The continuum intensity as a function of magnetic field

Abstract

Context. To deepen our understanding of the role of small-scale magnetic fields in active regions (ARs) and in the quiet Sun (QS) on the solar irradiance, it is fundamental to investigate the physical processes underlying their continuum brightness. Previous results showed that magnetic elements in the QS reach larger continuum intensities than in ARs at disk center, but left this difference unexplained. Aims. We use Hinode/SP disk center data to study the influence of the local amount of magnetic flux on the vigour of the convective flows and the continuum intensity contrasts. Methods. The apparent (i.e. averaged over a pixel) longitudinal field strength and line-of-sight (LOS) plasma velocity were retrieved by means of Milne-Eddington inversions (VFISV code). We analyzed a series of boxes taken over AR plages and the QS, to determine how the continuum intensity contrast of magnetic elements, the amplitude of the vertical flows and the box-averaged contrast were affected by the mean longitudinal field strength in the box (which scales with the total unsigned flux in the box).Results. Both the continuum brightness of the magnetic elements and the dispersion of the LOS velocities anti-correlate with the mean longitudinal field strength. This can be attributed to the "magnetic patches" (here defined as areas where the longitudinal field strength is above 100 G) carrying most of the flux in the boxes. There the velocity amplitude and the spatial scale of convection are reduced. Due to this hampered convective transport, these patches appear darker than their surroundings. Consequently, the average brightness of a box decreases as the the patches occupy a larger fraction of it and the amount of embedded flux thereby increases. Conclusions. Our results suggest that as the magnetic flux increases locally (e.g. from weak network to strong plage), the heating of the magnetic elements is reduced by the intermediate of a more suppressed convective energy transport within the larger and stronger magnetic patches. This, together with the known presence of larger magnetic features, could explain the previously found lower contrasts of the brightest magnetic elements in ARs compared to the QS. The inhibition of convection also affects the average continuum brightness of a photospheric region, so that at disk center, an area of photosphere in strong network or plage appears darker than a purely quiet one. This is qualitatively consistent with the predictions of 3D MHD simulations.

Brightness of the Sun’s small scale magnetic field: proximity effects

“…Observationally, the effect of magnetic concentrations on nearby convective flows is easily detectable through changes in granulation morphology and vertical velocities ("anomalous granulation", cf. Title et al 1986Title et al , 1992Kobel et al 2012). Concerns that these changes could also affect energy transport and hence surface flux have been around for some time (e.g., Spruit 1998).…”

“…6 is the opposite of the expected magnetic brightening effect. This shows that the simulations include effects that have not been detected so far in the observations (see however Kobel et al 2012). As a check on the reliability of the calculations, we invoke the procedure that was used in Schnerr & Spruit (2011) for measuring magnetic brightening in weak fields from high-resolution observations (see Sect.…”

“…This would constitute a complication to be accounted for in the interpretation of TSI variations. Direct detection of such effects at the required levels of a fraction of a percent is observationally quite challenging, but has recently become possible using space-based data (Kobel et al 2012).…”

Intensity contrast of solar network and faculae

“…The decline at low B l /μ aside, the observed trends suggests that brightness or temperature excess in both the lower and middle photosphere in flux tubes decreases with increasing magnetic filling factor (the continuum and line core formation height of the Fe I 6173 Å line is about 20 and 300 km respectively, Norton et al 2006). This may partly be due to the increasing average size of flux tubes; larger magnetic elements appear darker as lateral heating is less efficient (Spruit 1976;Spruit & Zwaan 1981;Grossmann-Doerth et al 1994) and magnetic suppression of surrounding convective energy transport is more severe (Kobel et al 2012). The total radiation flux derived from MHD simulations by Vögler (2005) exhibits a similar behaviour.…”

Properties of solar plage from a spatially coupled inversion of Hinode SP data

“…Title et al 1989;Narayan & Scharmer 2010). Morinaga et al (2008) and Kobel et al (2012) concluded that the high spatial density of the kG magnetic fields causes a suppression of the convection process.…”