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Electron trapping and acceleration by kinetic Alfvén waves in solar flares  

Ivan Zimovets   Submitted: 2016-08-25 05:56

Theoretical models and spacecraft observations of solar flares highlight the role of wave-particle interaction for non-local electron acceleration. In one scenario, the acceleration of a large electron population up to high energies is due to the transport of electromagnetic energy from the loop-top region down to the footpoints, which is then followed by the energy being released in dense plasma in the lower atmosphere. We consider one particular mechanism of non-linear electron acceleration by kinetic Alfvén waves. Here, waves are generated by plasma flows in the energy release region near the loop top. We estimate the efficiency of this mechanism and the energies of accelerated electrons. We use analytical estimates and test-particle modelling to investigate the effects of electron trapping and acceleration by kinetic Alfvén waves in the inhomogeneous plasma of the solar corona. We demonstrate that, for realistic wave amplitudes, electrons can be accelerated up to 10-1000 keV during their propagation along magnetic field lines. Here the electric field that is parallel to the direction of the background magnetic field is about 10 to 1000 times the amplitude of the Dreicer electric field. The acceleration mechanism strongly depends on electron scattering which is due to collisions that only take place near the loop footpoints. The non-linear wave-particle interaction can play an important role in the generation of relativistic electrons within flare loops. Electron trapping and coherent acceleration by kinetic Alfvén waves represent the energy cascade from large-scale plasma flows that originate at the loop-top region down to the electron scale. The non-diffusive character of the non-linear electron acceleration may be responsible for the fast generation of high-energy particles.

Authors: Artemyev A.V., Zimovets I.V., Rankin R.
Projects: None

Publication Status: A&A (published)
Last Modified: 2016-08-27 22:25
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Spatio-temporal dynamics of sources of hard X-ray pulsations in solar flares  

Ivan Zimovets   Submitted: 2016-08-25 05:48

We present systematic analysis of the spatio-temporal evolution of sources of hard X-ray (HXR) pulsations in solar flares. We concentrate on disk flares whose impulsive phase are accompanied by a series of more than three successive peaks (pulsations) of HXR emission detected in the RHESSI 50-100 keV energy channel with 4-second time cadence. 29 such flares observed from February 2002 to June 2015 with characteristic time differences between successive peaks P=8-270 s are studied. The main observational result of the analysis is that sources of HXR pulsations in all flares are not stationary, they demonstrate apparent movements/displacements in parental active regions from pulsation to pulsation. The flares can be subdivided into two main groups depending on the character of dynamics of HXR sources. The group-1 consists of 16 flares (55%) with the systematic dynamics of the HXR sources from pulsation to pulsation with respect to a magnetic polarity inversion line (MPIL), which has simple extended trace on the photosphere. The group-2 consists of 13 flares (45%) with more chaotic displacements of the HXR sources with respect to an MPIL having more complicated structure, and sometimes several MPILs are presented in parental active regions of such flares. Based on the observations we conclude that the mechanism of the flare HXR pulsations (at least with time differences of the considered range) is related to successive triggering of flare energy release process in different magnetic loops (or bundles of loops) of parental active regions. Group-1 flare regions consist of loops stacked into magnetic arcades extended along MPILs. Group-2 flare regions have more complicated magnetic structures and loops are arranged more chaotically and randomly there. We also found that at least 14 (88%) group-1 flares and 11 (85%) group-2 flares are accompanied by coronal mass ejections (CMEs), i.e. the absolute majority of the flares studied are eruptive events. This gives a strong indication that eruptive processes play important role in generation of HXR pulsations in flares. We suggest that an erupting flux rope can act as a trigger of flare energy release. Its successive interaction with different loops of a parental active region can lead to apparent motion of HXR sources and to a series of HXR pulsations. However, the exact mechanism responsible for the generation of pulsations remains unclear and requires more detailed investigation.

