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Author name code: archontis
ADS astronomy entries on 2022-09-14
author:"Archontis, Vasilis"
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Title: Emergence of magnetic flux in a partially ionized solar
atmosphere
Authors: Chouliaras, Georgios; Archontis, Vasilis; Syntelis, Petros
2022cosp...44.2552C Altcode:
We perform 3-D MHD simulations for a magnetic flux emergence experiment
to investigate whether the inclusion of ionization-recombination in
the equation of state affects the magnetic flux ascend to the solar
atmosphere. We modify the single-fluid MHD equations to include the
presence of neutrals on our environment configuration. We used the
Lare3D code to numerically solve this 3D time-dependent compressible,
resistive MHD equations in cartesian coordinates. Two different models
have been considered, the FIP which assumes a fully ionized plasma
and the PIP which assumes partially ionized plasma. We performed two
different runs with the same beta plasma to be able to observe the
differences in the behaviour of the ascend of each flux tube. The
axis of the PIP flux tube reaches lower altitudes in the convection
zone compared to the axis of the FIP due to the inclusion of the
ionization-recombination to the equation of state. The presence of
neutrals in the high convection zone affects the structure of the
rising magnetic field on the small scale. This different ascended
magnetic field on the photosphere has an impact on the evolution of the
bipolar region leading to polarities with more spherical and compact
shapes than the FIP model and shorter polarity inversion line. Overall
our magnetic flux emergence experiment revealed further the role of
partial ionization to the magnetic flux emergence into the dynamic
solar atmosphere.
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Title: Decoding the Pre-Eruptive Magnetic Field Configurations of
Coronal Mass Ejections
Authors: Patsourakos, S.; Vourlidas, A.; Török, T.; Kliem, B.;
Antiochos, S. K.; Archontis, V.; Aulanier, G.; Cheng, X.; Chintzoglou,
G.; Georgoulis, M. K.; Green, L. M.; Leake, J. E.; Moore, R.; Nindos,
A.; Syntelis, P.; Yardley, S. L.; Yurchyshyn, V.; Zhang, J.
2020SSRv..216..131P Altcode: 2020arXiv201010186P
A clear understanding of the nature of the pre-eruptive magnetic
field configurations of Coronal Mass Ejections (CMEs) is required
for understanding and eventually predicting solar eruptions. Only
two, but seemingly disparate, magnetic configurations are considered
viable; namely, sheared magnetic arcades (SMA) and magnetic flux ropes
(MFR). They can form via three physical mechanisms (flux emergence,
flux cancellation, helicity condensation). Whether the CME culprit
is an SMA or an MFR, however, has been strongly debated for thirty
years. We formed an International Space Science Institute (ISSI) team to
address and resolve this issue and report the outcome here. We review
the status of the field across modeling and observations, identify
the open and closed issues, compile lists of SMA and MFR observables
to be tested against observations and outline research activities
to close the gaps in our current understanding. We propose that the
combination of multi-viewpoint multi-thermal coronal observations
and multi-height vector magnetic field measurements is the optimal
approach for resolving the issue conclusively. We demonstrate the
approach using MHD simulations and synthetic coronal images.
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Title: Eruptions and flaring activity in emerging quadrupolar regions
Authors: Syntelis, P.; Lee, E. J.; Fairbairn, C. W.; Archontis, V.;
Hood, A. W.
2019A&A...630A.134S Altcode: 2019arXiv190901446S
Context. Solar observations suggest that some of the most dynamic
active regions are associated with complex photospheric magnetic
configurations such as quadrupolar regions, and especially those
that have a δ-spot configuration and a strong polarity inversion
line (PIL). <BR /> Aims: We study the formation and eruption
of magnetic flux ropes in quadrupolar regions. <BR /> Methods:
We performed 3D magnetohydrodynamics simulations of the partial
emergence of a highly twisted flux tube from the solar interior into a
non-magnetised stratified atmosphere. We introduced a density deficit
at two places along the length of the subphotospheric flux tube to
emerge as two Ω-shaped loops, forming a quadrupolar region. <BR />
Results: At the photosphere, the emerging flux forms two initially
separated bipoles, which later come in contact, forming a δ-spot
central region. Above the two bipoles, two magnetic lobes expand and
interact through a series of current sheets at the interface between
them. Two recurrent confined eruptions are produced. In both cases,
the reconnection between sheared low-lying field lines forms a flux
rope. The reconnection between the two lobes higher in the atmosphere
forms field lines that retract down and push against the flux rope,
creating a current sheet between them. It also forms field lines
that create a third magnetic lobe between the two emerged lobes,
that later acts as a strapping field. The flux rope eruptions are
triggered when the reconnection between the flux ropes and the field
above the ropes becomes efficient enough to remove the tension of the
overlying field. These reconnection events occur internally in the
quadrupolar system, as the atmosphere is non-magnetised. The flux
rope of the first, weaker, eruption almost fully reconnects with
the overlying field. The flux rope of the second, more energetic,
eruption is confined by the overlying strapping field. During the
second eruption, the flux rope is enhanced in size, flux, and twist,
similar to confined-flare-to-flux-rope observations. Proxies of the
emission reveal the two erupting filaments channels. A flare arcade
is only formed in the second eruption owing to the longer lasting and
more efficient reconnection at the current sheet below the flux rope.
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Title: Particle Acceleration and Heating in Regions of Magnetic
Flux Emergence
Authors: Isliker, H.; Archontis, V.; Vlahos, L.
2019ApJ...882...57I Altcode: 2019arXiv190704296I
The interaction between emerging and pre-existing magnetic fields
in the solar atmosphere can trigger several dynamic phenomena, such
as eruptions and jets. A key element during this interaction is
the formation of large-scale current sheets, and eventually their
fragmentation that leads to the creation of a strongly turbulent
environment. In this paper, we study the kinetic aspects of the
interaction (reconnection) between emerging and ambient magnetic
fields. We show that the statistical properties of the spontaneously
fragmented and fractal electric fields are responsible for the
efficient heating and acceleration of charged particles, which form a
power-law tail at high energies on sub-second timescales. A fraction
of the energized particles escapes from the acceleration volume, with
a super-hot component with a temperature close to 150 MK, and with a
power-law high-energy tail with an index between -2 and -3. We estimate
the transport coefficients in energy space from the dynamics of the
charged particles inside the fragmented and fractal electric fields, and
the solution of a fractional transport equation, as appropriate for a
strongly turbulent plasma, agrees with the test-particle simulations. We
also show that the acceleration mechanism is not related to Fermi
acceleration, and the Fokker-Planck equation is inconsistent and
not adequate as a transport model. Finally, we address the problem
of correlations between spatial transport and transport in energy
space. Our results confirm the observations reported for high-energy
particles (hard X-rays, type III bursts, and solar energetic particles)
during the emission of solar jets.
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Title: Introduction to the physics of solar eruptions and their
space weather impact
Authors: Archontis, Vasilis; Vlahos, Loukas
2019RSPTA.37790152A Altcode: 2019arXiv190508361A
The physical processes, which drive powerful solar eruptions, play an
important role in our understanding of the Sun-Earth connection. In this
Special Issue, we firstly discuss how magnetic fields emerge from the
solar interior to the solar surface, to build up active regions, which
commonly host large-scale coronal disturbances, such as coronal mass
ejections (CMEs). Then, we discuss the physical processes associated
with the driving and triggering of these eruptions, the propagation
of the large-scale magnetic disturbances through interplanetary
space and the interaction of CMEs with Earth's magnetic field. The
acceleration mechanisms for the solar energetic particles related to
explosive phenomena (e.g. flares and/or CMEs) in the solar corona
are also discussed. The main aim of this Issue, therefore, is to
encapsulate the present state-of-the-art in research related to the
genesis of solar eruptions and their space-weather implications. <P
/>This article is part of the theme issue `Solar eruptions and their
space weather impact'.
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Title: The emergence of magnetic flux and its role on the onset of
solar dynamic events
Authors: Archontis, V.; Syntelis, P.
2019RSPTA.37780387A Altcode: 2019arXiv190406274A
A plethora of solar dynamic events, such as the formation of active
regions, the emission of jets and the occurrence of eruptions is often
associated with the emergence of magnetic flux from the interior of
the Sun to the surface and above. Here, we present a short review
on the onset, driving and/or triggering of such events by magnetic
flux emergence. We briefly describe some key observational examples,
theoretical aspects and numerical simulations, towards revealing the
mechanisms that govern solar dynamics and activity related to flux
emergence. We show that the combination of important physical processes
like shearing and reconnection of magnetic fieldlines in emerging
flux regions or at their vicinity can power some of the most dynamic
phenomena in the Sun on various temporal and spatial scales. Based on
previous and recent observational and numerical studies, we highlight
that, in most cases, none of these processes alone can drive and also
trigger explosive phenomena releasing considerable amount of energy
towards the outer solar atmosphere and space, such as flares, jets and
large-scale eruptions (e.g. coronal mass ejections). In addition, one
has to take into account the physical properties of the emerging field
(e.g. strength, amount of flux, relative orientation to neighbouring
and pre-existing magnetic fields, etc.) in order to better understand
the exact role of magnetic flux emergence on the onset of solar dynamic
events. <P />This article is part of the theme issue `Solar eruptions
and their space weather impact'.
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Title: Ellerman bombs and UV bursts: transient events in chromospheric
current sheets
Authors: Hansteen, V.; Ortiz, A.; Archontis, V.; Carlsson, M.; Pereira,
T. M. D.; Bjørgen, J. P.
2019A&A...626A..33H Altcode: 2019arXiv190411524H
Context. Ellerman bombs (EBs), observed in the photospheric wings
of the Hα line, and UV bursts, observed in the transition region Si
IV line, are both brightenings related to flux emergence regions and
specifically to magnetic flux of opposite polarity that meet in the
photosphere. These two reconnection-related phenomena, nominally formed
far apart, occasionally occur in the same location and at the same
time, thus challenging our understanding of reconnection and heating
of the lower solar atmosphere. <BR /> Aims: We consider the formation
of an active region, including long fibrils and hot and dense coronal
plasma. The emergence of a untwisted magnetic flux sheet, injected 2.5
Mm below the photosphere, is studied as it pierces the photosphere
and interacts with the preexisting ambient field. Specifically, we
aim to study whether EBs and UV bursts are generated as a result of
such flux emergence and examine their physical relationship. <BR />
Methods: The Bifrost radiative magnetohydrodynamics code was used
to model flux emerging into a model atmosphere that contained a
fairly strong ambient field, constraining the emerging field to
a limited volume wherein multiple reconnection events occur as
the field breaks through the photosphere and expands into the outer
atmosphere. Synthetic spectra of the different reconnection events were
computed using the 1.5D RH code and the fully 3D MULTI3D code. <BR
/> Results: The formation of UV bursts and EBs at intensities and
with line profiles that are highly reminiscent of observed spectra
are understood to be a result of the reconnection of emerging flux
with itself in a long-lasting current sheet that extends over several
scale heights through the chromosphere. Synthetic spectra in the Hα
and Si IV 139.376 nm lines both show characteristics that are typical
of the observations. These synthetic diagnostics suggest that there
are no compelling reasons to assume that UV bursts occur in the
photosphere. Instead, EBs and UV bursts are occasionally formed at
opposite ends of a long current sheet that resides in an extended bubble
of cool gas. <P />The movie associated to Fig. 3 is available at <A
href="https://www.aanda.org/10.1051/0004-6361/201935376/olm">https://www.aanda.org</A>
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Title: Sheared Magnetic Arcades and the Pre-eruptive Magnetic
Configuration of Coronal Mass Ejections: Diagnostics, Challenges
and Future Observables
Authors: Patsourakos, Spiros; Vourlidas, A.; Anthiochos, S. K.;
Archontis, V.; Aulanier, G.; Cheng, X.; Chintzoglou, G.; Georgoulis,
M. K.; Green, L. M.; Kliem, B.; Leake, J.; Moore, R. L.; Nindos, A.;
Syntelis, P.; Torok, T.; Yardley, S. L.; Yurchyshyn, V.; Zhang, J.
