Relation between photospheric magnetic field and chromospheric emission

Abstract

Aims. We investigate the relationship between the photospheric magnetic field and the emission of the mid chromosphere of the Sun. Methods. We simultaneously observed the Stokes parameters of the photospheric iron line pair at 630.2 nm and the intensity profile of the chromospheric Ca  H line at 396.8 nm in a quiet Sun region at a heliocentric angle of 53• . Various line parameters have been deduced from the Ca  H line profile. The photospheric magnetic field vector has been reconstructed from an inversion of the measured Stokes profiles. After alignment of the Ca and Fe maps, a common mask has been created to define network and inter-network regions. We perform a statistical analysis of network and inter-network properties. The H-index is the integrated emission in a 0.1 nm band around the Ca core. We separate a non-magnetically, H non , and a magnetically, H mag , heated component from a non-heated component, H co in the H-index. Results. The average network and inter-network H-indices are equal to 12 and 10 pm, respectively. The emission in the network is correlated with the magnetic flux density, approaching a value of H ≈ 10 pm for vanishing flux. The inter-network magnetic field is dominated by weak field strengths with values down to 200 G and has a mean absolute flux density of about 11 Mx cm −2 . Conclusions. We find that a dominant fraction of the calcium emission caused by the heated atmosphere in the magnetic network has non-magnetic origin (H mag ≈ 2 pm, H non ≈ 3 pm). Considering the effect of straylight, the contribution from an atmosphere with no temperature rise to the H-index (H co ≈ 6 pm) is about half of the observed H-index in the inter-network. The H-index in the inter-network is not correlated to any property of the photospheric magnetic field, suggesting that magnetic flux concentrations have a negligible role in the chromospheric heating in this region. The height range of the thermal coupling between the photosphere and low/mid chromosphere increases in presence of magnetic field. In addition, we demonstrate that a poor signal-to-noise level in the Stokes profiles leads to a significant over-estimation of the magnetic field strength.

The relationship between chromospheric emissions and magnetic field strength

“…For instance, if we set a B threshold of 20 G we cannot distinguish between the quality of the linear and powerlaw fits for all chromospheric emissions analyzed here. Our conclusions agree with those presented by Rezaei et al (2007), who found from the POLIS measurements at 1 resolution a clear non-linear increase of the calcium core flux with magnetic flux for values B ≤ 100 G and a slow increase for higher values. They also demonstrated that the power-law exponent depends strongly on the value of the lower threshold.…”

“…Unfortunately, due to the very low B values in the internetwork, only a few percent of IN structures are associated with magnetogram signal exceeding the noise level. However, it should be noted that analysis of the cell interiors and their relation to the underlying magnetic field carried out in Rezaei et al (2007) for Ca ii H-line with lower noise and higher resolution has revealed similar results. This supports the idea that the role of magnetic field is negligible for bright internetwork grains (e.g.…”

“…Skumanich et al 1975;Schrijver et al 1989;Nindos & Zirin 1998;Harvey & White 1999;Ortiz & Rast 2005;Rezaei et al 2007). In the quiet-Sun chromosphere Skumanich et al (1975) and Nindos & Zirin (1998) found a linear correlation between the K-line intensities and the absolute values of the magnetic field strength.…”

“…Considering 4 types of brightness structures, which included Article published by EDP Sciences active regions, decaying active regions, the enhanced network and quiet (weak) network, Harvey & White (1999) ascertained that in both active and quiet Sun the relation between the magnetic flux density and calcium residual intensity follows a power law with an exponent of 0.5. Ortiz & Rast (2005) cited a powerlaw exponent value of 0.66 for the quiet Sun, while Rezaei et al (2007) obtained a lower exponent of 0.2 for locations in a quietSun region, and higher values of 0.4-0.5 for network locations, depending on the magnetic field threshold value. found power-law relations between excess flux densities in stellar chromospheric lines with differing temperature responses, with the exponents increasing with increasing difference between their temperatures of formation.…”

The magnetic flux of the quiet Sun internetwork as observed with the Tenerife infrared polarimeter

“…8) shows an enhanced fraction of these weak fields below 200 G, which implies that the detection limit due to the noise is not yet reached at 200 G. In the question of the seemingly preference for kG fields in the analysis of 630 nm data, Martínez González et al (2006b) cautioned that the thermodynamics must be treated consistently in an analysis of the 630 nm line pair in the weak field limit. Rezaei et al (2007) were able to derive a distribution without the bump using a SIR inversion of 630 nm data of lower spatial resolution than available nowadays in HINODE observations. It would be worthwhile to analyse the HINODE data without employing the Milne-Eddington (ME) approximation to determine whether the field strength distribution changes with a more sophisticated treatment of the thermodynamics.…”