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Papers by Valentina Zharkova

Proceedings of the International Astronomical Union, Sep 1, 2007

Proceedings of the International Astronomical Union, Sep 1, 2007

Solar Physics, May 1, 2006

EURASIP Journal on Advances in Signal Processing, Sep 14, 2005

Proceedings of the International Astronomical Union, Sep 1, 2007

Proceedings of the International Astronomical Union, Jul 1, 2017

Space Science Reviews, Nov 1, 2005

Solar Physics, Jul 22, 2008

arXiv (Cornell University), Jan 18, 2023

Monthly Notices of the Royal Astronomical Society, Mar 22, 2022

<div> <div> <div> <p... more <div> <div> <div> <p>We will overview particle motion in 3D Harris-type RCSs without and with magnetic islands using particle-in-cell (PIC) method considering the plasma feedback to electromagnetic fields. We evaluate particle energy gains and pitch angle distributions (PADs) of accelerated particles of both changes in different locations inside current sheets as seen under the different directions by a virtual spacecraft passing through. The RCS parameters are considered comparable to heliosphere and solar wind conditions. </p> <p>The energy gains and the PADs of particles are shown to change depending on a topology of magnetic fields.  We report separation of electrons from ions at acceleeration in current sheets with strong guiding fields  and formation of transit and bounced beams from the particles of the same charge. The  transit particles are shown to form  bi-directional energetic electron beams (strahls), while bounced particles are mainly account from driopout fluxes in the heliosphere. In topologies with weak guding field strahls are mainly present inside the magneticislands and located closely above/below the X-nullpoints in the inflow regions. As the guiding field becomes larger, the regions with bi-directional strahls are compressed towards small areas in the exhausts of current sheets. Mono-directional strahls with PADS along 0 or 180 degrees are found quasi-parallel to the magnetic field lines near the X-nullpoint due to the dominant Fermi-type magnetic curvature drift acceleration. Meanwhile, high-energy electrons confined inside magnetic islands create PADs about 90◦. </p> </div> </div> </div>

Proceedings of the International Astronomical Union, Jul 1, 2017

Astronomy and Astrophysics, Jun 16, 2011

Context. Microwave (MW) and hard X-ray (HXR) data are thought to be powerful means of investigati... more Context. Microwave (MW) and hard X-ray (HXR) data are thought to be powerful means of investigating the mechanisms of particle acceleration and precipitation in solar flares, reflecting different aspects of electron interaction with ambient particles in the presence of a magnetic field. Simultaneous simulation of HXR and MW emission with the same populations of electrons is still a big challenge for interpreting observations of real events. Recent progress in simulations of particle kinetics with the time-dependent Fokker-Planck (FP) approach offers an opportunity to produce this interpretation. Aims. In this paper we apply the FP kinetic model of precipitation of electron beam with energy range from 12 keV to 1.2 MeV to the interpretation of X-ray and MW emissions observed in the flare of 2001 March 10. Methods. The theoretical HXR and MW emissions were calculated by using the distribution functions of electron beams found by solving time-dependent approach in a converging magnetic field for anisotropic scattering of beam electrons on the ambient particles in Coloumb collisions and Ohmic losses. Results. The simultaneously observed HXR photon spectra and frequency distribution of MW emission and polarization were fit by those simulated from FP models that include the effects of electric field induced by beam electrons and precipitation onto a converging magnetic loop. Magnetic field strengths in the footpoints on the photosphere were updated with newly calibrated SOHO/MDI data. The observed HXR energy spectrum above 10 keV is shown to be a double power law that was precisely fit by the photon HXR spectrum simulated for the model including the self-induced electric field but without magnetic convergence. The MW emission simulated for different models of electron precipitation revealed a better fit (above 90% confidence level) to the observed distribution at higher frequencies for the models combining collisions and electric field effects with a moderate magnetic field convergence of two. The MW simulations were able to reproduce closely the main features of the MW emission observed at higher frequencies: the spectral index, the frequency of peak intensity and the frequency of the MW polarization reversal, while at lower frequencies the simulated MW intensities are lower than the observed ones.

