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Time variations of observed Hα line profiles and precipitation depths of non-thermal electrons in a solar flare  

Robert Falewicz   Submitted: 2017-09-01 03:20

We compare time variations of the Hα and X-ray emissions observed during the pre-impulsive and impulsive phases of the C1.1-class solar flare on 21 June 2013 with those of plasma parameters and synthesized X-ray emission from a one-dimensional hydro-dynamic numerical model of the flare. The numerical model was calculated assuming that the external energy is delivered to the flaring loop by non-thermal electrons. The Hα spectra and images were obtained using the Multi-channel Subtractive Double Pass spectrograph with a time resolution of 50~ms. The X-ray fluxes and spectra were recorded by the Reuven Ramaty High-Energy Solar Spectroscopic Imager ( RHESSI). Pre-flare geometric and thermodynamic parameters of the model and the delivered energy were estimated using RHESSI data. The time variations of the X-ray light curves in various energy bands and the those of the Hα intensities and line profiles were well correlated. The time scales of the observed variations agree with the calculated variations of the plasma parameters in the flaring loop footpoints, reflecting the time variations of the vertical extent of the energy deposition layer. Our result shows that the fast time variations of the Hα emission of the flaring kernels can be explained by momentary changes of the deposited energy flux and the variations of the penetration depths of the non-thermal electrons.

Authors: R. Falewicz, K. Radziszewski, P. Rudawy, A. Berlicki
Projects: None

Publication Status: accepted to ApJ
Last Modified: 2017-09-01 11:13
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2D MHD and 1D HD models of a solar flare - a comprehensive comparison of the results  

Robert Falewicz   Submitted: 2015-10-03 04:08

Without any doubt solar flaring loops possess a multi-thread internal structure that is poorly resolved and there are no means to observe heating episodes and thermodynamic evolution of the individual threads. These limitations cause fundamental problems in numerical modelling of flaring loops, such as selection of a structure and a number of threads, and an implementation of a proper model of the energy deposition process. A set of 1D hydrodynamic and 2D magnetohydrodynamic models of a flaring loop are developed to compare energy redistribution and plasma dynamics in the course of a prototypical solar flare. Basic parameters of the modeled loop are set according to the progenitor M1.8 flare recorded in the AR10126 on September 20, 2002 between 09:21 UT and 09:50 UT. The non-ideal 1D models include thermal conduction and radiative losses of the optically thin plasma as energy loss mechanisms, while the non-ideal 2D models take into account viscosity and thermal conduction as energy loss mechanisms only. The 2D models have a continuous distribution of the parameters of the plasma across the loop, and are powered by varying in time and space along and across the loop heating flux. We show that such 2D models are a borderline case of a multi-thread internal structure of the flaring loop, with a filling factor equal to one. Despite the assumptions used in applied 2D models, their overall success in replicating the observations suggests that they can be adopted as a correct approximation of the observed flaring structures.

Authors: R. Falewicz, P. Rudawy, K. Murawski, A. K. Srivastava
Projects: None

Publication Status: Accepted (ApJ)
Last Modified: 2015-10-07 12:57
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Plasma heating in solar flares and their soft and hard X-ray emissions  

Robert Falewicz   Submitted: 2014-05-26 03:51

In this paper, the energy budgets of two single-loop like flares observed in X-ray are analysed under the assumption that non-thermal electrons (NTEs) are the only source of plasma heating during all phases of both events. The flares were observed by RHESSI and GOES on February 20th, 2002 and June 2nd, 2002, respectively. Using a 1D hydrodynamic code for both flares the energy deposited in the chromosphere was derived applying RHESSI observational data. The use of the Fokker-Planck formalism permits the calculation of distributions of the non-thermal electrons in flaring loops, thus spatial distributions of the X-ray non-thermal emissions and integral fluxes for the selected energy ranges which were compared with the observed ones. Additionally, a comparative analysis of the spatial distributions of the signals in the RHESSI images was conducted for the footpoints and for the entire flare loops in selected energy ranges with these quantities fluxes obtained from the models. The best compatibility of the model and observations was obtained for the June 2nd, 2002 event in the 0.5-4 A GOES range and total fluxes in the 6-12 keV, 12-25 keV, 20-25 keV and 50-100 keV energy bands. Results of photometry of the individual flaring structures in a high energy range shows that the best compliance occurred for the June 2nd, 2002 flare, where the synthesized emissions were 30% or more higher than the observed emissions. For the February 20th, 2002 flare, synthesized emission is about 4 times lower than the observed one. However, in the low energy range the best conformity was obtained for the February 20th, 2002 flare, where emission from the model is about 11% lower than the observed one. The larger inconsistency occurs for the June 2nd, 2002 solar flare, where synthesized emission is about 12 times greater or even more than the observed emission. Some part of these differences may be caused by inevitable flaws of the applied methodology, like by an assumption that the model of the flare is symmetric and there are no differences in the emissions originating from the feet of the flare's loop and by relative simplicity of the applied numerical 1D code and procedures. No doubt a significant refinement of the applied numerical models and more sophisticated implementation of the various physical mechanisms involved are required to achieve a better agreement. Despite these problems, a collation of modelled results with observations shows that soft and hard X-ray emissions observed for analysed single-loop like events may be fully explained by electron beam-driven evaporation only.

