Thesis--Varnaseya Sanskrit Vishwavidyalaya. / Includes bibliographical references (p. [198]-200).

Thesis--Varnaseya Sanskrit Vishwavidyalaya. / Includes bibliographical references (p. [198]-200).

Thèse d'Astronomie : Sciences : Université, Faculté des sciences de Paris : 1834. / Titre provenant de l'écran-titre.

Faint patterns of light and shadow known as "shadow bands" are often seen for a few moments before, and for a similar period Just after, a total solar eclipse. The cause of these bands is not known at this time. One of the proposed mechanisms for producing these shadow bands is basic Fresnel edge diffraction.A computer program was developed by the writer to calculate the Fresnel patterns produced by the eclipse geometry. Certain assumptions, believed to be reasonable, were made in this development. The resulting patterns were compared to the available data on shadow bands, and were found to differ by roughly two orders of magnitude in most respects.The author concludes that basic Fresnel edge diffraction for visible wavelengths is not capable of producing shadow band like patterns.

Since the Kennelly- Heaviside hypothesis in 1902 of the existence of a partially conducting layer in the upper atmosphere was proved to be true by the experiments of APPLETON and BARNETT (1925) and BREIT and TUVE (1926), this region has become known as the ionosphere. The ionosphere was soon discovered to consist of, not one but several layers (Fig. 1) (i) A layer at a height of just over 100 km. called the E layer. (ii) A layer at a height of approximately 300km. called the F₂ layer. (iii) A layer at a height of approximately 200 km. called the F₁ layer; this layer differs from the other two in that it is only present during the day time in Summer. (iv) Occasional intense reflections from a height of about 100 km. are found - these cannot be attributed to the normal E layer and have received the name "Sporadic E". The presence of two E layers (E₁ and E₂) has been suggested by HALLIDAY (1936) and BEST and RATCLIFFE (l978) but until recently most workers still seem to attribute these reflections to Sporadic E. Recent measurement by rockets of the electron density at E layer heights still do not confirm whether such bifurcation exists in the E region. The diurnal and seasonal variations of the first three layers indicate that the sun is the chief agent in their production. It is generally agreed that these layers consist of ionised molecules or atoms and free electrons produced by radiation from the sun. The origin of Sporadic E ionisation is still obscure, but it is thought that these sudden increases in ionisation which occur in E layer heights are due to passing meteors. Recently it has also been suggested by SEDDON, PICKAR and JACKSON (1954) from rocket measurements that Sporadic E might be due to a steep electron density gradient above the B layer.

The behaviour of the ionosphere during a solar eclipse is of great interest because radiation from the sun is the cause of ionization in the upper atmosphere and it is useful to be able to conduct experiments where this radiation is cut off and restored in a known manner. Experimental results, especially those dealing with the F2 layer, have proved puzzling. Cusps which cannot be explained appear on the records obtained from ionosphere sounders and in the F2 region the electron density at a given height shows a maximum after the eclipse where one would expect it simply to rise to a steady value. An attempt is made in this thesis to explain some of the anomalies in terms of tilts in the ionospheric layers and minima of electron density or "valleys" between the ionospheric layers. The problem is attacked theoretically. Part I deals with the theoretical background to ionospheric physics in general and to this problem in particular. Standard methods of dealing with radio propagation in the ionosphere as well as some methods developed by the author are discussed. Part II deals directly with the effects of a solar eclipse on a theoretical ionosphere. Ionograms which would be obtained in the theoretical ionosphere are constructed. These are scaled by standard methods to show where errors may arise . It appears that tilts in the layers have only a small effect. The effect of the valley is, however, extremely important, giving rise to the apparent maximum of electron density in the F2 layer at a given height after the eclipse. This maximum does not in fact exist but arises from an error in the scaling method which ignores the possibility of a valley. Some records taken during the solar eclipse of 25 December, 1954 have been scaled. They support the conclusion reached theoretically.

Les régions d’interface du Soleil de la photosphère à la chromosphère et au delà de la basse couronne ont été étudiées à partir des spectres éclairs. Les éclipses sont les plus adaptées à ce type d’observation, car l’occultation a lieu en dehors de l’atmosphère terrestre et sont exemptes de lumière parasite. Les images Extrême-UV des régions du limbe obtenues récemment dans l’espace sont analysés avec des modèles hydrostatiques à une dimension, comme les modèles VAL, mais cette méthode ne tient pas compte du phénomène d’émergence du champ magnétique, associé au réseau chromosphérique qui est responsable de: i) les spicules et le milieu interspiculaire, ii) les jets coronaux et macrospicules. Un saut de température de 0.01 à 1 MK est observé autour de 2 Mm d’altitude plus loin, et produit plus loin le flot du vent solaire permanent. Le processus de chauffage responsable du saut de température et la source du vent solaire ne sont pas encore compris. Dans cette thèse, nous traitons ces problèmes à partir de spectres éclairs récents réalisés avec les technologies actuelles de détecteurs CCD rapides, images d’éclipse en lumière blanche et des images EUV obtenues avec des instruments de missions spatiales. Nous illustrons les mécanismes des émissions des raies à faible potentiel de première ionisation (FIP) présents dans les basses couches de l’atmosphère solaire. Nous identifions plus précisément les raies à bas FIP à la fois dans les interfaces, à l’intérieur et en dehors des protubérances. Nous caractérisons en détail les enveloppes d’hélium et la région de l’interface solaire. Nous discutons de l'enrichissement de la couronne en éléments low FIP. / The solar interface region from the photosphere to the chromosphere and to the lower corona has been studied using flash spectra obtained during solar total eclipses. Eclipses are very favourable for this type of observation as the occultation takes place outside the Earth atmosphere and are free of parasitic scattered light. Independently, EUV filtergrams of the limb region obtained in space were analyzed using one dimensional hydrostatic models like the VAL models but this method ignores the ubiquitous magnetic field emergence phenomenon associated with the chromospheric network and responsible for: i) spicules and interspicular regions, ii) coronal jets and macrospicules. The components of the solar interface region are dynamical and different type of waves and magnetic reconnections are suggested to be at work. A jump of temperature from 0.01 to 1 MK is observed near the 2 Mm heights and higher, further producing a permanent solar wind flow. The heating processes responsible for this temperature jump and for the flow are not yet fully understood. In this thesis, we reconsider these problems on the basis of original, superior flash spectra which benefit from present technology such as CCD detectors, white light (W-L) eclipse images and new EUV images obtained with space-borne instruments. We illustrate the mechanisms of low First Ionisation Potential (FIP) emission lines present in the low layers of the solar atmosphere and interfaces. We identify more precisely low FIP lines both inside and nearby prominences. We characterize in detail the He shells and the solar interface region. We discuss the enrichment of low FIP elements in the corona.