THEMIS is a joint operation from France (CNRS/INSU) and and Italy (INAF) national research agencies. It is located at Izaña, 2400 m, within the Teide Observatory from the Instituto de Astrofisica de Canarias, on the island of Tenerife. THEMIS stands for "Télescope Héliographique pour l'Etude du Magnétisme et des Instabilités Solaires". It is a 90 cm usefull aperture solar telescope, and currently the third larger in the world. Its specific design allows for high-accuracy spectropolarimetry of the solar surface together with monochromatic high resolution imaging.
An audiovisual about the telescope is available here.
After two decades of giant progress and findings in the internal structure of the Sun, the measurement and understanding of the solar magnetic fields has become the true burning issue at the dawn of the 21st century. There is now a rapidly growing interest for scientific topics like the generation and transport of the magnetic field from the base of the solar convective zone to the corona. Flux tubes existence and behavior, coronal heating, interplanetary magnetic field structure, space meteorology and solar-terrestrial relationships possibly connected to the solar activity cycles are major emerging issues of this 21st century.
From the development of the first instruments in France during the 50's, the solar group from Paris Observatory became an international leader for solar magnetic observations during the 70's. Within this group emerged in 1982 the project of a large solar telescope dedicated to the observation of the solar magnetism. The construction started in 1993 and the first light happened in 1996. The telescope is opened to the international community since 1999.
The f/16 Ritchey-Chretien telescope includes an alt-az mounting, a helium filled telescope tube, a Stokes polarimeter located at the prime focus. The multi-mode spectrograph, delivers routine vector polarimetry analysis with an accuracy ranging from 10-3 to 10-5 in some configurations. The spectrograph design allows the observation of up to 10 wavelengths simultaneously from 400 to 1100 nm, giving an opportunity to perform 3d inversion of the magnetic fields structure in the solar atmosphere. Themis currently offers 3 complementary observing modes:
- MTR mode for multiline spectropolarimetry
- MSDP mode allowing for multichannel substractive double-pass spectro-imaging with polarimetry
- IPM mode for a very narrow-band imaging with a universal birefringent filter followed by a Fabry-Perot interferometer.
Foreseable science steeming from THEMIS rely on progress in either the instrument or the data analysis or both. In terms of the instrument, evolution is driven towards increasing present accuracy and sensitivity without any major refurbishing of the instrumental design. Addressing image stability through a soon-to-be commissioned tip-tilt system and in-project adaptive optics will allow observers to routinely scan the solar surface with 0.5 arcsec resolution. This, which may seem an average number when compared to high-resolution imaging telescopes, is a major step forward when combined with the multiline spectropolarimetric capabilities of THEMIS. All study areas of THEMIS should benefit from this improvement, from active region to quiet sun and filaments. The improvement in image resolution should be necessarily accompanied of an improvement in polarimetric accuracy, mostly through control of seeing with high-pace modulating polarimeters (above 10Hz for a beginning).
THEMIS is also widening its spectral covering with experiments that show the capabilities of the instrument to perform up to 13000 Angstroms in the near IR. Coming work should explore the region up to 2 microns opening the telescope to a full new set of solar spectral lines rich with information and waiting to be observed simultaneously with visible lines, a possibility only open at THEMIS. In the other direction, THEMIS is also improving its photon budgets in the blue up to 4000 nm and exploring capabilities down to 3900
Angstroms where the well-known Ca H and K lines are found.
The sophistication of THEMIS data (4-dimensional data cubes in up to 10 spectral domains) is yet to be exploited. Present on-line data tools have already started a very significant progress in this direction by filling the gap between raw data and second level data products like magnetic field vector and thermodynamics of the solar atmosphere. The next big jump forward should combine different spectral lines into a coherent and comprehensive picture of not just a layer of the solar atmosphere but of a volume of that solar atmosphere. Such a goal should be understood in a philosophy of not providing the observer at THEMIS with just an instrument, but with a probe of the solar atmosphere. And as such, advanced data products should be readily available to the observer.
High resolution magnetic field vectors in filament channels in the photosphere are filled with parasitic polarities like the one shown in the figure (40x20 arcsecs approx). The field topology represented by the blue arrows reveals what at first sight seems to be like a dip in the magnetic line of force. Such a dip would be a place of choice for the accumulation of cool plasma escaping from the filament hanging over the channel 10000 km high. Those structures, called filament feet, would thus be explained by magnetic dips like the one seen by THEMIS and shown in the figure.
Magnetic fields can also be measured directly in prominences through polarimetric measurements in selected spectral lines of He and high control of scattered light in the telescope. THEMIS simultaneously measured the 4 Stokes parameters and in particular the circular polarization of the He D3 line which resulted in a preliminary field strength of 40G, stronger than