Constraining the magnetic vector in the quiet solar photosphere and the impact of instrumental degradation

Abstract

Context. With the advent of next generation high resolution telescopes, our understanding of how the magnetic field is organized in the internetwork (IN) photosphere is likely to advance significantly. Aims. We aim to evaluate the extent to which we can retrieve accurate information about the magnetic vector in the IN photosphere using inversion techniques. Methods. We use a snapshot produced from high resolution three-dimensional magnetohydrodynamic (MHD) simulations and employ the Stokes Inversions based on Response functions (SIR) code to produce synthetic observables in the same near infrared spectral window as observed by the GREGOR Infrared Spectrograph (GRIS), which contains the highly magnetically sensitive photospheric Fe I line pair at 15 648.52 Å and 15 652.87 Å. We then use a parallelized wrapper to SIR to perform nearly 14 million inversions of the synthetic spectra to test how well the ‘true’ MHD atmospheric parameters can be constrained statistically. Finally, we degrade the synthetic Stokes vector spectrally and spatially to GREGOR resolutions and examine how this influences real observations, considering the impact of stray light, spatial resolution and signal-to-noise (S/N) in particular. Results. We find that the depth-averaged parameters can be recovered by the inversions of the undegraded profiles, and by adding simple gradients to magnetic field strength, inclination, and line of sight velocity we show that an improvement in the χ2 value is achieved. We also evaluate the extent to which we can constrain these parameters at various optical depths, with the kinematic and thermodynamic parameters sensitive deeper in the atmosphere than the magnetic parameters. We find the S/N and spatial resolution both play a significant role in determining how the degraded atmosphere appears. At the same time, we find that the magnetic and kinematic parameters are invariant upon inclusion of an unpolarized stray light. We compare our results to recent IN observations obtained by GREGOR. We studied a linear polarization feature which resembles those recently observed by GRIS in terms of appearing as ‘loop-like’ structures and exhibiting very similar magnetic flux density. Thus, we demonstrate that realistic MHD simulations are capable of showing close agreement with real observations, and the symbiosis between them and observations continues to prove essential. We finally discuss the considerations that must be made for DKIST-era observations.

Exploring magnetic loops and serpentine fields in the quiet Sun with the GRIS-IFU

“…Thus, by setting the threshold at 5σ n , the dip in the γ distribution is created by excluding the weakest fields. This dip is also observed in the distribution from MU-RaM simulations by Campbell et al (2021b). For S2 value close to 90 • by SIR when a good fit is achieved, which is the reason for the central peak in the γ distribution.…”

“…This is an essential step to maximise the statistical chances of achieving the global χ 2 minimum solution for each Stokes vector. For the inversion, we adopt the same scheme as tested on real observations of the NIR line pair by Martínez González et al ( 2016); Campbell et al (2021a) and on synthetic observations by Campbell et al (2021b). This is a two-component inversion with one non-magnetic (i.e.…”

Exploring Magnetic Loops and Serpentine Fields in the Quiet Sun with the GRIS-IFU

“…This is an essential step to maximize the statistical chances of achieving the global χ 2 minimum solution for each Stokes vector. For the inversion, we adopt the same scheme as tested on real observations of the NIR line pair by Martínez González et al (2016), Campbell et al (2021a, and on synthetic observations by Campbell et al (2021b). This is a two-component inversion with one nonmagnetic model (i.e., the magnetic field strength, B, was set to zero and was not allowed to vary) and one magnetic model, which are combined by SIR in accordance with their respective filling factors, α, which is another free parameter.…”

“…Two-dimensional spectrographs like the GRIS-IFU are uniquely capable of providing time series imaging with high polarimetric sensitivity and spectral resolution simultaneously, with the size of the FOV and cadence acting as competing factors. Campbell et al (2021b) have predicted, using synthetic observations produced from MURaM simulations (Vögler et al 2005) degraded to GREGOR/GRIS-IFU resolutions, that increasing the total integration time of the observations will yield higher fractions of polarization and allow the magnetic inclination to be better constrained in a larger fraction of the FOV. We report observations calibrated based on these predictions.…”

Oscillations in the line-of-sight magnetic field strength in a pore observed by the GREGOR Infrared Spectrograph (GRIS)

“…By inverting multiple lines, with varying strengths, opacities, and g eff values, we can probe the atmosphere at a greater range of heights than if we inverted only one line. Campbell et al (2021b) The black contour outlines the pixels that were analysed using the wavelet method. The red crosses indicate the 15 pixels that displayed sustained high-amplitude oscillations within the period range studied here.…”