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Recent Advances in Understanding Particle Acceleration Processes in Solar Flares  

Valentina Zharkova   Submitted: 2010-11-29 11:17

We review basic theoretical concepts in particle acceleration, with particular emphasis on processes likely to occur in regions of magnetic reconnection. Several new developments are discussed, including detailed studies of reconnection in three-dimensional magnetic field configurations (e.g., current sheets, collapsing traps, separatrix regions) and stochastic acceleration in a turbulent environment. Fluid, test-particle, and particle-in-cell approaches are used and results compared. While these studies show considerable promise in accounting for the various observational manifestations of solar flares, they are limited by a number of factors, mostly relating to available computational power. Not the least of these issues is the need to explicitly incorporate the electrodynamic feedback of the accelerated particles themselves on the environment in which they are accelerated. A brief prognosis for future advancement is offered.

Authors: Zharkova V.V., K. Arzner, A. O. Benz, P.,Browning, C. Dauphin, A. G. Emslie, L., Fletcher, E. P. Kontar, G. Mann, M. Onofri, V. Petrosian, R. Turkmani, N. Vilmer, and L.Vlahos
Projects: RHESSI

Publication Status: Special Issue of Space Science Review of the RHESSI-inspired monograph, 'High-Energy Aspects of Solar Flares', accepted.
Last Modified: 2010-12-01 10:54
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MANIFESTATIONS OF ENERGETIC ELECTRONS WITH ANISOTROPIC DISTRIBUTIONS IN SOLAR FLARES. II. GYROSYNCHROTRON MICROWAVE EMISSION  

Valentina Zharkova   Submitted: 2010-10-15 08:19

We investigate the spectra and polarization of the gyrosynchrotron microwave (MW) emission generated by anisotropic electron beams in the solar corona. The electron distributions are selected from the steady propagation/precipitation model of beam electrons obtained from the time-dependent solutions of the Fokker-Planck equation taking into account particle anisotropic precipitation into a converging magnetic tube while losing energy in collisions and Ohmic losses induced by a self-induced electric field. We separate the effects of converging magnetic field from those of self-induced electric field for beams with different initial energy fluxes and spectral indices. The effect of returning electrons of the beam is negligible for the beams with relatively weak energy fluxes (F lsim 1010 erg cm"2 s"1), while it becomes very important for the electron beams with F ≳ 1012 erg cm"2 s"1. Electric field-induced losses lead to the increase of MW emission intensity, especially at larger viewing angles (θ ≳ 140?, looking at the loop from a side). The polarization remains typical for the beam-like distributions. The combined effect of the self-induced electric field and converging magnetic field reveals a noticeable (up to a factor of 10) increase of the emission intensity (for the viewing angles θ sime 140?-150?) in comparison with the models considering only collision factor, especially in the deeper precipitation layers (near the loop footpoints). Thus, considering the self-induced electric field is especially important for the resulting MW emission intensity, spectra shape, and polarization that can provide much closer correlation of simulations with observations in solar flares.

Authors: Kuznetsov A.A. and Zharkova V.V.
Projects: None

Publication Status: Astrophysical Journal, v 722, 1577-1588, 2010
Last Modified: 2010-10-15 10:23
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The effect of energetic particle beams on the chromospheric emission of the 25th July 2004 flare  