2019shin.confE.194P Altcode:
Our thinking about the pre-eruptive magnetic configuration of Coronal
Mass Ejections has been effectively dichotomized into two opposing
and often fiercely contested views: namely, sheared magnetic arcades
and magnetic flux ropes. Finding a solution to this issue will have
important implications for our understanding of CME initiation. We
first discuss the very value of embarking into the arcade vs. flux rope
dilemma and illustrate the corresponding challenges and difficulties to
address it. Next, we are compiling several observational diagnostics of
pre-eruptive sheared magnetic arcades stemming from theory/modeling,
discuss their merits, and highlight potential ambiguities that could
arise in their interpretation. We finally conclude with a discussion
of possible new observables, in the frame of upcoming or proposed
instrumentation, that could help to circumvent the issues we are
currently facing.
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Title: Recurrent CME-like Eruptions in Emerging Flux
Regions. II. Scaling of Energy and Collision of Successive Eruptions
Authors: Syntelis, P.; Archontis, V.; Tsinganos, K.
2019ApJ...876...61S Altcode: 2019arXiv190403923S
We present results of three-dimensional MHD simulations of recurrent
eruptions in emerging flux regions. The initial numerical setup is
the same as that in the work by Syntelis et al. Here, we perform
a parametric study on the magnetic field strength (B <SUB>0</SUB>)
of the emerging field. The kinetic energy of the produced ejective
eruptions in the emerging flux region ranges from 10<SUP>26</SUP>
to 10<SUP>28</SUP> erg, reaching up to the energies of small coronal
mass ejections. The kinetic and magnetic energies of the eruptions
scale linearly in a logarithmic plot. We find that the eruptions are
triggered earlier for higher B <SUB>0</SUB> and that B <SUB>0</SUB>
is not directly correlated to the frequency of occurrence of the
eruptions. Using large numerical domains, we show the initial stage
of the partial merging of two colliding erupting fields. The partial
merging occurs partly by the reconnection between the field lines of the
following and the leading eruption at the interface between them. We
also find that tether-cutting reconnection of the field lines of the
leading eruption underneath the following eruption magnetically links
the two eruptions. Shocks develop inside the leading eruption during
the collision.
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Title: Successful and Failed Flux Tube Emergence in the Solar Interior
Authors: Syntelis, P.; Archontis, V.; Hood, A.
2019ApJ...874...15S Altcode: 2019arXiv190207969S
We report on our 3D magnetohydrodynamic simulations of cylindrical
weakly twisted flux tubes emerging from 18 Mm below the photosphere. We
perform a parametric study by varying the initial magnetic field
strength (B <SUB>0</SUB>), radius (R), twist (α), and length of
the emerging part of the flux tube (λ) to investigate how these
parameters affect the transfer of the magnetic field from the
convection zone to the photosphere. We show that the efficiency
of emergence at the photosphere (i.e., how strong the photospheric
field will be in comparison to B <SUB>0</SUB>) depends not only on B
<SUB>0</SUB>, but also on the morphology of the emerging field and on
the twist. We show that parameters such as B <SUB>0</SUB> and magnetic
flux alone cannot determine whether a flux tube will emerge to the
solar surface. For instance, high-B <SUB>0</SUB> (weak-B <SUB>0</SUB>)
fields may fail (succeed) to emerge at the photosphere, depending
on their geometrical properties. We also show that the photospheric
magnetic field strength can vary greatly for flux tubes with the
same B <SUB>0</SUB> but different geometric properties. Moreover,
in some cases we have found scaling laws, whereby the magnetic field
strength scales with the local density as B ∝ ρ <SUP> κ </SUP>,
where κ ≈ 1 deeper in the convection zone and κ < 1 close to the
photosphere. The transition between the two values occurs approximately
when the local pressure scale (H <SUB> p </SUB>) becomes comparable
to the diameter of the flux tube (H <SUB> p </SUB> ≈ 2R). We derive
forms to explain how and when these scaling laws appear and compare
them with the numerical simulations.
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Title: Recurrent CME-like Eruptions in Emerging Flux Regions. I. On
the Mechanism of Eruptions
Authors: Syntelis, P.; Archontis, V.; Tsinganos, K.
2017ApJ...850...95S Altcode: 2017arXiv171110249S
We report on three-dimensional (3D) magnetohydrodynamic (MHD)
simulations of recurrent eruptions in emerging flux regions. We find
that reconnection of sheared field lines, along the polarity inversion
line of an emerging bipolar region, leads to the formation of a new
magnetic structure, which adopts the shape of a magnetic flux rope
(FR) during its rising motion. Initially, the FR undergoes a slow-rise
phase and, eventually, it experiences a fast-rise phase and ejective
eruption toward the outer solar atmosphere. In total, four eruptions
occur during the evolution of the system. For the first eruption, our
analysis indicates that the torus instability initiates the eruption
and that tether-cutting reconnection of the field lines, which envelop
the FR, triggers the rapid acceleration of the eruptive field. For the
following eruptions, we conjecture that it is the interplay between
tether-cutting reconnection and torus instability that causes the
onset of the various phases. We show the 3D shape of the erupting
fields, focusing more on how magnetic field lines reconnect during the
eruptions. We find that when the envelope field lines reconnect mainly
with themselves, hot and dense plasma is transferred closer to the core
of the erupting FR. The same area appears to be cooler and less dense
when the envelope field lines reconnect with neighboring sheared field
lines. The plasma density and temperature distribution, together with
the rising speeds, energies, and size of the erupting fields, indicate
that they may account for small-scale (mini) coronal mass ejections.
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Title: Bombs and Flares at the Surface and Lower Atmosphere of the Sun
Authors: Hansteen, V. H.; Archontis, V.; Pereira, T. M. D.; Carlsson,
M.; Rouppe van der Voort, L.; Leenaarts, J.
2017ApJ...839...22H Altcode: 2017arXiv170402872H
A spectacular manifestation of solar activity is the appearance of
transient brightenings in the far wings of the Hα line, known as
Ellerman bombs (EBs). Recent observations obtained by the Interface
Region Imaging Spectrograph have revealed another type of plasma
“bombs” (UV bursts) with high temperatures of perhaps up to 8 ×
10<SUP>4</SUP> K within the cooler lower solar atmosphere. Realistic
numerical modeling showing such events is needed to explain
their nature. Here, we report on 3D radiative magnetohydrodynamic
simulations of magnetic flux emergence in the solar atmosphere. We
find that ubiquitous reconnection between emerging bipolar magnetic
fields can trigger EBs in the photosphere, UV bursts in the mid/low
chromosphere and small (nano-/micro-) flares (10<SUP>6</SUP> K) in
the upper chromosphere. These results provide new insights into the
emergence and build up of the coronal magnetic field and the dynamics
and heating of the solar surface and lower atmosphere.
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Title: Solar Coronal Jets: Observations, Theory, and Modeling
Authors: Raouafi, N. E.; Patsourakos, S.; Pariat, E.; Young, P. R.;
Sterling, A. C.; Savcheva, A.; Shimojo, M.; Moreno-Insertis, F.;
DeVore, C. R.; Archontis, V.; Török, T.; Mason, H.; Curdt, W.;
Meyer, K.; Dalmasse, K.; Matsui, Y.
2016SSRv..201....1R Altcode: 2016arXiv160702108R; 2016SSRv..tmp...31R
Coronal jets represent important manifestations of ubiquitous solar
transients, which may be the source of significant mass and energy
input to the upper solar atmosphere and the solar wind. While
the energy involved in a jet-like event is smaller than that of
"nominal" solar flares and coronal mass ejections (CMEs), jets
share many common properties with these phenomena, in particular,
the explosive magnetically driven dynamics. Studies of jets could,
therefore, provide critical insight for understanding the larger,
more complex drivers of the solar activity. On the other side of the
size-spectrum, the study of jets could also supply important clues on
the physics of transients close or at the limit of the current spatial
resolution such as spicules. Furthermore, jet phenomena may hint to
basic process for heating the corona and accelerating the solar wind;
consequently their study gives us the opportunity to attack a broad
range of solar-heliospheric problems.
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Title: Ellerman Bombs and IRIS Bombs; In the photosphere and above
Authors: Hansteen, Viggo; Archontis, V.
2016usc..confE.118H Altcode:
The lower Solar atmosphere, consisting of the photosphere and
chromosphere, can occasionally show violent activity more often
associated with the magnetically dominated outer layers of the Sun;
the upper chromosphere, transition region and corona. However, in
regions of strong flux emergence, where Solar active regions are being
formed, one can see evidence of photospheric reconnection as the field
struggles to emerge through the non-buoyant photosphere and expand
into the atmosphere above. Ellerman bombs, short lived, brightness
enhancements in the outer wings of strong optical lines are thought to
be a result of such reconnection. Observations made with the NASA's
Interface Region Imaging Spectrograph, showed similar 'UV bursts' in
lines usually associated with the outer Solar atmosphere, while at the
same time clearly being situated below much cooler gas. We here present
a numerical model of flux emergence in which both Ellerman bombs and
perhaps IRIS bombs (UV bursts) are naturally and copiously produced.
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Title: Emergence of non-twisted magnetic fields in the Sun: Jets
and atmospheric response
Authors: Syntelis, P.; Archontis, V.; Gontikakis, C.; Tsinganos, K.
2015A&A...584A..10S Altcode: 2015arXiv150902850S
<BR /> Aims: We study the emergence of a non-twisted flux tube
from the solar interior into the solar atmosphere. We investigate
whether the length of the buoyant part of the flux tube (i.e. λ)
affects the emergence of the field and the dynamics of the evolving
magnetic flux system. <BR /> Methods: We perform three-dimensional
(3D), time-dependent, resistive, compressible magnetohydrodynamic
(MHD) simulations using the Lare3D code. <BR /> Results: We find that
there are considerable differences in the dynamics of the emergence
of a magnetic flux tube when λ is varied. In the solar interior,
for larger values of λ, the rising magnetic field emerges faster
and expands more due to its lower magnetic tension. As a result, its
field strength decreases and its emergence above the photosphere occurs
later than in the smaller λ case. However, in both cases, the emerging
field at the photosphere becomes unstable in two places, forming two
magnetic bipoles that interact dynamically during the evolution of the
system. Most of the dynamic phenomena occur at the current layer, which
is formed at the interface between the interacting bipoles. We find the
formation and ejection of plasmoids, the onset of successive jets from
the interface, and the impulsive heating of the plasma in the solar
atmosphere. We discuss the triggering mechanism of the jets and the
atmospheric response to the emergence of magnetic flux in the two cases.
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Title: Sunspot rotation. I. A consequence of flux emergence
Authors: Sturrock, Z.; Hood, A. W.; Archontis, V.; McNeill, C. M.
