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Quiet Sun EUV transient brightenings and turbulence
A panoramic view by EIT on board SOHO
Received 2 March 1998 / Accepted 25 May 1998
Since January 1996, the Extreme-Ultraviolet Imaging Telescope (EIT, onboard SOHO) has produced unique image sequences covering a wide field of view with a high temporal resolution, in the He ii transition region line and in several Fe coronal emission lines. Using two sequences acquired with cadences of the order of one minute and with durations ranging from one hour to several hours, we analyse the dynamical properties of the transition region and corona. We find evidence of turbulence both in spatial and in temporal power spectra suggesting that the plasma of the quiet solar atmosphere is in a permanent state of turbulence.
As predicted by numerical simulations, this turbulence has an highly intermittent nature. We find an unexpectedly large number of small-scale brightenings. The coronal brightenings are identified as the low energy counterparts of "X-ray network flares" observed with SXT on board Yohkoh. The thousands of brightenings observed by EIT in the transition region include many that are similar to "blinkers" observed with CDS, though we find a larger variety.
Thanks to EIT's wide spatial coverage, we can perform a large scale statistical study, complementing spectroscopic studies which give access to only a very small subsample of events. We present occurrence distributions of duration, size and radiative output of the brightenings as well as various correlations between these parameters. The energy injected by the brightenings into the solar atmosphere is insufficient to be in itself responsible for coronal heating. We discuss the importance of the ubiquitous small brightenings as perhaps the most visible aspect of yet undetected heating events higher up in the quiet corona.
Key words: Sun: corona Sun: transition region Sun: UV radiation turbulence
Send offprint requests to: David.Berghmans@ksb-orb.oma.be
Online publication: July 27, 1998