Intensity variations in EIT shutterless mode: Waves or flows?

¹ Centre for Plasma Astrophysics, K.U. Leuven, Celestijnenlaan 200 B, 3001 Leuven, Belgium
² Royal Observatory of Belgium, Ringlaan 3 Av. Circulaire, 1180 Brussels, Belgium e-mail: david.berghmans@oma.be
³ Mullard Space Science Laboratory, Holmbury St. Mary, Dorking, Surrey RH5 6NT, UK
⁴ Observatoire de Paris, LESIA, FRE2461 (CNRS), 92195 Meudon Cedex, France e-mail: lidia.vandriel@obspm.fr
⁵ Konkoly Observatory, PO Box 67, 1525 Budapest, Hungary

Corresponding author: A. De Groof, Anik.DeGroof@wis.kuleuven.ac.be

Received: 1 September 2003
Accepted: 3 November 2003

Abstract

On 11 July 2001 an EIT shutterless campaign was conducted which provided 120 high-cadence (68 s) 304 Å images of the north eastern quarter of the Sun. The most interesting feature seen in the data is an off-limb half loop structure along which systematic intensity variations are seen which appear to propagate from the top of the loop towards its footpoint. We investigate the underlying cause of these propagating disturbances, i.e. whether they are caused by waves or by plasma flows. First we identify 7 blobs with the highest intensities and follow them along the loop. By means of a location-time plot, bulk velocities can be measured at several locations along the loop. The velocity curve found this way is then compared with characteristic wave speeds and with the free-fall speed in order to deduce the nature of the intensity variations. Additional information on density and temperature is derived by measuring the relative intensity enhancements and comparing the EIT 304 Å sequence with Big Bear data and 171 Å data (TRACE/EIT). The combination of all these constraints gives us an insight on the nature and origin of these intensity variations. The idea of slow magneto-acoustic waves is rejected, and we find several arguments supporting that these intensity variations are due to flowing/falling plasma blobs.