2015A&A...582A..76S Altcode: 2015arXiv150802437S
Context. Solar eruptions and high flare activity often accompany the
rapid rotation of sunspots. The study of sunspot rotation and the
mechanisms driving this motion are therefore key to our understanding
of how the solar atmosphere attains the conditions necessary for large
energy release. <BR /> Aims: We aim to demonstrate and investigate the
rotation of sunspots in a 3D numerical experiment of the emergence
of a magnetic flux tube as it rises through the solar interior and
emerges into the atmosphere. Furthermore, we seek to show that the
sub-photospheric twist stored in the interior is injected into the solar
atmosphere by means of a definitive rotation of the sunspots. <BR />
Methods: A numerical experiment is performed to solve the 3D resistive
magnetohydrodynamic equations using a Lagrangian-Remap code. We
track the emergence of a toroidal flux tube as it rises through the
solar interior and emerges into the atmosphere investigating various
quantities related to both the magnetic field and plasma. <BR />
Results: Through detailed analysis of the numerical experiment,
we find clear evidence that the photospheric footprints or sunspots
of the flux tube undergo a rotation. Significant vertical vortical
motions are found to develop within the two polarity sources after
the field emerges. These rotational motions are found to leave the
interior portion of the field untwisted and twist up the atmospheric
portion of the field. This is shown by our analysis of the relative
magnetic helicity as a significant portion of the interior helicity is
transported to the atmosphere. In addition, there is a substantial
transport of magnetic energy to the atmosphere. Rotation angles
are also calculated by tracing selected fieldlines; the fieldlines
threading through the sunspot are found to rotate through angles
of up to 353° over the course of the experiment. We explain the
rotation by an unbalanced torque produced by the magnetic tension
force, rather than an apparent effect. <P />The movies associated
to Figs. 3, 5, and 11 are available in electronic form at <A
href="http://www.aanda.org/10.1051/0004-6361/201526521/olm">http://www.aanda.org</A>
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Title: Magnetic Flux Emergence Along the Solar Cycle
Authors: Schmieder, B.; Archontis, V.; Pariat, E.
2015sac..book..227S Altcode:
No abstract at ADS
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Title: Helical Blowout Jets in the Sun: Untwisting and Propagation
of Waves
Authors: Lee, E. J.; Archontis, V.; Hood, A. W.
2015ApJ...798L..10L Altcode: 2014arXiv1412.4853L
We report on a numerical experiment of the recurrent onset of helical
"blowout" jets in an emerging flux region. We find that these jets are
running with velocities of ~100-250 km s<SUP>-1</SUP> and they transfer
a vast amount of heavy plasma into the outer solar atmosphere. During
their emission, they undergo an untwisting motion as a result
of reconnection between the twisted emerging and the non-twisted
pre-existing magnetic field in the solar atmosphere. For the first
time in the context of blowout jets, we provide direct evidence that
their untwisting motion is associated with the propagation of torsional
Alfvén waves in the corona.
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Title: Validation and Benchmarking of a Practical Free Magnetic
Energy and Relative Magnetic Helicity Budget Calculation in Solar
Magnetic Structures
Authors: Moraitis, K.; Tziotziou, K.; Georgoulis, M. K.; Archontis, V.
2014SoPh..289.4453M Altcode: 2014arXiv1406.5381M; 2014SoPh..tmp..122M
In earlier works we introduced and tested a nonlinear force-free
(NLFF) method designed to self-consistently calculate the coronal
free magnetic energy and the relative magnetic helicity budgets of
observed solar magnetic structures. In principle, the method requires
only a single, photospheric or low-chromospheric, vector magnetogram
of a quiet-Sun patch or an active region and performs calculations
without three-dimensional magnetic and velocity-field information. In
this work we strictly validate this method using three-dimensional
coronal magnetic fields. Benchmarking employs both synthetic,
three-dimensional magnetohydrodynamic simulations and nonlinear
force-free field extrapolations of the active-region solar corona. Our
time-efficient NLFF method provides budgets that differ from those of
more demanding semi-analytical methods by a factor of approximately
three, at most. This difference is expected to come from the physical
concept and the construction of the method. Temporal correlations show
more discrepancies that are, however, soundly improved for more complex,
massive active regions, reaching correlation coefficients on the order
of, or exceeding, 0.9. In conclusion, we argue that our NLFF method
can be reliably used for a routine and fast calculation of the free
magnetic energy and relative magnetic helicity budgets in targeted
parts of the solar magnetized corona. As explained in this article and
in previous works, this is an asset that can lead to valuable insight
into the physics and triggering of solar eruptions.
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Title: Solar Flaring Activity and Coronal Heating.
Authors: Archontis, V.; Hansteen, V. H.
2014AGUFMSH53D..02A Altcode:
We report on the formation of small solar flares produced
by patchy magnetic reconnection between interacting magnetic
loops. Three-dimensional (3D) magnetohydrodynamic (MHD) numerical
experiments were performed, where a uniform magnetic flux sheet
was injected into a fully developed convective layer. The gradual
emergence of the field into the solar atmosphere results in a network
of magnetic loops, which interact dynamically forming current layers
at their interfaces. The formation and ejection of plasmoids out of
the current layers leads to patchy reconnection and the spontaneous
formation of several small (size ≈1-2 Mm) flares. We find that
these flares are short-lived (30 s-3 minutes) bursts of energy in the
range O(1025-1027) erg, which is basically the nanoflare-microflare
range. Their persistent formation and co-operative action and evolution
leads to recurrent emission of fast EUV/X-ray jets and considerable
plasma heating in the active corona.
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Title: Magnetic Flux Emergence Along the Solar Cycle
Authors: Schmieder, B.; Archontis, V.; Pariat, E.
2014SSRv..186..227S Altcode: 2014SSRv..tmp...47S
Flux emergence plays an important role along the solar cycle. Magnetic
flux emergence builds sunspot groups and solar activity. The sunspot
groups contribute to the large scale behaviour of the magnetic field
over the 11 year cycle and the reversal of the North and South magnetic
polarity every 22 years. The leading polarity of sunspot groups is
opposite in the North and South hemispheres and reverses for each
new solar cycle. However the hemispheric rule shows the conservation
of sign of the magnetic helicity with positive and negative magnetic
helicity in the South and North hemispheres, respectively. MHD models
of emerging flux have been developed over the past twenty years but
have not yet succeeded to reproduce solar observations. The emergence
of flux occurs through plasma layers of very high gradients of pressure
and changing of modes from a large β to a low β plasma (<1). With
the new armada of high spatial and temporal resolution instruments
on the ground and in space, emergence of magnetic flux is observed
in tremendous detail and followed during their transit through the
upper atmosphere. Signatures of flux emergence in the corona depend
on the pre-existing magnetic configuration and on the strength of the
emerging flux. We review in this paper new and established models as
well as the recent observations.
---------------------------------------------------------
Title: Validation of the magnetic energy vs. helicity scaling in
solar magnetic structures
Authors: Tziotziou, K.; Moraitis, K.; Georgoulis, M. K.; Archontis, V.
2014A&A...570L...1T Altcode: 2014arXiv1409.8117T
<BR /> Aims: We assess the validity of the free magnetic energy -
relative magnetic helicity diagram for solar magnetic structures. <BR />
Methods: We used two different methods of calculating the free magnetic
energy and the relative magnetic helicity budgets: a classical,
volume-calculation nonlinear force-free (NLFF) method applied to
finite coronal magnetic structures and a surface-calculation NLFF
derivation that relies on a single photospheric or chromospheric vector
magnetogram. Both methods were applied to two different data sets,
namely synthetic active-region cases obtained by three-dimensional
magneto-hydrodynamic (MHD) simulations and observed active-region cases,
which include both eruptive and noneruptive magnetic structures. <BR
/> Results: The derived energy-helicity diagram shows a consistent
monotonic scaling between relative helicity and free energy with
a scaling index 0.84 ± 0.05 for both data sets and calculation
methods. It also confirms the segregation between noneruptive and
eruptive active regions and the existence of thresholds in both free
energy and relative helicity for active regions to enter eruptive
territory. <BR /> Conclusions: We consider the previously reported
energy-helicity diagram of solar magnetic structures as adequately
validated and envision a significant role of the uncovered scaling in
future studies of solar magnetism.
---------------------------------------------------------
Title: Clusters of Small Eruptive Flares Produced by Magnetic
Reconnection in the Sun
Authors: Archontis, V.; Hansteen, V.
2014ApJ...788L...2A Altcode: 2014arXiv1405.6420A
We report on the formation of small solar flares produced by
patchy magnetic reconnection between interacting magnetic loops. A
three-dimensional (3D) magnetohydrodynamic (MHD) numerical experiment
was performed, where a uniform magnetic flux sheet was injected into
a fully developed convective layer. The gradual emergence of the
field into the solar atmosphere results in a network of magnetic
loops, which interact dynamically forming current layers at their
interfaces. The formation and ejection of plasmoids out of the
current layers leads to patchy reconnection and the spontaneous
formation of several small (size ≈1-2 Mm) flares. We find that
these flares are short-lived (30 s-3 minutes) bursts of energy in the
range O(10<SUP>25</SUP>-10<SUP>27</SUP>) erg, which is basically the
nanoflare-microflare range. Their persistent formation and co-operative
action and evolution leads to recurrent emission of fast EUV/X-ray
jets and considerable plasma heating in the active corona.
---------------------------------------------------------
Title: Distribution of electric currents in source regions of solar
eruptions
Authors: Torok, Tibor; Leake, James E.; Titov, Viacheslav; Archontis,
Vasilis; Mikic, Zoran; Linton, Mark; Dalmasse, Kevin; Aulanier,
Guillaume; Kliem, Bernhard
2014AAS...22431202T Altcode:
There has been a long-lasting debate on the question of whether or
not electric currents in the source regions of solar eruptions are
neutralized. That is, whether or not the direct coronal currents
connecting the photospheric polarities in such regions are surrounded
by return currents of equal amount and opposite direction. In order to
address this question, we consider several mechanisms of source region
formation (flux emergence, photospheric shearing/twisting flows,
and flux cancellation) and quantify the evolution of the electric
currents, using 3D MHD simulations. For the experiments conducted so
far, we find a clear dominance of the direct currents over the return
currents in all cases in which the models produce significant magnetic
shear along the source region's polarity inversion line. This suggests
that pre-eruptive magnetic configurations in strongly sheared active
regions and filament channels carry substantial net currents. We discuss
the implications of this result for the modeling of solar eruptions.
---------------------------------------------------------
Title: Recurrent Explosive Eruptions and the "Sigmoid-to-arcade"
Transformation in the Sun Driven by Dynamical Magnetic Flux Emergence
Authors: Archontis, V.; Hood, A. W.; Tsinganos, K.
2014ApJ...786L..21A Altcode: 2014arXiv1405.6955A
We report on three-dimensional MHD simulations of recurrent mini
coronal mass ejection (CME)-like eruptions in a small active region
(AR), which is formed by the dynamical emergence of a twisted (not kink
unstable) flux tube from the solar interior. The eruptions develop as a
result of the repeated formation and expulsion of new flux ropes due to
continuous emergence and reconnection of sheared field lines along the
polarity inversion line of the AR. The acceleration of the eruptions
is triggered by tether-cutting reconnection at the current sheet
underneath the erupting field. We find that each explosive eruption
is followed by reformation of a sigmoidal structure and a subsequent
"sigmoid-to-flare arcade" transformation in the AR. These results
might have implications for recurrent CMEs and eruptive sigmoids/flares
observations and theoretical studies.
---------------------------------------------------------
Title: Distribution of Electric Currents in Solar Active Regions
Authors: Török, T.; Leake, J. E.; Titov, V. S.; Archontis, V.;
Mikić, Z.; Linton, M. G.; Dalmasse, K.; Aulanier, G.; Kliem, B.
