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First Direct Exploration of Magnetic Fields in the Upper Solar Atmosphere – Ultraviolet spectropolarimetry opens a new window for solar physics research

23/05/2017 - 10:45
For the first time, scientists have explored the magnetic field in the upper solar atmosphere by observing the polarization of ultraviolet light from the Sun. They accomplished this by analyzing data taken by the CLASP sounding rocket experiment during its 5-minute flight in space on September 3, 2015. Now that ultraviolet spectropolarimetry, the method used in the CLASP project, has been proven to work, it can be used in future investigations of the magnetic fields in the upper chromosphere and the transition region to better understand activity in the solar atmosphere.
By analyzing the characteristics of light from the Sun, astronomers can determine how it has been emitted and scattered in the solar atmosphere, and thus determine the conditions in the solar atmosphere. Because magnetic fields are thought to play an important role in various types of solar activity, many precise measurements of the magnetic fields at the solar surface (the so-called photosphere) have been made, but not so many observations have measured the magnetic fields in the solar atmosphere above the surface. While visible light is emitted from the photosphere, ultraviolet (UV) light is emitted and scattered in the parts of the solar atmosphere known as the chromosphere and the transition region. This is why the CLASP project aims at using a spectroscopic line in the ultraviolet, the hydrogen Lyman-α line, to investigate the magnetic fields in the upper chromosphere and the transition region.
The CLASP international team (Japan, United States, Spain, and France) then developed a spectropolarimeter, an instrument which provides detailed wavelength (color) and polarization (orientation of the light waves) information for light passing through a narrow slit.  The French team from Institut d'Astrophysique Spatiale (CNRS / Université Paris-Sud) has contributed to the optical design of the instrument and to the procurement of the spectroscopic grating, the component that divides light as a function of its wavelength.
Figure 1 shows (bottom right) the position of the spectropolarimeter slit on a background image taken by the slit-jaw camera onboard CLASP during its flight; the diagrams on the left show the wavelength and polarization data for the hydrogen Lyman-α line. These diagrams show that the hydrogen Lyman-α line from the Sun is actually polarized. The researchers have found that some of the polarization characteristics match those predicted by the theoretical models of light scattering in the solar atmosphere. However, other such characteristics are unexpected, indicating that the structures of the upper chromosphere and transition region are more complicated than the structures included in the models. In particular, the team discovered that polarization varied on small spatial scales of 1/50 to 1/100 of the solar radius.
In addition, polarization produced by scattering processes is affected by the local magnetic fields. To investigate such an effect of the magnetic field on the measured polarization, the team observed 3 different wavelength ranges in which polarization has different sensitivities to magnetic field: the core of the hydrogen Lyman-α line (121.567 nm very sensitive), an ionized silicon emission line (120.65 nm, lowly sensitive), and the wing of the hydrogen Lyman-α spectral line (insensitive to magnetic field). The results for the 4 regions indicated in Figure 1 are plotted in Figure 2 and demonstrate that the large deviations from the expected scattering polarization in the Lyman-α core and the silicon line are in fact due to the magnetic fields, because the Lyman-α wing polarization remains almost constant.
These epoch-making results are the first to directly show that magnetic fields exist in the transition region. They also demonstrate that ultraviolet spectropolarimetry is effective in studying solar magnetic fields. Moreover, these results have shown that sounding rocket experiments like CLASP can play an important role in pioneering new techniques, even though they are small scale and short term compared to satellites.
Dr. Ryoko Ishikawa, project scientist for the Japanese CLASP team, describes the significance of the results, “The successful observation of polarization indicative of magnetic fields in the upper chromosphere and the transition region means that ultraviolet spectropolarimetry has opened a new window to such solar magnetic fields, allowing us to see new aspects of the Sun.”
Figure 1: The position of the CLASP spectropolarimeter slit (bottom right) and the polarization spectrum of the upper solar chromosphere and the transition region (left). (©NAOJ, JAXA, NASA/MSFC)

Figure 2: Comparison of the polarization of 3 spectral lines with differing sensitivities to magnetic fields. A, B, C, and D correspond to the areas labeled in Figure 1. (©NAOJ)


Scientific papers:
  • R. Kano, et. al. (2017). Discovery of Scattering Polarization in the Hydrogen Lyα Line of the Solar Disk Radiation, Astrophys. J. Lett., 839, 10.
  • R. Ishikawa, et. al. (2017). Indication of the Hanle Effect by Comparing the Scattering Polarization Observed by CLASP in the Lyman-α and Si III 120.65 nm Lines. Astrophys. J., 841, 31.
The CLASP Principal Investigators are:
  • Amy Winebarger (NASA Marshall Space Flight Center)
  • Ryouei Kano (National Astronomical Observatory of Japan)
  • Frédéric Auchère (Institut d'Astrophysique Spatiale, CNRS/Univ. Paris-Sud)
  • Javier Trujillo Bueno (Instituto de Astrofísica de Canarias)


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