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* News 04/04/20 * The archive is using a new backend database. This has thrown up a few SQL errors in the last few days. If you have any issues please email adavey@nso.edu with either the number of eprint you are trying to edit or a link to your preprint.

Sunquake with a second-bounce, other sunquakes and emission associated with X9.3 flare of 6 September 2017. II. Proposed interpretation  

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 Ĺ) 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

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  

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:

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  

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

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  

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

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  

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

Publication Status: Astronomy and Astrophysics, accepted
Last Modified: 2020-01-27 14:53
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Oscillations of the baseline of solar magnetic field and solar irradiance on a millennial timescale  

Valentina Zharkova   Submitted: 2019-06-30 14:57

Recently discovered long-term oscillations of the solar background magnetic field associated with double dynamo waves generated in inner and outer layers of the Sun indicate that the solar activity is heading in the next three decades (2019-2055) to a Modern grand minimum similar to Maunder one. On the other hand, a reconstruction of solar total irradiance suggests that since the Maunder minimum there is an increase in the cycle-averaged total solar irradiance (TSI) by a value of about 1?1.5 Wm-2 closely correlated with an increase of the baseline (average) terrestrial temperature. In order to understand these two opposite trends, we calculated the double dynamo summary curve of magnetic field variations backward one hundred thousand years allowing us to confirm strong oscillations of solar activity in regular (11 year) and recently reported grand (350?400 year) solar cycles caused by actions of the double solar dynamo. In addition, oscillations of the baseline (zero-line) of magnetic field with a period of 1950 ? 95 years (a super-grand cycle) are discovered by applying a running averaging filter to suppress large-scale oscillations of 11 year cycles. Latest minimum of the baseline oscillations is found to coincide with the grand solar minimum (the Maunder minimum) occurred before the current super-grand cycle start. Since then the baseline magnitude became slowly increasing towards its maximum at 2600 to be followed by its decrease and minimum at ~3700. These oscillations of the baseline solar magnetic field are found associated with a long-term solar inertial motion about the barycenter of the solar system and closely linked to an increase of solar irradiance and terrestrial temperature in the past two centuries. This trend is anticipated to continue in the next six centuries that can lead to a further natural increase of the terrestrial temperature by more than 2.5 ?C by 2600.

Authors: V. Zharkova, S.Shepherd, S. Zharkov and E. Popova
Projects: None

Publication Status: published in Scientific Reports, Springer Nature group
Last Modified: 2019-07-03 13:06
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Non-thermal hydrogen Lyman line and continuum emission in solar flares generated by electron beams  

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

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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Particle acceleration in coalescent and squashed magnetic islands I. Test particle approach  

Valentina Zharkova   Submitted: 2018-10-09 06:57

Aims. Magnetic reconnection in large Harris-type reconnecting current sheets (RCSs) with a single X-nullpoint often leads to occurrence of magnetic islands with multiple O- and X-nullpoints. Over time these magnetic islands become squashed, or coalescent with two islands merging, as it has been observed indirectly during coronal mass ejection and by in-situ observations in the heliosphere and magnetotail. These points emphasize the importance of understanding the basic energising processes of ambient particles dragged into current sheets with magnetic islands of different configuration. Methods. Trajectories of protons and electrons accelerated by a reconnection electric field are investigated using a test particle approach in RCSs with different 3D magnetic field topologies defined analytically for multiple X- and O-nullpoints. Trajectories, densities and energy distributions are explored for 106 thermal particles dragged into the current sheets from different sides and distances. Results. This study confrms that protons and electrons accelerated in magnetic islands in a presence of strong guiding field are ejected from a current sheet into the opposite semiplanes with respect to its midplane. Particles are found to escape O-nullpoints only through the neighbouring X-nullpoints along (not across) the midplane following the separation law for electrons and protons in a given magnetic topology. Particles gain energy either inside O-nullpoints or in a vicinity of X-nullpoints that often leads to electron clouds formed about the X-nullpoint between the O-nullpoints. Electrons are shown to be able to gain sub-relativistic energies in a single magnetic island. Energy spectra of accelerated particles are close to power laws with spectral indices varying from 1.1 to 2.4. The more squashed the islands the larger the difference between the energy gains by transit and bounced particles. Their energy spectra are often with double maxima leading to fast growing turbulence. Conclusions. Particles are shown to gain most energy in multiple X-nullpoints between O-nullpoints (or magnetic islands). This leads to a formation of electron clouds between magnetic islands. Particle energy gains are much larger in squashed islands than in coalescent ones. In summary, particle acceleration by a reconnection electric field in magnetic islands is much more effective than in an RCS with a single X-nullpoint.

