EDP Sciences
Free access
Volume 434, Number 3, May II 2005
Page(s) 1183 - 1189
Section The Sun
DOI http://dx.doi.org/10.1051/0004-6361:20041654

A&A 434, 1183-1189 (2005)
DOI: 10.1051/0004-6361:20041654

Absence of linear polarization in H$\alpha$ emission of solar flares

M. Bianda1, 2, A. O. Benz2, J. O. Stenflo2, 3, G. Küveler4 and R. Ramelli1

1  IRSOL, 6605 Locarno-Monti, Switzerland
    e-mail: mbianda@irsol.ch
2  Institute of Astronomy, ETH-Zentrum, 8092 Zürich, Switzerland
3  Faculty of Mathematics and Science, University of Zurich, 8057 Zürich, Switzerland
4  Fachhochschule Wiesbaden, Am Brückweg 26, 65428 Rüsselsheim, Germany

(Received 13 July 2004 / Accepted 17 January 2005)

High sensitivity observations of H$\alpha$ polarization of 30 flares of different sizes and disk positions are reported. Both filter and spectrographic techniques have been used. The ZIMPOL system eliminates spurious polarizations due to seeing and flat-field effects. We didn't find any clear linear polarization signature above our sensitivity level which was usually better than 0.1%. The observations include an X17.1 flare with gamma-ray lines reported by the RHESSI satellite. These results cast serious doubts on previous claims of linear polarization at the one percent level and more, attributed to impact polarization. The absence of linear polarization limits the anisotropy of energetic protons in the H$\alpha$ emitting region. The likely causes are isotropization by collisions with neutrals in the chromosphere and defocusing by the converging magnetic field.

Key words: polarization -- Sun: flares -- techniques: polarimetric

© ESO 2005