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The solar radiation input is the major energy source for the Earth's biosphere, and the direct driving force for atmospheric, and oceanic circulations. The Sun is a typical main sequence star with spectral class of G2, one of 100 billion stars in the galaxy system. The energy generated in the fusion processes in the inner core is transported though radiative processes in the radiation zone, and by convections in the convection zone to the photosphere, which is what we can see. The photosphere has a thickness equal to 500 km, a small fraction of the total solar radius that equals 6.6 x 105 km, and is often called the 'surface' of the Sun, and is the region from which solar energy is emitted to interplanetary space. The photosphere has an effective temperature of 5780°K.
The Sun is not a uniform fireball. Some areas on the Sun are darker, and some areas are brighter. These relatively darker areas are called sunspots with temperature 1500°K cooler than the sunÕs effective photospheric temperature. It was found that solar activity has an 11-year cycle with typically 50~150 number of sunspots during solar maximum and nearly zero in the solar minimum. Because of the lower temperature of sunspots in the photosphere, the presence of sunspots decreases the emitting energy to space. Surprisingly, at the solar maximum when sunspots are numerous, the average luminosity is larger. This is because the increase of brightness in the faculae (Latin meaning torches) areas surrounding the sunspots over power the darkness due to the sunspots, consequently leading to a brighter sun in solar maximum than in solar minimum. Another well known phenomenon is solar the 27-day rotation cycle observed from tracing the sunspots passage. The Sun-Earth climate relation has been a very hot topic since the discovery of sunspots with related subjects ranging from rainfall changes, lake level variations, river flow changes, drought cycles, storms, pressure systems, to biological phenomena such as insect populations, circumpolar mammal populations, seaweed density, agricultural yields etc. The two most well founded SunÐEarth's climate connection topics are the early faint young Sun paradox, and the Little Ice Age during the Maunder minimum time period. The total solar irradiance (TSI) arriving at the mean Sun-Earth distance is about 1368 W/m2 , and was often referred to as solar constant. Whether or not TSI is actually constant, or how it might vary, was much debated before satellite observations showed that it does indeed vary, though only by about 0.1% over the 11-year cycle. The spectral solar irradiance (SSI) in UV spectrum has been observed to vary during an 11-year solar cycle with much larger amplitude compared with the variability of TSI. Even though TSI and SSI in UV spectrum have been observed to vary during solar cycles, how the Sun varies (both TSI and whole spectrum SSI) and influences the EarthÕs climate over long time scales remains unresolved. The launch of the SORCE (Solar Radiation and Climate Experiment) satellite early in year 2003 started a new era of Sun-Earth climate research. Long term monitoring of both TSI and SSI is expected to lead to better understanding of the Sun Ð Earth's climate connection. The Goddard Sun-Climate Center has been formed recently. Scientists from the Climate and Radiation Branch, the Solar Physics Branch, GISS, and other organizations of the Goddard Space Flight Center, along with an external panel of experts, discuss and collaborate on issues in Sun-Climate research, and develop strategic plans for future research. Interdiciplinary efforts across Earth and Space science make this group unique in Earth-Sun exploration research. Contact: Guoyong Wen
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