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Author name code: olshevsky
ADS astronomy entries on 2022-09-14
author:"Olshevsky, Vyacheslav"
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Title: A Database of MMS Bow Shock Crossings Compiled Using Machine
Learning
Authors: Lalti, A.; Khotyaintsev, Yu. V.; Dimmock, A. P.; Johlander,
A.; Graham, D. B.; Olshevsky, V.
2022JGRA..12730454L Altcode: 2022arXiv220304680L
Identifying collisionless shock crossings in data sent from spacecraft
has so far been done manually or using basic algorithms. It is a
tedious job that shock physicists have to go through if they want to
conduct case studies or perform statistical studies. We use a machine
learning approach to automatically identify shock crossings from the
Magnetospheric Multiscale (MMS) spacecraft. We compiled a database
of 2,797 shock crossings, spanning a period from October 2015 to
December 2020, including various spacecraft-related and shock-related
parameters for each event. Furthermore, we show that the shock
crossings in the database are spread out in space, from the subsolar
point to the far flanks. On top of that, we show that they cover a
wide range of parameter space. We also present a possible scientific
application of the database by looking for correlations between ion
acceleration efficiency at shocks with different shock parameters,
such as the angle between the upstream magnetic field and the shock
normal θ<SUB>Bn</SUB> and the Alfvénic Mach number M<SUB>A</SUB>. We
find no clear correlation between the acceleration efficiency and
M<SUB>A</SUB>; however, we find that quasi-parallel shocks are more
efficient at accelerating ions than quasi-perpendicular shocks.
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Title: Automated Classification of Plasma Regions Using 3D Particle
Energy Distributions
Authors: Olshevsky, Vyacheslav; Khotyaintsev, Yuri V.; Lalti, Ahmad;
Divin, Andrey; Delzanno, Gian Luca; Anderzén, Sven; Herman, Pawel;
Chien, Steven W. D.; Avanov, Levon; Dimmock, Andrew P.; Markidis,
Stefano
2021JGRA..12629620O Altcode: 2019arXiv190805715O
We investigate the properties of the ion sky maps produced by the Dual
Ion Spectrometers (DIS) from the Fast Plasma Investigation (FPI). We
have trained a convolutional neural network classifier to predict
four regions crossed by the Magnetospheric Multiscale Mission (MMS) on
the dayside magnetosphere: solar wind, ion foreshock, magnetosheath,
and magnetopause using solely DIS spectrograms. The accuracy of the
classifier is >98%. We use the classifier to detect mixed plasma
regions, in particular to find the bow shock regions. A similar approach
can be used to identify the magnetopause crossings and reveal regions
prone to magnetic reconnection. Data processing through the trained
classifier is fast and efficient and thus can be used for classification
for the whole MMS database.
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Title: Kinetic Modeling in the Magnetosphere
Authors: Markidis, Stefano; Olshevsky, Vyacheslav; Tóth, Gábor;
Chen, Yuxi; Peng, Ivy Bo; Lapenta, Giovanni; Gombosi, Tamas
2021GMS...259..607M Altcode: 2020arXiv201206669M
This paper presents the state of the art of kinetic modeling
techniques for simulating plasma kinetic dynamics in magnetospheres. We
describe the critical numerical techniques for enabling large-scale
kinetic simulations of magnetospheres: parameter scaling, implicit
Particle-in-Cell schemes, and fluid-kinetic coupling. We show an
application of these techniques to study particle acceleration
and heating in asymmetric magnetic reconnection in the Ganymede
magnetosphere.
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Title: A comparison of methods for finding magnetic nulls in
simulations and in situ observations of space plasmas
Authors: Olshevsky, V.; Pontin, D. I.; Williams, B.; Parnell, C. E.;
Fu, H. S.; Liu, Y.; Yao, S.; Khotyaintsev, Y. V.
2020A&A...644A.150O Altcode: 2021arXiv210102014O
Context. Magnetic nulls are ubiquitous in space plasmas, and are
of interest as sites of localised energy dissipation or magnetic
reconnection. As such, a number of methods have been proposed for
detecting nulls in both simulation data and in situ spacecraft data
from Earth's magnetosphere. The same methods can be applied to detect
stagnation points in flow fields. <BR /> Aims: In this paper we describe
a systematic comparison of different methods for finding magnetic
nulls. The Poincaré index method, the first-order Taylor expansion
(FOTE) method, and the trilinear method are considered. <BR /> Methods:
We define a magnetic field containing fourteen magnetic nulls whose
positions and types are known to arbitrary precision. Furthermore,
we applied the selected techniques in order to find and classify
those nulls. Two situations are considered: one in which the magnetic
field is discretised on a rectangular grid, and the second in which the
magnetic field is discretised along synthetic "spacecraft trajectories"
within the domain. <BR /> Results: At present, FOTE and trilinear are
the most reliable methods for finding nulls in the spacecraft data
and in numerical simulations on Cartesian grids, respectively. The
Poincaré index method is suitable for simulations on both tetrahedral
and hexahedral meshes. <BR /> Conclusions: The proposed magnetic field
configuration can be used for grading and benchmarking the new and
existing tools for finding magnetic nulls and flow stagnation points.
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Title: Extending the FOTE Method to Three-dimensional Plasma Flow
Fields
Authors: Wang, Z.; Fu, H. S.; Olshevsky, V.; Liu, Y. Y.; Liu, C. M.;
Chen, Z. Z.
2020ApJS..249...10W Altcode:
In this study, we examine whether the First-Order Taylor Expansion
(FOTE) method can be applied to steady-state plasma flow fields in
space. We particularly examine whether this method (termed FOTE-V)
can be used to identify the flow critical points (including both
stagnation point and vortex center) and reconstruct the flow patterns
around these points. Quantitatively, we test the accuracy of this method
using 3D kinetic simulation data, and find the FOTE-V method can give
accurate reconstruction results within an area about 2 times the size
of the spacecraft tetrahedron, particularly when there are no clear
nonlinear flow structures in the simulation box. With simulation data,
we also reveal the ability of the FOTE-V method on reconstructing 3D
flow field topology of both radial-type null and spiral-type nulls. We
further test the accuracy of this method using measurements from NASA's
Magnetospheric Multi-scale (MMS) mission. In a current sheet crossing
event, the FOTE-V method successfully identifies the spiral-type
nulls in the reconnection exhaust region. In an EDR crossing event,
the FOTE-V method detects the stagnation point near the reconnection
center. We find these 3D flow structures are quasi-linear at the
MMS separation scale. Utilizing the continuity equation of the
steady flow, we define a parameter, $\alpha =\tfrac{{\rm{\nabla
}}\cdot \left(n{\boldsymbol{V}}\right)}{{\rm{\nabla }}\times
\left(n{\boldsymbol{V}}\right)}$ , to quantify the error of this
method—the smaller this parameter the better the results. This study
demonstrates that the plasma flows at small scale are indeed linear,
and thus the FOTE-V method can be applied to such flow fields. In
particular, this method will be useful to study stagnation points and
electron vortices in space plasmas.
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Title: A Fully Kinetic Perspective of Electron Acceleration around
a Weakly Outgassing Comet
Authors: Divin, Andrey; Deca, Jan; Eriksson, Anders; Henri, Pierre;
Lapenta, Giovanni; Olshevsky, Vyacheslav; Markidis, Stefano
2020ApJ...889L..33D Altcode:
The cometary mission Rosetta has shown the presence of
higher-than-expected suprathermal electron fluxes. In this study,
using 3D fully kinetic electromagnetic simulations of the interaction
of the solar wind with a comet, we constrain the kinetic mechanism
that is responsible for the bulk electron energization that creates
the suprathermal distribution from the warm background of solar wind
electrons. We identify and characterize the magnetic field-aligned
ambipolar electric field that ensures quasi-neutrality and traps warm
electrons. Solar wind electrons are accelerated to energies as high as
50-70 eV close to the comet nucleus without the need for wave-particle
or turbulent heating mechanisms. We find that the accelerating
potential controls the parallel electron temperature, total density,
and (to a lesser degree) the perpendicular electron temperature and
the magnetic field magnitude. Our self-consistent approach enables us
to better understand the underlying plasma processes that govern the
near-comet plasma environment.
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Title: Accelerating Magnetospheric Modeling with Heterogeneous
Hardware
Authors: Markidis, S.; Chien, S. W. D.; Olshevsky, V.
