MEASUREMENT OF CORONAL MAGNETIC TWISTS DURING LOOP EMERGENCE OF NOAA 8069

Abstract. Emerging coronal loops were studied with extreme ultraviolet observations performed by SOHO/EIT on 5 and 6 August 1997 for NOAA 8069. Physical parameters (size and twist) were determined by a new stereoscopic method. The flux tubes were measured twisted when first observed by EIT. After emerging, they de-twisted as they expanded, which corresponds to a minimization of the energy. Different scenarios which take into account the conservation of the magnetic helicity are discussed in relation with structure and temperature variations

A Nanoflare Distribution Generated by Repeated Relaxations Triggered by Kink Instability

Context: It is thought likely that vast numbers of nanoflares are responsible for the corona having a temperature of millions of degrees. Current observational technologies lack the resolving power to confirm the nanoflare hypothesis. An alternative approach is to construct a magnetohydrodynamic coronal loop model that has the ability to predict nanoflare energy distributions. Aims: This paper presents the initial results generated by such a model. It predicts heating events with a range of sizes, depending on where the instability threshold for linear kink modes is encountered. The aims are to calculate the distribution of event energies and to investigate whether kink instability can be predicted from a single parameter. Methods: The loop is represented as a straight line-tied cylinder. The twisting caused by random photospheric motions is captured by two parameters, representing the ratio of current density to field strength for specific regions of the loop. Dissipation of the loop's magnetic energy begins during the nonlinear stage of the instability, which develops as a consequence of current sheet reconnection. After flaring, the loop evolves to the state of lowest energy where, in accordance with relaxation theory, the ratio of current to field is constant throughout the loop and helicity is conserved. Results: The results suggest that instability cannot be predicted by any simple twist-derived property reaching a critical value. The model is applied such that the loop undergoes repeated episodes of instability followed by energy-releasing relaxation. Hence, an energy distribution of the nanoflares produced is collated. Conclusions: The final energy distribution features two nanoflare populations that follow different power laws. The power law index for the higher energy population is more than sufficient for coronal heating.Comment: 13 pages, 18 figure

Multiscale Edge Detection in the Corona

Coronal Mass Ejections (CMEs) are challenging objects to detect using automated techniques, due to their high velocity and diffuse, irregular morphology. A necessary step to automating the detection process is to first remove the subjectivity introduced by the observer used in the current, standard, CME detection and tracking method. Here we describe and demonstrate a multiscale edge detection technique that addresses this step and could serve as one part of an automated CME detection system. This method provides a way to objectively define a CME front with associated error estimates. These fronts can then be used to extract CME morphology and kinematics. We apply this technique to a CME observed on 18 April 2000 by the Large Angle Solar COronagraph experiment (LASCO) C2/C3 and a CME observed on 21 April 2002 by LASCO C2/C3 and the Transition Region and Coronal Explorer (TRACE). For the two examples in this work, the heights determined by the standard manual method are larger than those determined with the multiscale method by approximately 10% using LASCO data and approximately 20% using TRACE data.Comment: 14 pages, 7 figures, In Solar Physics Topical Issue "Solar Image Analysis and Visualization

EIT Observations of the Extreme Ultraviolet Sun

The Extreme Ultraviolet Imaging Telescope (EIT) on board the SOHO spacecraft has been operational since 2 January 1996. EIT observes the Sun over a 45 x 45 arc min field of view in four emission line groups: Feix, x, Fexii, Fexv, and Heii. A post-launch determination of the instrument flatfield, the instrument scattering function, and the instrument aging were necessary for the reduction and analysis of the data. The observed structures and their evolution in each of the four EUV bandpasses are characteristic of the peak emission temperature of the line(s) chosen for that bandpass. Reports on the initial results of a variety of analysis projects demonstrate the range of investigations now underway: EIT provides new observations of the corona in the temperature range of 1 to 2 MK. Temperature studies of the large-scale coronal features extend previous coronagraph work with low-noise temperature maps. Temperatures of radial, extended, plume-like structures in both the polar coronal hole and in a low latitude decaying active region were found to be cooler than the surrounding material. Active region loops were investigated in detail and found to be isothermal for the low loops but hottest at the loop tops for the large loops