Authors: Q.Xia and V. Zharkova
Projects: None

Publication Status: Astronomy and Astrophysics, accepted
Last Modified: 2018-10-10 13:24
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Lost and found sunquake in the 6 September 2011 flare caused by beam electrons  

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

Publication Status: Astronomy and Astrophysics, in press
Last Modified: 2018-08-07 11:10
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Reply to comment by Usoskin (2017) on the paper  

Valentina Zharkova   Submitted: 2017-10-10 05:25

In this communication we provide our answers to the comments by Usoskin (2017) on our recent paper (Popova et al, 2017a). We show that Principal Component Analysis (PCA) allows us to derive eigen vectors with eigen values assigned to variance of solar magnetic field waves from full disk solar magnetograms obtained in cycles 21-23 which came in pairs. The current paper (Popova et al, 2017a) adds the second pair of magnetic waves generated by quadruple magnetic sources. This allows us to recover a centennial cycle, in addition to the grand cycle, and to produce a closer fit to the solar and terrestrial activity features in the past millennium.

Authors: Zharkova V.V., Popova E., Shepherd S.J. and Zharkov S.
Projects:

Publication Status: accepted
Last Modified: 2017-10-12 16:26
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Non-thermal hydrogen Balmer and Paschen emission in solar flares generated by electron beams  

Valentina Zharkova   Submitted: 2017-10-10 05:22

Sharp rises of hard X-ray (HXR) emission accompanied by Hα line profiles with strong red shifts up to 4 Å from the central wavelength, often observed at the onset of flares with the Specola Solare Ticinese Telescope (STT) and the Swedish Solar Telescope (SST), are not fully explained by existing radiative models. Moreover, the observations of white light (WL) and Balmer contin- uum emission with the Interface Region Imaging Spectrograph (IRIS) reveal strong co-temporal enhancements and often nearly co-spatial with HXR emission. These effects indicate a fast ef- fective source of excitation and ionisation of hydrogen atoms in flaring atmospheres associated with HXR emission. In this paper we investigate electron beams as the agents accounting for the observed hydrogen line and continuum emission. Flaring atmospheres are considered to be produced by a 1D hydrodynamic response to the injection of an electron beam defining their kinetic temperatures, densities and macrovelocities. A radiative response in these atmospheres is simulated using a fully non-local ther- modynamic equilibrium (NLTE) approach for a 5 levels plus continuum hydrogen atom model, considering its excitation and ionisation by spontaneous, external and internal diffusive radiation and by inelastic collisions with thermal and beam electrons. Resulting intensities of hydrogen line and continuum emission were also calculated for Balmer and Paschen series. We find that inelastic collisions with beam electrons strongly increase excitation and ionisation of hydrogen atoms from the chromosphere to photosphere. This leads to an increase in Lyman continuum radiation, which has high optical thickness, and after the beam is off it governs hydrogen ionisation and leads to the long lasting order of magnitude enhancement of emission in Balmer and Paschen continua. The ratio of Balmer-to-other-continuum head intensities are found correlated with the beams's initial flux. The height distribution of contribution functions for Paschen continuum emission indicate a close correlation with the observations of heights of WL and HXR emission reported for limb flares. This process also leads to a strong increase of wing emission (Stark's wings) in Balmer and Paschen lines, that is superimposed on large red-shifted enhancements of Hα-Hγ line emission resulting from a downward motion by hydrodynamic shocks. The simulated line profiles are shown to closely fit the observations for different flaring events.

