THEMIS Solar Telescope Research Articles

Rotational radial shear in the low solar photosphere

Context.Radial differential rotation is an important physical ingredient in stellar dynamo theory. In the case of the Sun, heliosismology techniques have revealed the existence of a near-surface shear layer covering 15–20% of the upper convection zone. It was recently shown that the rotation velocity gradient is not uniform in this layer and that it displays a steep increase in a shallow layer near the surface.Aims.We report the detection of a rotation velocity depth-gradient in the low photosphere that is not accessible to heliosismology techniques.Methods.We applied differential interferometric methods to spectroscopic data obtained with the solar telescope THEMIS, which is equipped with an efficient adaptative optics system. The detection was based on the measurement of a systematic east-west shift between images of the solar granulation at different depths in the FeI 630.15 nm at the center of the solar disk. The same technique was applied to obtain the depth-difference between the images from their perspective shift when they are observed away from the disk center. Both THEMIS and HINODE/SOT data were used for the height-difference measurement, giving similar results.Results.At the center of the solar disk, we measured a systematic retrograde shift of the photospheric granular structures on the east-west axis and with no shift in the north-south direction. The retrograde shift increases linearly with height. We interpret these findings as a signature of a steep decrease in the angular velocity in the low photosphere.Conclusions.The rotational radial shear in the low solar photosphere is likely related to the dynamics of the subsurface shear layer. Its measurement yields a valuable constraint on the numerical simulations of the solar upper convection zone.

Exospheric Na distributions along the Mercury orbit with the THEMIS telescope

The Na exosphere of Mercury is characterized by the variability of the emission lines intensity and of its distribution in time scales from less than one hour to seasonal variations. While the faster variations, accounting for about 10–20% of fluctuations are probably linked to the planetary response to solar wind and Interplanetary Magnetic Field variability, the seasonal variations (up to about 80%) should be explained by complex mechanisms involving different surface release processes, loss, source and migrations of the exospheric Na atoms. Eventually, a Na annual cycle can be identified. In the past, ground-based observations and equatorial density from MESSENGER data have been analyzed. In this study, for a more extensive investigation of the exospheric Na features, we have studied the local time and latitudinal distributions of the exospheric Na column density as a function of the True Anomaly Angle (TAA) of Mercury by means of the extended dataset of images, collected from 2009 to 2013, by the THEMIS solar telescope. Our results show that the THEMIS images, in agreement with previous results, registered a strong general increase in sodium abundance at aphelion and a dawn ward emission predominance with respect to dusk ward and subsolar region between 90° and 150° TAA. This behavior can be explained by desorption of a sodium surface reservoir consisting of sodium that is pushed anti-sunward and condenses preferentially in the coldest regions. Our analyses shows a predominance of subsolar line-of-sight column density along the rest of Mercury's orbit. An unexpected relationship between Northward or Southward peak emission and both TAA and local time is also shown by our analysis. This result seems to contradict previous results obtained from different data sets and it is not easily explained, thus it requires further investigations.

Atmospheric circulation of Venus measured with visible imaging spectroscopy at the THEMIS observatory

Measuring the atmospheric circulation of Venus at different altitudes is important for understanding its complex dynamics, in particular the mechanisms driving super-rotation. Observationally, Doppler imaging spectroscopy is in principle the most reliable way to measure wind speeds of planetary atmospheres because it directly provides the projected speed of atmospheric particles. However, high-resolution imaging spectroscopy is challenging, especially in the visible domain, and most knowledge about atmospheric dynamics has been obtained with the cloud tracking technique. The objective of the present work is to measure the global properties of the atmospheric dynamics of Venus at the altitude of the uppermost clouds, which is probed by reflected solar lines in the visible domain. Our results are based on high-resolution spectroscopic observations with the long-slit spectrometer of the solar telescope THEMIS. We present the first instantaneous “radial-velocity snapshot” of any planet of the solar system in the visible domain, i.e., a complete radial-velocity map of the planet obtained by stacking data on less than 10% of its rotation period. From this, we measured the properties of the zonal and meridional winds, which we unambiguously detect. We identify a wind circulation pattern that significantly differs from previous knowledge about Venus. The zonal wind reveals a “hot spot” structure, featuring about 200 m s−1 at sunrise and 70 m s−1 at noon in the equatorial region. Regarding meridional winds, we detect an equator-to-pole meridional flow peaking at 45 m s−1 at mid-latitudes, i.e., about twice as large as what has been reported so far.

