Centre-to-limb variation of solar granulation in the infrared

¹ Instituto de Astrofísica de Canarias, 38205 La Laguna, Tenerife, Spain e-mail: msanchez@ll.iac.es; mva@ll.iac.es; jab@ll.iac.es
² Astronomical Institute, Academy of Sciences of the Czech Republic, 25165 Ondřejov, The Czech Republic e-mail: msobotka@asu.cas.cz

Corresponding author: M. Sanchez Cuberes, msanchez@ll.iac.es

Received: 25 January 2002
Accepted: 22 October 2002

Abstract

Broad-band images have been obtained at the extremes of the continuum absorption coefficient of the solar atmosphere, at λ0.8 μm and λ1.55 μm. Quiet regions have been observed for different heliocentric angles. The centre-to-limb variation of granulation contrast has been computed in both wavelength ranges and a steeper decrease in the contrast is obtained for λ0.8 μm than for λ1.55 μm. For positions near the solar limb the granulation contrast shows a tendency to increase at both wavelengths. Mean granular sizes vary from 1.25 arcsec² at the disc centre to 2 arcsec² at in the λ0.8 μm images and from 1.24 arcsec² to 1.85 arcsec² at λ1.55 μm. Observations close to the limb detect granular structures of 0. ´´96 or even smaller at a distance from the limb of 0. ´´32, equal to the diffraction limit of the telescope at λ8000 Å. Using an Eddington-Barbier approximation this implies a penetration of the temperature fluctuations associated with granulation up to approximately 220 km, although the calculation of response functions defines a broader interval.