Authors: Robert Falewicz
Projects: None

Publication Status: Accepted for publicaton in ApJ
Last Modified: 2014-05-26 13:02
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Plasma heating in the very early and decay phases of solar flares  

Robert Falewicz   Submitted: 2011-03-17 05:31

In this paper we analyze the energy budgets of two single-loop solar flares under the assumption that non-thermal electrons are the only source of plasma heating during all phases of both events. The flares were observed by the Ramaty High Energy Solar Spectroscopic Imager (RHESSI) and Geostationary Operational Environmental Satellite (GOES) on September 20, 2002 and March 17, 2002, respectively. For both investigated flares we derived the energy fluxes contained in non-thermal electron beams from the RHESSI observational data constrained by observed GOES light-curves. We showed that energy delivered by non-thermal electrons was fully sufficient to fulfil the energy budgets of the plasma during the pre-heating and impulsive phases of both flares as well as during the decay phase of one of them. We concluded that in the case of the investigated flares there was no need to use any additional ad-hoc heating mechanisms other than heating by non-thermal electrons.

Authors: Falewicz, R., Siarkowski, M. and Rudawy, P.
Projects: None

Publication Status: ApJ (in press)
Last Modified: 2011-03-17 13:49
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Plasma heating in the very early phase of solar flares  

Robert Falewicz   Submitted: 2009-10-05 06:21

In this paper we analyze soft and hard X-ray emission of the 2002 September 20 M1.8 GOES class solar flare observed by RHESSI and GOES satellites. In this flare event, soft X-ray emission precedes the onset of the main bulk hard X-ray emission by ~5 min. This suggests that an additional heating mechanism may be at work at the early beginning of the flare. However RHESSI spectra indicate presence of the non-thermal electrons also before impulsive phase. So, we assumed that a dominant energy transport mechanism during rise phase of solar flares is electron beam-driven evaporation. We used non-thermal electron beams derived from RHESSI spectra as the heating source in a hydrodynamic model of the analyzed flare. We showed that energy delivered by non-thermal electron beams is sufficient to heat the flare loop to temperatures in which it emits soft X-ray closely following the GOES 1-8A light-curve. We also analyze the number of non-thermal electrons, the low energy cut-off, electron spectral indices and the changes of these parameters with time.

Authors: M. Siarkowski, R. Falewicz, and P. Rudawy
Projects: RHESSI

Publication Status: accepted (ApJL)
Last Modified: 2009-10-05 09:41
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Temporal variations of the CaXIX spectra in solar flares  

