Geoscience Reference
In-Depth Information
The Record of Solar Forcing in Cosmogenic Isotope Data
Raimund Muscheler, Lund University, Sweden
Muscheler discussed how information can be recovered about the Sun's activity in the past from
studies of cosmogenic isotope data. The most common isotopes used in these studies are 10 Be in ice cores
and 14 C in tree rings, and they can be used together to separate changes in solar activity from differences
in climate. He noted that responses in the data are seen not only for solar modulation but also for
variations in Earth's geomagnetic field on timescales longer than 500 years. The challenge is to detect
reliable signals from these data sets for a particular time period. TSI can be considered to be more closely
tied to the closed flux (which is larger than the open-field fraction) and magnetic features on the Sun
compared to the modulation of the GCRs in the heliosphere by the Sun's open flux. He noted that when
talking about the cosmic-ray flux at Earth, Earth's magnetic field determines where and how much of the
cosmic -ray flux makes it into Earth's atmosphere. An estimate of the geomagnetic field intensity is
necessary along with isotope measurements to determine the solar modulation. In the atmosphere itself,
circulation and climate also affect the deposition of the isotopes.
14 C's response to variations in the solar cycle is affected by the fact that the 14 C is taken up in
CO 2 . Muscheler explained that CO 2 remains in the atmosphere for approximately 7-8 years and then has
a long residence time in the large reservoir of the ocean where it can continue to exchange with the
atmosphere and the biosphere before being trapped in tree rings. This leads to a dampening of the
smaller-scale variations. Because of this residence time in a large reservoir, 14 C is a less direct proxy for
cosmic rays, but it does have the advantage of being less subject to concerns about geographic influences
than is 10 Be. 14 C records a more global signal on 11-year-cycle timescales or longer. Muscheler stated
that even if changes in climate cause changes in the carbon cycles of the biosphere and the ocean, the 14 C
in the atmosphere changes at the same rate and so does not obscure a solar signal. Even though climate
influence is unlikely to be a major influence, a model of the carbon cycle is needed to calculate 14 C
production rates to make an assumption about the carbon cycle. Variations in 14 C in the atmosphere alone
do not provide a signal of the production of 14 C.
Muscheler discussed how 10 Be is produced by spallation processes in the atmosphere through
reactions with nitrogen and oxygen. These reactions require high-energy particles. Normally energies at
these levels are seen only in GCRs and in the relatively rare solar proton events (SPEs). These SPE
contributions are relatively short-lived and are generally believed to be undetectable in the climatological
record because they are obscured by other short cycles, such as the changes in 18 O caused by yearly
temperature changes.
Muscheler continued by explaining how 10 Be is produced in the stratosphere, becomes attached to
aerosols, and is then sensitive to stratosphere-troposphere exchange processes before being deposited and
trapped in ice cores. 10 Be is further complicated by the geomagnetic field configuration characteristic of
the location. High-latitude locations such as Greenland or Antarctica have little shielding, and so the
solar signal is relatively strong. At low latitudes there is little variation with solar cycle due to the
stronger geomagnetic shielding. A still unresolved issue is how to go from a local measurement with
some random variability to a globally representative value. Muscheler summarized 10 Be as a relatively
direct proxy for cosmic rays with significant noise associated with location and climate influences.
Muscheler discussed two commonly used data sets from the past 1,000 years for 10 Be. Both sets
of data show a largely consistent picture with the Maunder and Spörer Minima from activity indices seen
in the 10 Be record. There is a disagreement between researchers when looking at the records from the past
50 years. On the basis of one data set, it can be argued that today's base solar activity is high, but that it
is not unprecedented. Others claim that their reconstruction indicates a period of high solar activity in the
past 60 years that is unique in the past 1,150 years. Muscheler suggested that the difference between the
recent records is caused by one of them having been influenced by non-solar-related climate change.
Muscheler discussed evidence of long-term changes in solar activity over the past 10,000 years. There
are, however, uncertainties engendered by the comparison of the 10 Be to the 14 C record. These differences
may be due to changes in 10 Be transport, snow accumulation rates, carbon cycle uncertainties, or
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