arXiv subjects

B. Gelly

Publications and source records attributed to B. Gelly.

8 recordsLinked to original sources

arXiv2026

TH\'EMIS: a calibration-free solar telescope

Context. TH\'EMIS was initially built as a calibration-free telescope, with the polarisation analysis performed in F1 focus, before any oblique reflection. Recently, the telescope was improved with installation of an Adaptive Optics apparatus. Aims. We would like to show you that it remains a calibration-free telescope, even if the two beams able to provide the light polarisation, are now separated in front of the cameras only. Methods. This is obtained owing to an adaptation of all the oblique reflections along the ray path. Results. We present raw images taken across a sunspot, with three lines: two polarisable lines, which display the sunspot magnetic field polarisation, linear and circular, and one unpolarisable line, which does not display any polarisation, which shows that the telescope is essentially free of any instrumental polarisation, because, if this line had showed any polarisation, this polarisation would have been of instrumental origin. Those images were recorded on September 12, 2024. We also present vector magnetic field, vector current density and vector Lorentz force maps obtained in July 2025, which are in accordance with previous results: circular currents wrapping spots clockwise about a positive polarity spot and counterclockwise about a negative polarity spot, and centripetal Lorentz force maintaining sunspots. Conclusions. As a result, TH\'EMIS remains a calibration-free telescope.

astro-ph.IM↗

arXiv2026

White paper on the relevance of the European Solar Telescope (EST) for the French heliophysics

The project of the European Solar Telescope aims to provide a state-of-the art infrastructure to study the Sun and its interactions with Earth and the heliosphere. This 4.2m aperture telescope will be equipped with multi conjugate adaptive optics, light-polarisation analyser, imaging spectrograph and integral field unit spectrographs. It will provide unprecedented observations of the solar photosphere and chromosphere and of the dynamical events and features that pertains to the low solar atmosphere. The EST project is presently in a phase of crystallisation, aiming at the creation of an European Research Infrastructure Consortium. While the French community has continuously been associated with the development of the EST project, some specific scientific aspects are more particularly relevant for the French astrophysics and heliophysics communities. The present review highlights the scientific research axes of high interest from the French community that shall strongly benefit from EST. The later will not only advance numerous topics of solar physics, as well as solar adaptive optics developments, but will also provide unrivaled datasets of high interest in the framework of space weather. This review also aims to highlight the space weather use that can be done with future EST observations, that will be particularly relevant for French heliophysicists.

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arXiv2025

Rotational radial shear in the low solar photosphere. Direct detection from high-resolution spectro-imaging

Radial differential rotation is an important factor in stellar dynamo theory. In the Sun, helioseismology has revealed a near surface shear layer in the upper 5 to 10% of the convection zone. At low to midlatitudes, the rotation velocity gradient decreases sharply near the surface. A depth gradient in rotational velocity was recently detected in the low photosphere using a differential interferometric method on spectroscopic data. Granular structures at different depths in the FeI 630.15 nm line showed a systematic retrograde shift compared to continuum structures, suggesting a height-related decrease in angular velocity, dependent on the assumed granulation coherence time. We use a more direct approach to measure the differential rotational velocity at different photospheric heights. We performed spectroscopic scans of the same granular region in FeI 630.15 nm and CaI 616.2 nm lines and measured displacements of images at different line chords between consecutive scans. These observations require excellent seeing, stable adaptive optics, and scanning times shorter than the granulation lifetime. Adaptive optics stabilizes continuum images but not higher-altitude rotation differences. We used THEMIS and HINODE SOT FeI data to measure formation height differences via perspective shifts observed away from the disk center with the slit radially oriented. Measurements at disk center and $\pm$25{\deg} latitude along the central meridian show a parabolic decrease in rotational velocity with height, reaching about 16% slower rotation at 80 km above the continuum. No significant difference is found between the equator and $\pm$25{\deg} latitudes. The low photosphere is a transition zone between the convective and radiative layers. Our measurements provide new constraints on its dynamical behavior and valuable boundary conditions for numerical simulations of the Sun s upper convection zone.