Authors: Druett M. and Zharkova V.V.
Projects: None

Publication Status: Accepted
Last Modified: 2017-10-11 06:21
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Beam electrons as a source of Hα flare ribbons  

Valentina Zharkova   Submitted: 2017-06-27 07:18

The observations of solar flare onsets show rapid increase of hard and soft X-rays, ultra-violet emission with large Doppler blue shifts associated with plasma upflows, and Hα hydrogen emission with red shifts up to 1-4 Å. Modern radiative hydrodynamic models account well for blue-shifted emission, but struggle to reproduce closely the red-shifted Hα lines. Here we present a joint hydrodynamic and radiative model showing that during the first seconds of beam injection the effects caused by beam electrons can reproduce Hα line profiles with large red-shifts closely matching those observed in a C1.5 flare by the Swedish Solar Telescope. The model also accounts closely for timing and magnitude of upward motion to the corona observed 29 s after the event onset in 171 Å by the Atmospheric Imaging Assembly/Solar Dynamics Observatory.

Authors: Malcolm Druett, Eamon Scullion, Valentina Zharkova, Sarah Matthews, Sergei Zharkov & Luc Rouppe Van der Voort
Projects: None

Publication Status: published in Nature Communications
Last Modified: 2017-06-28 08:41
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On a role of quadruple component of magnetic field in defining solar activity in grand cycles  

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

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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Analytical solutions of continuity equation for joint collisional and Ohmic energy losses and their effects on hard X-ray emission.II. Mixed energy losses  

Valentina Zharkova   Submitted: 2016-03-08 04:49

In this paper we consider simultaneous analytical solutions of continuity equations for electron beam precipitation a) in collisional losses and b) in Ohmic losses, or mixed energy losses (MEL), by applying the iterative method to calculate the resulting differential densities at given precipitation depth. The differential densities of precipitating electrons derived from the analytical solutions for mixed energy losses reveal increased flattening at energies below 10-30 keV compared to a pure collisional case. This flattening becomes stronger with an increasing precipitation depth turning into a positive slope at greater precipitation depths in the chromosphere resulting in a differential density distribution with maximum that shifts towards higher energies with increase of a column depth. While the differential densities combining precipitating and returning electrons are higher at lower energies than those for a pure collisional case. The resulting hard X-ray (HXR) emission produced by the beams with different initial energy fluxes and spectral indices is calculated using the MEL approach for different ratios between the differential densities of precipitating and returning electrons. The number of returning electrons can be even further enhanced by a magnetic mirroring, not considered in the present model, while dominating at lower atmospheric depths where the magnetic convergence and magnitude are the highest. The proposed MEL approach provides an opportunity to account simultaneously for both collisional and ohmic losses in flaring events, which can be used for a quick spectral fitting of HXR spectra and evaluation of a fraction of returning electrons versus precipitating ones. The semi-analytical MEL approach is used for spectral fitting to RHESSI observations of nine C, M and X class flares revealing a close fit to the observations and good resemblance to numerical FP solutions.

Authors: Zharkova V.V. and Dobranskis R.R.
Projects: RHESSI

Publication Status: MNRAS, in press
Last Modified: 2016-03-08 12:17
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On the Generation of Hydrodynamic Shocks by Mixed Beams and Occurrence of Sunquakes in Flares  