Robert Falewicz   Submitted: 2009-09-25 13:10

Standard model of solar flares comprises a bulk expansion and rise of abruptly heated plasma (the chromospheric evaporation). Emission from plasma ascending along loops rooted on the visible solar disk should be often dominated, at least temporally, by a blue-shifted emission. However, there is only a very limited number of published observations of solar flares having spectra in which the blue-shifted component dominates the stationary one. In this work we compare observed X-ray spectra of three solar flares recorded during their impulsive phases and relevant synthetic spectra calculated using one-dimensional hydro-dynamic numerical model of these flares. The main aim of the work was to explain why numerous flares do not show blue-shifted spectra. We synthetised time series of BCS spectra of three solar flares in various moments of their evolution from the beginning of the impulsive phases beyond maxima of the X-ray emissions using 1D numerical model of the solar flares and standard software to calculate BCS synthetic spectra of the flaring plasma. The models of the flares were calculated using observed energy distributions of the non-thermal electron beams injected into the loops, initial values of the main physical parameters of the plasma confined in the loops and geometrical properties loops' estimated using available observational data. The synthesized BCS spectra of the flares were compared with the relevant observed BCS spectra. Taking into account the geometrical dependences of the line-of-sight velocities of the plasma moving along the flaring loop inclined toward the solar surface as well as a distribution of the investigated flares over the solar disk, we conclude that stationary component of the spectrum should be observed almost for all flares during their early phases of evolution. In opposite, the blue-shifted component of the spectrum could be not detected in flares having plasma rising along the flaring loop even with high velocity due to the geometrical dependences only. Our simulations based on realistic heating rates of plasma by non-thermal electrons indicate also that the upper chromosphere is heated by non-thermal electrons a few seconds before beginning of noticeable high-velocity bulk motions, and before this time plasma emits stationary component of the spectrum only. After the start of the up-flow motion, the blue-shifted component dominate temporally the synthetic spectra of the investigated flares at their early phases.

Authors: R. Falewicz, P. Rudawy, and M. Siarkowski
Projects: None

Publication Status: accepted (A&A)
Last Modified: 2009-09-28 09:47
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Relationship between non-thermal electron energy spectra and GOES classes  

Robert Falewicz   Submitted: 2009-04-05 12:21

We investigate the influence of the variations of energy spectrum of non-thermal electrons on the resulting GOES classes of solar flares. Twelve observed flares with various soft to hard X-ray emission ratios were modeled using different non-thermal electron energy distributions. Initial values of the flare physical parameters including geometrical properties were estimated using observations. We found that, for a fixed total energy of non-thermal electrons in a flare, the resulting GOES class of the flare can be changed significantly by varying the spectral index and low energy cut-off of the non-thermal electron distribution. Thus, the GOES class of a flare depends not only on the total non-thermal electrons energy but also on the electron beam parameters. For example, we were able to convert a M2.7 class solar flare into a merely C1.4 class one and a B8.1 class event into a C2.6 class flare. The results of our work also suggest that the level of correlation between the cumulative time integral of HXR and SXR fluxes can depend on the considered HXR energy range.

Authors: R. Falewicz, P. Rudawy, and M. Siarkowski
Projects: Yohkoh-HXT

Publication Status: A&A, accepted 2009 Mar 31
Last Modified: 2009-04-05 14:26
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On the causes of Hard X-ray asymmetry in Solar Flares  

Robert Falewicz   Submitted: 2006-09-28 03:28

Aims. Hard X-ray (HXR) emission in the footpoints of flaring loops often indicates an asymmetry, where brighter X-ray flux usually appears in the footpoint with a weaker magnetic field. This is explained by the fact that more electrons can reach the chromosphere in the weaker magnetic field. However, there are numerous exceptions to above rule, where the stronger HXR source is located in the loop?s foot that is rooted in the stronger magnetic field. In our paper we analyse three such exceptional events from the paper by Go et al. (2004). For all three events, we found evidence of the magnetic loops interactions near a brighter footpoint. Such loop interactions suggest that for these flares the energy release is located closer to one end of the loop. Methods. We analysed the process of electron transport in detail in the dense flaring loop as a possible cause of the observed asymmetry. Using the flare parameters derived from observations, we calculated how the collisional energy losses in the loop reduce the number of electrons impacting the chromosphere at the other footpoint. Results. We show that these calculations reproduce the observed flux ratios within the error limits in both footpoints for each of the analysed events. Thus we conclude that the injection of electrons near one of the footpoints, together with enhanced density in the flaring loop, can explain observed HXR footpoints asymmetry.

Authors: Falewicz, R & Siarkowski, M.
Projects: None

Publication Status: A&A (accepted)
Last Modified: 2006-09-28 11:02
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Abstracts by Author
Time variations of observed Hα line profiles and precipitation depths of non-thermal electrons in a solar flare
2D MHD and 1D HD models of a solar flare - a comprehensive comparison of the results
Plasma heating in solar flares and their soft and hard X-ray emissions
Plasma heating in the very early and decay phases of solar flares
Plasma heating in the very early phase of solar flares
Temporal variations of the CaXIX spectra in solar flares
Relationship between non-thermal electron energy spectra and GOES classes
On the causes of Hard X-ray asymmetry in Solar Flares

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