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The Sun's shape and brightness

Nature volume 392, pages 155–157 (1998)Cite this article

Abstract

The origin of the 11- and 22-year solar cycles remains one of the more mysterious aspects of the Sun. These cycles are probably driven by convection in the solar interior, but the convection zone is difficult to probe. Small departures from sphericity in the effective surface temperature of the Sun can in principle be used in this regard. Such variations, which are observed as changes in the surface brightness with solar latitude, may be caused by differences between the vertical and horizontal turbulent convective flows1 inside the Sun. Moreover, variations in the Sun's luminosity may be related to changes in conditions near the base of the convection zone2 that result from the magnetic (sunspot) cycle3. Here we present satellite data that show that the Sun's shape and temperature vary with latitude in an unexpectedly complex way. Although the solar oblateness shows no evidence of varying with the solar cycle, we find a significant hexadecapole shape term which may vary. We also see a variation of about 1.5 K in the surface temperature with latitude. Based on these results, we suggest that sensitive observations of brightness variations be used as a record of the surface ‘shadow’ of cyclical changes in the solar interior.

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Figure 1: Limb displacement, S(θ), for each of the 512 angular bins around the Sun, as derived from 1997 measurements.
Figure 2: The modern oblateness measurements versus time.
Figure 3: The effective limb temperature change versus position angle, θ, in 1997 is plotted here.

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Acknowledgements

We thank J. Saba, C. DeForest and J. Covington for their assistance in operating the MDI instrument; L. Allen, H. Schweitzer, J.-P. Olive and K. Miller for the planning and execution of SOHO roll manoeuvrers; and the SOHO flight operations team, including H. Benefield, N. Piston and B.Sapper, for making these activities run flawlessly. This work was supported by NASA. We dedicate this work to the memory of Robert H. Dicke, who inspired a generation of astrophysicists to consider the measurement and interpretation of the shape of the Sun.

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Authors and Affiliations

  1. Michigan State University, East Lansing, 48823, Michigan, USA

    J. R. Kuhn

  2. National Solar Observatory, Sunspot, 88349, New Mexico, USA

    J. R. Kuhn

  3. Stanford University, Stanford, 94305, California, USA

    R. I. Bush & P. Scherrer

  4. Jackson Community College, Jackson, 49201, Michigan, USA

    X. Scheick

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  1. J. R. Kuhn
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  2. R. I. Bush
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  4. P. Scherrer
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Correspondence to J. R. Kuhn.

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Kuhn, J., Bush, R., Scheick, X. et al. The Sun's shape and brightness. Nature 392, 155–157 (1998). https://doi.org/10.1038/32361

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