Injection et libération d'énergie libre, d'hélicité magnétique, et de courants électriques dans l'atmosphère solaire

The origin of solar flares is intrinsically related to the evolution of current-carrying magnetic fields. To better understand and quantify their role, I jointly studied three physical quantities whose properties quantify those of eruptive magnetic fields: relative magnetic helicity, electric currents, and free magnetic energy. The first part of my work introduces a method that I have developed to study the transfer of magnetic helicity into the solar atmosphere and locate the regions of opposite helicity flux, which are possibly responsible for the most energetic flares. In the second part of this thesis, I present a study on the generation and distribution of electric currents in current-carrying magnetic fields. Using 3D magnetohydrodynamics numerical simulations, I demonstrated that a net electric current – which is a key element of several solar flare models – can be naturally generated by the presence of a strong magnetic shear along the photospheric polarity inversion line. Finally, the third part of this manuscript describes a study of the properties of 3D magnetic reconnection in the framework of solar coronal jets. This work allows to relate the properties of magnetic reconnection with the dissipation of electric currents, the amount of released energy, and the amount of ejected helicity. The approach used here by combining three proxies of current-carrying magnetic fields, pave the way for studying the properties of the magnetic fields that are responsible for solar activity.L’origine des éruptions solaires vient de l’évolution de champs magnétiques porteurs de courants électriques. Pour comprendre et caractériser leur rôle, j’ai étudié conjointement trois grandeurs physiques dont les propriétés caractérisent celles des champs magnétiques éruptifs : l’hélicité magnétique relative, les courants électriques induits, et l’énergie magnétique libre. Tout d’abord, je présente une méthode que j’ai développée pour étudier le transfert d’hélicité magnétique dans l’atmosphère solaire et localiser les régions de flux d’hélicité opposés, i.e. les régions potentiellement à l’origine des éruptions solaires les plus énergétiques. J’expose ensuite les résultats de simulations numériques magnétohydrodynamiques 3D, portant sur l’étude des courants électriques dans les champs magnétiques éruptifs. Je montre qu’un courant électrique net dans un champ magnétique éruptif – élément clé des modèles d’éruptions solaires, est naturellement généré par la présence d’un cisaillement magnétique intense au niveau de la ligne d’inversion de polarité magnétique photosphérique. Enfin, je présente mes travaux sur l’étude des propriétés de la reconnexion magnétique en 3D, dans le cadre des jets coronaux. Ce travail permet de faire le lien entre les propriétés de la reconnexion magnétique, la dissipation des courants électriques induits, la quantité d’énergie libérée, et la quantité d’hélicité éjectée. Cette approche que j’ai utilisée en combinant trois proxys différents des caractéristiques des champs magnétiques éruptifs, ouvre de nouvelles perspectives pour étudier les propriétés des champs magnétiques responsables de l’activité solaire

Polarimetric measurements in prominences and “tornadoes” observed by THEMIS

Since 2013, coordinated campaigns with the THEMIS spectropolarimeter in Tenerife and other instruments (space based: Hinode/SOT, IRIS or ground based: Sac Peak, Meudon) are organized to observe prominences. THEMIS records spectropolarimetry at the He I D3 and we use the PCA inversion technique to derive their field strength, inclination and azimuth. All of the observed prominences are quiescent, as they were stable as filaments for at least three days and not eruptive. They present similar characteristics, they are highly dynamic and present horizontal magnetic fields. Statistically, the inclination from the local vertical is around 90 degrees, with some points around 60 and 120 degrees. The field strength is between 5 and 15 Gauss. We tested the effects of adding a turbulent field component to the horizontal field. For those pixels showing inclinations around 60 and 120 degrees, we find that such a model is compatible with the polarimetric observations. In some of these prominences, identified as “tornadoes” the field strength may reach 50 Gauss, and in the top of the tornadoes some points exhibit an inclination which cannot correspond to any model in our grid of models. We investigate different solutions

Distribution of Electric Currents in Solar Active Regions

There has been a long-lasting debate on the question of whether or not electric currents in solar active regions are neutralized. That is, whether or not the main (or direct) coronal currents connecting the active region polarities are surrounded by shielding (or return) currents of equal total value and opposite direction. Both theory and observations are not yet fully conclusive regarding this question, and numerical simulations have, surprisingly, barely been used to address it. Here we quantify the evolution of electric currents during the formation of a bipolar active region by considering a three-dimensional magnetohydrodynamic simulation of the emergence of a sub-photospheric, current-neutralized magnetic flux rope into the solar atmosphere. We find that a strong deviation from current neutralization develops simultaneously with the onset of significant flux emergence into the corona, accompanied by the development of substantial magnetic shear along the active region's polarity inversion line. After the region has formed and flux emergence has ceased, the strong magnetic fields in the region's center are connected solely by direct currents, and the total direct current is several times larger than the total return current. These results suggest that active regions, the main sources of coronal mass ejections and flares, are born with substantial net currents, in agreement with recent observations. Furthermore, they support eruption models that employ pre-eruption magnetic fields containing such currents.Comment: 6 pages, 5 figures, to appear in Astrophysical Journal Letter