2014ApJ...782L..10T Altcode: 2014arXiv1401.2931T
There has been a long-standing debate on the question of whether or
not electric currents in solar active regions are neutralized. That
is, whether or not the main (or direct) coronal currents connecting
the active region polarities are surrounded by shielding (or return)
currents of equal total value and opposite direction. Both theory and
observations are not yet fully conclusive regarding this question, and
numerical simulations have, surprisingly, barely been used to address
it. Here we quantify the evolution of electric currents during the
formation of a bipolar active region by considering a three-dimensional
magnetohydrodynamic simulation of the emergence of a sub-photospheric,
current-neutralized magnetic flux rope into the solar atmosphere. We
find that a strong deviation from current neutralization develops
simultaneously with the onset of significant flux emergence into the
corona, accompanied by the development of substantial magnetic shear
along the active region's polarity inversion line. After the region
has formed and flux emergence has ceased, the strong magnetic fields
in the region's center are connected solely by direct currents, and
the total direct current is several times larger than the total return
current. These results suggest that active regions, the main sources
of coronal mass ejections and flares, are born with substantial net
currents, in agreement with recent observations. Furthermore, they
support eruption models that employ pre-eruption magnetic fields
containing such currents.
---------------------------------------------------------
Title: Realistic 3D simulations of a small flare resulting from
flux emergence
Authors: Hansteen, Viggo; Archontis, Vasilis
2014cosp...40E1151H Altcode:
We have performed three-dimensional (3d) magnetohydrodynamic simulations
of magnetic flux emergence in a model that spans the convection zone
and into the outer solar atmosphere with the Bifrost code. This is a
“realistic” model, in the sense that the parameters and physical
effects that control the atmosphere can be used to produce diagnostics
that can be directly compared with observations. The emerging flux
leads to the formation of several current sheets as it rises into
the modeled corona. Multiple plasmoids are ejected from the current
sheets. Reconnection occurs impulsively, producing heating and fast
outflows near or in the current sheet, arranged in a manner reminiscent
of the CSHKP flare model. This includes a cusp like arcade and a flux
rope in the lower atmospere underneath the current sheet. We discuss
the evolution of the model and several synthetic observables.
---------------------------------------------------------
Title: Particle acceleration in regions of magnetic flux emergence:
a statistical approach using test-particle- and MHD-simulations
Authors: Vlahos, Loukas; Archontis, Vasilis; Isliker, Heinz
2014cosp...40E3539V Altcode:
We consider 3D nonlinear MHD simulations of an emerging flux tube, from
the convection zone into the corona, focusing on the coronal part of
the simulations. We first analyze the statistical nature and spatial
structure of the electric field, calculating histograms and making
use of iso-contour visualizations. Then test-particle simulations are
performed for electrons, in order to study heating and acceleration
phenomena, as well as to determine HXR emission. This study is done by
comparatively exploring quiet, turbulent explosive, and mildly explosive
phases of the MHD simulations. Also, the importance of collisional and
relativistic effects is assessed, and the role of the integration time
is investigated. Particular aim of this project is to verify the quasi-
linear assumptions made in standard transport models, and to identify
possible transport effects that cannot be captured with the latter. In
order to determine the relation of our results to Fermi acceleration
and Fokker-Planck modeling, we determine the standard transport
coefficients. After all, we find that the electric field of the MHD
simulations must be downscaled in order to prevent an un-physically
high degree of acceleration, and the value chosen for the scale factor
strongly affects the results. In different MHD time-instances we find
heating to take place, and acceleration that depends on the level of
MHD turbulence. Also, acceleration appears to be a transient phenomenon,
there is a kind of saturation effect, and the parallel dynamics clearly
dominate the energetics. The HXR spectra are not yet really compatible
with observations, we have though to further explore the scaling of
the electric field and the integration times used.
---------------------------------------------------------
Title: Free magnetic energy and relative magnetic helicity diagnostics
for the quality of NLFF field extrapolations
Authors: Moraitis, Kostas; Archontis, Vasilis; Tziotziou, Konstantinos;
Georgoulis, Manolis K.
2014cosp...40E2169M Altcode:
We calculate the instantaneous free magnetic energy and relative
magnetic helicity of solar active regions using two independent
approaches: a) a non-linear force-free (NLFF) method that requires
only a single photospheric vector magnetogram, and b) well known
semi-analytical formulas that require the full three-dimensional (3D)
magnetic field structure. The 3D field is obtained either from MHD
simulations, or from observed magnetograms via respective NLFF field
extrapolations. We find qualitative agreement between the two methods
and, quantitatively, a discrepancy not exceeding a factor of 4. The
comparison of the two methods reveals, as a byproduct, two independent
tests for the quality of a given force-free field extrapolation. We find
that not all extrapolations manage to achieve the force-free condition
in a valid, divergence-free, magnetic configuration. This research has
been co-financed by the European Union (European Social Fund - ESF)
and Greek national funds through the Operational Program "Education
and Lifelong Learning" of the National Strategic Reference Framework
(NSRF) - Research Funding Program: Thales. Investing in knowledge
society through the European Social Fund.
---------------------------------------------------------
Title: Free magnetic energy and relative magnetic helicity in active
and quiet solar regions and their role in solar dynamics
Authors: Tziotziou, Konstantinos; Archontis, Vasilis; Tsiropoula,
Georgia; Georgoulis, Manolis K.; Moraitis, Kostas; Kontogiannis,
Ioannis
2014cosp...40E3428T Altcode:
We present a novel non-linear force-free method for the calculation of
the instantaneous free magnetic energy and relative magnetic helicity
budgets of a solar region from a single photospheric/chromospheric
vector magnetogram. Our objective is to study the role of these
quantities both in solar eruptions and in quiet-Sun dynamics. The
validity of the method is tested using both observations and synthetic
magnetohydrodynamical (MHD) models. The method is applied for the
derivation of the energy-helicity (EH) diagram of solar active regions
(ARs) from a sample of 162 vector magnetograms corresponding to 42
different ARs, suggesting the existence of 4×10(31) erg and 2×10(42)
Mx(2) thresholds in free energy and relative helicity, respectively, for
ARs to enter eruptive territory. Furthermore, the dynamical evolution
of both quantities in eruptive NOAA AR 11158, using a high-cadence
5-day time series of vector magnetograms, suggests the formation of
increasingly helical pre-eruption structures and a causal relation
between flares and Coronal Mass Ejections (CMEs). The method is
also used to derive helicity and energy budgets in quiet Sun regions
and construct the respective EH diagram. Our results highlight the
importance of both energy and helicity in AR evolution and quiet-Sun
dynamics and instigate further research on the underlying physics with
three-dimensional MHD models. This work is supported by EU's Seventh
Framework Programme via a Marie Curie Fellowship.
---------------------------------------------------------
Title: The Emergence of Weakly Twisted Magnetic Fields in the Sun
Authors: Archontis, V.; Hood, A. W.; Tsinganos, K.
2013ApJ...778...42A Altcode:
We have studied the emergence of a weakly twisted magnetic flux tube
from the upper convection zone into the solar atmosphere. It is found
that the rising magnetized plasma does not undergo the classical, single
Ω-shaped loop emergence, but it becomes unstable in two places, forming
two magnetic lobes that are anchored in small-scale bipolar structures
at the photosphere, between the two main flux concentrations. The two
magnetic lobes rise and expand into the corona, forming an overall
undulating magnetic flux system. The dynamical interaction of the
lobes results in the triggering of high-speed and hot jets and the
formation of successive cool and hot loops that coexist in the emerging
flux region. Although the initial emerging field is weakly twisted,
a highly twisted magnetic flux rope is formed at the low atmosphere,
due to shearing and reconnection. The new flux rope (hereafter
post-emergence flux rope) does not erupt. It remains confined by
the overlying field. Although there is no ejective eruption of the
post-emergence rope, it is found that a considerable amount of axial
and azimuthal flux is transferred into the solar atmosphere during
the emergence of the magnetic field.
---------------------------------------------------------
Title: Magnetic helicity and free energy in solar active regions
Authors: Moraitis, K.; Georgoulis, M.; Tziotziou, K.; Archontis, V.
2013hell.confS..21M Altcode:
We study the evolution of the non-potential free magnetic energy
and relative magnetic helicity budgets in solar active regions
(ARs). For this we use a time-series of a three-dimensional, synthetic
AR produced by magnetohydrodynamical (MHD) simulations. As a first
step, we calculate the potential magnetic field that has the same
normal components with the MHD field along all boundaries of the AR,
by solving Laplace's equation. The free magnetic energy of the AR is
then easily derived. From the two fields, MHD and potential one, we
calculate the corresponding vector potentials with a recently proposed
integration method. The knowledge of both fields and their respective
vector potentials throughout the AR, allows us to estimate the relative
magnetic helicity budget of the AR. Following this procedure for each
snapshot of the AR, we reconstruct the evolution of free energy and
helicity in the AR. Our method reproduces, for a synthetic AR, the
energy/helicity relations known to hold in real active regions.
---------------------------------------------------------
Title: SDO Observations of Solar Jets
Authors: Moschou, S. P.; Tsinganos, K.; Vourlidas, A.; Archontis, V.
2013SoPh..284..427M Altcode: 2012SoPh..tmp..310M
We present an analysis of high cadence observations of solar jets
observed in the Extreme Ultraviolet (EUV), at 304 Å, with the
Atmospheric Imaging Assembly instrument aboard the Solar Dynamics
Observatory (SDO). The jets in our sample lie very close to the solar
limb to minimize projection effects. Two of the events show clear
helical patterns during ejection. We also find that some of the jets
are recurrent and that most of them cannot overcome solar gravity.
---------------------------------------------------------
Title: Erratum: "A Numerical Model of Standard to Blowout Jets"
<A href="/abs/2013ApJL..769L..21A">(2013, ApJL, 769, L21)</A>
Authors: Archontis, V.; Hood, A. W.
2013ApJ...770L..41A Altcode:
No abstract at ADS
---------------------------------------------------------
Title: A Numerical Model of Standard to Blowout Jets
Authors: Archontis, V.; Hood, A. W.
2013ApJ...769L..21A Altcode:
We report on three-dimensional (3D) MHD simulations of the formation
of jets produced during the emergence and eruption of solar magnetic
fields. The interaction between an emerging and an ambient magnetic
field in the solar atmosphere leads to (external) reconnection
and the formation of "standard" jets with an inverse Y-shaped
configuration. Eventually, low-atmosphere (internal) reconnection of
sheared fieldlines in the emerging flux region produces an erupting
magnetic flux rope and a reconnection jet underneath it. The erupting
plasma blows out the ambient field and, moreover, it unwinds as it
is ejected into the outer solar atmosphere. The fast emission of
the cool material that erupts together with the hot outflows due to
external/internal reconnection form a wider "blowout" jet. We show the
transition from "standard" to "blowout" jets and report on their 3D
structure. The physical plasma properties of the jets are consistent
with observational studies.
---------------------------------------------------------
Title: Modelling magnetic flux emergence in the solar convection zone
Authors: Bushby, P. J.; Archontis, V.
2012A&A...545A.107B Altcode: 2012arXiv1208.1667B
Context. Bipolar magnetic regions are formed when loops of magnetic flux
emerge at the solar photosphere. Magnetic buoyancy plays a crucial role
in this flux emergence process, particularly at larger scales. However
it is not yet clear to what extent the local convective motions
influence the evolution of rising loops of magnetic flux. <BR /> Aims:
Our aim is to investigate the flux emergence process in a simulation of
granular convection. In particular we aim to determine the circumstances
under which magnetic buoyancy enhances the flux emergence rate (which
is otherwise driven solely by the convective upflows). <BR /> Methods:
We used three-dimensional numerical simulations, solving the equations
of compressible magnetohydrodynamics in a horizontally-periodic
Cartesian domain. A horizontal magnetic flux tube was inserted into
fully developed hydrodynamic convection. We systematically varied
the initial field strength, the tube thickness, the initial entropy
distribution along the tube axis and the magnetic Reynolds number. <BR
/> Results: Focusing upon the low magnetic Prandtl number regime (Pm
< 1) at moderate magnetic Reynolds number, we find that the flux tube
is always susceptible to convective disruption to some extent. However,
stronger flux tubes tend to maintain their structure more effectively
than weaker ones. Magnetic buoyancy does enhance the flux emergence
rates in the strongest initial field cases, and this enhancement becomes
more pronounced when we increase the width of the flux tube. This
is also the case at higher magnetic Reynolds numbers, although the
flux emergence rates are generally lower in these less dissipative
simulations because the convective disruption of the flux tube is much
more effective in these cases. These simulations seem to be relatively
insensitive to the precise choice of initial conditions: for a given
flow, the evolution of the flux tube is determined primarily by the
initial magnetic field distribution and the magnetic Reynolds number.