Authors: Kuznetsov S.A., Zimovets I.V., Morgachev A.S., Struminsky A.B.
Projects: RHESSI,SDO-HMI,SoHO-MDI

Publication Status: Solar Physics (accepted)
Last Modified: 2016-08-27 22:26
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Spatially resolved observations of a coronal type II radio burst with multiple lanes  

Ivan Zimovets   Submitted: 2015-03-22 03:44

Relative dynamics of the radio sources of the metric type II burst with three emission lanes and coronal mass ejection (CME) occurred in the lower corona (r≲1.5R⊙) during the SOL2011-02-16T14:19 event is studied. The observational data of the Nançay Radioheliograph (NRH) and the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO) are used. These observations are also supplemented by the data sets obtained with the STEREO-A and -B, RHESSI, and GOES spacecraft, as well as with the ground-based solar radio spectrometers. It is found that the sources of the radio burst were located ahead of the expanding CME and had a complex spatial structure. The first and the second lanes were both emitted from the "magnetic funnel" - a bundle of open magnetic field lines separated the south and north systems of magnetic loops of the active region. Due to the projection effect and limited angular resolution of the NRH it is not possible to determine, whether the spatial locations of the radio sources of the two first emission lanes differed or not. It is argued that the observations support the hypothesis that the radio sources of the first and second lanes could be emitted respectively ahead of and behind a front of the same weak (the Alfvén Mach number MA≈1.1-1.2), fast mode, quasi-parallel piston MHD shock wave. However, the third lane of the burst was definitely emitted from a different place. Its radio sources were situated ahead of the north-west part of the CME propagated through the north system of magnetic loops. This indicates clearly that different emission lanes of the same type II burst can be a result of propagation of different parts of a single CME through regions with different physical conditions (geometries and plasma densities) in the lower corona.

Authors: Zimovets I.V., Sadykov V.M.
Projects: Nançay Radioheliograph,RHESSI,SDO-AIA,STEREO

Publication Status: Adv. Space Res. (published online, in press)
Last Modified: 2015-03-25 08:16
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Non-Thermal ``Burst-on-Tail'' of Long-Duration Solar Event on 26 October 2003  

Ivan Zimovets   Submitted: 2012-08-28 06:51

Observations of a rare long-duration solar event of GOES class X1.2 from 26 October 2003 are presented. This event showed a pronounced burst of hard X-ray and microwave emission, which was extremely delayed (>60 min) with respect to the main impulsive phase and did not have any significant response visible in soft X-ray emission. We refer to this phenomenon as a ``burst-on-tail''. Based on TRACE observations of the growing flare arcade and some simplified estimation, we explain why a reaction of active region plasma to accelerated electrons may change drastically over time. We suggest that, during the ``burst-on-tail'', non-thermal electrons were injected into magnetic loops of larger spatial scale than during the impulsive phase bursts, thus resulting in much smaller values of plasma temperature and emission measure in their coronal volume, and hence little soft X-ray flux. The nature of the long gap between the main impulsive phase and the ``burst-on-tail'' is, however, still an open question.

Authors: I. Zimovets, A. Struminsky
Projects: GOES X-rays ,Other,TRACE

Publication Status: Solar Physics (accepted)
Last Modified: 2012-08-28 12:31
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Spatially resolved observations of a split-band coronal type-II radio burst  

Ivan Zimovets   Submitted: 2012-08-28 02:59

Context. The origin of coronal type-II radio bursts and of their band-splitting are still not fully understood. Aims. To make progress in solving this problem on the basis of one extremely well observed solar eruptive event. Methods. The relative dynamics of multi-thermal eruptive plasmas, observed in detail by the SDO/AIA and of the harmonic type-II burst sources, observed by the NRH at ten frequencies from 445 to 151 MHz, is studied for the partially behind the limb event on 3 November 2010. Special attention is given to the band-splitting of the burst. Analysis is supplemented by investigation of coronal hard X-ray (HXR) sources observed by the RHESSI. Results. It is found that the flare impulsive phase was accompanied by the formation of a double coronal HXR source, whose upper part coincided with the hot (T~10 MK) eruptive plasma blob. The leading edge (LE) of the eruptive plasmas (T~1-2 MK) moved upward from the flare region with the speed of v=900-1400 km s-1. The type II burst source initially appeared just above the LE apex and moved with the same speed and in the same direction. After about 20 s it started to move about twice faster, but still in the same direction. At any given moment the low frequency component (LFC) source of the splitted type-II burst was situated above the high frequency component (HFC) source, which in turn was situated above the LE. It is also found that at a given frequency the HFC source was located slightly closer to the photosphere than the LFC source. Conclusions. The shock wave, which could be responsible for the observed type-II radio burst, was initially driven by the multi-temperature eruptive plasmas, but later transformed to a freely propagating blast shock wave. The most preferable interpretation of the type-II burst splitting is that its LFC was emitted from the upstream region of the shock, whereas the HFC - from the downstream region.

