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Author name code: toeroek
ADS astronomy entries on 2022-09-14
author:"Toeroek, Tibor"
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Title: A Magnetogram-matching Method for Energizing Magnetic Flux
Ropes Toward Eruption
Authors: Titov, V. S.; Downs, C.; Török, T.; Linker, J. A.
2022ApJ...936..121T Altcode: 2022arXiv220503982T
We propose a new "helicity-pumping" method for energizing coronal
equilibria that contain a magnetic flux rope (MFR) toward an
eruption. We achieve this in a sequence of magnetohydrodynamics
relaxations of small line-tied pulses of magnetic helicity, each of
which is simulated by a suitable rescaling of the current-carrying
part of the field. The whole procedure is "magnetogram-matching"
because it involves no changes to the normal component of the field
at the photospheric boundary. The method is illustrated by applying
it to an observed force-free configuration whose MFR is modeled
with our regularized Biot-Savart law method. We find that, in spite
of the bipolar character of the external field, the MFR eruption is
sustained by two reconnection processes. The first, which we refer to
as breakthrough reconnection, is analogous to breakout reconnection
in quadrupolar configurations. It occurs at a quasi-separator inside a
current layer that wraps around the erupting MFR and is caused by the
photospheric line-tying effect. The second process is the classical
flare reconnection, which develops at the second quasi-separator inside
a vertical current layer that is formed below the erupting MFR. Both
reconnection processes work in tandem with the magnetic forces of the
unstable MFR to propel it through the overlying ambient field, and their
interplay may also be relevant for the thermal processes occurring in
the plasma of solar flares. The considered example suggests that our
method will be beneficial for both the modeling of observed eruptive
events and theoretical studies of eruptions in idealized magnetic
configurations.
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Title: What are the Limits to Extreme Eruptions on the Sun?
Authors: Linker, Jon; Riley, Pete; Titov, Viacheslav; Lionello,
Roberto; Downs, Cooper; Torok, Tibor; Caplan, Ronald
2022cosp...44.1560L Altcode:
Extreme solar eruptions are of great interest, for both purely
scientific as well as practical reasons. The 1859 Carrington event
is often considered to represent the largest extreme of space weather
events. However, so-called superflares (energies from 1e33-1e35 ergs)
on other stars hint at even more acute possibilities. What are the
energy limits for solar eruptions? The energy that powers these events
is believed to be stored as free magnetic energy (energy above the
potential field state) prior to eruption. Therefore, the maximum free
energy that can be stored in an active region (AR) bounds the largest
possible eruption that can be released from a region. According to the
Aly-Sturrock theorem, the energy of a fully force-free field cannot
exceed the energy of the so-called open field. If the theorem holds,
this places an upper limit on the amount of free energy that can be
stored: the maximum free energy (MFE) is the difference between the open
field energy and the potential field energy of the active region. We
computed the MFE for some of the most flare productive ARs of solar
cycles 22-24 and found 6 cases where the maximum possible energy storage
was on the order of or greater than 1e34 ergs. In simulation studies,
we have found that while the MFE is indeed a useful upper bound,
it is generally larger than the maximum energy that can actually
be stored. We have found that the related theory of partially open
fields can provide a more stringent upper bound - the partially open
field energy (POFE). We calculate POFEs for several significant ARs of
cycles 22-24 and revisit their energy storage limits. Work supported
by NSF and NASA.
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Title: Suppression of Torus Instability on Cool Stars
Authors: Sun, Xudong; Derosa, Marc; Torok, Tibor
2022cosp...44.1389S Altcode:
Despite the frequent detection of stellar super flares, reports on
stellar coronal mass ejections (CMEs) are rare. This is in contrast with
our Sun, where almost all large flares are accompanied by a CME. Here,
we use an analytical coronal magnetic field model to demonstrate that
the torus instability, a leading mechanism for solar CMEs, tends to
be suppressed in stellar magnetic environment. Contributing factors
include larger starspots, stronger global dipole field, and more
closed magnetic geometry compared to the Sun. Suppression of the torus
instability may contribute to the low apparent CME rate on cool stars.
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Title: Early Dynamics and Trajectories of CMEs
Authors: Zhang, Jie; Torok, Tibor; Nikou, Eleni; Nazmus Sakib, Md;
Dhakal, Suman
2022cosp...44.1359Z Altcode:
How CMEs evolve right after their initiation is of high interest
for solar physics research and space weather predictions. Important
properties such as their speed and trajectory are largely set during
this early phase of evolution. However, the underlying conditions and
physical mechanisms remain poorly understood, both observationally and
theoretically. In this study, we perform a comprehensive investigation
of the kinematic, geometric and morphological evolution of CMEs from
the very beginning of their eruption near the surface of the Sun
continuously until they reach the outer corona. In particular, we
study three CMEs that originated near the disk center, but exhibited
significantly different behavior in terms of their rise direction
or deflection: the deflection angles (measured along both longitude
and latitude) varies from >20 degree to nearly zero degree for
these events. We employ a novel technique to determine the true early
propagation direction of disk-centered CMEs, using high cadence EUV
observation from SDO/AIA. Our technique is based on fitting the shape
of observed EUV waves, which are presumably driven by the expanding
magnetic flux rope. We further use STEREO/COR1 and COR2 observations,
in combination with SOHO/LASCO data, to determine the 3D trajectory
and morphology of CMEs in the outer corona. We also investigate the
properties of the magnetic field in the source regions of these CMEs, in
order to determine the causes of observed evolution both qualitatively
and quantitatively. We also compare our observational results with
data-constrained state-of-the-art 3D MHD numerical simulation of
specific events. Our work addresses the following scientific questions:
(1) What determines the trajectory of a rising CME? (2) Can we constrain
and/or predict the trajectory of a potential CME from models?
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Title: The 3D magnetic structure of CMEs throughout the extended
solar corona
Authors: Palmerio, Erika; Downs, Cooper; Torok, Tibor; Ben-Nun, Michal
2022cosp...44.1128P Altcode:
The magnetic fields that make up the internal structure of coronal
mass ejections (CMEs) are thought to be organised in a flux-rope
configuration consisting of twisted magnetic fields that wind about
a central axis. Remote-sensing observations of CMEs show a wide
range of morphologies, dynamics, and evolution, including rotation,
non-uniform expansion, deflection, and interaction with the ambient
solar wind. In-situ measurements, however, typically consist of a single
1D spacecraft trajectory through a large 3D structure (or few at best),
limiting our understanding of how the internal magnetic structure of
a given CME may vary in time and space. In this work, we analyse the
magnetic configuration of an idealised CME during its early evolution
in the range 1-30 Rs, using the Magnetohydrodynamic Algorithm Outside
a Sphere (MAS) code. The initial flux rope erupts in a simplified
coronal configuration, from a bipolar active region located under
the streamer belt, and propagates through a uniform background solar
wind. We place a fleet of synthetic spacecraft throughout the CME's
path at different combinations of heliocentric distance, latitude,
and longitude. We identify and examine flux-rope signatures in the
synthetic in-situ profiles, in order to characterise radial variations
as well as latitudinal/longitudinal ones. We find that, even in the
case of a simplified CME erupting under solar minimum-like conditions,
the sampling location significantly affects the global structure that
would be deduced from common flux-rope reconstruction and analysis
techniques used for in-situ measurements.
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Title: The Pre-Eruptive Structure and Initiation Mechanism(s) of CMEs
Authors: Torok, Tibor
2022cosp...44.2461T Altcode:
Coronal mass ejections (CMEs) are huge expulsions of magnetized
plasma from the low solar corona into interplanetary space, and
the main driver of space weather disturbances in the terrestrial
magnetosphere. Despite extensive research being conducted since the
discovery of CMEs about 50 years ago, many important aspects of these
enigmatic events are still not well understood. In this presentation, I
will address two of these aspects, namely the nature of the pre-eruptive
magnetic configuration and the physical mechanism(s) by which CME are
triggered and driven. Specifically, I will discuss how numerical (MHD)
simulations can help us to improve our understanding of CMEs.
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Title: The first widespread solar energetic particle event of solar
cycle 25 on 2020 November 29. Shock wave properties and the wide
distribution of solar energetic particles
Authors: Kouloumvakos, A.; Kwon, R. Y.; Rodríguez-García, L.; Lario,
D.; Dresing, N.; Kilpua, E. K. J.; Vainio, R.; Török, T.; Plotnikov,
I.; Rouillard, A. P.; Downs, C.; Linker, J. A.; Malandraki, O. E.;
Pinto, R. F.; Riley, P.; Allen, R. C.
2022A&A...660A..84K Altcode:
Context. On 2020 November 29, an eruptive event occurred in an active
region located behind the eastern solar limb as seen from Earth. The
event consisted of an M4.4 class flare, a coronal mass ejection,
an extreme ultraviolet (EUV) wave, and a white-light (WL) shock
wave. The eruption gave rise to the first widespread solar energetic
particle (SEP) event of solar cycle 25, which was observed at four
widely separated heliospheric locations (∼230°). <BR /> Aims: Our
aim is to better understand the source of this widespread SEP event,
examine the role of the coronal shock wave in the wide distribution
of SEPs, and investigate the shock wave properties at the field lines
magnetically connected to the spacecraft. <BR /> Methods: Using EUV
and WL data, we reconstructed the global three-dimensional structure of
the shock in the corona and computed its kinematics. We determined the
magnetic field configurations in the corona and interplanetary space,
inferred the magnetic connectivity of the spacecraft with the shock
surface, and derived the evolution of the shock parameters at the
connecting field lines. <BR /> Results: Remote sensing observations
show formation of the coronal shock wave occurring early during the
eruption, and its rapid propagation to distant locations. The results
of the shock wave modelling show multiple regions where a strong
shock has formed and efficient particle acceleration is expected
to take place. The pressure/shock wave is magnetically connected to
all spacecraft locations before or during the estimated SEP release
times. The release of the observed near-relativistic electrons occurs
predominantly close to the time when the pressure/shock wave connects to
the magnetic field lines or when the shock wave becomes supercritical,
whereas the proton release is significantly delayed with respect to the
time when the shock wave becomes supercritical, with the only exception
being the proton release at the Parker Solar Probe. <BR /> Conclusions:
Our results suggest that the shock wave plays an important role in the
spread of SEPs. Supercritical shock regions are connected to most of the
spacecraft. The particle increase at Earth, which is barely connected
to the wave, also suggests that the cross-field transport cannot be
ignored. The release of energetic electrons seems to occur close to
the time when the shock wave connects to, or becomes supercritical at,
the field lines connecting to the spacecraft. Energetic protons are
released with a time-delay relative to the time when the pressure/shock
wave connects to the spacecraft locations. We attribute this delay
to the time that it takes for the shock wave to accelerate protons
efficiently. <P />Movie associated to Fig. 2 is available at <A
href="https://www.aanda.org/10.1051/0004-6361/202142515/olm">https://www.aanda.org</A>
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Title: A Model of Homologous Confined and Ejective Eruptions Involving
Kink Instability and Flux Cancellation
Authors: Hassanin, Alshaimaa; Kliem, Bernhard; Seehafer, Norbert;
Török, Tibor
2022ApJ...929L..23H Altcode: 2022arXiv220411767H
In this study, we model a sequence of a confined and a full eruption,
employing the relaxed end state of the confined eruption of a
kink-unstable flux rope as the initial condition for the ejective
one. The full eruption, a model of a coronal mass ejection, develops
as a result of converging motions imposed at the photospheric
boundary, which drive flux cancellation. In this process, parts of
the positive and negative external flux converge toward the polarity
inversion line, reconnect, and cancel each other. Flux of the same
amount as the canceled flux transfers to a flux rope, increasing
the free magnetic energy of the coronal field. With sustained flux
cancellation and the associated progressive weakening of the magnetic
tension of the overlying flux, we find that a flux reduction of ≍11%
initiates the torus instability of the flux rope, which leads to a full
eruption. These results demonstrate that a homologous full eruption,
following a confined one, can be driven by flux cancellation.
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Title: Torus-stable zone above starspots
Authors: Sun, Xudong; Török, Tibor; DeRosa, Marc L.
2022MNRAS.509.5075S Altcode: 2021arXiv211103665S; 2021MNRAS.tmp.2934S
Whilst intense solar flares are almost always accompanied by a coronal
mass ejection (CME), reports on stellar CMEs are rare, despite the
frequent detection of stellar 'super flares'. The torus instability of
magnetic flux ropes is believed to be one of the main driving mechanisms
of solar CMEs. Suppression of the torus instability, due to a confining
background coronal magnetic field that decreases sufficiently slowly
with height, may contribute to the lack of stellar CME detection. Here,
we use the solar magnetic field as a template to estimate the vertical
extent of this 'torus-stable zone' (TSZ) above a stellar active
region. For an idealized potential field model comprising the fields
of a local bipole (mimicking a pair of starspots) and a global dipole,
we show that the upper bound of the TSZ increases with the bipole
size, the dipole strength, and the source surface radius where the
coronal field becomes radial. The boundaries of the TSZ depend on the
interplay between the spots' and the dipole's magnetic fields, which
provide the local- and global-scale confinement, respectively. They
range from about half the bipole size to a significant fraction of the
stellar radius. For smaller spots and an intermediate dipole field,
a secondary TSZ arises at a higher altitude, which may increase the
likelihood of 'failed eruptions'. Our results suggest that the low
apparent CME occurrence rate on cool stars is, at least partially,
due to the presence of extended TSZs.
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Title: Magnetogram-matching Energization and Eruption of Magnetic
Flux Ropes
Authors: Titov, Viacheslav; Downs, Cooper; Torok, Tibor; Linker, Jon
2021AGUFMSH32A..04T Altcode:
We propose a new technique for energizing coronal magnetic equilibria
toward eruptions. We achieve this via a sequence of MHD relaxations
of small line-tied pulses of magnetic helicity, each of which is
simulated by a suitable rescaling of the current-carrying part of
the field. The whole procedure is 'magnetogram-matching' because
it involves no changes to the normal component of the field at the
lower boundary. The technique is illustrated by application to bipolar
force-free configurations whose magnetic flux ropes (MFRs) are modeled
with our regularized Biot-Savart law method. We have found that, in
spite of the bipolar character of the ambient potential field in these
examples, the resulting MFR eruption is generally sustained by two
reconnection processes. The first, which we refer to as breakthrough
reconnection, is analogous to breakout reconnection in quadrupolar
configurations. It occurs at a quasi-separator field line located inside
the current layer that wraps around the erupting MFR, and results from
taking into account the line-tying effect at the photosphere. The second
process is the classical tether-cutting reconnection that develops at
the second quasi-separator inside a vertical current layer formed below
the erupting MFR. Both reconnection processes work in tandem to propel
the MFR through the overlying ambient field. The considered examples
suggest that our technique will be beneficial for both the modeling
of particular eruptive events and theoretical studies of eruptions
in idealized magnetic configurations. This research was supported by
NASA programs HTMS (award no. 80NSSC20K1274) and HSR (80NSSC19K0858
and 80NSSC20K1317); NASA/ NSF program DRIVE (80NSSC20K0604); and NSF
grants AGS-1135432, AGS-1923377, and ICER-1854790.
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Title: Propagation and Deflection of CMEs in Different Background
Magnetic Fields
Authors: Ben-Nun, Michal; Torok, Tibor; Downs, Cooper; Caplan, Ronald;
Lionello, Roberto
2021AGUFMSH35B2043B Altcode:
As suggested by Isenberg and Forbes (2007) and demonstrated numerically
by Kliem et al. (2012), the Lorentz forces stemming from the interaction
of the axial current in an erupting magnetic flux rope (MFR) with an
ambient magnetic-field component that has the same orientation as the
initial MFR axis lead to a rotation of the top part of the MFR about
its rise direction. In principle, the same mechanism can be applied to
CMEs that propagate in a unipolar radial field in the corona or inner
heliosphere. In such cases, however, the corresponding forces should
not lead to a rotation, but to a deflection of the CME front, thereby
significantly altering the CME's magnetic orientation. Apart from a
brief consideration in Lugaz et al. (2011), such deflections have, to
the best of our knowledge, not yet been studied systematically. Here
we employ three-dimensional (3D) idealized magnetohydrodynamic
(MHD) simulations to investigate this effect in background fields of
increasing complexity. We first consider a freely expanding toroidal
MFR in a uniform background field, as well as the propagation of a
compact, line-tied MFR in a unipolar radial field. In both cases,
we find significant deflections. We then use a more realistic setup,
in which we erupt an MFR from a localized, bipolar source region into
a global dipole field and solar wind, which allows for a significant
expansion of the MFR before it encounters an open field. We perform a
parametric study in which we vary the location and magnetic orientation
of the source region, as well as the handedness (helicity sign) of the
MFR. In this presentation, we discuss the influence of these parameters
on the CME trajectory, and on other important CME properties such as
its speed, morphology, and interaction (reconnection) with the ambient
magnetic field.
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Title: Theory and Models of Flare/CME Onset Via Flux Emergence and/or
Shear Flows
Authors: Linton, Mark; Torok, Tibor; Lynch, Benjamin
2021AGUFMSH35B2042L Altcode:
The onset of eruptive flare energy release requires both a buildup
of stored energy and a trigger for the release of that energy. This
talk will review key models of how this storage and release occurs in
solar eruptions, in particular for breakout eruptions and for torus
instability eruptions. In both cases, the eruptions require the buildup
of free magnetic energy in the form of sheared field. For the breakout
mechanism the energy is built up as sheared magnetic fields in coronal
arcades, while for the torus instability the energy is built up as a
combination of axial and twist field in coronal flux ropes. We will
review recent work on the buildup of this energy to eruptive states,
both via velocity shearing at the photosphere and via the emergence of
sheared flux from the convection zone into the corona. Then we will
review recent work exploring how the emergence of new magnetic flux
into the corona can act as a trigger for these eruptive events. Much
of the recent work to be discussed here is being carried out within
the framework of NASAs Living with a Star focused science team
on Understanding the Onset of Major Solar Eruptions. This work is
supported by the NASA Living with a Star program. Distribution A:
Approved for public release: distribution unlimited
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Title: Evolution of Non-neutralized Electric Currents in Eruptive
Solar Active Regions
Authors: Prazak, Michael; Downs, Cooper; Torok, Tibor; Jiong, Qiu;
Titov, Viacheslav
2021AGUFMSH35B2036P Altcode:
We study the evolution of several solar active regions (ARs)
that produced both fast (>600 km/s) and slow (<600 km/s)
coronal mass ejections (CMEs). We have been analyzing the vector
magnetic field measurements for the ARs during their disk passage,
and our preliminary results suggest that the ARs produced slow CMEs
throughout the observed period and fast CMEs only in the presence
of strong non-neutralized electric currents. We have also analyzed
the temporal and spatial evolution of non-neutralized currents in
NOAA AR 11305, which produced an M-class eruptive flare. Strong
non-neutralized currents, associated with emerging flux and shear
flows, were located along the magnetic polarity inversion line. The
observations suggest the presence of a magnetic flux rope (MFR), which
evolved and erupted, forming flare ribbons that encompass the areas
of strong current. The regularized Biot-Savart laws (rBSLs; Titov
et. al. 2018) formalism and a zero-$\beta$ magnetohydrodynamic (MHD)
model were employed to construct the aforementioned MFR and to follow
its evolution. The spatial location and the amount of non-neutralized
currents computed from the model well agree with the observational
measurements. This result suggests that the development of the MFR,
which can be identified by sustained and concentrated photospheric
non-neutralized electric currents, is an important element for the
production of the CME in this study. This work is supported by NASA
programs HSR (80NSSC20K1317) and HGI (80NSSC18K0622).
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Title: Partially Open Fields as the Energy Bounds for Solar Eruptions
Authors: Linker, Jon; Downs, Cooper; Caplan, Ronald; Kazachenko,
Maria; Torok, Tibor; Titov, Viacheslav; Lionello, Roberto; Riley, Pete
2021AGUFMSH42B..02L Altcode:
The energy source for major solar eruptions, such as flare and coronal
mass ejections (CMEs), is recognized to originate in the solar magnetic
field. Specifically, it is believed to be the release of the free
magnetic energy (energy above the potential field state) stored in
the field prior to eruption. A key question for both predicting future
eruptions and estimating their possible magnitude is, what is the bound
to this energy? The Aly-Sturrock theorem shows that the energy of a
fully force-free field cannot exceed the energy of the so-called open
field. If the theorem holds, this places an upper limit on the amount
of free energy that can be stored. In recent simulations, we have found
that the energy of a closely related field, the partially open field
(POF), can place a useful bound on the energy of an eruption from real
active regions, a much tighter constraint than the energy of the fully
open field. We are using a database of flare ribbons (Kazachenko et al.,
ApJ 845, 2017) to test this idea observationally. A flare ribbon mask
is defined as the area swept out by the ribbons during the flare. It
can serve as a proxy for the region of the field that opened during the
eruption. In a preliminary study, we used the ribbon masks to define
the POF for several large events originating in solar cycle 24 active
regions, and computed the energy of the POF. Our results suggested a
strong correlation of energy release in solar events and the POF. In
this presentation, we describe a continuation of this study, extending
it to a significant number of the M and X class flare occurring in
solar cycle 24. Work supported by NASA and NSF.
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Title: Flux rope reformation as a model for homologous solar flares
and coronal mass ejections
Authors: Saad Hassanin, Alshaimaa; Kliem, Bernhard; Torok, Tibor;
Seehafer, Norbert
2021AGUFMSH32A..09S Altcode:
In this study we model for the first time a sequence of a confined
and a full eruption, employing the flux rope reformed in the confined
eruption as the initial condition for the ejective one. The full
eruption develops as a result of imposed converging motions in the
photospheric boundary which drive flux cancellation. In this process,
a part of the positive and negative sunspot flux converge toward
the polarity inversion line, reconnect, and cancel each other. Flux
of the same amount as the canceled flux transfers to the flux rope,
building up free magnetic energy. With sustained flux cancellation
and the associated progressive weakening of the magnetic tension of
the overlying flux, we find that a flux reduction of 8.9% leads to the
ejective eruption. These results demonstrate that homologous eruptions,
eventually leading to a coronal mass ejection (CME), can be driven by
flux cancellation.
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Title: Tracking and Understanding the Trajectory of CMEs From Birth
to Late Stage
Authors: Zhang, Jie; Sakib, Md Nazmus; Torok, Tibor; Dhakal, Suman;
Nikou, Eleni
2021AGUFMSH35B2059Z Altcode:
We present a detailed observational study of the trajectory of coronal
mass ejections (CMEs) from their onset continuously to the late stage
of evolution in the outer corona. The 3-D trajectory of CMEs, along with
their sizes, kinematics and internal magnetic structure, determines the
full global state of a CME and how it evolves in the interplanetary
space. These properties are essential for understanding their
evolution, their interaction with the interplanetary magnetic field
and plasma, and ultimately their geoeffectiveness and space weather
consequences. Changes of the CME trajectory, often referred to as
deflection or channeling, have been observed mostly in the outer corona
and/or for events that originate near the solar limb. Measurements
of the early trajectory of CMEs that originate near the center of
the solar disk are rare, due to the limitation of coronagraphic
observations. However, these CMEs are of particular interest since
they are mor likely to hit the Earth and the likelihood of hitting is
sensitive to the trajectory. Here we present a novel technique for
determining CME trajectories near the solar surface, which employs
SDO/AIA observations and is based on the shape of coronal waves driven
by the CME expansion. We further use STEREO/COR1 and COR2 observations,
in combination with SOHO/LASCO data, to determine the 3D trajectory of
CMEs in the inner and outer corona, respectively. This study addresses
the following two main questions: (1) What are the rise directions of
CMEs right after the onset of the eruption and how do they change as
a function of height above the solar surface? (2) Are deviations from
a radial trajectory caused primarily by the deflection of the CME at
coronal holes or by the magnetic properties of the CME's source region?
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Title: Torus-Stable Zone Above Starspots
Authors: Sun, Xudong; Torok, Tibor; DeRosa, Marc
2021AGUFMSH32A..02S Altcode:
The torus instability (TI) of magnetic flux ropes is one of the main
driving mechanisms of solar coronal mass ejections (CMEs). If the
stabilizing background magnetic field decreases sufficiently slowly
with height, the TI will be suppressed. Here we estimate the vertical
extent of this "torus-stable zone" (TSZ) above starspots using the
solar magnetic field as a template. For a potential field comprising
a bipole as a pair of starspots and a global dipole, we show that the
upper bound of the TSZ increases with the bipole size, the dipole
strength, and the source surface radius where the coronal field
becomes radial. The values depend on the interplay between the spot
and dipole magnetic fields, which provide the local and global-scale
confinement, respectively. They range from about half the bipole size
to a significant fraction of the stellar radius. A secondary TSZ
sometimes arises at a higher altitude which may facilitate "failed
eruptions". The suppression of the TI may contribute to the lack of
CME detection on cool stars, as larger starspots, stronger dipole,
and more closed magnetic topology significantly expand the TSZ.
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Title: Optimization of Magnetic Flux Ropes Modeled with the
Regularized Biot-Savart Law Method
Authors: Titov, V. S.; Downs, C.; Török, T.; Linker, J. A.; Caplan,
R. M.; Lionello, R.
2021ApJS..255....9T Altcode: 2021arXiv210602789T
The so-called regularized Biot-Savart laws (RBSLs) provide an efficient
and flexible method for modeling pre-eruptive magnetic configurations
of coronal mass ejections (CMEs) whose characteristics are constrained
by observational images and magnetic field data. This method allows
one to calculate the field of magnetic flux ropes (MFRs) with small
circular cross sections and an arbitrary axis shape. The field of
the whole configuration is constructed as a superposition of (1)
such a flux-rope field and (2) an ambient potential field derived, for
example, from an observed magnetogram. The RBSL kernels are determined
from the requirement that the MFR field for a straight cylinder must
be exactly force free. For a curved MFR, however, the magnetic forces
are generally unbalanced over the whole path of the MFR. To minimize
these forces, we apply a modified Gauss-Newton method to find optimal
MFR parameters. This is done by iteratively adjusting the MFR axis
path and axial current. We then try to relax the resulting optimized
configuration in a subsequent line-tied zero-beta magnetohydrodynamic
simulation toward a force-free equilibrium. By considering two models of
the sigmoidal pre-eruption configuration for the 2009 February 13 CME,
we demonstrate how this approach works and what it is capable of. We
show, in particular, that the building blocks of the core magnetic
structure described by these models match morphological features
typically observed in such types of configurations. Our method will
be useful for both the modeling of particular eruptive events and
theoretical studies of idealized pre-eruptive MFR configurations.
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Title: The Upgraded RBSL Method Applied To The Modeling Of Sigmoidal
Pre-eruptive Magnetic Configurations
Authors: Titov, V. S.; Torok, T.; Downs, C.; Linker, J.; Caplan, R.;
Lionello, R.
2021AAS...23821312T Altcode:
The so-called regularized Biot-Savart laws (RBSLs, Titov et al.,
ApJL 2018) provide an efficient and flexible method for constructing
pre-eruptive configurations (PECs) whose characteristics are constrained
by remote-sensing observations. This method allows one to calculate the
field of magnetic flux ropes (MFRs) of small diameters and an arbitrary
axis shape. The field of the PEC is generally a superposition of (1)
such an MFR field, (2) an ambient potential field determined, e.g.,
by the radial field component of an observed magnetogram, and (3) a
so-called compensating potential field that counteracts perturbations
of the radial field by the MFR at the boundary. The constructed
PEC is then relaxed in a line-tied zero-beta MHD simulation toward a
force-free equilibrium. <P />We have recently upgraded our method in two
ways. First, we have reformulated the RBSLs so that the compensating
field can be neglected if the distance between the MFR footprints
is much less than the solar radius. Second, we have developed an
optimization method to minimize unbalanced magnetic forces prior to
the MHD relaxation of a modeled PEC. This minimization is obtained by
optimizing the shape and axial current of the corresponding MFR with
a modified Gauss-Newton method of least squares. <P />We apply the
upgraded method to construct sigmoidal PECs for the 2009 February 13
CME event. The resulting PECs have a complex core magnetic structure,
with the MFR nested within a sheared magnetic arcade. Both the MFR and
the arcade are bounded in the central region of the PECs by current
layers. Depending on the strength of the current in the pre-relaxed
MFR, the core of the final PEC can also contain a vertical current
layer, which is then embedded in the sheared arcade, underneath the
MFR. Our structural analysis reveals building blocks that match the
morphological features typically observed in bipolar PECs (e.g., Moore
et al., ApJ 2001) very well. This suggests that the method will not
only be beneficial as a tool for modeling solar eruptions, but also for
scientific studies that require a detailed understanding of the magnetic
structure of PECs. <P />*Research supported by NSF, NASA, and AFOSR
---------------------------------------------------------
Title: Torus-Stable Zone Above Starspots
Authors: Sun, X.; Torok, T.; DeRosa, M.
2021AAS...23820801S Altcode:
The torus instability (TI) of current-carrying magnetic flux tubes
is thought to drive many solar coronal mass ejections (CMEs). The
background magnetic field provides the stabilizing force: if it
decreases with height at a rate (decay index) slower than a critical
value, the TI may be suppressed. Here we estimate the vertical extent
of a "torus-stable zone" above starspots using a scaled model for the
Sun. For a potential-field model comprising a bipole (as a pair of
starspots) in alignment with a global dipole, we show that the upper
bound of this zone h<SUB>c</SUB> increases with the bipole size a,
the dipole field with harmonic coefficient g<SUB>10</SUB>, and the
source surface radius R<SUB>s</SUB> where the magnetic field becomes
radial. The value of h<SUB>c</SUB>, ranging from about 0.5a to a
significant fraction of the stellar radius, depends on the interplay
between the spot and dipole magnetic fields; its upper limit is set
by R<SUB>s</SUB>. Suppression of the TI may contribute to the lack of
CME detection from active cool stars, as larger starspots, stronger
dipole, and more closed magnetic topology significantly expand the
torus-stable zone.
---------------------------------------------------------
Title: Magnetic Field Curvature In A Filament Channel Derived From
Oscillation Measurements And MHD Modeling
Authors: Kucera, T. A.; Luna, M.; Torok, T.; Muglach, K.; Downs, C.;
Sun, X.; Thompson, B.; Karpen, J.; Gilbert, H.
2021AAS...23811306K Altcode:
We have used measurements of repeated large amplitude longitudinal
oscillations (LALOs) in an active region filament to diagnose the
curvature of the magnetic field in the filament channel and compared the
results with predictions of an MHD flux-rope model based on magnetograms
of the region. In May and June of 2014 Active Region 12076 exhibited a
complex of filaments undergoing repeated oscillations over the course
of twelve days. The central filament channel exhibited emerging and then
canceling magnetic flux that resulted in multiple activations, filament
eruptions, and eight oscillation events, which we analyzed using GONG
H-alpha data. Luna and Karpen (2012) model LALOs as oscillations of
magnetized filament plasma moving along dipped magnetic field lines
with gravity as a restoring force. Under this model the period of these
oscillations can be used to estimate the curvature of the magnetic
field in the location of the filament threads. Utilizing this, we find
that the measured periods in the central filament ranging from 34-74
minutes should correspond to magnetic field curvatures of about 30-136
Mm. We also derive radii of curvature for the central filament channel
using a flux-rope model that is based on an SDO/HMI magnetogram of the
region. The rope is constructed using the analytic expressions by Titov
et al. (2018) and then numerically relaxed towards a force-free state in
the zero-beta MHD approximation, where gravity and thermal pressure are
neglected. For comparison, we also employ a nonlinear force-free field
(NLFFF) extrapolation of the active region. We compare the results
of these different ways of attempting to determine the field in the
filament channel.
---------------------------------------------------------
Title: Torus-Stable Zone Above Starspots
Authors: Sun, Xudong; Török, Tibor; DeRosa, Marc
2021csss.confE..15S Altcode:
The torus instability (TI) of current-carrying magnetic flux tubes
is thought to drive many solar coronal mass ejections (CMEs). The
background magnetic field provides the stabilizing force: if it
decreases with height at a rate slower than a critical value, the
TI may be suppressed. Here we estimate the vertical extent of a
"torus-stable zone" above starspots using a scaled model for the
Sun. For a potential-field model comprising a bipole (as a pair of
starspots) in alignment with a global dipole, we show that the upper
bound of this zone h<SUB>c</SUB> increases with the bipole size a,
the dipole field with harmonic coefficient g<SUB>10</SUB>, and the
source surface radius R<SUB>s</SUB> where the magnetic field becomes
radial. The value of h<SUB>c</SUB>, ranging from about 0.5a to a
significant fraction of the stellar radius, depends on the interplay
between the spot and dipole magnetic fields; its upper limit is set
by R<SUB>s</SUB>. Suppression of the TI may contribute to the lack of
CME detection from active cool stars, as larger starspots, stronger
dipole, and more closed magnetic topology significantly expand the
torus-stable zone.