Valentina Zharkova   Submitted: 2010-10-15 08:16

In this paper a role of particle beams for generation of a close spatial and temporal correlation between hard H-ray (HXR) energy and Hα line emission bursts in the solar flare of 25 July 2004 is discussed. The light curves in HXR emission and Hα line revealed the pre-flare and main flare events correlated with the variations of a line-of-sight (LOS) magnetic flux. HXR emission shows the three main bursts during the main flare phase which are accompanied by Hα intensity increases. The latter appear in 10 locations (kernels) at different times in succession; the kernel locations are associated with strong magnetic sources situated on the opposite sides from the magnetic neutral line (MNL). The appearance of Hα kernels in the pre-flare event and those at the start of the main event are correlated very closely (within a few seconds) with the HXR emission observed by RHESSI (HXR+Hα kernels), while the appearance of some other ones, not associated with HXR emission, was delayed by ten of seconds. Electron beam parameters were derived from the RHESSI spectra in where the observed HXR photon flux was corrected for the self-induced electric field effect. Possible implications of this electric field on production of the transient magnetic fields observed during the flare are discussed. The Hα emission observed during the first burst was simulated in hydrodynamic atmospheres for 5 level plus continuum hydrogen atom with the full non-LTE approach combining radiative, thermal and non-thermal excitation and ionization by an electron beam with the derived parameters. The simulated temporal profiles of Hα emission produced by non-thermal hydrogen excitation revealed a very good fit to the observed ones in the HXR+Hα kernels and 10-20 s delays in the other kernels. This allows to speculate that Hα emission in these kernels is caused mainly by electrons in the HXR+Hα kernels and mainly by protons in the other ones. Key words: Sun: flares; Sun: particle beams; Sun:

Authors: Zharkova V.V., Kashapova L.K., Chornogor S.N. and Andrienko O,V.
Projects: RHESSI

Publication Status: Monthly Notices of the Royal Astronomical Society, in press
Last Modified: 2010-10-15 10:23
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Diagnostics of energetic electrons with anisotropic distributions in solar flares.I. Hard X-rays bremsstrahlung emission  

Valentina Zharkova   Submitted: 2010-02-16 08:20

Aims.The paper aims are to simulate steady-state distributions of electrons beams precipitating in collisional and Ohmic losses with pitch angle anisotropy into a flaring atmosphere with converging magnetic field and to apply these to the interpretation of HXR photon spectra, directivity and polarization observed for different photon energies and flare positions on the solar disk. Methods.Summary approximation method is applied to a time-dependent Fokker-Planck equation by splitting the temporal derivative equally between the derivatives in depth, energy and pitch angles and finding the solutions in forward and backward directions for each variable. Results. For softer beams, there is a noticeable flattening of the photon spectra at lower energies caused by the self-induced electric field that increases for larger viewing angles. For the models with an electric field, the HXR emission with lower energies (30 keV) becomes directed mainly upwards at upper atmospheric levels owing to the increased number of particles moving upwards, while in deeper layers it again becomes directed downwards. The polarization maximum shifts to higher energies with every precipitation depth approaching 25 keV for the models with pure collisions and 100 keV for the models with return currents. At deeper layers, the polarization decreases because of the isotropization of electrons by collisions. The maximum polarization is observed at the viewing angle of 90◦, becoming shifted to lower angles for softer beams. The integrated polarization and directivity shows a dependence on a magnetic field convergence for harder beams, while for softer beams the directivity is strongly affected by the self-induced electric field changing from a downward motion to an upward one at upper atmospheric depths. Conclusions. The proposed precipitation model for an electron beam with wider pitch angle dispersion of 0.2 taking into account collisional and Ohmic losses allowed us to fit the double power law HXR photon spectra with a spectrum flattening at lower energies observed in the flares of 20 and 23 July 2002. The observed directivity of HXR photons of 20 keV derived for a large number of flares located from the disk center to limb is also reproduced well by the theoretical directivity calculated for an electron beam with a very narrow pitch angle dispersion of 0.02. The simulated polarization of this narrowly-directed electron beam fits up to 90% of all the available polarimetric observations carried out at various locations across the solar disk.

