Solar Influences on Global Change (1994) / Chapter Skim
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6 UNDERSTANDING THE VARIABLE SUN
6 UNDERSTANDING THE VARIABLE SUN
Pages 95-110
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From page 95... ...
In this period were obtained the first long term records from space of the solar radiative energy inputs to the Earth that are critical for studying solar influences on global change: total solar irradiance and the solar UV spectral irradiance, as well as fluxes of energetic protons and electrons. Ground based measurements were also made of solar observables closely related to the energy inputs measured from space.
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From page 96... ...
. Yet contemporary measurements of solar energy inputs alone reveal little about future solar variability nor of past solar variations that might have influenced the paleoclimate record, which is the focus of the Earth System History science element of the USGCRP.
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From page 97... ...
generally, the solar activity cycle pertains to the periodic emergence of magnetic flux that generates not just sunspots, which are ciark, but a variety of phenomena, especially bright regions known as plages and faculae that radiate strongly at UV and EUV wavelengths. The dark sunspots and bright plages and faculae modify the radiation from the solar disk, thereby generating the variations observed by spaceborne solar radiometers.
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From page 98... ...
Although 27-day periodicity is attributable to the Sun's rotation, no physical models explain the emergence, evolution, and decay of the magnetic active regions on the solar surface that cause changes in the amplitude and phase of this cycle. Since the 27-day rotation modulates solar irradiance by changing
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From page 99... ...
Conceptually, a dynamo process is thought to underlie the Sun's magnetic cycle (e.g., DeLuca and Gilman, 1991) , and models have been constructed to provide insight into the specific basic solar characteristics that must change to produce the periodic magnetic flux tube emergence that is responsible for solar energy input variations to the Earth.
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From page 100... ...
Active regions (sunspots, plages and faculae, filaments, coronal holes, etc) modify the local intensity of the solar surface, resulting in an inhomogeneous solar disk whose radiation field is variable in both
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From page 101... ...
The radiative energy input to the Earth is the integral of the radiance from the entire solar hemisphere visible at the Earth -- that is, the irradiance. As a result of this integration, the interpretation of variations observed in the total and spectral solar irradiances involves constructing irradiance time series by combining the contributions from sunspots, plages, the bright magnetic network, and internetwork regions.
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From page 102... ...
Surface inhomogeneities associated with magnetic active regions can been seen as regions of enhanced brightness (plage and faculae) and also as small dark regions (sunspots)
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From page 103... ...
In each figure, the solid lines are the variations smoothed over approximately three 27-day solar rotations. Both the Ca II and He I time series are useful proxies for solar radiative output from bright faculae while the sunspot blocking is a parameterization of He emission deficit caused by dark sunspots.
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From page 104... ...
Much of the longer term variability during solar cycle 21 and the ascending phase of cycle 22 is also reproduced by this model, with the exception of the first years of the record. The longer term solar cycle changes occur because of a brightness component in addition to the sunspots and the brightest faculae associated with magnetic active regions.
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From page 105... ...
~J~' ~. a_ A CRIM ·~\ ""I , , , 90 80 85 YEAR FIGURE 6.4 Comparison of the SMM/ACRIM I and Nimbus 7/ERB total solar irradiance data with empirical variability models constructed from the respective radiometry.
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From page 106... ...
Coronal mass ejections periodically disrupt the quasistationary pattern of high and low speed flows from coronal holes and streamers. The projection of solar wind profiles at the Earth back to their origins on the Sun can therefore be ambiguous, which emphasizes, again, the importance of knowledge of solar magnetic field structure in determining the time profile of solar energy input at ~ AU.
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From page 107... ...
Until there is a solid appreciation of the connection between the cosmogonic isotope variations and the modulation of the energy output from the Sun by magnetic active region phenomena (sunspots, plages, network) , it will be difficult to construct reliable quantitative estimates of the strengths of extreme in solar energy, such as during the Maunder Minimum type episodes that appear to have occurred commonly (every 200 years or so)
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From page 108... ...
The goal of stellar dynamo models is to reproduce the periodic variation of flux tube emergence seen on the Sun and also to show the systematic latitude and polarity variations that occur as spots move from high latitudes toward the equator as the cycle progresses, with the polarity of sunspot pairs and the polar fields reversing from one cycle to the next. The necessary empirical boundary conditions for such moclels come from detailed knowledge of magnetic flux tubes, their distribution in both time and position on the Sun, and evolution of their energy transport seen in observations of radiative output.
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From page 109... ...
Knowledge of physical conditions on the Sun at these extreme is primitive because modern scientific observations have all been made during an era of high solar activity. Galileo's and others' discovery of sunspots at the beginning of the seventeenth century came at a time to document an ensuing period of low solar activity.
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From page 110... ...
But there is limited physical understanding of why these precursor methods should be appropriate except that the magnetic fields and corona near the solar poles change near solar maximum and hence may herald the onset of the new cycle before the next generation of sunspots appears. On century time scales, the periodicities of ~ ~ and 88 years identified n the sunspot record, together with the 208 year periodicity found in the |4C record, provide limited guidance to future solar behavior, such as the occurrence of the next Maunder Minimum.
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