Energy Release During Slow Long Duration Flares Observed by RHESSI

Slow Long Duration Events (SLDEs) are flares characterized by long duration of rising phase. In many such cases impulsive phase is weak with lack of typical short-lasting pulses. Instead of that smooth, long-lasting Hard X-ray (HXR) emission is observed. We analysed hard X-ray emission and morphology of six selected SLDEs. In our analysis we utilized data from RHESSI and GOES satellites. Physical parameters of HXR sources were obtained from imaging spectroscopy and were used for the energy balance analysis. Characteristic time of heating rate decrease, after reaching its maximum value, is very long, which explains long rising phase of these flares.Comment: Accepted for publication in Solar Physic

EUV and HXR Signatures of Electron Acceleration During the Failed Eruption of a Filament

We search for EUV brightenings in TRACE 171 {\AA} images and HXR bursts observed during failed eruptions. We expect that if an eruption is confined due to interaction with overlying magnetic structures then we should observe effects connected with reconnection between magnetic structures and acceleration of particles. We utilized TRACE observations of three well observed failed eruptions. EUV images were compared to 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 HXR photon spectral index of power-law fit to HXR data. We have found that EUV brightenings are closely related to the eruption velocity decrease, to HXR bursts and to episodes of hardening of HXR spectra. The EUV brightened areas are observed far from the flaring structure, in footpoints of large systems of loops observed 30-60 minutes after the maximum of a flare. These are not `post-flare' loops that are also observed but at significantly lower heights. The high lying systems of loops are observed at heights equal to height, at which eruption was observed to stop. We observed HXR source spatially correlated with EUV brightening only once. For other EUV brightened areas we estimated the expected brightness of HXR sources. We find that EUV brightenings are produced due to interaction between the erupting structure with overlying loops. The interaction is strong enough to heat the system of high loops. These loops cool down and are visible in EUV range about 30-60 minutes 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 enough dynamic range and sensitivity to enable such observations.Comment: A&A accepte

Investigation of quasi-periodic varaiations in hard X-rays of solar flares

The aim of the present paper is to use quasi-periodic oscillations in hard X-rays (HXRs) of solar flares as a diagnostic tool for investigation of impulsive electron acceleration. We have selected a number of flares which showed quasi-periodic oscillations in hard X-rays and their loop-top sources could be easily recognized in HXR images. We have considered MHD standing waves to explain the observed HXR oscillations. We interpret these HXR oscillations as being due to oscillations of magnetic traps within cusp-like magnetic structures. This is confirmed by a good correlation between periods of the oscillations and the sizes of the loop-top sources. We argue that a model of oscillating magnetic traps is adequate to explain the observations. During the compressions of a trap particles are accelerated, but during its expansions plasma, coming from chromospheric evaporation, fills the trap, which explains the large number of electrons being accelerated during a sequence of strong impulses. The advantage of our model of oscillating magnetic traps is that it can explain both the impulses of electron acceleration and quasi-periodicity of their distribution in time.Comment: 21 pages, 11 figures, 3 tables, submitted to Solar Physic

Investigation of long-duration arcade flares

Aims.We present the analysis of the three well-observed long-duration arcade flares (LDAFs). We aim to find general properties of LDAFs. In this paper results concerning morphology and physical parameters in these flares are shown. Methods.Yohkoh observations, Kitt Peak Vacuum Telescope (KPVT) magnetograms and GOES/SEM measurements are used to determine morphology and evolution of the analysed flares and physical parameters of their loop-top kernels (temperature, density, altitude). Results.We found that: (1) a so-called arcade channel may exist in LDAFs; (2) the energy release in LDAFs occurs in several places simultaneously and the reconnection process is not uniform along the flaring arcade; (3) a presence of $20{-}30$ MK hot plasma during the decay phase suggests that the reconnection occurs after the LDAF maximum

The interaction of a plasmoid with a loop-top kernel

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

Plasma dynamics in the flaring loop observed by RHESSI

Context. Hard X-rays (HXRs) contain the most direct information about the non-thermal electron population in solar flares. The approximation of the HXR emission mechanism (bremsstrahlung), known as the thick-target model, is well developed. It allows one to diagnose the physical conditions within a flaring structure. The thick-target model predicts that in flare foot points, we should observe lowering of HXR sources’ altitude with increasing energy. Aims. The foot point of HXR sources result from the direct interaction of non-thermal electron beams with plasma in the lower part of the solar atmosphere, where the density increases rapidly. Therefore, we can estimate the plasma density distribution along the non-thermal electron beam directly from the observations of the altitude-energy relation obtained for the HXR foot point sources. However, the relation is not only density-dependent. Its shape is also determined by the power-law distribution of non-thermal electrons. Additionally, during the impulsive phase, the plasma density and a degree of ionisation within foot points may change dramatically due to heating and chromospheric evaporation. For this reason, the interpretation of observed HXR foot point sources’ altitudes is not straightforward and needs detailed numerical modelling of the electron precipitation process. Methods. We present the results of numerical modelling of one well-observed solar flare. We used HXR observations obtained by RHESSI. The numerical model was calculated using the hydrodynamic 1D model with an application of the Fokker-Planck formalism for non-thermal beam precipitation. Results. HXR data were used to trace chromospheric density changes during a non-thermal emission burst, in detail. We have found that the amount of mass that evaporated from the chromosphere is in the range of 2.7 × 1013 − 4.0 × 1014 g. This is in good agreement with the ranges obtained from hydrodynamical modelling of a flaring loop (2.3 × 1013 − 3.3 × 1013 g), and from an analysis of observed emission measure in the loop top (3.9 × 1013 − 5.3 × 1013 g). Additionally, we used specific scaling laws which gave another estimation of the evaporated mass around 2 × 1014 g. Conclusions. Consistency between the obtained values shows that HXR images may provide an important constraint for models – a mass of plasma that evaporated due to a non-thermal electron beam depositing energy in the chromosphere. High-energy, non-thermal sources’ (above 20 keV in this case) positions fit the column density changes obtained from the hydrodynamical model perfectly. Density changes seem to be less affected by the electrons’ spectral index. The obtained results significantly improve our understanding of non-thermal electron beam precipitation and allow us to refine the energy balance in solar flare foot points during the impulsive phase

RHESSI observations of long-duration flares with long-lasting X-ray loop-top sources

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 1031   erg