Swiss Virtual Institute for Solar Science
Polarimetry
Polarimetry and Second Solar Spectrum
During the past decade ETH Zurich has developed novel instrumentation for high-precision imaging spectro-polarimetry, which allows for imaging of the full Stokes vector with a polarimetric accuracy of about 0.0005% in the degree of polarization, which is close to two orders of magnitude better than other such systems in the astrophysical world. This system (ZIMPOL "Zurich Imaging Polarimeter") has been applied with great success at the world's leading solar telescopes and opened the door to a new world of polarization phenomena that could not be accessed by previous instruments, in particular polarized signatures from coherent scattering processes (which produce the so-called "second solar spectrum", see figure below). The home base of ZIMPOL in Switzerland is Istituto Ricerche Solari Locarno (IRSOL) where the observing opportunities were recently upgraded with the Fabry-Perot filter and adaptive optic systems. The first and the second volume of "The Second Solar Spectrum" atlas covering the wavelength range of 3910 - 6995 Å has been recorded at IRSOL.
Polarimetric applications include exploring various aspects of solar magnetic fields, in particular its fractal-like and largely spatially unresolved fine structure, probing internal structure of sunspots and examining fundamental quantum physics phenomena that govern the formation of astrophysical spectra. One of the highlights is the first detection of the Hanle effect in many atomic and molecular lines.
The figure above shows a small, 20 Å section from the Atlas of the Second Solar Spectrum, which was compiled from ZIMPOL observations at IRSOL. The diagram labeled Stokes I/Ic represents the normalized intensity spectrum (which may be considered as the "first solar spectrum"), while the bottom diagram labeled Stokes Q/I represents the simultaneously recorded degree of linear polarization. This polarized spectrum is called the "second solar spectrum" since it has little resemblance with the intensity spectrum and is formed by very different physical processes, and therefore gives complementary information about the Sun as if it were an entirely new spectrum. The spectral features that are identified in the above diagram are due to molecular lines from the so-called Q branch of MgH.