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Abstract

High resolution observations of an isolated sunspot located near the centre of the solar disk (NOAA/AR 7783) were obtained on October 3, 1994. They were performed with the Multichannel Subtractive Double Pass (MSDP) Spectrograph installed at the focus of the VTT at Tenerife and operating in the Hα line. The penumbra and superpenumbra are made up of an apparently well-ordered pattern of elongated dark fibrils. Line profiles were reconstructed and monochromatic images and Doppler shift velocities were derived at several depths in the Hα line over a two dimensional field of view. Apart from the Doppler velocities the Hα line profiles are used for the computation of the line-of-sight velocities of the dark fibrils by two other methods. The first one involves the well-known Beckers' cloud model. The second is the photographic subtraction method based on the cloud model and expressing the “Doppler signal”. This method using the monochromatic intensities at two wavelengths on either side of the Hα line profile can give, in the general case, a quantitative picture of the velocity field, but under some assumptions, values of the velocities at different depths can be obtained. The values of the velocities obtained by this method are compared to those given by the Doppler shift method and by the cloud model, while comparison of the velocities at different depths can give a picture of the gradient of the velocity inside the dark penumbral fibrils.