Instrumentation for solar spectropolarimetry: state of the art and prospects

TL;DR

This paper reviews current and future instrumental techniques for solar spectropolarimetry, emphasizing high-resolution measurements crucial for understanding solar magnetism and its impact on space weather.

Contribution

It provides a comprehensive overview of instrumental solutions across different wavelengths and observing platforms, highlighting recent advances and design trade-offs.

Findings

High-resolution spectropolarimetric techniques enable detection of weak solar magnetic fields.

Instrumental innovations improve sensitivity and resolution in solar magnetic field measurements.

Different observational platforms offer complementary capabilities for solar spectropolarimetry.

Abstract

Given its unchallenged capabilities in terms of sensitivity and spatial resolution, the combination of imaging spectropolarimetry and numeric Stokes inversion represents 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 (sub-arcsec), spectral (<100 mA), and temporal (<10 s) resolution along with high polarimetric sensitivity (<0.001 of the intensity). We collect and discuss both well-established and upcoming instrumental solutions developed during the last decades to push solar observations toward 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 spectro-polarimeter, 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.