Plasma turbulence generated in 3D current sheet with magnetic islands |
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Valentina Zharkova Submitted: 2021-10-02 04:26
In this paper we aim to investigate the kinetic turbulence in a reconnecting current sheet (RCS) with X- and O-nullpoints and to explore its link to the features of accelerated particles. We carry out simulations of magnetic reconnection in a thin current sheet with 3D magnetic field topology affected by tearing instability until the formation of two large magnetic islands using particle-in-cell (PIC) approach. The model utilises a strong guiding field that leads to separation of the particles of opposite charges, generation of a strong polarisation electric field across the RCS and suppression of kink instability in the 'out-of-plane' direction. The accelerated particles of the same charge entering an RCS from the opposite edges are shown accelerated to different energies forming the `bump-in-tail' velocity distributions that, in turn, can generates plasma turbulence in different locations. The turbulence-generated waves produced by either electron or proton beams can be identified from the energy spectra of electromagnetic field fluctuations in the phase and frequency domains. From the phase space analysis we gather that the kinetic turbulence may be generated by accelerated particle beams, which are later found to evolve into a phase-space hole indicating the beam breakage. This happens at some distance from the particle entrance into an RCS, e.g. about 7di (ion inertial depth) for the electron beam and 12di for the proton beam. In a wavenumber space the spectral index of the power spectrum of the turbulent magnetic field near the ion inertial length is found to be -2.7 that is consistent with other estimations. The collective turbulence power spectra are consistent with the high-frequency fluctuations of perpendicular electric field, or upper hybrid waves, to occur in a vicinity of X-nullpoints, where the Langmuir (LW) can be generated by accelerated electrons with high growth rates, while further from X-nullponts or on the edges of magnetic islands, where electrons become ejected and start moving across the magnetic field lines, Bernstein waves can be generated. The frequency spectra of high and low-frequency waves are explored in the kinetic turbulence in parallel and perpendicular directions to the local magnetic field showing noticeable lower hybrid turbulence occurring between the electron's gyro- and plasma frequencies seen also in the wavelet spectra. Fluctuation of the perpendicular electric field component of turbulence can be consistent with the oblique whistler waves generated on the ambient density fluctuations by intense electron beams. This study brings attention to a key role of particle acceleration in generation kinetic turbulence inside current sheets.
Authors: Zharkova V. and Xia, Q,
Projects: None
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Publication Status: accepted to Frontiers in Space Physics
Last Modified: 2021-10-03 18:01
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Sunquake with a second-bounce, other sunquakes and emission associated with X9.3 flare of 6 September 2017. II. Proposed interpretation |
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Valentina Zharkova Submitted: 2020-05-27 04:09
In this paper we present the interpretation of the observations of the flare from 6 September 2017 reported in Paper 1. These include gamma-ray (GR), hard X-ray (HXR), soft X-rays (SXR), Lyα line, extreme ultraviolet (EUV), Hα, and white light (WL) emission, which were recorded during the two flaring events 1 (FE1) and 2 (FE2) that occurred at 11:55:37 UT (FE1) and 12:06:40 UT (FE2). Paper 1 also reported the first detection of the sunquake with first and second bounces of seismic waves combined with four other sunquakes in different locations supported with the observations of HXR, GR, EUV, Hα, and WL emission with strongly varying spatial resolution and temporal coverage. In the current Paper 2, we propose some likely scenarios for heating of flaring atmospheres in the footpoints with sunquakes which were supported with EUV and Hα emission. We used a range of parameters derived from the HXR, EUV, and Hα line observations to generate hydrodynamic models, which can account for the blueshifts derived from the EUV emission and the redshifts observed with the EUV Imaging Spectrometer (EIS) in the He II line and by the CRisp Imaging Spectro-Polarimeter (CRISP) in the Swedish Solar Telescope (SST) in Hα line emission. The parameters of hydrodynamic shocks produced by different beams in flaring atmospheres were used as the initial conditions for another type of hydrodynamic models that were developed for acoustic wave propagation in the solar interior. These models simulate the sets of acoustic waves produced in the interior by the hydrodynamic shocks from atmospheres above deposited in different footpoints of magnetic loops. The Hα line profiles with large redshifts in three kernels (two in FE1 and one in FE2) were interpreted with the full non-local thermodynamic equilibrium (NLTE) radiative simulations in all optically thick transitions (Lyman lines and continuum Hα, Hβ, and Pα) applied