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Astron. Astrophys. 321, 921-926 (1997)

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3. Spatial correlation in the slit direction

Guiding in the direction of the slit turns out to be rather difficult if individual prominence structures exhibit motions or evolutionary changes (as decay or birth). This is most impressively seen in the intensity variation of prominence-H (Fig. 1). Such prominences are not only hard to be guided on the slit, but also the numerical correlation procedure in the slit direction is almost impossible. A 'global' correlation of the total spatial intensity distribution in the spectra may introduce arbitrary lateral shifts, the correlation of one single structure may yield unrealistic shifts of the neighbouring structures.

[FIGURE] Fig. 1. Time variation of the spatial intensity distribution (along the slit) of the Ca [FORMULA]  8542 emission in prominence-H (left panel), showing strong lateral motions and evolutionary changes which make guiding on the slit as difficult as a spatial correlation during data reduction; right: prominence-C showing significant evolutionary variations

On the other side, Doppler measurement at a fixed spatial location are senseless for emission structures which drift away or decay, since Doppler shifts are only defined at locations with significant emissions. The determination of Doppler oscillations in such structures would require their 'stretching' by a local correlation procedure, individually applied to each intensity maximum. To our knowledge, such a local correlation has so far not been reported in former papers. A possible occurrence of power maxima due to the usual 'global' correlation is thus not yet considered.

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© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998