Instrumentation for solar spectropolarimetry: state of the art and prospects

Abstract

Given its unchallenged capabilities in terms of sensitivity and spatial resolution, the combination of imaging spectropolarimetry and numeric Stokes inversion represent the dominant technique currently used to remotely sense the physical properties of the solar atmosphere, and in particular, its important driving magnetic field. Solar magnetism manifests itself in a wide range of spatial, temporal and energetic scales. The ubiquitous but relatively small and weak fields of the so called quiet Sun are believed today to be crucial for answering many open questions in solar physics, some of which have substantial practical relevance due to the strong Sun-Earth connection. However, such fields are very challenging to detect because they require spectropolarimetric measurements with high spatial (subarcsec), spectral (< 100 m) and temporal (< 10 s) resolution along with high polarimetric sensitivity (< 0.1% of the intensity). In this review we collect and discuss both well-established and upcoming instrumental solutions developed during the last decades to push solar observations towards the above-mentioned parameter regime. This typically involves design trade-offs due to the high dimensionality of the data and signal-to-noise-ratio considerations, among others. We focus on the main three components that form a spectropolarimeter, namely, wavelength discriminators, the devices employed to encode the incoming polarization state into intensity images (polarization modulators), and the sensor technologies used to register them. We consider the instrumental solutions introduced to perform this kind of measurements at different optical wavelengths and from various observing locations, i.e., ground-based, from the stratosphere or near space.

Measuring Magnetic Field With Atacama Large Millimeter/Submillimeter Array

“…Currently, ALMA antennas are equipped with eight receivers, covering the range of wavelengths from 0.3 mm (Band 10, 950 GHz) to 3.6 mm (Band 3, 84 GHz). Two receivers for longer wavelengths are planned to be added, with one receiver up to 8.6 mm (Band 1, 35 GHz, Huang et al, 2016Huang et al, , 2018 being in construction, and the second one, for the wavelengths up to 4.6 mm (Band 2, 67-116 GHz, Yagoubov et al, 2020), being under development. The array is reconfigurable in multiple patterns (configurations) ranging in size from 150 m (compact) up to 16 km (extended), depending on the required sensitivity and spatial resolution.…”

The Visible Spectro-Polarimeter of the Daniel K. Inouye Solar Telescope

“…Most modern solar telescopes are equipped with spectro-polarimeters (Iglesias and Feller, 2019). Imaging spectro-polarimeters using Fabry-Pérot interferometers have become commonly available at many telescopes, such as the Triple Etalon Solar Spectrometer (Kentischer et al, 1998;Tritschler et al, 2002) at the German Vacuum Tower Telescope (VTT: von der Lühe, 1998), and the CRisp Imaging SpectroPolarimeter (Scharmer et al, 2008) at the Swedish 1-m Solar Telescope (SST: Scharmer et al, 2003).…”

A nonlinear solar magnetic field calibration method for the filter-based magnetograph by the residual network

“…At present, the solar magnetic field is generally measured indirectly by means of the Zeeman effect (Hale 1908) of the magnetically sensitive solar spectral lines. According to the different focal plane equipment, the optical magnetograph can be classified as the systems of the systems of the grating spectrographs (SGs) and filtergraphs (FGs) (Lin 2001;Iglesias & Feller 2019). Additionally, the integral field solutions are being developed to solve the inability of SGs and FGs to cover the desired spatial and spectral field of views simultaneously (Iglesias & Feller 2019).…”