---------------------------------------------------------
Title: Energetic Proton Propagation and Acceleration Simulated for
the Bastille Day Event of 2000 July 14
Authors: Young, Matthew A.; Schwadron, Nathan A.; Gorby, Matthew;
Linker, Jon; Caplan, Ronald M.; Downs, Cooper; Török, Tibor; Riley,
Pete; Lionello, Roberto; Titov, Viacheslav; Mewaldt, Richard A.;
Cohen, Christina M. S.
2021ApJ...909..160Y Altcode: 2020arXiv201209078Y
This work presents results from simulations of the 2000 July 14
("Bastille Day") solar proton event. We used the Energetic Particle
Radiation Environment Model (EPREM) and the CORona-HELiosphere (CORHEL)
software suite within the SPE Threat Assessment Tool (STAT) framework
to model proton acceleration to GeV energies due to the passage of a
CME through the low solar corona, and we compared the model results
to GOES-08 observations. The coupled simulation models particle
acceleration from 1 to 20 R<SUB>⊙</SUB>, after which it models only
particle transport. The simulation roughly reproduces the peak event
fluxes and the timing and spatial location of the energetic particle
event. While peak fluxes and overall variation within the first few
hours of the simulation agree well with observations, the modeled CME
moves beyond the inner simulation boundary after several hours. The
model therefore accurately describes the acceleration processes in
the low corona and resolves the sites of most rapid acceleration close
to the Sun. Plots of integral flux envelopes from multiple simulated
observers near Earth further improve the comparison to observations
and increase potential for predicting solar particle events. Broken
power-law fits to fluence spectra agree with diffusive acceleration
theory over the low energy range. Over the high energy range,
they demonstrate the variability in acceleration rate and mirror
the interevent variability observed in solar cycle 23 ground-level
enhancements. We discuss ways to improve STAT predictions, including
using corrected GOES energy bins and computing fits to the seed
spectrum. This paper demonstrates a predictive tool for simulating
low-coronal solar energetic particle acceleration.
---------------------------------------------------------
Title: Decay Index Profile and Coronal Mass Ejection Speed
Authors: Kliem, Bernhard; Zhang, Jie; Torok, Tibor; Chintzoglou,
Georgios
2021cosp...43E.997K Altcode:
The velocity of coronal mass ejections (CMEs) is one of the primary
parameters determining their potential geoeffectiveness. A great
majority of very fast CMEs receive their main acceleration already
in the corona. We study the magnetic source region structure for
a complete sample of 15 very fast CMEs (v > 1500 km/s) during
2000--2006, originating within 30 deg from central meridian. We find
a correlation between CME speed and the decay index profile of the
coronal field estimated by a PFSS extrapolation. The correlation
is considerably weaker for an extended sample that includes slower
CMEs. We also study how the decay index profile is related to the
structure of the photospheric field distribution. This is complemented
by a parametric simulation study of flux-rope eruptions using the
analytic Titov-D\'emoulin active-region model for simple bipolar and
quadrupolar source regions. The simulations provide simple relationships
between the photospheric field distribution and the coronal decay index
profile. They also help identifying source regions which are likely to
produce slow CMEs only, thus improving the correlation for the extended
CME sample. Very fast, moderate-velocity, and even confined eruptions
are found, and the conditions for their occurrence are quantified.
---------------------------------------------------------
Title: Optimization of Magnetic Flux Ropes Modeled with the RBSL
Method*
Authors: Titov, V. S.; Downs, C.; Torok, T.; Linker, J.; Caplan,
R. M.; Lionello, R.
2020AGUFMSH0440022T Altcode:
The so-called regularized Biot-Savart laws (RBSLs, Titov et al. 2018)
provide an efficient and flexible method for modeling pre-eruptive
magnetic configurations whose characteristics are constrained by
observational image and magnetic-field data. This method allows
one to calculate the field of magnetic flux ropes (MFRs) with small
circular cross-sections and an arbitrary axis shape. The field of
the whole configuration is constructed as a superposition of (1)
such a flux-rope field, (2) an ambient potential field determined,
for example, by the radial field component of an observed magnetogram,
and (3) a so-called compensating potential field that counteracts
deviations of the radial field caused by the axial current of the
MFR. The RBSL kernels are determined from the requirement that the MFR
field for a straight cylinder must be exactly force-free. For a curved
MFR, however, the magnetic forces are generally unbalanced over the
whole path of the MFR. To reduce this imbalance, we apply a modified
Gauss-Newton method to minimize the magnitude of the residual magnetic
forces per unit length and the unit axial current of the MFR. This is
done by iteratively adjusting the MFR axis path and axial current. We
then try to relax the resulting optimized configuration in a subsequent
line-tied zero-beta MHD simulation toward a force-free equilibrium. By
considering several examples, we demonstrate how this approach works
depending on the initial parameters of the MFR and the ambient magnetic
field. Our method will be beneficial for both the modeling of particular
eruptive events and theoretical studies of idealized pre-eruptive
magnetic configurations. <P />* This research is supported by NSF,
NASA's HSR, SBIR, and LWS Programs, and AFOSR
---------------------------------------------------------
Title: Solar and Heliospheric Models at the CCMC - An Update
Authors: MacNeice, P. J.; Chulaki, A.; Mendoza, A. M. M.; Mays, M. L.;
Weigand, C.; Arge, C. N.; Jones, S. I.; Linker, J.; Downs, C.; Torok,
T.; Fisher, G. H.; Cheung, C. M. M.
2020AGUFMSH0030015M Altcode:
The Community Coordinated Modeling Center (CCMC) at NASA Goddard Space
Flight Center is the world largest repository of models dedicated to
Space Weather Research and forecasting. In this presentation we provide
an update on new additions and updates to the CCMC's inventory of Solar
and Heliospheric models. In particular, we describe the latest version
of WSA, the CORHEL TDM model, and the CGEM model suite. The latest
version of WSA is now available to users through our Runs-On-Request
websites as well as through its continuous near realtime execution. It
can use input magnetograms from an extensive list of observatories,
including maps processed using the ADAPT surface flux evolution
model, and can return results and solar wind forecasts at all inner
planets and most inner heliospheric spacecraft locations. The CORHEL
TDM model enables users to design flux ropes embedded in coronal
fields in derived from observed magnetograms, and then follow the
evolution of the flux rope using a zero-beta MHD code. A future upgrade
(currently in development at PredSci) w ill support full thermodynamic
CME simulations. Finally, the CGEM model suite supports the generation
and application of boundary conditions for realistic driving of coronal
3D field models based on times series observations of photospheric
vector magnetogram data.
---------------------------------------------------------
Title: Coronal Acceleration of Large and Acute SEP Events
Authors: Young, M.; Schwadron, N.; Gorby, M.; Linker, J.; Caplan,
R. M.; Downs, C.; Torok, T.; Riley, P.; Lionello, R.; Titov, V. S.;
Mewaldt, R.; Cohen, C.
2020AGUFMSH012..03Y Altcode:
Solar energetic particle (SEP) events pose a serious threat to
spacecraft and astronauts throughout the heliosphere. On Earth, strong
events can harm aircraft avionics, communication, and navigation. In
space, energetic particles can be hazardous for crews of Low Earth Orbit
spacecraft and the International Space Station, especially when engaged
in extravehicular activity. One important goal when studying energetic
particles in the heliosphere is providing a meaningful estimate of
their flux at a the location of a particular observer. At Earth,
good estimates of both the energetic particle flux and the expected
intra-event variability can significantly improve our ability to protect
space-based assets without incurring unnecessary operational delays. The
largest SEP events typically arise in conjunction with X-class flares
and very fast coronal mass ejections (CMEs). One probable mechanism for
accelerating energetic particles that propagate to Earth is the shock
wave or compression that forms low in the corona during the passage of
a CME. After the shock wave or compression forms, it propagates outward
and accelerates particles over a finite space for a finite time. These
energetic particles can travel at a significant fraction of the speed of
light and reach an observer soon after an eruptive event if the observer
and the acceleration site are magnetically connected. This connectivity
depends, in turn, on both the structure of the coronal and heliospheric
magnetic field and the local shock properties. Furthermore, intra-event
variability depends on local properties (e.g., mean free path and
rigidity) along the field line where the shock wave or compression
accelerates particles, and the finite scales over which the shock wave
or compression operates. This work presents results from a simulation
of the extreme SEP event on 14 July (Bastille Day) 2000, using the
Energetic Particle Radiation Environment Model (EPREM) coupled to
the Magnetohydrodynamic Algorithm outside a Sphere (MAS) code from
Predictive Science Incorporated (PSI). We show how coronal variability
in acceleration rate and compression strength maps to variability at
1 au, with a focus on field lines connected to near-Earth observers,
and how this intra-event variability compares to observed inter-event
variability in GOES data.
---------------------------------------------------------
Title: Partially Open Fields and the Energy of Solar Eruptions
Authors: Linker, J.; Downs, C.; Caplan, R. M.; Torok, T.; Kazachenko,
M.; Titov, V. S.; Lionello, R.; Riley, P.
2020AGUFMSH046..01L Altcode:
It has long been recognized that the energy source for major solar
flares and coronal mass ejections (CMEs) is the solar magnetic field
within active regions. Specifically, it is believed to be the release
of the free magnetic energy (energy above the potential field state)
stored in the field prior to eruption. For estimates of the free energy
to provide a prognostic for future eruptions, we must know how much
energy an active region can store - Is there a bound to this energy? <P
/>The Aly-Sturrock theorem shows that the energy of a fully force-free
field cannot exceed the energy of the so-called open field. If the
theorem holds, this places an upper limit on the amount of free energy
that can be stored. In recent simulations, we have found that the energy
of a closely related field, the partially open field (POF), can place
a useful bound on the energy of an eruption from real active regions,
a much tighter constraint than the energy of the fully open field. A
database of flare ribbons (Kazachenko et al., ApJ 845, 2017) offers us
an opportunity to test this idea observationally. A flare ribbon mask
is defined as the area swept out by the ribbons during the flare. It
can serve as a proxy for the region of the field that opened during
the eruption. In this preliminary study, we use the ribbon masks to
define the POF for several large events originating in solar cycle 24
active regions, and compute the energy of the POF. We compare these
energies with the X-ray fluxes and CME energies for these events. <P
/>Work supported by NSF, NASA, and AFOSR.
---------------------------------------------------------
Title: MHD Modeling of an Observed Solar Filament
Authors: Torok, T.; Downs, C.; Titov, V. S.; Lionello, R.; Linker, J.
2020AGUFMSH041..04T Altcode:
The physical mechanisms by which solar prominences (or filaments) form
are still not well understood. The presently most favored scenario
invokes the evaporation of chromospheric plasma via localized heating
at the footprints of a magnetic flux rope (MFR) or sheared arcade, and
the subsequent condensation of this plasma in the corona due to thermal
non-equilibrium (TNE). This scenario has been modeled extensively in
one-dimensional (1D) hydrodynamic simulations along static magnetic
field lines and, very recently, also in fully 3D magnetohydrodynamic
(MHD) simulations, using idealized MFR configurations. However, such
configurations lack the complexity of real prominence magnetic fields,
which poses additional challenges. Here we report on our recent attempts
to employ data-constrained MHD simulations to model the formation of
observed filaments. To this end, we selected the filament that erupted
in a spectacular manner on June 7, 2011 in NOAA AR 11226. To model its
formation, we first develop a semi-realistic ("thermodynamic MHD") model
of the solar corona, using SDO/HMI data as boundary condition for the
magnetic field. Next, we insert an MFR constructed with the RBSL method
(Titov et al., 2018) into the source region of the filament. Finally,
after a short relaxation of the system, we impose localized heating in
the footprint regions of the MFR. In our study, we vary the geometry
and footprint locations of the MFR, as well as the amount of heating
in the respective MFR footprints, and investigate how these parameters
affect the formation of persistent plasma condensations. We compare
our results with simulations of prominence formation in idealized
MFR configurations, and we discuss the difficulties that arise once
realistic cases are considered.
---------------------------------------------------------
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.
---------------------------------------------------------
Title: Electric-current Neutralization and Eruptive Activity of
Solar Active Regions
Authors: Torok, Tibor; Liu, Yang; Leake, James E.; Sun, Xudong; Titov,
Viacheslav S.
2020EGUGA..22.1654T Altcode:
The physical conditions that determine the eruptive activity of
solar active regions (ARs) are still not well understood. Various
proxies for predicting eruptive activity have been suggested, with
relatively limited success. Moreover, it is presently unclear under
which conditions an eruption will remain confined to the low corona
or produce a coronal mass ejection (CME).Using vector magnetogram data
from SDO/HMI, we investigate the association between electric-current
neutralization and eruptive activity for a sample of ARs. We find
that the vast majority of CME-producing ARs are characterized by a
strongly non-neutralized total current, while the total current in
ARs that do not produce CMEs is almost perfectly neutralized, even
if those ARs produce strong (X-class) confined flares. This suggests
that the degree of current neutralization can serve as a good proxy
for assessing the ability of ARs to produce CMEs.
---------------------------------------------------------
Title: Initiation and Early Kinematic Evolution of Solar Eruptions
Authors: Cheng, X.; Zhang, J.; Kliem, B.; Török, T.; Xing, C.;
Zhou, Z. J.; Inhester, B.; Ding, M. D.
2020ApJ...894...85C Altcode: 2020arXiv200403790C
We investigate the initiation and early evolution of 12 solar eruptions,
including six active-region hot channel and six quiescent filament
eruptions, which were well observed by the Solar Dynamics Observatory,
as well as by the Solar Terrestrial Relations Observatory for the
latter. The sample includes one failed eruption and 11 coronal mass
ejections, with velocities ranging from 493 to 2140 km s<SUP>-1</SUP>. A
detailed analysis of the eruption kinematics yields the following main
results. (1) The early evolution of all events consists of a slow-rise
phase followed by a main-acceleration phase, the height-time profiles
of which differ markedly and can be best fit, respectively, by a linear
and an exponential function. This indicates that different physical
processes dominate in these phases, which is at variance with models
that involve a single process. (2) The kinematic evolution of the
eruptions tends to be synchronized with the flare light curve in both
phases. The synchronization is often but not always close. A delayed
onset of the impulsive flare phase is found in the majority of the
filament eruptions (five out of six). This delay and its trend to be
larger for slower eruptions favor ideal MHD instability models. (3)
The average decay index at the onset heights of the main acceleration
is close to the threshold of the torus instability for both groups
of events (although, it is based on a tentative coronal field model
for the hot channels), suggesting that this instability initiates and
possibly drives the main acceleration.
---------------------------------------------------------
Title: Prediction of Coronal Structure for the July 2, 2019 Total
Solar Eclipse: Comparison with Observations
Authors: Linker, J.; Downs, C.; Caplan, R. M.; Riley, P.; Titov,
V. S.; Lionello, R.; Torok, T.; Reyes, A.
2019AGUFMSH13A..04L Altcode:
We are fortunate to study the Sun at a time when so many space-based
assets observe the Sun and its corona remotely, as well as measuring
the solar wind (the interplanetary extension of the corona) in
situ. Yet even at this time, total solar eclipses offer an unparalleled
opportunity to observe the low and middle corona. These observations
in turn are a powerful test of coronal models. As is our tradition,
we used our global thermodynamic magnetohydrodynamic (MHD) model
to predict the structure of the solar corona prior to the July 2,
2019 total solar eclipse. The key observational input to the model are
measurements of the photospheric magnetic field. The model incorporates
a wave-turbulence driven (WTD) description of coronal heating and solar
wind acceleration, and shear/twist in the magnetic field at the observed
location of filament channels. We compare our prediction with available
eclipse images, and with SDO/AIA and STEREO EUVI data. A particular
emphasis in this prediction was the inclusion of parasitic polarities
in the polar regions, as this creates plume-like structures in the
simulated corona. Significant plumes were indeed observed, which we
compare with the modeled structures. <P />Research supported by NASA,
AFOSR, and NSF.
---------------------------------------------------------
Title: Modeling Magnetic Flux Ropes with the RBSL Method
Authors: Titov, V. S.; Downs, C.; Torok, T.; Caplan, R. M.; Linker,
J.; Lionello, R.; Reyes, A.
2019AGUFMSH33B3386T Altcode:
We describe progress in developing a method for smoothly embedding
magnetic flux ropes (MFRs) with various internal structures into ambient
potential magnetic fields. The method uses the so-called regularized
Biot-Savart laws (RBSLs, Titov et al. 2018), which enable one to
calculate the magnetic field produced by axial and azimuthal currents
flowing in a channel with a circular cross-section and an axis path
of arbitrary shape. In the latest version of our method, the whole
configuration is a superposition of the following three fields: 1)
the MFR field determined by the RBSLs, 2) an ambient potential field
determined, for example, by the radial field component of an observed
magnetogram, and 3) a so-called compensating potential field that
counteracts perturbations of the radial field at the surface by the
MFR field. To make the configuration as force-free as possible, the
method aims to optimize the MFR characteristics in two ways. First,
for a cylindrical force-free MFR, we determine the corresponding
kernels of the RBSLs for different profiles of the axial current
density and use those to calculate the magnetic field of a thin,
curved MFR, which has slightly imbalanced magnetic forces. Second, we
minimize this imbalance by iteratively adjusting the shape of the MFR,
based on the line density of magnetic forces that we calculate along
the MFR at every iteration. If the resulting optimized MFR is stable,
a subsequent line-tied zero-beta MHD relaxation will typically yield
a force-free MFR whose parameters are quite close to the initial
ones. MFR configurations produced in this way should be very useful
for modeling solar eruptions, because the initial MFR parameters are
largely determined by the specific properties of the source-region. Our
efficient method also facilitates parametric studies and the stability
analysis of pre-eruptive configurations. We demonstrate this by using an
idealized model of a toroidal-arc MFR, for which we derive the critical
decay index of its ambient field as a function of the MFR parameters.
---------------------------------------------------------
Title: Exploring Plasma Heating in the Current Sheet Region in a
Three-dimensional Coronal Mass Ejection Simulation
Authors: Reeves, Katharine K.; Török, Tibor; Mikić, Zoran; Linker,
Jon; Murphy, Nicholas A.
2019ApJ...887..103R Altcode: 2019arXiv191005386R
We simulate a coronal mass ejection using a three-dimensional
magnetohydrodynamic code that includes coronal heating, thermal
conduction, and radiative cooling in the energy equation. The magnetic
flux distribution at 1 R <SUB> s </SUB> is produced by a localized
subsurface dipole superimposed on a global dipole field, mimicking
the presence of an active region within the global corona. Transverse
electric fields are applied near the polarity inversion line to
introduce a transverse magnetic field, followed by the imposition of
a converging flow to form and destabilize a flux rope, producing an
eruption. We examine the quantities responsible for plasma heating and
cooling during the eruption, including thermal conduction, radiation,
adiabatic effects, coronal heating, and ohmic heating. We find that
ohmic heating is an important contributor to hot temperatures in the
current sheet region early in the eruption, but in the late phase,
adiabatic compression plays an important role in heating the plasma
there. Thermal conduction also plays an important role in the transport
of thermal energy away from the current sheet region throughout the
reconnection process, producing a “thermal halo” and widening the
region of high temperatures. We simulate emission from solar telescopes
for this eruption and find that there is evidence for emission from
heated plasma above the flare loops late in the eruption, when the
adiabatic heating is the dominant heating term. These results provide an
explanation for hot supra-arcade plasma sheets that are often observed
in X-rays and extreme ultraviolet wavelengths during the decay phase
of large flares.
---------------------------------------------------------
Title: Solar Eruptions Triggered by Flux Emergence
Authors: Torok, T.; Linton, M.; Leake, J. E.; Mikic, Z.; Titov, V. S.;
Lionello, R.
2019AGUFMSH33B3390T Altcode:
Observations have shown a clear association of prominence eruptions
and CMEs with the emergence of magnetic flux close to, or within,
filament channels. It has been suggested that reconnection triggered
by the emergence destroys the force balance between the magnetic
field in the filament channel and its ambient field, causing the
former to erupt. Magnetohydrodynamic (MHD) numerical simulations
support this scenario for two-dimensional (2D) coronal flux-rope
configurations. However, such simulations do not take into account 3D
effects such as the anchoring of the flux rope in the dense photosphere
or the occurrence of 3D MHD instabilities. Here we present the first
3D MHD simulations of (boundary-driven) flux emergence in the vicinity
of a pre-existing coronal flux rope. We find that three processes
are important for the evolution of the system: (1) expansion or
contraction of the coronal field due to the intrusion of new flux,
(2) reconnection between the emerging and pre-existing flux systems,
and (3) repulsion or attraction of the respective current channels. We
vary the position and orientation of the emerging flux and investigate
under which conditions these processes can trigger an eruption.
---------------------------------------------------------
Title: Data-Constrained Modeling of Eruptions in the Solar Corona:
Insights from 3D MHD Simulations
Authors: Downs, C.; Torok, T.; Titov, V. S.; Linker, J.
2019AGUFMSH32A..06D Altcode:
Understanding the energy storage and release processes of solar
eruptions, also known as Coronal Mass Ejections (CMEs), remains
challenging on multiple fronts. On one hand, the complexity of solar
active regions, and the myriad ways in which they present themselves,
makes it difficult to pin down the essential mechanisms of CMEs. On
the other hand, our primary way of observing the early stages of CMEs
is through remote sensing diagnostics, which provide only partial
inferences of the underlying plasma state. Sophisticated models that
capture both the energy storage and release processes in tandem with
remote sensing diagnostics are one way to approach this problem. Using
these ideas to frame our discussion, we present an overview of the
essential steps for constructing a case-study, data-constrained model of
a solar eruption using a 3D thermodynamic MHD model of the global solar
corona. Choices for boundary conditions, energy storage, initiation,
and the global coronal background all have significant consequences
for the ensuing evolution and interpretation of results. Despite these
complexities and challenges, we show how such modeling can be used
to directly connect the observable consequences of CMEs (EUV waves,
coronal dimming) to their underlying physical processes (CME expansion
and connectivity changes). Such modeling helps to unify our picture of
CMEs as they evolve and interact with disparate regions of the solar
atmosphere. Future prospects will also be discussed.
---------------------------------------------------------
Title: Stabilities of magnetic flux ropes anchored in the solar
surface
Authors: Qiu, J.; Downs, C.; Titov, V. S.; Torok, T.
2019AGUFMSH33B3391Q Altcode:
Eruptions of a magnetic flux rope in the solar corona can be
triggered by certain magnetohydrodynamic (MHD) instabilities, such
as the helical <P />kink instability or the torus instability. The
instability threshold depends on the geometry of the flux rope, and
it is not a-priori clear which instability will occur or dominate in a
given configuration. Furthermore, a magnetic flux rope is anchored in
the photosphere, and this line-tying condition imposes an essential
constraint on the stability analysis. The modified Titov-Demoulin
model (TDm, Titov et al. 2014) provides a three-parameter family of
approximate equilibria of a toroidal-arc magnetic flux rope anchored
in the photosphere that is smoothly embedded in an ambient bipolar
potential field in a spherical geometry. This study examines the
stability of a force-free TDm magnetic flux rope in dependence of its
geometric parameters. The global MHD MAS code is used to evolve the
flux ropes under the line-tying condition. For a given perturbation of
the flux rope, a configuration is not stable if it never returns to
an equilibrium or if it evolves into a new equilibrium that deviates
strongly from the initial equilibrium solution. Our preliminary study
suggests that configurations with an X-line below the flux rope are
usually not stable, and also confirms that a high-lying flux rope can
be kink-stable even for large twist (see, e.g., Torok et al. 2004).
---------------------------------------------------------
Title: Formation and Eruption of Magnetic Flux Ropes in the Solar
Corona
Authors: Linton, Mark; Torok, Tibor; Leake, James
2019AAS...23430201L Altcode:
Magnetic flux ropes in the solar corona are often thought to form
the basis for both long-lived coronal prominences and for eruptive
coronal mass ejections. In this presentation we discuss numerical
simulations exploring how such flux rope structures can both be created
and disrupted via the emergence of magnetic flux from the convection
zone into the corona. The presentation will focus on the formation of
these structures directly from flux emergence as well as the effects,
both stabilizing and eruptive, that newly emerging flux can have on
pre-existing coronal flux ropes. <P />This work is supported by the
NASA Living with a Star program and the Chief of Naval Research.
---------------------------------------------------------
Title: Bounding the Energy of Solar Eruptions
Authors: Linker, Jon A.; Downs, Cooper; Caplan, Ronald M.; Torok,
Tibor; Riley, Pete; Titov, Viacheslav; Lionello, Roberto; Mikic,
Zoran; Amari, Tahar
2019AAS...23431704L Altcode:
Major solar eruptions such as X-class flares and coronal mass ejections
(CMEs) are the fundamental source of solar energetic particles and
geomagnetic storms, and are thus key drivers of space weather at
Earth. The energy for solar eruptions is recognized to originate in
the solar magnetic field, and is believed to be stored as free magnetic
energy (energy above the potential field state) prior to eruption. Solar
active regions are the site of the most violent activity. Solar active
regions can store widely varying amounts of energy, so knowledge of
the free energy alone does not necessarily tell us when an eruption
is imminent. For estimates of the free energy to provide predictive
power, we must know how much energy a region can store - what is the
energy bound? <P />In recent work, we have found that the energy of a
particular field, the partially open field (POF), can place a useful
bound on the energy of an eruption from real active regions, a much
tighter constraint than the energy of the fully open field. However,
in general, it is difficult to solve for the POF. In this presentation,
we discuss methods for approximating the energy of this field, and
show a comparison of the approximation for a case where the solution
is known. We discuss the implications for understanding and predicting
major solar eruptions. <P />Research supported by NASA and AFOSR
---------------------------------------------------------
Title: Coupled MHD-Focused Transport Simulations for Modeling Solar
Particle Events
Authors: Linker, Jon A.; Caplan, Ronald M.; Schwadron, Nathan;
Gorby, Matthew; Downs, Cooper; Torok, Tibor; Lionello, Roberto;
Wijaya, Janvier
2019JPhCS1225a2007L Altcode: 2019arXiv190505299L
We describe the initial version of the Solar Particle Event (SPE) Threat
Assessment Tool or STAT. STAT relies on elements of Corona-Heliosphere
(CORHEL) and the Earth-Moon-Mars Radiation Environment Module (EMMREM),
and allows users to investigate coronal mass ejection (CME) driven
SPEs using coupled magnetohydrodynamic (MHD) and focused transport
solutions. At the present time STAT focuses on modeling solar energetic
particle (SEP) acceleration in and transport from the low corona,
where the highest energy SEP events are generated. We illustrate
STAT’s capabilities with a model of the July 14, 2000 “Bastille
Day” event, including innovative diagnostics for understanding the
three-dimensional distribution of particle fluxes and their relation
to the structure of the underlying CME driver. A preliminary comparison
with NOAA GOES measurements is shown.
---------------------------------------------------------
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.
---------------------------------------------------------
Title: GPU Acceleration of an Established Solar MHD Code using OpenACC
Authors: Caplan, R. M.; Linker, J. A.; Mikić, Z.; Downs, C.; Török,
T.; Titov, V. S.
2019JPhCS1225a2012C Altcode: 2018arXiv181102605C
GPU accelerators have had a notable impact on high-performance
computing across many disciplines. They provide high performance with
low cost/power, and therefore have become a primary compute resource
on many of the largest supercomputers. Here, we implement multi-GPU
acceleration into our Solar MHD code (MAS) using OpenACC in a fully
portable, single-source manner. Our preliminary implementation is
focused on MAS running in a reduced physics “zero-beta” mode. While
valuable on its own, our main goal is to pave the way for a full
physics, thermodynamic MHD implementation. We describe the OpenACC
implementation methodology and challenges. “Time-to-solution”
performance results of a production-level flux rope eruption
simulation on multi-CPU and multi-GPU systems are shown. We find that
the GPU-accelerated MAS code has the ability to run “zero-beta”
simulations on a single multi-GPU server at speeds previously requiring
multiple CPU server-nodes of a supercomputer.
---------------------------------------------------------
Title: What is the pre-eruptive structure of CMEs?
Authors: Torok, Tibor
2019shin.confE..45T Altcode:
Coronal mass ejections (CMEs) are, together with major flares,
the largest energy-release processes in the solar system and the
main driver of space-weather disturbances close to the Earth. While
CMEs have been studied for almost half a century, many open questions
remain, especially regarding their initiation and pre-eruptive magnetic
structure. There is little doubt that CMEs are organized as flux ropes,
but there still exists a heated debate on whether the flux rope exits
prior to the eruption or is formed (from a sheared arcade) during the
eruption process. In this talk I will review the main arguments in favor
of the flux-rope picture, and I will argue that our current perspective
of 'either flux rope or arcade' is too simplistic and should be revised.
---------------------------------------------------------
Title: Prediction of Coronal Structure for the July 2, 2019 Total
Solar Eclipse
Authors: Linker, Jon A.; Downs, C.; Caplan, R. M.; Riley, P.; Lionello,
R.; Titov, V. S.; Torok, T.; Reyes, A.
2019shin.confE..66L Altcode:
We are fortunate to study the Sun at a time when so many space-based
assets observe the Sun and its corona remotely, as well as measuring
the solar wind (the interplanetary extension of the corona) in
situ. Yet even at this time, total solar eclipses offer an unparalleled
opportunity to observe the low and middle corona. These observations
in turn are a powerful test of coronal models. As is our tradition,
we are predicting the structure of the solar corona for the July 2,
2019 total solar eclipse. We describe progress and challenges in
modeling the global thermodynamic and magnetic state of the solar
corona for the prediction. The key observational input to the model are
measurements of the photospheric magnetic field. The model incorporates
a wave-turbulence driven (WTD) description of coronal heating and solar
wind acceleration, and shear/twist in the field at the expected location
of filament channels. The prediction will include images of brightness,
polarization brightness, EUV, and X-rays. We describe comparisons of
our prediction with available ground-based observations of the eclipse,
as well as observations from SDO/AIA, Hinode/XRT, and STEREO/EUVI. <P
/>Research supported by NASA, AFOSR, and NSF.
---------------------------------------------------------
Title: Particle Radiation Sources, Propagation and Interactions
in Deep Space, at Earth, the Moon, Mars, and Beyond: Examples of
Radiation Interactions and Effects
Authors: Schwadron, Nathan A.; Cooper, John F.; Desai, Mihir; Downs,
Cooper; Gorby, Matt; Jordan, Andrew P.; Joyce, Colin J.; Kozarev,
Kamen; Linker, Jon A.; Mikíc, Zoran; Riley, Pete; Spence, Harlan E.;
Török, Tibor; Townsend, Lawrence W.; Wilson, Jody K.; Zeitlin, Cary
2019sfsw.book..257S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The Physical Processes of CME/ICME Evolution
Authors: Manchester, Ward, IV; Kilpua, Emilia K. J.; Liu, Ying D.;
Lugaz, Noé; Riley, Pete; Török, Tibor; Vršnak, Bojan
2019sfsw.book..165M Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The Origin, Early Evolution and Predictability of Solar
Eruptions
Authors: Green, Lucie M.; Török, Tibor; Vršnak, Bojan; Manchester,
Ward, IV; Veronig, Astrid
2019sfsw.book..113G Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Electric-current Neutralization and Eruptive Activity of
Solar Active Regions
Authors: Liu, Yang; Sun, Xudong; Torok, Tibor; Titov, Viacheslav S.;
Leake, James E.
2018csc..confE..39L Altcode:
The physical conditions that determine the eruptive activity of active
regions (ARs) are still not well understood. Various observational
proxies for predicting eruptive activity have been suggested, with
rather limited success. Moreover, it is presently unclear under which
conditions an eruption will remain confined to the low corona or
produce a coronal mass ejection (CME). Using vector magnetogram data
from SDO/HMI, we investigate the association between electric-current
neutralization and eruptive activity for four ARs. Two ARs produced
flares and CMEs, one produced only (very strong) confined flares,
and one did not exhibit significant eruptions. We find that both
CME-producing ARs are characterized by a strongly non-neutralized
total current, while the total current in the remaining ARs is almost
perfectly neutralized. This suggests that the degree of current
neutralization may serve as a proxy for assessing the ability of ARs
to produce CMEs.