2019AGUFMSM12B..07M Altcode:
The advent of supercomputers with multiple accelerators per
computational node is impacting the development of codes for
magnetospheric modeling. The two current largest supercomputers in
November 2018 Top500 list, Summit and Sierra, feature six and four
V100 NVIDIA GPUs per node respectively providing a theoretical
peak performance of 750 and 500 Tops/s (operations in mixed
precision) per node [1]. However, it is still unclear how codes for
magnetospheric modeling could take advantage of new heterogeneous
architectures. Widely-used massively parallel codes for magnetospheric
modeling are not yet capable of exploiting these new systems and
need to be redesigned. Two main aspects have to be considered: first,
the algorithms for magnetospheric modeling have to be reformulated to
use dense matrix operations; second, new algorithms have to cope with
low-precision calculations, still retaining acceptable accuracy. In
this talk, we review emerging heterogeneous architectures and present
our work in designing and developing new algorithmic changes in iPIC3D
[2], an implicit PIC code for magnetospheric modeling, to fully exploit
heterogeneous hardware. <P /><SUB>This work has received funding from
the European Commission H2020 program, Grant Agreement No. 801039
(EPiGRAM-HS, epigram-hs.eu)</SUB> <P />[1] Markidis, Stefano, Steven
Wei Der Chien, Erwin Laure, Ivy Bo Peng, and Jeffrey S. Vetter. "NVIDIA
tensor core programmability, performance & precision." In 2018 IEEE
International Parallel and Distributed Processing Symposium Workshops
(IPDPSW), pp. 522-531. IEEE, 2018. <P />[2] Markidis, Stefano, Giovanni
Lapenta, and Rizwan-uddin. "Multi-scale simulations of plasma with
iPIC3D." Mathematics and Computers in Simulation 80, no. 7 (2010):
1509-1519.
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Title: SOTE: A Nonlinear Method for Magnetic Topology Reconstruction
in Space Plasmas
Authors: Liu, Y. Y.; Fu, H. S.; Olshevsky, V.; Pontin, D. I.; Liu,
C. M.; Wang, Z.; Chen, G.; Dai, L.; Retino, A.
2019ApJS..244...31L Altcode:
Complex magnetic structures are ubiquitous in turbulent astrophysical
plasmas. Such structures can be host to many dynamic processes,
such as magnetic reconnection and energy dissipation. Thus,
revealing the 3D topologies of these structures is necessary. In
this study, we propose a new method to reconstruct complex magnetic
topologies in quasi-steady space plasmas, by utilizing eight-point
measurements of magnetic fields and particles. Such a method, based
on the Second-Order Taylor Expansion (SOTE) of a magnetic field, is
nonlinear; it is constrained by {{\nabla }}\cdot {\boldsymbol{B}}=0 and
{{\nabla }}× {\boldsymbol{B}}={μ }<SUB>0</SUB>{\boldsymbol{J}}, where
{\boldsymbol{J}}={ne}({{\boldsymbol{V}}}<SUB>{\boldsymbol{i</SUB>}}-{{\boldsymbol{V}}}<SUB>{\boldsymbol{e</SUB>}})
is from particle moments. A benchmark test of this method,
using the simulation data, shows that the method can give accurate
reconstruction results within an area about three times the size of a
spacecraft tetrahedron. By comparing to the previous First-Order Taylor
Expansion (FOTE) method, this method (SOTE) gives similar results for
reconstructing quasilinear structures but exhibits better accuracy in
reconstructing nonlinear structures. Such a method will be useful to
the multi-scale missions, such as the future European Space Agency's
“cross-scale” mission and China's “self-adaptive” mission. Also,
it can be applied to four-point missions, such as Cluster and the
Magnetospheric Multiscale Mission. We demonstrated how to apply this
method to the four-point missions. In principle, this method will
be useful to study shocks, magnetic holes, dipolarization fronts,
and other nonlinear structures in space plasmas.
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Title: Generation of Turbulence in Colliding Reconnection Jets
Authors: Pucci, Francesco; Matthaeus, William H.; Chasapis, A.;
Servidio, Sergio; Sorriso-Valvo, L.; Olshevsky, V.; Newman, D. L.;
Goldman, M. V.; Lapenta, Giovanni
2018ApJ...867...10P Altcode: 2018arXiv181013318P
The collision of magnetic reconnection jets is studied by means of a
three-dimensional numerical simulation at the kinetic scale, in the
presence of a strong guide field. We show that turbulence develops
due to the collision of jets, producing several current sheets in
reconnection outflows, aligned with the guide field direction. The
turbulence is mainly two-dimensional, with stronger gradients in the
plane perpendicular to the guide field and low wave-like activity
in the parallel direction. First, we provide a numerical method
to isolate the central turbulent region. Second, we analyze the
spatial second-order structure function and prove that turbulence
is confined in this region. Finally, we compute local magnetic and
electric frequency spectra, finding a trend in the subion range that
differs from typical cases for which the Taylor hypothesis is valid,
as well as wave activity in the range between ion and electron cyclotron
frequencies. Our results are relevant to understand observed collisions
of reconnection jets in space plasmas.
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Title: Properties of turbulence in the reconnection exhaust: numerical
simulations compared with observations
Authors: Pucci, F.; Servidio, S.; Sorriso-Valvo, L.; Olshevsky,
V.; Matthaeus, W. H.; Malara, F.; Goldman, M. V.; Newman, D. L.;
Lapenta, G.
2018arXiv181100005P Altcode:
The properties of the turbulence which develops in the outflows of
magnetic reconnection have been investigated using self-consistent
plasma simulations, in three dimensions. As commonly observed
in space plasmas, magnetic reconnection is characterized by the
presence of turbulence. Here we provide a direct comparison of our
simulations with reported observations of reconnection events in the
magnetotail investigating the properties of the electromagnetic field
and the energy conversion mechanisms. In particular, simulations show
the development of a turbulent cascade consistent with spacecraft
observations, statistics of the the dissipation mechanisms in the
turbulent outflows similar to the one observed in reconnection jets
in the magnetotail, and that the properties of turbulence vary as a
function of the distance from the reconnecting X-line.
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Title: Properties of Decaying Plasma Turbulence at Subproton Scales
Authors: Olshevsky, Vyacheslav; Servidio, Sergio; Pucci, Francesco;
Primavera, Leonardo; Lapenta, Giovanni
2018ApJ...860...11O Altcode:
We study the properties of plasma turbulence at subproton scales using
kinetic electromagnetic three-dimensional simulations with nonidentical
initial conditions. Particle-in-cell modeling of the Taylor-Green
vortex has been performed, starting from three different magnetic field
configurations. All simulations expose very similar energy evolution in
which the large-scale ion flows and magnetic structures deteriorate and
transfer their energy into particle heating. Heating is more intense
for electrons, decreasing the initial temperature ratio and leading to
temperature equipartition between the two species. A full turbulent
cascade, with a well-defined power-law shape at subproton scales, is
established within a characteristic turnover time. Spectral indices for
magnetic field fluctuations in two simulations are close to α <SUB>
B </SUB> ≈ 2.9, and are steeper in the remaining case with α <SUB>
B </SUB> ≈ 3.05. Energy is dissipated by a complex mixture of plasma
instabilities and magnetic reconnection and is milder in the latter
simulation. The number of magnetic nulls, and the dissipation pattern
observed in this case, differ from two others. Spectral indices for
the kinetic energy deviate from magnetic spectra by ≈1 in the first
simulation, and by ≈0.75 in two other runs. The difference between
magnetic and electric slopes confirm the previously observed value of
α <SUB> B </SUB> - α <SUB> E </SUB> ≈ 2.
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Title: A fully kinetic perspective of electron acceleration around
a weakly outgassing comet: Ohm's law
Authors: Deca, Jan; Divin, Andrey; Henri, Pierre; Eriksson, Anders;
Olshevsky, Vyacheslav; Markidis, Stefano; Horányi, Mihály
2018EGUGA..20.6571D Altcode:
When a comet is sufficiently close to the Sun, the sublimation
of ice leads to outgassing and the formation of a coma of gas and
dust. Ionisation of the outgassing material then results in mass-loading
of the solar wind and magnetic field draping around the cometary
nucleus. Here we present three-dimensional fully kinetic simulations
of the solar wind interaction with comet 67P/Churyumov- Gerasimenko
at a low-activity regime, before collisions have any impact on the
plasma dynamics. The interaction scales are well below the relevant ion
gyroradii. Non-equilibrium electron distributions develop. To first
order, the dynamical interaction is representative of a four-fluid
coupled system [Deca et al., PRL 2017, Divin et al., submitted]. Our
approach is self-consistent and allows to distill Ohm's law directly
from the electron dynamics in the simulation, rather than imposing
it beforehand. In the vicinity of the cometary nucleus, the balance
changes between the different terms in the equation. Deciphering the
relative importance of each term allows to identify the driving physics
in the various regions of the cometary plasma environment. For example,
we find that close to the outgassing nucleus the electron pressure
gradient dominates; that at sub-ion scales, the total electric field
is a superposition of the solar wind convective electric field, where
electrons are frozen-in, and the ambipolar electric field; that the
latter accelerates electrons parallel to the magnetic field and is
the source of/provides feedback to the electron pressure gradient that
balances the Ohm's law perpendicular to the magnetic field, and that
the role of electron inertia is negligible to balance the electric
field. In conclusion, Ohm's law shows us what happens to all ion and
electron species, and why.
