Some THEMIS-MTR observations of the second solar spectrum (2000 campaign)

Abstract

We report spectropolarimetric observations with the THEMIS telescope multi-lines operating mode (MTR) during the 2000 observational period from August 27th to September 1st. We measured the resonance polarization at the limb of a series of lines: 460.7 nm, D1 589.6 nm and D2 589.0 nm, D1 493.4 nm and D2 455.4 nm, 493.2 nm. The data analysis method is mainly described in Bommier & Rayrole ([CITE]), and has been completed by using the beam exchange facility as available in 2000 THEMIS, i.e., in a single Stokes parameter. A so-called “generalized beam exchange” technique has been settled on, for the full Stokes vector measurement under this limitation. The observations have been devoted to the measurement of the scattering polarization which is a linear polarization observed near the limb of the Quiet Sun, eventually modified by a weak magnetic field (the so-called Hanle effect). The entrance slit of the spectrograph has been oriented parallel to the tangential direction of the solar limb, and data have been averaged in time and along the spatial direction of the slit in order to increase the polarimetric resolution. Two different cameras have been used to record simultaneously the two polarization states exiting the beam-splitter. The results of our polarization measurements are in good agreement with those given in the second spectrum solar atlas of Gandorfer ([CITE]), based on 1999–2000 observations. Nevertheless, with regard to a quantification of the polarization signal, we found that the signal is systematically smaller than previous results obtained during the 1994–96 observational period and was also observed as decreasing during the 1998 observational period, as if a 11-year cyclic variation of the limb polarization occured. This signal variability obviously requires further observational and interpretative investigations. We noticed other differences to previous results, in particular, the linear polarization shape of the D1 line that also requires further observational investigation.