During the 2025 observation campaign of Brigitte Schmieder and Arek Berlicki, THEMIS targeted a small prominence. Prominence are emitting structures that can be observed at the solar limb, beyond the outer edge of the Sun's disk. Prominences and filaments are two aspects of an unique physical feature: a domain of cold chromosphere like plasma, magnetically confined in the much hotter solar corona. While this structure appears as dark when seen in contrast with the disk, and is then called a filament, it appears bright in contrast to the plane of sky. As a magnetised structure filament/prominence can erupt, releasing plasma and material toward the solar system. Understanding how the magnetic structure them is fundamental to comprehend their stability or lack off.

THEMIS is mainly a scanning spectrograph instrument, i.e. a very thin slit scans the region of interest, in order to obtain high resolution spectrograms. THEMIS images are thus reconstructed. While scanning, THEMIS is thus very sensitive to the effect of turbulence and the reconstructed image have necessarily a lower resolution that direct imaging. On the other hand, THEMIS can deliver simultaneous images at different wavelength.

While adaptive optics can generally be used to significantly improve on disc observations, such as with our own Themis AO, AOs fail when trying to observe over the limb because no structure there can be tracked by the AO. Recently our colleagues of the U.S. National Science Foundation National Solar Observatory and New Jersey Institute of Technology, observed beautiful prominence dynamics with the Goode Solar Telescope thanks to their new coronal adaptive optics in a direct imaging approach.

At THEMIS, during our 2025 campaign we tried a different approach. While we were scanning a prominence at the limb, we use TAO on an offset region which is close to the limb while still on the disc. TAO is a simple AO which isoplanatic region (region where most of the AO correction is done) is limited. Since the isoplanatic region and the region of scientific interest are far away, the AO correction may be limited at the prominence. We were however very please to see that TAO still provide significant improvements. As can be seen in this image of the month, the turbulence induced motions (in the direction of the slit) which are present when TAO is off, are strongly reduced when TAO is switched on. The limb appears much smoother and the prominence better resolved.

If one could sneak inside MTR2 spectrograph while THEMIS is observing, this is what one would see looking down. The picture shows the beautiful work of decomposing the white solar light into small ranges of electromagnetic spectrum that are of interest for researchers to study the physical properties of the Sun.

The white light beam coming from the THEMIS telescope (white light on the left) is first decomposed in a low resolution spectrum (rainbow on the middle left). A rigid mask placed on the light path, enables to select several bands within the solar spectrum that will be analysed (overlapping orange and red patch on the middle right). Finally, an echelle grating enables to strongly increase the dispersion (spacing) of the spectral domains of interest (separated and extended red and orange patch on the right). There spectral cameras are placed to record the high resolution spectrum.

With its a spectral resolving power, R, of about 200 000-300 000, THEMIS MTR2 spectrograph has one of the world's best resolving power in astrophysics. MTR2 has the ability to distinguish between two wavelengths separated by a small amount. THEMIS theoretical can produce simultaneously up to eight high resolution spectrograms (although most cases, only 2-4 are requested). The choice of wavelength domains is not pre-imposed to the visiting research scientist observing with THEMIS. The choice is left to the investigators, giving them a high level of freedom to study diverse topics in solar physics.

In June, during an observation campaign lead by researchers from the Paris Observatory (France) and the University of Wroclaw (Poland), THEMIS observed solar filaments. Solar filaments are large magnetised structure of the solar corona confining cold chromospheric-like plasma. Thanks to its specific magnetic structure, solar filaments plasma, of a temperature of about 10 000°K, “hangs” thermally isolated from the million °K solar corona. As this dense and cool plasma absorbs the light emitted from the lower solar layer, the filament appears dark relatively to the background.

The THEMIS observations, presented in the left panel, results from two reconstructed images obtained from two adjacent scans over the solar filament with the THEMIS spectrograph slit. The two scans, which have a 90“ range with a 0.5” spatial step, are then stitched together to obtain a larger field of view of about 110“x90”. Only the reconstructed image in the core of Hα line is displayed here, but THEMIS data allow to sample the full range of the Hα line with a spectral resolution of 4mÅ.THEMIS high-resolution observations are very complementary to the observations of the Meteospace/3SOLEIL solar surveillance service of OCA/CNRS-INSU, which provide full-Sun high cadence (every 10s) Hα observations, presented on the right panel.

Thanks to its high-resolution, as illustrated in the left panel, THEMIS permits to analyse the filamentary structure of the solar filament and understand its magnetic field thanks to THEMIS polarised measurements. In particular, the magnetic properties of the “barbs” of the filament, the features which extend away from the “spine” (the filament axis), remains ill understood and an active topic of research.

May was Mercury Month a theMis! : In May 2025 took place the usual annual observation campaign of Mercury, led by researchers from the Italian National Institute of Astrophysics (INAF/IAPS in Rome) in collaboration with scientist from the French Laboratoire Atmosphères, Observations Spatiales (LATMOS/CNRS-UVSQ-SU-CNES in Paris).

In a sequence of scans of the exosphere of Mercury obtained some years ago, THEMIS could follow the hourly evolution of the reconstructed distribution of the Sodium emission. The figure displays the intensity emission (in kiloRayleigh) after preliminary reduction, including bias and sky background subtraction, as well as spectral and flux calibrations. Solid white line highlights the disk of the planet, the cross indicating the center of the disk. Mercury disk is 6.0'' wide. The Sun is located on the left. The images show the two peaks of higher intensity at high hermian latitude in the direction of the Sun. These peaks of sodium emission are roughly co-spatial with the positions of the magnetic footprints. Their evolution is due to the link of such emission with the Mercury magnetosphere and the interaction with the varying solar wind particles penetrating the magnetosphere and flowing to the surface. Adapted from Mangano et al. 2013.

A selection of THEMIS sunspot observations obtained between 2022 and 2025 with the BroadBand Imaging Camera. Observations are performed using Themis Adaptive Optics and images are processed with a Knox-Thompson image-reconstruction method.

THEMIS observations of the east sunspot group of the active region NOAA 13981 on February 6th 2025. The left image is a Knox-Thompson reconstructed from red continuum broadband camera. The right panels are intensity maps reconstructed from spectral observations by the MTR2 spectrograph observing the Hα absorption line. Theses maps are reconstructed from a scan by the telescope of the active region. While these intensity images have a lower spatial resolution, they provide a wealth of spectroscopic information. The region was observed during a flare, as can be noted by the typical ribbon-shape emission in the Hα line centre. This emission is absent from the continuum emission, indicating a chromospheric emission phenomena. Meanwhile, the red wing maps (Hα+1Å and Hα+0.5Å) display dark absorbing material above the location of the flare ribbons, signature of down-flowing dense material, a phenomena nicknamed “coronal rain”. While performing observations with THEMIS, targeting this flaring active region has been permitted thanks thanks to the high-cadence Hα surveillance permitted by the METEOSPACE/3SOLEIL service.

Observation of the main sunspot of the active region NOAA 13959 on January 15th 2025. This white light broad band image in the red continuum highlights the two “light bridges” of the sunspot. Light-bridges are lanes of bright material that divide the sunspot umbra. The image is the results from 100 acquired snapshots processed with a Knox Thompson image reconstruction method.