Plasma Parameters in a Post-Eruptive Arcade Observed with CORONAS-F/SPIRIT, Yohkoh/SXT, SOHO/EIT, and in Microwaves

The SPectroheliographIc X-Ray Imaging Telescope (SPIRIT) aboard the CORONAS-F spacecraft sometimes observes in the Mg XII 8.42 Å line (Tₘax ~ 9 \,MK) large, bright features existing for many hours high in the corona. This fact suggests that plasma beta there might not be small. We identify such a feature observed on 2001 October 22 at a height of 100 Mm with a hot part of a post-eruptive arcade. Using multi-spectral data and radio astronomy methods, we estimate the plasma parameters in the arcade and reveal the coronal magnetic configuration. Several hours after the eruption, the temperature in the arcade was 6–8 MK, and the plasma density was (5 -- 10) · 10⁹ \,cm⁻³. We confirm the results by the accord of the quantities obtained from different observations using different methods and by the extrapolation of the photospheric magnetograms into the corona, from which we have obtained a magnetic field strength of about 7 G at that height. Hence, indeed β ≥ 1 in the post-eruptive arcade. The high-beta possibility had previously been stated, e.g., by Ichimoto et al. (1994), but still not investigated. We address this issue in terms of the “standard flare model” elaborated by Shibata and Yokoyama. During this event, Yohkoh/SXT observed McKenzie-Hudson dark moving features. Those features are filled with cold material, if they are not empty, and may affect the arcade.