Discovery of Scattering Polarization in the Hydrogen Lyα Line of the Solar Disk Radiation

Abstract

There is a thin transition region (TR) in the solar atmosphere where the temperature rises from 10,000 K in the chromosphere to millions of degrees in the corona. Little is known about the mechanisms that dominate this enigmatic region other than the magnetic field plays a key role. The magnetism of the TR can only be detected by polarimetric measurements of a few ultraviolet (UV) spectral lines, the Lyα line of neutral hydrogen at 121.6nm (the strongest line of the solar UV spectrum) being of particular interest given its sensitivity to the Hanle effect (the magnetic-field-induced modification of the scattering line polarization). We report the discovery of linear polarization produced by scattering processes in the Lyα line, obtained with the Chromospheric Lyman-Alpha SpectroPolarimeter (CLASP) rocket experiment. The Stokes profiles observed by CLASP in quiet regions of the solar disk show that the Q/I and U/I linear polarization signals are of the order of 0.1% in the line core and up to a few percent in the nearby wings, and that both have conspicuous spatial variations with scales of ∼10arcsec. These observations help constrain theoretical models of the chromosphere-corona TR and extrapolations of the magnetic field from photospheric magnetograms. In fact, the observed spatial variation from disk to limb of polarization at the line core and wings already challenge the predictions from three-dimensional magnetohydrodynamical models of the upper solar chromosphere.

A Statistical Inference Method for Interpreting the CLASP Observations

“…In general, the statistical models chosen for interpreting the CLASP observations should take into account the fact that there may exist correlations between the intensity and polarization signals. This has been predicted by Stěpán et al (2015) and later confirmed by CLASP (see Kano et al 2017), and in Trujillo Bueno et al (2018) we will take into account that plasma structures, such as internetwork and network regions, differ not only in intensity but also in their polarization signals. However, for the sake of simplicity, in this paper, we ignore such correlations.…”

“…Recently, the Chromospheric Lyman-Alpha Spec-troPolarimeter (CLASP) sounding rocket experiment provided the first ever successful measurement of the linear polarization Q/I and U/I profiles produced by scattering processes in the hydrogen Ly-α line of the solar disk radiation (Kano et al 2017), as well as in the Si iii resonance line at 1206Å (Ishikawa et al 2017). Such novel spectropolarimetric observations, with a spatial resolution of about 3 arcseconds, confirmed the following theoretical predictions for the Ly-α line Belluzzi et al 2012;Štěpán et al 2015):…”

Polarization Calibration of the Chromospheric Lyman-Alpha SpectroPolarimeter for a 0.1% Polarization Sensitivity in the VUV Range. Part II: In-Flight Calibration

“…Observations were conducted at the solar disc center for around 20 seconds, before re-pointing close to the limbs (i.e. main science target) for about 280 seconds (Kano et al, 2016). The disc center was observed to provide an in-flight calibration of the spurious polarization: the atomic-level polarization was expected to vanish at µ=1 (i.e.…”

CLASP Constraints on the Magnetization and Geometrical Complexity of the Chromosphere-Corona Transition Region

“…Moreover, such radiative transfer investigations pointed out that the line-center polarization is modified by the presence of magnetic fields in the chromosphere-corona TR via the Hanle effect. These theoretical investigations motivated the development of the Chromospheric Lyman-Alpha SpectroPolarimeter (CLASP), an international sounding rocket experiment that on 3rd September 2015 successfully measured the wavelength variation of the intensity and linear polarization of the Ly-α line in quiet regions of the solar disk (see Kano et al 2017, and references therein to the papers describing the instrument). CLASP observed the Ly-α Stokes profiles I, Q, and U with a spatial resolution of about 3 arcsec and a temporal resolution of 5 minutes.…”