Valentina Zharkova   Submitted: 2015-11-19 07:38

Observations of solar flares with sunquakes by space- and ground-based instruments reveal essentially different dynamics of seismic events in different flares. Some sunquakes are found to be closely associated with the locations of hard X-ray (HXR) and whitelight (WL) emission, while others are located outside either of them. In this article we investigate possible sources causing a seismic response in a form of hydrodynamic shocks produced by the injection of mixed (electron plus proton) beams, discuss the velocities of these shocks, and the depths where they deposit the bulk of their energy and momentum. The simulation of hydrodynamic shocks in flaring atmospheres induced by electron-rich and proton-rich beams reveals that the linear depth of the shock termination is shifted beneath the level of the quiet solar photosphere on a distance from 200 to 5000 km. The parameters of these atmospheric hydrodynamic shocks are used as initial condition for another hydrodynamic model developed for acoustic-wave propagation in the solar interior (Zharkov, Mon. Not. Roy. Astron. Soc. 431, 3414, 2013). The model reveals that the depth of energy and momentum deposition by the atmospheric shocks strongly affects the propagation velocity of the acoustic-wave packet in the interior. The locations of the first bounces from the photosphere of acoustic waves generated in the vicinity of a flare are seen as ripples on the solar surface, or sunquakes. Mixed proton-dominated beams are found to produce a strong supersonic shock at depths 200 ? 300 km under the level of the quiet-Sun photosphere and in this way produce well-observable acoustic waves, while electron-dominated beams create a slightly supersonic shock propagating down to 5000 km under the photosphere. This shock can only generate acoustic waves at the top layers beneath the photosphere since the shock velocity very quickly drops below the local sound speed. The distance 'delta' of the first bounce of the generated acoustic waves is discussed in relation to the minimal phase velocities of wave packets defined by the acoustic cutoff frequency and the parameters of atmospheric shock termination beneath the photosphere.

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

Publication Status: published, Solar Physics journal
Last Modified: 2015-11-20 15:38
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Heartbeat of the Sun from Principal Component Analysis and prediction of solar activity on a millennium timescale  

Valentina Zharkova   Submitted: 2015-11-01 10:33

We derive two principal components (PCs) of temporal magnetic field variations over the solar cycles 21?24 from full disk magnetograms covering about 39% of data variance, with σ = 0.67. These PCs are attributed to two main magnetic waves travelling from the opposite hemispheres with close frequencies and increasing phase shift. Using symbolic regeression analysis we also derive mathematical formulae for these waves and calculate their summary curve which we show is linked to solar activity index. Extrapolation of the PCs backward for 800 years reveals the two 350-year grand cycles superimposed on 22 year-cycles with the features showing a remarkable resemblance to sunspot activity reported in the past including the Maunder and Dalton minimum. The summary curve calculated for the next millennium predicts further three grand cycles with the closest grand minimum occurring in the forthcoming cycles 26?27 with the two magnetic field waves separating into the opposite hemispheres leading to strongly reduced solar activity. These grand cycle variations are probed by α - Ω dynamo model with meridional circulation. Dynamo waves are found generated with close frequencies whose interaction leads to beating effects responsible for the grand cycles (350?400 years) superimposed on a standard 22 year cycle. This approach opens a new era in investigation and confident prediction of solar activity on a millenium timescale.

Authors: Zharkova V.V., Shepherd S.J., Popova E., Zharkov S.I.
Projects: None

Publication Status: Nature Scientific Reports 5, 15689 (2015) doi:10.1038/srep15689
Last Modified: 2015-11-02 10:52
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Dynamical small-scale magnetic islands as a source of local acceleration of particles in the solar wind  

Valentina Zharkova   Submitted: 2015-11-01 10:31

We present observations of energetic particle flux increases up to 1 MeV at 1 AU, which cannot be associated with ordinary mechanisms of particle acceleration, such as acceleration at shocks or at the Sun. Such unusual energetic particle events very likely have a local origin. Multi-spacecraft observations show that numerous cases of energetic particle flux enhancements and spikes correspond to passages of spacecraft through areas filled with magnetic islands with a typical width ~0.010.001AU that experience dynamical merging or/and contraction. The presence of magnetic islands inside magnetically confined cavities in the solar wind may lead to local particle energization, especially in the case when the particles have already been pre-accelerated to keV energies, for example, at shocks or due to magnetic reconnection at the heliospheric current sheet. We consider different magnetic configurations that provide favourable conditions for both the appearance of small-scale magnetic islands and their confinement.

