Linear polarization of the  λ1031.92 coronal line

I. Constraints on the solar wind velocity field vector in the polar holes

¹ Max-Planck-Institut für Aeronomie, Max-Planck-Str. 2, 37191 Katlenburg-Lindau, Germany
² Istituto Nazionale di Astrofisica (INAF), Osservatorio Astronomico di Torino, 10025 Pino Torinese, Italy
³ Laboratoire d'Etude du Rayonnement et de la Matière en Astrophysique, Observatoire de Paris-Meudon, 92195 Meudon, France
⁴ Institut d'Astrophysique Spatiale, Université Paris XI, 91405 Orsay, France

Corresponding author: N.-E. Raouafi, Raouafi@linmpi.mpg.de

Received: 7 December 2001
Accepted: 24 April 2002

Abstract

In the first part of the present work, we report on the determination of the linear polarization parameters of the O vi  coronal line (hereafter O vi D₂). Spectropolarimetric observations done by the SUMER spectrometer on board the spacecraft SoHO during minimum activity of the solar cycle (March, 1996) in the south coronal hole were used to analyze the partial linear polarization of the D₂ resonance line scattered by the O vi coronal ions. Widths of the O vi D₂ and O vi 1037.61 (hereafter O vi D₁) lines and limb-brightening determined from data observations of SUMER/SoHO are also presented. The second part of this work is devoted to the interpretation of the determined polarization parameters in terms of the macroscopic velocity field vector of the O vi coronal ions that can be assimilated to the solar wind velocity field. Numerical results of the linear polarization parameters of the O vi D₂ coronal line are presented. Constraints on the strength and direction of the solar wind velocity vector are obtained by comparing the numerical and the observational results of the O vi D₂ coronal line polarization parameters. In conclusion, the effect of the macroscopic velocity field vector of the scattering ions (Doppler redistribution) provides a first set of results compatible with the observations. However, the inclination of the derived set of compatible field vectors, with respect to the solar vertical, is higher than expected in a coronal hole. The result will be improved by the inclusion of the magnetic field effect (Hanle effect), which will be treated in a further paper.