Etude de la couronne solaire en 3D et de son évolution avec SOHO/EIT

L'original de la these est disponible en CD Rom et comporte en plus des films (observations, animations, et simulations) en IDL et mpegThis thesis deals with 3D evolution of coronal structures based upon the ultraviolet telescope of SOHO : EIT.Anaglyphs and incertainties on a complete stereovision reconstruction are described.Stereoscopic methods for loop reconstruction were successfully made to find 3D parameters. With dynamical stereoscopy, physical conditions were derived for 30 loops of temperature around 1MK.A method which is able to derive twist variation were also built. Emerging loops were found highly twisted and they detwist as they grow. According to helicity conservation, this correspond to a transfert of twist into expansion.Long time twist evolution of magnetic flux tubes are followed in relation with flares as relaxation.Interaction between magnetic field lines were analysed. An example of reconnectionbetween open and closed field line were observed. Other interactions were found withmulti-wavelength observations : coronal holes borders (and thus CH) are better defined when an active region nearby is growing.Other imaging techniques were used to better take profit as possible of SOHO/EIT.A multiscale vision model (MVM) was applied with success to show small coronal structures evolutions hidden by the noise level.Pour mieux comprendre les structures de la couronne solaire de température élevée et leurs évolutions, des méthodes de vision à 3 dimensions à partir des images EUV de SOHO/EIT ont été développées. Des anaglyphes ont servi à mieux comprendre le rôle du champ magnétique et des intéractions entre champsfermés et ouverts. Il est apparu que la reconnection entre des trous coronaux (CH) et des régions actives (AR) peut expliquer certaines disparitions de boucles coronales fermées. De plus, des analyses multi-instruments ont montré que la structuration desCHs résulte parfois de changements dans l'activité magnétique voisine.Les mesures d'incertitudes sur les reconstructions par stéréovision n'ont pas permis de valider une méthode générale pour visualiser la couronne globalement en 3D. Par contre, le principe a pu être utilisé avec succès pour la reconstruction en 3 dimensions de boucles coronales. Ainsi après analyse des paramètres physiques de 30 boucles EUV de températures intermédiaires d'une région active, il est apparu que contrairement aux boucles chaudes en X, elles étaient en équilibre hydrostatique.Les boucles d'une région active émergentes sont apparues comme se détorsadant au fur et à mesure de son grandissement ce qui correspond à un transfert de l'hélicité.Des corrélations entre des brusques détorsadages et des éruptions ont aussi été établis dans les cas où la torsion initiale est trop importante et permet le développementd'instabilités.Ces études 3D vont pouvoir permettre de mieux contraindre les bilans énergétiques pour le chauffage coronal gràce à la conservation de l'hélicité et d'améliorer la prévision de la météorologie spatiale. L'utilisation de techniques d'imageries adaptées (commele dégrillage ou le Modèle de Vision Multi-échelle) a permis de faire ressortir certains détails dans la formation de CMEs par exemple

The Sun, The Earth, and the Space Weather

International audienceDetermining when GPS gives false positioning due to Space Weatherconsequences has been one of the aims in a study done by Team Nina 2002 during the Alpbach Summer School. For that, TEC maps (Total Electron Contents of the Earth atmosphere at a certain position) must be derived because the precision is invert proportional to the number of particles along the line of sight for satellites. These maps are sensible to the fluctuations day/night but also particles injections coming from the solar corona via Coronal Mass Ejections (CMEs) or Coronal Holes (CHs). Also a 3D prevision of which structures could erupt toward the magnetosphere are needed for these aspects of Space Weather. We first present the SOHO mission and its European data base MEDOC at IAS. SOHO has 12 instruments including a EUV imageur EIT and 3 coronagraphs LASCO. Densities and temperatures can be measured by SOHO/EIT wavelength ratios and the spectroscopic CHIANTI code. SOHO/EIT has observed CMEs evolution in projection. If the CMEs are coming toward the Earth, magnetosphere perturbations can be observed.The understanding and the forecasting need 3D observations and interpretations of such structures. As the solar corona is optically thin, thus the intensity is deduced from the emission measure integrated along the line of sight. This integrationcreates uncertainties when we want to determine precisely structures morphology and geometrical parameters. Portier-Fozzani [4, 5] has shown that the classical stereo methods do not work directly. Tomography methods would be the most appropriate for optical thin material, but because of the low number of different viewangles, these methods cannot be applied easily. Meanwhile, by introducing constraints to stereographics methods such as the geometrical shape of the object (example: loops = circle, plumes = conics,...), it is possible to derive some geometricalparameters. Afterward, the physical coherence of this model purely observational is checked. Thus in Aschwanden et al. (APJ. 515–842 (1999)), we could measure from a more sophisticated method the geometrical parameters of loops. From the EIT image ratio, physical parameters of potential loops are determined. Considering thepossibilities of small deformations, Portier-Fozzani, Demoulin et al. (Sp. Sci. Rev. 97, 51–54 (2001)) took into account that the loops could be twisted. They measured that in an emerging active region, loops appear first twisted and detwist as they expand.This result, if we assume that magnetic helicity has to be conserved, gives important stability criteria in function of the size and the twist degrees for coronal loops. Then we describe the future observations that the STEREO/SECCHI Mission will made. It is two satellites separating themselves from 22.5◦ per year. That will improve stereovision reconstruction that is actually limited to the high temporal dynamic of structures as CMEs involved in Space Weather