Magnetic fragments in the photosphere are in continuous motion and, due to the complex nature of the magnetic field in the solar atmosphere, these motions are likely to drive a lucrative coronal energy source: the passing of initially-unconnected opposite-polarity fragments that release energy through both closing and then re-opening the same fieldlines Three-dimensional, time-dependent MHD and potential models are used to investigate the passing of fragments in an overlying field  The processes of closing and opening the field generally occur through separator and separatrix reconnection, respectively The rates of flux reconnection in these processes are determined They are found to be dependent on the direction of the surrounding magnetic field relative to the motion of the fragments and the velocity of the sources In particular, separator reconnection rates (closing) and separatrix-surface reconnection rates (opening) are directly related to the rate of flux transport perpendicular to the current sheet (overlying field) The results suggest that both types of reconnection are fast with the peak rates of separator and separatrix reconnection occurring at 58% and 29% of the peak potential reconnection rate, respectively, when the sources are driven at a hundredth of the peak Alfven velocity in the box Moreover, the slower the system is driven the closer the flux reconnection rates are to the instantaneous potential rates Furthermore, there is a maximum reconnection rate for both types of reconnection as the driving speed tends to the Alfven speed with the separatrix reconnection rate typically half that of separator reconnection These results suggest that, on the Sun, reconnection driven by the passing of small-scale network and intranetwork fragments is a highly efficient process that is very likely to contribute significantly to the heating of the background solar corona  The three-dimensional reconnection processes are efficient because, unlike in two-dimensions, there are many places within the current sheets where reconnection can take place simultaneously giving rise to fine-scale structure along the boundaries between the open, closed and re-opened flux Furthermore, due to the complexity of the magnetic field above the photosphere the reconnection all takes place low down at less than a quarter of the separation of the initial fragments above the photosphere

https://www.aanda.org/articles/aa/pdf/2004/47/aa0350-03.pdf

Elementary heating events - magnetic interactions between two flux sources. II. Rates of flux reconnection

Cette these analyse la maniere dont l'emergence d'un nouvel objet de science, le Soleil, entraine une reorganisation des champs disciplinaires ainsi que la transformation des pratiques scientifiques et leurs interactions. Ce travail couvre une periode suffisamment longue afin de mieux restituer cette evolution complexe, allant de 1820 a 1930. Il ressort qu'une veritable physique du Soleil n'a pu emerger qu'avec la constitution de la thermodynamique. Elle rend ainsi possible de penser physiquement le Soleil. Avant cela, les sciences etaient encore separes selon leurs objets et leur methode. Mais ce nouvel objet se trouve avec un statut hybride, suspendu entre le domaine des sciences d'observation et des sciences d'experimentation. La physique solaire se developpe neanmoins a partir du milieu du XIXe siecle selon le rythme des decouvertes et des innovations techniques. Dans les annees 1860, Herve Faye et Angelo Secchi proposent de vastes syntheses, representatives d'un style typiquement humboldtien. Cependant, ces theories globales et qualitatives montrent leur limite au debut du XXe siecle. Les progres theoriques et instrumentaux, notamment avec la mise au point du bolometre de Samuel Langley, aboutissent a l'apparition de la notion de modele mathematique chez Eddington, et signe l'emergence d'une nouvelle approche theorique en astrophysique, et conjointement dans d'autres secteurs comme la cosmologie. Cette nouvelle approche fait plus que legitimer l'extrapolation des lois physiques aux astres, elle en fait de veritables laboratoires naturelles. Ainsi, au terme de cette periode, nous passons du Soleil hors du laboratoire au Soleil comme laboratoire.

