2001/01/26 by W. A. Dziembowski, P. R. Goode, Philip R. Goode +1 · 76 citations
Physics and Astronomy · #Astro and Planetary Science #Astrophysics #Computational physics #Coronal mass ejection #Isotropy #Magnetic field #Optics #Oscillation (cell signaling) #Photosphere #Physics #RADIUS #Solar and Space Plasma Dynamics #Solar radius #Solar wind #Stellar, planetary, and galactic studies #astro-ph
paper · pdf · doi:10.1086/320976
published in The Astrophysical Journal 553(2), 897-904 (IOP Publishing) · 10 pages (ApJ preprint style), 4 figures; accepted for publication in ApJ
arxiv created 2001/01/26 · openalex publication_date 2001/06/01 · arxiv updated 2009/12/01 · openalex created_date 2025/10/10 · openalex updated_date 2026/08/06
We have analyzed the full set of Solar and Heliospheric Observatory ( SOHO ) Michelson Doppler Imager (MDI) f - and p -mode oscillation frequencies from 1996 to date in a search for evidence of solar radius evolution during the rising phase of the current activity cycle. Just as Antia et al. in 2000, we find that a significant fraction of the f -mode frequency changes scale with frequency and that if these are interpreted in terms of a radius change, it implies a shrinking Sun. Our inferred rate of shrinkage is about 1.5 km yr -1 , which is somewhat smaller than found by Antia et al. We argue that this rate does not refer to the surface but, rather, to a layer extending roughly from 4 to 8 Mm beneath the visible surface. The rate of shrinking may be accounted for by an increasing radial component of the rms random magnetic field at a rate that depends on its radial distribution. If it were uniform, the required field would be ~7 kG. However, if it were inwardly increasing, then a 1 kG field at 8 Mm would suffice. To assess contribution to the solar radius change arising above 4 Mm, we analyzed the p -mode data. The evolution of the p -mode frequencies may be explained by a magnetic field growing with activity. Our finding here is very similar to that of Goldreich et al. (1991). If the change were isotropic, then a 0.2 kG increase, from activity minimum to maximum, is required at the photosphere, which would grow to about 1 kG at 1 Mm. If only the radial component of the field were to increase, then the requirement for the photospheric field increase is reduced to a modest 60-90 G. A relative decrease in temperature of the order of 10 -3 in the subphotospheric layers, or an equivalent decrease in the turbulent energy, would have a similar effect to the required inward growth of magnetic field change. The implications of the near-surface magnetic field changes depend on the anisotropy of the random magnetic field. If the field change is predominantly radial, then we infer an additional shrinking at a rate between 1.1 and 1.3 km yr -1 at the photosphere. If, on the other hand, the increase is isotropic, we find a competing expansion at a rate of 2.3 km yr -1 . In any case, variations in the Sun's radius in the activity cycle are at the level of 10 -5 or less and, hence, have a negligible contribution to the irradiance variations.