In this paper, we present the analysis of incoherent non-thermal radio emission from a sample of hot magnetic stars, ranging from early-B to early-A spectral type. Spanning a wide range of stellar parameters and wind properties, these stars display a commonality in their radio emission which presents new challenges to the wind scenario as originally conceived. It was thought that relativistic electrons, responsible for the radio emission, originate in current sheets formed, where the wind opens the magnetic field lines. However, the true mass-loss rates from the cooler stars are too small to explain the observed non-thermal broad-band radio spectra. Instead, we suggest the existence of a radiation belt located inside the inner magnetosphere, similar to that of Jupiter. Such a structure explains the overall indifference of the broad-band radio emissions on wind mass-loss rates. Further, correlating the radio luminosities from a larger sample of magnetic stars with their stellar parameters, the combined roles of rotation and magnetic properties have been empirically determined. Finally, our sample of early-type magnetic stars suggests a scaling relationship between the non-thermal radio luminosity and the electric voltage induced by the magnetosphere's co-rotation, which appears to hold for a broader range of stellar types with dipole-dominated magnetospheres (like the cases of the planet Jupiter and the ultracool dwarf stars and brown dwarfs). We conclude that well-ordered and stable rotating magnetospheres share a common physical mechanism for supporting the generation of non-thermal electrons.

Les circuits à base de jonctions Josephson comme les filtres à interférences quantiques, nommés SQIF (Superconducting Quantum Interference Filter), sont des capteurs très sensibles au champ magnétique. Les éléments de base d’un tel circuit sont les SQUID (Superconducting Quantum Interference Device). Aussi performants dans la détection de champ magnétique, ces derniers ne permettent pas de réaliser des mesures absolues. De plus, la nécessité d’un asservissement par une boucle à verrouillage de flux limite la bande de fréquence d’utilisation. Les SQIF n’ont pas cette limitation et permettent les mesures absolues de champ magnétique. Leur capacité à combiner une taille compacte, une très bonne sensibilité et une large bande fréquentielle d’utilisation fait de ces capteurs des sérieux concurrents aux antennes classiques. Des mesures expérimentales avec des SQIF HTS faits par la technologie de jonctions irradiées montrent qu’il est possible de réaliser la détection de signaux radiofréquence jusqu’au moins 5 GHz en configuration de champ proche et en environnement non magnétiquement blindé. Afin de réaliser l’adaptation d’impédance et améliorer les caractéristiques DC de ces capteurs, différentes géométries de réseau sont étudiées. L’étude permet de définir les paramètres d’importance dans la conception de circuits SQIF afin de réaliser des détecteurs radiofréquence performants. / Superconducting Quantum Interference Filters (SQIF) are Josephson circuits very sensitive to magnetic field. They are made of arrays of SQUIDs (Superconducting QUantum Interference Devices). The latter, when operated alone, doesn’t allows absolute magnetic field measurements and have to be used with a flux locked loop, which limits the frequency band of operation. SQIFs doesn’t have such limitations and they offer the possibility to combine compactness, sensitivity and wide band of frequency at the same time. Because of this, SQIFs are serious concurrents to classical antennas in microwave applications. Experimental measurements made with HTS SQIFs and irradiated Josephson junctions shows that it is possible to detect microwave signals up to 5 GHz in an unshielded environment, and near field configuration. To perform better detection, it is important to match impedance of circuits. In the goal to do this and to improve DC characteristics, different network geometries are studied. At the end this study allows to define which parameters are important in the design of SQIF circuits for microwave detection.

