CHROMOSPHERIC FINE-STRUCTURE - BLACK-AND-WHITE MOTTLES

Halpha observations of the chromosphere have been obtained on June 17, 1986 with a high spatial resolution using the MSDP (Multichannel Subt ractive Double Pass) spectrograph operating at Pic du Midi observatory . Various fine structures are observed in the chromosphere: bushes of fibrils around supergranules and fibrils (mottles) in the rosettes. Th ey are relatively stable, but line-of-sight velocities +/-5 - 10 km s- 1 are observed at the boundaries of supergranules (rosettes). Velocity images exhibit finer structures than intensity ones, with no cospatia l relationship between both of them. Typical Halpha line profiles of d ark and bright (black & white) mottles, constructed from 11 MSDP chann els, are used to derive the physical conditions in these structures. N LTE models of dark and bright mottles are represented by a grid of pro minence-like models of Gouttebroze et al. (1993). It is demonstrated t hat higher-pressure models (p(g) congruent-to 0.5 - 1 dyn cm-2) with t emperature around 10(4) K naturally explain the profiles of both dark and bright structures. By introducing a generalized two-cloud model wi th non-uniform source function, we account schematically for a shear-t ype velocity field. However, the velocities are found to be of a secon dary importance when the broad U-shaped profiles formation in consider ed. We conclude that both dark and bright mottles have a similar natur e and that their brightness difference is predominantly due to pressur e variations of the plasma injected into the rosette flux tubes. Final ly, we discuss the ambiguity of Halpha contrast profiles and comment o n applicability of the classical cloud model.