for flaring atmospheres with fast downward motions while considering thermal and non-thermal excitation and ionisation of hydrogen atoms by energetic power-law electron beams. The observed Hα line profiles in three kernels were fit with the simulate blue wing emission of the Hα line profiles shifted significantly (by 4-6 Å) towards the line red wings, because of strong downward motions with velocities about 300 km s-1 by the shocks generated in flaring atmospheres by powerful beams. The flaring atmosphere associated with the largest sunquake (seismic source 2 in FE1) is found consistent with being induced by a strong hydrodynamic shock produced by a mixed beam deposited at an angle of -30◦ from the local vertical. We explain the occurrence of a second bounce in the largest sunquake by a stronger momentum delivered by the shock generated in the flaring atmosphere by a mixed beam and deeper depths of the interior where this shock was deposited. Indeed, the shock with mixed beam parameters is found deposited deeply into the interior beneath the flaring atmosphere under the angle to the local vertical that would allow the acoustic waves generated in the direction closer to the surface to conserve enough energy for the second bounces from the interior layers and from the photosphere. The wave characteristics of seismic sources 1 and 3 (in FE1) were consistent with those produced by the shocks generated by similar mixed beams deposited at the angles -(0 - 10)◦ (seismic source 1) and +30◦ (seismic source 3) to the local vertical. The differences of seismic signatures produced in the flares of 6 September 2011 and 2017 are also discussed.
Authors: Valentina Zharkova, Sergei Zharkov, Malcolm Druett, Sarah Matthews, and Satoshi Inoue
Projects: None
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Publication Status: Published in A&A
Last Modified: 2020-05-27 13:13
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Sunquake with a second bounce, other sunquakes, and emission associated with the X9.3 flare of 6 September 2017. I. Observations |
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Valentina Zharkova Submitted: 2020-05-12 14:26
The 6 September 2017 X9.3 solar flare produced very unique observations of magnetic field transients and a few
seismic responses, or sunquakes, detected by the Helioseismic and Magnetic Imager (HMI) instrument aboard Solar Dynamic
Observatory (SDO) spacecraft, including the strongest sunquake ever reported. This flare was one of a few flares occurring
within a few days or hours in the same active region. Despite numerous reports of the fast variations of magnetic field, and
seismic and white light emission, no attempts were made to interpret the flare features using multi-wavelength observations.
In this study, we attempt to produce the summary of available observations of the most powerful flare of the 6 September
2017 obtained using instruments with different spatial resolutions (Paper 1) and to provide possible interpretation of the flaring
events, which occurred in the locations of some seismic sources (paper 2). We employed non-linear force-free field (NLFFF)
extrapolations followed by magnetohydrodynamic simulations in order to identify the presence of several magnetic flux ropes
prior to the initiation of this X9.3 flare. Sunquakes were observed using the directional holography and time–distance diagram
detection techniques. The high-resolution method to detect the Hα line kernels in the CRISP instrument at the diffraction level
limit was also applied. We explore the available gamma-ray (GR), hard X-ray (HXR), Lyman-α, and extreme ultra-violet (EUV)
emission for this flare comprising two flaring events observed by space- and ground-based instruments with different spatial
resolutions. For each flaring event we detect a few seismic sources, or sunquakes, using Dopplergrams from the HMI/SDO
instrument coinciding with the kernels of Hα line emission with strong redshifts and white light sources. The properties of
sunquakes were explored simultaneously with the observations of HXR (with KONUS/WIND and the Reuven Ramaty High
Energy Solar Spectroscopic Imager (RHESSI) payload), EUV (with the Atmospheric Imaging Assembly (AIA/SDO and the
EUV Imaging Spectrometer (EIS) aboard Hinode payload), Hα line emission (with the CRisp Imaging Spectro-Polarimeter
(CRISP) in the Swedish Solar Telescope (SST)), and white light emission (with HMI/SDO). The locations of sunquake and
Hα kernels are associated with the footpoints of magnetic flux ropes formed immediately before the X9.3 flare onset. For the
first time we present the detection of the largest sunquake ever recorded with the first and second bounces of acoustic waves
generated in the solar interior, the ripples of which appear at a short distance of 5-8 Mm from the initial flare location. Four
other sunquakes were also detected, one of which is likely to have occurred 10 minutes later in the same location as the largest
sunquake. Possible parameters of flaring atmospheres in the locations with sunquakes are discussed using available temporal
and spatial coverage of hard X-ray, gamma-ray, EUV, hydrogen Hα-line, and white light emission in preparation for their use
in an interpretation to be given in Paper 2.