Authors: Zharkova V.V., Kuznetsov A.A. and Siversky T.V
Projects: RHESSI

Publication Status: published in Astronomy and Astrophysics, v477/1
Last Modified: 2010-02-16 10:19
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Active latitudes for sunspot occurrences in the Solar Cycle 23  

Valentina Zharkova   Submitted: 2010-02-16 08:12

In this paper we carry out the principal component analysis of the sunspot and background magnetic fields in the cycle 23. There two main latitude pe- riodical components of the opposite polarities derived in sunspot magnetic flux which behave rather off-phase in the Northern and Southern hemispheres. These waves are shown to be modulated by the two primary waves of the back- ground magnetic field which have the opposite polarities and travel slightly off-phase in each hemisphere. In the cycle 23 the maximums in sunspot components correspond to the minimums in the BMF components. This points out to a presence of the two waves in BMF travelling in each hemisphere (possibly, in the opposite directions or having the opposite polarities) which affect the sunspot appearances in these hemispheres.

Authors: Zharkova V.V. and Zharkov S.I.
Projects: None

Publication Status: ADVANCES IN GEOSCIENCES (ADGEO), in press, World Scientific Publishing, 2010
Last Modified: 2010-02-16 10:19
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Diagnostics of energetic electrons with anisotropic distributions in solar flares I. Hard X-rays bremsstrahlung emission  

Valentina Zharkova   Submitted: 2009-11-12 12:39

Aims. The paper aims are to simulate steady-state distributions of electrons beams precipitating in collisional and Ohmic losses with pitch angle anisotropy into a flaring atmosphere with converging magnetic field and to apply these to the interpretation of HXR photon spectra, directivity and polarization observed for different photon energies and positions of flares. Methods. Summary approximation method is applied to time-dependent Fokker-Planck equation splitting a temporal derivative equally between the derivatives in depth, energy and pitch angles and finding the solutions in forward and backward directions for each variable. Results. For softer beams there is a noticeable flattening of the photon spectra at lower energies caused by the self- induced electric field which increases for larger viewing angles. For the models with electric field the HXR emission with lower energies (30 keV) becomes directed mainly upwards at upper atmospheric levels owing to the increased number of particles moving upwards, while at the deeper layers it becomes again directed downwards. The polarization maximum shifts to higher energies with every precipitation depth approaching 25 keV for the models with pure collisions and 100 keV for the models with return currents. At deeper layers the polarization decreases because of the izotropization of electrons by collisions. The maximum polarization is observed at the viewing angle of 90◦ becoming shifted to lower angles for softer beams. The integrated polarization and directivity shows dependence on a magnetic field convergence for harder beams while for softer beam the directivity is strongly affected by the self-induced electric field changing from downward to upward one at upper atmospheric depths. Conclusions. The proposed precipitation model for an electron beam with wider pitch angle dispersion of 0.2 taking into account collisional and Ohmic losses allowed us to fit the double power law HXR photon spectra with a spectrum flattening at lower energies observed in the flares of 20 and 23 July 2002. The observed directivity of HXR photons of 20 keV derived for a large number of flares located from the disk center to limb is also well reproduced by the theoretical directivity calculated for an electron beam with a very narrow pitch angle dispersion of 0.02. The simulated polarization of such the narrowly-directed electron beam fits up to 90% of all the available polarimetric observations carried out at various locations on the solar disk.

Authors: Zharkova V.V., Kuznetsov A.A. and Siversky T.V.
Projects: RHESSI

Publication Status: Astronomy and Astrophysics, accepted
Last Modified: 2009-11-12 17:04
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Stationary and impulsive injection of electron beams in converging magnetic field  

Valentina Zharkova   Submitted: 2009-06-22 08:37

Aims. In this work we study the evolution of an electron beam injected into the solar atmosphere. In particular we focus on the heating produced by the precipitating electrons. The influence of converging magnetic field with different spatial profiles is compared. Time evolution of short electron impulses with different initial distributions is studied. Methods. The time dependent Fokker-Planck equation for the electron distribution in the beam was numerically solved using the summary approximation method. Results.Previously studied approximations of the magnetic field profile are found to be ineffective. However, the magnetic field models proposed here can influence precipitating electrons and, thus, reduce the heating produced by them. It was also found that short electron impulses injected into the atmosphere can produce hard X-ray bursts with the timescales observed earlier during solar flares.