---------------------------------------------------------
Title: Global Non-Potential Magnetic Models of the Solar Corona
During the March 2015 Eclipse
Authors: Yeates, Anthony R.; Amari, Tahar; Contopoulos, Ioannis; Feng,
Xueshang; Mackay, Duncan H.; Mikić, Zoran; Wiegelmann, Thomas; Hutton,
Joseph; Lowder, Christopher A.; Morgan, Huw; Petrie, Gordon; Rachmeler,
Laurel A.; Upton, Lisa A.; Canou, Aurelien; Chopin, Pierre; Downs,
Cooper; Druckmüller, Miloslav; Linker, Jon A.; Seaton, Daniel B.;
Török, Tibor
2018SSRv..214...99Y Altcode: 2018arXiv180800785Y
Seven different models are applied to the same problem of simulating
the Sun's coronal magnetic field during the solar eclipse on 2015
March 20. All of the models are non-potential, allowing for free
magnetic energy, but the associated electric currents are developed
in significantly different ways. This is not a direct comparison
of the coronal modelling techniques, in that the different models
also use different photospheric boundary conditions, reflecting
the range of approaches currently used in the community. Despite
the significant differences, the results show broad agreement in the
overall magnetic topology. Among those models with significant volume
currents in much of the corona, there is general agreement that the
ratio of total to potential magnetic energy should be approximately
1.4. However, there are significant differences in the electric current
distributions; while static extrapolations are best able to reproduce
active regions, they are unable to recover sheared magnetic fields in
filament channels using currently available vector magnetogram data. By
contrast, time-evolving simulations can recover the filament channel
fields at the expense of not matching the observed vector magnetic
fields within active regions. We suggest that, at present, the best
approach may be a hybrid model using static extrapolations but with
additional energization informed by simplified evolution models. This
is demonstrated by one of the models.
---------------------------------------------------------
Title: Sequential Eruptions Triggered by Flux Emergence: Observations
and Modeling
Authors: Dacie, S.; Török, T.; Démoulin, P.; Linton, M. G.; Downs,
C.; van Driel-Gesztelyi, L.; Long, D. M.; Leake, J. E.
2018ApJ...862..117D Altcode: 2018arXiv180700020D
We describe and analyze observations by the Solar Dynamics Observatory
of the emergence of a small, bipolar active region within an area of
unipolar magnetic flux that was surrounded by a circular, quiescent
filament. Within only 8 hours from the start of the emergence, a
partial splitting of the filament and two consecutive coronal mass
ejections took place. We argue that all three dynamic events occurred
as a result of particular magnetic-reconnection episodes between
the emerging bipole and the pre-existing coronal magnetic field. To
substantiate our interpretation, we consider 3D magnetohydrodynamic
simulations that model the emergence of magnetic flux in the vicinity of
a large-scale coronal flux rope. The simulations qualitatively reproduce
most of the reconnection episodes suggested by the observations, as
well as the filament splitting, the first eruption, and the formation
of sheared/twisted fields that may have played a role in the second
eruption. Our results suggest that the position of emerging flux with
respect to the background magnetic configuration is a crucial factor for
the resulting evolution, while previous results suggest that parameters
such as the orientation or the amount of emerging flux are important
as well. This poses a challenge for predicting the onset of eruptions
that are triggered by flux emergence, and calls for a detailed survey
of the relevant parameter space by means of numerical simulations.
---------------------------------------------------------
Title: Predicting the corona for the 21 August 2017 total solar
eclipse
Authors: Mikić; , Zoran; Downs, Cooper; Linker, Jon A.; Caplan, Ronald
M.; Mackay, Duncan H.; Upton, Lisa A.; Riley, Pete; Lionello, Roberto;
Török, Tibor; Titov, Viacheslav S.; Wijaya, Janvier; Druckmüller,
Miloslav; Pasachoff, Jay M.; Carlos, Wendy
2018NatAs...2..913M Altcode: 2018NatAs.tmp..120M
The total solar eclipse that occurred on 21 August 2017 across the
United States provided an opportunity to test a magnetohydrodynamic
model of the solar corona driven by measured magnetic fields. We used
a new heating model based on the dissipation of Alfvén waves, and
a new energization mechanism to twist the magnetic field in filament
channels. We predicted what the corona would look like one week before
the eclipse. Here, we describe how this prediction was accomplished,
and show that it compared favourably with observations of the
eclipse in white light and extreme ultraviolet. The model allows us to
understand the relationship of observed features, including streamers,
coronal holes, prominences, polar plumes and thin rays, to the magnetic
field. We show that the discrepancies between the model and observations
arise from limitations in our ability to observe the Sun's magnetic
field. Predictions of this kind provide opportunities to improve the
models, forging the path to improved space weather prediction.
---------------------------------------------------------
Title: 3D MHD Modeling of Prominence Formation and Eruption
Authors: Torok, Tibor
2018cosp...42E3414T Altcode:
The formation of prominences in the solar corona is widely thought
to be caused by plasma evaporation and condensation via thermal
non-equilibrium. This process has been previously modeled by
one-dimensional hydrodynamic simulations along individual field lines
within static magnetic fields. A more realistic modeling of prominence
formation can be achieved through the use of three-dimensional
thermodynamic MHD simulations with time-dependent magnetic fields,
which has only become feasible within the last few years. In this talk
I will describe recent three-dimensional simulations that display the
formation and eruption of prominence-like structures. The capabilities
and limitations of these simulations will be discussed, along with
the next steps that lie ahead.
---------------------------------------------------------
Title: The formation and eruption of external flux ropes in emerging
active regions;
Authors: Leake, James Edward; Torok, Tibor
2018shin.confE..79L Altcode:
Solar eruptions such as CMEs and coronal jets arise from the presence of
filament channels (FCs), which are located above the polarity inversion
lines (PILs) of photospheric magnetic fields. Observational studies of
FC formation is limited to a handful of cases, and so the underlying
mechanism of FC formation has not yet been clarified
---------------------------------------------------------
Title: Generalizing the RBSL-method for Flux Ropes with Various
Current Profiles and Nonzero External Axial Field
Authors: Titov, Viacheslav; Linker, Jon; Mikic, Zoran; Downs, Cooper;
Torok, Tibor; Caplan, Ronald; Wijaya, Janvier
2018cosp...42E3391T Altcode:
Magnetic flux ropes (FRs) likely play a key role in prominence formation
and solar eruptions.It is therefore important to develop methods for
constructing FR configurations constrained by observational data.With
this aim, we have recently derived a pair of regularized Biot-Savart
laws (RBSLs; Titov et al. 2017) that allow one to efficiently calculate
the magnetic vector potential of an FR with circular cross-sections
and an axis of arbitrary shape.One of the RBSLs represents the axial
component of the vector potential produced by the axial current of the
FR, while the other represents the azimuthal component produced by the
axial flux of the FR.The kernels of the RBSLs are regularized at the
axis in such a way that, when the axis is straight, the RBSLs define a
cylindrical flux rope whose structure is exactly force-free.Therefore,
a curved thin FR defined by the RBSLs with the same kernels is
approximately force-free.Originally, we implemented the RBSLs only
for FRs that have a parabolic profile of the axial current and a
vanishing axial magnetic field at the FR surface.Here we present a
two-parametric generalization of the method that describes FRs with
various axial-current profiles and a nonvanishing external axial field
existing in sheared configurations.To benchmark this generalization, we
applied it first to simple configurations of a toroidal-arc FR embedded
into a potential background field, which are geometrically similar to
the model proposed by Titov & Démoulin (1999).We investigated the
numerical FR equilibria reached in zero-beta MHD relaxations of these
configurations in dependence of the initial axial-current profile
and the strength of the external axial field. We plan to apply the
generalized RBSLs to more realistic and complex configurations. Our
previous successful applications of the RBSLs for FRs with a parabolic
axial-current profile suggest the following. The shape of the FR
axis can be determined in more complicated cases by tracking the
observed polarity inversion line of the eruptions' source region and
estimating its height variation as well as other FR parameters by means
of a potential field extrapolated from the observed magnetogram. This
research was supported by NASA's HSR, LWS, and HGI programs,NSF grants
AGS-1560411 and AGS-1135432,and AFOSR contract FA9550-15-C-0001.
---------------------------------------------------------
Title: Data-constrained simulation of a Double-decker Eruption
Authors: Savcheva, Antonia; Kliem, B.; Downs, Copper; Torok, Tibor
2018shin.confE..91S Altcode:
We present the challenges we encountered in producing the initial
condition for the hypnotized double-decker flux rope eruption on
12/07/12. We then use this I.C. in Kliem-Torok zero-beta MHD simulation
to produce an eruption and reproduce overall magnetic field structure
of the eruption. We also produce a full thermodynamic simulation with
MAS of a simpler IC which also produces similarities to the observed
ribbons and dimmings. Both approaches have their advantages.
---------------------------------------------------------
Title: The Role of Emerging Magnetic Flux in the Initiation of
Solar Eruptions
Authors: Linton, Mark; Torok, Tibor; Leake, James; Schuck, Pete
2018cosp...42E2029L Altcode:
The eruption of solar magnetic fields, generating flares and coronal
mass ejections,and accelerating energetic particles, can have
significant effects on the Earth'sspace environment. The source
of these eruptions is ultimately the magnetic fieldwhich emerges
from the solar interior into the solar atmosphere to energize the
coronalmagnetic field. We will review theoretical and observational
evidence for thegeneration of eruptions by flux emergence. We will
then report on a series of numericalexperiments exploring how dynamic
flux emergence into either potential or pre-energizedcoronal fields
can lead to eruptions.This work was supported by the NASA Living with
a Star program.
---------------------------------------------------------
Title: Using MHD Simulations for Space-Weather Forecasting: Where
do we Stand?
Authors: Torok, Tibor; Linker, Jon; Mikic, Zoran; Riley, Pete; Titov,
Viacheslav; Lionello, Roberto; Downs, Cooper; Caplan, Ronald; Wijaya,
Janvier
2018cosp...42E3415T Altcode:
Coronal mass ejections (CMEs) are the main driver of space-weather
disturbances in the terrestrial magnetosphere. Predicting the impact
of CMEs before they arrive at Earth is one of the main challenges
of solar and heliospheric physics. A candidate tool for this purpose
are numerical simulations. State-of-the-art MHD simulations are now
capable of modeling CMEs all the way from Sun to Earth, but they
are computationally still too demanding to be used for real-time
modeling. At present, only a simplified model (ENLIL), which does not
include the corona and simulates CMEs as velocity perturbations, is used
for operational space-weather forecast. However, given the continuous
increase of computing power, more sophisticated simulations may become
available for this purpose in the near future, and first attempts
are currently made to prepare for operational use. A specific task at
hand is to evaluate the accuracy of these simulations in reproducing
in-situ measurements at Earth. I this presentation, we will briefly
review state-of-the-art CME simulations and discuss their predictive
capabilities and limitations. As an example, we will present a recent
Sun-to-Earth simulation of the well-known 14 July 2000 "Bastille-Day"
event, which produced a very strong geomagnetic storm.
---------------------------------------------------------
Title: Coordinated Observations and Modeling of Accelerated Particles
at the Sun and in the Inner Heliosphere
Authors: Schwadron, Nathan; Mays, M. Leila; Linker, Jon; Spence,
Harlan; Townsend, Lawrence; De Wet, Wouter; Gorby, Matthew; Wilson,
. Jody; Odstrcil, Dusan; Downs, Cooper; Torok, Tibor; Winslow, Reka;
Caplan, Ronald; Niehof, Jon
2018cosp...42E3043S Altcode:
Acute space radiation hazards pose one of the most serious risks to
future human and robotic exploration. Large solar energetic particle
(SEP) events are dangerous to astronauts and equipment. A fundamental
question remains as to how large SEP events are formed, how they are
related to coronal mass ejections (CMEs) and active region eruptions,
and what factors control the differential fluxes incident at Earth
and other observers during SEP events. The current evolution of the
Sun between solar cycles 23 and 24 and during cycle 24 remains highly
anomalous compared to previous periods of the space age. The Sun has
been abnormally quiet over a relatively long solar minimum when galactic
cosmic rays (GCRs) achieved the highest flux levels observed in the
space age, and the power, pressure, flux and magnetic flux of the solar
wind were at the lowest levels. Despite the continued paucity of solar
activity, one of the hardest solar events in almost a decade occurred
in September 2017 after more than a year of all-clear periods. The
2017 September event demonstrates the importance of large fluxes of
suprathermal seed populations and fast, large CMEs that drive shocks
and compressions low in the corona (<5 Rs) where large SEP events
are accelerated. The Coronal-Solar Wind Energetic Particle Acceleration
(C-SWEPA) modeling effort and the SPE Threat Assessment Tool (STAT)
combine the Earth-Moon-Mars Radiation Environment Modules (EMMREM)
that describe energetic particles and their effects, with the CORHEL
(Corona-Heliosphere) modeling suite developed by the Predictive Science,
Inc. (PSI) group. C-SWEPA has also developed coupling between EMMREM
and Enlil at the CCMC. The C-SWEPA and STAT projects have resulted
in coupled models that describe the conditions of the corona, solar
wind, CMEs, associated shocks, particle acceleration, and propagation
via physics-based modules. Recent simulations demonstrate how CMEs
form powerful compressions and shocks low in the corona that rapidly
accelerate high energy particles often up to the GeV energies required
for Ground Level Enhancements (GLEs). The most pronounced acceleration
of the CME occurs very close to the Sun (<2 Rs) causing extremely
strong compression on the flanks and nose of the CME. Results from
recent modeling and observational studies demonstrate that the size
of the shock or compression limits the break energy and longitudinal
distribution of SEPs. The new capabilities afforded by STAT and C-SWEPA
highlight the pathway toward prediction for large SEPs, and provide
an important resource for answering fundamental new questions likely
to arise from Parker Solar Probe and Solar Orbiter measurements.
---------------------------------------------------------
Title: Evaluating Uncertainties in Coronal Electron Temperature
and Radial Speed Measurements Using a Simulation of the Bastille
Day Eruption
Authors: Reginald, Nelson; St. Cyr, Orville; Davila, Joseph;
Rastaetter, Lutz; Török, Tibor
2018SoPh..293...82R Altcode:
Obtaining reliable measurements of plasma parameters in the Sun's
corona remains an important challenge for solar physics. We previously
presented a method for producing maps of electron temperature and
speed of the solar corona using K-corona brightness measurements made
through four color filters in visible light, which were tested for
their accuracies using models of a structured, yet steady corona. In
this article we test the same technique using a coronal model of the
Bastille Day (14 July 2000) coronal mass ejection, which also contains
quiet areas and streamers. We use the coronal electron density,
temperature, and flow speed contained in the model to determine two
K-coronal brightness ratios at (410.3, 390.0 nm) and (423.3, 398.7
nm) along more than 4000 lines of sight. Now assuming that for real
observations, the only information we have for each line of sight are
these two K-coronal brightness ratios, we use a spherically symmetric
model of the corona that contains no structures to interpret these
two ratios for electron temperature and speed. We then compare the
interpreted (or measured) values for each line of sight with the
true values from the model at the plane of the sky for that same line
of sight to determine the magnitude of the errors. We show that the
measured values closely match the true values in quiet areas. However,
in locations of coronal structures, the measured values are predictably
underestimated or overestimated compared to the true values, but can
nevertheless be used to determine the positions of the structures
with respect to the plane of the sky, in front or behind. Based on our
results, we propose that future white-light coronagraphs be equipped
to image the corona using four color filters in order to routinely
create coronal maps of electron density, temperature, and flow speed.
---------------------------------------------------------
Title: Partially Open Fields and Solar Eruptions
Authors: Linker, Jon; Mikic, Zoran; Downs, Cooper; Caplan, Ronald M.;
Riley, Pete; Torok, Tibor; Titov, Viacheslav S.; Lionello, Roberto;
Amari, Tahar
2018tess.conf10905L Altcode:
Partially Open Fields and Solar Eruptions* <P />Major solar eruptions
such as X-class flares and coronal mass ejections (CMEs) are the
progenitors of solar energetic particles and geomagnetic storms, and are
thus key drivers of space weather at Earth. The solar magnetic field
is the ultimate source of these massive events, the energy of which
is believed to be stored as free magnetic energy (energy above the
potential field state) prior to eruption. The amount of free magnetic
energy available in a given region is therefore a crucial indicator
of its propensity for eruption. However, solar active regions,
from which the largest events originate, can store widely varying
amounts of energy. Therefore, estimates of the free energy alone are
likely to be insufficient for knowing when a region will erupt; we
must also estimate the bounds on how much energy can be stored in a
given region. <P />The Aly-Sturrock theorem (Aly, ApJ 1991; Sturrock,
ApJ 1991) shows that the energy of a fully force-free field cannot
exceed the energy of the so-called open field. If the theorem holds,
this places an upper limit on the amount of free energy that can
be stored. In this paper, we describe how a closely related field,
the partially open field (Wolfson & Low ApJ 1992; Hu, ApJ 2004;
Aly & Amari, GAFD 2007), may place a much tighter bound on energy
storage and yield insights as to when major eruptions from an active
region are imminent (Amari et al., Nature, 2014). We demonstrate
the idea for AR9077, the source of the July 14, 2000 "Bastille Day"
flare/CME. <P />*Research supported by NASA and AFOSR
---------------------------------------------------------
Title: Evaluating Uncertainties in Coronal Electron Temperature and
Radial Speed Measurements Using a Simulation of the Bastille-Day
Eruption
Authors: Reginald, Nelson Leslie; St Cyr, O. C.; Davila, Joseph M.;
Rastaetter, Lutz; Torok, Tibor
2018tess.conf10410R Altcode:
Obtaining reliable measurements of plasma parameters in the Sun's corona
remains an important challenge for solar physics. We have previously
conducted field experiments using MACS and ISCORE instruments to create
maps of electron temperature and speed in the plane of the sky of
the solar corona using K-corona brightness measurements made through
four color filters in visible light. These instrumental techniques
were tested for their accuracy by conducting synthetic observations
on models that contained streamers and quiet areas and results were
presented in (Reginald et al., 2014, Solar Phys., 289, 2021). Here,
we present similar results from conducting synthetic observations
on a coronal model of the Bastille-Day (July 14, 2000) coronal mass
ejection that also contains streamers and quiet regions. We use the
coronal electron density, temperature, and flow speed contained in
the Bastille-Day model to determine two K-coronal brightness ratios at
(410.0, 390.0 nm) and (423.3, 398.7 nm) along more than 4000 lines of
sight on eight select frames. Now assuming that, for real observations,
the only information we have for each line of sight are these two
K-coronal brightness ratios, we then use a spherically symmetric model
of the corona that contains no structures to interpret these two ratios
for electron temperature and speed in the plane of the sky. Finally,
for each line of sight, we compare the interpreted (or measured)
value with the true value from the Bastille-Day model in the plane
of the sky to determine the magnitude of the error. An example of the
three step process applied on one frame in the Bastille-Day model is
shown in the image. We show that the measured values closely match
the true values in quiet areas. However, in locations of coronal
structures the measured values are predictably underestimated or
overestimated over the true values, but can nevertheless be used to
determine the positions of the structures with respect to the plane
of the sky, in front or behind. Our results show the potential for
future white-light coronagraphs be equipped with four color filters
instead of the customary single filter to produce synoptic maps of
electron density, temperature and flow speed in the plane of the sky.
---------------------------------------------------------
Title: Sun-To-Earth MHD Simulation of the 14 JULY 2000 "Bastille
Day" Eruption
Authors: Torok, Tibor; Downs, Cooper; Linker, Jon A.; Lionello,
Roberto; Titov, Viacheslav S.; Mikic, Zoran; Riley, Pete; Caplan,
Ron M.; Wijaya, Janvier
2018EGUGA..20.5564T Altcode:
Solar eruptions are the main driver of space-weather disturbances at
the Earth. Extreme events are of particular interest, not only because
of the scientific challenges they pose, but also because of their
possible societal consequences. Here we present a magnetohydrodynamic
(MHD) simulation of the 14 July 2000 “Bastille Day" eruption,
which produced a very strong geomagnetic storm. After constructing a
“thermodynamic" MHD model of the corona and solar wind, we insert a
magnetically stable flux rope along the polarity inversion line of
the eruption's source region and initiate the eruption by boundary
flows. More than 10<SUP>33</SUP> ergs of magnetic energy are released
in the eruption within a few minutes, driving a flare, an EUV wave, and
a coronal mass ejection (CME) that travels in the outer corona at ≈
1500 km s<SUP>-1</SUP>, close to the observed speed. We then propagate
the CME to Earth, using a heliospheric MHD code. Our simulation thus
provides the opportunity to test how well in situ observations of
extreme events are matched if the eruption is initiated from a stable
magnetic-equilibrium state. We find that the flux-rope center is very
similar in character to the observed magnetic cloud, but arrives
≈ 8.5 hours later and ≈ 15° too far to the North, with field
strengths that are too weak by a factor of ≈ 1.6. The front of the
flux rope is highly distorted, exhibiting localized magnetic-field
concentrations as it passes 1 AU. We discuss these properties with
regard to the development of space-weather predictions based on MHD
simulations of solar eruptions.
---------------------------------------------------------
Title: Sun-to-Earth MHD Simulation of the 2000 July 14 “Bastille
Day” Eruption
Authors: Török, Tibor; Downs, Cooper; Linker, Jon A.; Lionello, R.;
Titov, Viacheslav S.; Mikić, Zoran; Riley, Pete; Caplan, Ronald M.;
Wijaya, Janvier
2018ApJ...856...75T Altcode: 2018arXiv180105903T
Solar eruptions are the main driver of space-weather disturbances at
Earth. Extreme events are of particular interest, not only because
of the scientific challenges they pose, but also because of their
possible societal consequences. Here we present a magnetohydrodynamic
(MHD) simulation of the 2000 July 14 “Bastille Day” eruption,
which produced a very strong geomagnetic storm. After constructing
a “thermodynamic” MHD model of the corona and solar wind, we
insert a magnetically stable flux rope along the polarity inversion
line of the eruption’s source region and initiate the eruption
by boundary flows. More than 10<SUP>33</SUP> erg of magnetic energy
is released in the eruption within a few minutes, driving a flare,
an extreme-ultraviolet wave, and a coronal mass ejection (CME) that
travels in the outer corona at ≈1500 km s<SUP>-1</SUP>, close to the
observed speed. We then propagate the CME to Earth, using a heliospheric
MHD code. Our simulation thus provides the opportunity to test how well
in situ observations of extreme events are matched if the eruption is
initiated from a stable magnetic equilibrium state. We find that the
flux-rope center is very similar in character to the observed magnetic
cloud, but arrives ≈8.5 hr later and ≈15° too far to the north,
with field strengths that are too weak by a factor of ≈1.6. The front
of the flux rope is highly distorted, exhibiting localized magnetic
field concentrations as it passes 1 au. We discuss these properties
with regard to the development of space-weather predictions based on
MHD simulations of solar eruptions.
---------------------------------------------------------
Title: The Origin, Early Evolution and Predictability of Solar
Eruptions
Authors: Green, Lucie M.; Török, Tibor; Vršnak, Bojan; Manchester,
Ward; Veronig, Astrid
2018SSRv..214...46G Altcode: 2018arXiv180104608G
Coronal mass ejections (CMEs) were discovered in the early 1970s
when space-borne coronagraphs revealed that eruptions of plasma
are ejected from the Sun. Today, it is known that the Sun produces
eruptive flares, filament eruptions, coronal mass ejections and failed
eruptions; all thought to be due to a release of energy stored in
the coronal magnetic field during its drastic reconfiguration. This
review discusses the observations and physical mechanisms behind this
eruptive activity, with a view to making an assessment of the current
capability of forecasting these events for space weather risk and impact
mitigation. Whilst a wealth of observations exist, and detailed models
have been developed, there still exists a need to draw these approaches
together. In particular more realistic models are encouraged in order
to asses the full range of complexity of the solar atmosphere and the
criteria for which an eruption is formed. From the observational side,
a more detailed understanding of the role of photospheric flows and
reconnection is needed in order to identify the evolutionary path that
ultimately means a magnetic structure will erupt.
---------------------------------------------------------
Title: Regularized Biot-Savart Laws for Modeling Magnetic Flux Ropes
Authors: Titov, Viacheslav S.; Downs, Cooper; Mikić, Zoran; Török,
Tibor; Linker, Jon A.; Caplan, Ronald M.
2018ApJ...852L..21T Altcode: 2017arXiv171206708T
Many existing models assume that magnetic flux ropes play a key role
in solar flares and coronal mass ejections (CMEs). It is therefore
important to develop efficient methods for constructing flux-rope
configurations constrained by observed magnetic data and the morphology
of the pre-eruptive source region. For this purpose, we have derived
and implemented a compact analytical form that represents the magnetic
field of a thin flux rope with an axis of arbitrary shape and circular
cross-sections. This form implies that the flux rope carries axial
current I and axial flux F, so that the respective magnetic field is the
curl of the sum of axial and azimuthal vector potentials proportional
to I and F, respectively. We expressed the vector potentials in terms
of modified Biot-Savart laws, whose kernels are regularized at the
axis in such a way that, when the axis is straight, these laws define a
cylindrical force-free flux rope with a parabolic profile for the axial
current density. For the cases we have studied so far, we determined
the shape of the rope axis by following the polarity inversion line of
the eruptions’ source region, using observed magnetograms. The height
variation along the axis and other flux-rope parameters are estimated
by means of potential-field extrapolations. Using this heuristic
approach, we were able to construct pre-eruption configurations for
the 2009 February 13 and 2011 October 1 CME events. These applications
demonstrate the flexibility and efficiency of our new method for
energizing pre-eruptive configurations in simulations of CMEs.
---------------------------------------------------------
Title: Predicting the Magnetic Properties of ICMEs: A Pragmatic View
Authors: Riley, P.; Linker, J.; Ben-Nun, M.; Torok, T.; Ulrich, R. K.;
Russell, C. T.; Lai, H.; de Koning, C. A.; Pizzo, V. J.; Liu, Y.;
Hoeksema, J. T.
2017AGUFMSH31D..08R Altcode:
The southward component of the interplanetary magnetic field plays
a crucial role in being able to successfully predict space weather
phenomena. Yet, thus far, it has proven extremely difficult to forecast
with any degree of accuracy. In this presentation, we describe an
empirically-based modeling framework for estimating Bz values during
the passage of interplanetary coronal mass ejections (ICMEs). The model
includes: (1) an empirically-based estimate of the magnetic properties
of the flux rope in the low corona (including helicity and field
strength); (2) an empirically-based estimate of the dynamic properties
of the flux rope in the high corona (including direction, speed,
and mass); and (3) a physics-based estimate of the evolution of the
flux rope during its passage to 1 AU driven by the output from (1) and
(2). We compare model output with observations for a selection of events
to estimate the accuracy of this approach. Importantly, we pay specific
attention to the uncertainties introduced by the components within
the framework, separating intrinsic limitations from those that can
be improved upon, either by better observations or more sophisticated
modeling. Our analysis suggests that current observations/modeling are
insufficient for this empirically-based framework to provide reliable
and actionable prediction of the magnetic properties of ICMEs. We
suggest several paths that may lead to better forecasts.
---------------------------------------------------------
Title: 3D MHD Modeling of Prominence Formation by Plasma Evaporation
and Condensation
Authors: Torok, T.; Lionello, R.; Mikic, Z.; Downs, C.; Titov, V. S.
2017AGUFMSH41C..07T Altcode:
The formation of prominence material in the solar corona still belongs
to the open questions of solar physics. There exists a consensus
that prominence plasma has to be of chromospheric origin, but the
mechanisms by which it accumulates in the corona are still not well
understood. The presently most accepted scenario invokes the evaporation
of chromospheric plasma via foot point heating and its subsequent
condensation in the corona via thermal instabilities. This scenario
has been successfully modeled in 1D hydrodynamic simulations along
single field lines of a static magnetic field, but a more appropriate,
fully 3D treatment of the thermodynamics in time-dependent magnetic
fields was started just very recently by Xia et al. Our group at
PSI has recently begun to engage in this challenging task as well,
using our time-dependent, fully 3D thermodynamic MHD code MAS. For
our investigation we consider two different coronal flux-rope
configurations, using the analytical model by Titov and Démoulin and
a model in which an elongated flux rope is constructed by photospheric
flows. We investigate the plasma behavior for both configurations,
using heating models of different complexity, and accompany our analysis
by 1D loop simulations performed along selected field lines. In this
presentation, we outline our modeling approach and discuss the results
obtained so far.
---------------------------------------------------------
Title: Ion Charge States in the July 14, 2000 CME: MHD Simulations
Authors: Lionello, R.; Riley, P.; Torok, T.; Linker, J.; Mikic, Z.;
Raymond, J. C.; Shen, C.
2017AGUFMSH11B2438L Altcode:
In situ measurements of ion fractional charge states at 1 AU and
elsewhere can provide important information about electron temperatures
back in the corona, since, once "frozen in," the charge states remain
essentially unaltered as they travel through the solar wind. For
example, high-ionization states suggest that the plasma originated
from hotter regions on the solar corona. However, connecting these
in situ measurements with remote spectroscopic observations has
proven difficult. Using a global MHD model of the solar corona and
heliosphere, which includes the self-consistent calculation of minor
ion charge states, we compute the fractional charge state profiles of
several ions associated with the CME that occurred on July 14, 2000
and the ambient solar wind. Our approach is based on non-equilibrium
ionization calculations, which are more accurate than the standard
ionization equilibrium way of computing charge states. We follow the
evolution of these profiles, together with the magnetofluid parameters
as the plasma propagates from the low corona to 1 AU. We discuss the
results of the CME simulations, compare them with in situ measurements,
and relate them to theories for the origin of CMEs.
---------------------------------------------------------
Title: Thermal energy creation and transport and X-ray/EUV emission
in a thermodynamic MHD CME simulation
Authors: Reeves, K.; Mikic, Z.; Torok, T.; Linker, J.; Murphy, N. A.
2017AGUFMSH11C..07R Altcode:
We model a CME using the PSI 3D numerical MHD code that includes
coronal heating, thermal conduction and radiative cooling in the
energy equation. The magnetic flux distribution at 1 Rs is produced by
a localized subsurface dipole superimposed on a global dipole field,
mimicking the presence of an active region within the global corona. We
introduce transverse electric fields near the neutral line in the
active region to form a flux rope, then a converging flow is imposed
that causes the eruption. We follow the formation and evolution of
the current sheet and find that instabilities set in soon after the
reconnection commences. We simulate XRT and AIA EUV emission and find
that the instabilities manifest as bright features emanating from the
reconnection region. We examine the quantities responsible for plasma
heating and cooling during the eruption, including thermal conduction,
radiation, adiabatic compression and expansion, coronal heating and
ohmic heating due to dissipation of currents. We find that the adiabatic
compression plays an important role in heating the plasma around the
current sheet, especially in the later stages of the eruption when the
instabilities are present. Thermal conduction also plays an important
role in the transport of thermal energy away from the current sheet
region throughout the reconnection process.
---------------------------------------------------------
Title: Regularized Biot-Savart Laws for Modeling Magnetic
Configurations with Flux Ropes
Authors: Titov, V. S.; Downs, C.; Mikic, Z.; Torok, T.; Linker, J.
2017AGUFMSH12A..06T Altcode:
Many existing models assume that magnetic flux ropes play a key role
in solar flares and coronal mass ejections (CMEs). It is therefore
important to develop efficient methods for constructing flux-rope
configurations constrained by observed magnetic data and the initial
morphology of CMEs. For this purpose, we have derived and implemented
a compact analytical form that represents the magnetic field of
a thin flux rope with an axis of arbitrary shape and a circular
cross-section. This form implies that the flux rope carries axial
current I and axial flux F, so that the respective magnetic field is the
curl of the sum of toroidal and poloidal vector potentials proportional
to I and F, respectively. We expressed the vector potentials in terms
of modified Biot-Savart laws whose kernels are regularized at the axis
in such a way that these laws define a cylindrical force-free flux
rope with a parabolic profile of the axial current density, when the
axis is straight. For the cases we have studied so far, we determined
the shape of the rope axis by following the polarity inversion line of
the eruptions' source region, using observed magnetograms. The height
variation along the axis and other flux-rope parameters are estimated
by means of potential field extrapolations. Using this heuristic
approach, we were able to construct pre-eruption configurations for
the 2009 February13 and 2011 October 1 CME events. These applications
demonstrate the flexibility and efficiency of our new method for
energizing pre-eruptive configurations in MHD simulations of CMEs. We
discuss possible ways of optimizing the axis paths and other extensions
of the method in order to make it more useful and robust. Research
supported by NSF, NASA's HSR and LWS Programs, and AFOSR.
---------------------------------------------------------
Title: Modeling the 21 August 2017 Total Solar Eclipse: Prediction
Results and New Techniques
Authors: Downs, C.; Mikic, Z.; Caplan, R. M.; Linker, J.; Lionello,
R.; Torok, T.; Titov, V. S.; Riley, P.; MacKay, D.; Upton, L.