Motion of Photospheric Matter within the Active Region Site with Two Ellerman Bombs

Examination results for specific features of variation of line-of-sight plasma velocities in different photosphere layers of the NOAA 11024 active region (AR) under the influence of two evolving Ellerman bombs (EB-1 and EB-2) are presented. Spectroscopic data with high spatial and temporal resolution were obtained with the French–Italian THEMIS solar telescope on July 4, 2009. The observation time was 20 min. The AR was undergoing a sharp increase in activity on the day of observations, and the EBs evolved in the region of one of the three magnetic fluxes that were emerging at the time. The brightness of EB-1 decreased during observations, while EB-2 got brighter. The λ ≈ 630 nm spectral region, which includes Fraunhofer lines forming within a wide range of photospheric heights (neutral iron lines Fe I λ 630.15, 630.25, and 630.35 nm and titanium line Ti I λ 630.38 nm), was used. Changes in the velocity and direction of motion of matter in the region of Ellerman bombs and in their immediate vicinity at different photosphere levels and at different stages of EB evolution were determined and analyzed. It was found that upflows were predominant at all levels of the AR photosphere. At the same time, line-of-sight velocities Vlos and the amplitude of their oscillations decreased considerably in the region of EBs. Apparently, downward matter flows reduced the velocity of upward plasma motion. This suggests that small-scale downward flows induced by magnetic reconnections were superimposed onto the large-scale upward motion of plasma of the new magnetic flux. The profile shape of photospheric lines reinforces this conclusion. The profiles of strong lines had a red asymmetry. The matter velocity determined based on the shift of a prominent component in the red wing of the Fe I λ 630.35 nm line profiles was as high as 2 km/s. Such a Vlos distribution in the EB regions indicates that they consisted of several jets moving with different velocities in different directions. The line-of-sight velocity in the central part of EB-1 and EB-2 varied from –1 to 0 km/s and from –1 to 0.2 km/s in the upper photospheric layer and from –1.6 to –0.2 km/s and from –1.1 to 0.25 km/s in the lower layer of the photosphere, respectively. Vlos variations in the vicinity of these Ellerman bombs were oscillatory, and the interval between oscillations was ~5 min. The pattern of quasiperiodic Vlos oscillations was disturbed in the EB regions: they often occurred in antiphase. It may be concluded that the excitation caused by a pulsed energy release as a result of successive magnetic reconnections associated with the emergence of the new magnetic flux propagated from the EB-1 site along a magnetic loop and triggered the formation of EB-2, and the Ellerman bombs then evolved as a physically connected pair. The studied features of the line-of-sight velocity variation of chromospheric and photospheric matter indicate that during the evolved of EBs differently directed motions of matter there was: it moved upward in the lower chromospheric layer, while downward flows reduced the velocity of ascending plasma at the photospheric level. Magnetic reconnections occurring in the layer between the upper photosphere and the lower chromosphere, where the core of the Hα line was formed, could induce such a velocity distribution.

Small-Scale Magnetic Features in the Active Region NOAA 11024

To reveal and study magnetic features in the active region NOAA 11024, temporal changes in the profiles of the Stokes parameter V of photospheric lines Fe I λ 630.15 nm, Fe I λ 630.25 nm, Fe I λ 630.35 nm, and Ti I λ 630.38 nm were analyzed. Data of spectropolarimetric observations carried out with a high temporal and spatial resolution at the French-Italian solar telescope THEMIS (Tenerife Island, Spain) on July 4, 2009, were used. The examined area of the active region includes two pores of opposite polarity and two plages. One of the pores was rapidly developing. For 20 minutes of observations, eruptive processes, such as three Ellerman bombs and chromospheric surges, took place in this area. The V profiles of the photospheric lines substantially differ in different places of the active region. They are normal in most cases. In the pores, their amplitude increases toward outer edges of the pores. The maximum amplitude was noticed in the profiles at the boundary between the emerging pore and the Ellerman bomb. It was rapidly increasing with time. Two small patches with abnormal V profiles of the photospheric lines Fe I λ 630.15 and 630.25 nm have been found. The amplitude of the profiles in these patches was reduced. The amplitude and the shape of the profiles changed with time. One of the patches was between the pores, while the other was in the plage area. During the observations, the polarity of the photospheric magnetic field in these patches changed. Evidence of new, emerging, small-scale magnetic fluxes of opposite polarity was obtained. Due to the emergence of these fluxes, magnetic reconnections started and chromospheric surges and an Ellerman bomb appeared. It has been found that there are strong oscillations in the amplitude of the V profiles of photospheric lines Fe I λ 630.15 and 630.25 nm during Ellerman bombs, which may indicate the pulsed energy realize.