Authors: Siversky, T.V. and Zharkova, V.V.
Projects: RHESSI

Publication Status: Astronomy and Astrophysics, in press
Last Modified: 2009-06-22 09:04
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Particle acceleration in a reconnecting current sheet: PIC simulation.  

Valentina Zharkova   Submitted: 2009-03-02 12:20

The acceleration of protons and electrons in a reconnecting current sheet (RCS) is simulated with a particle-in-cell (PIC) 2D3V code for the proton-to-electron mass ratio of 100. The electro-magnetic configuration forming the RCS incorporates all three components of the magnetic field (including the guiding field)and a drifted electric field. PIC simulations reveal that there is a polarisation electric field that appears during acceleration owing to a separation of electrons from protons towards the midplane of the RCS. If the plasma density is low, the polarisation field is weak and the particle trajectories in the PIC simulations are similar to those in the test particle (TP) approach. For the higher plasma density the polarisation field is stronger and it affects the trajectories of protons by increasing their orbits during acceleration. This field also leads to a less asymmetrical abundances of ejected protons towards the midplane in comparison with the TP approach. For a given magnetic topology electrons in PIC simulations are ejected to the same semispace as protons, contrary to the TP results. This happens because the polarisation field extends far beyond the thickness of a current sheet. This field decelerates the electrons, which are initially ejected into the semispace opposite to the protons, returns them back to the RCS, and, eventually, leads to the electron ejection into the same semispace as protons. Energy distribution of the ejected electrons is rather wide with a single peak, contrary to the two-peak narrow-energy distribution obtained in the TP approach. In the case of a strong guiding field, the mean energy of the ejected electrons is found to be smaller than it is predicted analytically and by the TP simulations. The beam of accelerated electrons is also found to generate turbulent electric field in a form of Langmuir waves.

Authors: T.V.Siversky and V.V.Zharkova
Projects: None

Publication Status: Journal of Plasma Physics - accepted
Last Modified: 2009-03-02 13:17
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The effect of magnetic topology on particle acceleration in 3D current sheet. I. Test particle approach.  

Valentina Zharkova   Submitted: 2008-10-13 13:50

Electron and proton acceleration by a drifted uper-Dreicer electric field is investigated in a strongly compressed non-neutral reconnecting current sheet (NRCS). The guiding field is assumed to be constant within an RCSand parallel to the direction of drifted electric field. The other two magnetic field components, transverse and tangential, are considered to vary exponentially and linearly with distances from the X null-point. The proton and electron energy spectra are calcu- lated numerically in a model RCS with different magnetic field topologies by solving a motion equation in the test particle approach with some test with Particle-in-cell (PIC) approach. Three mechanisms of particle acceleration inside an RCS are con- sidered: a drifted electric field caused by the plasma inflows formed during a mag- netic reconnection process, a polarization electric field induced by the accelerated protons and electrons extracted from the PIC simulations and turbulent electric filed induced by some instabilities generated by accelerated particles. Electron and proton densities, energy spectra inside an RCS and at ejection are found to be strongly affected by the magnetic field topology: for stronger magnetic fields the spectra are softer having a small higher energy cutoff while for weaker magnetic field the spectra are harder with much bigger upper cutoff energies. Depending on the magnetic component ratios and drifted electric field magnitude, particles are found ejected either as quasi-thermal flows with very high temperatures or as fo- cused power law beams. A polarization field is found to reduce the acceleration time inside an RCS and to increase the energy by a few orders of magnitude compared to the energy gained by particles at acceleration by a pure drifted electric field. The turbulent electric field induced at ejection by the two beam instabilities of the same kind particles leads to a few-folded increase of particle number with higher energies leading to flattening of their energy spectra.

