Solar prominence polarimetry

Abstract

We measure the resonance polarization in solar prominences in Halpha, Hbeta and HeD3. A two-dimensional set-up with narrow-band filter, polarization analyzer and CCD camera is used to take prominence images in polarized light at high spatial resolution. Placed on a coude telescope's hour axis, the observations near the equinoxia are free from purely instrumental polarization. Above the 0.1% noise limit, the Balmer lines do not show a polarization in contrast to the HeD3 line. Here, we determine the complete polarization profile after exchange of filter and CCD with the spectrograph, keeping the polarization analyzer fixed. In most prominences the Stokes-U and -Q profiles are not similar to Stokes-I: occasionally the blue and the red components of the emission are equal or even show a reverse ratio. This fits calculations for magnetic field strengths of the order of 50 Gaubeta being markedly stronger than commonly assumed.