Authors: I. Zimovets, N. Vilmer, A. C.-L. Chian, I. Sharykin, A. Struminsky
Projects: Nançay Radioheliograph,RHESSI,SDO-AIA

Publication Status: A&A (accepted)
Last Modified: 2012-08-28 12:33
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Observations of quasi-periodic solar X-ray emission as a result of MHD oscillations in a system of multiple flare loops  

Ivan Zimovets   Submitted: 2009-10-05 06:34

We investigate the solar flare of 20 October 2002. The flare was accompanied by quasi-periodic pulsations (QPP) of both thermal and nonthermal hard X-ray emissions (HXR) observed by RHESSI in the 3-50 keV energy range. Analysis of the HXR time profiles in different energy channels made with the Lomb periodogram indicates two statistically significant time periods of about 16 and 36 seconds. The 36-second QPP were observed only in the nonthermal HXR emission in the impulsive phase of the flare. The 16-second QPP were more pronounced in the thermal HXR emission and were observed both in the impulsive and in the decay phases of the flare. Imaging analysis of the flare region, the determined time periods of the QPP and the estimated physical parameters of magnetic loops in the flare region allow us to interpret the observations as follows. 1) In the impulsive phase energy was released and electrons were accelerated by successive acts with the average time period of about 36 seconds in different parts of two spatially separated, but interacting loop systems of the flare region. 2) The 36-second periodicity of energy release could be caused by the action of fast MHD oscillations in the loops connecting these flaring sites. 3) During the first explosive acts of energy release the MHD oscillations (most probably the sausage mode) with time period of 16 seconds were excited in one system of the flare loops. 4) These oscillations were maintained by the subsequent explosive acts of energy release in the impulsive phase and were completely damped in the decay phase of the flare.

Authors: Zimovets, I.V., Struminsky, A.B.
Projects: RHESSI

Publication Status: SolPhys(submitted)
Last Modified: 2009-10-05 09:41
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Imaging Observations of Quasi-Periodic Pulsatory Non-Thermal Emission in Two-Ribbon Solar Flares  

Ivan Zimovets   Submitted: 2009-06-20 02:51

Using RHESSI and some auxiliary observations we examine possible connections between the spatial and temporal structure of non-thermal hard X-ray (HXR) emission sources from the two-ribbon flares of 29 May 2003 and 19 January 2005. In each of these events quasi-periodic pulsations (QPP) with time period of 1-3 minutes are evident in both hard X-rays and microwaves. The sources of non-thermal HXR emission are situated mainly at the footpoints of the flare arcade loops observed by TRACE and the SOHO/EIT instrument in the EUV range. At least one of the sources moves systematically during and after the QPP-phase in each flare. The sources move predominantly parallel to the magnetic inversion line during the 29 May flare and along flare ribbons during the QPP-phase of both flares. By contrast, the sources start to show movement perpendicular to the flare ribbons with the velocity comparable to the velocity of along the ribbons movement after the QPP-phase. The sources of each pulse are localized in distinct parts of the ribbon during the QPP-phase. The measured velocity of the sources and the estimated energy release rate do not correlate well with the flux of the HXR emission calculated from these sources. The sources of microwaves and thermal HXRs are situated near the apex of the flare loop arcade and are not stationary either. Almost all of the QPP as well as some pulses of non-thermal HXR emission during the post-QPP-phase reveal the soft-hard-soft spectral behavior indicating separate acts of electron acceleration and injection. In our opinion at least two different flare scenarios based on the Nakariakov et al. (2006, Astron. Astrophys. 452, 343) model and on the idea of current-carrying loop coalescence are suitable for interpreting the observations. However, it is currently not possible to choose the preferred one owing to observational limitations.

Authors: Zimovets, I.V., Struminsky, A.B.
Projects: RHESSI,SoHO-EIT,SoHO-MDI,TRACE

Publication Status: SolPhys(accepted)
Last Modified: 2009-06-20 11:56
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Abstracts by Author
Electron trapping and acceleration by kinetic Alfv?n waves in solar flares
Spatio-temporal dynamics of sources of hard X-ray pulsations in solar flares
Spatially resolved observations of a coronal type II radio burst with multiple lanes
Non-Thermal ``Burst-on-Tail'' of Long-Duration Solar Event on 26 October 2003
Spatially resolved observations of a split-band coronal type-II radio burst
Observations of quasi-periodic solar X-ray emission as a result of MHD oscillations in a system of multiple flare loops
Imaging Observations of Quasi-Periodic Pulsatory Non-Thermal Emission in Two-Ribbon Solar Flares

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