摘要：Context. The contribution of quiet-Sun regions to the solar irradiance variability is currently unclear. Some solar-cycle variations of the quiet-Sun physical structure, such as the temperature gradient or the photospheric radius, might affect the irradiance.
Aims. We intend to investigate possible variations of the photospheric temperature gradient with magnetic activity.
Methods. We used high-resolution center-to-limb observations of the FeI 630.15 nm line profile in the quiet Sun performed onboard the Hinode satellite on 2007, December 19, and on 2013, December 7, that is, close to a minimum and a maximum of magnetic activity, respectively. We analyzed samples of 10″ × 10″ internetwork regions. The wings of the FeI 630.15 nm line were used in a non-standard way to recover images at roughly constant continuum optical depths above the continuum formation level. The image formation height is derived from measuring its perspective shift with respect to the continuum image, both observed away from disk center. The measurement relies on a cross-spectral method that is not limited by the spatial resolution of the SOT telescope and does not rely on any radiative transfer computation. The radiation temperature measured in the images is related to the photospheric temperature at their respective formation height.
Results. The method allows us to investigate the temperature gradient in the low photosphere at altitudes of between 0 and 60 km above the 500 nm continuum formation height. In this layer the internetwork temperature gradient appears steeper in our 2013 sample than in the sample of 2007 in the northern hemisphere, whereas we detect no significant change in the southern hemisphere. We argue that this might be related to some strong hemispheric asymmetry of the magnetic activity at the solar maximum of cycle 24.
Conclusions. Structural changes have been observed in numerical simulations of the magneto-convection at the surface of the Sun where the increase of the ambient sub-surface magnetic fields leads to some steepening of the temperature gradient in the internetwork. Hemispheric asymmetry of the activity has been reported for the last solar cycles with successive dominant north and south hemisphere during the activity maximum. Our results seem consistent with this global physical picture, but need to be confirmed by additional studies.
关键词：techniques: high angular resolution;techniques: spectroscopic;Sun: photosphere;Sun: fundamental parameters