Cette thèse, intitulée "Assimilation de données et modélisation stochastique dans la réanalyse des données géomagnétiques satellitaires", propose de retrouver des informations sur l'état du noyau terrestre à la frontière noyau-manteau, en combinant, d'une part, des contraintes spatiales issues de modélisations numériques directes, et d'autre part, des informations temporelles provenant d'équations stochastiques.Cet objectif est réalisé grâce à l'utilisation de méthodes inverses, et à la construction d'un algorithme d'assimilation de données à état augmenté.L'algorithme répond à la double volonté d'être flexible, c'est-à-dire de pouvoir intégrer plusieurs sources de données ou d'informations, et simple d'utilisation, c'est-à-dire d'être peu coûteux en temps de calcul et facilement modifiable.Ce travail s'inscrit dans la continuité des études menées par la communauté en assimilation de données géomagnétiques, ainsi que dans l'opportunité d'utiliser les dernières données satellitaires provenant de la récente mission Swarm (2014-....).Il a été effectué en collaboration avec Julien Aubert (IPGP), qui a fourni les contraintes spatiales issues du modèle Coupled-Earth, ainsi qu'avec Christopher C. Finlay (DTU) et Magnus Hammer (DTU), qui ont fourni les données satellitaires et d'observatoires au sol.Les principaux aboutissements de cette thèse sont la conception d'un algorithme simple et fonctionnel, validé lors d'expériences jumelles synthétiques (publié), et appliqué tout d'abord aux coefficients de Gauss d'un modèle géomagnétique, avant d'être étendu vers les données satellitaires des missions CHAMP et Swarm.Mon algorithme est capable de retrouver de l'information sur les quantités mesurées du noyau mais également sur des quantités non-observées telles que les écoulements du fer liquide ou la diffusion magnétique.Ce travail amène également à la production d'un modèle de champ magnétique et de champ de vitesse à la frontière noyau-manteau qui n'est pas classiquement régularisé.Ce modèle géomagnétique montre des résultats globalement similaires au modèle de référence CHAOS-6, et est cohérent avec les autres travaux de la communauté.Ainsi, les cartes de champ magnétique et de vitesse obtenues à la surface du noyau attestent que le champ magnétique est majoritairement dirigé par l'advection, et confirment la présence persistante d'un gyre équatorial associé avec un hémisphère Pacifique plus calme mais plus dynamique pour le champ de vitesse.La diffusion magnétique retrouvée est concentrée sous l'Indonésie et l'Océan Indien.Fondamentalement, ma thèse démontre l'importance de la prise en compte des erreurs de modélisation dans l'assimilation de données géomagnétiques, avec l'apparition de biais importants et une sous-estimation des erreurs a posteriori lorsque ces erreurs sont négligées.Finalement, le travail présenté tout au long de ce manuscrit restant préliminaire, il ouvre la voie vers une utilisation accrue des mesures géomagnétiques, avec notamment la future publication d'un code libre qui permettra la comparaison systématique des résultats obtenus avec ceux de la communauté. / This thesis, entitled {sc Data Assimilation and Stochastic Modelling in Geomagnetic Satellite Data Reanalysis}, intends to retrieve information on the state of the Earth's core at the Core-Mantle-Boundary, by combining, first, spatial constraints coming from direct numerical simulations, and second, temporal information coming from stochastic equations.This purpose is achieved through inverse methods and a data assimilation augmented state algorithm.The proposed algorithm is designed to be flexible, textit{i.e.} able to integrate several types of data or constraints, and to be simple, textit{i.e.} with low computation time and easy to modify.This work fits in with the other studies on the geomagnetic data assimilation of the community, and with the opportunity to use the last satellite data from Swarm spacecraft (2014-....).We have worked in collaboration with Julien Aubert (IPGP), who has provided the spatial constraints from Coupled-Earth dynamo, and with Christopher C. Finlay (DTU) and Magnus Hammer (DTU), who have provided the satellites and ground observatories data.The major outcomes of this thesis are the design of a functional algorithm, validated through synthetic twin experiments (published), and applied, first, to the Gauss coefficients of a geomagnetic model, and second, to the measures of the CHAMP and Swarm missions.My algorithm is able to retrieve information, not only on the measured quantities, but also on the unobserved quantities like the core flows or the magnetic diffusion.This work has led to the production of a magnetic field and core flows model at the core surface which is not classically regularized.The geomagnetic field model shows results that are globally similar to the CHAOS-6 reference field model, and that are coherent with the other studies of the community.Thus, the maps of the magnetic field and the velocity field obtained, confirm that the dipole decay is principally driven by advection, and display the persistent presence of the Atlantic gyre associated with a Pacific hemisphere less energetic.The inverted magnetic diffusion is concentrated under Indonesia and Indian Ocean.Fundamentally, my thesis demonstrate the importance of taking into account the modelling errors in the geomagnetic data assimilation, which leads to strong biases and an underestimation of the textit{a posteriori} errors when those errors are neglected.Finally, the work presented in this manuscript is preliminary, and it paves the way toward an increased use of the satellite data, with in particular, the free release of my code in order to compared the results with the ones obtained by the community.

