Les bulles chromospheriques

Résumé

Nous étudions des structures chromosphériques qui ont été identifiées sur des filtrogrammes pris au bord du disque solaire dans la raie Hα, et que nous appelons ‘bulles’ (figures 1 et 2). Un certain nombre de paramètres ont été mesurés (tableau I et figure 4), et une discussion est développée en ce qui concerne les hauteurs maximum et les diamètres des bulles. Nous comparons les bulles avec d'autres structures chromosphériques (tableau II). Finalement nous examinons deux hypothèses sur la formation des bulles.

Abstract

The purpose of this work is to study some characteristics of roundish features which are observed on Hα filtergrams, above the solar chromosphere, at the limb. The ‘Bulles’ - chromospheric bubbles - so called for their apparent shape, move upwards into the low corona and fade out when their material is completely ionised (Figures 1 to 3). The physical data for nine observed bubbles listed in Table I are: lifetime (2), average height (3), ascending velocity (4), lateral velocity (5), angle of the movement with the normal to the solar limb (6), intensity in Hα (7) and diameters (8). The time variations of some of these properties are pictured in Figure 4. It is noted that all measured velocities are super-sonic. We discuss the relation between the ascending velocity and the maximum height reached by the bubbles (Figure 5) and compare it with the one obtained by Rush and Roberts for spicules. The study of their diameters shows that the vertical diameter is generally greater than the horizontal one. Their ratio is a function of speed and becomes equal to unity when the bubble velocity reaches 70 km s⁻¹ (Figure 6). The bubbles seem to diffuse in the corona as spicules do. We also compare the properties of bubbles with some already known chromospheric features (Table II) and there seems to be a link between bubbles and Becker's grains. Finally, we, however, propose two hypothesis regarding the origin of bubbles: (i) they are related to spicules, or (ii) they are accelerated in the low chromosphere by the divergence of the magnetic field.