2017AGUFMSH13B2475D Altcode:
As has been our tradition for past solar eclipses, we conducted a
high resolution magnetohydrodynamic (MHD) simulation of the corona
to predict the appearance of the 21 August 2017 solar eclipse. In
this presentation, we discuss our model setup and our forward
modeled predictions for the corona's appearance, including images
of polarized brightness and EUV/soft X-Ray emission. We show how
the combination of forward modeled observables and knowledge of the
underlying magnetic field from the model can be used to interpret
the structures seen during the eclipse. We also discuss two new
features added to this year's prediction. First, in an attempt to
improve the morphological shape of streamers in the low corona,
we energize the large-scale magnetic field by emerging shear and
canceling flux within filament channels. The handedness of the shear
is deduced from a magnetofrictional model, which is driven by the
evolving photospheric field produced by the Advective Flux Transport
model. Second, we apply our new wave-turbulence-driven (WTD) model for
coronal heating. This model has substantially fewer free parameters
than previous empirical heating models, but is inherently sensitive to
the 3D geometry and connectivity of the magnetic field--a key property
for modeling the thermal-magnetic structure of the corona. We examine
the effect of these considerations on forward modeled observables,
and present them in the context of our final 2017 eclipse prediction
(www.predsci.com/corona/aug2017eclipse). Research supported by NASA's
Heliophysics Supporting Research and Living With a Star Programs.
---------------------------------------------------------
Title: Particle Radiation Sources, Propagation and Interactions
in Deep Space, at Earth, the Moon, Mars, and Beyond: Examples of
Radiation Interactions and Effects
Authors: Schwadron, Nathan A.; Cooper, John F.; Desai, Mihir; Downs,
Cooper; Gorby, Matt; Jordan, Andrew P.; Joyce, Colin J.; Kozarev,
Kamen; Linker, Jon A.; Mikíc, Zoran; Riley, Pete; Spence, Harlan E.;
Török, Tibor; Townsend, Lawrence W.; Wilson, Jody K.; Zeitlin, Cary
2017SSRv..212.1069S Altcode: 2017SSRv..tmp...63S
Particle radiation has significant effects for astronauts, satellites
and planetary bodies throughout the Solar System. Acute space radiation
hazards pose risks to human and robotic exploration. This radiation also
naturally weathers the exposed surface regolith of the Moon, the two
moons of Mars, and other airless bodies, and contributes to chemical
evolution of planetary atmospheres at Earth, Mars, Venus, Titan, and
Pluto. We provide a select review of recent areas of research covering
the origin of SEPs from coronal mass ejections low in the corona,
propagation of events through the solar system during the anomalously
weak solar cycle 24 and important examples of radiation interactions
for Earth, other planets and airless bodies such as the Moon.
---------------------------------------------------------
Title: The Physical Processes of CME/ICME Evolution
Authors: Manchester, Ward; Kilpua, Emilia K. J.; Liu, Ying D.; Lugaz,
Noé; Riley, Pete; Török, Tibor; Vršnak, Bojan
2017SSRv..212.1159M Altcode: 2017SSRv..tmp...90M
As observed in Thomson-scattered white light, coronal mass ejections
(CMEs) are manifest as large-scale expulsions of plasma magnetically
driven from the corona in the most energetic eruptions from the
Sun. It remains a tantalizing mystery as to how these erupting magnetic
fields evolve to form the complex structures we observe in the solar
wind at Earth. Here, we strive to provide a fresh perspective on the
post-eruption and interplanetary evolution of CMEs, focusing on the
physical processes that define the many complex interactions of the
ejected plasma with its surroundings as it departs the corona and
propagates through the heliosphere. We summarize the ways CMEs and
their interplanetary CMEs (ICMEs) are rotated, reconfigured, deformed,
deflected, decelerated and disguised during their journey through the
solar wind. This study then leads to consideration of how structures
originating in coronal eruptions can be connected to their far removed
interplanetary counterparts. Given that ICMEs are the drivers of most
geomagnetic storms (and the sole driver of extreme storms), this work
provides a guide to the processes that must be considered in making
space weather forecasts from remote observations of the corona.
---------------------------------------------------------
Title: Electric-current Neutralization, Magnetic Shear, and Eruptive
Activity in Solar Active Regions
Authors: Liu, Yang; Sun, Xudong; Török, Tibor; Titov, Viacheslav S.;
Leake, James E.
2017ApJ...846L...6L Altcode: 2017arXiv170804411L
The physical conditions that determine whether or not solar active
regions (ARs) produce strong flares and coronal mass ejections (CMEs)
are not yet well understood. Here, we investigate the association
between electric-current neutralization, magnetic shear along polarity
inversion lines (PILs), and eruptive activity in four ARs: two emerging
and two well-developed ones. We find that the CME-producing ARs are
characterized by a strongly non-neutralized total current, while the
total current in the ARs that did not produce CMEs is almost perfectly
neutralized. The difference in the PIL shear between these two groups
is much less pronounced, which suggests that the degree of current
neutralization may serve as a better proxy for assessing the ability
of ARs to produce CMEs.
---------------------------------------------------------
Title: Neutralization of Electric Current, Magnetic Shear, and
Eruptive Activity in Solar Active Regions
Authors: Liu, Yang; Sun, Xudong; Torok, Tibor; Titov, Viacheslav;
Leake, James E.
2017SPD....4830002L Altcode:
There has been an ongoing debate on whether or not the electric currents
in solar active regions (ARs) are neutralized. Current-neutralization
means that the direct coronal currents that connect the AR polarity
centers are surrounded by return currents of equal total strength
and opposite direction, i.e. the net current is zero. Using data from
SDO/HMI, we analyze the direct and return currents in four ARs; two
eruptive ones and two non-eruptive ones. The eruptive ARs produced
strong flares and CMEs (successful eruptions), while the non-eruptive
ARs include one quiet AR that produced no strong eruptions and one that
produced a series of failed eruptions. It is found that the eruptive
ARs have strong net currents and large shear of the magnetic field near
their polarity inversion lines (PILs). In contrast, the currents in
the non-eruptive ARs are well neutralized, and the PIL-shear is rather
small. This agrees with MHD simulations that demonstrate a relationship
between the level of current neutralization and the amount of magnetic
shear near the PIL. We discuss the implications of these results for
the capability of ARs to produce strong eruptions.
---------------------------------------------------------
Title: New Techniques Used in Modeling the 2017 Total Solar Eclipse:
Energizing and Heating the Large-Scale Corona
Authors: Downs, Cooper; Mikic, Zoran; Linker, Jon A.; Caplan, Ronald
M.; Lionello, Roberto; Torok, Tibor; Titov, Viacheslav; Riley, Pete;
Mackay, Duncan; Upton, Lisa
2017SPD....4820802D Altcode:
Over the past two decades, our group has used a magnetohydrodynamic
(MHD) model of the corona to predict the appearance of total solar
eclipses. In this presentation we detail recent innovations and
new techniques applied to our prediction model for the August 21,
2017 total solar eclipse. First, we have developed a method for
capturing the large-scale energized fields typical of the corona,
namely the sheared/twisted fields built up through long-term processes
of differential rotation and flux-emergence/cancellation. Using
inferences of the location and chirality of filament channels (deduced
from a magnetofrictional model driven by the evolving photospheric
field produced by the Advective Flux Transport model), we tailor a
customized boundary electric field profile that will emerge shear along
the desired portions of polarity inversion lines (PILs) and cancel flux
to create long twisted flux systems low in the corona. This method
has the potential to improve the morphological shape of streamers in
the low solar corona. Second, we apply, for the first time in our
eclipse prediction simulations, a new wave-turbulence-dissipation
(WTD) based model for coronal heating. This model has substantially
fewer free parameters than previous empirical heating models, but is
inherently sensitive to the 3D geometry and connectivity of the coronal
field---a key property for modeling/predicting the thermal-magnetic
structure of the solar corona. Overall, we will examine the effect
of these considerations on white-light and EUV observables from the
simulations, and present them in the context of our final 2017 eclipse
prediction model.Research supported by NASA's Heliophysics Supporting
Research and Living With a Star Programs.
---------------------------------------------------------
Title: Regularized Biot-Savart Laws for Modeling Magnetic Flux Ropes
Authors: Titov, Viacheslav; Downs, Cooper; Mikic, Zoran; Torok, Tibor;
Linker, Jon A.
2017SPD....4840606T Altcode:
Many existing models assume that magnetic flux ropes play a key role
in solar flares and coronal mass ejections (CMEs). It is therefore
important to develop efficient methods for constructing flux-rope
configurations constrained by observed magnetic data and the initial
morphology of CMEs. As our new step in this direction, we have derived
and implemented a compact analytical form that represents the magnetic
field of a thin flux rope with an axis of arbitrary shape and a circular
cross-section. This form implies that the flux rope carries axial
current I and axial flux F, so that the respective magnetic field is a
curl of the sum of toroidal and poloidal vector potentials proportional
to I and F, respectively. The vector potentials are expressed in terms
of Biot-Savart laws whose kernels are regularized at the rope axis. We
regularized them in such a way that for a straight-line axis the form
provides a cylindrical force-free flux rope with a parabolic profile of
the axial current density. So far, we set the shape of the rope axis
by tracking the polarity inversion lines of observed magnetograms and
estimating its height and other parameters of the rope from a calculated
potential field above these lines. In spite of this heuristic approach,
we were able to successfully construct pre-eruption configurations for
the 2009 February13 and 2011 October 1 CME events. These applications
demonstrate that our regularized Biot-Savart laws are indeed a very
flexible and efficient method for energizing initial configurations
in MHD simulations of CMEs. We discuss possible ways of optimizing
the axis paths and other extensions of the method in order to make it
more useful and robust.Research supported by NSF, NASA's HSR and LWS
Programs, and AFOSR.
---------------------------------------------------------
Title: Prediction of the Solar Corona for the 2017 August 21 Total
Solar Eclipse
Authors: Mikic, Zoran; Downs, Cooper; Linker, Jon A.; Caplan, Ronald
M.; Lionello, Roberto; Torok, Tibor; Titov, Viacheslav; Riley, Pete;
Mackay, Duncan; Upton, Lisa
2017SPD....4820801M Altcode:
It has become our tradition to predict the structure of the corona
prior to eclipses, using a magnetohydrodynamic (MHD) model based on
measurements of photospheric magnetic fields on the Sun. We plan to
continue this tradition for the August 21, 2017 total solar eclipse that
will sweep across the United States. We will predict the structure of
the corona using SDO/HMI photospheric magnetic field data, including
images of polarization brightness, magnetic field line traces, and
images of simulated emission in EUV and X-rays. These images can be
compared directly with observations of the total eclipse, as well as
observations from SDO/AIA, Hinode/XRT, and STEREO/EUVI. This year we
will attempt to energize the magnetic field within filament channels
for a more realistic prediction, by constructing flux ropes at the
locations where filament channels are observed. The handedness of the
flux ropes will be deduced from a magnetofrictional model driven by the
evolving photospheric field produced by the Advective Flux Transport
model.Research supported by NASA's Heliophysics Supporting Research
and Living With a Star Programs.
---------------------------------------------------------
Title: 2010 August 1-2 Sympathetic Eruptions. II. Magnetic Topology
of the MHD Background Field
Authors: Titov, Viacheslav S.; Mikić, Zoran; Török, Tibor; Linker,
Jon A.; Panasenco, Olga
2017ApJ...845..141T Altcode: 2017arXiv170707773T
Using a potential field source-surface (PFSS) model, we recently
analyzed the global topology of the background coronal magnetic field
for a sequence of coronal mass ejections (CMEs) that occurred on
2010 August 1-2. Here we repeat this analysis for the background field
reproduced by a magnetohydrodynamic (MHD) model that incorporates plasma
thermodynamics. As for the PFSS model, we find that all three CME source
regions contain a coronal hole (CH) that is separated from neighboring
CHs by topologically very similar pseudo-streamer structures. However,
the two models yield very different results for the size, shape,
and flux of the CHs. We find that the helmet-streamer cusp line,
which corresponds to a source-surface null line in the PFSS model,
is structurally unstable and does not form in the MHD model. Our
analysis indicates that, generally, in MHD configurations, this line
instead consists of a multiple-null separator passing along the edge
of disconnected-flux regions. Some of these regions are transient
and may be the origin of the so-called streamer blobs. We show that
the core topological structure of such blobs is a three-dimensional
“plasmoid” consisting of two conjoined flux ropes of opposite
handedness, which connect at a spiral null point of the magnetic
field. Our analysis reveals that such plasmoids also appear in
pseudo-streamers on much smaller scales. These new insights into the
coronal magnetic topology provide some intriguing implications for solar
energetic particle events and for the properties of the slow solar wind.
---------------------------------------------------------
Title: Magnetic Source Region Characteristics Influencing the Velocity
of Solar Eruptions in the Corona
Authors: Kliem, B.; Chintzoglou, G.; Torok, T.; Zhang, J.; Downs, C.
2016AGUFMSH13B2292K Altcode:
The velocity of coronal mass ejections (CMEs) is one of the primary
parameters determining their potential geoeffectiveness. The great
majority of very fast CMEs receive their main acceleration already in
the corona. We study the magnetic source region structure for a complete
sample of 15 very fast CMEs (v > 1500 km/s) during 2000-2006,
originating within 30 deg from central meridian and find a correlation
between CME speed and the decay index profile of the coronal field
estimated by a PFSS extrapolation. Such a correlation is not found
for a comparison sample of slower CMEs. We also study how the decay
index profile is related to the structure of the photospheric field
distribution. This is complemented by a parametric simulation study
of flux rope eruptions using the analytic Titov-Demoulin active-region
model for simple bipolar and quadrupolar source regions, which provide
simple relationships between the photospheric field distribution and
the coronal decay index profile. Very fast, moderate-velocity, and even
confined eruptions are found. Detailed, data-constrained MHD modeling
of a very fast and a relatively slow CME, including a comparison of
their source region characteristics, will also be presented. Support
by NSF and NASA's LWS program is acknowledged.
---------------------------------------------------------
Title: Core Dimming Regions as Probes of Magnetic Connectivity and
Reconfiguration.
Authors: Downs, C.; Titov, V. S.; Jiong, Q.; Torok, T.; Linker, J.;
Mikic, Z.
2016AGUFMSH12B..05D Altcode:
The early onset and evolution of a Coronal Mass Ejection (CME)
is a process that features essential coupling between the erupting
flux-system and the ambient corona. In this presentation we will
discuss the deep coronal dimming signatures of three contrasting
case-study events, and relate these signatures to the pre-event magnetic
configuration. We model each event by inserting a stable flux-rope
into the erupting region and then relaxing the configuration with
a full-sun zero-beta MHD model. Structural analysis of the magnetic
field, including maps of the squashing factor (Q), field line heights,
and the overall connectivity can be used to paint a detailed picture
of the likely eruption process, including where and why deep dimming
features appear. We argue that such features are likely probes of the
reconnection process between erupting magnetic flux and surrounding
coronal magnetic fields--a process relevant to understanding the
dynamic magnetic connectivity of CMEs and flux-ropes in the heliosphere.
---------------------------------------------------------
Title: Radiation Environments for Future Human Exploration Throughout
the Solar System.
Authors: Schwadron, N.; Gorby, M.; Linker, J.; Riley, P.; Torok,
T.; Downs, C.; Spence, H. E.; Desai, M. I.; Mikic, Z.; Joyce, C. J.;
Kozarev, K. A.; Townsend, L. W.; Wimmer-Schweingruber, R. F.
2016AGUFMSA41B2371S Altcode:
Acute space radiation hazards pose one of the most serious risks to
future human and robotic exploration. The ability to predict when and
where large events will occur is necessary in order to mitigate their
hazards. The largest events are usually associated with complex sunspot
groups (also known as active regions) that harbor strong, stressed
magnetic fields. Highly energetic protons accelerated very low in the
corona by the passage of coronal mass ejection (CME)-driven compressions
or shocks and from flares travel near the speed of light, arriving
at Earth minutes after the eruptive event. Whether these particles
actually reach Earth, the Moon, Mars (or any other point) depends on
their transport in the interplanetary magnetic field and their magnetic
connection to the shock. Recent contemporaneous observations during
the largest events in almost a decade show the unique longitudinal
distributions of this ionizing radiation broadly distributed from
sources near the Sun and yet highly isolated during the passage of CME
shocks. Over the last decade, we have observed space weather events
as the solar wind exhibits extremely low densities and magnetic field
strengths, representing states that have never been observed during the
space age. The highly abnormal solar activity during cycles 23 and 24
has caused the longest solar minimum in over 80 years and continues
into the unusually small solar maximum of cycle 24. As a result of
the remarkably weak solar activity, we have also observed the highest
fluxes of galactic cosmic rays in the space age and relatively small
particle radiation events. We have used observations from LRO/CRaTER to
examine the implications of these highly unusual solar conditions for
human space exploration throughout the inner solar system. While these
conditions are not a show-stopper for long-duration missions (e.g., to
the Moon, an asteroid, or Mars), galactic cosmic ray radiation remains
a significant and worsening factor that limits mission durations. If the
heliospheric magnetic field continues to weaken over time, as is likely,
then allowable mission durations will decrease correspondingly. Thus, we
examine the rapidly changing radiation environment and its implications
for human exploration destinations throughout the inner solar system.
---------------------------------------------------------
Title: The Impact of Coronal Jets on the Solar Wind and Magnetic
Structures in the Inner Heliosphere.
Authors: Lionello, R.; Torok, T.; Titov, V. S.; Linker, J.; Mikic,
Z.; Leake, J. E.; Linton, M.
2016AGUFMSH53A..06L Altcode:
Transient, collimated plasma eruptions, so-called coronal (or X-ray)
jets, are observed low in the corona in EUV and soft X-ray bands. They
are thought to be triggered by reconnection between closed and open
magnetic fields, although their formation mechanisms are not yet
fully understood. However, coronal jets are also observed to extend
to several solar radii, suggesting that they may provide a still
undetermined contribution to the solar wind. We simulate coronal jets
with our "thermodynamic" full MHD model of the solar corona by driving
the emergence of a magnetic flux rope into an open coronal magnetic
field. We study the impact of jets to the solar wind by varying the
field strength of the emerging flux rope, and we follow the propagation
of ejected magnetic structures into the inner heliosphere.
---------------------------------------------------------
Title: Sun-to-Earth MHD Modeling of Powerful Solar Eruptions
Authors: Torok, T.; Downs, C.; Linker, J.; Lionello, R.; Titov, V. S.;
Riley, P.; Mikic, Z.
2016AGUFMSH14A..05T Altcode:
Large solar eruptions that produce strong flares and powerful coronal
mass ejections are the main driver of space weather disturbances close
to the Earth. One of the main goals of numerical simulations of such
events is therefore to reproduce their in-situ signatures at 1 AU.This
requires a sophisticated model of the pre-eruptive configuration, the
initiation and early evolution of the eruption, and the large-scale
magnetic and plasma environment in which the eruption propagates. We
have been conducting magnetohydrodynamic (MHD) simulations that comply
with these requirements. We first produce a steady-state MHD solution
of the background corona that incorporates photospheric magnetic field
measurements, realistic energy transfer in the corona, and the solar
wind. We then use the recently developed, modified flux-rope model by
Titov et al. to insert a stable flux rope into the source region of the
eruption, while preserving the original magnetogram. Several instances
of the model can be combined to account for source regions with a highly
curved and elongated polarity inversion line (PIL). The eruption is
then initiated by imposing plasma flows that slowly converge towards
the PIL. Finally, we propagate the eruption to Earth, by coupling the
coronal simulation to our heliospheric MHD code. In this presentation
we illustrate our method for the famous "Bastille Day" event of July
14, 2000, which produced an X5.7 flare, a fast halo CME, andan intense
geomagnetic storm. We assess the quality of the simulation by comparing
synthetic satellite images with the observations, and we discuss how
well it reproduces the in-situ measurements at 1 AU. We also briefly
present our ongoing modeling effort for the more recent event of July
12, 2012, which was observed in great detail all the way from Sun
to Earth.
---------------------------------------------------------
Title: A Thin-Flux-Rope Approximation as a Basis for Modeling of Pre-
and Post-Eruptive Magnetic Configurations
Authors: Titov, V. S.; Mikic, Z.; Torok, T.; Linker, J.
2016AGUFMSH13C2313T Altcode:
Many existing models of solar flares and coronal mass ejections (CMEs)
assume a key role of magnetic flux ropes in these phenomena. It is
therefore important to have efficient methods for constructing flux-rope
configurations consistent with the observed photospheric magnetic data
and morphology of CMEs. As our new step in this direction, we propose
an analytical formulation that succinctly represents the magnetic
field of a thin flux rope, which has an axis of arbitrary shape and
a circular cross-section with the diameter slowly varying along the
axis. This representation implies also that the flux rope carries
axial current I and axial flux F, so that the respective magnetic
field is a curl of the sum of toroidal and poloidal vector potentials
proportional to I and F, respectively. Each of the two potentials
is individually expressed in terms of a modified Biot-Savart law
with separate kernels, both regularized at the rope axis. We argue
that the proposed representation is flexible enough to be used in
MHD simulations for initializing pre-eruptive configurations in the
low corona or post-eruptive configurations (interplanetary CMEs) in
the heliosphere. We discuss the potential advantages of our approach,
and the subsequent steps to be performed, to develop a fully operative
and highly competitive method compared to existing methods. Research
supported by NSF, NASA's HSR and LWS Programs, and AFOSR.
---------------------------------------------------------
Title: Tracking Changes in Magnetic Topology in MHD Simulations
Authors: Mikic, Z.; Titov, V. S.; Lionello, R.; Torok, T.; Linker,
J.; Downs, C.
2016AGUFMSH43B2570M Altcode:
The topology of the coronal magnetic field plays a key role in the
properties of the corona and the source of the slow solar wind. The
concept of slip-back mapping (Titov et al. 2009) has been applied
to detect open, closed, and disconnected flux systems formed by
reconnection of coronal magnetic fields during a given time interval. In
particular, this technique can identify regions where closed magnetic
field lines became open (e.g., via interchange reconnection), and
conversely, where open field lines became closed. We will describe the
application of this technique to the analysis of 3D MHD simulations
(including those of coronal jets and the propagation of "blobs" in the
solar wind). Research supported by NASA's Living With a Star Program.
---------------------------------------------------------
Title: Coupling of Coronal and Heliospheric Magnetohydrodynamic
Models: Solution Comparisons and Verification
Authors: Merkin, V. G.; Lionello, R.; Lyon, J. G.; Linker, J.; Török,
T.; Downs, C.
2016ApJ...831...23M Altcode:
Two well-established magnetohydrodynamic (MHD) codes are coupled
to model the solar corona and the inner heliosphere. The corona is
simulated using the MHD algorithm outside a sphere (MAS) model. The
Lyon-Fedder-Mobarry (LFM) model is used in the heliosphere. The
interface between the models is placed in a spherical shell above the
critical point and allows both models to work in either a rotating
or an inertial frame. Numerical tests are presented examining the
coupled model solutions from 20 to 50 solar radii. The heliospheric
simulations are run with both LFM and the MAS extension into the
heliosphere, and use the same polytropic coronal MAS solutions as the
inner boundary condition. The coronal simulations are performed for
idealized magnetic configurations, with an out-of-equilibrium flux rope
inserted into an axisymmetric background, with and without including
the solar rotation. The temporal evolution at the inner boundary of
the LFM and MAS solutions is shown to be nearly identical, as are the
steady-state background solutions, prior to the insertion of the flux
rope. However, after the coronal mass ejection has propagated through
the significant portion of the simulation domain, the heliospheric
solutions diverge. Additional simulations with different resolution are
then performed and show that the MAS heliospheric solutions approach
those of LFM when run with progressively higher resolution. Following
these detailed tests, a more realistic simulation driven by the
thermodynamic coronal MAS is presented, which includes solar rotation
and an azimuthally asymmetric background and extends to the Earth’s
orbit.
---------------------------------------------------------
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.
---------------------------------------------------------
Title: The Contribution of Coronal Jets to the Solar Wind
Authors: Lionello, R.; Török, T.; Titov, V. S.; Leake, J. E.;
Mikić, Z.; Linker, J. A.; Linton, M. G.
2016ApJ...831L...2L Altcode: 2016arXiv161003134L
Transient collimated plasma eruptions in the solar corona, commonly
known as coronal (or X-ray) jets, are among the most interesting
manifestations of solar activity. It has been suggested that these
events contribute to the mass and energy content of the corona and
solar wind, but the extent of these contributions remains uncertain. We
have recently modeled the formation and evolution of coronal jets
using a three-dimensional (3D) magnetohydrodynamic (MHD) code with
thermodynamics in a large spherical domain that includes the solar
wind. Our model is coupled to 3D MHD flux-emergence simulations, I.e.,
we use boundary conditions provided by such simulations to drive a
time-dependent coronal evolution. The model includes parametric coronal
heating, radiative losses, and thermal conduction, which enables us to
simulate the dynamics and plasma properties of coronal jets in a more
realistic manner than done so far. Here, we employ these simulations to
calculate the amount of mass and energy transported by coronal jets into
the outer corona and inner heliosphere. Based on observed jet-occurrence
rates, we then estimate the total contribution of coronal jets to the
mass and energy content of the solar wind to (0.4-3.0)% and (0.3-1.0)%,
respectively. Our results are largely consistent with the few previous
rough estimates obtained from observations, supporting the conjecture
that coronal jets provide only a small amount of mass and energy to the
solar wind. We emphasize, however, that more advanced observations and
simulations (including parametric studies) are needed to substantiate
this conjecture.
---------------------------------------------------------
Title: Unexpectedly Strong Lorentz-Force Impulse Observed During a
Solar Eruption
Authors: Sun, X.; Fisher, G.; Torok, T.; Hoeksema, J. T.; Li, Y.;
CGEM Team
2016usc..confE..12S Altcode:
For fast coronal mass ejections (CMEs), the acceleration phase takes
place in the low corona; the momentum process is presumably dominated by
the Lorentz force. Using ultra-high-cadence vector magnetic data from
the Helioseismic and Magnetic Imager (HMI) and numerical simulations,
we show that the observed fast-evolving photospheric field can be used
to characterize the impulse of the Lorentz force during a CME. While the
peak Lorentz force concurs with the maximum ejecta acceleration, the
observed total force impulse surprisingly exceeds the CME momentum by
over an order of magnitude. We conjecture that most of the Lorentz force
impulse is "trapped" in the thin layer of the photosphere above the HMI
line-formation height and is counter-balanced by gravity. This implies
a consequent upward plasma motion which we coin "gentle photospheric
upwelling". The unexpected effect dominates the momentum processes,
but is negligible for the energy budget, suggesting a complex coupling
between different layers of the solar atmosphere during CMEs.
---------------------------------------------------------
Title: The Thermodynamics of Coronal Jets and Their Contribution to
the Solar Wind
Authors: Lionello, Roberto; Török, Tibor; Titov, Viacheslav; Linker,
Jon A.; Mikic, Zoran; James E.; Linton, Mark
2016usc..confE..11L Altcode:
Coronal (or X-ray) jets are transient, collimated plasma eruptions
that are observed low in the corona in EUV and soft X-ray bands. It is
widely accepted that they are triggered by reconnection between closed
and open magnetic fields, but their detailed formation mechanisms are
still under debate. Since coronal jets are often seen to extend to
several solar radii, it has been suggested that they may contribute to
powering the solar wind, but the amount of this contribution remains
largely uncertain. Here we present the first MHD simulations of coronal
jets that include the solar wind and a realistic description of the
energy transfer in the corona ("thermodynamic MHD"). The evolution in
our model is driven by the emergence of a magnetic flux rope into an
open magnetic field. We find different types of jets in our simulations,
and discuss their respective formation mechanisms, morphologies, and
emission properties. We also analyze their energy and mass contributions
to the solar wind, and compare them with existing estimations obtained
from observations.
---------------------------------------------------------
Title: Core Dimming Regions as Probes of Magnetic Connectivity and
Reconfiguration.
Authors: Downs, Cooper; Titov, Viacheslav; Qiu, Jiong; Török, Tibor;
Linker Zoran Mikić, Jon A.
2016shin.confE.135D Altcode:
The early onset and evolution of a Coronal Mass Ejection (CME) is a
process that features intimate coupling between the erupting flux-system
and the ambient corona. In this presentation we will discuss the deep
coronal dimming signatures of three contrasting case-study events, and
relate these signatures to the pre-event magnetic configuration. We
model each event by inserting a stable flux-rope into the erupting
region and then relaxing the configuration with a full-sun zero-beta
MHD model. Structural analysis of the magnetic field, including
maps of generalized squashing factor (Q), field line length, and
overall connectivity can be used to paint a detailed picture of
the likely eruption process, including where and why deep dimming
features appear. We argue that such features are likely probes of the
reconnection process between erupting magnetic flux and surrounding
coronal magnetic fields.
---------------------------------------------------------
Title: Unexpectedly Large Lorentz-Force Impulse Observed During a
Solar Eruption
Authors: Sun, Xudong; Fisher, George; Torok, Tibor; Hoeksema, Todd;
Li, Yan; CGEM Team
2016shin.confE.158S Altcode:
For fast coronal mass ejections (CMEs), the acceleration phase takes
place in the low corona; the momentum process is presumably dominated by
the Lorentz force. Using ultra-high-cadence vector magnetic data from
the Helioseismic and Magnetic Imager (HMI) and numerical simulations,
we show that the observed fast-evolving photospheric field can be used
to characterize the impulse of the Lorentz force during a CME. While the
peak Lorentz force concurs with the maximum ejecta acceleration, the
observed total force impulse surprisingly exceeds the CME momentum by
over an order of magnitude. We conjecture that most of the Lorentz force
impulse is "trapped" in the thin layer of the photosphere above the HMI
line-formation height and is counter-balanced by gravity. This implies
a consequent upward plasma motion which we coin "gentle photospheric
upwelling". The unexpected effect dominates the momentum processes,
but is negligible for the energy budget, suggesting a complex coupling
between different layers of the solar atmosphere during CMEs.
---------------------------------------------------------
Title: Field Line Structure of Separatrix and Qausi-Separatrix
Magnetic Surfaces in the Solar Corona
Authors: Titov, Viacheslav S.; Mikić, Zoran; Downs, Cooper; Török,
Tibor; Lionello, Roberto; Linker, Jon A.
2016shin.confE.132T Altcode:
The analysis of the magnetic field topology provides a key framework for
understanding complex phenomena in the solar atmosphere and other cosmic
plasmas where the magnetic field plays an active role. This analysis
is facilitated by the calculation of the so-called squashing factor Q
on the surfaces that bound or cross the magnetic configuration under
study. The Q-factor is a dimensionless quantity that characterizes
the divergence of the field lines on the way between their boundary
end points. For realistic configurations, the Q-maps reveal intricate
networks of high-Q lines, which are, in turn, the cross-sections of
separatrix and quasi-separatrix surfaces present in the magnetic
configuration. <P />The sheer complexity of Q-maps can often be
difficult to interpret. To mitigate this problem, we have developed
a new technique that allows one to efficiently compute the field
line structure of the (quasi-)separatrix surfaces by starting from
their high-Q lines. The underlying algorithm iteratively determines
sets of field-line pairs that bracket null, minimum, and bald-patch
points. Convergence of the algorithm towards the high-Q line on either
side automatically yields approximation of the (quasi-)separatrix
surfaces. We demonstrate the outstanding capabilities of this
technique by reconstructing the magnetic topology for a number of
on-going projects at Predictive Science Inc., which include coronal
mass ejections, streamers, streamer blobs, pseudo-streamers, and
coronal jets. <P />Research supported by NSF/SHINE and NSF/FESD,
and by NASAś HSR and LWS Programs.
---------------------------------------------------------
Title: Unexpectedly Strong Lorentz-Force Impulse Observed During a
Solar Eruption
Authors: Sun, Xudong; Fisher, George H.; Torok, Tibor; Hoeksema,
Jon Todd; Li, Yan; CGEM Team
2016SPD....47.0628S Altcode:
For fast coronal mass ejections (CMEs), the acceleration phase takes
place in the low corona; the momentum process is presumably dominated by
the Lorentz force. Using ultra-high-cadence vector magnetic data from
the Helioseismic and Magnetic Imager (HMI) and numerical simulations,
we show that the observed fast-evolving photospheric field can be used
to characterize the impulse of the Lorentz force during a CME. While the
peak Lorentz force concurs with the maximum ejecta acceleration, the
observed total force impulse surprisingly exceeds the CME momentum by
over an order of magnitude. We conjecture that most of the Lorentz force
impulse is "trapped" in the thin layer of the photosphere above the HMI
line-formation height and is counter-balanced by gravity. This implies
a consequent upward plasma motion which we coin "gentle photospheric
upwelling". The unexpected effect dominates the momentum processes,
but is negligible for the energy budget, suggesting a complex coupling
between different layers of the solar atmosphere during CMEs.
