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Astron. Astrophys. 355, 769-780 (2000)

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1. Introduction

Systems of flare loops appear shortly after the onset of solar flares first in the soft X-ray and the EUV band and later on, H[FORMULA] loop prominences for events on the limb or typical dark loops on the disk usually appear (Bray et al. 1991). These loops are most conspicuous in the gradual phase of flares and can persist for many hours. This is also the reason why they are called in literature `post-flare loops - PFL' although their appearance during the impulsive phase is not unusual.

Sturrock (1968), Kopp & Pneumann (1976), Forbes & Malherbe (1986) and others proposed that PFL can result from a gradual reconnection of magnetic fields high in the solar corona which produces successively higher magnetic arches in the corona and at the same time, energy is released at the reconnection site (cusp) above the loop system. This energy is then transported downwards, along the magnetic field lines, either by thermal conduction or by beams of accelerated particles. As a consequence, the chromosphere and transition region are heated and a flow of evaporated hot plasma fills the corresponding magnetic loop system. A hot PFL is thus created. As the reconnection continues, this loop is separated from the energy input and starts to cool. In the meantime, a new hot PFL is originating above it.

PFL systems have been extensively observed particularly in H[FORMULA] and in soft X-rays, especially after the launch of Yohkoh with its Soft X-ray Telescope (SXT) (Tsuneta et al. 1991). H[FORMULA] images show plasma at temperatures around [FORMULA] K and provide very good spatial resolution useful for studies of the structural and dynamic properties of PFL (e.g. Wiik et al. 1996). From these observations it is also possible to determine the emission measure of cool plasma in the loop system (Schmieder et al. 1996). On the other hand, soft X-ray images, which lack the spatial resolution of H[FORMULA] images, show plasma with temperatures of the order of [FORMULA] - [FORMULA] K and they are useful for approximate temperature and emission measure analysis. The relation between cool (H[FORMULA]) and hot plasma in PFL systems was described in Schmieder et al. (1995).

Observations of PFL systems in EUV lines provide an excellent opportunity to study the behaviour of post-flare loop plasma at intermediate temperatures. EUV spectra also allow to apply efficient electron density, emission measure and temperature diagnostic methods (e.g. Mason & Monsignori Fossi 1994). Several studies of this kind have been published. Dere & Cook (1979) studied the time evolution of differential emission measure and electron density during the decay of a flare using data from Solrad 9 and Skylab. The spatial distribution of EUV emission and electron density was studied by Cheng (1980) using Skylab data. A displacement of loops visible in lines with high and low formation temperatures was found. Similar results based on data from SMM and H[FORMULA] observations were also published by vestka et al. (1987).

Using observations from SOHO Coronal Diagnostic Spectrometer (CDS) and Yohkoh SXT, we examine in this paper a decaying PFL system in its final phase when the loops are fading and becoming invisible. From SXT observations we derive the time evolution of the temperature and the emission measure in the hot part of the system during its decay. The CDS data was used to determine the vertical thermal structure of the examined loop system using the temperature sensitive line pair of Fe XVI at 360.8 Å and Si XII at 520.7 Å and its electron density from the density sensitive line pair of Fe XIV 334.2/353.8. From integrated intensities of several allowed lines with different formation temperatures the emission measures were calculated. From these measurements we estimated the geometrical filling factor at the top of the PFL system in Fe XIV lines. The values of the temperature and the electron density were then used to estimate the cooling time of the system from its initial temperature down to [FORMULA] K.

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© European Southern Observatory (ESO) 2000

Online publication: March 9, 2000