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Title: Numerical simulation of the solar wind-Moon interaction using
3D Particle-in-Cell (PIC) simulations.
Authors: Ahmadi, Tara; Divin, Andrey; Deca, Jan; Lue, Charles;
Olshevsky, Vyacheslav; Markidis, Stefano; Semenov, Vladimir
2018EGUGA..2016798A Altcode:
We present results of three-dimensional Particle-in-Cell (PIC)
simulations of quiet solar wind-Moon interaction using full-particle
electromagnetic implicit code iPIC3D. The Moon is taken as a
passive absorber of the inflowing particles, without intrinsic
magnetic fields or resistivity. We show that (similar to past 1D,
2D PIC and 3D hybrid studies) the large-scale Lunar wake with nearly
zero density is formed, which is bounded by strong rarefaction and
compression waves attached to the Moon. We investigate in detail
velocity distribution functions (VDFs) and ion and electron moments
in the wake, including regions with very small macroparticle count. In
order to reconstruct the distributions in low-density wake, we sample
VDFs using a backward Liouville method by tracing particles back in
time in quasisteady electric and magnetic fields taken from original 3D
PIC simulation. Obtained VDFs display large degree of anisotropy and
nongyrotropy and reveal fine-scale features which can be interpreted
as the Moon's shadow in velocity domain.
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Title: Nonlinear waves and instabilities leading to secondary
reconnection in reconnection outflows
Authors: Lapenta, Giovanni; Pucci, Francesco; Olshevsky, Vyacheslav;
Servidio, Sergio; Sorriso-Valvo, Luca; Newman, David L.; Goldman,
Martin V.
2018JPlPh..84a7103L Altcode: 2018arXiv180808612L
Reconnection outflows have been under intense recent scrutiny, from
in situ observations and from simulations. These regions are host
to a variety of instabilities and intense energy exchanges, often
even superior to the main reconnection site. We report here a number
of results drawn from an investigation of simulations. First, the
outflows are observed to become unstable to drift instabilities. Second,
these instabilities lead to the formation of secondary reconnection
sites. Third, the secondary processes are responsible for large energy
exchanges and particle energization. Finally, the particle distribution
function are modified to become non-Maxwellian and include multiple
interpenetrating populations.
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Title: Simulating the Solar Wind Interaction with Comet
67P/Churyumov-Gerasimenko: Latest Results
Authors: Deca, J.; Divin, A. V.; Henri, P.; Eriksson, A. I.; Markidis,
S.; Olshevsky, V.; Goldstein, R.; Myllys, M. E.; Horanyi, M.
2017AGUFM.P51D2628D Altcode:
First observed in 1969, comet 67P/Churyumov-Gerasimenko was escorted
for almost two years along its 6.45-yr elliptical orbit by ESA's
Rosetta orbiter spacecraft. When a comet is sufficiently close to
the Sun, the sublimation of ice leads to an outgassing atmosphere
and the formation of a coma, and a dust and plasma tail. Comets are
critical to decipher the physics of gas release processes in space. The
latter result in mass-loaded plasmas, which more than three decades
after the Active Magnetospheric Particle Tracer Explorers (AMPTE)
space release experiments are still not fully understood. Using a 3D
fully kinetic approach, we study the solar wind interaction with comet
67P/Churyumov-Gerasimenko, focusing in particular on the ion-electron
dynamics for various outgassing rates. A detailed kinetic treatment of
the electron dynamics is critical to fully capture the complex physics
of mass-loading plasmas and to describe the strongly inhomogeneous
plasma dynamics observed by Rosetta, down to electron kinetic scales.
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Title: Properties of Turbulence in the Reconnection Exhaust: Numerical
Simulations Compared with Observations
Authors: Pucci, F.; Servidio, S.; Sorriso-Valvo, L.; Olshevsky,
V.; Matthaeus, W. H.; Malara, F.; Goldman, M. V.; Newman, D. L.;
Lapenta, G.
2017ApJ...841...60P Altcode:
The properties of the turbulence that develops in the outflows of
magnetic reconnection have been investigated using self-consistent
plasma simulations, in three dimensions. As commonly observed in space
plasmas, magnetic reconnection is characterized by the presence of
turbulence. Here we provide a direct comparison of our simulations
with reported observations of reconnection events in the magnetotail,
investigating the properties of the electromagnetic field and the energy
conversion mechanisms. In particular, simulations show the development
of a turbulent cascade consistent with spacecraft observations,
statistics of the dissipation mechanisms in the turbulent outflows
similar to the ones observed in reconnection jets in the magnetotail,
and that the properties of turbulence vary as a function of the distance
from the reconnecting X-line.
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Title: Electron and Ion Dynamics of the Solar Wind Interaction with
a Weakly Outgassing Comet
Authors: Deca, Jan; Divin, Andrey; Henri, Pierre; Eriksson, Anders;
Markidis, Stefano; Olshevsky, Vyacheslav; Horányi, Mihály
2017PhRvL.118t5101D Altcode:
Using a 3D fully kinetic approach, we disentangle and explain
the ion and electron dynamics of the solar wind interaction with a
weakly outgassing comet. We show that, to first order, the dynamical
interaction is representative of a four-fluid coupled system. We
self-consistently simulate and identify the origin of the warm and
suprathermal electron distributions observed by ESA's Rosetta mission
to comet 67P/Churyumov-Gerasimenko and conclude that a detailed kinetic
treatment of the electron dynamics is critical to fully capture the
complex physics of mass-loading plasmas.
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Title: Three dimensional Particle-in-Cell (PIC) simulations of the
67P environment
Authors: Divin, Andrey; Deca, Jan; Henri, Pierre; Horanyi, Mihaly;
Markidis, Stefano; Lapenta, Giovanni; Olshevsky, Vyacheslav; Eriksson,
Anders
2017EGUGA..19.1556D Altcode:
ESA's Rosetta orbiter spacecraft escorted comet
67P/Churyumov-Gerasimenko for two years, carrying 21 scientific
instruments. Five of those were dedicated to plasma measurements. The
mission revealed for the first time, and in unprecedented detail,
the fascinating evolution of a comet and its interaction with our Sun
as it races along its 6.45yr elliptical orbit around the Sun. Using a
self-consistent 3-D fully kinetic electromagnetic particle-in-cell
approach, we focus on the global cometary environment and, in
particular, on the collisionless electron-kinetic interaction. We
include cometary ions and electrons produced by the ionization of
the outgassing cometary atmosphere in addition to the solar wind ion
and electron plasma flow. We approximate mass-loading of the cold
cometary ion and electron populations using a 1/r relation with
distance to the comet with a total neutral production rate of Q =
1026 s-1. Our simulation results disentangle for the first time
the kinetic ion and electron dynamics of the solar wind interaction
with a weakly outgassing comet. The simulated global structure of
the solar wind-comet interaction confirms the results reported in
hybrid simulations of the induced cometary magnetosphere. Moreover,
we show that cometary and solar wind electrons neutralize the solar
wind protons and cometary ions, respectively, in the region of influence
around the comet, representing to first order a four-fluid behavior. The
electron energy distribution close to the comet is shown to be a mix
of cometary and solar wind electrons that appear as, respectively,
a thermal and a suprathermal components. Analyzing ion and electron
energy distribution functions, and comparing with plasma measurements
from ESA's Rosetta mission to comet 67P/Churyumov-Gerasimenko, we
conclude that a detailed kinetic treatment of the electron dynamics is
critical to fully capture the complex physics of mass-loading plasmas.
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Title: The Role of Electron Dynamics in the Solar Wind Interaction
with Comet 67P/Churyumov-Gerasimenko at 3 AU
Authors: Deca, J.; Divin, A.; Henri, P.; Eriksson, A.; Markidis, S.;
Olshevsky, V.; Horányi, M.