---------------------------------------------------------
Title: Magnetic flux emergence and associated dynamic phenomena in
the Sun
Authors: Archontis, V.
2012RSPTA.370.3088A Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The Creation of Outflowing Plasma in the Corona at Emerging
Flux Regions: Comparing Observations and Simulations
Authors: Harra, L. K.; Archontis, V.; Pedram, E.; Hood, A. W.; Shelton,
D. L.; van Driel-Gesztelyi, L.
2012SoPh..278...47H Altcode:
In this paper we analyse the flux emergence that occurred in the
following polarity area of an active region on 1 - 2 December
2006. Observations have revealed the existence of fast outflows
at the edge of the emerging flux region. We have performed 3-D
numerical simulations to study the mechanisms responsible for these
flows. The results indicate that these outflows are reconnection jets
or pressure-driven outflows, depending on the relative orientation
of the magnetic fields in contact (i.e. the emerging flux and the
active region's field which is favourable for reconnection on the
west side and nearly parallel with the pre-existing field on the east
side of the emerging flux). In the observations, the flows are larger
on the west side until late in the flux emergence, when the reverse
is true. The simulations show that the flows are faster on the west
side, but do not show the east flows increasing with time. There is an
asymmetry in the expansion of the emerging flux region, which is also
seen in the observations. The west side of the emerging flux region
expands faster into the corona than the other side. In the simulations,
efficient magnetic reconnection occurs on the west side, with new loops
being created containing strong downflows that are clearly seen in the
observations. On the other side, the simulations show strong compression
as the dominant mechanism for the generation of flows. There is evidence
of these flows in the observations, but the flows are stronger than
the simulations predict at the later stages. There could be additional
small-angle reconnection that adds to the flows from the compression,
as well as reconnection occurring in larger loops that lie across the
whole active region.
---------------------------------------------------------
Title: 3D MHD Flux Emergence Experiments: Idealised Models and
Coronal Interactions
Authors: Hood, A. W.; Archontis, V.; MacTaggart, D.
2012SoPh..278....3H Altcode: 2011arXiv1103.3685H
This paper reviews some of the many 3D numerical experiments of the
emergence of magnetic fields from the solar interior and the subsequent
interaction with the pre-existing coronal magnetic field. The models
described here are idealised, in the sense that the internal energy
equation only involves the adiabatic, Ohmic and viscous shock heating
terms. However, provided the main aim is to investigate the dynamical
evolution, this is adequate. Many interesting observational phenomena
are explained by these models in a self-consistent manner.
---------------------------------------------------------
Title: Magnetic flux emergence: a precursor of solar plasma expulsion
Authors: Archontis, V.; Hood, A. W.
2012A&A...537A..62A Altcode:
<BR /> Aims: We model the emergence of magnetized plasma from the
top of the convection zone to the lower corona. We investigate the
eruption of coronal flux ropes above emerging flux regions. <BR
/> Methods: We performed three-dimensional numerical experiments
in which the time-dependent and resistive equations of MHD are
solved self-consistently, using the Lare3D code. <BR /> Results: A
subphotospheric magnetic flux tube rises from the convectively unstable
layer into the solar surface, followed by the formation and eruption
of a new flux rope into the corona. Firstly, we examined the case where
the corona is field-free. The expansion of the emerging field forms an
envelope sheath that surrounds the newly formed flux rope. The erupting
ropes are confined by the envelope field. The eruptions are driven by
the gradient of the gas pressure and the tension of fieldlines that
reconnect within the emerging flux region. The amount of the initial
twist of the emerging field and the dense plasma that is lifted up,
determine the height-time profile of the erupting ropes. Secondly,
we examined the case of emergence into a pre-existing magnetic field
in the upper solar atmosphere. A variety of different ambient field
configurations was used in the experiments. External reconnection
between the emerging and the pre-existing field may result in the
removal of sufficient flux from the interacting fields and the full
ejection of the flux ropes. <BR /> Conclusions: The results indicate
that the relative contact angle of the interacting flux systems and
their field strengths are crucial parameters that ultimately affect
the evolution of the eruption of the rope into the higher solar
atmosphere. One important result is that for any contact angle that
favors reconnection, ejective eruptions occur earlier when the ambient
field is relatively strong. In many cases, the erupting plasma adopts
an S-like configuration. The sigmoidal structure accelerates during the
fast eruption of the rope. The acceleration is enhanced by the external
and internal reconnection of fieldlines during the rising motion of the
rope. A key result is that in the reconnection-favored cases, the flux
ropes experience ejective eruptions when the envelope flux is reduced
(owing to removal by external reconnection) below that of the erupting
flux rope. If the envelope flux stays higher than the erupting flux,
the magnetic flux rope remains confined by the envelope field.
---------------------------------------------------------
Title: Evolution of Electric Currents during Active Region Formation
Authors: Torok, T.; Archontis, V.; Titov, V. S.
2011AGUFMSH33C..08T Altcode:
In previous work it has been shown that the emergence of twisted
magnetic flux tubes into the corona can lead to the formation of
both stable and eruptive coronal flux ropes, either by the rigid
emergence of the tube or by shear flows and reconnection occurring
within its expanding upper part. Such an intrusion of new magnetic
flux into the corona naturally produces return currents that flow
in the opposite direction of the flux rope current. It has been
argued that such return currents significantly change the local force
balance -- thus could prevent the flux rope from eruption -- and that
therefore coronal flux rope models that employ a non-neutralized flux
rope current are not suitable to model filament eruptions or coronal
mass ejections. Recently, however, Georgoulis et al. have shown from
observations that strong non-neutralized currents can exist close
to the polarity inversion lines of active regions, particularly in
regions that produce eruptions. This raises the question on the physical
origin of such non-neutralized currents. In this talk, we will present
results from our investigation of the evolution of photospheric and
coronal electric currents in the course of the formation of active
regions and coronal flux ropes, using the flux emergence simulations by
Archontis et al., and we will discuss the implications of our results
for coronal eruptions.
---------------------------------------------------------
Title: Flux Emergence and Associated Dynamic Events in the Sun
Authors: Archontis, V.
2010ASPC..424....3A Altcode:
One of the most important processes, responsible for many dynamical
phenomena observed in the Sun, is the emergence of magnetic flux
from the solar interior in active regions and the modification of the
coronal magnetic field in response to the emergence. In fact, magnetic
flux emergence might be responsible for the appearance of small-scale
events (e.g., compact flares, plasmoids, active-region-associated
X-ray brightenings) and large-scale events (e.g., X-class flares
and CMEs). However, it is clear that the question of how exactly the
magnetic fields rise through the convection zone of the Sun and emerge
through the photosphere and chromosphere into the corona has still not
been solved. Studying the process of flux emergence is an important
step towards the understanding of the dynamic coupling between the
solar interior and the outer solar atmosphere. This paper provides a
brief review of some numerical models, which have been used to study
the process of magnetic flux emergence into the outer solar atmosphere
and the dynamics of associated explosive events.
---------------------------------------------------------
Title: Study of a Solar Active Region Jet
Authors: Gontikakis, C.; Archontis, V.; Tsinganos, K.
2010ASPC..424...19G Altcode:
We present the observations of an active region jet originating from the
east side of NOAA 8531 on May 15 1999. The observations include a series
of TRACE 171 Å filtergrams, and simultaneous observations from SUMER
in Ne VIII, 770 Å, C IV 1548 Å, as well as MDI magnetograms. The
observations were compared with the results of a 3D MHD numerical
simulation of magnetic flux emergence and its subsequent reconnection
with preexisting magnetic flux. The numerical simulation reproduces
the observed 100 km/s outflow at the right temperature range
(0.6-1×10<SUP>6</SUP> Kelvin). Moreover, the observations seem to
suggest that the jet plasma falls back on the solar surface along an
active region loop, in agreement with our model.
---------------------------------------------------------
Title: Flux emergence and coronal eruption
Authors: Archontis, V.; Hood, A. W.
2010A&A...514A..56A Altcode: 2010arXiv1003.2333A
<BR /> Aims: Our aim is to study the photospheric flux distribution
of a twisted flux tube that emerges from the solar interior. We also
report on the eruption of a new flux rope when the emerging tube rises
into a pre-existing magnetic field in the corona. <BR /> Methods: To
study the evolution, we use 3D numerical simulations by solving the
time-dependent and resistive MHD equations. We qualitatively compare
our numerical results with MDI magnetograms of emerging flux at the
solar surface. <BR /> Results: We find that the photospheric magnetic
flux distribution consists of two regions of opposite polarities and
elongated magnetic tails on the two sides of the polarity inversion
line (PIL), depending on the azimuthal nature of the emerging field
lines and the initial field strength of the rising tube. Their shape
is progressively deformed due to plasma motions towards the PIL. Our
results are in qualitative agreement with observational studies of
magnetic flux emergence in active regions (ARs). Moreover, if the
initial twist of the emerging tube is small, the photospheric magnetic
field develops an undulating shape and does not possess tails. In all
cases, we find that a new flux rope is formed above the original axis
of the emerging tube that may erupt into the corona, depending on the
strength of the ambient field.
---------------------------------------------------------
Title: Recurrent solar jets in active regions
Authors: Archontis, V.; Tsinganos, K.; Gontikakis, C.
2010A&A...512L...2A Altcode: 2010arXiv1003.2349A
<BR /> Aims: We study the emergence of a toroidal flux tube into the
solar atmosphere and its interaction with a pre-existing field of
an active region. We investigate the emission of jets as a result of
repeated reconnection events between colliding magnetic fields. <BR
/> Methods: We perform 3D simulations by solving the time-dependent,
resistive MHD equations in a highly stratified atmosphere. <BR />
Results: A small active region field is constructed by the emergence
of a toroidal magnetic flux tube. A current structure is build up
and reconnection sets in when new emerging flux comes into contact
with the ambient field of the active region. The topology of the
magnetic field around the current structure is drastically modified
during reconnection. The modification results in a formation of new
magnetic systems that eventually collide and reconnect. We find that
reconnection jets are taking place in successive recurrent phases in
directions perpendicular to each other, while in each phase they release
magnetic energy and hot plasma into the solar atmosphere. After a series
of recurrent appearance of jets, the system approaches an equilibrium
where the efficiency of the reconnection is substantially reduced. We
deduce that the emergence of new magnetic flux introduces a perturbation
to the active region field, which in turn causes reconnection between
neighboring magnetic fields and the release of the trapped energy in the
form of jet-like emissions. This is the first time that self-consistent
recurrency of jets in active regions is shown in a three-dimensional
experiment of magnetic flux emergence.
---------------------------------------------------------
Title: Formation of Ellerman bombs due to 3D flux emergence
Authors: Archontis, V.; Hood, A. W.
