Discovery of Scattering Polarization in the Hydrogen Lyman-$\alpha$ 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 Lyman-$\alpha$ 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 Lyman-$\alpha$ line, obtained with the Chromospheric Lyman-Alpha Spectro-Polarimeter (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 $\sim 10$ 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.

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Cited by in corpus (26)

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• Measuring the magnetic origins of solar flares, CMEs and Space Weather
• Quiet-Sun hydrogen Lyman-alpha line profile derived from SOHO/SUMER solar-disk observations
• Modeling the scattering polarization of the solar Ca i 4227 Å line with angle-dependent partial frequency redistribution
• Mg ii h&k spectra of an enhanced network region simulated with the MURaM-ChE code. Results using 1.5D synthesis
• Magnetic sensitivity in the wing scattering polarization signals of the hydrogen Lyman-alpha line of the solar disk radiation
• The transfer of polarized radiation in resonance lines with partial frequency redistribution, J-state interference, and arbitrary magnetic fields. A radiative transfer code and useful approximations
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• Empirical relations between the intensities of Lyman lines of H and He${}^{+}$
• Structure and dynamics of the internetwork solar chromosphere: results of a small-scale dynamo simulation
• Comparison of Solar Fine Structure Observed Simultaneously in Ly-α and Mg II h
• Spectral lines in FUV and EUV for diagnosing coronal magnetic field
• The magnetic sensitivity of the (250-278nm) Fe II polarization spectrum
• Evidence for the Operation of the Hanle and Magneto-Optical Effects in the Scattering Polarization Signals Observed by CLASP2 Across the Mg II h and k Lines
• Assessment of the CRD approximation for the observer's frame RIII redistribution matrix
• Diagnostic potential of wavelength-integrated scattering polarisation signals of the solar He II Ly-alpha line
• A numerical approach for modelling the polarisation signals of strong resonance lines with partial frequency redistribution. Numerical applications to two-term atoms and plane-parallel atmospheres
• Determining the Magnetic Field in the Atmosphere of a Solar Active Region Observed by the CLASP2.1 Sounding Rocket Experiment
• A Statistical Inference Method for Interpreting the CLASP Observations
• Magnetic Imaging of the Outer Solar Atmosphere (MImOSA): Unlocking the driver of the dynamics in the upper solar atmosphere
• Fast and accurate approximation of the angle-averaged redistribution function for polarized radiation
• The potential of the wavelength-integrated scattering polarization of the hydrogen Ly-alpha line for probing the solar chromosphere
• Rayleigh Scattering in Spectral Series with L-Term Interference