Astronomy and Astrophysics, Feb 16, 2005

The effect of a viewing angle on the hydroden H α-line impact polarisation is investigated in a p... more The effect of a viewing angle on the hydroden H α-line impact polarisation is investigated in a plane vertical atmosphere arbitrary located on the solar disk. The impact polarisation is assumed to be caused by precipitating beam electrons with pitch-angular anisotropy steadily injected into the flaring atmosphere from its top. The polarisation is calculated for a 3 level plus continuum hydrogen atom affected by Zeeman splitting in a moderate magnetic field taking into account depolarising effects of diffusive radiation and collisions with thermal electrons. The H α polarisation profiles are affected by electron beams only in the line cores whereas the wings are fully depolarized by the collisions with thermal electrons despite the extended wing emission, or "moustaches", caused by beam electrons. The full (integrated in wavelength) H α-line linear polarisation, caused by moderate electron beams, is shown to be 2−20% and either negative or positive depending on the position of a flaring loop on the solar disk and the direction of an emitted photon from the local magnetic field. The polarisation plane is projected onto a viewing angle ψ, being a superposition of the flare location on a solar disk and the magnetic field deviation from vertical on the solar surface. For viewing angles less then 50 • the H α-line impact polarisation is negative increasing up to −10% towards smaller angles, meaning that the polarisation is mostly perpendicular to the plane B × K where B is the magnetic field induction and K is the photon momentum vector. For viewing angles greater than 60 • the measured impact polarisation becomes positive, sharply increasing up to 20% towards the limb. In the range of 50−60 • the observed impact polarisation goes through a zero point despite the actual presence of beam electrons in the flaring atmosphere. The theoretical predictions of the dependence of polarisation degree on viewing angle fit remarkably well the observations of H α-line linear polarisation in small-scale flaring events such as moustaches or Ellerman bombs, located in different positions on a solar disk.

The long-term millennial oscillations of the baseline solar background magnetic field (SBMF) an... more The long-term millennial oscillations of the baseline solar background magnetic field (SBMF) and the ephemeris of the Sun-Earth distances are compared with the oscillations of solar irradiance at the terrestrial biomass (Hallstatt's cycle). Based the Sun-Earth distances derived from the current JPL ephemeris based on solar inertial motion and gravitational effects on the Sun by four large planets: Jupiter, Saturn, Neptune and Uranus we demonstrate the S-E distance is reduced by 0.005 au in the millennium M1 600-1600 and 0.011 au in millennium M2 1600-2600. We show that variations of the Sun-Earth distances are accountable for the increase of the solar irradiance by about 20−2520-2520−25 Wm−2Wm^{-2}Wm−2 since 1700 that will continue to last until 2500. he decrease of the S-E distance per century in the current millennium follows the rate of the terrestrial temperature increase reported since MM. We evaluate that this difference of the Sun-Earth distances caused by SIM leads to the d...

Journal of Atmospheric and Solar-Terrestrial Physics, 2018

In this communication we provide our answers to the comments by Usoskin (2017) on our recent pape... more 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.

Cornell University - arXiv, Mar 7, 2022

We compare the frequencies of volcanic eruptions in the past 270 years with the variations of sol... more We compare the frequencies of volcanic eruptions in the past 270 years with the variations of solar activity and summary curve of principal components of the solar background magnetic field (SBMF) derived from the WSO synoptic magnetic maps. Quartile distributions of volcanic eruption frequencies over the four phases of a 11 year cycle (growth, maximum, descent and minimum) reveal higher numbers of eruptions occurring at the maxima of SBMF of southern polarity with some increases at the minima of sunspot numbers. The frequency analysis of volcanic eruptions with Morlet wavelet reveals that the period of 22 years is more pronounced than 11 years. Comparison of the volcanic frequencies with the summary curve of SBMF for 11 cycles after 1868 (excluding 1950-1980 affected by the open nuclear bomb testing) reveals a strong correlation with the coefficient of-0.84 (within a confidence interval of 95%), while for 8 cycles in the early period of 1750-1868 the correlation becomes nearly twice lower. This lower correlation was likely to be caused by the geomagnetic jerk and migration of the Earth's magnetic pole to lower latitudes. The maxima of volcanic eruptions are shown to occur during solar activity cycles with the southern magnetic polarity that, in turn, can be associated with stronger disturbances of the geomagnetic field leading to the eruption number increase. The next anticipated maximum of volcanic eruptions is expected to occur during cycle 26 (2031-2042), when the solar background magnetic field will have a southern magnetic polarity. These volcanic eruptions can contribute to terrestrial cooling during the modern grand solar minimum (GSM) (2020-2053).

