 
isotope - around 5700 years - one can then deduce the concentration of 
radioactive carbon present in the atmosphere when the growth ring was 
formed. As radioactive carbon is mainly produced by cosmic particles, 
which in turn are kept away from the Earth to a greater or lesser extent 
by the magnetic field of the sun - the more active the sun, the better 
it shields the Earth - it is possible to deduce solar activity from a 
change in the concentration of C14 in the atmosphere.

Better results through modern detection techniques

Precise measurements of a change in that already very small 
concentration, however, resemble the search for a grain of dust on a 
needle in a huge haystack. "The only measurements of that kind were made 
in the 80's and 90's", says Lukas Wacker, "but only for the last 400 
years and using the extremely laborious counting method". In that 
method, radioactive decay events of C14 in a sample are directly counted 
using a Geiger counter, which requires a relatively large amount of 
material and, owing to the long half-life of C14, even more time. "Using 
modern accelerator mass spectrometry we were now able to measure the C14 
concentration to within 0.1 percent in just a few hours with tree-ring 
samples that were a thousand times smaller", adds PhD student Nicolas 
Brehm, who was responsible for those analyses.

In accelerator mass spectrometry, C14 and C12 atoms (the "normal", 
non-radioactive carbon; C14, by contrast, contains two additional 
neutrons in its nucleus) of the tree material are first electrically 
charged and then accelerated by an electric potential of several 
thousand volts, after which they are sent through a magnetic field. In 
that magnetic field the two carbon isotopes, which have different 
masses, are deflected to different degrees and can thus be counted 
separately. To eventually obtain the desired information on solar 
activity from that raw data, the researchers have to perform some 
intricate statistical analysis on it and further process the results 
using computer models.

Regular eleven-year cycle over a millennium

This procedure enabled the researchers to seamlessly reconstruct solar 
activity from 969 to 1933. From that reconstruction they could confirm 
the regularity of the eleven-year cycle as well as the fact that the 
amplitude of that cycle (by how much the solar activity goes up and 
down) is also smaller during long-lasting solar minima. Such insights 
are important for a better understanding of the internal dynamics of the 
sun. The measurement results also allowed a confirmation of the solar 
energetic proton event of 993. In such an event, highly accelerated 
protons that reach the Earth during a solar flare cause a slight 
overproduction of C14. Moreover, the research team also found evidence 
of two further, as yet unknown events in 1052 and 1279. This could 
indicate that such events - which can severely disturb electronic 
circuits on Earth and in satellites - happen more frequently than 
previously thought.

More precise dating by the C14 method

As tree ring archives exist for the past 14'000 years, in the near 
future the researchers want to use their method to determine the yearly 
C14 concentrations all the way back to the end of the last ice age. As a 
kind of "extra", the data in the new study can be used for dating 
organic material much more precisely using the C14 method and have 
already been included in the latest edition of the internationally 
recognized radio carbon calibration curves (IntCal). "ETH had not been 
involved in that reference database before", says Lukas Wacker, "but 
with our new results we have now contributed a third of the measurements 
in one go."

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