The continuum intensity as a function of magnetic field

Abstract

Context. Small-scale magnetic fields are major contributors to the solar irradiance variations. Hence, the continuum intensity contrast of magnetic elements in the quiet Sun (QS) network and in active region (AR) plage is an essential quantity that needs to be measured reliably. Aims. By using Hinode/SP disk center data at a constant, high spatial resolution, we aim at updating results of earlier ground-based studies of contrast vs. magnetogram signal, and to look for systematic differences between AR plages and QS network. Methods. The field strength, filling factor and inclination of the field was retrieved by means of a Milne-Eddington inversion (VFISV code). As in earlier studies, we then performed a pixel-by-pixel study of 630.2 nm continuum contrast vs. apparent (i.e. averaged over a pixel) longitudinal magnetic field over large fields of view in ARs and in the QS. Results. The continuum contrast of magnetic elements reaches larger values in the QS (on average 3.7%) than in ARs (on average 1.3%). This could not be attributed to any systematic difference in the chosen contrast references, so that it mainly reflects an intrinsic brightness difference. The larger contrasts in the QS are in agreement with earlier, lower resolution results, although our values are larger due to our better spatial resolution. At Hinode's spatial resolution, moreover, the relationship between contrast and apparent longitudinal field strength exhibits a peak at around 700 G in both the QS and ARs, whereas earlier lower resolution studies only found a peak in the QS and a monotonic decrease in ARs. We attribute this discrepancy both to our careful removal of the pores and their close surroundings affected by the telescope diffraction, as well as to the enhanced spatial resolution and very low scattered light of the Hinode Solar Optical Telescope. We verified that the magnetic elements producing the peak in the contrast curve are rather vertical in the AR and in the QS, so that the larger contrasts in the QS cannot be explained by larger inclinations, as had been proposed earlier.The opposite polarities in ARs do not exhibit any noticeable difference in inclination either, although they reach different contrasts when the amount of flux is significantly unbalanced between the polarities. Conclusions. According to our inversions, the magnetic elements producing the peak of the contrast curves have similar properties (field strength, inclination, filling factor) in ARs and in the QS, so that the larger brightness of magnetic elements in the QS remains unexplained. Indirect evidence suggests that the contrast difference is not primarily due to any difference in average size of the magnetic elements. A possible explanation lies in the different efficiencies of convective energy transport in the QS and in ARs, which will be the topic of a second paper.

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

“…For pixel-based values, a comparison of the mean relations between the disk-center brightness contrast (with respect to the average quiet Sun) and the magnetic field strength from observations and simulations at their respective spatial resolution reveals a striking disagreement. The simulations basically show a monotonic increase of contrast with field strength , while the observations exhibit a maximum contrast (which may even be negative) at intermediate field strength (or magnetogram signal) and a decrease towards stronger field values (e.g., Title et al 1992;Topka et al 1992;Lawrence et al 1993;Narayan & Scharmer 2010;Kobel et al 2011), even if dark pores are explicitely excluded. On the other hand, if only bright features are considered, observers find increasing brightness (or saturation) with growing field strength (Viticchié et al 2010;Berger et al 2007).…”

“…1, upper panel) to determine the I − B relation for small-scale magnetic features (i.e., excluding pores) in a plage region. The data were treated in exactly the same way as described by Kobel et al (2011) in order to obtain maps of the magnetic field and of the intensity in the red continuum of the 630.25 nm line. This included an inversion of the Stokes spectra with the VFISV code developed by Borrero et al (2011), which provides the "apparent" strength of the line-of-sight component of the field, B LOS = Bα cos γ, where B is the "intrinsic" field strength, α is the fractional amount of light that comes from the magnetized plasma, and γ is the inclination angle with respect to the observer (here, the vertical direction).…”

The possible origin of facular brightness in the solar atmosphere

“…the entire profile was used to calculate the dependencies, which should give more precise results than in Title et al (1992) and Topka et al (1992), where the combination of non-simultaneous observations in the two wings of a spectral line was used. In a newer observations by Hinode, with much higher spatial resolution and when the magnetic field strength (not flux) was obtained using inversions, Kobel et al (2011) find that the relationship between contrast and apparent longitudinal field strength exhibits a peak at around 700 G, both for the quiet Sun network and active region plage, while earlier studies only found a monotonic decrease in active regions. 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).…”

“…This latter dependence makes it possible to test theoretical models by means of observations. Indeed, observations of the solar disc center given in Frazier (1971), Title et al (1992), Topka et al (1992Topka et al ( , 1997, Montagne et al (1996), Kobel et al (2011), Kostyk (2013) show that the absolute value of continuum contrast of facular regions (without considering separately granules and intergranular lanes) depends on the magnetic field, though the observational findings are somewhat contradictory. Frazier (1971) shows that the continuum contrast of the facula at the solar disc center and the magnetogram signal are correlated in a complicated manner.…”

“…High spatial resolution observations reveal that facular regions break into clusters of bright points, small pores, and facular granular cells (Dunn & Zirker 1973;Title et al 1992;Berger et al 2004;Lites et al 2004;Narayan & Scharmer 2010;Kobel et al 2011;Viticchié et al 2011). It is thought that these features appear as a consequence of the presence of small-scale magnetic elements (or flux tubes) of the size of hundreds of kilometers and magnetic field strength of the order of 1−2 kG (Stenflo 1973;Solanki 1993).…”