Aims. We study the interaction between a downward moving plasmoid and a loop-top kernel recognized in the 30 November 2000 flare. Such an interaction is predicted by some numerical models of solar flares. Methods. Using X-ray observations from Yohkoh and GOES, EUV observations from SOHO, and radio observations from Ondřejov, we perform multi-wavelength analysis of this interaction. Results. The Yohkoh/SXT and SOHO/EIT images indicate that the growing flare loop with the loop-top kernel and the above-lying plasmoid were formed as a result of the interaction of two extended arcade-loops. While the flare loop was growing upwards, the plasmoid moved downwards with the velocity of about 16 km s −1 and interacted with the loop-top kernel. Many details of this interaction are found, e.g., an increase of the X-ray and decimetric radio fluxes and an increase of the plasma temperature at the interaction site. Just after the coalescence of the plasmoid with the loop-top kernel, the 1-2 GHz pulsating radio structure and hard X-ray source above the coalescence site were observed. The analyzed temperature maps indicate flows of heated plasma around the plasmoid to the location of the X-ray and radio source. These observations are in agreement with predictions from numerical modelling.
Aims. We search for extreme-ultraviolet (EUV) brightenings in TRACE 171 Å images and hard X-ray (HXR) bursts observed during failed eruptions. We expect that if an eruption is confined by interaction with overlaying magnetic structures, we should observe effects caused by reconnection between magnetic structures and acceleration of particles. Methods. We used TRACE observations of three well-observed failed eruptions. A semi-automated method was used to search for abrupt brightness changes in the TRACE field of view. The EUV images were compared to the HXR spatial distribution reconstructed from YOHKOH/HXT and RHESSI data. The EUV light curves of a selected area were compared to height profiles of eruption, HXR emission, and the HXR photon spectral index of a power-law fit to the HXR data. Results. We have found that EUV brightenings are closely related to the eruption velocity decrease, to HXR bursts, and to episodes of hardening of the HXR spectra. The EUV-brightened areas are observed far from the flaring structure, in footpoints of large systems of loops observed 30-60 min after the maximum of a flare. These are not "post-flare" loops, which are also observed, but at significantly lower heights. The high-lying systems of loops are observed at heights equal to the height at which the eruption was observed to stop. We observed only one HXR source that was spatially correlated with EUV brightening. For other EUV-brightened areas we estimated the expected brightness of HXR sources. Conclusions. We find that EUV brightenings are produced by interaction between the erupting structure with overlaying loops. The interaction is strong enough to heat the system of high loops. These loops cool and are visible in the EUV range about 30-60 min later. The estimated brightness of HXR sources associated with EUV brightenings shows that they are too weak to be detected with present instruments. However, next-generation instruments will have sufficient dynamic range and sensitivity to enable such observations.
Aims. We analyse three well-observed long-duration arcade flares (LDAFs) to find their general properties. Results concerning energy release in LDAFs (energy balance and diagnostic diagrams) are shown. Methods. Yohkoh observations and Kitt Peak Vacuum Telescope magnetograms are used to determine physical parameters of flare loop-top kernels (temperature, density, altitude). Using these parameters the energy release is studied in detail. Results. We found that: (1) the energy release in LDAFs occurs during their whole decay phase; (2) the kernel heating (E H ) is of the order of 0.01-1 erg cm −3 s −1 ; (3) the kernels usually cannot be distinguished in SXT images when E H is drops below a particular value. This value depends on the altitude of the kernels.
Context. The Yohkoh /HXT observations of long duration events (LDEs) have shown that the HXR emission (14−23 keV) is present for tens of minutes after flare maximum. As a result, some heating process is expected to exist during that time. The higher energy resolution of RHESSI compared to HXT allow us to analyse LDEs in a more comprehensive way. Aims. We selected three LDEs observed by RHESSI to answer the questions of how long HXR emission can be present, where it is emitted, what its nature is and how much energy should be released to sustain the emission. Methods. We used RHESSI data to reconstruct images of the selected flares with an energy resolution as high as 1 keV. Next we estimated physical parameters of HXR sources through imaging spectroscopy. The physical parameters obtained were then used to calculate the energy balance of the observed sources. Results. We found that HXR thermal emission can be present for many hours after LDE flare maximum. The emission comes from large and hot loop-top sources. The total energy that must be released to sustain the emission of the sources is as high as 10 31 erg.
Slow long-duration events (SLDEs) are flares characterized by the long duration of their rising phase. In many such cases the impulsive phase is weak without typical shortlasting pulses. Instead, smooth, long-lasting hard X-ray (HXR) emission is observed. We analyzed hard X-ray emission and morphology of six selected SLDEs. In our analysis we utilized data from the RHESSI and GOES satellites. The physical parameters of HXR sources were obtained from imaging spectroscopy and were used for the energy balance analysis. The characteristic decay time of the heating rate, after reaching its maximum value, is very long, which explains the long rising phase of these flares.
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