Astron. Astrophys. 335, L97-L100 (1998)

2. Observations

We analyse high resolution MDI data (pixel size 0.605 arcsec) obtained in a big active region (NOAA 7999) located close to solar disc centre. The data analysed here consist of a time series recorded on 27 Nov., 1996 at a cadence of one minute and lasting 1.5 hours. They are composed of simultaneous observations of the continuum intensity, a proxy of the magnetic flux (magnetogram) and the line shift. Such data have the advantage that they show true solar variations in the absence of seeing fluctuations. This removes an important source of noise. In order to follow the same spatial points, we correct the data for solar rotation before constructing the time series.

Fig. 1 shows maps of each of the observed quantities. The contour levels drawn on each of them show the umbral and penumbral boundaries derived from the continuum intensity. The leading spot is on the right, the following on the left in the image.

Fig. 1. Typical intensity, magnetogram and velocity images of the active region under consideration. The scales on the axes are in arcseconds. The contours represent the umbral and penumbral boundaries derived from the brightness image.

The lower frame depicts the Doppler velocity. The Evershed outflow can be clearly seen in spite of the fact that the region is located very close to disc centre (the central solar meridian is located at position in this figure, i.e. slightly left of the leftmost sunspot, the equator lies at ).

© European Southern Observatory (ESO) 1998

Online publication: June 26, 1998
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