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Physical parameters and flows along chromospheric penumbral fibrils
Received 28 October 1999 / Accepted 21 March 2000
High resolution observations of a sunspot region were obtained on October 3, 1994 with the Multichannel Subtractive Double Pass (MSDP) spectrograph. This instrument installed at the focus of the Vacuum Tower Telescope (VTT) at Tenerife (Canary Islands) operated in the H line. Intensity fluctuations and Doppler shift velocities at several wavelengths were derived over a two dimensional field of view. The observed intensity profiles were matched with theoretical ones using a technique proposed by Tsiropoula et al. 1999, which enables the variation of the source function inside the structures and the derivation of some physical parameters like the source function, the Doppler width, the optical depth and the line-of-sight velocity. This technique was applied to the dark fibrils surrounding the sunspot umbra. Once these parameters are estimated several other parameters can be determined like population densities at levels 1, 2, 3 (, , ), total particle density of hydrogen, , electron density, , electron temperature, , gas pressure, p, total column mass, m, mass density, , sound speed, etc. Furthermore, using a simple geometrical model we estimated from the line-of-sight velocity the flow velocity along the dark fibrils assuming different sets of inclination angles of the velocity vector with respect to the vertical. We found that there is a flow from the outer edge of the fibrils (in the side of the penumbra), to their inner edge (umbra side), which is consistent to the siphon flow. Moreover, we found a subsonic flow for the entire extent of two of the fibrils considered and for all inclination angle sets, while for one fibril we found a subsonic flow in the outer edge and supersonic velocity in the inner edge for some of the inclination angle sets.
Key words: Sun: chromosphere Sun: sunspots
© European Southern Observatory (ESO) 2000
Online publication: June 5, 2000