Solar perturbations transits in Mercury exosphere

The link existing between the dayside Na exospheric patterns of Mercury and the solar wind-magnetosphere-surface interactions is investigated thanks to the synergy of Earth-based observations with the THEMIS solar telescope and the in-situ measurements of the Interplanetary Magnetic Field (IMF) and proton fluxes at the magnetic cusp regions by MESSENGER. Frequently, two-peak patterns of variable intensity are observed, located at high latitudes in both hemispheres. Occasionally, Na signal is instead diffused above the sub-solar region. In a special case, the Na signal is diffused above the subsolar region, when the MESSENGER data detect the transit of two Coronal Mass Ejections (CMEs). Na emission patterns results to be clearly related to the solar wind conditions at Mercury. Hence, the Na exospheric patterns, observed from ground, could be considered as a natural monitor of solar disturbances when transiting near Mercury.

Spectral study of a pair of Ellerman bombs

Results of the analysis of spectral observations of two Ellerman bombs in the Hα line are presented. These bombs (EB-1 and EB-2) appeared and evolved in the active region NOAA 11024 in the emerging magnetic flux area. The spectral data of a high temporal and spatial resolution were obtained with the French–Italian solar telescope THEMIS (Tenerife Island, Spain; THEMIS stands for Telescope Heliographique pour l’Etude du Magnetisme et des Instabilites Solaires) on July 4, 2009. The Hα-line profiles obtained for different periods of the Ellerman bombs (EBs) evolution were asymmetrical, demonstrating the emission excess in the long-wavelength wing. The intensity variations in the line wings indicate both the gradual and pulsed release of energy in the course of EBs. Temporal variations in the line-of-sight velocities Vlos of the chromospheric material at a level of the Hα-core formation showed two periods in the velocity enhancement, containing several individual peaks. The maximum line-of-sight velocity of the material was–9 and 8 km/s toward and from the observer, respectively. Rapid upward and downward plasma streams (where Vlos reaches–80 and 50 km/s, respectively) were sometimes observed. The Ellerman bombs were accompanied by small chromospheric ejections (surges) lasting for 0.5–1.5 min. A fine structure of EBs was found in the Hα-line spectra obtained during 4 min, when the intensity in the line wings sharply increased. The peculiarities of variations in the intensity of the Hα-line wings and the line-of-sight velocity of the chromospheric material suggest that two investigated EBs appeared and evolved as a physically connected pair. Our results support the model wherein the magnetic reconnection in the lower atmospheric layers is considered as a triggering mechanism for the EB formation.

Plasma motions in the solar loop of emerging magnetic flux

The results of analyzing variations in the line-of-sight (LOS) velocities in the solar loop at photospheric and chromospheric levels in the region of emerging magnetic flux for the evolving active region NOAA 11024 are reported. The analysis combines the data of multiwave spectropolarimetric observations that were carried out on July 4, 2009, (Tenerife, Spain) using THEMIS solar telescope and the data obtained with GOES, SOHO, and STEREO cosmic satellites. A complex sequence of active events has been studied: formation of the Ellerman bomb, B1 X-ray microflare, and four chromospheric surges that were formed as a result of magnetic reconnection caused by new emerging magnetic flux. The Ellerman bomb was formed in the vicinity of a growing pore. Variations in the velocity V LOS of the EB had an oscillation character for chromosphere and photosphere. Before the microflare, the average velocities of the upward and downward plasma fluxes in one leg of the magnetic loop were nearly the same—26 km/s. During the microflare, the velocity V LOS of the ascending and descending flows increased up to −33 and 50 km/s, respectively. Variations in line-of-sight velocity of a plasma in the second leg of the magnetic loop correlated well with variations of V LOS in the region of microflare, but they occurred 1.5 minutes later. During the time of observations, four chromospheric ejections of matter were formed and three of them occurred in the region of Ellerman’s bomb formation. Sharp variations in the soft X-ray intensity occurred during these ejections. At photospheric level, variations in the line-of-sight velocity of plasma in the legs of the loop occurred in the opposite direction. In the region of the first leg, velocity V LOS diminished from −1.8 to −0.4 km/s, while the velocity increased from −0.6 to −2.6 km/s in the region of the second leg.