---------------------------------------------------------
Title: The Contribution of Jets to Coronal and Solar Wind Energetics:
MHD Simulations
Authors: Lionello, Roberto; Torok, Tibor; Titov, Viacheslav; Linker,
Jon A.; Mikic, Zoran; Leake, James E.; Linton, Mark
2016SPD....4740202L Altcode:
Transient collimated plasma eruptions in the corona, commonly known as
coronal jets, are among the most interesting manifestations of solar
activity.We use the 3D MHD model with thermodynamics developed at PSI
to investigate the origin, dynamics, and plasma properties of coronal
jets.Our model is coupled with 3D MHD flux emergence simulations,
i.e, we use boundary conditions provided by such simulations to
drive a time-dependent coronal evolution. It includes parametric
coronal heating, radiative losses, and thermal conduction in the
energy equations.This enables us to simulate the energy transfer in
coronal jets in a more realistic manner than done so far and to study
the amount of energy and mass transported by these phenomena into
the higher corona and inner heliosphere. We discuss our results and
compare them with previous estimations obtained from observations.
---------------------------------------------------------
Title: Modeling Jets in the Corona and Solar Wind
Authors: Torok, Tibor; Lionello, Roberto; Titov, Viacheslav S.; Leake,
James E.; Mikic, Zoran; Linker, Jon A.; Linton, Mark G.
2016EGUGA..18.2692T Altcode: 2015arXiv151109350T
Coronal jets are transient, collimated eruptions that occur in
regions of open or semi-open magnetic fields in the solar corona. Our
understanding of these events has significantly improved in recent
years, owing to improved observational capabilities and numerical
simulations. Yet, several important questions concerning coronal jets
remain largely unanswered. For example: What exactly are the physical
mechanisms that heat and accelerate the plasma? And to what extent
do jets contribute to the heating of the corona and in providing
mass and energy to the fast solar wind? Here we present a "new
generation" of coronal-jet simulations that will allow us to address
such questions in more detail than before. In contrast to previous
simulations, our code models the large-scale corona in a spherical
domain, uses an advanced description of the energy transfer in the
corona ("thermodynamic MHD"), and includes the solar wind. As a first
application, we consider a purely radial coronal magnetic field and
a simple coronal heating function that decreases exponentially with
height above the surface. We produce so-called standard and blowout
jets by continuously driving the system at the lower boundary with data
extracted from flux-emergence simulations. We discuss the formation,
dynamics, and evolution of the jets, as well as their contribution to
coronal heating and the solar wind.
---------------------------------------------------------
Title: Modeling Jets in the Corona and Solar Wind
Authors: Török, T.; Lionello, R.; Titov, V. S.; Leake, J. E.;
Mikić, Z.; Linker, J. A.; Linton, M. G.
2016ASPC..504..185T Altcode:
Coronal jets are transient, collimated eruptions that occur in
regions of predominantly open magnetic field in the solar corona. Our
understanding of these events has greatly evolved in recent years but
several open questions, such as the contribution of coronal jets to the
solar wind, remain. Here we present an overview of the observations and
numerical modeling of coronal jets, followed by a brief description of
"next-generation" simulations that include an advanced description
of the energy transfer in the corona ("thermodynamic MHD"), large
spherical computational domains, and the solar wind. These new models
will allow us to address some of the open questions.
---------------------------------------------------------
Title: Fast Wave Trains Associated with Solar Eruptions: Insights
from 3D Thermodynamic MHD Simulations
Authors: Downs, C.; Liu, W.; Torok, T.; Linker, J.; Mikic, Z.;
Ofman, L.
2015AGUFMSH22A..06D Altcode:
EUV imaging observations during the SDO/AIA era have provided new
insights into a variety of wave phenomena occurring in the low
solar corona. One example is the observation of quasi-periodic,
fast-propagating wave trains that are associated with solar eruptions,
including flares and CMEs. While there has been considerable
progress in understanding such waves from both an observational
and theoretical perspective, it remains a challenge to pin down
their physical origin. In this work, we detail our results from
a case-study 3D thermodynamic MHD simulation of a coronal mass
ejection where quasi-periodic wave trains are generated during the
simulated eruption. We find a direct correlation between the onset of
non-steady reconnection in the flare current sheet and the generation
of quasi-periodic wave train signatures when patchy, collimated
downflows interact with the flare arcade. Via forward modeling of
SDO/AIA observables, we explore how the appearance of the wave trains
is affected by line-of-sight integration and the multi-thermal nature
of the coronal medium. We also examine how the wave trains themselves
are channeled by natural waveguides formed in 3D by the non-uniform
background magnetic field. While the physical association of the
reconnection dynamics to the generation of quasi-periodic wave trains
appears to be a compelling result, unanswered questions posed from
recent observations as well as future prospects will be discussed.
---------------------------------------------------------
Title: Coupling MHD Simulations of CMEs to SEP Models
Authors: Torok, T.; Gorby, M.; Linker, J.; Schwadron, N.
2015AGUFMSH11A2376T Altcode:
Large Solar Energetic Particle events (SEPs) are a main space
weather hazard and extremely dangerous to astronauts and electronic
equipmentin space. They are typically associated with fast Coronal
Mass Ejections (CMEs). Recent results indicate that SEPs can be
generated already inthe early phase of CME expansion low in the
corona, but the underlyingphysical mechanisms are not yet well
understood. State-of-the-artmagnetohydrodynamic (MHD) simulations of
CME initiation and evolution,combined with numerical models of particle
acceleration and propagation,provide a powerful tool to investigate
these mechanisms. In this talk, we present recent developments in
the coupling of CORHEL/MAS thermodynamicMHD simulations of fast CMEs
to the EPREM particle code, and we discuss the insights that can be
gained from such a combined modeling approach.
---------------------------------------------------------
Title: Erratum: “Slow Rise and Partial Eruption of a
Double-decker Filament. I Observations and Interpretation'<A
href="/abs/2012ApJ...756...59L">(2012, ApJ, 756, 59)</A>
Authors: Liu, Rui; Kliem, Bernhard; Török, Tibor; Liu, Chang; Titov,
Viacheslav S.; Lionello, Roberto; Linker, Jon A.; Wang, Haimin
2015ApJ...814..164L Altcode:
No abstract at ADS
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Title: Thermodynamic MHD Simulations of Jets in the Solar Corona
and Inner Heliosphere
Authors: Lionello, R.; Torok, T.; Titov, V. S.; Linker, J.; Mikic,
Z.; Leake, J. E.; Linton, M.
2015AGUFMSH11F..02L Altcode:
Coronal jets are transient, collimated plasma ejections that occur
predominantly in coronal holes and are observed in EUV, soft X-ray,
and occasionally in white-light coronagraphs. While these intriguing
phenomena have been studied and modeled for more than two decades, the
details of their formation mechanism(s) are not yet fully understood,
and their potential role for the generation of the fast solar wind
remains largely elusive. <P />Here we present 3D MHD simulations of
coronal jets which are performed in a large computational domain (up
to 20 solar radii) and incorporate the effects of thermal conduction,
radiative cooling, empirical coronal heating, and the solar wind. These
features allow us to model the plasma properties and energy transfer of
coronal jets in a more realistic manner than done so far, and to study
the amount of energy and mass transported by these phenomena into the
higher corona and inner heliosphere. <P />In order to produce a jet,
we consider a simple, purely radial background magnetic field and
slowly introduce a magnetic flux rope into the coronal configuration
by coupling our model to dynamic flux emergence simulations at the
lower boundary of the computational domain. We find two types of jets
in our simulations: a very impulsive event reminiscent of so-called
blowout jets and a slowly developing, more extended event that produces
a long-lasting signature in the corona. We present synthetic satellite
images for both types of events and discuss their respective formation
mechanisms. Our analysis is supported by a detailed investigation of
the magnetic topology for the blowout-type case and of the transport
of energy and plasma into the higher corona and inner heliosphere for
the long-lasting event.
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Title: Slip versus Field-Line Mapping in Describing 3D Reconnection
of Coronal Magnetic Fields
Authors: Titov, V. S.; Mikic, Z.; Torok, T.; Downs, C.; Lionello,
R.; Linker, J.
2015AGUFMSH43A2421T Altcode:
We demonstrate two techniques for describing the structure of the
coronal magnetic field and its evolution due to reconnection in
numerical 3D simulations of the solar corona and CMEs. These techniques
employ two types of mapping of the boundary of the computational
domain on itself. One of them is defined at a given time moment via
connections of the magnetic field lines to their opposite endpoints. The
other mapping, called slip mapping, relates field line endpoints at two
different time moments and allows one to identify the slippage of plasma
elements due to resistivity across field lines for a given time interval
(Titov et al. 2009). The distortion of each of these mappings can be
measured by using the so-called squashing factor Q (Titov 2007). The
high-Q layers computed for the first and second mappings define,
respectively, (quasi-)separatrix surfaces and reconnection fronts in
evolving magnetic configurations. Analyzing these structural features,
we are able to reveal topologically different domains and reconnected
flux systems in the configurations, in particular, open, closed and
disconnected magnetic flux tubes, as well as quantify the related
magnetic flux transfer. Comparison with observations makes it possible
also to relate these features to observed morphological elements
such as flare loops and ribbons, and EUV dimmings. We illustrate
these general techniques by applying them to particular data-driven
MHD simulations. *Research supported by NASA's HSR and LWS Programs,
and NSF/SHINE and NSF/FESD.
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Title: How Much Energy Can Be Stored in Solar Active Region Magnetic
Fields?
Authors: Linker, J.; Downs, C.; Torok, T.; Titov, V. S.; Lionello,
R.; Mikic, Z.; Riley, P.
2015AGUFMSH52A..08L Altcode:
Major solar eruptions such as X-class flares and very fast coronal
mass ejections usually originate in active regions on the Sun. The
energy that powers these events is believed to be stored as free
magnetic energy (energy above the potential field state) prior to
eruption. While coronal magnetic fields are not in general force-free,
active regions have very strong magnetic fields and at low coronal
heights the plasma beta is therefore very small, making the field (in
equilibrium) essentially force-free. The Aly-Sturrock theorem shows that
the energy of a fully force-free field cannot exceed the energy of the
so-called open field. If the theorem holds, this places an upper limit
on the amount of free energy that can be stored: the maximum free energy
(MFE) is the difference between the open field energy and the potential
field energy of the active region. In thermodynamic MHD simulations of
a major eruption (the July 14, 2000 'Bastille' day event) and a modest
event (February 13, 2009, we have found that the MFE indeed bounds the
energy stored prior to eruption. We compute the MFE for major eruptive
events in cycles 23 and 24 to investigate the maximum amount of energy
that can be stored in solar active regions.Research supported by AFOSR,
NASA, and NSF.
---------------------------------------------------------
Title: Diagnosing the Properties of the Solar Wind using Magnetic
Topology
Authors: Mikic, Z.; Titov, V. S.; Lionello, R.; Downs, C.; Linker,
J.; Torok, T.; Riley, P.
2015AGUFMSH31C2436M Altcode:
Recent work suggests that the topology of the coronal magnetic field
plays a key role in the source and properties of the slow solar wind,
through the collection of separatrix surfaces and quasi-separatrix
layers (QSLs) that define the S-web (Antiochos et al. 2011; Linker et
al. 2011; Titov et al. 2011). We have accumulated extensive experience
with using the squashing factor Q to analyze the underlying structural
skeleton of the coronal magnetic field, to identify magnetic null
points, separator field lines, QSLs, and separatrix surfaces, and their
relationship with the topology of coronal hole boundaries. This will
be extended by implementing slip mapping (Titov et al. 2009) to detect
open, closed, and disconnected flux systems that are formed due to
magnetic reconnection in a coronal model driven by both the differential
rotation and evolution of the photospheric magnetic field. This idea
is based on using forward and backward differences in time between
the field line mapping expected from ideal MHD motions and the actual
mapping to diagnose magnetic reconnection. This technique can identify
regions in the photosphere where closed magnetic field lines are about
to open (e.g., via interchange reconnection), and conversely, where open
field lines are about to close. We will use these concepts to develop
tools that relate the changing magnetic topology to the properties of
the solar wind, to plan and interpret Solar Probe Plus and Solar Orbiter
observations. Research supported by NASA's Living With a Star Program.
---------------------------------------------------------
Title: Particle Acceleration at Low Coronal Compression Regions
and Shocks
Authors: Schwadron, N. A.; Lee, M. A.; Gorby, M.; Lugaz, N.; Spence,
H. E.; Desai, M.; Török, T.; Downs, C.; Linker, J.; Lionello,
R.; Mikić, Z.; Riley, P.; Giacalone, J.; Jokipii, J. R.; Kota, J.;
Kozarev, K.
2015ApJ...810...97S Altcode:
We present a study on particle acceleration in the low corona
associated with the expansion and acceleration of coronal mass ejections
(CMEs). Because CME expansion regions low in the corona are effective
accelerators over a finite spatial region, we show that there is a
rigidity regime where particles effectively diffuse away and escape
from the acceleration sites using analytic solutions to the Parker
transport equation. This leads to the formation of broken power-law
distributions. Based on our analytic solutions, we find a natural
ordering of the break energy and second power-law slope (above the
break energy) as a function of the scattering characteristics. These
relations provide testable predictions for the particle acceleration
from low in the corona. Our initial analysis of solar energetic particle
observations suggests a range of shock compression ratios and rigidity
dependencies that give rise to the solar energetic particle (SEP)
events studied. The wide range of characteristics inferred suggests
competing mechanisms at work in SEP acceleration. Thus, CME expansion
and acceleration in the low corona may naturally give rise to rapid
particle acceleration and broken power-law distributions in large
SEP events.
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Title: The Origin of Net Electric Currents in Solar Active Regions
Authors: Dalmasse, K.; Aulanier, G.; Démoulin, P.; Kliem, B.; Török,
T.; Pariat, E.
2015ApJ...810...17D Altcode: 2015arXiv150705060D
There is a recurring question in solar physics regarding whether or not
electric currents are neutralized in active regions (ARs). This question
was recently revisited using three-dimensional (3D) magnetohydrodynamic
(MHD) numerical simulations of magnetic flux emergence into the solar
atmosphere. Such simulations showed that flux emergence can generate
a substantial net current in ARs. Other sources of AR currents are
photospheric horizontal flows. Our aim is to determine the conditions
for the occurrence of net versus neutralized currents with this second
mechanism. Using 3D MHD simulations, we systematically impose line-tied,
quasi-static, photospheric twisting and shearing motions to a bipolar
potential magnetic field. We find that such flows: (1) produce
both direct and return currents, (2) induce very weak compression
currents—not observed in 2.5D—in the ambient field present in the
close vicinity of the current-carrying field, and (3) can generate
force-free magnetic fields with a net current. We demonstrate that
neutralized currents are in general produced only in the absence of
magnetic shear at the photospheric polarity inversion line—a special
condition that is rarely observed. We conclude that photospheric flows,
as magnetic flux emergence, can build up net currents in the solar
atmosphere, in agreement with recent observations. These results thus
provide support for eruption models based on pre-eruption magnetic
fields that possess a net coronal current.
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Title: Numerical Modeling of Single and Sympathetic Solar Eruptions
Authors: Torok, Tibor
2015IAUGA..2251036T Altcode:
Solar eruptions such as coronal mass ejections (CMEs) and eruptive
flares are the largest energy release processes in the solar
system and the main driver of space weather disturbances near
Earth. While eruptions are solitary, occasionally they appear to
be connected in so-called sympathetic eruptions. The numerical
modeling of (single) eruptions has significantly improved in the
recent years. Magnetohydrodynamic (MHD) simulations are now capable of
modeling the whole evolution of observed CMEs, from their onset in the
low corona up to their arrival at Earth, with an unprecedented degree
of realism. The modeling of sympathetic eruptions has very recently
began as well, but those simulations are still very idealized. In
this talk, I will summarize the current state of CME simulations and
discuss briefly the next steps ahead.
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Title: How Much Energy Can Be Stored in Active Region Magnetic Fields?
Authors: Linker, Jon A.; Torok, Tibor; Downs, Cooper; Titov,
Viacheslav; Lionello, Roberto; Riley, Pete; Mikic, Zoran
2015shin.confE..77L Altcode:
Major solar eruptions such as X-class flares and very fast coronal
mass ejections usually originate in active regions on the Sun. The
energy that powers these events is believed to be stored as free
magnetic energy (energy above the potential field state) prior to
eruption. While coronal magnetic fields are not in general force-free,
active regions have very strong magnetic fields and at low coronal
heights the plasma beta is very small, making the field (in equilibrium)
essentially force-free. The Aly-Sturrock theorem shows that the energy
of a force-free field cannot exceed the energy of the so-called open
field. If the theorem holds, this places an upper limit on the amount
of free energy that can be stored. We investigate the magnetic energy
storage and release in full thermodynamic MHD simulations of a major
event (the July 14, 2000 'Bastille' day event) and a modest event
(February 13, 2009) and relate it to the potential and open field
energies for these active regions. We discuss the usefulness of the
open field energy as a guide to how much energy can be stored in an
active region.
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Title: Thermodynamic 3D MHD Modeling of Coronal Jets
Authors: Lionello, Roberto; Torok, Tibor; Titov, Viacheslav S.; Leake,
James E.; Linton, Mark G.; Linker, Jon A.; Mikic, Zoran
2015shin.confE..32L Altcode:
Transient collimated plasma eruptions in the corona, so-called
'standard' and 'blowout' coronal jets, are among the most intriguing
manifestations of solar activity. We use the PSI 'thermodynamic'
3D MHD model to improve our understanding of the origin, dynamics,
and plasma properties of coronal jets. Our code models the corona by
taking into account thermal conduction, radiative cooling, empirical
coronal heating, and the solar wind. <P />These properties enable us to
simulate the energy transfer in coronal jets in a more realistic manner
than done so far, and to study the amount of energy and mass transported
by these phenomena into the higher corona and solar wind. Here we couple
our model with 3D MHD flux emergence simulations, i.e, we use boundary
conditions provided by such simulations to drive a time-dependent
coronal evolution. In particular, we study the topological properties
of the magnetic fields associated with jets, how the jet appears in
EUV and soft X-ray emission, and its signature in the inner heliosphere.
---------------------------------------------------------
Title: Connecting the evolution and properties of CMEs to their low
coronal signatures. A modeling case study of the ‘simple’ Feb
13 2009 event
Authors: Downs, Cooper; Török, Tibor; Titov, Viacheslav; Liu, Wei;
Linker, Jon; Mikić, Zoran
2015TESS....130401D Altcode:
The early onset and and evolution of a CME is a process that features
an intimate coupling between the erupting flux-system and the
ambient corona. For this reason low coronal signatures that we often
observe in the EUV can be used to infer information on the physical
nature and evolution of CMEs. In this presentation we will discuss
a 3D thermodynamic MHD simulation of the Feb 13 2009 eruption,
which occurred from an isolated region during solar minimum and
produced well characterized EUV wave and transient coronal dimming
features. Using observations as a guide, we simulate the entire
evolution of the eruption and global corona, starting from the initial
stable configuration through onset and evolution to the post-eruptive
reconfiguration. With a particular focus on coronal dimmings, we
track how the connectivity of the erupting flux-rope evolves with
time and how this relates to corresponding dimmings in synthetic EUV
observables. We find that the appearance of the core dimming regions
and their migration over time can be related to when and where the
erupting rope reconnects with itself and the adjacent arcade. Other
aspects related to CME evolution, such as the generation of an EUV
wave and quasi-periodic fast-propagating waves are also discussed.
---------------------------------------------------------
Title: Electric current neutralization in solar active regions
Authors: Dalmasse, Kévin; Aulanier, Guillaume; Török, Tibor;
Démoulin, Pascal; Pariat, Etienne; Kliem, Bernhard
2015TESS....111303D Altcode:
There is a recurring question in solar physics of whether or not
photospheric vertical electric currents are neutralized in solar active
regions, i.e., whether or not the total electric current integrated
over a single magnetic polarity of an active region vanishes. While
different arguments have been proposed in favor of, or against, the
neutralization of electric currents, both theory and observations are
still not fully conclusive. Providing the answer to this question is
crucial for theoretical models of solar eruptions. Indeed, if currents
are neutralized in active regions, then any eruption model based on net
- i.e., non-zero - electric currents, such as the torus instability,
requires further consideration. We address the question of electric
current neutralization in active regions using 3D zero-beta MHD
simulations of line-tied, slow photospheric driving motions imposed
on an initially potential magnetic field. We compare our results to a
recent study of the build-up of coronal electric currents in an MHD
simulation of the emergence of a current-neutralized twisted flux
tube into the solar atmosphere. Our parametric study shows that, in
accordance with the flux emergence simulation, photospheric motions are
associated with the formation of both direct and return currents. It
further shows that both processes (flux emergence and photospheric
flows) can lead to the formation of strong net currents in the solar
corona, and that the non-neutralization of electric currents is related
to the presence of magnetic shear at the polarity inversion line. We
discuss the implications of our results for the observations and for
theoretical models of solar eruptions.
---------------------------------------------------------
Title: Magnetic Topology of the Global MHD Configuration on 2010
August 1-2
Authors: Titov, V. S.; Mikic, Z.; Torok, T.; Linker, J.; Panasenco, O.
2014AGUFMSH23A4148T Altcode:
It appears that the global magnetic topology of the solar corona
predetermines to a large extent the magnetic flux transfer during
solar eruptions. We have recently analyzed the global topology for
a source-surface model of the background magnetic field at the time
of the 2010 August 1-2 sympathetic CMEs (Titov et al. 2012). Now we
extend this analysis to a more accurate thermodynamic MHD model of
the solar corona. As for the source-surface model, we find a similar
triplet of pseudo-streamers in the source regions of the eruptions. The
new study confirms that all these pseudo-streamers contain separatrix
curtains that fan out from a basic magnetic null point, individual
for each of the pseudo-streamers. In combination with the associated
separatrix domes, these separatrix curtains fully isolate adjacent
coronal holes of the like polarity from each other. However, the size
and shape of the coronal holes, as well as their open magnetic fluxes
and the fluxes in the lobes of the separatrix domes, are very different
for the two models. The definition of the open separator field lines,
where the (interchange) reconnection between open and closed magnetic
flux takes place, is also modified, since the structurally unstable
source-surface null lines do not exist anymore in the MHD model. In
spite of all these differences, we reassert our earlier hypothesis
that magnetic reconnection at these nulls and the associated separators
likely plays a key role in coupling the successive eruptions observed
by SDO and STEREO. The results obtained provide further validation of
our recent simplified MHD model of sympathetic eruptions (Török et
al. 2011). Research supported by NASA's Heliophysics Theory and LWS
Programs, and NSF/SHINE and NSF/FESD.
---------------------------------------------------------
Title: Propagation and Evolution of Interplanetary Magnetic Clouds:
Global Simulations and Comparisons with Observations
Authors: Riley, P.; Ben-Nun, M.; Linker, J.; Torok, T.; Lionello,
R.; Downs, C.
2014AGUFMSH42A..05R Altcode:
In this talk, we explore the evolution of interplanetary coronal mass
ejections (ICMEs), and fast magnetic clouds (MCs) in particular. We
address three specific issues. First, What are the large-scale forces
acting on ejecta as they travel from the Sun to 1 AU through a realistic
ambient solar wind, and how does they affect the large-scale structure
of the event? Second, what are the dominant waves/shocks associated with
fast ICMEs? And third, how are the properties of ICMEs different during
cycle 24 than during the previous cycle? To accomplish these objectives,
we employ a variety of numerical approaches, including global resistive
MHD models that incorporate realistic energy transport processes. We
also compare and contrast model results with both remote solar and
in-situ measurements of ICMEs at 1 AU and elsewhere, including the
so-called “Bastille Day” event of July 14, 2000, and the more recent
“extreme ICME” observed by STEREO-A on July 23, 2012.
---------------------------------------------------------
Title: Towards a Thermodynamic 3D MHD Model of Coronal Jets
Authors: Lionello, R.; Torok, T.; Linker, J.; Mikic, Z.
2014AGUFMSH53D..06L Altcode:
Transient collimated plasma eruptions in the corona, so-called
"standard" and "blowout" coronal jets, are among the most intriguing
manifestations of solar activity. We have begun to use the PSI
"thermodynamic" 3D MHD model to improve our understanding of the
origin, dynamics, and plasma properties of coronal jets. Our code
models the corona by taking into account thermal conduction, radiative
cooling, empirical coronal heating, and the solar wind, and it is
capable of using observed magnetograms as boundary condition for the
magnetic field. Furthermore, the model is coupled with 3D MHD flux
emergence simulations, i.e it can use boundary conditions provided by
such simulations to drive a time-dependent coronal evolution. These
properties enable us to simulate the energy transfer in coronal jetsin
a more realistic manner. We will present preliminary results.
---------------------------------------------------------
Title: Sympathetic solar eruptions in quadrupolar magnetic
configurations
Authors: Torok, T.; Titov, V. S.; Panasenco, O.
2014AGUFMSH23A4146T Altcode:
Observations by SDO/AIA have renewed the interest in sympathetic
solareruptions, i.e., of eruptions that occur simultaneously (or in
shortsuccession) at different source regions in the corona. Recently,
Toroket al. (2011) developed an idealized numerical model for
the triggermechanisms of sympathetic eruptions in so-called
pseudo-streamers, whichconsist of a tri-polar magnetic configuration
with a parasitic polarityin their center. Here we extend the work
by Torok et al. by investigating sympathetic eruptions in (the
topologically somewhat more complex) quadrupolar configurations,
using MHD simulations. We consider both symmetric and asymmetric
initial configurations that contain two or three flux ropes within the
quadrupole. We find, differentto Torok et al. (2011), that magnetic
reconnection induced by a firsteruption cannot just trigger, but also
prevent subsequent eruptions. In addition, a (relatively modest)
asymmetry of the configuration may fully suppress the occurrence
of successive full eruptions, i.e., of coronal mass ejections. We
discuss the implications of these results for our understanding of
sympathetic eruptions.
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Title: Particle Acceleration in the Low Corona Over Broad Longitudes:
Coupling MHD and 3D Particle Simulations
Authors: Gorby, M.; Schwadron, N.; Torok, T.; Downs, C.; Lionello,
R.; Linker, J.; Titov, V. S.; Mikic, Z.; Riley, P.; Desai, M. I.;
Dayeh, M. A.
2014AGUFMSH21B4127G Altcode:
Recent work on the coupling between the Energetic Particle Radiation
Environment Module (EPREM, a 3D energetic particle model) and
Magnetohydrodynamics Around a Sphere (MAS, an MHD code developed
at Predictive Science, Inc.) has demonstrated the efficacy of
compression regions around fast coronal mass ejections (CMEs) for
particle acceleration low in the corona (∼ 3 - 6 solar radii). These
couplings show rapid particle acceleration over a broad longitudinal
extent (∼ 80 degrees) resulting from the pile-up of magnetic flux in
the compression regions and their subsequent expansion. The challenge
for forming large SEP events in such compression-acceleration scenarios
is to have enhanced scattering within the acceleration region while
also allowing for efficient escape of accelerated particles downstream
(away from the Sun) from the compression region. We present here
the most recent simulation results including energetic particle and
CME plasma profiles, the subsequent flux and dosages at 1AU, and an
analysis of the compressional regions as efficient accelerators.
---------------------------------------------------------
Title: Slow Rise and Partial Eruption of a Double-decker
Filament. II. A Double Flux Rope Model
Authors: Kliem, Bernhard; Török, Tibor; Titov, Viacheslav S.;
Lionello, Roberto; Linker, Jon A.; Liu, Rui; Liu, Chang; Wang, Haimin
2014ApJ...792..107K Altcode: 2014arXiv1407.2272K
Force-free equilibria containing two vertically arranged magnetic flux
ropes of like chirality and current direction are considered as a model
for split filaments/prominences and filament-sigmoid systems. Such
equilibria are constructed analytically through an extension of the
methods developed in Titov & Démoulin and numerically through an
evolutionary sequence including shear flows, flux emergence, and flux
cancellation in the photospheric boundary. It is demonstrated that
the analytical equilibria are stable if an external toroidal (shear)
field component exceeding a threshold value is included. If this
component decreases sufficiently, then both flux ropes turn unstable
for conditions typical of solar active regions, with the lower rope
typically becoming unstable first. Either both flux ropes erupt upward,
or only the upper rope erupts while the lower rope reconnects with
the ambient flux low in the corona and is destroyed. However, for
shear field strengths staying somewhat above the threshold value,
the configuration also admits evolutions which lead to partial
eruptions with only the upper flux rope becoming unstable and the
lower one remaining in place. This can be triggered by a transfer of
flux and current from the lower to the upper rope, as suggested by
the observations of a split filament in Paper I. It can also result
from tether-cutting reconnection with the ambient flux at the X-type
structure between the flux ropes, which similarly influences their
stability properties in opposite ways. This is demonstrated for the
numerically constructed equilibrium.
---------------------------------------------------------
Title: A Method for Embedding Circular Force-free Flux Ropes in
Potential Magnetic Fields
Authors: Titov, V. S.; Török, T.; Mikic, Z.; Linker, J. A.
2014ApJ...790..163T Altcode:
We propose a method for constructing approximate force-free equilibria
in pre-eruptive configurations in which a thin force-free flux rope is
embedded into a locally bipolar-type potential magnetic field. The flux
rope is assumed to have a circular-arc axis, a circular cross-section,
and electric current that is either concentrated in a thin layer at the
boundary of the rope or smoothly distributed across it with a maximum
of the current density at the center. The entire solution is described
in terms of the magnetic vector potential in order to facilitate
the implementation of the method in numerical magnetohydrodynamic
(MHD) codes that evolve the vector potential rather than the magnetic
field itself. The parameters of the flux rope can be chosen so that
its subsequent MHD relaxation under photospheric line-tied boundary
conditions leads to nearly exact numerical equilibria. To show the
capabilities of our method, we apply it to several cases with different
ambient magnetic fields and internal flux-rope structures. These
examples demonstrate that the proposed method is a useful tool for
initializing data-driven simulations of solar eruptions.
---------------------------------------------------------
Title: Catastrophe versus Instability for the Eruption of a Toroidal
Solar Magnetic Flux Rope
Authors: Kliem, B.; Lin, J.; Forbes, T. G.; Priest, E. R.; Török, T.
2014ApJ...789...46K Altcode: 2014arXiv1404.5922K
The onset of a solar eruption is formulated here as either a magnetic
catastrophe or as an instability. Both start with the same equation of
force balance governing the underlying equilibria. Using a toroidal
flux rope in an external bipolar or quadrupolar field as a model
for the current-carrying flux, we demonstrate the occurrence of a
fold catastrophe by loss of equilibrium for several representative
evolutionary sequences in the stable domain of parameter space. We
verify that this catastrophe and the torus instability occur at the same
point; they are thus equivalent descriptions for the onset condition
of solar eruptions.
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Title: Developing 3D CME Models
Authors: Mikic, Zoran; Torok, Tibor; Titov, Viacheslav; Linker,
Jon A.; Reeves, Kathy
2014AAS...22421808M Altcode:
We describe the development of CME models in three dimensions,
including the energization of active regions and the initiation
of eruptions via flux cancellation. We contrast the dynamics from
idealized zero-beta models with more sophisticated models based on
thermodynamic solutions. We explore the effect of the strength of the
magnetic field in the active region (or, more appropriately, the amount
of smoothing applied to the observed magnetic field), the profiles
for transverse field emergence or applied shear, and the nature of
the flux cancellation, on the dynamics of eruptions. In particular,
our interest is in understanding which effects lead to fast CMEs.
---------------------------------------------------------
Title: The evolution of writhe in kink-unstable flux ropes and
erupting filaments
Authors: Török, T.; Kliem, B.; Berger, M. A.; Linton, M. G.;
Démoulin, P.; van Driel-Gesztelyi, L.
2014PPCF...56f4012T Altcode: 2014arXiv1403.1565T
The helical kink instability of a twisted magnetic flux tube has been
suggested as a trigger mechanism for solar filament eruptions and
coronal mass ejections (CMEs). In order to investigate if estimations
of the pre-emptive twist can be obtained from observations of writhe
in such events, we quantitatively analyze the conversion of twist into
writhe in the course of the instability, using numerical simulations. We
consider the line tied, cylindrically symmetric Gold-Hoyle flux rope
model and measure the writhe using the formulae by Berger and Prior
which express the quantity as a single integral in space. We find that
the amount of twist converted into writhe does not simply scale with
the initial flux rope twist, but depends mainly on the growth rates
of the instability eigenmodes of higher longitudinal order than the
basic mode. The saturation levels of the writhe, as well as the shapes
of the kinked flux ropes, are very similar for considerable ranges of
initial flux rope twists, which essentially precludes estimations of
pre-eruptive twist from measurements of writhe. However, our simulations
suggest an upper twist limit of ∼6π for the majority of filaments
prior to their eruption.
---------------------------------------------------------
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: Coronal Magnetic Reconnection Driven by CME Expansion—the
2011 June 7 Event
Authors: van Driel-Gesztelyi, L.; Baker, D.; Török, T.; Pariat, E.;
Green, L. M.; Williams, D. R.; Carlyle, J.; Valori, G.; Démoulin,
P.; Kliem, B.; Long, D. M.; Matthews, S. A.; Malherbe, J. -M.
