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Picture of the month November 2002:
The left figure shows intensity profiles of a Fraunhofer absorption line at 557.6 nm (Fe I, g=0). Both profiles stem from a sunspot penumbra at a heliocentric angle of 24 degrees (distance from disk center) and have been measured with TESOS (see picture of the month September 2002). The red profile stems from the limb-side penumbra, and the blue profile from the disk-center-side penumbra. While the line core (intensity minimum) of both lines are at the same wavelength, the line wing (intermediate intensities) of the limb-side profile is shifted towards longer wavelengths (red-shifted), while the line wing of the disk-center-side profile is shifted towards shorter wavelengths (blue-shifted). Therby, the two profiles are asymmetric.
The right figure displays the bisectors of the two profiles. A bisector is used to measure the asymmetry of a line profile and represents the wavelength shifts of the line at specific intensity levels. Interpreting the wavelength shifts as Doppler shifts, we can assign velocities to the shifts which correspond to the line-of-sight component of the plasma velocity. From the right figure it is seen that the line core is at rest while the Doppler shift increases linearly towards positive (red-shifted) values on the limb side, and decreases towards negative (blue-shifted) values on the disk-center side.
The intensity profile of an absorption line probes different layers of the solar atmosphere and as a rule of thumb, the line core (region of lowest intensity) stems from high atmospheric layers, and as the intensity increases along the line profile, the information contributing to the line profile stems from deeper and deeper layers. Hence, the increasing shift of the bisectors towards the line wing indicates, that the flow velocity has its maximum in the very deep atmospheric layers. It is known that the flow (called Evershed flow) is predominantly horizontal, but the fact that the limb-side bisector exhibits higher velocities in the line wing (1.5 km/s) than the center-side bisector (-0.9 km/s), indicates the presence of a downflow component within the penumbra.
The left image shows the best filtergram of the scan (at a wavelength in the continuum). The right image displays a map of the line-of-sight component of the (Doppler) velocity, obtained from the line wing (bright equals to a blue shift and dark equals to a red shift). Technical details are described in the picture of the month in September 2002.
Observations: T. Berkefeld, K. Mikurda und A. Tritschler
Text: Rolf Schlichenmaier