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Astron. Astrophys. 342, 563-574 (1999)

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Analysis and comparison of loop structures imaged with NIXT and Yohkoh/SXT

V. Di Matteo 1, F. Reale 2, G. Peres 2 and L. Golub 3

1 Osservatorio Astronomico, Piazza del Parlamento 1, I-90134 Palermo, Italy (e-mail: dimatteo@oapa.astropa.unipa.it)
2 Dipartimento di Scienze Fisiche e Astronomiche, Università di Palermo, Sezione di Astronomia, Piazza del Parlamento 1, I-90134 Palermo, Italy (e-mail: reale@oapa.astropa.unipa.it; peres@oapa.astropa.unipa.it)
3 Smithsonian Astrophysical Observatory, 60 Garden Street, Cambridge, MA 02138, USA (e-mail: golub@corona.harvard.edu)

Received 30 June 1998 / Accepted 16 October 1998


We analyze and compare five coronal regions simultaneously observed by NIXT and Yohkoh/SXT on April 12, 1993. The compact loop structures (length [FORMULA] cm) imaged in three regions with NIXT and with SXT have a good general morphological correspondence. A large scale ([FORMULA] cm) and an intermediate scale ([FORMULA] cm) structure observed in the NIXT image have no obvious counterpart in the SXT image. The pressure of the loop plasma detected by NIXT is derived from the brightness profile along the loops by applying a method based on loop models. The pressure of the loop plasma in the SXT band has also been derived from loop models on the basis of the temperature obtained from the standard Yohkoh data analysis. NIXT pressures are systematically lower than those found with SXT. By comparing the measured total loop luminosity to that expected on the basis of loop models, we constrain quantitatively the volume filling factor of the plasma emitting respectively in the NIXT and SXT bands. The filling factors obtained in the NIXT band for the compact and intermediate loops are very low ([FORMULA]), but they are of the order of unity for the large structure. The low filling factors suggest a strong loop filamentation. The filling factors for the compact structures are instead of the order of unity in the SXT band. We discuss our results and propose a scenario for their interpretation.

Key words: Sun: X-rays, gamma rays – Sun: corona

Send offprint requests to: F. Reale

© European Southern Observatory (ESO) 1999

Online publication: February 22, 1999