Polarimetric measurements in prominences and “tornadoes” observed by THEMIS

Since 2013, coordinated campaigns with the THEMIS spectropolarimeter in Tenerife and other instruments (space based: Hinode/SOT, IRIS or ground based: Sac Peak, Meudon) are organized to observe prominences. THEMIS records spectropolarimetry at the He I D3 and we use the PCA inversion technique to derive their field strength, inclination and azimuth. All of the observed prominences are quiescent, as they were stable as filaments for at least three days and not eruptive. They present similar characteristics, they are highly dynamic and present horizontal magnetic fields. Statistically, the inclination from the local vertical is around 90 degrees, with some points around 60 and 120 degrees. The field strength is between 5 and 15 Gauss. We tested the effects of adding a turbulent field component to the horizontal field. For those pixels showing inclinations around 60 and 120 degrees, we find that such a model is compatible with the polarimetric observations. In some of these prominences, identified as “tornadoes” the field strength may reach 50 Gauss, and in the top of the tornadoes some points exhibit an inclination which cannot correspond to any model in our grid of models. We investigate different solutions

Optical instrumentation for chromospheric monitoring during solar cycle 25 at Paris and Côte d'Azur observatories

International audienceWe present the observing program proposed by Paris and Côte d’Azur Observatories for monitoring solar activity during the upcoming cycle 25 and providing near real time images and movies of the chromosphere for space-weather research and applications. Two optical instruments are fully dedicated to this task and we summarize their capabilities. Short-term and fast-cadence observations of the chromosphere will be performed automatically at Calern observatory (Côte d’Azur), where dynamic events, as flare development, Moreton waves, filament instabilities and Coronal Mass Ejections onset, will be tracked. This new set of telescopes will operate in 2021 with narrow bandpass filters selecting Hα and CaII K lines. We present the instrumental design and a simulation of future images. At Meudon, the Spectroheliograph is well adapted to the long-term and low-cadence survey of chromospheric activity by recently improved and optimized spectroscopic means. Surface scans deliver daily (x, y, λ) datacubes of Hα, CaII K and CaII H line profiles. We describe the nature of available data and emphasize the new calibration method of spectra

Designing a New Coronal Magnetic Field Energy Diagnostic

International audienceIn the solar corona, the free energy, i.e., the excess in magnetic energy over a ground-state potential field, forms the reservoir of energy that can be released during solar flares and coronal mass ejections. Such free energy provides a measure of the magnetic field nonpotentiality. Recent theoretical and observational studies indicate that the presence of nonpotential magnetic fields is imprinted into the structures of infrared, off-limb, coronal polarization. In this paper, we investigate the possibility of exploiting such observations for mapping and studying the accumulation and release of coronal free magnetic energy, with the goal of developing a new tool for identifying "hot spots" of coronal free energy such as those associated with twisted and/or sheared coronal magnetic fields. We applied forward modeling of infrared coronal polarimetry to three-dimensional models of nonpotential and potential magnetic fields. From these we defined a quantitative diagnostic of nonpotentiality that in the future could be calculated from a comparison of infrared, off-limb, coronal polarization observations from, e.g., the Coronal Multi-channel Polarimeter or the Daniel K. Inouye Solar Telescope, and the corresponding polarization signal forward-modeled from a potential field extrapolated from photospheric magnetograms. We considered the relative diagnostic potential of linear and circular polarization, and the sensitivities of these diagnostics to coronal density distributions and assumed boundary conditions of the potential field. Our work confirms the capacity of polarization measurements for diagnosing nonpotentiality and free energy in the solar corona

Forward Modeling of a Pseudostreamer

International audienceIn this paper, we present an analysis of a pseudostreamer embedding a filament cavity, observed on 2015 April 18 on the solar southwest limb. We use the flux-rope insertion method to construct nonlinear force-free field (NLFFF) models constrained by observed Solar Dynamics Observatory (SDO)/AIA coronal structures and the SDO/Helioseismic Magnetic Imager photospheric magnetogram. The resulting magnetic field models are forward-modeled to produce synthetic data directly comparable to Mauna Loa Solar Observatory/Coronal Multichannel Polarimeter (CoMP) observations of the intensity and linear polarization of the Fe XIII 1074.7 nm infrared coronal emission line using FORWARD. In addition, we determine the location of quasi-separatrix layers in the magnetic models, producing a Q-map from which the signatures of magnetic null points and separatrices can be identified. An apparent magnetic null observed in linear polarization by CoMP is reproduced by the model and appears in the region of the 2D-projected magnetic null in the Q-map. Further, we find that the height of the CoMP null is better reproduced by our NLFFF model than by the synthetic data we produce with potential-field source-surface models, implying the presence of a flux rope in the northern lobe of the pseudostreamer