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arXiv2020

Evolution of exploding granules from coordinated observations by THEMIS, IRIS, SDO/HMI, and HINODE, and a simulation

Exploding granules constitute the strongest horizontal flows on the quiet Sun and contribute to the structure of the surface horizontal velocity fields which build the large-scale organization of the discrete magnetic field. In this work we explore exploding granule expansion through the observations of the ground-based THEMIS telescope, IRIS, SDO, and the Hinode space-borne instruments, and finally with the magnetohydrodynamics simulation. We evaluate the detection and the expansion of exploding granules at several wavelengths and at various spatial and temporal resolutions. To analyze the different temporal sequences, two methods of image segmentation are applied to select the granules. The first allows us to follow individually the exploding granules observed simultaneously by THEMIS, IRIS, and SDO. The second uses long time independent sequences from THEMIS, IRIS, SDO, Hinode, and a simulation. In the first method (called manual) the segmentation isolates the cell of the granules (bright granules and intergranular parts), while in the second method (called statistical) only the bright part of the granules are isolated. The results obtained with simultaneous or distinct temporal observations using the two methods of segmentation are in good agreement. The granule area evolves linearly with an expansion velocity that decreases with the radius. A rapid decrease in the velocity expansion in the first two minutes is observed. The detection and measurement of the dynamics of the explosive granules can be performed from ground- and space-based instruments. Our work reveals the usefulness of SDO data, with low spatial resolution, to study the dynamics of the exploding granules all over the solar surface

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arXiv2016

Magnetic field in atypical prominence structures: Bubble, tornado and eruption

Spectropolarimetric observations of prominences have been obtained with the THEMIS telescope during four years of coordinated campaigns. Our aim is now to understand the conditions of the cool plasma and magnetism in `atypical' prominences, namely when the measured inclination of the magnetic field departs, to some extent, from the predominantly horizontal field found in `typical' prominences. What is the role of the magnetic field in these prominence types? Are plasma dynamics more important in these cases than the magnetic support? We focus our study on three types of `atypical' prominences (tornadoes, bubbles and jet-like prominence eruptions) that have all been observed by THEMIS in the He I D_3 line, from which the Stokes parameters can be derived. The magnetic field strength, inclination and azimuth in each pixel are obtained by using the Principal Component Analysis inversion method on a model of single scattering in the presence of the Hanle effect. The magnetic field in tornadoes is found to be more or less horizontal, whereas for the eruptive prominence it is mostly vertical. We estimate a tendency towards higher values of magnetic field strength inside the bubbles than outside in the surrounding prominence. In all of the models in our database, only one magnetic field orientation is considered for each pixel. While sufficient for most of the main prominence body, this assumption appears to be oversimplified in atypical prominence structures. We should consider these observations as the result of superposition of multiple magnetic fields, possibly even with a turbulent field component.

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arXiv2013

Helioseismology with PICARD

PICARD is a CNES micro-satellite launched in June 2010 (Thuillier at al. 2006). Its main goal is to measure the solar shape, total and spectral irradiance during the ascending phase of the activity cycle. The SODISM telescope onboard PICARD also allows us to conduct a program for helioseismology in intensity at 535.7 nm (Corbard et al. 2008). One-minute cadence low-resolution full images are available for a so-called medium-$l$ program, and high-resolution images of the limb recorded every 2 minutes are used to study mode amplification near the limb in the perspective of g-mode search. First analyses and results from these two programs are presented here.

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arXiv2010

Venus wind map at cloud top level with the MTR/THEMIS visible spectrometer. I. Instrumental performance and first results

Solar light gets scattered at cloud top level in Venus' atmosphere, in the visible range, which corresponds to the altitude of 67 km. We present Doppler velocity measurements performed with the high resolution spectrometer MTR of the Solar telescope THEMIS (Teide Observatory, Canary Island) on the sodium D2 solar line (5890 Å). Observations lasted only 49 min because of cloudy weather. However, we could assess the instrumental velocity sensitivity, 31 m/s per pixel of 1 arcsec, and give a value of the amplitude of zonal wind at equator at 151 +/- 16 m/s.

astro-ph.EP↗

arXiv2005

The magnetism of the solar interior for a complete MHD solar vision

The solar magnetism is no more considered as a purely superficial phenomenon. The SoHO community has shown that the length of the solar cycle depends on the transition region between radiation and convection. Nevertheless, the internal solar (stellar) magnetism stays poorly known. Starting in 2008, the American instrument HMI/SDO and the European microsatellite PICARD will enrich our view of the Sun-Earth relationship. Thus obtaining a complete MHD solar picture is a clear objective for the next decades and it requires complementary observations of the dynamics of the radiative zone. For that ambitious goal, space prototypes are being developed to improve gravity mode detection. The Sun is unique to progress on the topology of deep internal magnetic fields and to understand the complex mechanisms which provoke photospheric and coronal magnetic changes and possible longer cycles important for human life. We propose the following roadmap in Europe to contribute to this "impressive" revolution in Astronomy and in our Sun-Earth relationship: SoHO (1995-2007), PICARD (2008-2010), DynaMICS (2009-2017) in parallel to SDO (2008-2017) then a world-class mission located at the L1 orbit or above the solar poles.

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