spectropolarimetry

Spectropolarimetry is a technique that measures the polarization state of light as a function of wavelength, combining elements of spectroscopy and polarimetry. The data are typically expressed as spectra of the Stokes parameters I, Q, U, and V, where I is the total intensity, Q and U describe linear polarization, and V describes circular polarization. By recording polarization across spectral features, spectropolarimetry probes magnetic fields, scattering geometries, and the optical properties of astronomical and atmospheric media.

In astronomy and solar physics, spectropolarimetry is used to infer magnetic fields via the Zeeman and Hanle effects, to study the geometry of circumstellar material, stellar atmospheres, and explosive transients, and to characterize polarization produced by scattering in disks, winds, or cometary comae. Spectropolarimetric signatures across spectral lines carry information about field strength, orientation, and atmospheric conditions. In planetary science and remote sensing, it helps characterize planetary atmospheres and surfaces through the polarization of reflected light. Time-resolved and spatially resolved spectropolarimetry support surveys and targeted studies of diverse objects.

Instrumentation typically combines a spectrograph with a polarimeter. A polarization modulator (such as a wave plate or liquid-crystal retarder) and a polarization analyzer (e.g., a Wollaston prism) split the light into orthogonal components that are measured by a detector. Data reduction must correct for instrumental polarization, telescope-induced changes, and atmospheric or interstellar depolarization, often using calibration with known polarized and unpolarized sources. The resulting data enable magnetic-field inversions, scattering analyses, and characterization of atmospheric and astronomical media.