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Astron. Astrophys. 348, 251-260 (1999)

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Transient events in the EUV transition region and chromosphere

P.T. Gallagher 1,2, K.J.H. Phillips 2, L.K. Harra-Murnion 3, F. Baudin 4 and F.P. Keenan 1

1 Department of Pure and Applied Physics, The Queen's University of Belfast, Belfast, BT7 1NN, Ireland
2 Space Science Department, Rutherford Appleton Laboratory, Chilton, Didcot, Oxon. OX11 0QX, UK
3 Mullard Space Science Laboratory, Holmbury St Mary, Dorking, Surrey RH5 6NT, UK
4 Institut d'Astrophysique Spatiale, Université Paris XI-CNRS, Bât. 121, F-91405 Orsay Cedex, France

Received 2 March 1999 / Accepted 12 May 1999


Rapid time cadence observations of the quiet Sun extreme ultraviolet emission, observed by the CDS instrument on SOHO , are discussed. Numerous transient brightenings are observed in network features in both a transition region line (OV 629.73 Å) and a chromospheric line (HeI 584.33 Å), indicating a dynamic coupling between the chromospheric and transition region network. Their durations are between 80 and 200 s and dimensions 6 000-10 000 km. A wavelet analysis reveals a tendency for semi-periodic behaviour, with excess power at a frequency of about 4 mHz. The variations are much less evident in the internetwork or cell regions, although they are again semi-periodic. Relative line-of-sight velocities have also been derived from the data, the CDS spectral resolution allowing a precision of between 4.7 and 6 km s-1. There is a clear association of brightenings in the network with downflows of [FORMULA] 13 km s-1 at 250 000 K with some events having velocities of up to [FORMULA] 20 km s-1, these being measured relative to the average quiet Sun emission. Within the internetwork, we also find a weak correlation between events seen in the HeI (584.33 Å) and the OV (629.73 Å) lines. In this case, the events have a smaller size ([FORMULA] 2 000 km), amplitude (both in terms of their intensity and velocity), and also show a higher frequency of about 6 mHz.

The apparent differing properties of network and internetwork events implies that both these regions are heated by two distinct mechanisms. In the case of the internetwork, these results further confirm that acoustic waves propagating up from the photosphere and forming shocks in the overlying atmosphere are the most likely heating mechanism. For the network, it is apparent that the heating required must be in excess of that supplied by acoustic shocks. Our view is that the network events are produced by nanoflare-like magnetic reconnections in the corona, or possibly excitation due to a spicule-type event in which there is a repeated rebound.

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

Send offprint requests to: Peter Gallagher (P.Gallagher@qub.ac.uk)

© European Southern Observatory (ESO) 1999

Online publication: July 16, 1999