Authors: O. V. Khabarova, G. P. Zank, G. Li, J. A. le Roux, G. M.Webb, A. Dosch, O. E. Malandraki, Zharkova V.V.
Projects: None

Publication Status: Journal of Physics: Conference Series (JPCS), 642, 012033
Last Modified: 2015-11-02 10:52
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Particle acceleration in 3D single current sheets formed in the solar corona and heliosphere: PIC approach  

Valentina Zharkova   Submitted: 2015-11-01 10:28

Acceleration of protons and electrons in a reconnecting current sheet (RCS) is investigated with the test particle and particle-in-cell (PIC) approaches in a 3D magnetic topology. PIC simulations confirm a spatial separation of electrons and protons with respect to the midplane depending on the guiding field. Simulation reveals that the separation occurs in magnetic topologies with strong guiding fields and lasts as long as the particles are kept dragged into a current sheet. This separation produces a polarisation electric field induced by the plasma feedback to a presence of accelerated particles, which shape can change from symmetric towards the midplane (for weak guiding field) to fully asymmetric (for strong guiding field). Particles are found accelerated at a midplane of any current sheets present in the heliosphere to the energies up to hundred keV for electrons and hundred MeV for protons. The maximum energy gained by particles during their motion inside the current sheet is defined by its magnetic field topology (the ratio of magnetic field components), the side and location from the X-nullpoint, where the particles enter a current sheet. In strong magnetic fields of the solar corona with weaker guiding fields, electrons are found circulating about the midplane to large distances where proton are getting accelerated, creating about the current sheet midplane clouds of high energy electrons, which can be the source of hard X-ray emission in the coronal sources of flares. These electrons are ejected into the same footpoint as protons after the latter reach the energy sufficient to break from a current sheet. In a weaker magnetic field of the heliosphere the bounced electrons with lower energies cannot reach the midplane turning instead at some distance D before the current sheet midplane by 180 degrees from their initial motion. Also the beams of accelerated transit and bounced particles are found to generate turbulent electric fields in a form of Langmuir waves (electrons) or ion-acoustic waves (protons).

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

Publication Status: Journal of Physics, Conference Series (JPCS), 2015, 642, 012032
Last Modified: 2015-11-02 10:52
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Updated analytical solutions of continuity equation for electron beams precipitation. I. Pure collisional and pure ohmic energy losses  

Valentina Zharkova   Submitted: 2015-08-14 10:35

We present updated analytical solutions of continuity equations for power-law beam electrons precipitating in (a) purely collisional losses and (b) purely ohmic losses. The solutions of continuity equation (CE) normalized on electron density presented in Dobranskis & Zharkova are found by method of characteristics eliminating a mistake in the density characteristic pointed out by Emslie et al. The corrected electron beam differential densities (DD) for collisions are shown to have energy spectra with the index of -(γ + 1)/2, coinciding with the one derived from the inverse problem solution by Brown, while being lower by 1/2 than the index of -γ/2 obtained from CE for electron flux. This leads to a decrease of the index of mean electron spectra from -(γ - 2.5) (CE for flux) to -(γ - 2.0) (CE for electron density). The similar method is applied to CE for electrons precipitating in electric field induced by the beam itself. For the first time, the electron energy spectra are calculated for both constant and variable electric fields by using CE for electron density. We derive electron DD for precipitating electrons (moving towards the photosphere, μ = +1) and 'returning' electrons (moving towards the corona, μ = -1). The indices of DD energy spectra are reduced from -γ - 1 (CE for flux) to -γ (CE for electron density). While the index of mean electron spectra is increased by 0.5, from -γ + 0.5 (CE for flux) to -γ + 1(CE for electron density). Hard X-ray intensities are also calculated for relativistic cross-section for the updated differential spectra revealing closer resemblance to numerical Fokker-Planck (FP) solutions.