On the spectropolarimetric signature of FeH in the laboratory and in sunspots

RésuméLaboratory spectra showing Zeeman patterns in some FeH lines susceptible to be used as magnetic probes in cool stellar atmostpheres have been recorded in the laboratory, and molecular Landé factors obtained from analysis. These Landé factors have been used to model some lines recorded in Stokes V polarisation in sunspot spectra at the solar telescope THEMIS.

Dynamical evolution of sodium anisotropies in the exosphere of Mercury

Despite the recent MESSENGER measurements, the exosphere of Mercury, the tenuous collisionless cloud of gas surrounding the planet, still needs a comprehensive view of its generation mechanisms. In fact, it is the result of the diverse interactions between many systems, which are: the surface, the interplanetary medium (Solar wind, photons and meteoroids), the planetary and the interplanetary magnetic fields. Many ground-based observations allowed the detection of intense and variable sodium emissions at global and local spatial scales, the latter being mostly concentrated in the mid-to-high latitude regions. These regions are indeed the preferred location of solar wind precipitation on the surface of the planet.In the present paper, by using high resolution Na observations obtained at the Canary Islands with the THEMIS solar telescope, we analyze the variability of the sodium exosphere on time-scale of 1h and investigate the possible mechanisms that could explain the exospheric sodium emission distribution and its dynamics. A new procedure of analysis is applied to make the data fully comparable among each other and allow a proper analysis of the evolution of the observed peak. Our interpretation relates the observed North/South sodium asymmetries to the combined effects of plasma and photons impacts onto the Mercury's surface and of sodium diffusion through the upper layer of the surface. Simulations of the exosphere generation and of the possible coupling between IMF and planetary magnetosphere suggest that observations could be interpreted as the effects of a combination of both the magnetic reconnection regimes of pulsed and quasi-steady reconnection. In addition to this, a progressive broadening of the peak regions together with an increase of the equatorial region seem to corroborate the role of photon stimulated desorption, in association with the ion sputtering and with subsequent global sodium migration around Mercury, as the causes of the observed Na peak regions evolution.

Mercury exosphere. III: Energetic characterization of its sodium component

Mercury’s sodium exosphere has been observed only few times with high spectral resolution from ground based observatories enabling the analysis of the emission spectra. These observations highlighted the energetic state of the sodium exospheric atoms relative to the surface temperature. More recently, the Doppler shift of the exospheric Na atoms was measured and interpreted as consistent with an exosphere moving outwards from the subsolar point (Potter, A.E., Morgan, T.H., Killen, R.E. [2009]. Icarus 204, 355–367). Using THEMIS solar telescope, we observed Mercury’s sodium exosphere with very high spectral resolution at two opposite positions of its orbit. Using this very high spectral resolution and the scanning capabilities of THEMIS, we were able to reconstruct the 2D spatial distributions of the Doppler shifts and widths of the sodium atomic Na D2 and D1 lines. These observations revealed surprisingly large Doppler shift as well as spectral width consistent with previous observations.Starting from our 3D model of Mercury Na exosphere (Mercury Exosphere Global Circulation Model, Leblanc, F., Johnson, R.E. [2010]. Icarus 209, 280–300), we coupled this model with a 3D radiative transfer model described in a companion paper (Chaufray, J.Y., Leblanc, F. [2013]. Icarus, submitted for publication) which allows us to properly treat the non-maxwellian state of the simulated sodium exospheric population. Comparisons between THEMIS observations and simulations suggest that the previously observed energetic state of the Na exosphere might be essentially explained by a state of the Na exospheric atoms far from thermal equilibrium along with the Doppler shift dispersion of the Na atoms induced by the solar radiation pressure. However, the Doppler shift of the spectral lines cannot be explained by our modelling, suggesting either an exosphere spatially structured very differently than in our model or the inaccuracy of the spectral calibration when deriving the Doppler shift.

Laboratory determination of Landé factors for the molecular radical FeH

We report laboratory measurements of the Zeeman response of lines in the 0-0 Wing-Ford band of the F-X system ( λ ~ 1 μ m) of FeH, measured in magnetic fields 0.3 – 0.5 Tesla. New Lande factors are used to deduce the magnetic field in sunspots from Stokes V profiles recorded at the solar telescope THEMIS. The magnetic field deduced from atomic lines (Ti, Fe) is slightly higher than that found from FeH.