2014ApJ...788...85V Altcode: 2014arXiv1406.3153V
Coronal mass ejections (CMEs) erupt and expand in a magnetically
structured solar corona. Various indirect observational pieces of
evidence have shown that the magnetic field of CMEs reconnects with
surrounding magnetic fields, forming, e.g., dimming regions distant
from the CME source regions. Analyzing Solar Dynamics Observatory
(SDO) observations of the eruption from AR 11226 on 2011 June 7, we
present the first direct evidence of coronal magnetic reconnection
between the fields of two adjacent active regions during a CME. The
observations are presented jointly with a data-constrained numerical
simulation, demonstrating the formation/intensification of current
sheets along a hyperbolic flux tube at the interface between the CME
and the neighboring AR 11227. Reconnection resulted in the formation of
new magnetic connections between the erupting magnetic structure from
AR 11226 and the neighboring active region AR 11227 about 200 Mm from
the eruption site. The onset of reconnection first becomes apparent
in the SDO/AIA images when filament plasma, originally contained
within the erupting flux rope, is redirected toward remote areas in
AR 11227, tracing the change of large-scale magnetic connectivity. The
location of the coronal reconnection region becomes bright and directly
observable at SDO/AIA wavelengths, owing to the presence of down-flowing
cool, dense (10<SUP>10</SUP> cm<SUP>-3</SUP>) filament plasma in its
vicinity. The high-density plasma around the reconnection region is
heated to coronal temperatures, presumably by slow-mode shocks and
Coulomb collisions. These results provide the first direct observational
evidence that CMEs reconnect with surrounding magnetic structures,
leading to a large-scale reconfiguration of the coronal magnetic field.
---------------------------------------------------------
Title: Thermal energy creation and transport and X-ray/EUV emission
in a thermodynamic MHD CME simulation
Authors: Reeves, Kathy; Mikić, Zoran; Linker, Jon; Török, Tibor
2014shin.confE...2R Altcode:
We model a CME using a 3D numerical MHD code that includes coronal
heating, thermal conduction and radiative cooling in the energy
equation. We first develop a global coronal solution (from 1 to 20 Rs)
to serve as the initial condition for the CME simulation. The magnetic
flux distribution at 1 Rs is produced by a localized subsurface dipole
superimposed on a global dipole field, to mimic the presence of an
active region within the global corona. The resulting configuration
has solar wind emanating from the open field regions, dense plasma in
the streamer belt, and hot plasma in the active region. We introduce
transverse electric fields near the neutral line in the active region
to form a flux rope, then a converging flow is imposed that causes the
eruption. We follow the quantities responsible for plasma heating
and cooling during the eruption, including thermal conduction,
radiation, adiabatic compression and expansion, coronal heating
and ohmic heating due to dissipation of currents. We find that the
adiabatic compression plays an important role in heating plasma around
the current sheet and in the collapsing reconnected loops under the
erupting flux rope. Thermal conduction also plays an important role in
the transport of thermal energy. We follow the formation and evolution
of the current sheet and simulate emissions in the X-ray and extreme
ultra-violet wavelengths in order to determine signatures of current
sheet energetics in observations from the XRT on the Hinode satellite
and the AIA instrument on the Solar Dynamics Observatory.
---------------------------------------------------------
Title: A Method for Embedding Circular Force-Free Flux Ropes in
Potential Magnetic Fields
Authors: Titov, Viacheslav; Torok, Tibor; Mikic, Zoran; Linker, Jon A.
2014AAS...22421204T Altcode:
We propose a method for constructing approximate force-free equilibria
in pre-eruptive configurations that locally are a bipolar-type
potential magnetic field with a thin force-free flux rope embedded
inside it. The flux rope is assumed to have a circular-arc axis,
circular cross-section, and electric current that is either concentrated
in a thin layer at the boundary of the rope or smoothly distributed
across it with a maximum of the current density at the center.The
entire solution is described in terms of the magnetic vector
potential in order to facilitate the implementation of the method
in numerical magnetohydrodynamic (MHD) codes that evolve the vector
potential rather than the magnetic field itself. The parameters of
the flux rope can be chosen so that its subsequent MHD relaxation
under photospheric line-tied boundary conditions leads to nearly
exact numerical equilibria. To show the capabilities of our method,
we apply it to several cases with different ambient magnetic fields
and internal flux-rope structures. These examples demonstrate that
the proposed method is a useful tool for initializing data-driven
simulations of solar eruptions.
---------------------------------------------------------
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: Global Magnetic Topology and Large-Scale Dynamics of the
Solar Corona
Authors: Titov, Viacheslav; Linker, Jon; Mikic, Zoran; Riley, Pete;
Lionello, Roberto; Downs, Cooper; Torok, Tibor
2014cosp...40E3350T Altcode:
We consider the global topology of the coronal magnetic field
in relation to the large-scale dynamics of the solar corona. Our
consideration includes recent results on the structural analysis
of this field determined in two different approximations, namely,
potential field source surface model and solar magnetohydrodynamic
model. We identify similarities and differences between structural
features of the magnetic field obtained in these two models and discuss
their implications for understanding various large-scale phenomena in
the solar corona. The underlying magnetic topology manifests itself
in a variety of observed morphological features such as streamers,
pseudo-streamers or unipolar streamers, EUV dimmings, flare ribbons,
coronal holes, and jets. For each of them, the related magnetic
configuration has specific structural features, whose presence has to be
not only identified but also verified on its independence from the used
field model in order to reliably predict the impact of such features on
physical processes in the corona. Among them are magnetic null points
and minima, bald patches, separatrix surfaces and quasi-separatrix
layers, and open and closed separator field lines. These features form
a structural skeleton of the coronal magnetic field and are directly
involved through the ubiquitous process of magnetic reconnection in many
solar dynamic phenomena such as coronal mass ejections, solar wind,
acceleration and transport of energetic particles. We will pinpoint
and elucidate in our overview some of such involvements that have
recently received a considerable attention in our ongoing projects at
Predictive Science.
---------------------------------------------------------
Title: Electric currents in solar active regions
Authors: Dalmasse, Kévin; Pariat, Etienne; Kliem, Bernhard; Aulanier,
Guillaume; Demoulin, Pascal; Torok, Tibor
2014cosp...40E.613D Altcode:
There is a recurring question in solar physics about whether or not
photospheric vertical electric currents are neutralized in solar active
regions, i.e. if the total electric current integrated over a single
photospheric magnetic polarity of an active region vanishes. Different
arguments have been proposed in favor of, or against, the neutralization
of electric currents, but both theory and observations are still not
fully conclusive. The answer to this question has implications for
eruption models. Indeed, if currents are neutralized in active regions,
then any eruption model based on non-neutralized electric currents,
such as the torus instability, would need to be further analyzed. We
addressed the question of electric currents neutralization in active
regions using 3D zero-beta, line-tied, slow driving motions of an
initially potential magnetic field. We compared our results to a recent
study of electric currents build-up in a MHD numerical simulation of the
emergence of a current-neutralized twisted flux tube. Our parametric
analyses show that, as for the emergence, photospheric motions are
associated with the formation of both direct and return currents. It
further shows that both processes can lead to the formation of strong
net currents in the solar corona, and that the non-neutralization of
electric currents is related to the presence of magnetic shear at the
polarity inversion line. We will discuss the implications of our results
for the observations and for the different solar eruption models.
---------------------------------------------------------
Title: Time-Dependent Coupled Coronal-Solar Wind-SEP Modeling
Authors: Linker, Jon; Mikic, Zoran; Schwadron, Nathan; Riley, Pete;
Gorby, Matthew; Lionello, Roberto; Downs, Cooper; Torok, Tibor
2014cosp...40E1840L Altcode:
Solar energetic particle (SEP) events are important space weather
phenomena. SEPs can damage satellite instrumentation, and they can be
hazardous for crews of Low Earth Orbit spacecraft and the International
Space Station, especially when engaged in extravehicular activity. The
acceleration and transport of SEPs is intimately tied to the evolution
and propagation of coronal mass ejections (CMEs) and their associated
shock waves. In this presentation, we describe an approach to modeling
CMEs in the corona and inner heliosphere, together with modeling of
SEP acceleration and transport. CMEs are initiated and followed in
a realistic corona and solar wind using the MAS MHD code, and SEPs
are modeled using EPREM, a 3D energetic particle transport code. The
particles are not truly coupled to the MHD solution, in the sense
that the electric and magnetic fields from the MHD computation drive
the solutions of the focused transport equation. We show initial
comparisons with typical CME observations and SEP data, and discuss
the strengths and limitations of this approach.
---------------------------------------------------------
Title: Magnetic reconnection driven by filament eruption in the 7
June 2011 event
Authors: van Driel-Gesztelyi, L.; Baker, D.; Török, T.; Pariat, E.;
Green, L. M.; Williams, D. R.; Carlyle, J.; Valori, G.; Démoulin,
P.; Matthews, S. A.; Kliem, B.; Malherbe, J. -M.
2014IAUS..300..502V Altcode:
During an unusually massive filament eruption on 7 June 2011,
SDO/AIA imaged for the first time significant EUV emission around a
magnetic reconnection region in the solar corona. The reconnection
occurred between magnetic fields of the laterally expanding CME
and a neighbouring active region. A pre-existing quasi-separatrix
layer was activated in the process. This scenario is supported by
data-constrained numerical simulations of the eruption. Observations
show that dense cool filament plasma was re-directed and heated in
situ, producing coronal-temperature emission around the reconnection
region. These results provide the first direct observational evidence,
supported by MHD simulations and magnetic modelling, that a large-scale
re-configuration of the coronal magnetic field takes place during
solar eruptions via the process of magnetic reconnection.
---------------------------------------------------------
Title: Initiation of Coronal Mass Ejections by Sunspot Rotation
Authors: Valori, G.; Török, T.; Temmer, M.; Veronig, A. M.; van
Driel-Gesztelyi, L.; Vršnak, B.
2014IAUS..300..201V Altcode:
We report observations of a filament eruption, two-ribbon flare, and
coronal mass ejection (CME) that occurred in Active Region NOAA 10898
on 6 July 2006. The filament was located South of a strong sunspot that
dominated the region. In the evolution leading up to the eruption, and
for some time after it, a counter-clockwise rotation of the sunspot of
about 30 degrees was observed. We suggest that the rotation triggered
the eruption by progressively expanding the magnetic field above the
filament. To test this scenario, we study the effect of twisting
the initially potential field overlying a pre-existing flux rope,
using three-dimensional zero-β MHD simulations. We consider a magnetic
configuration whose photospheric flux distribution and coronal structure
is guided by the observations and a potential field extrapolation. We
find that the twisting leads to the expansion of the overlying field. As
a consequence of the progressively reduced magnetic tension, the flux
rope quasi-statically adapts to the changed environmental field, rising
slowly. Once the tension is sufficiently reduced, a distinct second
phase of evolution occurs where the flux rope enters an unstable regime
characterized by a strong acceleration. Our simulation thus suggests
a new mechanism for the triggering of eruptions in the vicinity of
rotating sunspots.
---------------------------------------------------------
Title: Simulations of Emerging Magnetic Flux. I. The Formation of
Stable Coronal Flux Ropes
Authors: Leake, James E.; Linton, Mark G.; Török, Tibor
2013ApJ...778...99L Altcode: 2013arXiv1308.6204L
We present results from three-dimensional visco-resistive
magnetohydrodynamic simulations of the emergence of a convection zone
magnetic flux tube into a solar atmosphere containing a pre-existing
dipole coronal field, which is orientated to minimize reconnection with
the emerging field. We observe that the emergence process is capable
of producing a coronal flux rope by the transfer of twist from the
convection zone, as found in previous simulations. We find that this
flux rope is stable, with no evidence of a fast rise, and that its
ultimate height in the corona is determined by the strength of the
pre-existing dipole field. We also find that although the electric
currents in the initial convection zone flux tube are almost perfectly
neutralized, the resultant coronal flux rope carries a significant
net current. These results suggest that flux tube emergence is capable
of creating non-current-neutralized stable flux ropes in the corona,
tethered by overlying potential fields, a magnetic configuration that
is believed to be the source of coronal mass ejections.
---------------------------------------------------------
Title: Magnetohydrodynamic Simulations of Interplanetary Coronal
Mass Ejections
Authors: Lionello, Roberto; Downs, Cooper; Linker, Jon A.; Török,
Tibor; Riley, Pete; Mikić, Zoran
2013ApJ...777...76L Altcode:
We describe a new MHD model for the propagation of interplanetary
coronal mass ejections (ICMEs) in the solar wind. Accurately following
the propagation of ICMEs is important for determining space weather
conditions. Our model solves the MHD equations in spherical coordinates
from a lower boundary above the critical point to Earth and beyond. On
this spherical surface, we prescribe the magnetic field, velocity,
density, and temperature calculated typically directly from a coronal
MHD model as time-dependent boundary conditions. However, any model
that can provide such quantities either in the inertial or rotating
frame of the Sun is suitable. We present two validations of the
technique employed in our new model and a more realistic simulation
of the propagation of an ICME from the Sun to Earth.
---------------------------------------------------------
Title: Initiation of Coronal Mass Ejections by Sunspot Rotation
Authors: Török, T.; Temmer, M.; Valori, G.; Veronig, A. M.; van
Driel-Gesztelyi, L.; Vršnak, B.
2013SoPh..286..453T Altcode: 2014arXiv1401.2922T
We study a filament eruption, two-ribbon flare, and coronal mass
ejection (CME) that occurred in NOAA Active Region 10898 on 6 July
2006. The filament was located South of a strong sunspot that dominated
the region. In the evolution leading up to the eruption, and for some
time after it, a counter-clockwise rotation of the sunspot of about
30 degrees was observed. We suggest that the rotation triggered the
eruption by progressively expanding the magnetic field above the
filament. To test this scenario, we study the effect of twisting
the initially potential field overlying a pre-existing flux-rope,
using three-dimensional zero-β MHD simulations. We first consider
a relatively simple and symmetric system, and then study a more
complex and asymmetric magnetic configuration, whose photospheric-flux
distribution and coronal structure are guided by the observations and a
potential field extrapolation. In both cases, we find that the twisting
leads to the expansion of the overlying field. As a consequence of the
progressively reduced magnetic tension, the flux-rope quasi-statically
adapts to the changed environmental field, rising slowly. Once the
tension is sufficiently reduced, a distinct second phase of evolution
occurs where the flux-rope enters an unstable regime characterised by
a strong acceleration. Our simulations thus suggest a new mechanism
for the triggering of eruptions in the vicinity of rotating sunspots.
---------------------------------------------------------
Title: Modeling Solar Eruptions: Where Do We stand?
Authors: Torok, Tibor
2013SPD....4430101T Altcode:
Solar flares and coronal mass ejections involve massive releases of
energies into the heliosphere and are the main driver of space weather
disturbances near Earth. It is now well accepted that these enigmatic
events are manifestations of a sudden and violent disruption of the
Sun's coronal magnetic field. However, although such eruptions have
been studied for many years, the detailed physical mechanisms by which
they are initiated and driven are not yet fully understood; primarily
because of our present inability to accurately measure magnetic fields
in the corona. Numerical models have become a powerful tool to help us
overcome this limitation. Global simulations of solar eruptions are
particularly challenging, because of the enormous disparity of the
relevant scales. While the steady advance of computational power has
enabled us to model eruptions with ever increasing detail and realism,
many questions remain unanswered. In this talk, I review what we have
learned from numerical modeling about the physical processes associated
with solar eruptions and I will discuss the current limitations and
future prospects of models.
---------------------------------------------------------
Title: The challenge in making models of fast CMEs
Authors: Mikić, Zoran; Török, Tibor; Titov, Viacheslav; Linker,
Jon A.; Lionello, Roberto; Downs, Cooper; Riley, Pete
2013AIPC.1539...42M Altcode:
It has been a challenge to explain theoretically how fast CMEs
(exceeding ~ 1,000km/s) occur. Our numerical models suggest that it
is not easy to release enough magnetic energy impulsively from an
active region. We have been studying CME models that are constrained
by observed magnetic fields, with realistic coronal plasma density
and temperature profiles, as derived from thermodynamic models of
the corona. We find that to get fast CMEs, the important parameters
are the magnetic energy density, the magnetic field drop-off index,
and the Alfvén speed profile in active regions. We describe how we
energize active regions, and how we subsequently initiate CMEs via
flux cancellation. We contrast CMEs from idealized zero-beta models
with more sophisticated models based on thermodynamic solutions.
---------------------------------------------------------
Title: Which magnetic topologies are favorable for an efficient
acceleration and escape of SEPs?
Authors: Titov, Viacheslav S.; Linker, Jon A.; Mikić, Zoran; Török,
Tibor; Lionello, Roberto
2013shin.confE.129T Altcode:
We assume that unstable magnetic flux ropes are the drivers of solar
flares and CMEs producing SEPs. The natural sites for the acceleration
of SEPs are current sheets and shocks that are formed in the solar
corona around these flux ropes during their eruptions. The location of
the current sheets and shocks in turn depends on the structure of the
background magnetic field ambient to the erupting flux ropes. This
raises an important question on which topologies of the background
field are favorable for an efficient production and escape of SEPs. We
propose that such topologies are inherent to pseudo-streamers, whose
lobes often harbor magnetic flux ropes. The pseudo-streamers possess
closed and open separator field lines, where current sheets have to
be formed whenever the harbored flux ropes start to erupt. These are
good preconditions for both the acceleration and transport of SEPs in
the open-field corona. In addition, the pseudo-streamers' structure
is prone to the generation of sympathetic flux-rope eruptions, which
can produce widely separated but well-synchronized beams of SEPs.
---------------------------------------------------------
Title: Modeling Solar Eruptions: Where Do We Stand?
Authors: Torok, Tibor
2013AAS...22220001T Altcode:
Solar flares and coronal mass ejections involve massive releases of
energies into the heliosphere and are the main driver of space weather
disturbances near Earth. It is now well accepted that these enigmatic
events are manifestations of a sudden and violent disruption of the
Sun's coronal magnetic field. However, although such eruptions have
been studied for many years, the detailed physical mechanisms by which
they are initiated and driven are not yet fully understood; primarily
because of our present inability to accurately measure magnetic fields
in the corona. Numerical models have become a powerful tool to help us
overcome this limitation. Global simulations of solar eruptions are
particularly challenging, because of the enormous disparity of the
relevant scales. While the steady advance of computational power has
enabled us to model eruptions with ever increasing detail and realism,
many questions remain unanswered. In this talk, I review what we have
learned from numerical modeling about the physical processes associated
with solar eruptions and I will discuss the current limitations and
future prospects of models.
---------------------------------------------------------
Title: Pseudo-Streamer Structures in the 2010 August 1-2 CMEs:
PFSS verses MHD model.
Authors: Titov, Viacheslav S.; Mikić, Zoran; Török, Tibor; Linker,
Jon A.; Panasenco, Olga
2013shin.confE.130T Altcode:
We upgrade our previous potential field source-surface (PFSS) model of
the background magnetic field in the 2010 August 1-2 sympathetic CMEs
to a more accurate thermodynamic MHD model of the solar corona. For
this new model, we verify our earlier results on the structure of the
large-scale magnetic field, making a similar topological analysis of the
field as before. We identify the similarities and differences between
the two configurations, particularly, for the eruptive regions with
three pseudo-streamers that we have found before. The new study confirms
that all these pseudo-streamers indeed contain vertical separatrix
surfaces located between two adjacent disconnected coronal holes. Of
special interest to us are the magnetic null points and separator
field lines belonging to such separatrix surfaces. These topological
features exist in both PFSS and MHD models, albeit in different
forms. We reassert our earlier hypothesis that magnetic reconnection
at these nulls and separators likely plays a key role in establishing
a physical connection between the successive eruptions observed by
SDO and STEREO. The results obtained provide further validation of
our recent simplified MHD model of sympathetic eruptions (Török et
al. 2011). <P />Work supported by Lockheed Martin, NASA's Heliophysics
Theory and SR&T programs, and SHINE NSF Grant AGS-1156119.
---------------------------------------------------------
Title: Numerical modeling of fast CMEs from Sun to Earth
Authors: Torok, Tibor; Downs, Cooper; Lionello, Roberto; Linker,
Jon A.; Titov, Viacheslav S.; Mikic, Zoran; Riley, Pete
2013EGUGA..1512485T Altcode:
Coronal mass ejections (CMEs) are the main driver of space weather
disturbances near Earth. The most severe disturbances are caused
by fast CMEs with coronal speeds in excess of 1000 km/s and magnetic
orientations favorable for interaction with the Earth's magnetosphere. A
proper assessment of the impact of CMEs from numerical simulations
requires the self-consistent modeling of both CME initiation and
its propagation through interplanetary space. Such simulations are
very challenging, in particular because of the enormous disparity of
scales involved. Here we present our recent attempts to model fast
CMEs all the way from Sun to Earth. We first simulate the initiation
and propagation of CMEs in the corona using our "thermodynamic" MHD
model, which includes empirical coronal heating, thermal conduction,
and radiation losses. After the initial configuration, consisting of a
large-scale dipole field and an idealized active region, is relaxed to
a steady-state solar wind solution, we insert a flux rope in magnetic
equilibrium into the active region and trigger its eruption by imposing
localized converging flows. We perform a small series of simulations,
varying the geometry and field strength of the flux rope. The resulting
CMEs produce a shock low in the corona and reach peak velocities of
up to 3000 km/s, after which they slow down to constant propagation
speeds of 1000 km/s or less. We then use our recently developed
heliospheric model to simulate the further propagation to 1 AU for
one of the model CMEs.
---------------------------------------------------------
Title: Can We Predict the Geoeffectiveness of CMEs?
Authors: Linker, Jon; Lionello, Roberto; Downs, Cooper; Mikic, Zoran;
Torok, Tibor; Titov, Viacheslav; Riley, Pete
2013enss.confE..11L Altcode:
Coronal Mass Ejections (CMEs) are immense eruptions of plasma and
magnetic field that are propelled outward from the sun, sometimes
with velocities greater than 2000 km/s. They are also responsible for
some of the most severe space weather at Earth, including geomagnetic
storms. Modeling CMEs from Sun to Earth is especially challenging,
because of the enormous disparity of scales involved. At the present
time, both NOAA SWPC and the CCMC use the WSA-Enlil model with "cone
model" CMEs to predict the arrival of possibly geoeffective CMEs at
Earth. This model has no embedded magnetic fields in the CME, and
therefore does not successfully predict the magnitude and direction
of Bz. In this paper, we outline a possible approach to this problem,
using coupled coronal and heliospheric simulations of coronal mass
ejections. Research supported by NASA, NSF, and AFOSR.
---------------------------------------------------------
Title: Characterizing the Magnetic Topology of Solar Eruptions
Authors: Titov, Viacheslav S.; Mikic, Zoran; Torok, Tibor; Linker,
Jon A.; Lionello, Roberto; Riley, Pete
2013enss.confE..15T Altcode:
Numerical MHD simulations of solar eruptions have made it possible
to model the evolution of magnetic configurations with considerable
realism. However, a comprehensive understanding of these complex
configurations requires the development of sophisticated techniques to
analyze the three-dimensional magnetic field structure. We describe
the current state of the art in this kind of analysis, with detailed
illustrations from on-going projects at Predictive Science. Separatrix
surfaces and quasi-separatrix layers form a structural skeleton of
magnetic configurations by dividing them into multiple components
with a simple topology. We discuss the principles and capabilities
of our techniques for analyzing the structural skeletons in erupting
configurations. In particular, we show how these techniques allow one:
(1) to identify erupting and non-erupting strands of the flux ropes; (2)
to determine the global topological flux cells in which such flux ropes
reside, and how they interact in successive eruptions; (3) to calculate
evolving magnetic fluxes for each component of these configurations;
(4) to relate certain structural features to observational features,
such as H-alpha flare ribbons, extreme-ultraviolet dimmings, and X-ray
sigmoids in solar eruptions. The ability to compare our results with
observations enables us to verify the accuracy of the MHD models and
to understand how the coronal magnetic field opens during eruptions.
---------------------------------------------------------
Title: A Multi-spacecraft View of a Giant Filament Eruption during
2009 September 26/27
Authors: Gosain, Sanjay; Schmieder, Brigitte; Artzner, Guy; Bogachev,
Sergei; Török, Tibor
2012ApJ...761...25G Altcode: 2012arXiv1210.6686G
We analyze multi-spacecraft observations of a giant filament eruption
that occurred during 2009 September 26 and 27. The filament eruption was
associated with a relatively slow coronal mass ejection. The filament
consisted of a large and a small part, and both parts erupted nearly
simultaneously. Here we focus on the eruption associated with the
larger part of the filament. The STEREO satellites were separated
by about 117° during this event, so we additionally used SoHO/EIT
and CORONAS/TESIS observations as a third eye (Earth view) to aid our
measurements. We measure the plane-of-sky trajectory of the filament as
seen from STEREO-A and TESIS viewpoints. Using a simple trigonometric
relation, we then use these measurements to estimate the true direction
of propagation of the filament which allows us to derive the true
R/R <SUB>⊙</SUB>-time profile of the filament apex. Furthermore, we
develop a new tomographic method that can potentially provide a more
robust three-dimensional (3D) reconstruction by exploiting multiple
simultaneous views. We apply this method also to investigate the 3D
evolution of the top part of filament. We expect this method to be
useful when SDO and STEREO observations are combined. We then analyze
the kinematics of the eruptive filament during its rapid acceleration
phase by fitting different functional forms to the height-time
data derived from the two methods. We find that for both methods an
exponential function fits the rise profile of the filament slightly
better than parabolic or cubic functions. Finally, we confront these
results with the predictions of theoretical eruption models.
---------------------------------------------------------
Title: Pseudo-Streamer Magnetic Topologies in the 2010 August 1-2 CMEs
Authors: Titov, V. S.; Mikic, Z.; Torok, T.; Linker, J. A.;
Panasenco, O.
2012AGUFMSH51A2211T Altcode:
We upgrade our previous source-surface model of the background magnetic
field in the 2010 August 1-2 sympathetic CMEs to a more accurate
thermodynamic MHD model of the solar corona. For this new model,
we verify our earlier results on the structure of the large-scale
magnetic field, making a similar topological analysis of the field
as before. We identify the similarities and differences between the
two configurations, particularly, for the eruptive regions with three
pseudo-streamers that we have found before. The new study confirms
that all these pseudo-streamers indeed contain vertical separatrix
surfaces located between two adjacent disconnected coronal holes. Of
special interest to us are the magnetic null points and separator field
lines belonging to such separatrix surfaces. We reassert our earlier
hypothesis that magnetic reconnection at these nulls and separators
likely plays a key role in establishing a physical connection between
the successive eruptions observed by SDO and STEREO. The results
obtained provide further validation of our recent simplified MHD
model of sympathetic eruptions (Török et al. 2011). Work supported
by NASA's Heliophysics Theory and SR&T programs, and SHINE NSF
Grant AGS-1156119.
---------------------------------------------------------
Title: Prediction of the Solar Corona for the 2012 November 13 Total
Solar Eclipse
Authors: Mikic, Z.; Linker, J. A.; Downs, C.; Lionello, R.; Riley,
P.; Titov, V. S.; Torok, T.
2012AGUFMSH33A2218M Altcode:
It has become our tradition to predict the structure of the corona
prior to eclipses, using a magnetohydrodynamic (MHD) model based on
measurements of photospheric magnetic fields on the Sun. We plan to
continue this tradition by predicting the structure of the corona for
the November 13, 2012 total solar eclipse, using SDO/HMI photospheric
magnetic field data. We will predict the structure of the corona,
including images of polarization brightness, magnetic field line traces,
and images of simulated emission in EUV and X-rays. These images can
be compared directly with observations of the total eclipse, as well
as observations from SDO/AIA, Hinode/XRT, and STEREO/EUVI. Research
supported by NASA's Heliophysics Theory and Living With a Star Programs,
and NSF/FESD.
---------------------------------------------------------
Title: Using multi-wavelength observations to constrain CME
simulations
Authors: Torok, T.; Mikic, Z.; Titov, V. S.; Linker, J. A.; Downs,
C.; Lionello, R.; Riley, P.
2012AGUFMSH33E..01T Altcode:
The steady growth of computing power now provides the possibility
to model coronal mass ejections (CMEs) at different levels of
complexity. Present CME simulations range from relatively simple
zero-beta calculations, which consider idealized configurations
to isolate the basic physical mechanisms at work in CMEs, to
semi-realistic "thermodynamic" MHD simulations of specific events
that allow us to confront the model results directly with the
observations. In this talk, we will discuss the respective benefits
of these different approaches. As an example, we will consider the
well-known sympathetic eruptions event on 2010, August 1, which our
group has been modeling using various degrees of approximation. In
particular, we will illustrate how we employed the observations (i)
to set up the respective initial magnetic configurations and (ii)
to validate the simulation results.
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Title: Magnetohydrodynamic Simulations of Interplanetary Coronal
Mass Ejections
Authors: Lionello, R.; Downs, C.; Linker, J. A.; Torok, T.; Mikic, Z.
2012AGUFMSH41B2117L Altcode:
Accurately following the propagation of Interplanetary Coronal Mass
Ejections (ICME) is very important for determining space weather
conditions. These are known to impact the functioning of satellites
or create a dangerous environment for astronauts in orbit around the
Earth. Here we describe how we simulate with our MHD numerical model in
spherical coordinates the propagation of ICMEs from the critical point
to Earth and beyond. We first obtain the boundary conditions to apply at
the lower boundaries using the results of simulations of coronal mass
ejections. These are normally derived from the coronal version of our
own model, but any other model that can provide the components of the
magnetic field and the velocity, density, and pressure of the plasma
can be used. Then we calculate the propagation of the disturbance in
interplanetary space.
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Title: A Parametric Study of Erupting Flux Rope Rotation. Modeling
the "Cartwheel CME" on 9 April 2008
Authors: Kliem, B.; Török, T.; Thompson, W. T.
2012SoPh..281..137K Altcode: 2011arXiv1112.3389K; 2012SoPh..tmp...91K
The rotation of erupting filaments in the solar corona is addressed
through a parametric simulation study of unstable, rotating flux ropes
in bipolar force-free initial equilibrium. The Lorentz force due to
the external shear-field component and the relaxation of tension in the
twisted field are the major contributors to the rotation in this model,
while reconnection with the ambient field is of minor importance, due
to the field's simple structure. In the low-beta corona, the rotation is
not guided by the changing orientation of the vertical field component's
polarity inversion line with height. The model yields strong initial
rotations which saturate in the corona and differ qualitatively from
the profile of rotation vs. height obtained in a recent simulation of
an eruption without preexisting flux rope. Both major mechanisms writhe
the flux rope axis, converting part of the initial twist helicity,
and produce rotation profiles which, to a large part, are very similar
within a range of shear-twist combinations. A difference lies in the
tendency of twist-driven rotation to saturate at lower heights than
shear-driven rotation. For parameters characteristic of the source
regions of erupting filaments and coronal mass ejections, the shear
field is found to be the dominant origin of rotations in the corona
and to be required if the rotation reaches angles of order 90 degrees
and higher; it dominates even if the twist exceeds the threshold of
the helical kink instability. The contributions by shear and twist to
the total rotation can be disentangled in the analysis of observations
if the rotation and rise profiles are simultaneously compared with
model calculations. The resulting twist estimate allows one to judge
whether the helical kink instability occurred. This is demonstrated
for the erupting prominence in the "Cartwheel CME" on 9 April 2008,
which has shown a rotation of ≈ 115<SUP>∘</SUP> up to a height of
1.5 R<SUB>⊙</SUB> above the photosphere. Out of a range of initial
equilibria which include strongly kink-unstable (twist Φ=5π), weakly
kink-unstable (Φ=3.5π), and kink-stable (Φ=2.5π) configurations,
only the evolution of the weakly kink-unstable flux rope matches the
observations in their entirety.
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Title: 2010 August 1-2 Sympathetic Eruptions. I. Magnetic Topology
of the Source-surface Background Field
Authors: Titov, V. S.; Mikic, Z.; Török, T.; Linker, J. A.;
Panasenco, O.
2012ApJ...759...70T Altcode: 2012arXiv1209.5797T
A sequence of apparently coupled eruptions was observed on 2010 August
1-2 by Solar Dynamics Observatory and STEREO. The eruptions were closely
synchronized with one another, even though some of them occurred at
widely separated locations. In an attempt to identify a plausible reason
for such synchronization, we study the large-scale structure of the
background magnetic configuration. The coronal field was computed from
the photospheric magnetic field observed at the appropriate time period
by using the potential field source-surface model. We investigate the
resulting field structure by analyzing the so-called squashing factor
calculated at the photospheric and source-surface boundaries, as well as
at different coronal cross-sections. Using this information as a guide,
we determine the underlying structural skeleton of the configuration,
including separatrix and quasi-separatrix surfaces. Our analysis
reveals, in particular, several pseudo-streamers in the regions where
the eruptions occurred. Of special interest to us are the magnetic
null points and separators associated with the pseudo-streamers. We
propose that magnetic reconnection triggered along these separators
by the first eruption likely played a key role in establishing the
assumed link between the sequential eruptions. The present work
substantiates our recent simplified magnetohydrodynamic model of
sympathetic eruptions and provides a guide for further deeper study
of these phenomena. Several important implications of our results for
the S-web model of the slow solar wind are also addressed.