2009A&A...508.1469A Altcode:
Aims. We investigate the emergence of a “sea-serpent” magnetic field
into the outer solar atmosphere and the connection between undulating
fieldlines and formation of Ellerman bombs.<BR /> Methods: We perform
3D numerical experiments solving the time-dependent and resistive
MHD equations.<BR /> Results: A sub-photospheric magnetic flux sheet
develops undulations due to the Parker instability. It rises from
the convectively unstable sub-photospheric layer and emerges into the
highly stratified atmosphere through successive reconnection events
along the undulating system. Brightenings with the characteristics of
Ellerman bombs are produced due to reconnection, which occurs during
the emergence of the field. At an advanced stage of the evolution of
the system, the resistive emergence leads to the formation of long,
arch-like magnetic fields that expand into the corona. The enhancement
of the magnetic field at the low atmosphere and episodes of emergence
of new magnetic flux are also discussed.<BR />
---------------------------------------------------------
Title: Observations and 3D MHD simulations of a solar active
region jet
Authors: Gontikakis, C.; Archontis, V.; Tsinganos, K.
2009A&A...506L..45G Altcode:
Aims: We study an active region jet originating from NOAA 8531 on
May 15 1999. We perform 3D MHD numerical simulations of magnetic flux
emergence and its subsequent reconnection with preexisting magnetic
flux. Then, we compare the physical properties of the observed jet with
the reconnecting outflow produced in the numerical model. <BR />Methods:
We report observations of this jet using a series of TRACE 171 Å
filtergrams, simultaneous observations from SUMER in Ne viii 770Å and
C iv 1548 Å as well as MDI magnetograms. In the numerical simulation,
the full compressible and resistive MHD equations are solved, including
viscous and Ohmic heating. <BR />Results: A high-velocity upflow
(≃100 km s<SUP>-1</SUP>) is observed after the emergence of new
magnetic flux at the edge of the active region. The jet is recorded
over a range of temperatures between 10<SUP>5</SUP> K and 1.5 ×
10<SUP>6</SUP> K. In our numerical experiments, we find that the jet
is the result of magnetic reconnection between newly emerging flux and
the preexisting magnetic field of the active region. <BR />Conclusions:
The hot and high-velocity bidirectional flows occur as a result of the
interaction between oppositely directed magnetic fields. Observations
and numerical results are strongly suggestive of effective reconnection
process being responsible for producing jets when emerging flux appears
in solar active regions.
---------------------------------------------------------
Title: The emergence of toroidal flux tubes from beneath the solar
photosphere
Authors: Hood, A. W.; Archontis, V.; Galsgaard, K.; Moreno-Insertis, F.
2009A&A...503..999H Altcode:
Context: Models of flux emergence frequently use a twisted cylindrical
loop as the initial starting configuration and ignore the coupling
between the radiation field and plasma. In these models, the axis of
the original tube never emerges through the photosphere. Without the
axis emerging, it is very difficult to form a realistic sunspot. <BR
/>Aims: The aim is to use a toroidal flux loop, placed beneath the solar
photosphere and study whether the axis of the system emerges fully into
the atmosphere. The toroidal curvature means that the plasma can drain
more effectively than in a straight cylindrical tube. <BR />Methods:
Three-dimensional magnetohydrodynamic numerical simulations of an
emerging magnetic flux tube are presented for an initial toroidal loop
model. The simulations use a Lagrangian-Remap code that is particularly
suited to dealing with shocks and strong current sheets. <BR />Results:
The evolution of the toroidal loop is followed and the characteristics
of the emergence process are compared with the traditional cylindrical
loops. The flux sources seen at the photosphere are more circular,
and there are less shearing motions in the upper photosphere. When the
initial magnetic field strength is relatively weak the evolution of the
system is similar to the cylindrical loop case, with the formation of
a new flux rope above the photosphere. A striking result is that for
large values of field strength the axial field of the toroidal loop
emerges fully into the corona. This is reported for the first time in
experiments of flux emergence in a highly stratified atmosphere that do
not solve self-consistently the radiation transfer problem. In addition,
the new flux rope forms below the original axis of the toroidal tube
when the field strength is sufficiently strong.
---------------------------------------------------------
Title: On the Structure and Evolution of Complexity in Sigmoids:
A Flux Emergence Model
Authors: Archontis, V.; Hood, A. W.; Savcheva, A.; Golub, L.;
Deluca, E.
2009ApJ...691.1276A Altcode:
Sigmoids are structures with a forward or inverse S-shape, generally
observed in the solar corona in soft X-ray emission. It is believed that
the appearance of a sigmoid in an active region is an important factor
in eruptive activity. The association of sigmoids with dynamic phenomena
such as flares and coronal mass ejections (CMEs) make the study of
sigmoids important. Recent observations of a coronal sigmoid, obtained
with the X-Ray Telescope (XRT) on board Hinode, showed the formation
and eruption phase with high spatial resolution. These observations
revealed that the topological structure of the sigmoid is complex:
it consists of many differently oriented loops that all together
form two opposite J-like bundles or an overall S-shaped structure. A
series of theoretical and numerical models have been proposed, over
the past years, to explain the nature of sigmoids but there is no
explanation on how the aforementioned complexity in sigmoids is built
up. In this paper, we present a flux emergence model that leads to the
formation of a sigmoid, whose structure and evolution of complexity
are in good qualitative agreement with the recent observations. For
the initial state of the experiment a twisted flux tube is placed
below the photosphere. A density deficit along the axis of the tube
makes the system buoyant in the middle and it adopts an Ω-shape as it
rises toward the outer atmosphere. During the evolution of the system,
expanding field lines that touch the photosphere at bald-patches (BPs)
form two seperatrix surfaces where dissipation is enhanced and current
sheets are formed. Originally, each of the BP seperatrix surfaces
has a J-like shape. Each one of the J's consist of reconnected field
lines with different shapes and different relative orientation. The
further dynamical evolution of the emerging flux tube results in the
occurrence of many sites that resemble rotational discontinuities. Thus,
additional current layers are formed inside the rising magnetized volume
increasing the complexity of the system. The reconnected field lines
along these layers form an overall S-shaped structure. The reconnection
process continues to occur leading to the formation of another current
concentration in the middle of the sigmoid where a flaring episode
occurs. This central brightening is accompanied by the eruption of a
flux rope from the central area of the sigmoid and the appearance of
"post-flare" loops underneath the current structure.
---------------------------------------------------------
Title: Eruption of magnetic flux ropes during flux emergence
Authors: Archontis, V.; Török, T.
2008A&A...492L..35A Altcode: 2008arXiv0811.1134A
Aims: We investigate the formation of flux ropes in a flux
emergence region and their rise into the outer atmosphere of the
Sun. <BR />Methods: We perform 3D numerical experiments by solving
the time-dependent and resistive MHD equations. <BR />Results: A
sub-photospheric twisted flux tube rises from the solar interior
and expands into the corona. A flux rope is formed within the
expanding field, due to shearing and reconnection of field lines at
low atmospheric heights. If the tube emerges into a non-magnetized
atmosphere, the flux rope rises, but remains confined inside the
expanding magnetized volume. In contrast, if the expanding tube is
allowed to reconnect with a pre-existing coronal field, the flux rope
experiences a full eruption with a rise profile that is in qualitative
agreement with erupting filaments and Coronal Mass Ejections.
---------------------------------------------------------
Title: NLFF Model of a Coronal Sigmoid
Authors: Savcheva, A. S.; Archontis, V.; van Ballegooijen, A.
2008AGUSMSP31A..05S Altcode:
Between Feb 10 and 12, 2007, the X-ray telescope on Hinode produced
some very high-cadence and high- resolution observations of a prominent
coronal sigmoid. Here we show our results from computing a NLFF model
of the sigmoid and compare it qualitatively to the XRT and TRACE
observations. In addition we include some preliminary qualitative
and quantitative results from 2.5D flux emergence simulation. We also
discuss the future goals of this project.
---------------------------------------------------------
Title: Magnetic flux emergence in the Sun
Authors: Archontis, V.
2008JGRA..113.3S04A Altcode:
Space weather research is closely connected with the study of the solar
magnetic activity. In past years, many solar missions (e.g., YOHKOH,
SOHO, TRACE, and RHESSI) have provided outstanding observations,
which have been used to improve our understanding of the structure
and the dynamical evolution of solar magnetic fields. In addition,
the newly launched solar missions (e.g., Hinode and STEREO) will
study the interaction between the emerging magnetic field and the
preexisting field in the corona (increasing our understanding of
the causes of solar variability), and they will also observe the
three-dimensional evolution of solar eruptions as they leave the Sun
and move into the interplanetary space. One of the most important
processes, responsible for many dynamical phenomena observed in the
Sun, is the emergence of magnetic flux from the solar interior in
active regions and the modification of the coronal magnetic field in
response to the emergence. In fact, magnetic flux emergence might be
responsible for the appearance of small-scale events (e.g., compact
flares, plasmoids, and active-region-associated X-ray brightenings)
and large-scale events (e.g., X-class flares and CMEs), which are major
drivers of space weather. However, it is clear that the question of
how exactly the magnetic fields rise through the convection zone of
the Sun and emerge through the photosphere and chromosphere into the
corona has still not been solved. It is believed that understanding the
process of flux emergence is an important step toward the understanding
of the initiation mechanism of eruptive events in the Sun, which is
another topic of great debate. This paper provides a brief review of
the theory and the numerical models, which have been used to study
the process of magnetic flux emergence into the outer atmosphere of
the Sun. We underline the similarities and differences between these
models, and we compare the basic features of the numerical results
with observations. Finally, we review the recent progress and discuss
what further developments are required in the models to best describe
the essential physics in the process of flux emergence.
---------------------------------------------------------
Title: A Flux Emergence Model for Solar Eruptions
Authors: Archontis, V.; Hood, A. W.
2008ApJ...674L.113A Altcode: 2008arXiv0801.1649A
We have simulated the three-dimensional (3D) emergence and interaction
of two twisted flux tubes, which rise from the interior into the outer
atmosphere of the Sun. We present evidence for the multiple formation
and eruption of flux ropes inside the emerging flux systems and hot
arcade-like structures in between them. Their formation is due to
internal reconnection, occurring between oppositely directed, highly
stretched, and sheared field lines at photospheric heights. Most of
the eruptions escape into the corona, but some are confined and fade
away without leaving the low atmosphere. As these flux ropes erupt,
new reconnected field lines accumulate around the main axis of the
initial magnetic flux systems. We also show the complex 3D field-line
geometry and the structure of the multiple current sheets, which form
as a result of the reconnection between the emerging flux systems.
---------------------------------------------------------
Title: Magnetic flux emergence in the Sun
Authors: Archontis, Vasilis
2008cosp...37..117A Altcode: 2008cosp.meet..117A
One of the most important processes, responsible for many dynamical
phenomena observed in the Sun, is the emergence of magnetic flux
from the solar interior in active regions and the modification of
the coronal magnetic field in response to the emergence. In fact,
magnetic flux emergence might be responsible for the appearance of
small-scale events (e.g., compact flares, plasmoids, active region
associated X-ray brightenings) and large-scale events (e.g., X-class
flares and CMEs). However, it is clear that the question of how exactly
the magnetic fields rise through the convection zone of the Sun and
emerge through the photosphere and chromosphere into the corona has
still not been solved. It is believed that understanding the process
of flux emergence is an important step towards the understanding of
the initiation mechanism of eruptive events in the Sun. In this talk,
we review the recent progress and discuss what further developments
are required in numerical experiments to best describe the essential
physics in the process of flux emergence.
---------------------------------------------------------
Title: The Effect of the Relative Orientation between the Coronal
Field and New Emerging Flux. I. Global Properties
Authors: Galsgaard, K.; Archontis, V.; Moreno-Insertis, F.; Hood, A. W.
