astronomical work on Sun-like stars, whether they exhibit quiet periods, and whether their brightness is diminished during quiet periods.5

The observed sunspot number has been demonstrated to be negatively correlated with the cosmic-ray flux. The cosmic-ray flux reaching Earth’s surface is modulated by the strength of the solar wind. It is now understood that this decrease in cosmic rays is due to changes in the magnetic field geometry in the heliosphere, the bubble blown in the interstellar medium by the solar wind.6 Higher levels of solar activity lead to a decrease in the cosmic-ray flux at Earth. Cosmic rays are potentially implicated in climate change on Earth because as they penetrate Earth’s atmosphere they leave behind an ionized path that could serve as a source of condensation centers that in turn affect cloudiness and Earth’s albedo (reflectivity of solar radiation).7 Cosmic rays may also have an effect on the global electric circuit of Earth’s atmosphere that is caused by thunderstorms separating charge from the surface to the troposphere.8 Research is being conducted on these potential mechanisms and their possible relevance as a climate-forcing agent. Furthermore, solar energetic particle (SEP) events, created at the shock front of coronal mass ejections (CMEs), for example, can influence the composition of the upper atmosphere. During a SEP event, solar protons and electrons follow Earth’s magnetic field lines toward the poles. The higher-energy particles can penetrate well into the stratosphere where they ionize the atmosphere, producing nitrogen oxides, whereas lower-energy particles can create nitrogen oxides in the lower thermosphere and mesosphere that then descend into the polar stratosphere. These nitrogen oxides can destroy ozone, thus altering not only the chemistry but also the radiative balance of that region.

THE MEASUREMENT RECORD FROM SPACE

The TSI and the spectral solar irradiance are the two most important measurements of the Sun’s output as it impacts climate. The continuous 33-year record of total solar irradiance from space-based observations is shown in Figure 1.1. This data record is the result of overlapping measurements from several instruments flown on different missions. Measurements made by individual radiometers providing the data shown in 9 A 2005 workshop conducted at the National Institute of Standards and Technology in Gaithersburg, Maryland,10 sparked investigations into the effects of diffraction, scattered light, and aperture area measurements on the differences between instrument results.

Evident in this combined, recalibrated record is an 11-year cycle with peak-to-peak amplitude of approximately 0.07 percent and variations greater by a factor of two to three that are associated with short-term transits of sunspots due to solar rotation (the lower panel of Figure 1.1). Measurement continuity has enabled successive radiometric time series obtained from different space missions to be intercalibrated to produce a 33-year-long composite TSI record. The need for such intercalibration makes

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5 P.G. Judge, and S.H. Saar, The outer solar atmosphere during the Maunder Minimum: A stellar perspective, The Astrophysical Journal 663:643, 2007.

6 L.A. Fisk, K.P. Wenzel, A. Balough, R.A. Burger, A.C. Cummings, P. Evenson, B. Heber, J.R. Jokipii, M.B. Krainev, and J. Kota, et al., Global processes that determine cosmic ray modulation. Space Science Review 83:179-214, 1998.

7 R. Harrison, The global atmospheric electric circuit and climate, Surveys in Geophysics 25:441-484, 2004.

8 L.I. Dorman and I.V. Dorman, Possible influence of cosmic rays on climate through thunderstorm clouds, Advances in Space Research 35(3):476-483, 2005.

9 M. Fligge and S.K. Solanki, The solar spectral irradiance since 1700, Geophysical Research Letters 27:2157, 2000.

10 J. Butler, B.C. Johnson, J.P. Rice, E.L. Shirley, and R.A. Barnes, Sources of differences in on-orbital total solar irradiance measurements and description of a proposed laboratory intercomparison, Journal of Research of National Institute of Standards and Technology 113:187-203, 2008.



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