The brightness of magnetic field concentrations in the quiet Sun

Abstract

In addition to the "facular" brightening of active regions, the quiet Sun also contains a small scale magnetic field with associated brightenings in continuum radiation. We measure this contribution of quiet regions to the Sun's brightness from high spatial resolution (0 . 16-0 . 32) observations of the Swedish 1-m Solar Telescope (SST) and Hinode satellite. The line-of-sight magnetic field and continuum intensity near Fe i 6302.5 Å are used to quantify the correlation between field strength and brightness. The data show that magnetic flux density contains a significant amount of intrinsically weak fields that contribute little to brightness. We show that with data of high spatial resolution a calibration of magnetic flux density as a proxy for brightness excess is possible. In the SST data, the magnetic brightening of a quiet region with an average (unsigned) flux density of 10 G is about 0.15%. In the Hinode data, and in SST data reduced to Hinode resolution, the measured brightening is some 40% lower. With appropriate correction for resolution, magnetic flux density can be used as a reliable proxy in regions of small scale mixed polarity. The measured brightness effect is larger than the variation of irradiance over a solar cycle. It is not clear, however, if this quiet Sun contribution actually varies significantly.

Is there a non-monotonic relation between photospheric brightness and magnetic field strength in solar plage regions?

“…For 0.5 m telescope aperture, the mean contrast reaches a maximum at around 1000 G and then decreases again, so that at this resolution strong magnetic features appear up to 10% darker than the mean intensity. In summary, while the mean contrast curve at original simulation resolution confirms the expectations outlined in the introduction (in the absence of pores), for reduced resolution it saturates or even turns over for strong fields, which is in accordance with observational results at similar spatial resolution (Narayan & Scharmer 2010;Kobel et al 2011;Schnerr & Spruit 2011). The contrast peak occurs at a somewhat higher field than in the observations.…”

“…13), although the variable environment of the magnetic features leads to considerable scatter. On the other hand, observations (Title et al 1989;Topka et al 1992;Title et al 1992;Lawrence et al 1993;Stangl & Hirzberger 2005;Narayan & Scharmer 2010;Schnerr & Spruit 2011) typically show a different relation between magnetic field signal and contrast (relative difference to the average disk-center brightness). Either the contrast is found to be monotonically decreasing with increasing field (possibly with a "knee" at intermediate values) or the relation is not monotonic: after initially increasing for weak fields, the contrast reaches a maximum and then decreases again for stronger fields, even if pores are explicitly removed from the analysis (Kobel et al 2011).…”

Center-to-limb variation of the area covered by magnetic bright points in the quiet Sun

“…6b), assuming BPs with a typical contrast on the order of 1.15. This figure for the contribution of the quiet Sun magnetic fields to TSI is similar to the estimate in Schnerr & Spruit (2011) mentioned in Sect. 1.…”

On the relation between continuum brightness and magnetic field in solar active regions

“…The "fishhook" shape of the curves for low field values reflects the transition from very weakly magnetized regions (granules, newly formed intergranular lanes) with field strengths A118, page 4 of 5 below 150 G to the flux of a few hundred G advected into the intergranular lanes that still has too low field strength to affect the local brightness (Schnerr & Spruit 2011).…”

“…The relation between the local values of brightness and magnetic field strength at the solar surface is a crucial ingredient for quantitative models and reconstructions of solar (ir)radiance (Fröhlich & Lean 2004;Domingo et al 2009;Schnerr & Spruit 2011;Ball et al 2012). Comprehensive numerical simulations of near-surface magneto-convection can be used to study the effect of the magnetic field on the local radiance, including spatial scales that are not resolved by observations.…”