Variation of the temperature gradient in the solar photosphere with magnetic activity

Abstract

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.

Critical Science Plan for the Daniel K. Inouye Solar Telescope (DKIST)

“…While out-of-phase variations have been explained in terms of a change in the mean photospheric temperature gradient with cycle, which atmospheric components contribute most to that mean change remains unclear, particularly as absolute irradiance measurements are disk integrated. Addressing these uncertainties requires developing both an understanding of the statistical distributions of small-scale elements and reliable techniques for measuring the photospheric temperature gradient (Faurobert, Balasubramanian, and Ricort, 2016;Criscuoli and Foukal, 2017).…”

“…Beyond individual magnetic-element contributions, field-induced variations of the temperature gradient in the deep solar photosphere (below 60 km above the 500 nm continuum) may contribute to cycle-dependent irradiance variability (e.g. Faurobert, Balasubramanian, and Ricort, 2016, and the references therein). Structural changes are suggested by observations of the acoustic p-modes frequencies, which show solar-cycle variation (e.g.…”

A new spectroscopic method for measuring the temperature gradient in the solar photosphere

“…We first measure the depth difference between the formation height of the line-cord images and the continuum level. This is achieved exactly in the same way as described in Paper I (see also Faurobert et al 2016), that is, we measure the perspective shift along the radial direction between images taken in a line wing and in the continuum when they are observed away from disk center, that is, at heliocentric angles different from 0. We stress that the depth-difference is obtained in km or in second of arc, without any reference to a model.…”

“…In other words, even if the temperature gradient would vary on a geometrical scale, its variations on the continuum optical-depth scale would be very small and the center-to-limb variation of the continuum intensity would be hardly detectable. This effect is described in greater detail in Faurobert et al (2016) and it may bring on some insights with regard to the negative results of Criscuoli & Foukal (2017). This is why the method we have developed is based on a completely different approach.…”

“…The point spread functions (PSFs) in the different situations are obtained from the PAOLA software (Jolissaint 2006(Jolissaint , 2010 and for the characteristics of the adaptive optics (AO), we refer to the description of the system that will operate on DKIST, which was presented in Johnson et al (2016) and Marino et al (2016) In Sect. 2, we explain the procedure we introduced to correct the line profile for the Zeeman broadening of the FeI 630.15 nm line and we recall very briefly the main steps of the measurement method that was presented in more detail in the previous papers, namely , Faurobert et al (2018, Faurobert et al ( , 2016. Section 3 is devoted to the presentation of the results.…”