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-37191 Katlenburg-Lindau, Germany
3Kiepenheuer-Institut für Sonnenphysik, D-79104 Freiburg, Germany
The instrumentally caused periodicities of the spectral line Doppler velocities, determined with the SUMER spectrometer onboard SOHO, are presented. 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 slight changes of the SUMER mechanical structure. Comparison with the housekeeping heating and temperature data shows that these so-called "thermoelastic oscillations" are caused by the changes of the internal SUMER temperature within the range of 1#10.15 K as it is controlled by the heaters in a closed loop.
Consequences of this instrumental effect for the SUMER measurements of the chromospheric and transition region spectral lines are discussed. A procedure how to calibrate the effect from the data post-facto is proposed and it is applied to the particular SUMER data set.
Key words: Sun: transition region - Sun : chromosphere - instrumentation: SUMER.
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