2017LPI....48.1315D Altcode:
Using a self-consistent 3D full-kinetic PIC approach, we disentangle
the ion and electron dynamics of the solar wind interaction with a
weakly outgassing comet.
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Title: Intermittent energy dissipation by turbulent reconnection
Authors: Fu, H. S.; Vaivads, A.; Khotyaintsev, Y. V.; André, M.;
Cao, J. B.; Olshevsky, V.; Eastwood, J. P.; Retinò, A.
2017GeoRL..44...37F Altcode:
Magnetic reconnection—the process responsible for many explosive
phenomena in both nature and laboratory—is efficient at dissipating
magnetic energy into particle energy. To date, exactly how this
dissipation happens remains unclear, owing to the scarcity of multipoint
measurements of the "diffusion region" at the sub-ion scale. Here we
report such a measurement by Cluster—four spacecraft with separation
of 1/5 ion scale. We discover numerous current filaments and magnetic
nulls inside the diffusion region of magnetic reconnection, with
the strongest currents appearing at spiral nulls (O-lines) and the
separatrices. Inside each current filament, kinetic-scale turbulence
is significantly increased and the energy dissipation, E' ṡ j, is
100 times larger than the typical value. At the jet reversal point,
where radial nulls (X-lines) are detected, the current, turbulence,
and energy dissipations are surprisingly small. All these features
clearly demonstrate that energy dissipation in magnetic reconnection
occurs at O-lines but not X-lines.
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Title: A new model for the electron pressure nongyrotropy in the
outer electron diffusion region
Authors: Divin, A.; Semenov, V.; Korovinskiy, D.; Markidis, S.; Deca,
J.; Olshevsky, V.; Lapenta, G.
2016GeoRL..4310565D Altcode:
We present a new model to describe the electron pressure nongyrotropy
inside the electron diffusion region (EDR) in an antiparallel magnetic
reconnection scenario. A combination of particle-in-cell simulations
and analytical estimates is used to identify such a component of the
electron pressure tensor in the rotated coordinates, which is nearly
invariant along the outflow direction between the X line and the
electron remagnetization points in the outer EDR. It is shown that
the EDR two-scale structure (inner and outer parts) is formed due to
superposition of the nongyrotropic meandering electron population and
gyrotropic electron population with large anisotropy parallel to the
magnetic field upstream of the EDR. Inside the inner EDR the influence
of the pressure anisotropy can largely be ignored. In the outer EDR,
a thin electron layer with electron flow speed exceeding the E ×
B drift velocity is supported by large-momentum flux produced by
the electron pressure anisotropy upstream of the EDR. We find that
this fast electron exhaust flow with |V<SUB>e</SUB>×B|>|E| is in
fact a constituent part of the EDR, a finding which will steer the
interpretation of the Magnetospheric Multiscale Mission (MMS) data.
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Title: Magnetic nulls in three-dimensional kinetic simulations of
space plasmas
Authors: Olshevsky, Vyacheslav; Deca, Jan; Divin, Andrey; Peng, Ivy
Bo; Markidis, Stefano; Innocenti, Maria Elena; Cazzola, Emanuele;
Lapenta, Giovanni
2016EGUGA..18.4053O Altcode:
We present a survey of magnetic nulls and associated energy dissipation
in different three-dimensional kinetic particle-in-cell simulations of
space plasmas. The configurations under study include: a traditional
Harris current sheet and current sheets with asymmetric density
distribution, dipolar and quadrupolar planetary magnetospheres,
lunar magnetic anomalies, and decaying turbulence. Nulls are
detected in the simulation snapshots by the topological degree
method. In all runs except the quadrupolar magnetospere the dominating
majority of nulls are of spiral topological type. When supported by
strong currents, these nulls indicate the regions of strong energy
dissipation. Dissipation, often accompanied by the changes in magnetic
topology, is caused by plasma instabilities in the current channels
or on their interfaces. Radial nulls show less activity, they can be
created or destroyed in pairs, via topological bifurcations. Although
such events demonstrate energy release, they are rather rare and
short-living. An important implication of our study to observations
is that magnetic topology should not be considered independently of
other plasma properties such as currents.
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Title: Magnetic Null Points in Kinetic Simulations of Space Plasmas
Authors: Olshevsky, Vyacheslav; Deca, Jan; Divin, Andrey; Peng, Ivy
Bo; Markidis, Stefano; Innocenti, Maria Elena; Cazzola, Emanuele;
Lapenta, Giovanni
2016ApJ...819...52O Altcode: 2015arXiv151202018O
We present a systematic attempt to study magnetic null points and the
associated magnetic energy conversion in kinetic particle-in-cell
simulations of various plasma configurations. We address
three-dimensional simulations performed with the semi-implicit kinetic
electromagnetic code iPic3D in different setups: variations of a Harris
current sheet, dipolar and quadrupolar magnetospheres interacting with
the solar wind, and a relaxing turbulent configuration with multiple
null points. Spiral nulls are more likely created in space plasmas: in
all our simulations except lunar magnetic anomaly (LMA) and quadrupolar
mini-magnetosphere the number of spiral nulls prevails over the number
of radial nulls by a factor of 3-9. We show that often magnetic nulls
do not indicate the regions of intensive energy dissipation. Energy
dissipation events caused by topological bifurcations at radial nulls
are rather rare and short-lived. The so-called X-lines formed by the
radial nulls in the Harris current sheet and LMA simulations are rather
stable and do not exhibit any energy dissipation. Energy dissipation
is more powerful in the vicinity of spiral nulls enclosed by magnetic
flux ropes with strong currents at their axes (their cross sections
resemble 2D magnetic islands). These null lines reminiscent of Z-pinches
efficiently dissipate magnetic energy due to secondary instabilities
such as the two-stream or kinking instability, accompanied by changes
in magnetic topology. Current enhancements accompanied by spiral nulls
may signal magnetic energy conversion sites in the observational data.
---------------------------------------------------------
Title: How to Find Magnetic Nulls and Reconstruct Field Topology
with MMS Data?
Authors: Fu, H.; Vaivads, A.; Khotyaintsev, Y. V.; Olshevsky, V.;
Andre, M.; Cao, J.; Huang, S.; Retino, A.; Lapenta, G.
2015AGUFMSM51A2517F Altcode:
In this study, we apply a new method—the first-order Taylor expansion
(FOTE)—to find magnetic nulls and reconstruct magnetic field topology,
in order to use it with the data from the forth-coming MMS mission. We
compare this method with the previously used Poincare index (PI),
and find that they are generally consistent, except that the PI method
can only find a null inside the spacecraft (SC) tetrahedron, while the
FOTE method can find a null both inside and outside the tetrahedron and
also deduce its drift velocity. In addition, the FOTE method can (1)
avoid limitations of the PI method such as data resolution, instrument
uncertainty (Bz offset), and SC separation; (2) identify 3D null types
(A, B, As, and Bs) and determine whether these types can degenerate
into 2D (X and O); (3) reconstruct the magnetic field topology. We
quantitively test the accuracy of FOTE in positioning magnetic nulls
and reconstructing field topology, by using the data from 3D kinetic
simulations. The influences of SC separation (0.05~1 di) and null-SC
distance (0~1 di) on the accuracy are both considered. We find
that: (1) for an isolated null, the method is accurate when the SC
separation is smaller than 1 di, and the null-SC distance is smaller
than 0.25~0.5 di; (2) for a null pair, the accuracy is same as in
the isolated-null situation, except at the separator line, where the
field is nonlinear. We define a parameter in terms of the eigenvalues
of the null to quantify the quality of our method—the smaller this
parameter the better the results. Comparing to the previously used one,
this parameter is more relevant for null identification. Using the new
method, we reconstruct the magnetic field topology around a radial-type
null and a spiral-type null, and find that the topologies are well
consistent with those predicted in theory. We therefore suggest using
this method to find magnetic nulls and reconstruct field topology with
four-point measurements, particularly from Cluster and the forth-coming
MMS mission. For the MMS mission, this null-finding algorithm can be
used to trigger its burst-mode measurements.
---------------------------------------------------------
Title: Null Points in Three-Dimensional Kinetic Simulations of
Magnetic Reconnection
Authors: Deca, J.; Olshevsky, V.; Divin, A. V.; Innocenti, M. E.;
Cazzola, E.; Peng, B.; Markidis, S.; Ormvråk, M.; Lapenta, G.
