THEMIS/MSDP magnetic field measurements

¹ Observatoire de Paris, Section de Meudon, LESIA, 92195 Meudon Principal Cedex, France e-mail: arkadiusz.berlicki@obspm.fr
² Astronomical Institute of the Wrocław University, ul. Kopernika 11, 51-622 Wrocław, Poland
³ Institute of Theoretical Astrophysics, University of Oslo, Blindern, 0315 Oslo, Norway

Received: 25 April 2005
Accepted: 11 July 2005

Abstract

We present an analysis of longitudinal magnetic field measurements using the spectral data obtained with the French – Italian polarisation free telescope THEMIS working in Multichannel Subtractive Double Pass (MSDP) mode. We also used SOHO/MDI data to extend our analysis. THEMIS observations in the MSDP mode allows us to perform imaging spectropolarimetry characterized by high spatial and time resolution. In our analysis we used the observations of solar active region NOAA 10484 performed on October 20, 2003. From THEMIS 2D spectral images recorded in Na D₁ 5896 Å line we obtained the longitudinal magnetic field in the active region. The value of the magnetic field was calculated at different distances from the Na D₁ line centre. We determine the LOS magnetic field at different heights in the solar atmosphere. SOHO/MDI observations provide the longitudinal magnetic field in Ni I (6768 Å) line. THEMIS/MSDP measurements at  Å are similar to SOHO/MDI results. Gradients of longitudinal magnetic fields derived from MSDP measurements at and  Å exhibit different behaviours according to solar targets. A decrease with height is seen in spot umbrae and penumbrae, while the gradient in facular and network areas suggests a slight increase of the longitudinal magnetic field, which might be explained by the 3D-structure of canopies.