THEMIS Solar Telescope

Measuring magnetic field signature in the solar atmosphere is the “raison d'être” of THEMIS. Since its renovation, the THEMIS team has had as a main objective of producing vector magnetograms (maps of the magnetic field vector) while observing with the THEMIS adaptive optics (TAO). This December 2025 image of the month, presents one of the ultimate steps toward this goal, presenting the so-called Stokes maps that describe the polarisation of the light. The figure shows that in the centre of the Fe I line (at 630,1 nm) a strong polarization signal is captured by the spectropolarimetric measurement of THEMIS in active region NOAA 14100, obtained in May, indicating of the presence of magnetic field. In the continuum near that line, no polarisation signal is observed, confirming the absence of polarisation contamination in THEMIS light path. From such Stokes map, a magnetic inversion code shall then be used to determine the magnetic fields that could induce these light polarization signal, and thus produce magnetograms with TAO.