Authors: S. Zharkov, S. Matthews, V. Zharkova, M. Druett, S. Inoue, I. E. Dammasch, C. Macrae
Projects:
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Publication Status: AStronomy and Astrophysics, in press
Last Modified: 2020-05-14 08:57
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Particle acceleration and transport during 3D CME eruptions |
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Valentina Zharkova Submitted: 2020-04-26 03:23
We calculate particle acceleration during corona mass ejection (CME) eruptions using combined magnetohydrody- namic (MHD) and test-particle models. The 2.5D/3D CMEs are generated via the breakout mechanism. In this scenario a reconnection at the “breakout” current sheet (CS) above the flux rope initiates the CME eruption by destabilizing a quasi-static force balance. Reconnection at the flare CS below the erupting flux rope drives the fast acceleration of the CME, which forms flare loops below and produces the energetic particles observed in flares. For test-particle simulations, two times are selected during the impulsive and decay phases of the eruption. Particles are revealed to be accelerated more efficiently in the flare CS rather than in the breakout CS even in the presence of large magnetic islands. Particles are first accelerated in the CSs (with or without magnetic islands) by the reconnection electric field mainly through particle curvature drift. We find, as expected, that accelerated particles precipitate into the chromosphere, or become trapped in the loop top by magnetic mirrors, or escape to interplanetary space along open field lines. Some trapped particles are reaccelerated, either via reinjection to the flare CS or through a local Betatron-type acceleration associated with compression of the magnetic field. The energetic particles produce rela- tively hard energy spectra during the impulsive phase. During the gradual phase, the relaxation of magnetic field shear reduces the guiding field in the flare CS, which leads to a decrease in particle energization efficiency. Important implications of our results for observations of particle acceleration in the solar coronal jets are also discussed.
Authors: Qian Xia, Joel Dahlin, Valentina Zharkova and Spiro Antiochos
Projects: None
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Publication Status: Astrophysica Journal, in press
Last Modified: 2020-04-27 10:45
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Counter-streaming strahls and heat flux dropouts as possible signatures of local particle acceleration in the solar wind |
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Valentina Zharkova Submitted: 2020-03-20 04:58
Suprathermal electrons with energies of ~70eV and above are observed at 1 AU as dispersionless halo electrons and magnetic field-aligned beams of strahls. For a long time, it has been thought that the both populations originate only from the solar corona, and the only active process impacting their properties in the solar wind is scattering. This view has consequently impacted interpretation of typical patterns of pitch-angle distributions (PADs) of suprathermal electrons. Meanwhile, recent observational studies supported by numerical simulations have shown that there is an unaccounted population of electrons accelerated to suprathermal energies at reconnecting current sheets (RCSs) and 3D dynamical plasmoids (or 2D magnetic islands (MIs)) directly in the heliosphere. We present multi-spacecraft observations of counterstreaming strahls and heat-flux dropouts in PADs within a region filled with plasmoids and RCSs unaffected by interplanetary shocks, comparing observed PAD features with those predicted by particle-in-cell simulations. We show typical PAD patterns determined by local acceleration of thermal-core electrons up to hundreds eV. Resulting PAD views depend on properties and topology of particular RCSs, MIs, and plasma/magnetic field parameters. Our study suggests that solar-wind-borne suprathermal electrons co-exist with those of solar origin. Therefore, some of heat flux dropout and bi- directional strahl events can be explained by local dynamical processes involving magnetic reconnection. Possible implications of the results for the interpretation of the actively-debated strahl/halo relative density decrease with heliocentric distance and puzzling features of suprathermal electrons observed at crossings of the heliospheric current sheet and cometary comas are also discussed.
Authors: O. Khabarova, V. Zharkova, Q. Xia, and O. E. Malandraki
Projects: Wind
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Publication Status: Astrophysical Journal Letters, in press
Last Modified: 2020-04-04 11:49
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Particle acceleration in coalescent and squashed magnetic islands II. Particle-In-cell approach |
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Valentina Zharkova Submitted: 2020-01-27 09:04
Aims. Particles are known to have efficient acceleration in reconnecting current sheets with multiple magnetic islands,
formed during a reconnection process. Using test particle approach, the recent investigation of particle dynamics in
3D magnetic islands, or current sheets with multiple X- and O-null points revealed that the particle energy gains are
higher in squashed magnetic islands than in coalescent ones. However, this approach did not consider the ambient
plasma feedback to the presence of accelerated particles, which aects their distributions within the acceleration region.