Authors: R. R. Dobranskis and V. V. Zharkova
Projects: None

Publication Status: published: MNRAS, 2015,453 (1): 229-241
Last Modified: 2015-08-17 15:12
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Additional acceleration of solar-wind particles in current sheets of the heliosphere  

Valentina Zharkova   Submitted: 2015-04-21 07:22

Particles of fast solar wind in the vicinity of the heliospheric current sheet (HCS) or in a front of interplanetary coronal mass ejections (ICMEs) often reveal very peculiar energy or velocity profiles, density distributions with double or triple peaks, and well-defined streams of electrons occurring around or far away from these events. In order to interpret the parameters of energetic particles (both ions and electrons) measured by the WIND spacecraft during the HCS crossings, a comparison of the data was carried out with 3-D particle-in-cell (PIC) simulations for the relevant magnetic topology (Zharkova and Khabarova, 2012). The simulations showed that all the observed particle-energy distributions, densities, ion peak velocities, electron pitch angles and directivities can be fitted with the same model if the heliospheric current sheet is in a status of continuous magnetic reconnection. In this paper we present further observations of the solar-wind particles being accelerated to rather higher energies while passing through the HCS and the evidence that this acceleration happens well before the appearance of the corotating interacting region (CIR), which passes through the spacecraft position hours later. We show that the measured particle characteristics (ion velocity, electron pitch angles and the distance at which electrons are turned from the HCS) are in agreement with the simulations of additional particle acceleration in a reconnecting HCS with a strong guiding field as measured by WIND. A few examples are also presented showing additional acceleration of solar-wind particles during their passage through current sheets formed in a front of ICMEs. This additional acceleration at the ICME current sheets can explain the anticorrelation of ion and electron fluxes frequently observed around the ICME's leading front. Furthermore, it may provide a plausible explanation of the appearance of bidirectional "strahls" (field-aligned most energetic suprathermal electrons) at the leading edge of ICMEs as energetic electrons generated during a magnetic reconnection at the ICME-front current sheet.

Authors: Zharkova V.V. and Khabarova O.
Projects: None

Publication Status: Ann. Geophys., 33, 457-470, 2015, published
Last Modified: 2015-04-21 10:46
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Abstracts by Author
Sunquake with a second-bounce, other sunquakes and emission associated with X9.3 flare of 6 September 2017. II. Proposed interpretation
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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Oscillations of the baseline of solar magnetic field and solar irradiance on a millennial timescale
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Particle acceleration in coalescent and squashed magnetic islands I. Test particle approach
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Reply to comment by Usoskin (2017) on the paper
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Analytical solutions of continuity equation for joint collisional and Ohmic energy losses and their effects on hard X-ray emission.II. Mixed energy losses
On the Generation of Hydrodynamic Shocks by Mixed Beams and Occurrence of Sunquakes in Flares
Heartbeat of the Sun from Principal Component Analysis and prediction of solar activity on a millennium timescale
Dynamical small-scale magnetic islands as a source of local acceleration of particles in the solar wind
Particle acceleration in 3D single current sheets formed in the solar corona and heliosphere: PIC approach
Updated analytical solutions of continuity equation for electron beams precipitation. I. Pure collisional and pure ohmic energy losses
Additional acceleration of solar-wind particles in current sheets of the heliosphere
PREDICTION OF SOLAR ACTIVITY FROM SOLAR BACKGROUND MAGNETIC FIELD VARIATIONS IN CYCLES 21?23
ERRATUM: "Exact Analytical Solutions of Continuity Equation for Electron Beams Precipitating in Coulomb Collisions"
Exact Analytical Solutions of Continuity Equation for Electron Beams Precipitating in Coulomb Collisions
Probing latitudinal variations of the solar magnetic field in cycles 21--23 by Parker's Two-Layer Dynamo Model with meridional circulation}
Principal Component Analysis of Background and Sunspot Magnetic Field Variations During Solar Cycles 21-23
Diagnostics of electron beam properties from the simultaneous hard X-ray and microwave emission in the 10 March 2001 flare
The effects of electron beam induced electric field on the generation of Langmuir turbulence in flaring atmospheres
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