A Substantial Amount of Hidden Magnetic Energy in the Quiet Sun

Deciphering and understanding the small-scale magnetic activity of the quiet solar photosphere should help to solve many of the key problems of solar and stellar physics, such as the magnetic coupling to the outer atmosphere and the coronal heating. At present, we can see only ${\sim}1$ of the complex magnetism of the quiet Sun, which highlights the need to develop a reliable way to investigate the remaining 99%. Here we report three-dimensional radiative tranfer modelling of scattering polarization in atomic and molecular lines that indicates the presence of hidden, mixed-polarity fields on subresolution scales. Combining this modelling with recent observational data we find a ubiquitous tangled magnetic field with an average strength of ${\sim}130$ G, which is much stronger in the intergranular regions of solar surface convection than in the granular regions. So the average magnetic energy density in the quiet solar photosphere is at least two orders of magnitude greater than that derived from simplistic one-dimensional investigations, and sufficient to balance radiative energy losses from the solar chromosphere.Comment: 21 pages and 2 figures (letter published in Nature on July 15, 2004

Can we explain non-typical solar flares?

We used multi-wavelength high-resolution data from ARIES, THEMIS, and SDO instruments, to analyze a non-standard, C3.3 class flare produced within the active region NOAA 11589 on 2012 October 16. Magnetic flux emergence and cancellation were continuously detected within the active region, the latter leading to the formation of two filaments. Our aim is to identify the origins of the flare taking into account the complex dynamics of its close surroundings. We analyzed the magnetic topology of the active region using a linear force-free field extrapolation to derive its 3D magnetic configuration and the location of quasi-separatrix layers (QSLs) which are preferential sites for flaring activity. Because the active region's magnetic field was nonlinear force-free, we completed a parametric study using different linear force-free field extrapolations to demonstrate the robustness of the derived QSLs. The topological analysis shows that the active region presented a complex magnetic configuration comprising several QSLs. The considered data set suggests that an emerging flux episode played a key role for triggering the flare. The emerging flux likely activated the complex system of QSLs leading to multiple coronal magnetic reconnections within the QSLs. This scenario accounts for the observed signatures: the two extended flare-ribbons developed at locations matched by the photospheric footprints of the QSLs, and were accompanied with flare loops that formed above the two filaments which played no important role in the flare dynamics. This is a typical example of a complex flare that can a-priori show standard flare signatures that are nevertheless impossible to interpret with any standard model of eruptive or confined flare. We find that a topological analysis however permitted to unveil the development of such complex sets of flare signatures.Comment: 13 pages, Accepted in A&

Instrumentation for solar spectropolarimetry: state of the art and prospects

Given its unchallenged capabilities in terms of sensitivity and spatial resolution, the combination of imaging spectropolarimetry and numeric Stokes inversion represents the dominant technique currently used to remotely sense the physical properties of the solar atmosphere and, in particular, its important driving magnetic field. Solar magnetism manifests itself in a wide range of spatial, temporal, and energetic scales. The ubiquitous but relatively small and weak fields of the so-called quiet Sun are believed today to be crucial for answering many open questions in solar physics, some of which have substantial practical relevance due to the strong Sun?Earth connection. However, such fields are very challenging to detect because they require spectropolarimetric measurements with high spatial (sub-arcsec), spectral (<100  mÅ), and temporal (<10  s) resolution along with high polarimetric sensitivity (<0.1  %   of the intensity). We collect and discuss both well-established and upcoming instrumental solutions developed during the last decades to push solar observations toward the above-mentioned parameter regime. This typically involves design trade-offs due to the high dimensionality of the data and signal-to-noise-ratio considerations, among others. We focus on the main three components that form a spectropolarimeter, namely, wavelength discriminators, the devices employed to encode the incoming polarization state into intensity images (polarization modulators), and the sensor technologies used to register them. We consider the instrumental solutions introduced to perform this kind of measurements at different optical wavelengths and from various observing locations, i.e., ground-based, from the stratosphere or near space.Fil: Iglesias, Francisco Andres. Universidad Tecnológica Nacional. Facultad Regional de Mendoza; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mendoza; ArgentinaFil: Feller, Alex. Max Planck Institut Fur Sonnensystemforschung; Alemani

IBIS 2.0 Science Description

The Interferometric BIdimensional Spectrometer 2.0 (IBIS 2.0) is a focal plane instrument which will be developed to acquire high cadence spectroscopic and spectropolarimetric images of the solar photosphere and chromosphere. Its previous version, named IBIS, was installed at the focal plane of the Dunn Solar Telescope of the National Solar Observatory in New Mexico (USA). It used two FPI in a classic mount and operated over the range 580 – 860 nm. IBIS 2.0 provides an important opportunity to investigate many open questions regarding the physics of the solar atmosphere, with particular attention to the phenomena visible in the photosphere and chromosphere. Moreover, IBIS 2.0 could represent a first step to develop a new instrument for the next generation telescopes. A brief overview of the project is available in [RD4]. A Science Working Group (SWG) has been charged by the project with the task of identifying the key science goals for the new version of the instrument and defining the corresponding science requirements that are needed to accomplish those goals. This document reports the outcome of such a Science Working Group