Methods. In the current paper, we use the particle-in-cell (PIC) approach to investigate further particle acceleration
in 3D Harris-type reconnecting current sheets with coalescent (merging) and squashed (contracting) magnetic islands
with different magnetic field topologies, ambient densities ranging 108 - 1012 m^3, proton-to-electron mass ratios, and
island aspect ratios.
Results. In current sheets with single or multiple X-nullpoints, accelerated particles of opposite charges are separated
and ejected into the opposite semiplanes from the current sheet midplane, generating a strong polarisation electric field
across a current sheet. Particles of the same charge form two populations: transit and bounced particles, with very
different energy and asymmetric pitch-angle distributions, which can be distinguished from observations. In some cases
the difference in energy gains by transit and bounced particles leads to turbulence generated by Buneman instability. In
magnetic island topology, the different reconnection electric fields in squashed and coalescent islands impose different
particle drift motions. This makes particle acceleration more efficient in squashed magnetic islands than in coalescent
ones. The spectral indices of electron energy spectra are 4.2 for coalescent and 4.0 for squashed islands, which
are lower than reported from the test-particle approach. The particles accelerated in magnetic islands are found trapped
in the midplane of squashed islands, and shifted as clouds towards the X-nullpoints in coalescent ones.
Conclusions. In reconnecting current sheets with multiple X- and O-nullpoints particles are found accelerated on a
much shorter spatial scale and gain higher energies than near a single X-nullpoint. The distinct density and pitch-
angle distributions of particles with high and low energy detected with the PIC approach can help to distinguish the
observational features of accelerated particles.
Authors: Xia, Q. and Zharkova V.V.
Projects: None
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Publication Status: Astronomy and Astrophysics, accepted
Last Modified: 2020-01-27 14:53
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Non-thermal hydrogen Lyman line and continuum emission in solar flares generated by electron beams |
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Valentina Zharkova Submitted: 2019-02-07 04:53
Aims.
Hydrogen Lyman continuum emission is greatly enhanced in the impulsive kernels of solar flares, with observations of Lyman
lines showing impulsive brightening and both red and blue wing asymmetries, based on the images with low spatial resolution.
A spate of proposed instruments will study Lyman emission in more detail from bright, impulsive flare kernels. In support of new
instrumentation we aim to apply an improved interpretation of Lyman emission with the hydrodynamic radiative code, HYDRO2GEN,
which has already successfully explained Hα
emission with large redshifts and sources of white light emission in solar flares. The
simulations can interpret the existing observations and propose observations in the forthcoming missions.
Methods.
A flaring atmosphere is considered to be produced by a 1D hydrodynamic response to injection of an electron beam,
defining depth variations of electron and ion kinetic temperatures, densities, and macro-velocities. Radiative responses in this flaring
atmosphere affected by the beams with different parameters are simulated using a fully non-local thermodynamic equilibrium (NLTE)
approach for a five-level plus continuum model hydrogen atom with excitation and ionisation by spontaneous, external, and internal
diffusive radiation, and by inelastic collisions with thermal and beam electrons. Integral radiative transfer equations for all optically
thick transitions are solved using the L2 approximation simultaneously with steady state equations.
Results.
During a beam injection in the impulsive phase there is a large increase of collisional ionisation and excitation by non-thermal electrons that strongly (by orders of magnitude) increases excitation and the ionisation degree of hydrogen atoms from all
atomic levels. These non-thermal collisions combined with plasma heating caused by beam electrons lead to an increase in Lyman
line and continuum radiation, which is highly optically thick. During a beam injection phase the Lyman continuum emission is greatly
enhanced in a large range of wavelengths resulting in a flattened distribution of Lyman continuum over wavelengths. After the beam
is switched off, Lyman continuum emission, because of its large opacity, sustains, for a very long time, the high ionisation degree
of the flaring plasma gained during the beam injection. This leads to a long enhancement of hydrogen ionisation, occurrence of
white light flares, and an increase of Lyman line emission in cores and wings, whose shapes are moved closer to those from complete redistribution (CRD) in frequencies, and away from the partial ones (PRD) derived in the non-flaring atmospheres. In addition, Lyman
line profiles can reflect macro-motions of a flaring atmosphere caused by downward hydrodynamic shocks produced in response to the beam injection reflected in the enhancements of Ly-line red wing emission. These red-shifted Ly-line profiles are often followed
by the enhancement of Ly-line blue wing emission caused by the chromospheric evaporation. The ratio of the integrated intensities in the Lyα and Lyβ lines is lower for more powerful flares and agrees with reported values from observations, except in the impulsive
phase in flaring kernels which were not resolved in previous observations, in which the ratio is even lower. These results can help
observers to design the future observations in Lyman lines and continuum emission in flaring atmospheres.