Le soleil comme laboratoire des pratiques scientifiques

History has the power to expose the origin and evolution of scientific ideas. How did humanity come to visualize the Sun as a gaseous plasma? Why is its interior thought to contain blackbody radiation? Who were the first people to postulate that the density of the solar body varied greatly with depth? When did mankind first conceive that the solar surface was merely an illusion? What were the foundations of such thoughts? In this regard, a detailed review of the Sun’s thermodynamic history provides both a necessary exposition of the circumstance which accompanied the acceptance of the gaseous models and a sound basis for discussing modern solar theories. It also becomes an invitation to reconsider the phase of the photosphere. As such, in this work, the contributions of Pierre Simon Laplace, Alexander Wilson, William Herschel, Hermann von Helmholtz, Herbert Spencer, Richard Christopher Carrington, John Frederick William Herschel, Father Pietro Angelo Secchi, Herve August Etienne Albans Faye, Edward Frankland, Joseph Norman Lockyer, Warren de la Rue, Balfour Stewart, Benjamin Loewy, and Gustav Robert Kirchhoff, relative to the evolution of modern stellar models, will be discussed. Six great pillars created a gaseous Sun: 1) Laplace’s Nebular Hypothesis, 2) Helmholtz’ contraction theory of energy production, 3) Andrew’s elucidation of critical temperatures, 4) Kirchhoff’s formulation of his law of thermal emission, 5) Plucker and Hittorf’s discovery of pressure broadening in gases, and 6) the evolution of the stellar equations of state. As these are reviewed, this work will venture to highlight not only the genesis of these revolutionary ideas, but also the forces which drove great men to advance a gaseous Sun.

A Thermodynamic History of the Solar Constitution — I: The Journey to a Gaseous Sun

/pdf/a-thermodynamic-history-of-the-solar-constitution-i-the-1y290w104g.pdf

A Thermodynamic History of the Solar Constitution - I: The Journey to a Gaseous Sun

In this work, the development of solar theory is followed from the concept that the Sun was an ethereal nuclear body with a partially condensed photosphere to the creation of a fully gaseous object. An overview will be presented of the liquid Sun. A powerful lineage has brought us the gaseous Sun and two of its main authors were the direct scientific descendants of Gustav Robert Kirchhoff: Franz Arthur Friedrich Schuster and Arthur Stanley Eddington. It will be discovered that the seminal ideas of Father Secchi and Herve Faye were not abandoned by astronomy until the beginning of 20th century. The central role of carbon in early solar physics will also be highlighted by revisiting George Johnstone Stoney. The evolution of the gaseous models will be outlined, along with the contributions of Johann Karl Friedrich Zollner, James Clerk Maxwell, Jonathan Homer Lane, August Ritter, William Thomson, William Huggins, William Edward Wilson, George Francis FitzGerald, Jacob Robert Emden, Frank Washington Very, Karl Schwarzschild, and Edward Arthur Milne. Finally, with the aid of Edward Arthur Milne, the work of James Hopwood Jeans, the last modern advocate of a liquid Sun, will be rediscovered. Jeans was a staunch advocate of the condensed phase, but deprived of a proper building block, he would eventually abandon his non-gaseous stars. For his part, Subrahmanyan Chandrasekhar would spend nine years of his life studying homogeneous liquid masses. These were precisely the kind of objects which Jeans had considered for his liquid stars.

/pdf/a-thermodynamic-history-of-the-solar-constitution-ii-the-28nk12xy1b.pdf

A Thermodynamic History of the Solar Constitution - II: The Theory of a Gaseous Sun and Jeans’ Failed Liquid Alternative

THE object of the investigation discussed in NATURE (No. 101, pp. 449–452) being merely that of ascertaining whether the incandescent matter contained in the solar atmosphere transmits radiant heat of sufficient energy to admit of thermometric measurement, no particular statement was deemed necessary regarding the spectrum which appeared on the bulb of the focal thermometer after shutting out the rays from the photosphere during the experiments. The appearance of this spectrum has in the meantime been carefully considered. Its extent and position suggest that the depth of the solar atmosphere far exceeds the limits hitherto assumed.

The Solar Atmosphere

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The Solar Atmosphere