THE SUN AND SOLAR VARIABILITY—PAST AND PRESENT
Over the past 30 years substantial advances have been made in understanding variability in solar
output. Perhaps most dramatic has been the improved insight into the classic question posed by William
Herschel two centuries ago: Does the Sun's brightness vary? 1 Information on total solar irradiance (TSI)
behavior has advanced from inferential knowledge for times prior to 1980 to the current understanding of
its variation on scales from minutes to the 11-year cycle.
Solar variability is closely related to the structure and magnitude of the solar magnetic field, and
so the ability to reconstruct past solar outputs, or predict them, is only as good as the understanding of
how the solar magnetic field varies in time and location on the Sun. The past 20 years have seen great
strides in the ability to model the large- and smaller-scale structure and variability of the solar magnetic
field. 2 These developments in models have been supported by the ability to make measurements of the
solar magnetic field.
Precise helioseismic measurements reveal the complex depth dependence of solar rotation
throughout the convection zone and well into the radiative core. However, translation of these advances
into improved understanding of the dynamo processes that generate solar magnetism has proven more
difficult. 3 There is still no precise predictive model of the dynamo that drives solar magnetism over the
11-year cycle or of its modulation envelope over centuries and millennia.
The most rapid advances in this area are coming from simulations of magneto-convection on
small scales in relatively shallow layers. 4,5 Their extension to the much deeper layers of the convective
and tachocline zones that are most likely to generate the 11-year sunspot cycle is not yet possible with
today's computing power.
At the September 2011 workshop, presentations on this topic included discussions of advances in
solar radiometry, an assessment of solar influences on Earth's climate change, heliospheric phenomena
responsible for cosmic ray modulation, and the behavior of quiet Sun contributions to solar irradiance on
timescales ranging from years to thousands of years.