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this record vulnerable to loss in the event of a gap in measurements. Proxy records of radioisotopes
provide evidence of long-term change in solar activity, but these must be tuned and extrapolated from the
existing TSI data record; however, based on present understanding, the irradiance variations inferred from
them are no greater than those observed radiometrically over recent solar cycles. New evidence now
suggests that secular variations of larger amplitude may have occurred on multi-decadal to millennial
timescales. The Intergovernmental Panel on Climate Change Fourth Assessment Report estimated the
direct radiative forcing due to changes in solar output since 1750 to be ~0.12 W m −2 (0.06 to 0.30);
corresponding to a change in TSI of ~0.5 W m −2 from its baseline value of 1360 W m -2 ) with a factor-of-
two uncertainty. 11 Shortly before 1750, the Maunder Minimum may have caused greater changes in solar
Continuous measurements of solar ultraviolet radiation began in 1978 with the Nimbus-7 Solar
Backscatter Ultraviolet (SBUV). 12 These measurements were followed by those from the Solar
Mesosphere Explorer (SME), 13 NOAA-9 SBUV/2, NOAA-11 SBUV/2, the Upper Atmosphere Research
Satellite (UARS) Solar Stellar Intercomparison Experiment (SOLSTICE), 14 and the Solar Ultraviolet
Spectral Irradiance Monitor (SUSIM). 15 The present-day SORCE SOLSTICE and SORCE Spectral
Irradiance Monitor (SIM) extend this continuous (albeit differing in spectral coverage, resolution, and
instrument accuracies and stabilities) record of the solar ultraviolet and its variability.
Although the ultraviolet region of the spectrum provides only a small fraction of the TSI,
ultraviolet irradiance can change by several percent over the solar cycle, and thus represents an important
source of modulation of the energy deposition and composition in the middle and upper atmosphere.
Ultraviolet irradiance both changes the radiative balance of the atmosphere and affects the shape of the
spectrum of radiation reaching the lower atmosphere. Such variations are thought to drive the top-down
coupling mechanism.
The record of measurement of the continuous, full solar irradiance spectrum, which is much
shorter than the record of TSI, commenced with measurements by the Spectral Irradiance Monitor on the
SORCE satellite in 2003. Results have indicated that ultraviolet trends during cycle 23 were larger than
those observed in previous cycles, and were compensated by trends in other bands that increased with
decreasing solar activity. 16 Spectral observations from SIM suggest a very different response in Earth's
atmosphere because of this compensating spectral behavior, suggesting that further modeling studies and
analysis of existing atmospheric observations may be needed, as well as continued validations of these
new observations.
Research into possible mechanisms of Sun-climate coupling has taken several paths. Progress is
hampered by incomplete understanding of solar variability, climate, and their complex interaction.
11 S. Solomon et al., Technical Summary in Climate Change 2007: The Physical Science Basis. Contribution of
Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change , 2007.
12 B.M. Schlesinger and R. P. Cebula, Solar variation 1979-1987 estimated from an empirical model for
changes with time in the sensitivity of the solar backscatter ultraviolet instrument, Journal of Geophysical Research
97:119-10,134, 1992.
13 G.J. Rottman, Observations of solar UV and EUV variability, Advances in Space Research 8:53-66, 1988.
14 G.J. Rottman, T. Woods, M. Snow, and G. DeToma, The solar cycle variation in ultraviolet irradiance,
Advances in Space Research 27:1927-1932, 2001.
15 L.E. Floyd, D. K. Prinz, P. C. Crane, and L. C. Herring, Solar UV irradiance variation during cycles 22 and
23, Advances in Space Research 29:1957-1962, 2002.
16 J. Harder, J.M. Fontenla, P. Pilewskie, E.C. Richard, T.N. Woods, Trends in solar spectral irradiance
variability in the visible and infrared, Geophysical Research Letters 36: L07801, 2009.
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