2015AGUFMSH43A2420D Altcode:
Kinetic particle-in-cell simulations are the primary tool for studying
magnetic reconnection in space plasmas. Magnetic null points are
believed to be the preferred locations in space where magnetic
reconnection is luckily to happen, and are in the focus of interest
of space missions such as Cluster and MMS. Simulations of magnetic
reconnection in various configurations performed with the implicit
particle-in-cell code iPic3D revealed that nulls are ubiquitious
in these models. We apply the Poincare index technique to locate
and identify the topological characteristics of the magnetic null
points in different three-dimensional simulations. We investigate
the relevance of magnetic nulls to energy dissipation, turbulence and
plasma instabilities. In particular, we found out that magnetic nulls
of spiral type associated with magnetic islands and flux ropes play
more important role in the energy release than the radial nulls. This
finding is in accordance with some recent MHD simulations and in situ
observations of Cluster spacecraft.
---------------------------------------------------------
Title: Where should MMS look for the electron and ion diffusion
regions?
Authors: Lapenta, G.; Goldman, M. V.; Newman, D. L.; Olshevsky, V.
2015AGUFMSH54A..06L Altcode:
Our message is that if we think of reconnection with the usual cartoon,
the MMS mission should follow the advice of Indiana Jones: X never
marks the spot. Based on 3D fully kinetic simulations started with a
well defined x-line, we observe that reconnection transitions towards a
more chaotic regime. Two fronts develop downstream of the x-line where
the outflow meets the pre-existing plasma. In the fronts an instability
develops caused by the local gradients of the density. The consequence
is the break up of the fronts in a fashion similar to the classical
fluid Rayleigh-Taylor instability with the formation of "fingers" of
plasma and embedded magnetic fields. These fingers interact and produce
secondary reconnection sites. We present several different diagnostics
that prove the existence of these secondary reconnection sites. Each
site is surrounded by its own electron diffusion region.At the fronts
the ions are generally not magnetized and considerable ion slippage is
present. The discovery we present is that electrons are also slipping,
forming localized diffusion regions near secondary reconnection sites
[1].The consequence of this discovery is twofold. First, the instability
in the fronts has strong energetic implications. We observe that the
energy transfer locally is very strong, an order of magnitude stronger
than in the "X" line. However, this energy transfer is of both signs
as it is natural for a wavy rippling with regions of magnetic to
kinetic and regions of kinetic to magnetic energy conversion.Second,
and most important for this session, is that MMS should not limit the
search for electron diffusion regions to the location marked with X
in all reconnection cartoons. Our simulations predict more numerous
and perhaps more easily measurable electron diffusion regions in the
fronts. [1] Lapenta, G et al., Nature Physics 11, 690-695 (2015)
---------------------------------------------------------
Title: Energy Dissipation in Magnetic Null Points at Kinetic Scales
Authors: Olshevsky, Vyacheslav; Divin, Andrey; Eriksson, Elin;
Markidis, Stefano; Lapenta, Giovanni
2015ApJ...807..155O Altcode: 2015arXiv150907961O
We use kinetic particle-in-cell and MHD simulations supported by an
observational data set to investigate magnetic reconnection in clusters
of null points in space plasma. The magnetic configuration under
investigation is driven by fast adiabatic flux rope compression that
dissipates almost half of the initial magnetic field energy. In this
phase powerful currents are excited producing secondary instabilities,
and the system is brought into a state of “intermittent turbulence”
within a few ion gyro-periods. Reconnection events are distributed
all over the simulation domain and energy dissipation is rather
volume-filling. Numerous spiral null points interconnected via
their spines form null lines embedded into magnetic flux ropes;
null point pairs demonstrate the signatures of torsional spine
reconnection. However, energy dissipation mainly happens in the
shear layers formed by adjacent flux ropes with oppositely directed
currents. In these regions radial null pairs are spontaneously emerging
and vanishing, associated with electron streams and small-scale current
sheets. The number of spiral nulls in the simulation outweighs the
number of radial nulls by a factor of 5-10, in accordance with Cluster
observations in the Earth's magnetosheath. Twisted magnetic fields
with embedded spiral null points might indicate the regions of major
energy dissipation for future space missions such as the Magnetospheric
Multiscale Mission.
---------------------------------------------------------
Title: How to find magnetic nulls and reconstruct field topology
with MMS data?
Authors: Fu, H. S.; Vaivads, A.; Khotyaintsev, Y. V.; Olshevsky, V.;
André, M.; Cao, J. B.; Huang, S. Y.; Retinò, A.; Lapenta, G.
2015JGRA..120.3758F Altcode:
In this study, we apply a new method—the first-order Taylor expansion
(FOTE)—to find magnetic nulls and reconstruct magnetic field topology,
in order to use it with the data from the forthcoming MMS mission. We
compare this method with the previously used Poincare index (PI),
and find that they are generally consistent, except that the PI method
can only find a null inside the spacecraft (SC) tetrahedron, while the
FOTE method can find a null both inside and outside the tetrahedron and
also deduce its drift velocity. In addition, the FOTE method can (1)
avoid limitations of the PI method such as data resolution, instrument
uncertainty (Bz offset), and SC separation; (2) identify 3-D null types
(A, B, As, and Bs) and determine whether these types can degenerate
into 2-D (X and O); (3) reconstruct the magnetic field topology. We
quantitatively test the accuracy of FOTE in positioning magnetic nulls
and reconstructing field topology by using the data from 3-D kinetic
simulations. The influences of SC separation (0.05~1 d<SUB>i</SUB>)
and null-SC distance (0~1 d<SUB>i</SUB>) on the accuracy are both
considered. We find that (1) for an isolated null, the method is
accurate when the SC separation is smaller than 1 d<SUB>i</SUB>, and
the null-SC distance is smaller than 0.25~0.5 d<SUB>i</SUB>; (2) for
a null pair, the accuracy is same as in the isolated-null situation,
except at the separator line, where the field is nonlinear. We define
a parameter ξ ≡ |( λ<SUB>1</SUB> + λ<SUB>2</SUB> + λ<SUB>3</SUB>
)|/|λ|<SUB>max</SUB> in terms of the eigenvalues (λ<SUB>i</SUB>)
of the null to quantify the quality of our method—the smaller this
parameter the better the results. Comparing to the previously used
parameter (η≡|∇ ṡ B|/|∇ × B|), ξ is more relevant for null
identification. Using the new method, we reconstruct the magnetic
field topology around a radial-type null and a spiral-type null, and
find that the topologies are well consistent with those predicted
in theory. We therefore suggest using this method to find magnetic
nulls and reconstruct field topology with four-point measurements,
particularly from Cluster and the forthcoming MMS mission. For the
MMS mission, this null-finding algorithm can be used to trigger its
burst-mode measurements.
---------------------------------------------------------
Title: Oscillatory patterns in three-dimensional kinetic simulations
of space plasma
Authors: Olshevsky, Vyacheslav; Deca, Jan; Divin, Andrey; Lapenta,
Giovanni; Markidis, Stefano
2015EGUGA..17.9026O Altcode:
We analyse kinetic simulations of the relaxation of a magnetic field
configuration with multiple null-points. The power spectral density
of the magnetic field is dissipative and exhibits two breaks: at
ion-inertial and at electron-gyration scales; the slopes are steeper
than observed in solar wind. Although different simulations in the same
configuration show similar energetics, the local evolution pattern is
rather chaotic. Most of the null-points in the simulations are of the
spiral type, they are surrounded by twisted field lines, and powerful
currents establish through them forming Z-pinches. Various instabilities
are associated with the current channels, especially prominent is the
kinking which drives secondary magnetic reconnection that dissipates
the magnetic energy. In some regions the current channels produce
thin secondary threads that show lower hybrid drift-like oscillatory
characteristics. Oscillatory patterns are also detected at the halo
boundary above dipolar lunar anomalies in 3-D kinetic simulations. It
is found that they are (at least partially) in relation to the position
of the B=0 line across the halo formed due to the opposing directions
of the dipolar and interplanetary magnetic field in the simulation
set-up, as well is to the strength of both fields and the solar wind
parameters. We investigate and compare the detailed characteristics
of small-scale wave patterns in both 3D simulations of null points
and lunar magnetic anomalies.