Proceedings of the International Astronomical Union, Sep 1, 2007

Proceedings of the International Astronomical Union, Sep 1, 2007

Solar Physics, May 1, 2006

EURASIP Journal on Advances in Signal Processing, Sep 14, 2005

Proceedings of the International Astronomical Union, Sep 1, 2007

Proceedings of the International Astronomical Union, Jul 1, 2017

Space Science Reviews, Nov 1, 2005

Solar Physics, Jul 22, 2008

arXiv (Cornell University), Jan 18, 2023

Monthly Notices of the Royal Astronomical Society, Mar 22, 2022

<div> <div> <div> <p... more <div> <div> <div> <p>We will overview particle motion in 3D Harris-type RCSs without and with magnetic islands using particle-in-cell (PIC) method considering the plasma feedback to electromagnetic fields. We evaluate particle energy gains and pitch angle distributions (PADs) of accelerated particles of both changes in different locations inside current sheets as seen under the different directions by a virtual spacecraft passing through. The RCS parameters are considered comparable to heliosphere and solar wind conditions. </p> <p>The energy gains and the PADs of particles are shown to change depending on a topology of magnetic fields.  We report separation of electrons from ions at acceleeration in current sheets with strong guiding fields  and formation of transit and bounced beams from the particles of the same charge. The  transit particles are shown to form  bi-directional energetic electron beams (strahls), while bounced particles are mainly account from driopout fluxes in the heliosphere. In topologies with weak guding field strahls are mainly present inside the magneticislands and located closely above/below the X-nullpoints in the inflow regions. As the guiding field becomes larger, the regions with bi-directional strahls are compressed towards small areas in the exhausts of current sheets. Mono-directional strahls with PADS along 0 or 180 degrees are found quasi-parallel to the magnetic field lines near the X-nullpoint due to the dominant Fermi-type magnetic curvature drift acceleration. Meanwhile, high-energy electrons confined inside magnetic islands create PADs about 90◦. </p> </div> </div> </div>

Proceedings of the International Astronomical Union, Jul 1, 2017

Astronomy and Astrophysics, Jun 16, 2011

Context. Microwave (MW) and hard X-ray (HXR) data are thought to be powerful means of investigati... more Context. Microwave (MW) and hard X-ray (HXR) data are thought to be powerful means of investigating the mechanisms of particle acceleration and precipitation in solar flares, reflecting different aspects of electron interaction with ambient particles in the presence of a magnetic field. Simultaneous simulation of HXR and MW emission with the same populations of electrons is still a big challenge for interpreting observations of real events. Recent progress in simulations of particle kinetics with the time-dependent Fokker-Planck (FP) approach offers an opportunity to produce this interpretation. Aims. In this paper we apply the FP kinetic model of precipitation of electron beam with energy range from 12 keV to 1.2 MeV to the interpretation of X-ray and MW emissions observed in the flare of 2001 March 10. Methods. The theoretical HXR and MW emissions were calculated by using the distribution functions of electron beams found by solving time-dependent approach in a converging magnetic field for anisotropic scattering of beam electrons on the ambient particles in Coloumb collisions and Ohmic losses. Results. The simultaneously observed HXR photon spectra and frequency distribution of MW emission and polarization were fit by those simulated from FP models that include the effects of electric field induced by beam electrons and precipitation onto a converging magnetic loop. Magnetic field strengths in the footpoints on the photosphere were updated with newly calibrated SOHO/MDI data. The observed HXR energy spectrum above 10 keV is shown to be a double power law that was precisely fit by the photon HXR spectrum simulated for the model including the self-induced electric field but without magnetic convergence. The MW emission simulated for different models of electron precipitation revealed a better fit (above 90% confidence level) to the observed distribution at higher frequencies for the models combining collisions and electric field effects with a moderate magnetic field convergence of two. The MW simulations were able to reproduce closely the main features of the MW emission observed at higher frequencies: the spectral index, the frequency of peak intensity and the frequency of the MW polarization reversal, while at lower frequencies the simulated MW intensities are lower than the observed ones.