Authors: Zharkova, V.V. and Agapitov, O.M.
Projects: None

Publication Status: Journal of Plasma Physics, accepted
Last Modified: 2008-11-14 18:19
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Subject will be restored when possible  

Valentina Zharkova   Submitted: 2008-09-10 16:11

Electron and proton acceleration by a super-Driecer electric field is investigated in a reconnecting current sheet (RCS) with guiding field. The particle trajectories and energy spectra are calculated numerically for different magnetic field topologies by solving motion equation in the test particle and PIC approaches. Three mechanisms of particle acceleration inside an RCS are considered: a drifted electric field caused by the plasma inflows formed during a magnetic reconnection process, polarization electric field induced by the accelerated protons and electrons extracted from PIC simulations and turbulent electric field induced by accelerated particles. Electron and proton densities, energy spectra inside an RCS and at ejection are found to be strongly affected by a magnetic field topology and induced electric fields leading to a strong increases of the energy of accelerated particles in addition to the energy gained by the drifted electric field.

Authors: V. V. Zharkova, T. Siverskyi and O. I. Agapitov?
Projects: None

Publication Status: AIP Proceedsings, in press
Last Modified: 2008-09-11 07:37
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Subject will be restored when possible  

Valentina Zharkova   Submitted: 2008-09-10 16:11

Electron and proton acceleration by a super-Driecer electric field is investigated in a reconnecting current sheet (RCS) with guiding field. The particle trajectories and energy spectra are calculated numerically for different magnetic field topologies by solving motion equation in the test particle and PIC approaches. Three mechanisms of particle acceleration inside an RCS are considered: a drifted electric field caused by the plasma inflows formed during a magnetic reconnection process, polarization electric field induced by the accelerated protons and electrons extracted from PIC simulations and turbulent electric field induced by accelerated particles. Electron and proton densities, energy spectra inside an RCS and at ejection are found to be strongly affected by a magnetic field topology and induced electric fields leading to a strong increases of the energy of accelerated particles in addition to the energy gained by the drifted electric field.

Authors: V.V.Zharkova, T. Siverskyi and O.I. Agapitov
Projects:

Publication Status: AIP Proceedsings, 7th IGPP/SSL Conference, in press
Last Modified: 2008-09-12 00:57
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Stages of particle acceleration in a 3Dreconnecting current sheet  

Valentina Zharkova   Submitted: 2008-09-10 16:11

Electron and proton acceleration by a super-Driecer electric field is investigated in a reconnecting current sheet (RCS) with guiding field. The particle trajectories and energy spectra are calculated numerically for different magnetic field topologies by solving motion equation in the test particle and PIC approaches. Three mechanisms of particle acceleration inside an RCS are considered: a drifted electric field caused by the plasma inflows formed during a magnetic reconnection process, polarization electric field induced by the accelerated protons and electrons extracted from PIC simulations and turbulent electric field induced by accelerated particles. Electron and proton densities, energy spectra inside an RCS and at ejection are found to be strongly affected by a magnetic field topology and induced electric fields leading to a strong increases of the energy of accelerated particles in addition to the energy gained by the drifted electric field.

Authors: V.V.Zharkova, T. Siverskyi and O.I. Agapitov
Projects: None

Publication Status: AIP Proceedsings, 7th IGPP/SSL Conference, in press
Last Modified: 2008-09-12 00:58
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Subject will be restored when possible  