Mercury exosphere I. Global circulation model of its sodium component

Our understanding of Mercury’s sodium exosphere has improved considerably in the last 5 years thanks to new observations (Schleicher, H., Wiedemann, G., Wöhl, H., Berkefeld, T., Soltau, D. [2004]. Astron. Astrophys. 425, 1119–1124) and to the publication of a summary of the large set of ground based observations (Potter, A.E., Killen, R.M., Morgan, T.H. [2007]. Icarus 186, 571–580; Potter, A.E., Killen, R.M. [2008]. Icarus 194, 1–12; Potter et al., 2009). In particular, the non-uniformity in longitude of the dayside sodium distribution (the dawn/dusk asymmetry) has now been clearly observed. This suggests that Mercury’s sodium exosphere is partly driven by a global day to nightside migration of the volatiles. One of the key questions remaining is the nature of the prevailing sodium ejection mechanisms. Because of the uncertain parameters for each ejection mechanisms, solving this problem has been difficult as indicated by the numerous papers over the last 15 years with very different conclusions. In addition, the variation of the size and of the spatial distribution of the surface reservoir (Leblanc, F., Johnson, R.E. [2003]. Icarus 164, 261–281) varies with distance from the Sun affecting the importance of each ejection mechanism on Mercury’s orbital position. We here present an updated version of the Leblanc and Johnson (Leblanc, F., Johnson, R.E. [2003]. Icarus 164, 261–281) model. We take into account the two populations of sodium in the surface reservoir (Hunten, D.M., Morgan, T.M., Shemansky, D.M. [1988]. The Mercury atmosphere. In: Vilas, F., Chapman, C., Matthews, M. (Eds.), Mercury. University of Arizona Press, Tucson, pp. 562–612), one ambient population (physisorbed in the regolith with low binding energy) and one source population (chemisorbed coming from grain interior or from fresh dust brought to the surface and characterized by a higher binding energy). We also incorporate a better description of the solar wind sputtering variation with solar conditions. The results of a large number of simulations of the sodium exosphere are compared with the measured annual cycle of Mercury sodium emission brightness. These measurements were obtained from the published data by Potter et al. (Potter, A.E., Killen, R.M., Morgan, T.H. [2007]. Icarus 186, 571–580) as well as from our own data obtained during the last 2 years using THEMIS solar telescope. These data show that: the annual cycle in the emission brightness is roughly the same from 1 year to another; there are significant discrepancies between what would be observed if the exospheric content were constant; and the annual cycle of Mercury’s sodium exosphere strongly depends on its position in its orbit so that there are seasons in Mercury’s exosphere. Based on these comparisons we derived the principal signatures for each ejection mechanism during a Mercury year and show that none of the ejection mechanisms dominates over the whole year. Rather, particular features of the annual cycle of the sodium intensity appear to be induced by one, temporarily dominant, ejection mechanism. Based on this analysis, we are able to roughly explain the annual cycle of Mercury’s exospheric sodium emission brightness. We also derive a set of parameters defining those ejection mechanisms which best reproduce this cycle. For our best case, Mercury’s exosphere content varies from ∼1.6 ± 0.1 × 10 28 Na atoms at TAA = 140° and 70° respectively to ∼4.5 ± 0.3 × 10 28 Na atoms at TAA = 180° and 0°. In addition, Mercury’s exospheric surface reservoir contains ∼1 × 10 31 Na atoms at TAA = 300° and at TAA = 170° with up to three times more sodium atoms trapped in Mercury’s nightside than in its dayside surface.

Short‐term variations of Mercury's Na exosphere observed with very high spectral resolution

Short time variations of Mercury's exosphere cannot be tracked easily from ground based observatories because of the difficulty of distinguishing them from Earth atmospheric effects. On July 13th 2008, using THEMIS solar telescope, we were able to simultaneously measure brightness, Doppler shift and width of the exospheric sodium D2 emission line during half a day with a resolving power of ∼370,000. Mercury's exosphere displayed an emission brightness peak in the Northern hemisphere which vanished in few hours and a more persistent Southern Hemispheric peak. The bulk Doppler shift of the exosphere suggests a period of strong escape from Mercury. The global changes of the Doppler shift and of the Doppler width suggest that a cloud of sodium atoms ejected before or at the beginning of our sequence of observations passed through THEMIS field of view moving anti‐sunward. A preferentially southern ejection of sodium atoms leading to the observed persistent southern emission peak is consistent with the orientation of the Interplanetary Magnetic Field during that period.

