Geoscience Reference
In-Depth Information
Minimum-like episode lasting 70 years. Foukal asserted that further work is required on this network-
area behavior during extended activity minima.
Foukal stressed that there is no evidence for the large (~0.3 percent) increase in TSI during the
early 20th century reported in a recent, widely quoted, study based on 10 Be. 13 That level of increase in
TSI would require the complete disappearance of the quiet network and internetwork going back in time
to 1900. This requirement contradicts the presence of a fully developed network on Ca K
spectroheliograms available since the 1890s. 14 Foukal asserted that this model, which also predicts strong
TSI driving of climate throughout the Holocene, cannot be correct.
Heliospheric Phenomena Responsible for Cosmic Ray Modulation at the Earth
Joe Giacalone, University of Arizona
Galactic cosmic rays are expected to be essentially isotropic in the space immediately outside the
heliosphere and penetrate the solar system with nearly equal probability from all directions. As GCRs
enter the solar system, they undergo both energetic and intensity variations resulting from their interaction
with the solar wind, interplanetary magnetic field, and the heliosphere (Figure 2.4). As Joe Giacalone
discussed, GCRs interact with atomic nuclei in Earth's atmosphere, creating a secondary cosmic ray
(neutron) that can interact with a background very abundant in oxygen and nitrogen to form 14 C or 10 Be,
among other nuclei. The GCR intensity at the top of Earth's atmosphere is anti-correlated with the
number of sunspots—GCR intensity is higher when there are fewer sunspots. This anti-correlation is the
basis for using 14 C and 10 Be deposited in tree rings and ice cores as a proxy measure of solar activity
dating back thousands of years. In addition to the 11-year GCR cycle, there is also a 22-year variation
related to the polarity of the solar magnetic field. These variations can be understood through the physics
of charged-particle motion in turbulent electric and magnetic fields associated with the solar wind plasma.
Even during the Maunder Minimum when sunspots were scarce, there were still polarity reversals, and
there were still enhancements of GCRs in the 11-year cycle such that a stronger magnetic field yielded
smaller GCR flux. The highest flux of GCRs on record occurred during the most recent solar minimum.
The radioisotope record in 14 C and 10 Be provides information on the varying concentration of
these isotopes in the biosphere over past millennia. This information can be translated into a time series
showing the variation in flux of the GCRs mainly responsible for the formation of these isotopes. The
resulting GCR flux variation has been used in Sun-climate studies in two separate ways. It can be used
directly to study the possible effect of GCR variation on atmospheric electrification and cloud formation.
As pointed out in the presentation by Giacalone, recent findings from laboratory measurements at CERN
have brought more attention to studies of cosmic ray effects. He also described the various influences
causing GCR flux variation on timescales from days to millennia. Sudden changes in the solar magnetic
field or in the resulting heliospheric structures account for the rapid changes. The 11- and 22-year cycles
are caused by more gradual variation of the heliospheric field strength and complexity, and in the number
of solar eruptive events over the sunspot cycle. Supernova eruptions and other changes in the interstellar
medium play a role on millennial timescales because of the large distances between stars and the diffusive
transport of the GCR population. Nearby supernovae can create short-lived pulses, but these are
exceptionally rare.
13 A. Shapiro, W. Schmutz, E. Rozanov, M. Schoell, M. Haberreiter, A.V. Shapiro, and S. Nyeki, A new
approach to the long-term reconstruction of the solar irradiance leads to large historical solar forcing. Astronomy and
Astrophysics 529:67, 2011.
14 P. Foukal, A new look at solar irradiance variation, Solar Physics 279:365-381, 2012.
Search WWH ::

Custom Search