Authors: Druett, M.K. and Zharkova V.V..
Projects: BBSO/NST
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Publication Status: Astronomy and Astrophysics, in press, DOI: https://doi.org/10.1051/0004-6361/201732427
Last Modified: 2019-02-07 10:23
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Lost and found sunquake in the 6 September 2011 flare caused by beam electrons |
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Valentina Zharkova Submitted: 2018-08-07 07:54
Active region NOAA 11283 produced two X-class flares on 6 and 7 September 2011 that have been well studied
by many authors. The X2.1 class flare occurred on September 6, 2011 and was associated with the first of two homologous
white light flares produced by this region, but no sunquake was found with it despite the one being detected in the second
flare of 7 September 2011. In this paper we present the first observation of a sunquake for the 6 September 2011 flare detected
via statistical significance analysis of egression power and verified via directional holography and time-distance diagram. The
surface wavefront exhibits directional preference in the north-west direction We interpret this sunquake and the associated
flare emission with a combination of a radiative hydrodynamic model of a flaring atmosphere heated by electron beam and
a hydrodynamic model of acoustic wave generation in the solar interior generated by a supersonic shock. The hydrodynamic
model of the flaring atmosphere produces a hydrodynamic shock travelling with supersonic velocities towards the photosphere
and beneath. For the first time we derive velocities (up to 140 km s-1) and onset time (about 50 seconds after flare onset) of
the shock deposition at given depths of the interior. The shock parameters are confirmed by the radiative signatures in hard
X-rays and white light emission observed from this flare. The shock propagation in the interior beneath the flare is found to
generate acoustic waves elongated in the direction of shock propagation, that results in an anisotropic wavefront seen on the
solar surface. Matching the detected seismic signatures on the solar surface with the acoustic wave front model derived for the
simulated shock velocities, we infer that the shock has to be deposited under an angle of about 30◦
to the local solar vertical.
Hence, the improved seismic detection technique combined with the double hydrodynamic model reported in this study opens
new perspectives for observation and interpretation of seismic signatures in solar flares.
Authors: Connor Macrae, Sergei Zharkov, Valentina Zharkova, Malcolm Druett, Sarah Matthews, and Tomoko Kawate
Projects: RHESSI,SDO-AIA,SDO-HMI
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Publication Status: Astronomy and Astrophysics, in press
Last Modified: 2018-08-07 11:10
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On a role of quadruple component of magnetic field in defining solar activity in grand cycles |
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Valentina Zharkova Submitted: 2017-05-16 06:03
In this paper we revise our prediction of solar activity using a solar background magnetic field as a proxy by the inclusion of eigen vectors of solar magnetic waves produced by quadruple magnetic sources, in addition to the principal eigen modes generated by two-layer dipole sources (Zharkova et al., 2015). By considering the interference of two dipole and one quadruple waves we produce the revised summary curve for the last 400 years accounting for the additional minima of solar activity occurred at the beginning of 19th (Dalton minimum) and 20th centuries. Using the dynamo model with meridional circulation and selecting the directions of circulation for quadruple waves, we estimate the parameters of quadrupole waves best fitting the observations in the past grand cycle. The comparison shows that the quadruple wave has to be generated in the inner layer of the solar convective zone, in order to provide the additional minima observed in 19 and 20 centuries, thus, naturally accounting for Gleissberg centennial cycle. The dynamo wave simulated for the dipole and quadruple sources reveals much closer correspondence of the resulting summary curve derived from the principal components of magnetic field variations to the solar activity oscillations derived from the average sunspot numbers in the current grand cycle.
Authors: Popova, E., Zharkova V.V., Shepherd S.J. and Zharkov S.I.
Projects: None
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Publication Status: Journal of Atmospheric and Solar-Terrestrial Physics, http://dx.doi.org/10.1016/j.jastp.2017.05.006 in press
Last Modified: 2017-05-18 11:32
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