A Novel Investigation of the Small-scale Magnetic Activity of the Quiet Sun via the Hanle Effect in the Sr i 4607 Å Line

Abstract

One of the key research problems in stellar physics is to decipher the small-scale magnetic activity of the quiet solar atmosphere. Recent magneto-convection simulations that account for small-scale dynamo action have provided three-dimensional (3D) models of the solar photosphere characterized by a high degree of small-scale magnetic activity, similar to that found through theoretical interpretation of the scattering polarization observed in the Sr i 4607Å line. Here we present the results of a novel investigation of the Hanle effect in this resonance line, based on 3D radiative transfer calculations in a high-resolution magneto-convection model having most of the convection zone magnetized close to the equipartition and a surface mean field strength B ≈170 G. The Hanle effect produced by the model's magnetic field depolarizes the zero-field scattering polarization signals significantly, to the extent that the center-to-limb variation of the calculated spatially-averaged polarization amplitudes is compatible with the observations. The standard deviation of the horizontal fluctuations of the calculated scattering polarization signals is very sensitive to the model's magnetic field and we find that the predicted spatial variations are sufficiently sizable so as to be able to detect them, especially with the next generation of solar telescopes. We find that at all on-disk positions the theoretical scattering polarization signals are anti-correlated with the continuum intensity. To facilitate reaching new observational breakthroughs, we show how the theoretically predicted polarization signals and spatial variations are modified when deteriorating the signal-to-noise ratio and the spectral and spatial resolutions of the simulated observations.

Solar Disk Center Shows Scattering Polarization in the Sr i 4607 Å Line

“…The most striking result is that we find the scattering polarization to be anticorrelating with the radiation field's quadrupole tensor element J 2 2 estimated from the observed Stokes I map, which is dominated by the continuum radiation in our data. We also find that the spatial structure of the scattering polarization is subgranular, in agreement with the theoretical predictions by Trujillo Shchukina (2007) anddel Pino Alemán et al (2018). This means that atomic polarization is not destroyed completely even in very dense layers of the quiet solar photosphere, confirming the results from numerical calculations reported by Trujillo Bueno & Shchukina (2007);del Pino Alemán et al (2018).…”

“…The RMS of the magnetic field strength ∆B rms over the observed field of view is such that 2(3∆B rms /22.8 G) 10 in the collisionless limit (here, the critical field 22.8 G appears from expressing the Larmor frequency in units of the Einstein coefficient for spontaneous emission A u = 2 × 10 8 s −1 , a factor of two is related to the quadrupolar component of the radiation field and the majority of magnetic field strength values, assuming a Gaussian distribution, lies between ±3∆B rms ). Collisions contribute additionally to relax atomic polarization and the critical field increases by a factor of (1 + + δ), where and δ are the rates of inelastic and elastic depolarizing collisions, respectively, normalized to A u (Landi Degl 'Innocenti & Landolfi 2004 Sainz et al 2014;del Pino Alemán et al 2018). Note that with increasing δ, the scattering polarization generated is also reduced and if δ 1, it would be completely wiped out.…”

“…However, resolving subgranular scales is necessary in order to resolve the polarization emerging from the axial symmetry breaking of the radiation field introduced by the thermal inhomogeneity due to granulation. Investigations based on the Hanle effect (Trujillo del Pino Alemán et al 2018) and…”

Quiet Sun magnetic fields: an observational view

“…Thus, observing the Sr i line at high spatial resolution is of considerable interest to distinguish between scenarios. The latest investigations are based on small-scale dynamo simulations and confirm that the scattering polarization of the Sr i 460.7 nm line is very sensitive to the model magnetic field, showing spatial variations that should be detectable with the next generation of solar telescopes (del Pino Alemán et al 2018). Interestingly, the scattering polarization signals resulting from the simulations are anticorrelated with the continuum intensity everywhere on the disk.…”

The Dimmest State of the Sun

“…Internetwork intensity is given by the mean intensity of the SSD‐only run snapshots, and facular intensity by the intensity of the facular features apparent in the snapshots of the facular runs. MURaM simulations are well suited for the current purpose, having been demonstrated in multiple studies to reproduce the observed properties of the internetwork magnetic field (Danilovic et al, 2010, 2016; del Pino Alemán et al, 2018; Rempel, 2014; Schüssler & Vögler, 2008) and of faculae and network (Afram et al, 2011; Danilovic et al, 2013; Riethmüller et al, 2014; Shelyag et al, 2004; Yeo et al, 2017).…”