Comet McNaught C/2006 P1: observation of the sodium emission by the solar telescope THEMIS

Comet McNaught C/2006 P1 was the brightest comet of the last forty years when reaching its perihelion at an heliocentric distance of 0.17 AU. Two days before this perihelion, at an heliocentric distance of 0.2 AU, Themis, a French-Italian solar telescope in the Canary Islands, Spain, observed the Comet sodium emission of McNaught. The measured maximum sodium brightness of the D2 emission line peaked at 900 Mega-Rayleigh. The spatial distribution of the sodium emission with respect to the nucleus of the comet is in agreement with previous observations. It displays a clear sunward-tailward asymmetry that suggests a dichotomy of the sodium sources between a source close to the nucleus and an extended source most probably corresponding to the dust tail. The spatial distribution along the slit of the width and speed of the Doppler Na distribution also suggests such a dichotomy. The sodium ejection rate inferred from this observation agrees with the value of the ejection rate extrapolated from comet Hale-Bopp, taking into account the heliocentric distance of comet McNaught and its significantly larger dust release. If we suppose a similar concentration of sodium atoms in both comets, this observation suggests that the sodium ejection rate from comets McNaught and Hale-Bopp is proportional to the solar flux. Therefore the most probable ejection mechanisms are photo-sputtering, solar wind sputtering, or cometary ion sputtering, and not thermal desorption.

Design and implementation of an image stabilization device at the THEMIS solar telescope

An image stabilizer has been inserted into the optical path of the THEMIS solar telescope. THEMIS is a Ritchey–Chretien reflector telescope using an altazimuthal mount and closely tied to its spectrograph. The optical and mechanical design, implementation and system tests are described, and emphasis is put on the complexity of situations that this stabilizer must accept, including the scanning of the solar surface while stabilizing. The current closed-loop crossover frequency of the device is 65 Hz at − 3 dB on all typical solar scenes.

Differential speckle interferometry: in-depth analysis of the solar photosphere

Aims.We present the results of an experiment performed at the solar telescope THEMIS in 2002 to measure the depth over which the solar granulation extends in the photosphere. Methods: Observations made in the 523.3 nm and 557.6 nm photospheric non-magnetic iron lines were correlated with images in the continuum using spectrograms. The difference in depth between the different levels in the photosphere is projected into a difference of position along the slit of the spectrograph, using a perspective effect similar to the well-known Wilson effect for sunspots. This requires measuring displacements, ones much smaller than the telescope resolution. This is made possible by using a differential speckle interferometric technique, cross-correlating images taken in the continuum and the line. The method is not adapted to following displacements of structures in the core of strong lines, due to their difference in shapes with the structures observed in the continuum. In this case, a sequential cross-spectrum method is developed to cross-correlate images taken at close wavelengths. Results: The raw results are surprising: displacements measured in the blue and the red wings of a line have opposite signs! North and South observations, however, clearly show the expected behavior attributed to a perspective effect. After a description of the observations, we give a first interpretation of the results. The main part of the observed displacement comes from the effect of unresolved Doppler shifts produced by horizontal velocities in the solar photosphere. The perspective effect we seek appears as a second-order term; we find that its amplitude is 2 or 3 times larger than predicted by theoretical 1D models. In the core of strong lines we detect a contrast inversion that also shows up in the cross-correlation function as an anti-correlation peak at line center. Conclusions: .This first use of the differential speckle interferometry technique on the Sun is quite promising for 3D studies at high spatial resolution. Further observations with very good image quality are needed to take advantage of this new technique. THEMIS is operated on the Island of Tenerife by CNRS-CNR in the Spanish Observatorio del Teide of the Instituto de Astrofisica de Canarias.

Diagnostics of Solar Magnetic fields based on the Hanle effect

The Hanle effect was first used in solar physics to determine weak magnetic fields in prominences (see Leroy 1981). More recently the development of a new generation of very accurate spectro-polarimeters has open a new window for the diagnostics of weak magnetic fields in the solar photosphere and chromosphere, based on the Hanle effect. I present here such recent research programs performed with the solar telescope THEMIS and with other polarimeters such as ZIMPOL and the LOCARNO polarimeter.