Study of the magnetic structure of active region filaments

El objetivo de esta tesis es el estudio de la estructura magnética de filamentos solares en regiones activas (RAs) y esclarecer su formación y evolución. Mientras que se han observado en numerosas ocasiones (y con diferentes resoluciones espaciales) filamentos fuera de RAs y sobre el limbo solar, el número de observaciones de filamentos en RAs es realmente escaso en la literatura. Los filamentos en RAs son fenómenos importantes que pueden dar lugar a eyecciones de masa coronal (EMC) cuyo material puede ser expulsado hacia el espacio con velocidades muy grandes pudiendo afectar a la Tierra. Las EMC están asociadas con cambios globales del campo magnético en la corona. En base a estos objetivos, se han estudiado en detalle datos espectropolarimétricos de los cuatro perfiles de Stokes en la región espectral de 10830 A. Las observaciones fueron tomadas en julio de 2005 con el polarímetro TIP en la VTT en Tenerife. Se realizaron observaciones de un filamento en una RA. El filamento se encontraba sobre la línea de inversión de polaridad (LIP) perteneciente a una RA que se encontraba en su fase de decaimiento. La región espectral de 10830 A ofrece la ventaja de disponer de varias líneas espectrales relevantes que se pueden observar de manera simultánea. Entre estas líneas se encuentra el triplete cromosférico de helio, una línea fotosférica de silicio y dos líneas telúricas. Durante la campaña de observación se tomaron dos mapas el 3 de julio y siete mapas y una serie temporal el 5 de julio. Estos datos se han preparado cuidadosamente para ser analizados mediante diferentes códigos de inversión. Como resultado de las inversiones de los cuatro parámetros de Stokes (I,Q,U,V) del triplete de helio 10830 A se han obtenido los valores del campo magnético más fuertes jamás registrados (600-700 Gauss). El siguiente paso consistió en determinar la estructura magnética, en el sistema de referencia local del Sol, en la cromosfera. Para ello fue necesario resolver la ambigüedad de los 180 grados. Existen varios métodos para solucionar este problema. En esta tesis se usó la herramienta AZAM. Las imágenes de helio permitieron distinguir dos áreas diferentes del filamento: (1) un área que presenta un filamento con forma alargada y grosor fino que corresponde al eje principal del filamento y (2) un área donde el filamento aparece difuso y extenso. Desde el punto de vista magnético, la primera parte presenta líneas de campo alargadas (paralelas) al eje del filamento mientras que en la segunda parte las líneas de campo pasan gradualmente de ser paralelas al eje a una configuración de polaridad normal. Para entender la estructura magnética global del filamento se incluyó en el análisis la línea de silicio 10827 A, la cual nos proporcionó la distribución magnética en la fotosfera. Dicha estructura presenta las siguientes características: (1) debajo del eje del filamento la configuración de polaridad es inversa, (2) debajo del filamento difuso se observan penumbras huérfanas y poros. Las líneas de campo en estas estructuras son alargadas y paralelas a la LIP. Es de destacar que, por primera vez, se obtiene en un filamento de una RA el vector del campo magnético simultáneamente en la cromosfera y fotosfera. Del análisis del vector del campo magnético se dedujo que el filamento es soportado por líneas de campo con forma de hélice. El eje principal de la hélice se encuentra, en una parte del filamento en la cromosfera mientras que en la otra en la fotosfera. El eje en la fotosfera da lugar a la creación de penumbras huérfanas. Para confirmar la estructura magnética propuesta, se realizaron extrapolaciones del campo magnético bajo la aproximación no lineal y libre de fuerzas. Como novedad, al disponer de magnetogramas vectoriales en dos alturas diferentes, estas extrapolaciones se pudieron realizar desde la fotosfera y también desde la cromosfera. Las líneas de campo obtenidas en ambas extrapolaciones son consistentes entre ellas y además confirmaron la estructura magnética propuesta con forma de hélice. Al disponer de magnetogramas v

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

Some THEMIS-MTR observations of the second solar spectrum (2000 campaign)

We report spectropolarimetric observations with the THEMIS telescope multi-lines operating mode (MTR) during the 2000 observational period from August 27th to September 1st. We measured the resonance polarization at the limb of a series of lines: \ion{Sr}{i} 460.7 nm, \ion{Na}{i} D1 589.6 nm and D2 589.0 nm, \ion{Ba}{ii} D1 493.4 nm and D2 455.4 nm, \ion{C}{i} 493.2 nm. The data analysis method is mainly described in Bommier & Rayrole ([CITE]), and has been completed by using the beam exchange facility as available in 2000 THEMIS, i.e., in a single Stokes parameter. A so-called “generalized beam exchange” technique has been settled on, for the full Stokes vector measurement under this limitation. The observations have been devoted to the measurement of the scattering polarization which is a linear polarization observed near the limb of the Quiet Sun, eventually modified by a weak magnetic field (the so-called Hanle effect). The entrance slit of the spectrograph has been oriented parallel to the tangential direction of the solar limb, and data have been averaged in time and along the spatial direction of the slit in order to increase the polarimetric resolution. Two different cameras have been used to record simultaneously the two polarization states exiting the beam-splitter. The results of our polarization measurements are in good agreement with those given in the second spectrum solar atlas of Gandorfer ([CITE]), based on 1999–2000 observations. Nevertheless, with regard to a quantification of the polarization signal, we found that the signal is systematically smaller than previous results obtained during the 1994–96 observational period and was also observed as decreasing during the 1998 observational period, as if a 11-year cyclic variation of the limb polarization occured. This signal variability obviously requires further observational and interpretative investigations. We noticed other differences to previous results, in particular, the linear polarization shape of the \ion{Na}{i} D1 line that also requires further observational investigation