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Title: Contracting and Erupting Components of Sigmoidal Active Regions
Authors: Liu, Rui; Liu, Chang; Török, Tibor; Wang, Yuming; Wang,
Haimin
2012ApJ...757..150L Altcode: 2012arXiv1208.0640L
It has recently been noted that solar eruptions can be associated with
the contraction of coronal loops that are not involved in magnetic
reconnection processes. In this paper, we investigate five coronal
eruptions originating from four sigmoidal active regions, using
high-cadence, high-resolution narrowband EUV images obtained by the
Solar Dynamic Observatory (SDO). The magnitudes of the flares associated
with the eruptions range from GOES class B to class X. Owing to the
high-sensitivity and broad temperature coverage of the Atmospheric
Imaging Assembly (AIA) on board SDO, we are able to identify both the
contracting and erupting components of the eruptions: the former is
observed in cold AIA channels as the contracting coronal loops overlying
the elbows of the sigmoid, and the latter is preferentially observed
in warm/hot AIA channels as an expanding bubble originating from the
center of the sigmoid. The initiation of eruption always precedes the
contraction, and in the energetically mild events (B- and C-flares),
it also precedes the increase in GOES soft X-ray fluxes. In the more
energetic events, the eruption is simultaneous with the impulsive phase
of the nonthermal hard X-ray emission. These observations confirm that
loop contraction is an integrated process in eruptions with partially
opened arcades. The consequence of contraction is a new equilibrium with
reduced magnetic energy, as the contracting loops never regain their
original positions. The contracting process is a direct consequence of
flare energy release, as evidenced by the strong correlation of the
maximal contracting speed, and strong anti-correlation of the time
delay of contraction relative to expansion, with the peak soft X-ray
flux. This is also implied by the relationship between contraction
and expansion, i.e., their timing and speed.
---------------------------------------------------------
Title: Slow Rise and Partial Eruption of a Double-decker
Filament. I. Observations and Interpretation
Authors: Liu, Rui; Kliem, Bernhard; Török, Tibor; Liu, Chang; Titov,
Viacheslav S.; Lionello, Roberto; Linker, Jon A.; Wang, Haimin
2012ApJ...756...59L Altcode: 2012arXiv1207.1757L
We study an active-region dextral filament that was composed of
two branches separated in height by about 13 Mm, as inferred from
three-dimensional reconstruction by combining SDO and STEREO-B
observations. This "double-decker" configuration sustained for days
before the upper branch erupted with a GOES-class M1.0 flare on 2010
August 7. Analyzing this evolution, we obtain the following main
results. (1) During the hours before the eruption, filament threads
within the lower branch were observed to intermittently brighten up,
lift upward, and then merge with the upper branch. The merging process
contributed magnetic flux and current to the upper branch, resulting
in its quasi-static ascent. (2) This transfer might serve as the
key mechanism for the upper branch to lose equilibrium by reaching
the limiting flux that can be stably held down by the overlying
field or by reaching the threshold of the torus instability. (3)
The erupting branch first straightened from a reverse S shape that
followed the polarity inversion line and then writhed into a forward S
shape. This shows a transfer of left-handed helicity in a sequence of
writhe-twist-writhe. The fact that the initial writhe is converted into
the twist of the flux rope excludes the helical kink instability as the
trigger process of the eruption, but supports the occurrence of the
instability in the main phase, which is indeed indicated by the very
strong writhing motion. (4) A hard X-ray sigmoid, likely of coronal
origin, formed in the gap between the two original filament branches
in the impulsive phase of the associated flare. This supports a model
of transient sigmoids forming in the vertical flare current sheet. (5)
Left-handed magnetic helicity is inferred for both branches of the
dextral filament. (6) Two types of force-free magnetic configurations
are compatible with the data, a double flux rope equilibrium and a
single flux rope situated above a loop arcade.
---------------------------------------------------------
Title: Reconstruction of 3D Coronal Magnetic Structures from
THEMIS/MTR and Hinode/SOT Vector Maps
Authors: Schmieder, B.; Guo, Y.; Aulanier, G.; Démoulin, P.; Török,
T.; Bommier, V.; Wiegelmann, T.; Gosain, S.
2012ASPC..454..363S Altcode:
Coordinated campaigns using THEMIS, Hinode, and other instruments have
allowed us to study the magnetic fields of faculae, filaments, and
active regions. In a first case, we modelled the 3D magnetic field in a
flaring active region with a nonlinear force-free field extrapolation,
using magnetic vectors observed by THEMIS/MTR as boundary condition. In
order to construct a consistent bottom boundary for the model, we
first removed the 180 degree ambiguity of the transverse fields and
minimized the force and torque in the observed vector fields. We found
a twisted magnetic flux rope, well aligned with the polarity inversion
line and a part of an Hα filament, and located where a large flare is
initiated about two hours later. In a second case, Hinode/SOT allowed
us to detect fine flux concentrations in faculae, while MTR provided us
with magnetic information at different levels in the atmosphere. The
polarimetry analysis of the MTR and SOT data gave consistent results,
using both UNNOFIT and MELANIE inversion codes.
---------------------------------------------------------
Title: MHD modeling of the solar corona: Progress and challenges
Authors: Linker, Jon; Mikic, Zoran; Lionello, Roberto; Riley, Pete;
Titov, Viacheslav; Torok, Tibor
2012cosp...39.1090L Altcode: 2012cosp.meet.1090L
The Sun and its activity is the ultimate driver of space weather at
Earth. This influence occurs not only via eruptive phenomena such as
coronal mass ejections, but also through the structure of the corona
itself, which forms the genesis of fast solar wind streams that trigger
recurrent geomagnetic activity. Coronal structure also determines the
connection of the ambient interplanetary magnetic field to CME-related
shocks and impulsive solar flares, and thus controls where solar
energetic particles propagate. In this talk we describe both the
present state of the art and new directions in coronal modeling for
both dynamic and slowly varying phenomena. We discuss the challenges to
incorporating these capabilities into future space weather forecasting
and specification models. Supported by NASA through the HTP, LWS,
and SR&T programs, by NSF through the FESD and CISM programs,
and by the AFOSR Space Science program.
---------------------------------------------------------
Title: Magnetic Topology of Pseudo-Streamers in the 2010 August 1-2
Eruption Events
Authors: Titov, Viacheslav S.; Mikic, Zoran; Torok, Tibor; Linker,
Jon A.; Panasenco, Olga
2012shin.confE.160T Altcode:
A sequence of apparently coupled eruptions was observed on 2010 August
1-2 by SDO and STEREO. The eruptions were closely synchronized, even
though some of them occurred very far from each other. Trying to
identify a plausible reason for such synchronization, we study the
large-scale structure of the background magnetic field. The latter
was computed from the photospheric magnetic field observed at the
appropriate time period by using the potential field source-surface
model.For the resulting configuration, we determine its structural
skeleton, which includes all separatrix and quasi-separatrix
surfaces. Analyzing them, we reveal three pseudo-streamers in the
regions where the eruptions occurred. Of special interest to us are
the magnetic null points and separator field lines associated with
these pseudo-streamers. We propose that magnetic reconnection at
such nulls and separators played likely a key role in establishing
the physical link between the successive eruptions. Work supported
by NASA's Heliophysics Theory and SR&T programs, and SHINE NSF
Grant AGS-1156119.
---------------------------------------------------------
Title: Sympathetic Eruptive Events and Pseudostreamers
Authors: Panasenco, Olga; Titov, Viacheslav; Mikić, Zoran; Török,
Tibor; de Toma, Giuliana; Velli, Marco
2012shin.confE.162P Altcode:
Sequences of apparently coupled CMEs triggered by sympathetic eruptions
of solar filaments are usually observed when the initial coronal
magnetic configuration above the source region contains at least
one coronal pseudostreamer. We study in detail an example of such a
sympathetic event observed on 27-28 July 2011 by SDO and STEREO. This
involved five filaments and caused four individual filament eruptions
and one partial eruption. The eruptions were closely synchronized,
even though some occurred at widely separated locations. In an attempt
to identify a plausible reason of such a synchronization, we study the
large-scale structure of the background PFSS magnetic fields, computed
from the observed photospheric magnetic field (SDO/HMI) during the
appropriate time period. We investigate the magnetic connectivities in
these configurations by calculating and analyzing the distributions of
the so-called squashing factor at the photospheric and source-surface
boundaries, as well as other cross-sections at different heights. This
allows us to get a comprehensive understanding of the underlying
structural skeleton of the magnetic configuration. In particular,
our analysis reveals two pseudostreamer magnetic configurations in the
region where the eruptions occurred. Of special interest to us are the
magnetic null points and separators located at the intersection of the
separatrix domes and curtains of the pseudostreamers. We assume that
magnetic reconnection induced by the first eruption at these locations
played likely a major role in establishing the postulated link between
the different eruptions in sequence. The close relationship between the
sympathetic eruptions and pseudostreamer configurations are supported
by a statistical study covering the SDO era (2010-2012).
---------------------------------------------------------
Title: The slow rise phase preceding solar eruptions
Authors: Torok, Tibor
2012shin.confE.214T Altcode:
The eruption of active region filaments and quiescent prominences
is oftenpreceded by a pre-eruptive phase, during which the filament
or prominencerises slowly for minutes to hours (at a speed of about
1-10 km/s) beforeit rapidly accelerates and becomes ejected from the
Sun. Limb observationsof quiescent prominences have shown that their
pre-eruptive rise is typicallyaccompanied by the slow expansion of
the surrounding cavity, during which theprominence/cavity system may
undergo morphological changes.The physics underlying this pre-eruptive
evolution is not yet well understood.It seems likely that we are
observing signatures of a magnetic flux rope which,presumably driven
by slow reconnection, is about to approach a critical heightat which
it cannot longer remain stable and therefore erupts.In this talk,
I will further detail this scenario, and I will briefly presenta
number of eruption simulations that include the pre-eruptive phase,
with theaim of motivating a discussion on how such simulations may
help us to interpret the observations.
---------------------------------------------------------
Title: Global energy diagnostics, current sheet formation and
reconnection outflow jets in a thermodynamic 3D MHD CME simulation
Authors: Reeves, Kathy; Mikić, Zoran; Linker, Jon; Török, Tibor;
Murphy, Nick
2012shin.confE..40R Altcode:
We model a CME using a 3D numerical MHD code that includes coronal
heating, thermal conduction and radiative cooling in the energy
equation. We first develop a global coronal solution (from 1 to 20
Rs) to serve as the initial condition for the CME simulation. The
magnetic flux distribution at 1 Rs consists of a local subsurface
dipole superimposed on global dipole, to mimic the presence of an
active region within the global corona. The resulting configuration
has solar wind emanating from the open field regions, dense plasma in
the streamer belt, and hot plasma in the active region. We introduce
transverse electric fields near the neutral line in the active
region to form a flux rope, then a converging flow is imposed that
causes the eruption. We examine the global energy budget for this
simulated eruption, including the magnetic, kinetic, internal and
gravitational potential energies, coronal heating, ohmic heating,
flow of Poynting flux across the simulation boundaries, and losses due
to radiation. These diagnostic are useful in assessing whether such
simulations reproduce the characteristics of CME observations. We also
follow the formation and evolution of the current sheet and reconnection
outflow jets in this model.
---------------------------------------------------------
Title: Observation & Modeling of An Erupting Double-Decker
Filament
Authors: Liu, Rui; Kliem, B.; Toeroek, T.; Liu, C.; Titov, V. S.;
Lionello, R.; Linker, J. A.; Wang, H.
2012AAS...22032203L Altcode:
We study an active-region dextral filament which was composed of two
branches separated in height by about 13 Mm. This “double-decker”
configuration sustained for days before the upper branch erupted on
2010 August 7. Main results are as follows. 1) During hours before
the eruption, filament threads within the lower branch were observed
to intermittently brighten up, lift upward, and then merge with the
upper branch. The merging process contributed magnetic flux to the
upper branch, resulting in its quasi-static ascent. 2) This flux
transfer might serve as the key mechanism for the upper branch to
lose equilibrium by reaching the limiting flux that can be stably held
down by the overlying field or by reaching the threshold of the torus
instability. 3) The erupting branch first straightened from a reverse S
shape that followed the polarity <P />inversion line and then writhed
into a forward S shape. This shows a transfer of left-handed helicity
in a sequence of writhe-twist-writhe. The fact that the initial writhe
is converted into the twist of the flux rope excludes the helical
kink instability as the trigger process of the eruption, but allows
for a role of the instability in the main phase. 4) A hard X-ray
sigmoid, likely of coronal origin, formed in the gap between the two
original filament branches in the impulsive phase of the associated
flare. This supports a model of transient sigmoids forming in the
vertical flare current sheet. 5) Using MHD modeling, we demonstrate
that a configuration with two force-free flux ropes of like handedness
can form in the slow-rise phase before an eruption and that it admits
stable equilibria as well as the instability of only the upper rope.
---------------------------------------------------------
Title: Observations and simulations of the sympathetic eruptions on
2010 August 1
Authors: Torok, T.; Mikic, Z.; Panasenco, O.; Titov, V. S.; Reeves,
K. K.; Velli, M.; Linker, J. A.; de Toma, G.
2012EGUGA..14.3270T Altcode:
During the rise of the new solar cycle, the Sun has produced a number
of so-called sympathetic eruptions, i.e., eruptions that occur close in
time in different source regions. While it has become clear in recent
years that in many of such events the individual eruptions must be
magnetically connected, the exact nature of these connections is not
yet understood. A particularly beautiful case, which consisted of half
a dozen individual eruptions, was observed by STEREO and SDO on 2010
August 1. Here we focus on a subset of two large, consecutive filament
eruptions that were preceded by a nearby CME. We first summarize the
main features of these events and then present 3D MHD simulations
that were designed to model such a chain of eruptions. The simulations
suggest that the two filament eruptions were triggered by two successive
reconnection events, each of which was induced by the previous eruption,
and thus provide a new mechanism for sympathetic eruptions.
---------------------------------------------------------
Title: Global MHD Models of the Corona and Solar Wind
Authors: Mikic, Z.; Linker, J. A.; Lionello, R.; Riley, P.; Titov,
V. S.; Torok, T.
2012decs.confE..85M Altcode:
Magnetohydrodynamic (MHD) models are useful in understanding the
properties of the global solar corona. They typically use measured
photospheric magnetic fields and an empirical specification of coronal
heating. Comparisons of simulated EUV and X-ray emission from such
models with observations (such as SOHO/EIT, Hinode/XRT, STEREO/EUVI, and
SDO/AIA) can provide a tight constraint on coronal heating models. We
will describe how these models can be used to improve our understanding
of the process that heats the corona.
---------------------------------------------------------
Title: 3D Reconstruction of a Rotating Erupting Prominence
Authors: Thompson, W. T.; Kliem, B.; Török, T.
2012SoPh..276..241T Altcode: 2011arXiv1112.3388T
A bright prominence associated with a coronal mass ejection (CME)
was seen erupting from the Sun on 9 April 2008. This prominence was
tracked by both the Solar Terrestrial Relations Observatory (STEREO)
EUVI and COR1 telescopes, and was seen to rotate about the line of
sight as it erupted; therefore, the event has been nicknamed the
"Cartwheel CME." The threads of the prominence in the core of the
CME quite clearly indicate the structure of a weakly to moderately
twisted flux rope throughout the field of view, up to heliocentric
heights of 4 solar radii. Although the STEREO separation was 48°,
it was possible to match some sharp features in the later part of the
eruption as seen in the 304 Å line in EUVI and in the Hα-sensitive
bandpass of COR1 by both STEREO Ahead and Behind. These features could
then be traced out in three-dimensional space, and reprojected into
a view in which the eruption is directed toward the observer. The
reconstructed view shows that the alignment of the prominence to the
vertical axis rotates as it rises up to a leading-edge height of ≈
2.5 solar radii, and then remains approximately constant. The alignment
at 2.5 solar radii differs by about 115° from the original filament
orientation inferred from Hα and EUV data, and the height profile
of the rotation, obtained here for the first time, shows that two
thirds of the total rotation are reached within ≈ 0.5 solar radii
above the photosphere. These features are well reproduced by numerical
simulations of an unstable moderately twisted flux rope embedded in
external flux with a relatively strong shear field component.
---------------------------------------------------------
Title: MHD Modeling of the Sympathetic Eruptions Observed on August
1, 2010
Authors: Mikic, Z.; Torok, T.; Titov, V. S.; Linker, J. A.; Lionello,
R.; Riley, P.
2011AGUFMSH41B..04M Altcode:
The multiple solar eruptions observed by SDO on August 1, 2010 present a
special challenge to theoretical models of CME initiation. SDO captured
in detail a remarkable chain of sympathetic eruptions that involved
the entire visible hemisphere of the Sun (Schrijver et al. 2011). It
consisted of several flares and six filament eruptions/CMEs, and
triggered a geomagnetic storm on August 3 (de Toma et al. 2010). This
series of eruptions was also observed by the two STEREO spacecraft. This
collection of observations presents a unique opportunity to understand
sympathetic eruptions theoretically. We will present 3D MHD simulations
of these events that have helped us to understand the possible
mechanisms by which the various filament eruptions/CMEs may be linked,
with particular emphasis on the global topology of the coronal magnetic
field in which these structures are embedded.
---------------------------------------------------------
Title: Comparing MHD Simulations of the Solar Corona and the Solar
Wind with Data
Authors: Lionello, R.; Linker, J. A.; Mikic, Z.; Riley, P.; Titov,
V. S.; Torok, T.
2011AGUFMSH41B..02L Altcode:
Our global three-dimensional magnetohydrodynamic (MHD) model of the
solar corona and the solar wind has been extensively used to model
the properties of the magnetic field and of the plasma, from Sun
to Earth and beyond. The key observational input to the model is the
incorporation of observed photospheric magnetic fields into the boundary
conditions. We have studied the geometrical and topological properties
of the magnetic field (e.g., the location and evolution of corona holes,
the reproduction of streamer structure, the location of the heliospheric
current sheet, etc.) and its dynamical reconfiguration (e.g., eruptions
and CMEs propagation). Direct comparison with observations have been
made in the corona by calculation of emission in several EUV and X-ray
bands, both for loops and the global corona. We have also compared the
simulated speed, density, temperature, and magnetic field in the solar
wind with in situ observations. We will discuss the insights obtained
on the strengths and limitations of the models from these comparisons.
---------------------------------------------------------
Title: How do Heliospheric Remote-Sensing Observations Limit Magnetic
Flux Rope Models?
Authors: Riley, P.; Torok, T.; Mikic, Z.; Linker, J. A.; Lionello,
R.; Titov, V. S.
2011AGUFMSH24A..02R Altcode:
In-situ measurements of coronal mass ejecta (CMEs) display a range of
properties, only some of which can be accounted for by current global
MHD models. In fact, first-principle models that include the initiation
and eruption of the ejecta necessarily produce well-defined flux ropes,
whereas only a fraction of CMEs observed in-situ appear to contain a
flux rope. In this talk, we summarize our current understanding of
the observed properties of interplanetary flux ropes and ejecta in
general. We explore ideas that the dichotomy between CMEs and flux
ropes might be due to: (1) an observational selection effect, that is,
all CMEs do in fact contain flux ropes and that the trajectory of the
spacecraft through the event is what determines whether a flux rope is
also encountered; (2) interactions of an erupting flux rope with itself
or between neighboring flux ropes to produce complex structures in which
the flux-rope structure has been significantly modified or destroyed;
(3) an evolutionary process, such as relaxation to a low plasma-beta
state, which governs whether a flux rope is present or not; or (4)
the existence of two (or more) intrinsic mechanisms for producing CMEs,
some of which produce flux ropes and some that do not. To assess these
ideas, we compare model results with a selection of CMEs observed by
the Ulysses, ACE, and STEREO spacecraft.
---------------------------------------------------------
Title: Advances in Modeling the initiation and evolution of CMEs
through the Solar WInd
Authors: Riley, P.; Mikic, Z.; Linker, J. A.; Torok, T.; Lionello,
R.; Titov, V. S.
2011AGUFMSH53C..05R Altcode:
Over the last decade, several factors have led to remarkable gains
in our ability to realistically model a coronal mass ejection (CME)
all the way from the solar surface to 1 AU, or beyond. First,
global models of the ambient solar corona and inner heliosphere
have improved dramatically. The algorithms have transitioned from
simple polytropic prescriptions to rich thermodynamic models that
can reproduce the essential features of remote solar observations and
in-situ measurements. Second, theories of CME initiation, and their
implementation into numerical models, have developed to the point
that a range of complex mechanisms can now be simulated with great
fidelity. Third, the original serial codes are now fully parallelized
allowing them to recruit thousands of processors, and with this,
the ability to simulate events on unprecedented temporal and spatial
scales. And fourth, successive NASA-led missions are returning ever-more
resolved and accurate photospheric magnetic field observations from
which boundary conditions can be derived. In this talk, we show how
these factors have allowed us to produce event-specific simulations
that provide genuine insight into the initiation and evolution of
CMEs, and contrast these results with what was "state-of-the-art"
only 10 years ago. We close by speculating on what the next advances
in global CME models might be.
---------------------------------------------------------
Title: Magnetic Topology of the Sympathetic CMEs Observed on 27 July
2011 and 1 August 2010
Authors: Titov, V. S.; Mikic, Z.; Torok, T.; Linker, J. A.;
Panasenco, O.
2011AGUFMSH43B1949T Altcode:
Two fascinating sequences of apparently coupled CMEs were observed
on 27-28 July 2011 and 1-2 August 2010 by SDO and STEREO. The latter
sequence has recently been described at length by Schrijver &
Title (2011). In both CME sequences, the individual eruptions were
closely synchronized with one another, even though some of them
occurred at widely separated locations. In an attempt to identify a
plausible reason of such a synchronization, we study the large-scale
structure of the background PFSS magnetic fields that were computed
from the observed photospheric magnetic field at the appropriate
time period. We investigate the magnetic connectivities in these
configurations by calculating and analyzing the distributions of the
so-called squashing factor at the photospheric and source-surface
boundaries, as well as at different cross-sections. This allows us
to get a comprehensive understanding of the underlying structural
skeleton of the magnetic configuration. In particular, our analysis
reveals several pseudo-streamers in the regions where the eruptions
occurred. Of special interest to us are the magnetic null points and
separators located at the intersection of the separatrix domes and
curtains of the pseudo-streamers. We assume that magnetic reconnection
induced by the first eruption at these locations played likely a major
role in establishing the postulated link between the eruptions in both
CME sequences. Our recent simplified MHD model of sympathetic eruptions
supports this assumption (Török et al. 2011). In the present study,
we try to further verify it by comparing the background magnetic
topologies of the two sequences of CMEs. Work supported by NASA and
the Center for Integrated Space Weather Modeling (an NSF Science and
Technology Center).
---------------------------------------------------------
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: A Model for Magnetically Coupled Sympathetic Eruptions
Authors: Török, T.; Panasenco, O.; Titov, V. S.; Mikić, Z.; Reeves,
K. K.; Velli, M.; Linker, J. A.; De Toma, G.
2011ApJ...739L..63T Altcode: 2011arXiv1108.2069T
Sympathetic eruptions on the Sun have been observed for several decades,
but the mechanisms by which one eruption can trigger another remain
poorly understood. We present a three-dimensional MHD simulation that
suggests two possible magnetic trigger mechanisms for sympathetic
eruptions. We consider a configuration that contains two coronal flux
ropes located within a pseudo-streamer and one rope located next to
it. A sequence of eruptions is initiated by triggering the eruption of
the flux rope next to the streamer. The expansion of the rope leads
to two consecutive reconnection events, each of which triggers the
eruption of a flux rope by removing a sufficient amount of overlying
flux. The simulation qualitatively reproduces important aspects of the
global sympathetic event on 2010 August 1 and provides a scenario for
the so-called twin filament eruptions. The suggested mechanisms are
also applicable for sympathetic eruptions occurring in other magnetic
configurations.
---------------------------------------------------------
Title: A filament supported by different magnetic field configurations
Authors: Guo, Y.; Schmieder, B.; Démoulin, P.; Wiegelmann, T.;
Aulanier, G.; Török, T.; Bommier, V.
2011IAUS..273..328G Altcode:
A nonlinear force-free magnetic field extrapolation of vector
magnetogram data obtained by THEMIS/MTR on 2005 May 27 suggests the
simultaneous existence of different magnetic configurations within
one active region filament: one part of the filament is supported by
field line dips within a flux rope, while the other part is located
in dips within an arcade structure. Although the axial field chirality
(dextral) and the magnetic helicity (negative) are the same along the
whole filament, the chiralities of the filament barbs at different
sections are opposite, i.e., right-bearing in the flux rope part and
left-bearing in the arcade part. This argues against past suggestions
that different barb chiralities imply different signs of helicity of
the underlying magnetic field. This new finding about the chirality of
filaments will be useful to associate eruptive filaments and magnetic
cloud using the helicity parameter in the Space Weather Science.
---------------------------------------------------------
Title: Solar activity due to magnetic complexity of active regions
Authors: Schmieder, Brigitte; Mandrini, Cristina; Chandra, Ramesh;
Démoulin, Pascal; Török, Tibor; Pariat, Etienne; Uddin, Wahab
2011IAUS..273..164S Altcode:
Active regions (ARs), involved in the Halloween events during
October-November 2003, were the source of unusual activity during
the following solar rotation. The flares on 18-20 November 2003 that
occur in the AR NOAA10501 were accompanied by coronal mass ejections
associated to some particularly geoeffective magnetic clouds. <P />Our
analysis of the magnetic flux and helicity injection revealed that
a new emerging bipole and consequent shearing motions continuously
energized the region during its disk passage. The stored energy was
eventually released through the interaction of the various systems
of magnetic loops by several magnetic reconnection events. Active
events on November 18 (filament eruptions and CMEs) were originated by
shearing motions along a section of the filament channel that injected
magnetic helicity with sign opposite to that of the AR. Two homologous
flares, that occurred on November 20, were apparently triggered by
different mechanisms as inferred from the flare ribbons evolution
(filament eruption and CMEs). We studied in detail the behaviour of
two North-South oriented filaments on November 20 2003. They merged
and split following a process suggestive of `sling-shot' reconnection
between two coronal flux ropes. We successfully tested this scenario
in a 3D MHD simulation that is presented in this paper.
---------------------------------------------------------
Title: A model for sympathetic eruptions
Authors: Torok, Tibor; Panasenco, O.; Titov, V. S.; Mikic, Z.; Velli,
M.; Linker, J.; De Toma, G.
2011shin.confE.125T Altcode:
Apart from single eruptions originating in localized source regions,
the Sun sometimes produces so-called sympathetic events, which consist
of several individual eruptions occurring almost simultaneously
in different source regions. The close temporal correlation of the
individual eruptions in such events indicates a causal link between
them, but the mechanisms by which one eruption can trigger another
one remain largely a mystery. <P />A particularly beautiful example
of a global sympathetic event was recently observed by the Solar
Dynamics Observatory (SDO) on 1 August 2010. It included a small
filament eruption and CME that was shortly after followed by the
nearby subsequent eruptions of two large adjacent (twin) filaments,
indicating that these three eruptions were physically connected. A
coronal potential field extrapolation reveals that the twin filaments
were located in the lobes of a so-called pseudostreamer prior to
their eruptions. <P />Here we present a 3D MHD simulation of the
successive eruption of two magnetic flux ropes in such a pseudostreamer
configuration. The two eruptions are triggered by the simulated eruption
of a third flux rope in the vicinity of the pseudostreamer. The
simulation qualitatively reproduces the CME and subsequent twin
filament eruption on 1 August 2010 and suggests that these events
were indeed physically connected. Furthermore, it provides a generic
scenario for the frequently observed twin filament eruptions in coronal
pseudostreamers and suggests a mechanism by which such eruptions can
be triggered in the first place. Our results thus provide an important
step for a better understanding of sympathetic eruptions.
---------------------------------------------------------
Title: Energetics of a Simulated 3D Eruption Using the PSI MAS Code
Authors: Reeves, Kathy K.; Török, Tibor; Linker, Zoran Mikić Jon
2011shin.confE..17R Altcode:
We examine the energy flow in a three-dimensional simulation of a
coronal mass ejection. The CME is simulated using the Predictive
Science Inc., MAS code. Various forms of energy are tracked as a
function of time over the whole simulation volume as the eruption
progresses. Current sheets are mapped to begin the process of locating
the site of energy release for the simulated flare. We also simulate
the response of the 6 EUV channels from the AIA telescopes on SDO
during the simulated eruption.
---------------------------------------------------------
Title: Structural Skeleton of the Background Magnetic Field During
Sympathetic Eruptions on 1-2 August 2010
Authors: Titov, Viacheslav S.; Mikić, Zoran; Török, Tibor; Linker,
Jon A.
2011shin.confE.131T Altcode:
The Solar Dynamics Observatory observed on 1-2 August 2010 an
interesting sequence of coronal mass ejections (CMEs) (Schrijver &
Title, 2011). These CMEs were closely synchronized with one another,
even though some of them occurred at remote locations. Therefore,
it is tempting to assume that these events were causally linked. In
an attempt to identify a plausible reason of such a link, we study
a large-scale structure of the background magnetic field that has
been computed from the observed photospheric magnetic field at the
appropriate time period. For this purpose, we investigate the respective
magnetic connectivity in the obtained configuration by calculating
and analyzing the distributions of the so-called squashing factor at
the boundaries as well as at different cross-sections. This allows
us to get a comprehensive understanding of the underlying structural
skeleton of the magnetic configuration. In particular, we have found
that five of the six erupting flux ropes were located inside the domes
of three pseudostreamers adjoint to the active region AR 11094. The
stalks of the pseudostreamers passed along the fan separatrix surfaces
emanating upward from the respective magnetic null points. We assume
that magnetic reconnection at these null points played likely a major
role in establishing a hypothetical causal link between the indicated
CMEs. The obtained topological framework provides a solid guide for
further numerical modeling and analysis of the observational data of
these events. <P />Work supported by NASA and the Center for Integrated
Space Weather Modeling (an NSF Science and Technology Center).
---------------------------------------------------------
Title: Actors of the main activity in large complex centres during
the 23 solar cycle maximum
Authors: Schmieder, B.; Démoulin, P.; Pariat, E.; Török, T.;
Molodij, G.; Mandrini, C. H.; Dasso, S.; Chandra, R.; Uddin, W.;
Kumar, P.; Manoharan, P. K.; Venkatakrishnan, P.; Srivastava, N.
2011AdSpR..47.2081S Altcode:
During the maximum of Solar Cycle 23, large active regions had a long
life, spanning several solar rotations, and produced large numbers of
X-class flares and CMEs, some of them associated to magnetic clouds
(MCs). This is the case for the Halloween active regions in 2003. The
most geoeffective MC of the cycle (Dst = -457) had its source during
the disk passage of one of these active regions (NOAA 10501) on
18 November 2003. Such an activity was presumably due to continuous
emerging magnetic flux that was observed during this passage. Moreover,
the region exhibited a complex topology with multiple domains of
different magnetic helicities. The complexity was observed to reach
such unprecedented levels that a detailed multi-wavelength analysis
is necessary to precisely identify the solar sources of CMEs and
MCs. Magnetic clouds are identified using in situ measurements and
interplanetary scintillation (IPS) data. Results from these two
different sets of data are also compared.
---------------------------------------------------------
Title: 3d Mhd Simulation Of Sympathetic Eruptions On 1 August 2010
Authors: Torok, Tibor; Panasenco, O.; Titov, V.; Mikic, Z.; Reeves,
K.; Velli, M.; Linker, J.; de Toma, G.