Within this past decade, advances in spectropolarimetric analyses allowed mapping surface characteristics of nearby non-resolved stars with unique characteristics. In this study, we attempt mapping of the magnetic field structure and surface distribution of elements for such a star, the magnetic Ap star phi Dra. Using high-precision photometric data from TESS satellite, we improve its rotation period. Spectrum synthesis calculations allow compilation of a list of lines present in its spectrum. The resulting synthetic spectrum and observed NARVAL spectra are used to re-estimate element abundances. Least-squares deconvolved (LSD) intensity and circular polarisation profiles are then computed from a sample of 1260 metal lines. To determine which element(s) are most suitable for in-depth analysis, variability of LSD profiles is studied qualitatively for Fe, Cr and Si. The longitudinal magnetic field of phi Dra is calculated from LSD circular polarisation profiles. Stellar magnetic field maps and distributions of Fe concentration are derived through Zeeman Doppler Imaging (ZDI). The resulting maps of this study show five areas of high Fe concentrations, in the Northern stellar hemisphere. The magnetic field topology of phi Dra resulting from our analysis is that of an offset dipole with small quadrupole contributions. Our abundance and magnetic maps suggest correlation between high concentrations of Fe and high magnetic field strength. The field is primarily radial in 4 out of 5 such regions, contrary to theoretical expectations.

No description available.

We present new ATCA multiwavelength radio measurements (range 2.1–21.2 GHz) of the early-type magnetic star ρ Oph A, performed in 2019 March during three different observing sessions. These new ATCA observations evidence a clear rotational modulation of the stellar radio emission and the detection of coherent auroral radio emission from ρ Oph A at 2.1 GHz. We collected high-resolution optical spectra of ρ Oph A acquired by several instruments over a time span of about 10 yr. We also report new magnetic field measurements of ρ Oph A that, together with the radio light curves and the temporal variation of the equivalent width of the He I line (λ = 5015 Å), were used to constrain the rotation period and the stellar magnetic field geometry. The above results have been used to model the stellar radio emission, modelling that allowed us to constrain the physical condition of ρ Oph A’s magnetosphere. Past XMM–Newton measurements showed periodic X-ray pulses from ρ Oph A. We correlate the X-ray light curve with the magnetic field geometry of ρ Oph A. The already published XMM–Newton data have been re-analysed showing that the X-ray spectra of ρ Oph A are compatible with the presence of a non-thermal X-ray component. We discuss a scenario where the emission phenomena occurring at the extremes of the electromagnetic spectrum, radio and X-ray, are directly induced by the same plasma process. We interpret the observed X-ray and radio features of ρ Oph A as having an auroral origin.

The non-thermal radio emission of main-sequence early-type stars is a signature of stellar magnetism. We present multiwavelength (1.6-16.7 GHz) ATCA measurements of the early-type magnetic star ρ OphC, which is a flat-spectrum non-thermal radio source. The ρ OphC radio emission is partially circularly polarized with a steep spectral dependence: the fraction of polarized emission is about 60 at the lowest frequency sub-band (1.6 GHz) while is undetected at 16.7 GHz. This is clear evidence of coherent Auroral Radio Emission (ARE) from the ρ OphC magnetosphere. Interestingly, the detection of the ρ OphC's ARE is not related to a peculiar rotational phase. This is a consequence of the stellar geometry, which makes the strongly anisotropic radiation beam of the amplified radiation always pointed towards Earth. The circular polarization sign evidences mainly amplification of the ordinary mode of the electromagnetic wave, consistent with a maser amplification occurring within dense regions. This is indirect evidence of the plasma evaporation from the polar caps, a phenomenon responsible for the thermal X-ray aurorae. ρ OphC is not the first early-type magnetic star showing the O-mode dominated ARE but is the first star with the ARE always on view.

