Solar Radiation
Solar radiation 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 convection in the convection zone to the photosphere, which is what we see. The photosphere is often called the “surface” of the Sun, and is the region from which solar energy is emitted to interplanetary space. It has a thickness of 500 km, a small fraction of the total solar radius of 6.6 x 105 km. 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 sun’s effective photospheric temperature. It was found that solar activity has an 11-year cycle with typically 50-150 numbers of sunspots during solar maximum and nearly zero during the solar minimum. Because of the lower temperature of sunspots in the photosphere, the presence of sunspots reduces 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. Other well-known phenomena are the 27-day rotation cycle observed from tracing the sunspots passage and the 22-year cycle of reversal of magnetic polarity of sunspot pairs.
The Sun-Earth climate relation has been a very hot topic since the discovery of sunspots. The related subjects range 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 Sun-Earth'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 1361 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. The satellite observations showed that it does indeed vary, though only by about 0.1% over the 11-year cycle. The spectral solar irradiance (SSI) at UV (ultraviolet) wavelength 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 at UV wavelengths have been observed to vary during solar cycles, how the Sun varies (both TSI and whole spectrum SSI) and how solar variations influence the Earth’s climate over long time scales remain unresolved. The launch of SORCE (Solar Radiation and Climate Experiment) satellite in early 2003 started a new era of Sun-Earth climate research. Long-term monitoring of both TSI and SSI is leading to better understanding of the Sun-Earth's climate connection.
There are two important findings from SORCE. First, the high accurate TIM (Total Irradiance Monitor) on SORCE reveals a much lower TSI of ~1361 W/m2 as compared to ~1366 W/m2 from earlier observations [Kopp et al., 2005]. The difference in global average is about the same as the relative radiative forcing of CO2 since the industrial revolution. Second, the SIM (Spectral Irradiance Monitor) observed SSI does not vary all in-phase with solar activity as compared to the in-phase variations as we have understood from proxy reconstructions [Harder et al., 2009]. The first discovery is critical in examining the energy budget of the planet Earth and isolating the climate change due to human activities. The second finding is crucial in understanding the physical mechanisms of the impact of solar variation on Earth’s climate. Based on SIM observations Cahalan et al. [2010] demonstrate remarkable different climate responses (stratosphere, troposphere, ocean mixed layer) to SORCE-based and proxy-based SSI variations. The out-of-phase SSI variations also have implications to re-examine the connection of the Sun and stratosphere, troposphere, biosphere, ocean, and Earth’s climate. So far SORCE has experimented the descending phase of solar cycle 23. Scientists in sun-climate community are eager to see variations in both TSI and SSI in the rising phase of solar cycle 24.
The Goddard Sun-Climate Center has been established. Scientists from the Climate and Radiation Lab, the Solar Physics Lab, 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. Interdisciplinary efforts across Earth and Space science make this group unique in Earth-Sun exploration research.
Contact: Guoyong Wen