2007ApJ...666..516G Altcode: 2007arXiv0705.1097G
The emergence of magnetic flux from the convection zone into the
corona is an important process for the dynamical evolution of the
coronal magnetic field. In this paper we extend our previous numerical
investigations, by looking at the process of flux interaction as an
initially twisted flux tube emerges into a plane-parallel, coronal
magnetic field. Significant differences are found in the dynamical
appearance and evolution of the emergence process depending on the
relative orientation between the rising flux system and any preexisting
coronal field. When the flux systems are nearly antiparallel, the
experiments show substantial reconnection and demonstrate clear
signatures of a high-temperature plasma located in the high-velocity
outflow regions extending from the reconnection region. However, the
cases that have a more parallel orientation of the flux systems show
very limited reconnection and none of the associated features. Despite
the very different amount of reconnection between the two flux systems,
it is found that the emerging flux that is still connected to the
original tube reaches the same height as a function of time. As a
compensation for the loss of tube flux, a clear difference is found
in the extent of the emerging loop in the direction perpendicular to
the main axis of the initial flux tube. Increasing amounts of magnetic
reconnection decrease the volume, which confines the remaining tube
flux.
---------------------------------------------------------
Title: Nonlinear MHD dynamo operating at equipartition
Authors: Archontis, V.; Dorch, S. B. F.; Nordlund, Å.
2007A&A...472..715A Altcode:
Context: We present results from non linear MHD dynamo experiments with
a three-dimensional steady and smooth flow that drives fast dynamo
action in the kinematic regime. In the saturation regime, the system
yields strong magnetic fields, which undergo transitions between an
energy-equipartition and a turbulent state. The generation and evolution
of such strong magnetic fields is relevant for the understanding of
dynamo action that occurs in stars and other astrophysical objects. <BR
/>Aims: We study the mode of operation of this dynamo, in the linear
and non-linear saturation regimes. We also consider the effect of
varying the magnetic and fluid Reymolds number on the non-linear
behaviour of the system. <BR />Methods: We perform three-dimensional
non-linear MHD simulations and visualization using a high resolution
numerical scheme. <BR />Results: We find that this dynamo has a
high growth rate in the linear regime, and that it can saturate at
a level significantly higher than intermittent turbulent dynamos,
namely at energy equipartition, for high values of the magnetic and
fluid Reynolds numbers. The equipartition solution however does not
remain time-independent during the simulation but exhibits a much
more intricate behaviour than previously thought. There are periods
in time where the solution is smooth and close to energy-equipartition
and others where it becomes turbulent. Similarities and differences in
the way the magnetic field is amplified and sustained for experiments
with varying Reynolds numbers are discussed. <BR />Conclusions: Strong
magnetic fields, in near equipartition, can be generated also by a
non-turbulent dynamo. A striking result is that the saturation state
of this dynamo reveals interesting transitions between turbulent and
laminar states.
---------------------------------------------------------
Title: Emergence and interaction of twisted flux tubes in the Sun
Authors: Archontis, V.; Hood, A. W.; Brady, C.
2007A&A...466..367A Altcode:
Aims: We present results from numerical simulations that study
the interaction of a pair of twisted, buoyant magnetic flux tubes,
which rise from the solar interior into the outer atmosphere of the
Sun. The aim of our new model is to reproduce some of the dynamic
solar phenomena in a self-consistent manner. <BR />Methods: We perform
non-linear simulations in 2.5D numerical experiments by solving the
compressible and resistive MHD equations using a Lagrangian remap,
shock capturing code (Lare2D). For some aspects of the problem, we
consider the evolution of the system using both uniform and locally
enhanced resistivity. <BR />Results: The two flux tubes start to rise
at the same time but from a different height below the photosphere. The
leading (first) tube, which is originally located nearer to the surface,
rises and eventually expands above the photosphere forming a magnetized
atmosphere for the upcoming system (second tube). Current sheets,
high-velocity reconnection jets, plasmoids, loop brightnenings and
arcade flare-like structures are formed, for the first time in such
numerical experiments, self-consistently by the emergence, expansion
and the dynamical interaction between the two emerging flux systems.
---------------------------------------------------------
Title: Ellerman Bombs and Jets Associated with Resistive Flux
Emergence
Authors: Isobe, H.; Tripathi, D.; Archontis, V.
2007ApJ...657L..53I Altcode:
Using two-dimensional (2D) magnetohydrodynamic simulations we study
the effects of resistive processes in the dynamics of magnetic
flux emergence and its relation to Ellerman bombs and other dynamic
phenomena in the Sun. The widely accepted scenario of flux emergence
is the formation and expansion of Ω-shaped loops due to the Parker
instability. Since the Parker instability has the largest growth rate at
finite wavelength λ<SUB>p</SUB>~10H-20H, where H is the scale height
(~200 km in the solar photosphere), a number of magnetic loops may
rise from the initial flux sheet if it is sufficiently long. This
process is shown in our numerical simulations. The multiple emerging
loops expand in the atmosphere and interact with each other, leading
to magnetic reconnection. At first reconnection occurs in the lower
atmosphere, which allows the sinking part of the flux sheet to emerge
above the photosphere. This reconnection also causes local heating
that may account for Ellerman bombs. In the later stage, reconnection
between the expanding loops occurs at higher levels of the atmosphere
and creates high-temperature reconnection jets, and eventually a large
(>>λ<SUB>p</SUB>) coronal loop is formed. Cool and dense plasma
structures, which are similar to Hα surges, are also formed. This
is not because of magnetic reconnection but due to the compression of
the plasma in between the expanding loops.
---------------------------------------------------------
Title: 3D simulations identifying the effects of varying the twist
and field strength of an emerging flux tube
Authors: Murray, M. J.; Hood, A. W.; Moreno-Insertis, F.; Galsgaard,
K.; Archontis, V.
2006A&A...460..909M Altcode:
Aims.We investigate the effects of varying the magnetic field strength
and the twist of a flux tube as it rises through the solar interior
and emerges into the atmosphere.<BR /> Methods: .Using a 3D numerical
MHD code, we consider a simple stratified model, comprising of one
solar interior layer and three overlying atmospheric layers. We set
a horizontal, twisted flux tube in the lowest layer. The specific
balance of forces chosen results in the tube being fully buoyant and
the temperature is decreased in the ends of the tube to encourage the
formation of an Ω-shape along the tube's length. We vary the magnetic
field strength and twist independently of each other so as to give clear
results of the individual effects of each parameter.<BR /> Results:
.We find a self-similar evolution in the rise and emergence of the flux
tube when the magnetic field strength of the tube is modified. During
the rise through the solar interior, the height of the crest and
axis, the velocity of the crest and axis, and the decrease in the
magnetic field strength of the axis of the tube are directly dependent
upon the initial magnetic field strength given to the tube. No such
self-similarity is evident when the twist of the flux tube is changed,
due to the complex interaction of the tension force on the rise of the
tube. For low magnetic field strength and twist values, we find that
the tube cannot fully emerge into the atmosphere once it reaches the
top of the interior since the buoyancy instability criterion cannot
be fulfilled. For those tubes that do advance into the atmosphere,
when the magnetic field strength has been modified, we find further
self-similar behaviour in the amount of tube flux transported into
the atmosphere. For the tubes that do emerge, the variation in the
twist results in the buoyancy instability, and subsequent emergence,
occurring at different locations along the tube's length.<BR />
---------------------------------------------------------
Title: New Results on an Equipartition Dynamo
Authors: Dorch, S. B. F.; Archontis, V.
2006IAUJD...8E...4D Altcode:
This contribution presents results from numerical computer experiments
with a 3-d steady sine flow (with zero mean helicity) that drives fast
dynamo action. The mode of operation of this so-called “no-cosines"
dynamo (recently dubbed “the Archontis dynamo"" by David Galloway)
was studied during linear and non-linear saturation regimes. The means
were 3-d non-linear MHD simulations and visualization using the high
resolution numerical scheme by Nordlund, Galsgaard and others. We have
found that the dynamo has a high growth rate in the linear regime, and
that it can saturate at a level significantly higher that intermittent
turbulent dynamos: Namely very close to energy equipartition for
high Reynolds numbers. The equipartition solution however is not
turbulent but a laminar solution that acts as an attractor to other
modes. Similarities and differences, in the way the magnetic field
is amplified and sustained, between experiments with varying Reynolds
numbers are illustrated. The conclusion is that strong astrophysical
magnetic fields at equipartition are not necessarily generated by
turbulent dynamos.
---------------------------------------------------------
Title: Three-dimensional Plasmoid Evolution in the Solar Atmosphere
Authors: Archontis, V.; Galsgaard, K.; Moreno-Insertis, F.; Hood, A. W.
2006ApJ...645L.161A Altcode:
We present clear evidence of the formation of three-dimensional (3D)
plasmoids in the current sheet between two magnetic flux systems in a
3D numerical experiment of flux emergence into the solar atmosphere and
study their properties and time evolution. Plasmoids are most likely
the result of resistive tearing mode instabilities. They adopt the
shape of a solenoid contained within the current sheet: the solenoid
is tightly wound when the field in the two flux systems is close to
antiparallel. The plasmoids are expelled to the sides of the sheet as
a result of a reconnection imbalance between the two x-lines on their
sides. We show the complex, 3D field line geometry in various plasmoids:
individual plasmoid field lines have external linkages to the flux
system on either side of the current sheet; we also find field lines
that go through a few plasmoids in succession, probably indicating
that the field line has resulted from multiple reconnection events.
---------------------------------------------------------
Title: Flux emergence and interaction with a coronal field: 3D
MHD simulations
Authors: Archontis, V.; Moreno-Insertis, F.; Galsgaard, K.; Hood, A. W.
2006IAUS..233...53A Altcode:
The dynamic process of magnetic flux emergence from the solar interior
to the outer atmosphere may well be related with eruptive phenomena
and intense events of the Solar activity. However, the physics of the
emergence is not still well understood. Thus, we have performed 3D MHD
simulations to study the rising motion of a twisted flux tube from the
convection zone of the Sun and its interaction with a preexisting
coronal magnetic field. The results show that the reconnection
process depends criticaly on the initial relative orientation between
the two magnetic flux systems into contact. On the other hand, the
overal process of emergence depends mostly on the dynamics of the
sub-photospheric plasma.
---------------------------------------------------------
Title: The Three-dimensional Interaction between Emerging Magnetic
Flux and a Large-Scale Coronal Field: Reconnection, Current Sheets,
and Jets
Authors: Archontis, V.; Moreno-Insertis, F.; Galsgaard, K.; Hood, A. W.
2005ApJ...635.1299A Altcode:
Using MHD numerical experiments in three dimensions, we study the
emergence of a bipolar magnetic region from the solar interior
into a model corona containing a large-scale, horizontal magnetic
field. An arch-shaped concentrated current sheet is formed at the
interface between the rising magnetized plasma and the ambient
coronal field. Three-dimensional reconnection takes place along
the current sheet, so that the corona and the photosphere become
magnetically connected, a process repeatedly observed in recent
satellite missions. We show the structure and evolution of the
current sheet and how it changes in time from a simple tangential
discontinuity to a rotational discontinuity with no null surface. We
find clear indications that individual reconnection events in this
three-dimensional environment in the advanced stage are not one-off
events, but instead take place in a continuous fashion, with each
field line changing connectivity during a finite time interval. We
also show that many individual field lines of the rising tube undergo
multiple processes of reconnection at different points in the corona,
thus creating photospheric pockets for the coronal field. We calculate
global measures for the amount of subphotospheric flux that becomes
linked to the corona during the experiment and find that most of
the original subphotospheric flux becomes connected to coronal field
lines. The ejection of plasma from the reconnection site gives rise to
high-speed and high-temperature jets. The acceleration mechanism for
those jets is akin to that found in previous two-dimensional models,
but the geometry of the jets bears a clear three-dimensional imprint,
having a curved-sheet appearance with a sharp interface to the overlying
coronal magnetic field system. Temperatures and velocities of the jets
in the simulations are commensurate with those measured in soft X-rays
by the Yohkoh satellite.
