Letter to the Editor

Solar prominence polarimetry

¹ Universitäts-Sternwarte, Geismarlandstraße 11, 37083 Göttingen, Germany
² Istituto Ricerche Solari, Via Patocchi, 6644 Locarno-Orselina, Switzerland

Corresponding author: E. Wiehr, ewiehr@uni-sw.gwdg.de

Received: 7 April 2003
Accepted: 6 May 2003

Abstract

We measure the resonance polarization in solar prominences in Hα, Hβ 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 coudé 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 Gauß being markedly stronger than commonly assumed.