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Astron. Astrophys. 353, 1083-1093 (2000)

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EUV brightness variations in the quiet Sun

A. Brkovi 1, I. Rüedi 2, S.K. Solanki 3, A. Fludra 4, R.A. Harrison 4, M.C.E. Huber 5, J.O. Stenflo 1 and K. Stucki 1

1 Institute of Astronomy, ETH-Zentrum, 8092 Zürich, Switzerland
2 Physikalisch-Meteorologisches Observatorium Davos, World Radiation Center, 7260 Davos Dorf, Switzerland
3 Max-Planck-Institut für Aeronomie, Max-Planck-Strasse 2, 37191 Katlenburg-Lindau, Germany
4 Space Science Department, Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire OX11 0QX, UK
5 ESA, Space Science Deptartment, ESTEC, PO Box 299, 2200 AG Noordwijk, The Netherlands

Received 16 July 1999 / Accepted 18 October 1999


The Coronal Diagnostic Spectrometer (CDS) onboard the SOHO satellite has been used to obtain movies of quiet Sun regions at disc centre. These movies were used to study brightness variations of solar features at three different temperatures sampled simultaneously in the chromospheric He I 584.3 Å ([FORMULA] K), the transition region O V 629.7 Å ([FORMULA] K) and coronal Mg IX 368.1 Å ([FORMULA] K) lines.

In all parts of the quiet Sun, from darkest intranetwork to brightest network, we find significant variability in the He I and O V line, while the variability in the Mg IX line is more marginal. The relative variability, defined by rms of intensity normalised to the local intensity, is independent of brightness and strongest in the transition region line. Thus the relative variability is the same in the network and the intranetwork. More than half of the points on the solar surface show a relative variability, determined over a period of 4 hours, greater than 15.5% for the O V line, but only 5% of the points exhibit a variability above 25%. Most of the variability appears to take place on time-scales between 5 and 80 minutes for the He I and O V lines.

Clear signs of "high variability" events are found. For these events the variability as a function of time seen in the different lines shows a good correlation. The correlation is higher for more variable events. These events coincide with the (time averaged) brightest points on the solar surface, i.e. they occur in the network. The spatial positions of the most variable points are identical in all the lines.

Key words: Sun: chromosphere – Sun: corona – Sun: transition region – Sun: UV radiation

Send offprint requests to: brkovic@astro.phys.ethz.ch

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© European Southern Observatory (ESO) 2000

Online publication: January 18, 2000