Collisional influence on the differential Hanle effect method applied to the second solar spectrum of the A–X (0, 0) band of MgH

¹ Laboratoire d'Étude du Rayonnement et de la Matière en Astrophysique, CNRS UMR 8112 – LERMA, Observatoire de Paris, Section de Meudon, 92195 Meudon, France e-mail: V.Bommier@obspm.fr
² Università degli Studi di Firenze, Dipartimento di Astronomia e Scienza dello Spazio, Largo E. Fermi 2, 50125 Firenze, Italy
³ Laboratoire d'Études Spatiales et d'Instrumentation en Astrophysique, CNRS UMR 8109 – LESIA, Observatoire de Paris, Section de Meudon, 92195 Meudon, France

Received: 25 November 2005
Accepted: 28 July 2006

Abstract

Aims.This paper presents an analysis of the  lines of the Q band of the A-X (0, 0) transition of MgH, whose linear polarization was measured 4 arcsec inside the solar limb in a quiet region (North Pole) with THEMIS on 21 November 2004.

Methods.This analysis is performed as follows: a) the Hanle effect  parameter is derived by applying the differential Hanle effect method between the two extreme pairs of lines. Assuming no depolarizing collisions, a magnetic field strength follows, which is found to be 9.2 Gauss, in agreement with previous observations of the same kind; b) this  parameter is entered in a code solving the NLTE polarized radiative transfer equations, and the other depolarizing parameter, namely the depolarizing collision rate, is then derived by adjusting the computed polarization to the observed one. Thus an average value of the rate per colliding hydrogen atom   10^-9 cm³ s^-1 is obtained for the upper levels of the 12 lines (standard deviation 0.21  10^-9 cm³ s^-1). The corresponding model-dependent depolarizing rate is   10⁷ s^-1 at  km; c) this depolarizing rate is now introduced in the conversion of the  parameter in terms of magnetic field strength: an average turbulent field strength of 29 ± 12 Gauss is derived as the final value, at height  ± 80 km where the polarization is formed. The Hönl-London factors of the lines under interest have been recalculated, leading to detect an error of a factor 2 in the recent literature.

Results.The derived value  ± 12 Gauss at  ± 80 km is in fairly good agreement with the previous determinations based on the interpretation of the  4607 Å limb polarization, which has led to fields in the range 35-60 Gauss.

Conclusions.Given the error bars, it seems unnecessary to put forward different formation regions for the and MgH lines.