Astronomy and Astrophysics, Feb 16, 2005

The effect of a viewing angle on the hydroden H α-line impact polarisation is investigated in a p... more The effect of a viewing angle on the hydroden H α-line impact polarisation is investigated in a plane vertical atmosphere arbitrary located on the solar disk. The impact polarisation is assumed to be caused by precipitating beam electrons with pitch-angular anisotropy steadily injected into the flaring atmosphere from its top. The polarisation is calculated for a 3 level plus continuum hydrogen atom affected by Zeeman splitting in a moderate magnetic field taking into account depolarising effects of diffusive radiation and collisions with thermal electrons. The H α polarisation profiles are affected by electron beams only in the line cores whereas the wings are fully depolarized by the collisions with thermal electrons despite the extended wing emission, or "moustaches", caused by beam electrons. The full (integrated in wavelength) H α-line linear polarisation, caused by moderate electron beams, is shown to be 2−20% and either negative or positive depending on the position of a flaring loop on the solar disk and the direction of an emitted photon from the local magnetic field. The polarisation plane is projected onto a viewing angle ψ, being a superposition of the flare location on a solar disk and the magnetic field deviation from vertical on the solar surface. For viewing angles less then 50 • the H α-line impact polarisation is negative increasing up to −10% towards smaller angles, meaning that the polarisation is mostly perpendicular to the plane B × K where B is the magnetic field induction and K is the photon momentum vector. For viewing angles greater than 60 • the measured impact polarisation becomes positive, sharply increasing up to 20% towards the limb. In the range of 50−60 • the observed impact polarisation goes through a zero point despite the actual presence of beam electrons in the flaring atmosphere. The theoretical predictions of the dependence of polarisation degree on viewing angle fit remarkably well the observations of H α-line linear polarisation in small-scale flaring events such as moustaches or Ellerman bombs, located in different positions on a solar disk.

The long-term millennial oscillations of the baseline solar background magnetic field (SBMF) an... more The long-term millennial oscillations of the baseline solar background magnetic field (SBMF) and the ephemeris of the Sun-Earth distances are compared with the oscillations of solar irradiance at the terrestrial biomass (Hallstatt's cycle). Based the Sun-Earth distances derived from the current JPL ephemeris based on solar inertial motion and gravitational effects on the Sun by four large planets: Jupiter, Saturn, Neptune and Uranus we demonstrate the S-E distance is reduced by 0.005 au in the millennium M1 600-1600 and 0.011 au in millennium M2 1600-2600. We show that variations of the Sun-Earth distances are accountable for the increase of the solar irradiance by about 20−2520-2520−25 Wm−2Wm^{-2}Wm−2 since 1700 that will continue to last until 2500. he decrease of the S-E distance per century in the current millennium follows the rate of the terrestrial temperature increase reported since MM. We evaluate that this difference of the Sun-Earth distances caused by SIM leads to the d...

Journal of Atmospheric and Solar-Terrestrial Physics, 2018

In this communication we provide our answers to the comments by Usoskin (2017) on our recent pape... more 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.

Cornell University - arXiv, Mar 7, 2022

We compare the frequencies of volcanic eruptions in the past 270 years with the variations of sol... more We compare the frequencies of volcanic eruptions in the past 270 years with the variations of solar activity and summary curve of principal components of the solar background magnetic field (SBMF) derived from the WSO synoptic magnetic maps. Quartile distributions of volcanic eruption frequencies over the four phases of a 11 year cycle (growth, maximum, descent and minimum) reveal higher numbers of eruptions occurring at the maxima of SBMF of southern polarity with some increases at the minima of sunspot numbers. The frequency analysis of volcanic eruptions with Morlet wavelet reveals that the period of 22 years is more pronounced than 11 years. Comparison of the volcanic frequencies with the summary curve of SBMF for 11 cycles after 1868 (excluding 1950-1980 affected by the open nuclear bomb testing) reveals a strong correlation with the coefficient of-0.84 (within a confidence interval of 95%), while for 8 cycles in the early period of 1750-1868 the correlation becomes nearly twice lower. This lower correlation was likely to be caused by the geomagnetic jerk and migration of the Earth's magnetic pole to lower latitudes. The maxima of volcanic eruptions are shown to occur during solar activity cycles with the southern magnetic polarity that, in turn, can be associated with stronger disturbances of the geomagnetic field leading to the eruption number increase. The next anticipated maximum of volcanic eruptions is expected to occur during cycle 26 (2031-2042), when the solar background magnetic field will have a southern magnetic polarity. These volcanic eruptions can contribute to terrestrial cooling during the modern grand solar minimum (GSM) (2020-2053).