An instrument designed to record polarization in the region 120-150 nm of the solar spectrum was launched on the satellite Intercosmos-16, July 27, 1976. The aim was to search for resonance-line polarization that is caused by coherent scattering. Oblique reflections at gold-and aluminium-coated mirrors in the instrument were used to analyze the polarization. The average polarization of the La solar limb was found to be less than 1%. It is indicated how future improved VUV polarization measurements may be a diagnostic tool for chromospheric and coronal magnetic fields and for the three-dimensional geometry of the emitting structures.
1. Scientific Background
Solar spectral lines may be polarized by three different mechanisms: (1) The Zeeman effect. A magnetic field splits the line in oppositely polarized components. (2) Coherent scattering in the lines. The polarization is changed by magnetic fields through the so-called Hanle effect, but is largest in the absence of fields. (3) Impact polarization. The excitation is caused by a directed electron beam. This is only expected to occur in solar flares.
Due to the A 2 dependence of the Zeeman effect, it is small in the vacuum ultraviolet (VUV) and gives rise to line polarization of only about 0.1% or less for the magnetic fields that we expect to occur in the chromosphere and corona. The conditions are, however, favorable in the VUV for polarization caused by coherent