---------------------------------------------------------
Title: Role of Z-pinches in magnetic reconnection in space plasmas
Authors: Olshevsky, Vyacheslav; Lapenta, Giovanni; Markidis, Stefano;
Divin, Andrey
2015JPlPh..81a3205O Altcode: 2015arXiv150907962O
A widely accepted scenario of magnetic reconnection in collisionless
space plasmas is the breakage of magnetic field lines in X-points. In
laboratory, reconnection is commonly studied in pinches, current
channels embedded into twisted magnetic fields. No model of magnetic
reconnection in space plasmas considers both null-points and pinches
as peers. We have performed a particle-in-cell simulation of magnetic
reconnection in a three-dimensional configuration where null-points
are present initially, and Z-pinches are formed during the simulation
along the lines of spiral null-points. The non-spiral null-points are
more stable than spiral ones, and no substantial energy dissipation is
associated with them. On the contrary, turbulent magnetic reconnection
in the pinches causes the magnetic energy to decay at a rate of ~1.5%
per ion gyro period. Dissipation in similar structures is a likely
scenario in space plasmas with large fraction of spiral null-points.
---------------------------------------------------------
Title: Turbulent convection in the Sun: modeling in unstructured
meshes
Authors: Olshevsky, Vyacheslav; Liang, Chunlei; Ham, Frank
2014arXiv1412.7318O Altcode:
We adopted an unstructured hydrodynamical solver CharLES to the problem
of global convection in the Sun. With the aim to investigate the
properties of solar turbulent convection and reproduce differential
rotation pattern. We performed simulations in two spherical shells,
with 1.3 and 10 million cells. In the first, coarse mesh, the solution
does not reproduce realistic convection, and is dominated by numerical
effects. In the second mesh, thermal conduction leads to cooling of
bottom layers, that could not be compensated by solar irradiance. More
simulations in the 10M cells mesh should be performed to investigate the
influence of transport coefficients and numerical effects. Our estimate
of the code performance suggests, that realistic simulations in even
finer grids could be performed for reasonable computational cost.
---------------------------------------------------------
Title: Role of Z-pinches in magnetic reconnection in space plasmas
Authors: Olshevsky, Vyacheslav; Lapenta, Giovanni; Markidis, Stefano;
Divin, Andrey
2014AAS...22440902O Altcode:
A generally accepted scenario of magnetic reconnection in space plasmas
is the breakage of magnetic field lines in X-points. In laboratory,
reconnection is widely studied in pinches, current channels embedded
into twisted magnetic fields. No model of magnetic reconnection in
space plasmas considers both null-points and pinches as peers. We have
performed a particle-in-cell simulation of magnetic reconnection in a
three-dimensional configuration where null-points are present initially,
and Z-pinches are formed during the simulation. The X-points are
relatively stable, and no substantial energy dissipation is associated
with them. On the contrary, turbulent magnetic reconnection driven by
kinking of the pinches causes the magnetic energy to decay at a rate
of 1.5% per ion gyro period. Current channels and twisted magnetic
fields are ubiquitous in turbulent space plasmas, so pinches can be
responsible for the observed high magnetic reconnection rates.
---------------------------------------------------------
Title: Influence of pinches on magnetic reconnection in turbulent
space plasmas
Authors: Olshevsky, Vyacheslav; Lapenta, Giovanni; Markidis, Stefano;
Divin, Andrey
2014cosp...40E2370O Altcode:
A generally accepted scenario of magnetic reconnection in space plasmas
is the breakage of magnetic field lines in X-points. In laboratory,
reconnection is widely studied in pinches, current channels embedded
into twisted magnetic fields. No model of magnetic reconnection in
space plasmas considers both null-points and pinches as peers. We have
performed a particle-in-cell simulation of magnetic reconnection in a
three-dimensional configuration where null-points are present nitially,
and Z-pinches are formed during the simulation. The X-points are
relatively stable, and no substantial energy dissipation is associated
with them. On contrary, turbulent magnetic reconnection in the pinches
causes the magnetic energy to decay at a rate of approximately 1.5
percent per ion gyro period. Current channels and twisted magnetic
fields are ubiquitous in turbulent space plasmas, so pinches can be
responsible for the observed high magnetic reconnection rates.
---------------------------------------------------------
Title: 2D collisionless magnetic reconnection: background density
dependence.
Authors: Divin, Andrey; Lapenta, Giovanni; Markidis, Stefano; André,
Mats; Khotyaintsev, Yuri; Olshevsky, Vyacheslav; Vaivads, Andris
2014cosp...40E.715D Altcode:
Even the simplest 2D configuration susceptible to magnetic reconnection
(namely, Harris current sheet), possesses a number of "free parameters"
that determine the dynamics and energetics of the process. Among
such parameters are T_i/T_e ratio, guide field value, current sheet
thickness, etc. In this report we systematically study the effect of
changing the background density (from n_b/n_0=0.5 to n_b/n_0=0.003),
which covers the range of lobe density values observed in the Earth's
magnetotail. We performed two-dimensional Particle-in-Cell (PIC)
simulations using implicit parallel code iPIC3D with double-periodic
configuration. Increase of the jet front magnetic field (B_z) with n_b
decrease is in agreement with 2D simulations reported previously. The
elevated B_z region (magnetic field component normal to the current
sheet) is several times larger than the initial current layer
thickness, whereas large normal electric field (E_x) area is focused
between the front and current sheet edge. Normal electric field has
a bipolar profile (in the X direction), intensity scales roughly as
(n_b/n_0)(-1/2) with changing n_b. In the low density case intense
waves are generated near magnetic reconnection separatrices, what can
be explained either by the separatrix electron flow disruption or by
electron holes propagation.
---------------------------------------------------------
Title: Energetics of Kinetic Reconnection in a Three-Dimensional
Null-Point Cluster
Authors: Olshevsky, V.; Lapenta, G.; Markidis, S.
2013PhRvL.111d5002O Altcode: 2015arXiv150907969O
We perform three-dimensional particle-in-cell simulations of magnetic
reconnection with multiple magnetic null points. Magnetic field energy
conversion into kinetic energy is about five times higher than in
traditional Harris sheet configuration. More than 85% of initial
magnetic field energy is transferred to particle energy during 25
reversed ion cyclofrequencies. Magnetic reconnection in the cluster of
null points evolves in three phases. During the first phase, ion beams
are excited, then give part of their energy back to the magnetic field
in the second phase. In the third phase, magnetic reconnection occurs
in many small patches around the current channels formed along the
stripes of a low magnetic field. Magnetic reconnection in null points
essentially presents three-dimensional features, with no two-dimensional
symmetries or current sheets.
---------------------------------------------------------
Title: SWIFF: Space weather integrated forecasting framework
Authors: Lapenta, Giovanni; Pierrard, Viviane; Keppens, Rony; Markidis,
Stefano; Poedts, Stefaan; Šebek, Ondřej; Trávníček, Pavel M.;
Henri, Pierre; Califano, Francesco; Pegoraro, Francesco; Faganello,
Matteo; Olshevsky, Vyacheslav; Restante, Anna Lisa; Nordlund, Åke;
Trier Frederiksen, Jacob; Mackay, Duncan H.; Parnell, Clare E.;
Bemporad, Alessandro; Susino, Roberto; Borremans, Kris
2013JSWSC...3A..05L Altcode:
SWIFF is a project funded by the Seventh Framework Programme of the
European Commission to study the mathematical-physics models that
form the basis for space weather forecasting. The phenomena of space
weather span a tremendous scale of densities and temperature with
scales ranging 10 orders of magnitude in space and time. Additionally
even in local regions there are concurrent processes developing at
the electron, ion and global scales strongly interacting with each
other. The fundamental challenge in modelling space weather is the
need to address multiple physics and multiple scales. Here we present
our approach to take existing expertise in fluid and kinetic models to
produce an integrated mathematical approach and software infrastructure
that allows fluid and kinetic processes to be modelled together. SWIFF
aims also at using this new infrastructure to model specific coupled
processes at the Solar Corona, in the interplanetary space and in the
interaction at the Earth magnetosphere.
---------------------------------------------------------
Title: Homologous CME: a multispacecraft approach supported by
simulations
Authors: Sanna, L.; Lapenta, G.; Steed, K.; Olshevsky, V.; Restante, A.
