THEMIS plays a role in developing and testing advanced observational techniques and instrumentation, mostly in the field of solar physics, particularly in the field of spectropolarimetry and adaptive optics.

The telescope specializes in spectroscopy and spectropolarimetry, analyzing the electromagnetic spectrum and the polarization of the light to infer multiple properties of the object that is observed, such as its composition, its physical characteristics and its magnetic field.

THEMIS provides detailed information about the Sun's magnetic field in the lowest layers of the solar atmosphere (the photosphere and chromosphere), as well as around planets.

THEMIS can realize a high spectral resolution observations and possess unique capabilities to measure the full Stokes parameters (the set of values that describe the polarization state of the light which is received by THEMIS), providing a complete characterisation of the target magnetic field. Its design minimizes instrumental polarization, ensuring highly accurate measurements of magnetic field.

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The main objectives of THEMIS in solar physics are:

THEMIS is dedicated to investigating the magnetic field of the Sun, which drives much of the solar activity, including sunspots, filaments and protuberances, solar flares, and solar eruptions. Thanks to its spectropolarimetric capabilities, THEMIS perform magnetic field measurements of the visible light that has been polarised either by the Zeeman effect, with the Hanle effect (“second solar spectrum”), and other atomic-level quantum processes. THEMIS enables the measure of the three components of the magnetic field vector with high precision, in different layers of the low solar atmosphere. THEMIS enables researcher to carry investigations on the very physics of magnetic field measurements.

THEMIS capabilities allow measurements both:

• in the so-called quiet Sun, thanks to THEMIS sensitivity and capacity to measure weak magnetic signals (<10 G), easily contaminated by Earth atmospheric turbulence.
• in active regions, in solar sunspots, where kG fields are present at the heart of the sunspot umbra, magnetic fields which powers solar active regions.

THEMIS provides insights into the processes governing the magnetic field's generation, dynamics and role in solar activity. THEMIS allows the study of the emergence of magnetic flux in the solar corona. Indeed, the solar magnetic field is generated in the solar interior and need to be transported toward the solar atmosphere. THEMIS aims to address the following questions:

• How is the magnetic field transported from the solar interior to the atmosphere, passing through the photosphere and chromosphere?
• What is the signature of flux emergence at different spatial scales?
• What are the different sources and formation mechanism of the magnetic field at distinct spatial scales: global dynamo localised in the tachocline, local dynamo within granules, …

THEMIS also allows to measure the magnetic field of solar prominences. Prominences are structures observed above the limb of the solar disk, emitting chromospheric lines. The cool and dense (relatively to its coronal environment) chromospheric-like plasma of prominences are maintained in the corona because of the complex structure of the magnetic field. THEMIS has unique capabilities to observe and understand prominences thanks to stabilised off-limb observation and multi-wavelength spectropolarimetric observations. THEMIS can measure their magnetic field (with Zeeman effect) enabling the understanding of their topology. THEMIS also allows for measurements using the Hanle effect and other atomic-level quantum correlations.

THEMIS aims to understand the coupling between different layers of the solar atmosphere and how energy is transported from the interior to the outer corona. More specifically, THEMIS aims at understanding the magnetic coupling between the photosphere and chromosphere, addressing the following questions:

• Why and how does the chromosphere heat up?
• How does the interaction between these two layers determines the dynamics and temperature of the chromosphere?

In order answer these questions, THEMIS permits simultaneous multi-wavelength observations and the determination of the chromospheric magnetic field thanks to inversion of magnetic sensitive chromospheric lines such as He I and Ca II.

THEMIS investigates transient events and dynamic processes in the Sun's atmosphere, including wave propagation, plasma flows, instabilities, and impulsively emitting features. THEMIS can observe small scale active events, such as bright points, Ellerman bombs, chromospheric jets, which are occurring in the low solar atmosphere and are emitting in photospheric and chromospheric lines to which THEMIS is design to observe. THEMIS also permits to observe the low-atmospheric response of larger coronal eruptions, such as the flares ribbons.

By measuring the magnetic field that fuels solar activity, THEMIS provides insights both on the geometry of the magnetic field that stabilise large solar structures, as well as on the mechanisms and instabilities that can catastrophically destabilised them, leading to powerful solar eruptions. Either directly through spectropolarimetric imaging, or by spatially scanning a solar active region with spectroscopic slit, THEMIS can produce magnetic field maps that can be used as input for 3D reconstruction methods of the coronal magnetic field (i.e. magnetic extrapolations), enabling to access the 3D magnetic structures and study its link with eruptiveness.

Finnaly, by monitoring long-term changes in solar magnetism, THEMIS contributes to understanding the solar cycle, including the 11-year activity cycle/22-year magnetic cycle and its impact on space weather. This research helps model how solar activity influences the Earth's magnetosphere and climate.

By improving our understanding of the Sun's magnetic activity and its role in driving solar phenomena, THEMIS contributes valuable data for space weather prediction and understanding the Sun-Earth connection.

While the THEMIS is primarily designed and optimized for studying the Sun, it does have the capability to observe planets and other objects of the solar system in particular those who are closer to the Sun than Earth such as Mercury, Venus and some comets during their perihelion.

Observation of such objects with night telescope is challenging. Being in relatively close apparent proximity to the Sun on the celestial sphere, their observations must either be done during daylight, time during which night telescopes are not designed to operate, or during twilight, hence with an important remaining sky’s brightness leading to a reduced contrast. In addition, at twilight, these objects are usually close to the horizon, their lights going through a longer depth in the Earth atmosphere and hence subject to more scattering, turbulence and in fine distortion of the image.

THEMIS thus offers without equivalent ground-based observational capabilities of these near-Sun objects. Since 2005, the user community has expanded THEMIS science for daytime observations of Mercury and Venus, supporting planetary space missions (e.g., Messenger, BepiColombo), and sporadic observations of comets near perihelion.

THEMIS's high spectral resolution can allow detailed observations of the atmospheres of planets, including scattering phenomena in their atmospheres. and the measurement of chemical compositions via spectroscopy.

For Mercury, THEMIS provides unique spatial and temporal resolutions and an optimal operational cycle. Since 2007, significant progress has been made in understanding fluctuations in Mercury’s sodium exosphere content. Several maps per day are possible, offering the best-known spatial resolution to date (8 pixels across the diameter), with implications for regolith degassing mechanisms.

THEMIS specializes in solar spectropolarimetry, that can be applied to study the magnetic properties of other objects, including planets, if they reflect or emit sufficient polarized light.

Of specific interest is the study of Mercury with polarimetry to understand the interaction of Mercury exosphere with the solar wind magnetic field. This is a key objective of the BepiColombo (ESA) space mission.