This thesis reports an in-fiber magnetic field sensor that is able to detect magnetic fields as low as 2 A/m at a spatial resolution of 1 mm. The small sensor consists of a magneto-optic slab waveguide, bismuth-doped rare earth iron garnet (Bi-RIG) that is coupled to an optical fiber. By coupling light from the fiber to the slab waveguide, it becomes an in-fiber magnetic field sensor. This is due to the Magneto-Optic Kerr effect; a change in refractive index is proportional to the applied magnetic field. When an AC field is applied, an AC component in the output power can be detected by a spectrum analyzer. The novelties of Magneto-Optic Slab Coupled Optical Sensor (MO-SCOS) devices include their small compact nature and a dielectric structure allowing low electromagnetic interference. Due to their compact size they are capable of placement within devices to measure interior electromagnetic fields immeasurable by other sensors that are either too large for internal placement or disruptive of the internal fields due to metallic structure. This work also reports progress on EO SCOS development. The EO sensor has found application in new environments including the electromagnetic rail gun, and a dual-axis sensor.

A rather strong mean longitudinal magnetic field of the order of a few hundred gauss was detected a few years ago in the Of?p star CPD −28° 2561 using FORS2 (FOcal Reducer low dispersion Spectrograph 2) low-resolution spectropolarimetric observations. In this work, we present additional low-resolution spectropolarimetric observations obtained during several weeks in 2013 December using FORS 2 mounted at the 8-m Antu telescope of the Very Large Telescope (VLT). These observations cover a little less than half of the stellar rotation period of 73.41 d mentioned in the literature. The behaviour of the mean longitudinal magnetic field is consistent with the assumption of a single-wave variation during the stellar rotation cycle, indicating a dominant dipolar contribution to the magnetic field topology. The estimated polar strength of the surface dipole Bd is larger than 1.15 kG. Further, we compared the behaviour of the line profiles of various elements at different rotation phases associated with different magnetic field strengths. The strongest contribution of the emission component is observed at the phases when the magnetic field shows a negative or positive extremum. The comparison of the spectral behaviour of CPD −28° 2561 with that of another Of?p star, HD 148937 of similar spectral type, reveals remarkable differences in the degree of variability between both stars. Finally, we present new X-ray observations obtained with the Suzaku X-ray Observatory. We report that the star is X-ray bright with log LX/Lbol ≈ −5.7. The low-resolution X-ray spectra reveal the presence of a plasma heated up to 24 MK. We associate the 24 MK plasma in CPD −28° 2561 with the presence of a kG strong magnetic field capable to confine stellar wind.

We report the detection of a strong, organized magnetic field in the secondary component of the massive O8III/I+O7.5V/III double-lined spectroscopic binary system HD 47129 (Plaskett's star) in the context of the Magnetism in Massive Stars survey. Eight independent Stokes V observations were acquired using the Echelle SpectroPolarimetric Device for the Observations of Stars (ESPaDOnS) spectropolarimeter at the Canada–France–Hawaii Telescope and the Narval spectropolarimeter at the Télescope Bernard Lyot. Using least-squares deconvolution we obtain definite detections of signal in Stokes V in three observations. No significant signal is detected in the diagnostic null (N) spectra. The Zeeman signatures are broad and track the radial velocity of the secondary component; we therefore conclude that the rapidly rotating secondary component is the magnetized star. Correcting the polarized spectra for the line and continuum of the (sharp-lined) primary, we measured the longitudinal magnetic field from each observation. The longitudinal field of the secondary is variable and exhibits extreme values of −810 ± 150 and +680 ± 190 G, implying a minimum surface dipole polar strength of 2850 ± 500 G. In contrast, we derive an upper limit (3σ) to the primary's surface magnetic field of 230 G. The combination of a strong magnetic field and rapid rotation leads us to conclude that the secondary hosts a centrifugal magnetosphere fed through a magnetically confined wind. We revisit the properties of the optical line profiles and X-ray emission – previously interpreted as a consequence of colliding stellar winds – in this context. We conclude that HD 47129 represents a heretofore unique stellar system – a close, massive binary with a rapidly rotating, magnetized component – that will be a rich target for further study.