Magnetofrictional Modeling of an Erupting Pseudostreamer

International audienceIn this study, we present the magnetic configuration of an erupting pseudostreamer observed on 2015 April 19, on the southwest limb of the Sun, with a prominence cavity embedded inside. The eruption resulted in a partial halo coronal mass ejection. The prominence eruption begins with a slow rise and then evolves to a fast-rise phase. We analyze this erupting pseudostreamer using the flux-rope insertion method and magnetofrictional relaxation to establish a sequence of plausible out-of-equilibrium magnetic configurations. This approach allows the direct incorporation of observations of structures seen in the corona (filament and cavity) to appropriately model the pseudostreamer based on SDO/HMI line-of-sight photospheric magnetograms. We also perform a topological analysis in order to determine the location of quasiseparatrix layers (QSLs) in the models, producing Q-maps to examine how the QSL locations progress in the higher iterations. We found that the axial flux in our best-fit unstable model was a factor of 20 times higher than we found in our marginally stable case. We computed the average magnetic field strength of the prominence and found that the unstable model exhibits twice the average field strength of the stable model. The eruption height from our modeling matches very well with the prominence eruption height measured from the AIA observation. The Q-maps derived from the model reproduce structures observed in LASCO/C2. Thus, the modeling and topological analysis results are fully consistent with the observed morphological features, implying that we have captured the large magnetic structure of the erupting filament in our magnetofrictional simulation

Modeling the Early Evolution of a Slow Coronal Mass Ejection Imaged by the Parker Solar Probe

International audienceDuring its first solar encounter, the Parker Solar Probe (PSP) acquired unprecedented up-close imaging of a small coronal mass ejection (CME) propagating in the forming slow solar wind. The CME originated as a cavity imaged in extreme ultraviolet that moved very slowly (<50 km s −1) to 3-5 solar radii (R e), where it then accelerated to supersonic speeds. We present a new model of an erupting flux rope (FR) that computes the forces acting on its expansion with a computation of its internal magnetic field in three dimensions. The latter is accomplished by solving the Grad-Shafranov equation inside two-dimensional cross sections of the FR. We use this model to interpret the kinematic evolution and morphology of the CME imaged by PSP. We investigate the relative role of toroidal forces, momentum coupling, and buoyancy for different assumptions on the initial properties of the CME. The best agreement between the dynamic evolution of the observed and simulated FR is obtained by modeling the two-phase eruption process as the result of two episodes of poloidal flux injection. Each episode, possibly induced by magnetic reconnection, boosted the toroidal forces accelerating the FR out of the corona. We also find that the drag induced by the accelerating solar wind could account for about half of the acceleration experienced by the FR. We use the model to interpret the presence of a small dark cavity, clearly imaged by PSP deep inside the CME, as a low-density region dominated by its strong axial magnetic fields

Relating Streamer Flows to Density and Magnetic Structures at the Parker Solar Probe

International audienceThe physical mechanisms that produce the slow solar wind are still highly debated. Parker Solar Probe's (PSP's) second solar encounter provided a new opportunity to relate in situ measurements of the nascent slow solar wind with white-light images of streamer flows. We exploit data taken by the Solar and Heliospheric Observatory, the Solar TErrestrial RElations Observatory (STEREO), and the Wide Imager on Solar Probe to reveal for the first time a close link between imaged streamer flows and the high-density plasma measured by the Solar Wind Electrons Alphas and Protons (SWEAP) experiment. We identify different types of slow winds measured by PSP that we relate to the spacecraft's magnetic connectivity (or not) to streamer flows. SWEAP measured high-density and highly variable plasma when PSP was well connected to streamers but more tenuous wind with much weaker density variations when it exited streamer flows. STEREO imaging of the release and propagation of small transients from the Sun to PSP reveals that the spacecraft was continually impacted by the southern edge of streamer transients. The impact of specific density structures is marked by a higher occurrence of magnetic field reversals measured by the FIELDS magnetometers. Magnetic reversals are associated with much stronger density variations inside than outside streamer flows. We tentatively interpret these findings in terms of magnetic reconnection between open magnetic fields and coronal loops with different properties, providing support for the formation of a subset of the slow wind by magnetic reconnection