Sunspot bright rings and the thermal diffusivity of solar convection

✍ Scribed by L. A. Fowler; P. Foukal; T. Duvall

• Publisher: Springer
• Year: 1983
• Tongue: English
• Weight: 644 KB
• Volume: 84
• Category: Article
• ISSN: 0038-0938
• DOI: 10.1007/bf00157440

✦ Synopsis

We have used the 512 channel diode array and vacuum telescope at KPNO to study the photospheric intensity distribution around sunspots, for comparison with isotherms predicted by convective blocking models of heat flow. Raster scan observations of 10 spots on 18 days were carried out in 1980 and 1981. Continuum passbands of 0.25 A width were selected to avoid contamination by weak Fraunhofer lines, whose strength is sensitive to the presence of magnetic faculae often found near spots. Our observations show no evidence of extended bright rings around the spots at the level of 1-2 ~o, as reported in one recent study using photographic photometry and much wider passbands. But 6 of the 10 spots we measured show marginally significant (2-3a) bright rings of peak amplitude 0.1-0.3~o. We are not able to explain these rings as a result of either residual facular signal, or instrumental effects. The excess radiative flux in these rings is small compared to the missing flux in the spot umbra and penumbra. We compare the brightness of the observed rings with peak brightnesses calculated from models of heat flow around spots of various depths and radii. Even if the spot is assumed to be unrealistically shallow, a detectable bright ring requires that the effective thermal conductivity (and/or its depth gradient) in layers surrounding the spot be significantly lower than the values indicated by mixing length models of the solar convection zone.