2012AGUFMSH51A2212S Altcode:
Using in situ and remote observations from multiple space crafts
(STEREO, SDO and Venus Express) provides the opportunity to to study
homologous CMEs. For example, on 7 August 2010, a halo CME originating
from NOAA AR11093 was observed remotely by STEREO B. Seven days later
this active region erupted again, and a halo CME was observed remotely
by STEREO A on 14 August 2010. In this and in similar other examples,
we show that multiple eruptions are associated with reverse S-shaped
flux rope structures and display a number of typical large-scale
features related to CMEs, including coronal dimmings and EUV waves. By
combining remote sensing and in situ observations of the ejecta,
we consider the structure and heliospheric evolution of these CMEs
and their interplanetary counterparts. The work is complemented by
a theoretical investigation where observed features are replicated
and clarified by simulation. This work is part of the eHeroes project
(www.eheroes.eu), funded by the European Commission, under the grant
agreement eHeroes (project n° 284461)
---------------------------------------------------------
Title: Kinetic structure of collisionless reconnection: hybrid
simulations
Authors: Šebek, O.; Trávníček, P. M.; Hellinger, P.; Lapenta,
G.; Keppens, R.; Olshevsky, V.; Restante, A. L.; Hendrix, T.
2012EGUGA..14.8382S Altcode:
Magnetic reconnection is a fundamental process observed in various
space plasma systems, such as, for example, interface between planetary
magnetosphere and solar wind at the dayside magnetopause. We study
magnetic reconnection by means of two-dimensional hybrid approach
(kinetic ions and fluid electrons). Our initial configuration consists
of Harris equilibrium layer with small amplitude perturbation of
magnetic field. These perturbations are origins of the formation of
magnetic islands. In this study we focus on the role of ionic kinetic
effects during the reconnection process, we examine the temperature
anisotropy and gyrotropy of the ion velocity distribution functions. We
discuss the importance of these kinetic effects by comparing the results
from hybrid simulations with the results from magneto-hydrodynamic
(MHD) simulations results.
---------------------------------------------------------
Title: Numerical simulation of propagation of the MHD waves in
sunspots
Authors: Parchevsky, K.; Kosovichev, A.; Khomenko, E.; Olshevsky,
V.; Collados, M.
2010HiA....15..354P Altcode:
We present results of numerical 3D simulation of propagation of MHD
waves in sunspots. We used two self consistent magnetohydrostatic
background models of sunspots. There are two main differences
between these models: (i) the topology of the magnetic field and
(ii) dependence of the horizontal profile of the sound speed on
depth. The model with convex shape of the magnetic field lines near
the photosphere has non-zero horizorntal perturbations of the sound
speed up to the depth of 7.5 Mm (deep model). In the model with concave
shape of the magnetic field lines near the photosphere Δ c/c is close
to zero everywhere below 2 Mm (shallow model). Strong Alfven wave is
generated at the wave source location in the deep model. This wave is
almost unnoticeable in the shallow model. Using filtering technique
we separated magnetoacoustic and magnetogravity waves. It is shown,
that inside the sunspot magnetoacoustic and magnetogravity waves
are not spatially separated unlike the case of the horizontally
uniform background model. The sunspot causes anisotropy of the
amplitude distribution along the wavefront and changes the shape
of the wavefront. The amplitude of the waves is reduced inside the
sunspot. This effect is stronger for the magnetogravity waves than for
magnetoacoustic waves. The shape of the wavefront of the magnetogravity
waves is distorted stronger as well. The deep model causes bigger
anisotropy for both mgnetoacoustic and magneto gravity waves than the
shallow model.
---------------------------------------------------------
Title: The Irkutsk Barium filter for narrow-band wide-field
high-resolution solar images at the Dutch Open Telescope
Authors: Hammerschlag, Robert H.; Skomorovsky, Valery I.; Bettonvil,
Felix C. M.; Kushtal, Galina I.; Olshevsky, Vyacheslav L.; Rutten,
Robert J.; Jägers, Aswin P. L.; Sliepen, Guus; Snik, Frans
2010SPIE.7735E..85H Altcode: 2010SPIE.7735E.265H
A wide-field birefringent filter for the barium II line at 455.4nm is
developed in Irkutsk. The Barium line is excellent for Doppler-shift
measurements because of low thermal line-broadening and steep
flanks of the line profile. The filter width is 0.008nm and the
filter is tunable over 0.4nm through the whole line and far enough
in the neighboring regions. A fast tuning system with servomotor is
developed at the Dutch Open Telescope (DOT). Observations are done
in speckle mode with 10 images per second and Keller-VonDerLühe
reconstruction using synchronous images of a nearby bluecontinuum
channel at 450.5nm. Simultaneous observation of several line positions,
typically 3 or 5, are made with this combination of fast tuning and
speckle. All polarizers are birefringent prisms which largely reduced
the light loss compared to polarizing sheets. The advantage of this
filter over Fabry-Perot filters is its wide field due to a large
permitted entrance angle and no need of polishing extremely precise
surfaces. The BaII observations at the DOT occur simultaneously with
those of a fast-tunable birefringent H-alpha filter. This gives the
unique possibility of simultaneous speckle-reconstructed observations
of velocities in photosphere (BaII) and chromosphere (H-alpha).
---------------------------------------------------------
Title: Numerical Simulation of Excitation and Propagation of
Helioseismic MHD Waves in Magnetostatic Models of Sunspots
Authors: Parchevsky, K.; Kosovichev, A.; Khomenko, E.; Olshevsky,
V.; Collados, M.
2010arXiv1002.1117P Altcode:
We present comparison of numerical simulations of propagation of
MHD waves,excited by subphotospheric perturbations, in two different
("deep" and "shallow") magnetostatic models of the sunspots. The "deep"
sunspot model distorts both the shape of the wavefront and its amplitude
stronger than the "shallow" model. For both sunspot models, the surface
gravity waves (f-mode) are affected by the sunspots stronger than
the acoustic p-modes. The wave amplitude inside the sunspot depends
on the photospheric strength of the magnetic field and the distance
of the source from the sunspot axis. For the source located at 9 Mm
from the center of the sunspot, the wave amplitude increases when
the wavefront passes through the central part of the sunspot. For
the source distance of 12 Mm, the wave amplitude inside the sunspot
is always smaller than outside. For the same source distance from
the sunspot center but for the models with different strength of the
magnetic field, the wave amplitude inside the sunspot increases with
the strength of the magnetic field. The simulations show that unlike
the case of the uniform inclined background magnetic field, the p-
and f-mode waves are not spatially separated inside the sunspot where
the magnetic field is strongly non-uniform. These properties have to
be taken into account for interpretation of observations of MHD waves
traveling through sunspot regions.
---------------------------------------------------------
Title: The solar Ba{II} 4554 Å line as a Doppler diagnostic: NLTE
analysis in 3D hydrodynamical model
Authors: Shchukina, N. G.; Olshevsky, V. L.; Khomenko, E. V.
2009A&A...506.1393S Altcode: 2009arXiv0905.0985S
Aims: The aim of this paper is to analyse the validity of the
Dopplergram and λ-meter techniques for the Doppler diagnostics
of solar photospheric velocities using the Ba II 4554 Å line. <BR
/>Methods: Both techniques are evaluated by means of NLTE radiative
transfer calculations of the Ba II 4554 Å line in a three-dimensional
hydrodynamical model of solar convection. We consider the cases of
spatially unsmeared profiles and the profiles smeared to the resolution
of ground-based observations. <BR />Results: We find that: (i)
speckle-reconstructed Dopplergram velocities reproduce the “true”
velocities well at heights around 300 km, except for intergranular lanes
with strong downflows where the velocity can be overestimated; (ii)
the λ-meter velocities give a good representation of the “true”
velocities through the whole photosphere, both under the original and
reduced spatial resolutions. The velocities derived from the inner wing
of smeared Ba II 4554 Å line profiles are more reliable than those for
the outer wing. Only under high spatial resolution does the inner wing
velocities calculated in intergranular regions give an underestimate
(or even a sign reversal) compared with the model velocities; (iii)
NLTE effects should be taken into account in modelling the Ba II 4554
Å line profiles. Such effects are more pronounced in intergranular
regions. <BR />Conclusions: Our analysis supports the opinion that the
Dopplergram technique applied to the Ba II 4554 Å line is a valuable
tool for the Doppler diagnostics of the middle photosphere around
300 km. The λ-meter technique applied to this line gives us a good
opportunity to “trace” the non-thermal motions along the whole
photosphere up to the temperature minimum and lower chromosphere. <P
/>Appendix is only available in electronic form at http://www.aanda.org
---------------------------------------------------------
Title: Theoretical Modeling of Propagation of Magnetoacoustic Waves
in Magnetic Regions Below Sunspots
Authors: Khomenko, E.; Kosovichev, A.; Collados, M.; Parchevsky, K.;
Olshevsky, V.
