Three-lobed near-infrared Stokes V profiles in the quiet Sun

Abstract

Context. The 1.5-m GREGOR solar telescope can resolve structures as small as 0.4" at near-infrared (NIR) wavelengths on the Sun. At this spatial resolution the polarized solar spectrum shows complex patterns, such as large horizontal and/or vertical variations of the physical parameters in the solar photosphere. Aims. We investigate a region of the quiet solar photosphere exhibiting three-lobed Stokes V profiles in the Fe I spectral line at 15648Å. The data were acquired with the GRIS spectropolarimeter attached to the GREGOR telescope. We aim at investigating the thermal, kinematic and magnetic properties of the atmosphere responsible for these measured complex signals.Methods. The SIR inversion code is employed to retrieve the physical parameters of the lower solar photosphere from the observed polarization signals. We follow two different approaches. On the one hand, we consider that the multi-lobe circular polarization signals are only produced by the line-of-sight variation of the physical parameters. We therefore invert the data assuming a single atmospheric component that occupies the entire resolution element in the horizontal plane and where the physical parameters vary with optical depth τ (i.e., line-of-sight). On the other hand, we consider that the multi-lobe circular polarization signals are produced not by the optical depth variations of the physical parameters but instead by their horizontal variations. Here we invert the data assuming that the resolution element is occupied by two different atmospheric components where the kinematic and magnetic properties are constant along the line-of-sight. Results. Both approaches reveal some common features about the topology responsible for the observed three-lobed Stokes V signals: both a strong (> 1000 Gauss) and a very weak (< 10 Gauss) magnetic field with opposite polarities and harboring flows directed in opposite directions must co-exist (either vertically or horizontally interlaced) within the resolution element.

Temporal evolution of small-scale internetwork magnetic fields in the solar photosphere

“…Therefore, we have found linear polarization located between opposite polarity vertical fields in both ROI A and C, and it is certainly conceivable that this may be characteristic of how the deep photospheric magnetic field is organized at these scales and resolution. Mixed polarity Stokes V profiles have also been observed in the infrared in previous studies (Khomenko et al (2003);Martínez González et al (2016); Kiess et al (2018)). Multi-lobed Stokes V profiles have been found in highly blue-or red-shifted Stokes profiles, indicative of supersonic magnetic flows at possible sites of reconnection (Borrero et al 2013).…”

“…However, the question remains unresolved as to whether this substructure is generated by gradients along the LOS or across the solar surface. Kiess et al (2018) inverted these data, focusing Stokes V profiles with more than two lobes, indicative of the presence of opposite polarities co-existing either vertically (along the LOS) or horizontally within the resolution element.…”

Using the infrared iron lines to probe solar subsurface convection

“…) and this property of the infrared lines has been exploited to probe deep into the solar photosphere, in the sunspot penumbrae, by Borrero et al Article number, page 1 of 11 arXiv:1904.07306v1 [astro-ph.SR] 15 Apr 2019 A&A proofs: manuscript no. Deep_convection_arxiv (2016), but also in the quiet Sun (Martínez González et al 2007Lagg et al 2016;Martínez González et al 2016;Kiess et al 2018). Also, the mean solar spectrum of these lines was modeled, either to infer atomic parameters (Borrero et al 2003) or to verify state-of-the-art MHD simulations (Beck et al 2013).…”

Formation and morphology of anomalous solar circular polarization

“…Observing quiet-sun intergranular lanes, Kiess et al (2018) found near-ubiquituous three-lobe Stokes V signals in the 1564.8 nm Fe i line. Carrying out detailed modeling and inversions, these authors concluded that two magnetic field components with very different strengths (9 G and 1147 G) coexisting in the same pixel are required to explain the signals.…”

“…Despite this, we think that the preliminar results support our model as a good starting point for explaining the signals without assuming spatiotemporally unresolved components. Kiess et al 2018). Red profiles: NLTE synthesis obtained with our model assuming a two-level atom for the transition e 7 D 1n 7 D o 1 , with A u = 1.94 · 10 −6 , effective Landé factor g eff L = 3, negligible damping constant a = 0, T = 8000 K, and w 1 0 = −0.03.…”