Detailed comparison of downflows seen both in EIT 30.4 nm and Big Bear Hα movies

¹ Centre for Plasma Astrophysics, KU Leuven, Celestijnenlaan 200B, 3001 Leuven, Belgium e-mail: [Anik.DeGroof;Stefaan.Poedts]@wis.kuleuven.be
² Institute of Theoretical Astrophysics, University of Oslo, PO Box 1029 Blindern, 0315 Oslo, Norway e-mail: Daniel.Mueller@astro.uio.no
³ Center of Mathematics for Applications, University of Oslo, PO Box 1053 Blindern, 0316 Oslo, Norway
⁴ European Space Agency, Research and Scientific Support Department c/o NASA Goddard Space Flight Center, Mail Code 612.5, Greenbelt, MD 20771, USA
⁵ Royal Observatory of Belgium, Ringlaan 3 Av. Circulaire, 1180 Brussels, Belgium e-mail: david.berghmans@oma.be

Received: 25 March 2005
Accepted: 19 July 2005

Abstract

An EIT shutterless campaign was conducted on 11 July 2001 and provided 120 high-cadence (68 s) 30.4 nm images of the north-eastern quarter of the Sun. Systematic intensity variations are seen which appear to propagate along an off-disk loop-like structure. In this paper we study the nature of these intensity variations by confronting the EIT observations studied in De Groof et al. (2004, A&A, 415, 1141) with simultaneous Hα images from Big Bear Solar Observatory. With the goal to carefully co-register the two image sets, we introduce a technique designed to compare data of two different instruments. The image series are first co-aligned and later overplotted in order to visualize and compare the behaviour of the propagating disturbances in both data sets. Since the same intensity variations are seen in the EIT 30.4 nm and in the Hα images, we confirm the interpretation of De Groof et al. (2004, A&A, 415, 1141) that we are observing downflows of relatively cool plasma. The origin of the downflows is explained by numerical simulations of “catastrophic cooling” in a coronal loop which is heated predominantly at its footpoints.