EDP Sciences
Free access
Volume 373, Number 3, July III 2001
Page(s) 1056 - 1072
Section The Sun
DOI http://dx.doi.org/10.1051/0004-6361:20010652

A&A 373, 1056-1072 (2001)
DOI: 10.1051/0004-6361:20010652

Analysis of blinkers and EUV brightenings in the quiet Sun observed with CDS

A. Brkovic1, S. K. Solanki2 and I. Rüedi3

1  Institute of Astronomy, ETH-Zentrum, 8092 Zürich, Switzerland
2  Max-Planck-Institut für Aeronomie, Max-Planck-Str. 2, 37191 Katlenburg-Lindau, Germany
3  Physikalisch-Meteorologisches Observatorium Davos, World Radiation Center, 7260 Davos Dorf, Switzerland

(Received 28 February 2001 / Accepted 23 April 2001)

Movies of quiet Sun regions at disc centre obtained with the Coronal Diagnostic Spectrometer (CDS) onboard the SOHO spacecraft are used to study the properties of transient brightenings seen in the extreme ultraviolet (EUV), so-called blinkers, at three different temperatures sampled simultaneously in the chromospheric He I 584.3 Å( $2 \times 10^4$ K), the transition region O V 629.7 Å( $2.5 \times 10^5$ K) and coronal Mg IX 368.1 Å(106 K) lines. Blinkers, here defined somewhat differently than in previous studies, were clearly detected in the O V and He I lines. Brightenings of the Mg IX line were also seen. A thorough analysis of blinker properties is carried out and their detailed properties are determined. Blinkers are found to be present in both bright (network) and dark (intranetwork) regions, but their number density is larger in the brighter areas (in O V) although the rest of their properties appear to be unaffected. The average sizes of brightenings range from 2.8 Mm2 in Mg IX, 12.4 Mm2 in He I to 23.5 Mm2 in O V. The durations of blinkers are in the range 3-110 min, with the average durations being 23 min in He I, about 16 min in O V and 12 min in Mg IX. The frequency distributions of ratio of peak to background intensity, excess energy and size follow power laws with exponents < -5 for the intensity ratio, and between -1 and -3 for the other two parameters. The correlation coefficients between pairs of ratio, energy and size are at least 0.5, while other pairs of parameters describing the blinkers appear to be uncorrelated. The best correlation is between size and energy. The blinker durations exhibit a distribution whose form is compatible with a log-normal function. Finally, blinkers in the 3 lines (i.e. 3 temperature regimes) are poorly correlated; with the correlation coefficient being always less than 0.4. This suggests that to a large extent the transition region reacts independently of the corona and chromosphere to energy deposition, so that these parts of the atmosphere are at least partly decoupled from each other. This agrees with the expectations from models having separate transition-region loops, but contradicts the classical picture of the transition region, as being heated dominantly by energy conduction from the corona.

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

Offprint request: A. Brkovic, brkovic@astro.phys.ethz.ch

© ESO 2001