J. Rybák1, W. Curdt2, A. Kucera1,
U. Schühle2, H. Wöhl3
1Astronomical Institute of the Slovak Academy of Sciences,
SK-05960 Tatranská Lomnica, Slovakia
2Max-Planck-Institut für Aeronomie, D-37189 Katlenburg-Lindau, Germany
3Kiepenheuer-Institut für Sonnenphysik, D-79104 Freiburg, Germany
The observational method can cause periodicities in the spectral line intensities and velocities, determined with the SUMER spectrometer onboard SOHO. On long-term measurements of O I, C II and O VI lines, originating in the upper solar atmosphere, it is shown that the reason of these periodicities was the spectrometer slit position stepping of the solar rotation compensation. The standard rotation compensation procedure, performed on-line by the instrument, was selected for these long-term observations. The residual periodical motion of the slit position on the solar disk was taking place and it produced the regular variations of the measured line intensities and velocities. Unfortunately, these regular variations significantly enhanced amplitudes in the frequency range where the real solar oscillations were already determined!
Consequences of this effect for SUMER measurements of the chromospheric and transition region dynamics are discussed. Procedures how to exclude the effect from the data post-facto as well as how to modify the future SUMER oscillations measurements in order to avoid the effect are proposed.
Key words: Sun: transition region - Sun : chromosphere - Sun: dynamics - instrumentation: SUMER.
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