Chromospheric fine structure: Black & white mottles

Details Chromospheric fine structure: Black & white mottles Chromospheric fine structure: Black & white mottles

Feb 1994

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994a%26a...282..939h&link_type=abstract

Astronomy and Astrophysics (ISSN 0004-6361), vol. 282, no. 3, p. 939-954

Astronomy and Astrophysics

Astrophysics

27

Chromosphere, Fine Structure, H Alpha Line, Solar Atmosphere, Space Plasmas, Stellar Models, Stellar Spectra, Astronomical Spectroscopy, Mathematical Models, Radiative Transfer, Spectrographs, Thermodynamic Equilibrium

Scientific paper

H-alpha observations of the chromosphere have been obtained on June 17, 1986 with a high spatial resolution using the MSDP (Multichannel Subtractive 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. They are relatively stable, but line-of-sight velocities +/- 5-10 km/s are observed at the boundaries of supergranules (rosettes). Velocity images exhibit finer structures than intensity ones, with no cospatial relationship between both of them. Typical H-alpha line profiles of dark and bright (black & white) mottles, constructed from 11 MSDP channels, are used to derive the physical conditions in these structures. Non-local thermodynamic equilibrium (NLTE) models of dark and bright mottles are represented by a grid of prominence-like models of Goutebroze et al. (1993). It is demonstrated that higher-pressure models(pg approximately = 0.5-1 dyn/sq cm) with temperature around 104 K naturally explain the profiles of both dark and bright structures. By introducing a generalized two-cloud model with non-uniform source function, we account schematically for a shear-type velocity field. However, the velocities are found to be of a secondary importance when the broad U-shaped profiles formation is considered. We conclude that dark and bright mottles have a similar nature and that their brightness difference is predominantly due to pressure variations of the plasma injected into the rosette flux tubes. Finally, we discuss the ambiguity of H-alpha contrast profiles and comment on applicability of the classical cloud model.

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