---------------------------------------------------------
Title: Magnetic Flux Emergence and its Interaction with AN Existing
Coronal Field
Authors: Galsgaard, K.; Moreno-Insertis, F.; Archontis, V.; Hood, A.
2005ESASP.596E..27G Altcode: 2005ccmf.confE..27G
No abstract at ADS
---------------------------------------------------------
Title: Magnetic Flux Emergence and its Interaction with AN Existing
Coronal Field
Authors: Galsgaard, K.; Moreno-Insertis, F.; Archontis, V.; Hood, A.
2005ESASP.596E..55G Altcode: 2005ccmf.confE..55G
No abstract at ADS
---------------------------------------------------------
Title: A Three-dimensional Study of Reconnection, Current Sheets,
and Jets Resulting from Magnetic Flux Emergence in the Sun
Authors: Galsgaard, K.; Moreno-Insertis, F.; Archontis, V.; Hood, A.
2005ApJ...618L.153G Altcode: 2004astro.ph.10057G
We present the results of a set of three-dimensional numerical
simulations of magnetic flux emergence from below the photosphere
and into the corona. The corona includes a uniform and horizontal
magnetic field as a model for a preexisting large-scale coronal
magnetic system. Cases with different relative orientations of the
upcoming and coronal fields are studied. Upon contact, a concentrated
current sheet with the shape of an arch is formed at the interface
that marks the positions of maximum jump in the field vector between
the two systems. Relative angles above 90° yield abundant magnetic
reconnection and plasma heating. The reconnection is seen to be
intrinsically three-dimensional in nature and to be accompanied by
marked local heating. It generates collimated high-speed outflows
only a short distance from the reconnection site, and these propagate
along the ambient magnetic field lines as jets. As a result of the
reconnection, magnetic field lines from the magnetized plasma below
the surface end up connecting to coronal field lines, thus causing a
profound change in the connectivity of the magnetic regions in the
corona. The experiments presented here yield a number of features
repeatedly observed with the TRACE and Yohkoh satellites, such as the
establishment of connectivity between emergent and preexisting active
regions, local heating, and high-velocity outflows.
---------------------------------------------------------
Title: A non-helical dynamo - MHD simulations of dynamo action by
a non-helical flow
Authors: Archontis, V.; Dorch, S. B. F.
2005HiA....13..136A Altcode:
We performed numerical 3-d MHD simulations to study whether or not
the presence of helicity is a necessary ingredient for fast dynamo
action. A steady 3-d flow with no mean helicity is used and is turned
out that apart from the high growth rate in the linear regime the
dynamo saturates at a level significantly higher that the intermittent
turbulent dynamos. It becomes clear from this example that the precense
of a mean helicity is not at all a requirement for dynamo action but
it is rather the stretching ability of the flow that amplifies the
magnetic energy in an exponential manner.
---------------------------------------------------------
Title: Flux Emergence from the Solar Interior Into a Uniformly
Magnetized Corona
Authors: Moreno-Insertis, F.; Galsgaard, K.; Archontis, V.; Hood, A.
2004ESASP.575..216M Altcode: 2004soho...15..216M
No abstract at ADS
---------------------------------------------------------
Title: 3D MHD Simulations on Magnetic Flux Emergence
Authors: Archontis, V.; Moreno-Insertis, F.; Galsgaard, K.; Hood, A.
2004ESASP.575..342A Altcode: 2004soho...15..342A
No abstract at ADS
---------------------------------------------------------
Title: Emergence of magnetic flux from the convection zone into
the corona
Authors: Archontis, V.; Moreno-Insertis, F.; Galsgaard, K.; Hood,
A.; O'Shea, E.
2004A&A...426.1047A Altcode:
Numerical experiments of the emergence of magnetic flux from the
uppermost layers of the solar interior to the photosphere and its
further eruption into the low atmosphere and corona are carried out. We
use idealized models for the initial stratification and magnetic
field distribution below the photosphere similar to those used for
multidimensional flux emergence experiments in the literature. The
energy equation is adiabatic except for the inclusion of ohmic and
viscous dissipation terms, which, however, become important only at
interfaces and reconnection sites. Three-dimensional experiments for the
eruption of magnetic flux both into an unmagnetized corona and into a
corona with a preexisting ambient horizontal field are presented. The
shocks preceding the rising plasma present the classical structure of
nonlinear Lamb waves. The expansion of the matter when rising into the
atmosphere takes place preferentially in the horizontal directions: a
flattened (or oval) low plasma-β ball ensues, in which the field lines
describe loops in the corona with increasing inclination away from the
vertical as one goes toward the sides of the structure. Magnetograms
and velocity field distributions on horizontal planes are presented
simultaneously for the solar interior and various levels in the
atmosphere. Since the background pressure and density drop over many
orders of magnitude with increasing height, the adiabatic expansion
of the rising plasma yields very low temperatures. To avoid this, the
entropy of the rising fluid elements should be increased to the high
values of the original atmosphere via heating mechanisms not included in
the present numerical experiments. The eruption of magnetic flux into
a corona with a preexisting magnetic field pointing in the horizontal
direction yields a clear case of essentially three-dimensional
reconnection when the upcoming and ambient field systems come into
contact. The coronal ambient field is chosen at time t=0 perpendicular
to the direction of the tube axis and thus, given the twist of the
magnetic tube, almost anti-parallel to the field lines at the upper
boundary of the rising plasma ball. A thin, dome-shaped current layer
is formed at the interface between the two flux systems, in which ohmic
dissipation and heating are taking place. The reconnection proceeds
by merging successive layers on both sides of the reconnection site;
however, this occurs not only at the cusp of the interface, but, also,
gradually along its sides in the direction transverse to the ambient
magnetic field. The topology of the magnetic field in the atmosphere
is thereby modified: the reconnected field lines typically are part of
the flanks of the tube below the photosphere but then join the ambient
field system in the corona and reach the boundaries of the domain as
horizontal field lines.
---------------------------------------------------------
Title: On the Saturation of Astrophysical Dynamos: Numerical
Experiments with the No-Cosines Flow
Authors: Dorch, S. B. F.; Archontis, V.
2004SoPh..224..171D Altcode: 2004astro.ph..9193D; 2005SoPh..224..171D
In the context of astrophysical dynamos we illustrate that the
no-cosines flow, with zero mean helicity, can drive fast dynamo action
and we study the dynamo's mode of operation during both the linear
and non-linear saturation regimes. It turns out that in addition to
a high growth rate in the linear regime, the dynamo saturates at a
level significantly higher than normal turbulent dynamos, namely at
exact equipartition when the magnetic Prandtl number Pr<SUB>m</SUB>∼
1. Visualization of the magnetic and velocity fields at saturation
will help us to understand some of the aspects of the non-linear
dynamo problem.
---------------------------------------------------------
Title: Dynamo action in turbulent flows
Authors: Archontis, V.; Dorch, S. B. F.; Nordlund, Å.
2003A&A...410..759A Altcode: 2003astro.ph..6069A
We present results from numerical simulations of nonlinear MHD dynamo
action produced by three-dimensional flows that become turbulent for
high values of the fluid Reynolds number. The magnitude of the forcing
function driving the flow is allowed to evolve with time in such way as
to maintain an approximately constant velocity amplitude (and average
kinetic energy) when the flow becomes hydrodynamically unstable. It
is found that the saturation level of the dynamo increases with the
fluid Reynolds number (at constant magnetic Prandtl number), and that
the average growth rate approaches an asymptotic value for high fluid
Reynolds number. The generation and destruction of magnetic field is
examined during the laminar and turbulent phase of the flow and it is
found that in the neighborhood of strong magnetic flux “cigars" Joule
dissipation is balanced by the work done against the Lorentz force,
while the steady increase of magnetic energy occurs mainly through
work done in the weak part of the magnetic field.
---------------------------------------------------------
Title: Helicity and Dynamo Action
Authors: Archontis, Vasilis D.; Dorch, Bertil F.
2003IAUJD...3E..10A Altcode:
We performed numerical 3-d MHD simulations to study whether or not
the presence of helicity is a necessary ingredient for fast dynamo
action. A steady 3-d flow with no mean helicity is used and is turned
out that apart from the high growth rate in the linear regime the
dynamo saturates at a level significantly higher that the intermittent
turbulent dynamos. It becomes clear from this example that the precense
of a mean helicity is not at all a requirement for dynamo action but
it is rather the stretching ability of the flow that amplifies the
magnetic energy in an exponential manner.
---------------------------------------------------------
Title: Numerical simulations of kinematic dynamo action
Authors: Archontis, V.; Dorch, S. B. F.; Nordlund, Å.
2003A&A...397..393A Altcode: 2002astro.ph..4208A
Numerical simulations of kinematic dynamo action in steady and
three-dimensional ABC flows are presented with special focus on the
difference in growth rates between cases with single and multiple
periods of the prescribed velocity field. It is found that the
difference in growth rate (apart from a trivial factor stemming from
a scaling of the rate of strain with the wavenumber of the velocity
field) is due to differences in the recycling of the weakest part of the
magnetic field. The single wavelength classical ABC-flow experiments
impose stronger symmetry requirements, which results in a suppression
of the growth rate. The experiments with larger wave number achieve
growth rates that are more compatible with the turn-over time scale
by breaking the symmetry of the resulting dynamo-generated magnetic
field. Differences in topology in cases with and without stagnation
points in the imposed velocity field are also investigated, and it is
found that the cigar-like structures that develop in the classical
A=B=C dynamos are replaced by ribbon structures in cases where the
flow is without stagnation points.
---------------------------------------------------------
Title: Dynamo action in turbulent flows
Authors: Archontis, V.; Nordlund, Å.
2002ESASP.505...95A Altcode: 2002solm.conf...95A; 2002IAUCo.188...95A
We present results from numerical simulations of kinematic and nonlinear
MHD dynamo action produced by turbulent flows. Traditionally, turbulence
was thought to be essential to dynamo action. There is new evidence
that indicates that laminar and turbulent dynamos are surprisingly
similar, with growth rates similar to large scale turn-over time in
both cases. An analysis of the Lorentz work and Joule dissipation
shows that dynamo occurs primarily in regions where the field is weak
by bending and stretching the magnetic field lines.
---------------------------------------------------------
Title: Linear, non-linear and turbulent dynamos
Authors: Archontis, Vasilis
2000PhDT.......179A Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Linear, Non-Linear and Turbulent Dynamos
Authors: Archontis, V.
2000PhDT..........A Altcode:
The properties of magnetic structures in linear, non-linear
and turbulent dynamos are studied by applying numerical 3-D
magneto-hydrodynamical simulations to parametrized smooth flows,
such as e.g. ABC flows.
---------------------------------------------------------
Title: 3D simulations of twisted magnetic flux ropes
Authors: Dorch, S. B. F.; Archontis, V.; Nordlund, Å.
1999A&A...352L..79D Altcode:
Several numerical simulations of buoyant 2D and 3D twisted flux ropes
have been performed. It is found that the apex region of an anchored
3D flux rope behaves similarly to the simpler case of a 2D horizontal
twisted flux tube while the overall structure of such a 3D flux rope
developes quite differently. Upon emergence a characteristic S-shape
of the magnetic field lines is displayed in agreement with observations
in soft X-ray.
---------------------------------------------------------
Title: Numerical Simulations of Dynamos Associated with ABC Flows
Authors: Archontis, V.; Dorch, B.
1999ASPC..178....1A Altcode: 1999sdnc.conf....1A
No abstract at ADS