Valentina Zharkova   Submitted: 2008-02-17 12:44

The mechanisms responsible for observational features associated with solar quakes are explored for different classes of flares and the role of high energy particles in both the non-thermal excitation and ionization and Coulomb and Ohmic heating of the ambient plasma is compared for different beam parameters. On the one hand, only hard electron beams with high energy fluxes are found producing excessive non-thermal hydrogen ionization by 4-5 orders of magnitude higher than in the quiet atmosphere. This excessive ionization leads to the white light flares associated with seismic emission appearing simultaneously with hard X-ray emission and, as a result, to a strong increase on Ni line emission observed as the seismic emission with the holographic technique. On the other hand, the ambient plasma hydrodynamic response to heating by such beam electrons forms the hydrodynamic shocks just below the transition region, in the upper chromosphere and they travel with a supersonic velocity for up to 5 minutes before can reach the photosphere. These hydrodynamic responses caused by the beam electrons are maximised at the lower chromosphere for moderate electron beams because of their smaller Ohmic losses in the upper atmosphere comparing to those for higher energy electron beams whose bulk energy being deposited in the transition region. These shocks caused by electron beams can explain the observations of seismic emission by TD diagrams and holographic methods in M and C-class flares while to account for the quakes in X-class flares high energy quasi-thermal protons or power law proton beams either sole or blended with electron ones are the most likely agents. Non-thermal ionization and excitation of lower atmospheric levels during the beam injection followed by thermo-conductive heating after the beam is stopped can contribute to the seismic signatures observed with the holographic technique caused by strong non-thermal ionization and back-warming heating occurring in the shock while it loses its energy in the optically thick line and continuous emission.

Authors: V.V.Zharkova
Projects: RHESSI

Publication Status: Solar Physics, accepted
Last Modified: 2008-02-18 10:04
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Subject will be restored when possible  

Valentina Zharkova   Submitted: 2008-02-17 12:42

The mechanisms responsible for observational features associated with solar quakes are explored for different classes of flares and the role of high energy particles in both the non-thermal excitation and ionization and Coulomb and Ohmic heating of the ambient plasma is compared for different beam parameters. On the one hand, only hard electron beams with high energy fluxes are found producing extensive non-thermal hydrogen ionization by 4 orders of magnitude higher than in the quiet atmosphere. This excessive ionization leads to the white light flares associated with seismic emission appearing simultaneously with hard X-ray emission and, as a result, to a strong increase on Ni line emission observed as the seismic emission with the holographic technique. On the other hand, the ambient plasma hydrodynamic response to heating by such beam electrons forms the hydrodynamic shocks just below the transition region, in the upper chromosphere and they travel with a supersonic velocity for up to 5 minutes before can reach the photosphere. These hydrodynamic responses caused by the beam electrons are maximised at the lower chromosphere for moderate electron beams because of their smaller Ohmic losses in the upper atmosphere comparing to those for higher energy electron beams whose bulk energy being deposited in the transition region. These shocks caused by electron beams can explain the observations of seismic emission by TD diagrams and holographic methods in M and C-class flares while to account for the quakes in X-class flares high energy quasi-thermal protons or power law proton beams either sole or blended with electron ones are the most likely agents. Non-thermal ionization and excitation of lower atmospheric levels during the beam injection followed by thermo-conductive heating after the beam is stopped can contribute to the seismic signatures observed with the holographic technique caused by strong non-thermal ionization and back-warming heating occurring in the shock while it loses its energy in the optically thick line and continuous emission.

Authors: V.V.Zharkova
Projects: RHESSI

Publication Status: Solar Physics, accepted
Last Modified: 2008-02-17 12:42
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Abstracts by Author
Recent Advances in Understanding Particle Acceleration Processes in Solar Flares
MANIFESTATIONS OF ENERGETIC ELECTRONS WITH ANISOTROPIC DISTRIBUTIONS IN SOLAR FLARES. II. GYROSYNCHROTRON MICROWAVE EMISSION
The effect of energetic particle beams on the chromospheric emission of the 25th July 2004 flare
Diagnostics of energetic electrons with anisotropic distributions in solar flares.I. Hard X-rays bremsstrahlung emission
Active latitudes for sunspot occurrences in the Solar Cycle 23
Diagnostics of energetic electrons with anisotropic distributions in solar flares I. Hard X-rays bremsstrahlung emission
Stationary and impulsive injection of electron beams in converging magnetic field
Particle acceleration in a reconnecting current sheet: PIC simulation.
The effect of magnetic topology on particle acceleration in 3D current sheet. I. Test particle approach.
Subject will be restored when possible
Subject will be restored when possible
Stages of particle acceleration in a 3D reconnecting current sheet
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