Properties of convective motions in facular regions

Abstract

Aims. We study the properties of solar granulation in a facular region from the photosphere up to the lower chromosphere. Our aim is to investigate the dependence of granular structure on magnetic field strength. Methods. We used observations obtained at the German Vacuum Tower Telescope (Observatorio del Teide, Tenerife) using two different instruments: the Triple Etalon SOlar Spectrometer (TESOS) to measure velocity and intensity variations along the photosphere in the Ba ii 4554 Å line; and, simultaneously, the Tenerife Infrared Polarimeter (TIP-II) to the measure Stokes parameters and the magnetic field strength at the lower photosphere in the Fe i 1.56 μm lines. Results. We find that the convective velocities of granules in the facular area decrease with magnetic field while the convective velocities of intergranular lanes increase with the field strength. Similar to the quiet areas, there is a contrast and velocity sign reversal taking place in the middle photosphere. The reversal heights depend on the magnetic field strength and are, on average, about 100 km higher than in the quiet regions. The correlation between convective velocity and intensity decreases with magnetic field at the bottom photosphere, but increases in the upper photosphere. The contrast of intergranular lanes observed close to the disk center is almost independent of the magnetic field strength. Conclusions. The strong magnetic field of the facular area seems to stabilize the convection and to promote more effective energy transfer in the upper layers of the solar atmosphere, since the convective elements reach greater heights.

The possible origin of facular brightness in the solar atmosphere

“…Following our previous papers in this series (Kostik & Khomenko 2012;2013), we used observations performed at the German Vacuum Tower Telescope (VTT), located at the Observatorio del Teide in Izaña, Tenerife. The dataset was taken on 13 November 2007.…”

“…The line core images in Ba ii (middle panel) show suspicious brightness in the same locations as in the Ca ii H filtergram that correspond to the facular area under study. The data treatment is described in detail in Kostik & Khomenko (2012, 2013). In the current work, we study temporal variations in an area whose size is 5 .…”

“…14 heights in the photosphere, see Shchukina et al 2009). These are defined as follows (Kostik & Khomenko 2012):…”

“…The contrast possibly depends on the size of magnetic elements and not on their strength (the intrinsic strength is more or less constant in the kG range). Kostik & Khomenko (2012) claim that the observed dependence between brightness and contrast is a consequence of the different distributions of magnetic field strength in granules and intergranular lanes. The histogram of the magnetic field in the facula has two maxima, one at the hecto-G and one at the kG value.…”

“…If one considers the dependence between the contrast and the magnetic field strength, selecting only intergranular lanes, then it turns out that the contrast remains practically unchanged with the magnetic field increasing from 400 to 1600 G, see Fig. 4 in Kostik & Khomenko (2012). Given the above, the observational dependence between the brightness and other quantities related to granulation needs further studies to clarify if solar faculae are indeed conglomerates of magnetic flux tubes.…”