On spectropolarimetric measurements with visible lines

Abstract

The ability of new instruments for providing accurate inferences of vector magnetic fields and line-of-sight velocities of the solar plasma depends a great deal on the sensitivity to these physical quantities of the spectral lines chosen to be measured. Recently, doubts have been raised about visible Stokes profiles to provide a clear distinction between weak fields and strong ones filling a small fraction of the observed area. The goal of this paper is to give qualitative and quantitative arguments that help in settling the debate since several instruments that employ visible lines are either operating or planned for the near future. The sensitivity of the Stokes profiles is calculated through the response functions (e.g. Ruiz Cobo & Del Toro Iniesta, 1994). Both theoretical and empirical evidences are gathered in favor of the reliability of visible Stokes profiles. The response functions are used as well for estimating the uncertainties in the physical quantities due to noise in the observations. A useful formula has been derived that takes into account the measurement technique (number of polarization measurements, polarimetric efficiencies, number of wavelength samples), the model assumptions (number of free parameters, the filling factor), and the radiative transfer (response functions). We conclude that a scenario with a weak magnetic field can reasonably be distinguished with visible lines from another with a strong field but a similar Stokes $V$ amplitude, provided that the Milne-Eddington approximation is good enough to describe the solar atmosphere and that the polarization signal is at least three or four times larger than the typical rms noise of $10^{-3}{I}_{\mathrm{c}}$ reached in the observations.

Accepted for publication in The Astrophysical Journal

References in corpus (10)

• The Solar Optical Telescope for the Hinode Mission: An Overview
• Quiet Sun internetwork magnetic fields from the inversion of Hinode measurements
• On the validity of the 630 nm Fe I nm lines for the magnetometry of the internetwork quiet Sun
• The usefulness of analytic response functions
• Quiet Sun magnetic fields from simultaneous inversions of visible and infrared spectropolarimetric observations
• Internetwork magnetic field distribution from simultaneous 1.56 micron and 630 nm observations
• A 3-D sunspot model derived from an inversion of spectropolarimetric observations and its implications for the penumbral heating
• Quiet Sun magnetic fields from space-borne observations: simulating Hinode's case
• Temporal evolution of the Evershed flow in sunspots. I. Observational characterization of Evershed clouds
• Temporal evolution of the Evershed flow in sunspots. II. Physical properties and nature of Evershed clouds

Cited by in corpus (9)

• Inversion of the radiative transfer equation for polarized light
• Analysis of Quiet-Sun Internetwork Magnetic Fields Based on Linear Polarization Signals
• Inferring the magnetic field vector in the quiet Sun. II. Interpreting results from the inversion of Stokes profiles
• Applicability of Milne-Eddington inversions to high spatial resolution observations of the quiet Sun
• Pervasive Linear Polarization Signals in the Quiet Sun
• Assessing the Behavior of Modern Solar Magnetographs and Spectropolarimeters
• Modelling and Interpreting The Effects of Spatial Resolution on Solar Magnetic Field Maps
• A modified Milne-Eddington approximation for a qualitative interpretation of chromospheric spectral lines
• Diagnostics for spectropolarimetry and magnetography