2009ApJ...694..411K Altcode: 2008arXiv0809.0278K
We use two-dimensional numerical simulations and eikonal approximation
to study properties of magnetohydrodynamic (MHD) waves traveling below
the solar surface through the magnetic structure of sunspots. We
consider a series of magnetostatic models of sunspots of different
magnetic field strengths, from 10 Mm below the photosphere to the
low chromosphere. The purpose of these studies is to quantify the
effect of the magnetic field on local helioseismology measurements
by modeling waves excited by subphotospheric sources. Time-distance
propagation diagrams and wave travel times are calculated for models
of various field strengths and compared to the nonmagnetic case. The
results clearly indicate that the observed time-distance helioseismology
signals in sunspot regions correspond to fast MHD waves. The slow MHD
waves form a distinctly different pattern in the time-distance diagram,
which has not been detected in observations. The numerical results are
in good agreement with the solution in the short-wavelength (eikonal)
approximation, providing its validation. The frequency dependence of
the travel times is in good qualitative agreement with observations.
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Title: Seismology of Sunspots: An Interplay between Temperature and
Magnetic Field Structures
Authors: Olshevsky, V.; Khomenko, E.; Collados, M.
2008ESPM...12..3.2O Altcode:
Using a numerical three-dimensional MHD modelling of magneto-acoustic
wave propagation in a realistic magnetostatic sunspot model we
investigate the influence of the magnetic field on the parameters
measured by local helioseismology. We find that the variations of
temperature as well as the presence of the magnetic field cause
important changes to the wave travel times. Magnetic field speeds up
the waves to considerable amount, while the temperature depression
within a sunspot causes the opposite action. The calculated travel time
differences between the unmagnetized and magnetized atmospheres lie in
the range typically obtained from local helioseismology correlation
analysis. Our numerical results are also in agreement with the
analytical calculations of the travel times applying WKB technique.
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Title: The Ba II [lambda]4554 resonance line and solar granulation
Authors: Olshevsky, V. L.; Shchukina, N. G.
2007msfa.conf..307O Altcode:
We present the results of an investigation of the impact of NLTE effects
and of granulation inhomogeneities on the solar Ba II [lambda]4554 Å
line. Our analysis is based on both the classical one-dimensional (1D)
solar atmosphere models and on the new generation of three-dimensional
(3D) hydrodynamical models. We show that NLTE and 3D effects have to
be taken into account for reliable diagnostics of the solar atmosphere
using this line. We analyse the influence of different parameters on the
line shape. It turns out to be most sensitive to collisional broadening
and barium abundance. Uncertainties in the oscillator strength, micro-
and macroturbulence (in 1D-case) have a secondary importance. We have
derived the barium abundance assuming NLTE. We find ABa = 2.16 in good
agreement with the recent result of Asplund et al. (2005).
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Title: Numerical modeling of MHD wave propagation in sunspots:
a 3D case
Authors: Olshevsky, V.; Khomenko, E.; Collados, M.
2007msfa.conf..347O Altcode:
We present the first results of a 3D numerical modeling of linear MHD
wave propagation in a realistic sunspot model. In our simulations,
a piston located at the base of the photosphere generates waves with
a certain period. The ratio between the acoustic and the Alfven speed,
cS /vA, decreases from much larger than one at the photosphere to much
lower than one in the chromosphere in our simulation domain. Waves
propagate through the region where cS << vA, where mode
transformation is observed. At a somewhat higher region, where cS =
vA, the fast (magnetic) mode reflects back to the photosphere due
to the vertical and horizontal gradients of vA. The slow (acoustic)
mode propagates to the upper layers and increases its velocity
amplitude. Unlike the 2D simulations, the Alfven mode is also generated
by the piston and experiences transformations at the cS = vA layer. The
behaviour of this mode requires further study.
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Title: Line-of-sight velocity distributions of elliptical galaxies
from collisionless mergers
Authors: González-García, A. C.; Balcells, M.; Olshevsky, V. S.
2006MNRAS.372L..78G Altcode: 2006astro.ph..9365G; 2006MNRAS.tmpL..95G
We analyse the skewness of the line-of-sight velocity distributions in
model elliptical galaxies built through collisionless galaxy mergers. We
build the models using large N-body simulations of mergers between
either two spiral or two elliptical galaxies. Our aim is to investigate
whether the observed ranges of skewness coefficient (h<SUB>3</SUB>)
and the rotational support (V/σ), as well as the anticorrelation
between h<SUB>3</SUB> and V, may be reproduced through collisionless
mergers. Previous attempts using N-body simulations failed to reach
V/σ ~ 1-2 and corresponding high h<SUB>3</SUB> values, which suggested
that gas dynamics and ensuing star formation might be needed in order
explain the skewness properties of ellipticals through mergers. Here
we show that high V/σ and high h<SUB>3</SUB> are reproduced in
collisionless spiral-spiral mergers whenever a central bulge allows
the discs to retain some of their original angular momentum during the
merger. We also show that elliptical-elliptical mergers, unless merging
from a high-angular momentum orbit, reproduce the strong skewness
observed in non-rotating, giant, boxy ellipticals. The behaviour of
the h<SUB>3</SUB> coefficient therefore associates rapidly-rotating
discy ellipticals to disc-disc mergers, and associates boxy,
slowly rotating giant ellipticals to elliptical-elliptical mergers,
a framework generally consistent with the expectations of hierarchical
galaxy formation.
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Title: Non-LTE Formation of Ba II Resonance Lines
Authors: Olshevsky, V. L.; Shchukina, N. G.
2006IAUJD...3E..16O Altcode:
We investigate the formation of the resonance lines of ionized Barium
in the Solar atmosphere. We have constructed atomic model of Ba II,
which includes 40 energy levels, 99 bound-bound and 39 bound-free
transitions. Using the numerical solution of radiative transfer equation
together with the statistical equilibrium equations we calculate
emergent profiles of the Ba II 4554 A resonance line under different
conditions. Line profiles are calculated in both standard 1D and 3D
atmospheric models in LTE and non-LTE approximations. We found that
non-LTE effects are very important for this line. Main NLTE mechanisms
are the resonance line scattering and photon losses. In 1D the line
shape is most sensitive to the following parameters: Barium abundance,
collisional strengths and macroturbulent velocity. Sensitivity to the
uncertainties in oscillator strengths is not so high. Taking precise
theoretical values of collisional strengths by Barklem & O'Mara
(1998) we found the best agreement with observations for the abundance
close to the "classical" value of 2.13. In 3D calculated profiles
are in a good agreement with the observations. In 1D the agreement
is worse. Thus it is critical for the modelling of Ba II lines to use
the 3D non-LTE radiative transfer.
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Title: Granular and Intergranular Oscillations from the Observations
of BA II Resonance Line
Authors: Olshevsky, V. L.
2005ESASP.596E..79O Altcode: 2005ccmf.confE..79O
No abstract at ADS
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Title: Ba II 4554A Resonance Line Formation in the Solar Atmosphere
Authors: Olshevsky, V.; Shchukina, N.
2005ysc..conf...71O Altcode: 2005yosc.conf...71O
We investigate non-LTE radiative transfer in the the Ba II 4554 A
line in the Solar atmosphere. This line is particularly suited to
measure non thermal motions in the solar photosphere. It is also used
in Stokes vector spectropolarimetry. NLTE spectral formation modeling
is a necessary step in the formulation of reliable quantitative data
interpretation through numerical inversion. The first phase of the work
includes creation of a reliable model of the Ba II atom. It includes:
energy levels, oscillator strengths for the selected bb-transitions and
bf-transitions probabilities. We also take into account the hyper fine
structure and isotopic splitting. Next step is to calculate departure
coefficients for Ba II through numerical solution of the radiative
transfer equations. When these coefficients are derived it is possible
to apply inversion techniques to spectral observations in this line.
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Title: Temperature oscillations in solar photosphere caused by
propagation of acoustic-gravity waves of small amplitudes
Authors: Olshevsky, V. L.
2004KFNT...20..396O Altcode:
We analysed temperature and velocity oscillations in the solar
photosphere which are caused by propagation of acoustic-gravity
waves. We calculate the oscillations using analytical solution of the
system of HD equations for isothermally stratified one-dimensional
atmosphere model. We investigated how well temperature oscillations in
the photosphere can be described by propagation of the small-amplitude
acoustic-gravity waves and how well the inversion methods can
reproduce small-amplitude temperature oscillations. It is shown that
the calculated velocity oscillations are in good agreement with the
observed ones in most of the photosphere for frequencies of 2.5 to
4.5 mHz. Calculated temperature oscillations are in agreement with
observed oscillations only in the medium photosphere, at heights from
300 to 400 km.