2011SPD....42.0908T Altcode: 2011BAAS..43S.0908T
Apart from single eruptions originating in localized source regions, the
Sun sometimes produces so-called sympathetic events, which consist of
<P />several individual eruptions occurring <P />almost simultaneously
in different source regions. The close temporal vicinity of the
individual eruptions in such events indicates the <P />existence of
a causal link between them, but the mechanisms by which one eruption
can trigger another one remain largely a mystery. A particularly
beautiful example of a global sympathetic event was recently observed
by the Solar Dynamics Observatory (SDO) on 1 August 2010. It included
a small filament eruption and CME that was closely followed by the
eruptions of two large adjacent (twin) filaments, indicating that these
three eruptions were physically connected. A coronal potential field
extrapolation revealed that the twin filaments were located in the
lobes of a so-called pseudostreamer prior to their eruptions. Here we
present a 3D MHD simulation of the successive eruption of two magnetic
flux ropes in such a pseudostreamer configuration. The two eruptions are
triggered by the simulated eruption of a third flux rope in the vicinity
of the pseudostreamer. The simulation qualitatively reproduces the CME
and subsequent twin filament eruption on 1 August 2010 and suggests that
these events were indeed physically connected. Furthermore, it provides
a generic scenario for the frequently observed twin filament eruptions
in coronal pseudostreamers and suggests a mechanism by which such
eruptions can be triggered in the first place. Our results thus provide
an important step for a better understanding of sympathetic eruptions.
---------------------------------------------------------
Title: Magnetic Topology of the Source Surface Potential Field on
1 August 2010
Authors: Titov, Viacheslav; Mikic, Z.; Torok, T.; Linker, J. A.
2011SPD....42.2303T Altcode: 2011BAAS..43S.2303T
A sequence of coronal mass ejections was recently observed by the Solar
Dynamics Observatory (SDO) on 1 August 2010. The events were closely
synchronized with one another, even though some of them occured at
rather different locations. Therefore, it is tempting to assume that
these events were causally linked with each other. In an attempt to
verify this assumption and identify a plausible reason of such a link,
we study the topological structure of the source surface potential
field that has been computed from the observed photospheric magnetic
field at the appropriate time period. For this purpose, we investigate
the respective magnetic connectivity in the obtained configuration by
calculating and analyzing the distributions of the so-called squashing
factor at the boundaries as well as at different cross-sections. This
allows us to get a comprehensive understanding of the underlying
structural skeleton of the magnetic cofiguration and identify the
robust topological features that likely establish the assumed causal
link in the indicated events. The obtained topological framework also
provides a solid guide for further numerical modeling and analysis of
the observational data of these eruptions.
---------------------------------------------------------
Title: Homologous Flares and Magnetic Field Topology in Active Region
NOAA 10501 on 20 November 2003
Authors: Chandra, R.; Schmieder, B.; Mandrini, C. H.; Démoulin, P.;
Pariat, E.; Török, T.; Uddin, W.
2011SoPh..269...83C Altcode: 2010arXiv1011.1187C; 2010SoPh..tmp..249C
We present and interpret observations of two morphologically homologous
flares that occurred in active region (AR) NOAA 10501 on 20 November
2003. Both flares displayed four homologous Hα ribbons and were
both accompanied by coronal mass ejections (CMEs). The central flare
ribbons were located at the site of an emerging bipole in the centre
of the active region. The negative polarity of this bipole fragmented
in two main pieces, one rotating around the positive polarity by
≈ 110° within 32 hours. We model the coronal magnetic field and
compute its topology, using as boundary condition the magnetogram
closest in time to each flare. In particular, we calculate the
location of quasi-separatrix layers (QSLs) in order to understand the
connectivity between the flare ribbons. Though several polarities were
present in AR 10501, the global magnetic field topology corresponds
to a quadrupolar magnetic field distribution without magnetic null
points. For both flares, the photospheric traces of QSLs are similar
and match well the locations of the four Hα ribbons. This globally
unchanged topology and the continuous shearing by the rotating bipole
are two key factors responsible for the flare homology. However, our
analyses also indicate that different magnetic connectivity domains
of the quadrupolar configuration become unstable during each flare,
so that magnetic reconnection proceeds differently in both events.
---------------------------------------------------------
Title: A single picture for solar coronal outflows and radio noise
storms
Authors: Del Zanna, G.; Aulanier, G.; Klein, K. -L.; Török, T.
2011A&A...526A.137D Altcode:
We propose a unified interpretation for persistent coronal outflows
and metric radio noise storms, two phenomena typically observed in
association with quiescent solar active regions. Our interpretation
is based on multi-wavelength observations of two such regions as
they crossed the meridian in May and July 2007. For both regions,
we observe a persistent pattern of blue-shifted coronal emission in
high-temperature lines with Hinode/EIS, and a radio noise storm with the
Nançay Radioheliograph. The observations are supplemented by potential
and linear force-free extrapolations of the photospheric magnetic
field over large computational boxes, and by a detailed analysis of
the coronal magnetic field topology. We find true separatrices in the
coronal field and null points high in the corona, which are preferential
locations for magnetic reconnection and electron acceleration. We
suggest that the continuous growth of active regions maintains a steady
reconnection across the separatrices at the null point. This interchange
reconnection occurs between closed, high-density loops in the core of
the active region and neighbouring open, low-density flux tubes. Thus,
the reconnection creates strong pressure imbalances which are the main
drivers of plasma upflows. Furthermore, the acceleration of low-energy
electrons in the interchange reconnection region sustains the radio
noise storm in the closed loop areas, as well as weak type III emission
along the open field lines. For both active regions studied, we find a
remarkable agreement between the observed places of persistent coronal
outflows and radio noise storms with their locations as predicted by
our interpretation.
---------------------------------------------------------
Title: Filament Interaction Modeled by Flux Rope Reconnection
Authors: Török, T.; Chandra, R.; Pariat, E.; Démoulin, P.;
Schmieder, B.; Aulanier, G.; Linton, M. G.; Mandrini, C. H.
2011ApJ...728...65T Altcode:
Hα observations of solar active region NOAA 10501 on 2003 November
20 revealed a very uncommon dynamic process: during the development
of a nearby flare, two adjacent elongated filaments approached each
other, merged at their middle sections, and separated again, thereby
forming stable configurations with new footpoint connections. The
observed dynamic pattern is indicative of "slingshot" reconnection
between two magnetic flux ropes. We test this scenario by means
of a three-dimensional zero β magnetohydrodynamic simulation,
using a modified version of the coronal flux rope model by Titov
and Démoulin as the initial condition for the magnetic field. To
this end, a configuration is constructed that contains two flux
ropes which are oriented side-by-side and are embedded in an ambient
potential field. The choice of the magnetic orientation of the flux
ropes and of the topology of the potential field is guided by the
observations. Quasi-static boundary flows are then imposed to bring
the middle sections of the flux ropes into contact. After sufficient
driving, the ropes reconnect and two new flux ropes are formed,
which now connect the former adjacent flux rope footpoints of opposite
polarity. The corresponding evolution of filament material is modeled
by calculating the positions of field line dips at all times. The dips
follow the morphological evolution of the flux ropes, in qualitative
agreement with the observed filaments.
---------------------------------------------------------
Title: Study of solar flares and filament interaction in NOAA 10501
on 20 November, 2003
Authors: Chandra, R.; Schmieder, B.; Mandrini, C. H.; Démoulin, P.;
Pariat, E.; Török, T.; Aulanier, G.; Uddin, W.; Linton, M. G.
2011ASInC...2..323C Altcode:
We analyze the observations of two flares from NOAA AR 10501 on 20
November, 2003. The flares are homologous, exhibit four ribbons and
are located in a quadrupolar magnetic configuration. The evolution
of the ribbons suggests that the first eruption is triggered by
"tether cutting" (with subsequent quadrupolar reconnection as in the
"magnetic breakout" model), whereas the second one is consistent
with the "magnetic breakout" model. Another interesting feature of
our observations is the interaction of two filaments elongated in the
north-south direction. The filaments merge at their central parts and
afterwards change their orientation to the east-west direction. This
merging and splitting is closely related to the evolution found in an
MHD simulation as a result of reconnection between two flux ropes.
---------------------------------------------------------
Title: Writhing and rotation of erupting prominences and CMEs
Authors: Torok, T.; Kliem, B.; Thompson, W. T.; Berger, M. A.
2010AGUFMSH43C..01T Altcode:
Erupting prominences often exhibit a writhing motion as they rise in
the corona to become the core of a coronal mass ejection (CME). The
writhing points towards the presence of a magnetic flux rope whose top
part rotates about the direction of ascent. Understanding what causes
the writhing, and which parameters determine its amount, will help
us to 1) constrain CME initiation models and 2) predict the magnetic
orientation of CMEs when they hit the Earth. Two mechanisms have been
suggested to cause significant writhing/rotation in the low corona,
namely the helical kink instability (KI) and the interaction of the
shear field component of the ambient coronal field with the flux rope
current. Here we present the first height profile of the rotation of an
erupting prominence, obtained from STEREO observations. The prominence
rotated by about 120 degree from its pre-eruptive orientation, until
it reached a heliocentric height of about 2.5 solar radii. The data
are compared to a series of numerical simulations that study the
corresponding rotation of an erupting magnetic flux rope, by varying
the initial flux rope twist and the external shear field component. The
parameter range compatible with the data is constrained by the observed
rotation-height and height-time profiles. It is found that, for the set
of geometrical model parameters considered here, the observed strong
rotation cannot be caused by the KI alone, but requires the presence
of a shear field component. Moreover, the simulations suggest that the
contribution of the shear field component was dominant in the observed
event, indicating that the flux rope was only moderately twisted. We
also briefly present the first measurements of the evolution of twist
and writhe in numerical simulations of confined and ejective flux rope
eruptions, and we discuss the implications of the results for filament
eruptions and CMEs.
---------------------------------------------------------
Title: Driving Mechanism and Onset Condition of a Confined Eruption
Authors: Guo, Y.; Ding, M. D.; Schmieder, B.; Li, H.; Török, T.;
Wiegelmann, T.
2010ApJ...725L..38G Altcode:
We study a confined eruption accompanied by an M1.1 flare in solar
active region (AR) NOAA 10767 on 2005 May 27, where a pre-eruptive
magnetic flux rope was reported in a nonlinear force-free field (NLFFF)
extrapolation. The observations show a strong writhing motion of the
erupting structure, suggesting that a flux rope was indeed present
and converted some of its twist into writhe in the course of the
eruption. Using the NLFFF extrapolation, we calculate the twist of
the pre-eruptive flux rope and find that it is in very good agreement
with thresholds of the helical kink instability found in numerical
simulations. We conclude that the activation and rise of the flux
rope were triggered and driven by the instability. Using a potential
field extrapolation, we also estimate the height distribution of the
decay index of the external magnetic field in the AR 1 hr prior to the
eruption. We find that the decay index stays below the threshold for
the torus instability for a significant height range above the erupting
flux rope. This provides a possible explanation for the confinement
of the eruption to the low corona.
---------------------------------------------------------
Title: 3D Study of Solar Eruptions Using SDO and STEREO Observations
Authors: de Toma, G.; Reinard, A. A.; Gibson, S. E.; Burkepile, J.;
Fan, Y.; Torok, T.
2010AGUFMSH23A1834D Altcode:
Combination of data from the recently launched SDO and the two STEREO
spacecraft -that are now at about 80deg from the Sun-Earth direction-
offers the unprecedented opportunity to observe simultaneously the
region where a CME originates and the CME moving outward in the
plane-of-the-sky. This allows us to compute trajectories for the
CME and the associated eruptive prominence and, at the same time, to
study the on-disk CME manifestations such as flares, dimming regions,
and coronal waves with very high spatial and temporal resolution. We
present examples of Earth-directed CMEs, when the CME can be traced
from the Sun to the Earth, that take advantage of this unique satellite
configuration.
---------------------------------------------------------
Title: Observational and numerical study of the 25 July 2004 event
Authors: Soenen, A.; Jacobs, C.; Poedts, S.; van Driel-Gesztelyi,
L.; Torok, T.; Lapenta, G.
2010AGUFMSH23B1843S Altcode:
We study the 25 July 2004 event. By analyzing SOHO EIT images we
establish a basic understanding of the large-scale interaction going
on during this event. Magnetic reconnection between the expanding
CME and the Southern hemispheric active regions (AR) will connect the
leading polarities of the two ARs, lead to brightening in the ARs and
transport CME field line foot points to distant ARs (observable as
dimming at the foot points).We reproduce the large scale interactions
during this event using three-dimensional magneto-hydrodynamic (MHD)
simulations. We superimpose a magnetic source region that resembles
the SOHO MDI images on a basic wind model. By emerging new flux at the
centre of this region we initiate a Coronal Mass Ejection (CME). We
monitor the evolution of this CME and study its interaction with the
source region.
---------------------------------------------------------
Title: Reconnection of a Kinking Flux Rope Triggering the Ejection
of a Microwave and Hard X-Ray Source II. Numerical Modeling
Authors: Kliem, B.; Linton, M. G.; Török, T.; Karlický, M.
2010SoPh..266...91K Altcode: 2010SoPh..tmp..149K; 2010arXiv1007.2147K
Numerical simulations of the helical (m=1) kink instability of an
arched, line-tied flux rope demonstrate that the helical deformation
enforces reconnection between the legs of the rope if modes with two
helical turns are dominant as a result of high initial twist in the
range Φ≳6π. Such a reconnection is complex, involving also the
ambient field. In addition to breaking up the original rope, it can
form a new, low-lying, less twisted flux rope. The new flux rope is
pushed downward by the reconnection outflow, which typically forces it
to break as well by reconnecting with the ambient field. The top part
of the original rope, largely rooted in the sources of the ambient
flux after the break-up, can fully erupt or be halted at low heights,
producing a "failed eruption." The helical current sheet associated with
the instability is squeezed between the approaching legs, temporarily
forming a double current sheet. The leg - leg reconnection proceeds
at a high rate, producing sufficiently strong electric fields that it
would be able to accelerate particles. It may also form plasmoids, or
plasmoid-like structures, which trap energetic particles and propagate
out of the reconnection region up to the top of the erupting flux rope
along the helical current sheet. The kinking of a highly twisted flux
rope involving leg - leg reconnection can explain key features of an
eruptive but partially occulted solar flare on 18 April 2001, which
ejected a relatively compact hard X-ray and microwave source and was
associated with a fast coronal mass ejection.
---------------------------------------------------------
Title: Testing magnetofrictional extrapolation with the
Titov-Démoulin model of solar active regions
Authors: Valori, G.; Kliem, B.; Török, T.; Titov, V. S.
2010A&A...519A..44V Altcode: 2010arXiv1005.0254V
We examine the nonlinear magnetofrictional extrapolation scheme
using the solar active region model by Titov and Démoulin as test
field. This model consists of an arched, line-tied current channel
held in force-free equilibrium by the potential field of a bipolar
flux distribution in the bottom boundary. A modified version with a
parabolic current density profile is employed here. We find that the
equilibrium is reconstructed with very high accuracy in a representative
range of parameter space, using only the vector field in the bottom
boundary as input. Structural features formed in the interface
between the flux rope and the surrounding arcade - “hyperbolic
flux tube” and “bald patch separatrix surface” - are reliably
reproduced, as are the flux rope twist and the energy and helicity of
the configuration. This demonstrates that force-free fields containing
these basic structural elements of solar active regions can be obtained
by extrapolation. The influence of the chosen initial condition on
the accuracy of reconstruction is also addressed, confirming that the
initial field that best matches the external potential field of the
model quite naturally leads to the best reconstruction. Extrapolating
the magnetogram of a Titov-Démoulin equilibrium in the unstable
range of parameter space yields a sequence of two opposing evolutionary
phases, which clearly indicate the unstable nature of the configuration:
a partial buildup of the flux rope with rising free energy is followed
by destruction of the rope, losing most of the free energy.
---------------------------------------------------------
Title: Symmetric Coronal Jets: A Reconnection-controlled Study
Authors: Rachmeler, L. A.; Pariat, E.; DeForest, C. E.; Antiochos,
S.; Török, T.
2010ApJ...715.1556R Altcode:
Current models and observations imply that reconnection is a key
mechanism for destabilization and initiation of coronal jets. We evolve
a system described by the theoretical symmetric jet formation model
using two different numerical codes with the goal of studying the
role of reconnection in this system. One of the codes is the Eulerian
adaptive mesh code ARMS, which simulates magnetic reconnection through
numerical diffusion. The quasi-Lagrangian FLUX code, on the other hand,
is ideal and able to evolve the system without reconnection. The ideal
nature of FLUX allows us to provide a control case of evolution without
reconnection. We find that during the initial symmetric and ideal phase
of evolution, both codes produce very similar morphologies and energy
growth. The symmetry is then broken by a kink-like motion of the axis
of rotation, after which the two systems diverge. In ARMS, current
sheets formed and reconnection rapidly released the stored magnetic
energy. In FLUX, the closed field remained approximately constant
in height while expanding in width and did not release any magnetic
energy. We find that the symmetry threshold is an ideal property of the
system, but the lack of energy release implies that the observed kink is
not an instability. Because of the confined nature of the FLUX system,
we conclude that reconnection is indeed necessary for jet formation
in symmetric jet models in a uniform coronal background field.
---------------------------------------------------------
Title: The writhe of helical structures in the solar corona
Authors: Török, T.; Berger, M. A.; Kliem, B.
2010A&A...516A..49T Altcode: 2010arXiv1004.3918T
Context. Helicity is a fundamental property of magnetic fields,
conserved in ideal MHD. In flux rope geometry, it consists of twist and
writhe helicity. Despite the common occurrence of helical structures
in the solar atmosphere, little is known about how their shape relates
to the writhe, which fraction of helicity is contained in writhe,
and how much helicity is exchanged between twist and writhe when
they erupt. <BR /> Aims: Here we perform a quantitative investigation
of these questions relevant for coronal flux ropes. <BR /> Methods:
The decomposition of the writhe of a curve into local and nonlocal
components greatly facilitates its computation. We use it to study
the relation between writhe and projected S shape of helical curves
and to measure writhe and twist in numerical simulations of flux
rope instabilities. The results are discussed with regard to filament
eruptions and coronal mass ejections (CMEs). <BR /> Results: (1) We
demonstrate that the relation between writhe and projected S shape
is not unique in principle, but that the ambiguity does not affect
low-lying structures, thus supporting the established empirical rule
which associates stable forward (reverse) S shaped structures low
in the corona with positive (negative) helicity. (2) Kink-unstable
erupting flux ropes are found to transform a far smaller fraction
of their twist helicity into writhe helicity than often assumed. (3)
Confined flux rope eruptions tend to show stronger writhe at low heights
than ejective eruptions (CMEs). This argues against suggestions that
the writhing facilitates the rise of the rope through the overlying
field. (4) Erupting filaments which are S shaped already before the
eruption and keep the sign of their axis writhe (which is expected if
field of one chirality dominates the source volume of the eruption),
must reverse their S shape in the course of the rise. Implications
for the occurrence of the helical kink instability in such events are
discussed. (5) The writhe of rising loops can easily be estimated
from the angle of rotation about the direction of ascent, once the
apex height exceeds the footpoint separation significantly. <BR />
Conclusions: Writhe can straightforwardly be computed for numerical
data and can often be estimated from observations. It is useful in
interpreting S shaped coronal structures and in constraining models
of eruptions.
---------------------------------------------------------
Title: Coexisting Flux Rope and Dipped Arcade Sections Along One
Solar Filament
Authors: Guo, Y.; Schmieder, B.; Démoulin, P.; Wiegelmann, T.;
Aulanier, G.; Török, T.; Bommier, V.
2010ApJ...714..343G Altcode:
We compute the three-dimensional magnetic field of an active
region in order to study the magnetic configuration of active region
filaments. The nonlinear force-free field model is adopted to compute
the magnetic field above the photosphere, where the vector magnetic
field was observed by THEMIS/MTR on 2005 May 27. We propose a new
method to remove the 180° ambiguity of the transverse field. Next, we
analyze the implications of the preprocessing of the data by minimizing
the total force and torque in the observed vector fields. This step
provides a consistent bottom boundary condition for the nonlinear
force-free field model. Then, using the optimization method to compute
the coronal field, we find a magnetic flux rope along the polarity
inversion line. The magnetic flux rope aligns well with part of an Hα
filament, while the total distribution of the magnetic dips coincides
with the whole Hα filament. This implies that the magnetic field
structure in one section of the filament is a flux rope, while the
other is a sheared arcade. The arcade induced a left-bearing filament
in the magnetic field of negative helicity, which is opposite to the
chirality of barbs that a flux rope would induce in a magnetic field
of the same helicity sign. The field strength in the center of the flux
rope is about 700 G, and the twist of the field lines is ~1.4 turns.
---------------------------------------------------------
Title: Formation of Torus-Unstable Flux Ropes and Electric Currents
in Erupting Sigmoids
Authors: Aulanier, G.; Török, T.; Démoulin, P.; DeLuca, E. E.
2010ApJ...708..314A Altcode:
We analyze the physical mechanisms that form a three-dimensional
coronal flux rope and later cause its eruption. This is achieved by a
zero-β magnetohydrodynamic (MHD) simulation of an initially potential,
asymmetric bipolar field, which evolves by means of simultaneous slow
magnetic field diffusion and sub-Alfvénic, line-tied shearing motions
in the photosphere. As in similar models, flux-cancellation-driven
photospheric reconnection in a bald-patch (BP) separatrix transforms the
sheared arcades into a slowly rising and stable flux rope. A bifurcation
from a BP to a quasi-separatrix layer (QSL) topology occurs later on in
the evolution, while the flux rope keeps growing and slowly rising,
now due to shear-driven coronal slip-running reconnection, which
is of tether-cutting type and takes place in the QSL. As the flux
rope reaches the altitude at which the decay index -∂ln B/∂ln z
of the potential field exceeds ~3/2, it rapidly accelerates upward,
while the overlying arcade eventually develops an inverse tear-drop
shape, as observed in coronal mass ejections (CMEs). This transition
to eruption is in accordance with the onset criterion of the torus
instability. Thus, we find that photospheric flux-cancellation and
tether-cutting coronal reconnection do not trigger CMEs in bipolar
magnetic fields, but are key pre-eruptive mechanisms for flux ropes to
build up and to rise to the critical height above the photosphere at
which the torus instability causes the eruption. In order to interpret
recent Hinode X-Ray Telescope observations of an erupting sigmoid, we
produce simplified synthetic soft X-ray images from the distribution
of the electric currents in the simulation. We find that a bright
sigmoidal envelope is formed by pairs of J-shaped field lines in the
pre-eruptive stage. These field lines form through the BP reconnection
and merge later on into S-shaped loops through the tether-cutting
reconnection. During the eruption, the central part of the sigmoid
brightens due to the formation of a vertical current layer in the wake
of the erupting flux rope. Slip-running reconnection in this layer
yields the formation of flare loops. A rapid decrease of currents due
to field line expansion, together with the increase of narrow currents
in the reconnecting QSL, yields the sigmoid hooks to thin in the early
stages of the eruption. Finally, a slightly rotating erupting loop-like
feature (ELLF) detaches from the center of the sigmoid. Most of this
ELLF is not associated with the erupting flux rope, but with a current
shell that develops within expanding field lines above the rope. Only
the short, curved end of the ELLF corresponds to a part of the flux
rope. We argue that the features found in the simulation are generic
for the formation and eruption of soft X-ray sigmoids.
---------------------------------------------------------
Title: Magnetic Flux Rope Eruption: Non Equilibrium versus Torus
Instability
Authors: Demoulin, Pascal; Aulanier, Guillaume; Toeroek, Tibor;
Deluca, Edward
2010cosp...38.1855D Altcode: 2010cosp.meet.1855D
The coronal magnetic configuration of an active region typically
evolves quietly during few days before becoming suddenly eruptive
and launching a CME. The precise origin of the eruption is still
debated. Among other mechanisms, it has been long proposed that a
loss of equilibrium, or an ideal MHD instability such as the torus
instability, could be responsible for the sudden eruptivity. We first
revisit both approaches with simple analytical models as well as with
a 3D MHD simulation of an initially potential bipolar field, which
evolves by means of simultaneous slow magnetic field diffusion and
shearing motions in the photosphere. Reconnection of sheared arcade
leads to the formation of a twisted flux rope, which corresponds to an
electric current channel. We find that the electric current distribution
and the field-line organization present in the MHD simulation provide
an explanation for the recent X-rays Hinode observations of erupting
sigmoidal regions. Next, we show analytically that the loss of
equilibrium and the torus instability are two different views of the
same physical mechanism. We compare the instability thresholds in the
limit of straight and circular current channels, finding that they are
closely comparable for thick current channels (as present in the MHD
simulation and as expected in the corona) while these thresholds are
well distinct at the limit of very thin current channels (as typically
found in previous studies). Finally, including photospheric line tying
of the current channel in the analytical models permits to have a
closer comparison between instability thresholds found analytically
and by the MHD simulation.
---------------------------------------------------------
Title: Ejective events from a complex active region
Authors: Mandrini, Cristina H.; Chandra, Ramesh; Pariat, Etienne;
Schmieder, Brigitte; Demoulin, Pascal; Toeroek, Tibor; Uddin, Wahab
2010cosp...38.1886M Altcode: 2010cosp.meet.1886M
On 18 and 20 November 2003 active region (AR) 10501 produced a series of
M flares all of them associated with coronal mass ejections (CMEs). The
particularity of this AR is that while observational tracers of the
magnetic helicity sign indicate that the large scale field in the
region had a negative magnetic helicity sign, the MC associated
to the most intense flare/CME on November 18 showed the opposite
sign. Furthermore, the filaments observed on November 20 present
morphological characteristics that correspond to a negative magnetic
helicity sign, the rotation of the polarities of an emerging bipole
indicate negative magnetic helicity sign injection; however, the flare
ribbons observed after two homologous events can be connected either
by field lines computed using a positive or a negative helicity sign
magnetic field. We combine Hα, EUV, hard X-rays, and magnetic field
data analysis with magnetic field modelling, and magnetic helicity
injection computations to understand the origin of the helicity
sign discrepancies discussed above. On November 20 magnetic field
modeling and topology computations (in particular, the location of
quasi-separatrix layers in relation to flare ribbons and evolution)
give us clues about the CME initiation process.
---------------------------------------------------------
Title: Actors of the main activity of large complex centres during
the 23 Solar Cycle maximum
Authors: Schmieder, Brigitte; Chandra, Ramesh; Demoulin, Pascal;
Mandrini, Cristina H.; Venkatakrishnan, P.; Manoharan, P. K.; Uddin,
Wahab; Pariat, Etienne; Toeroek, Tibor; Molodij, Guillaume; Kumar, P.
2010cosp...38.1861S Altcode: 2010cosp.meet.1861S
During the maximum of the last Solar Cycle solar cycle 23, large
active regions had a long life spanning several solar rotations and
produced a large number of X-ray class flares, CMEs and Magnetic
clouds (MC). This was the case for the Halloween active regions in
2003. The most geoeffective magnetic cloud of the cycle (Dst=-457)
has its source in one passage of the active region (NOAA 10501) on
November 18, 2003. Such an activity is presumably due to continuous
emerging magnetic flux that was observed during this passage. Moreover,
the region exhibited a complex topology with multiple domains of
distinct magnetic helicities. The complexity is observed to reach
such unprecedented levels that a detailed multi wavelength analysis
is necessary to precisely identify the sources of CMEs and MCs.
---------------------------------------------------------
Title: Intensification of Plasma Upflows in an Active Region---Coronal
Hole Complex: A CME Precursor
Authors: Baker, D.; van Driel-Gesztelyi, L.; Murray, M. J.; Green,
L. M.; Török, T.; Sun, J.
2009ASPC..415...75B Altcode:
We investigate the plasma flows resulting from the interaction between
a mature active region (AR) and a surrounding equatorial coronal hole
(CH) observed by Hinode's EIS and XRT from 15 to 18 October 2007. For 3
days, EIS velocity maps showed upflows at the AR's eastern and western
edges that were consistently between 5 and 10 km s<SUP>-1</SUP>, whereas
downflows of up to 30 km s<SUP>-1</SUP> were seen in AR loops. However,
on 18 October, velocity profiles of hotter coronal lines revealed
intensification in upflow velocities of up to 18 km s<SUP>-1</SUP>
at the AR's western footpoints 4.5 hours prior to a CME. We compare
the AR's plasma flows with 2.5D MHD numerical simulations of the
magnetic configuration, which show that expansion of the mature AR's
loops drives upflows along the neighboring CH field. Further, the
intensification of upflows observed on the AR's western side prior to
a CME is interpreted to be the result of the expansion of a flux rope
containing a filament further compressing the neighboring CH field.
---------------------------------------------------------
Title: Fan-Spine Topology Formation Through Two-Step Reconnection
Driven by Twisted Flux Emergence
Authors: Török, T.; Aulanier, G.; Schmieder, B.; Reeves, K. K.;
Golub, L.
2009ApJ...704..485T Altcode: 2009arXiv0909.2235T
We address the formation of three-dimensional nullpoint topologies
in the solar corona by combining Hinode/X-ray Telescope (XRT)
observations of a small dynamic limb event, which occurred beside
a non-erupting prominence cavity, with a three-dimensional (3D)
zero-β magnetohydrodynamics (MHD) simulation. To this end, we model
the boundary-driven "kinematic" emergence of a compact, intense,
and uniformly twisted flux tube into a potential field arcade that
overlies a weakly twisted coronal flux rope. The expansion of the
emerging flux in the corona gives rise to the formation of a nullpoint
at the interface of the emerging and the pre-existing fields. We unveil
a two-step reconnection process at the nullpoint that eventually yields
the formation of a broad 3D fan-spine configuration above the emerging
bipole. The first reconnection involves emerging fields and a set of
large-scale arcade field lines. It results in the launch of a torsional
MHD wave that propagates along the arcades, and in the formation of
a sheared loop system on one side of the emerging flux. The second
reconnection occurs between these newly formed loops and remote arcade
fields, and yields the formation of a second loop system on the opposite
side of the emerging flux. The two loop systems collectively display
an anenome pattern that is located below the fan surface. The flux that
surrounds the inner spine field line of the nullpoint retains a fraction
of the emerged twist, while the remaining twist is evacuated along
the reconnected arcades. The nature and timing of the features which
occur in the simulation do qualititatively reproduce those observed
by XRT in the particular event studied in this paper. Moreover, the
two-step reconnection process suggests a new consistent and generic
model for the formation of anemone regions in the solar corona.
---------------------------------------------------------
Title: 3D Reconstruction of an Erupting Prominence
Authors: Thompson, William T.; Kliem, B.; Toeroek, T.
2009SPD....40.2111T Altcode:
A bright prominence associated with a coronal mass ejection was seen
erupting from the Sun on April 9, 2008. This prominence was tracked
in both the STEREO EUVI and COR1 telescopes, and was seen to rotate or
“swirl” as it erupted. Although the STEREO separation was 48 degrees,
it was possible to match some sharp features in the later part of the
eruption as seen in the 304 A line in EUVI by both STEREO Ahead and
Behind. These features could then be traced out in three-dimensional
space, and reprojected into a view in which the eruption is directed
towards the observer. The reconstructed view shows that the alignment
of the prominence rotates as it rises through the EUVI field-of-view
out to 1.4 solar radii, and then remains constant as seen by COR1. The
alignment at 1.4 solar radii differed by about 120 degrees from the
original filament orientation. We will match the filament observations
against a model of the event as kink instability in a flux rope.
---------------------------------------------------------
Title: Flux Emergence as a Trigger of Coronal Mass Ejections
Authors: Linton, Mark; Torok, T.
2009SPD....40.2202L Altcode:
We will present preliminary results from an investigation of Coronal
Mass Ejections (CMEs) triggered by flux emergence. Related numerical
simulations rely so far mostly on the kinematic, i.e. boundary-driven,
emergence of magnetic flux at a photospheric boundary into the
corona. We are investigating the viability of these models by studying
the dynamical emergence of flux from a convection zone, through
a temperature minimum photospheric region, into the corona. We are
focusing on two CME models: the magnetic breakout model, and the ideal
MHD torus instability model. Results from the breakout CME study will
be shown at this meeting in the presentation by Leake et al. Here, we
will show first results from the torus CME model. We build on recent
simulations by Torok, where the torus instability of a pre-existing,
stable coronal flux rope is triggered by kinematic flux emergence. We
will report on our work to emerge flux dynamically in an equivalent
configuration.
---------------------------------------------------------
Title: Solar prominences
Authors: Schmieder, Brigitte; Aulanier, Guillaume; Török, Tibor
2009IAUS..257..223S Altcode:
Solar filaments (or prominences) are magnetic structures in the
corona. They can be represented by twisted flux ropes in a bipolar
magnetic environment. In such models, the dipped field lines of the
flux rope carry the filament material and parasitic polarities in the
filament channel are responsible for the existence of the lateral feet
of prominences. <P />Very simple laws do exist for the chirality of
filaments, the so-called “filament chirality rules”: commonly
dextral/sinistral filaments corresponding to left- (resp. right)
hand magnetic twists are in the North/South hemisphere. Combining
these rules with 3D weakly twisted flux tube models, the sign of the
magnetic helicity in several filaments were identified. These rules
were also applied to the 180° disambiguation of the direction of the
photospheric transverse magnetic field around filaments using THEMIS
vector magnetograph data (López Ariste et al. 2006). Consequently,
an unprecedented evidence of horizontal magnetic support in filament
feet has been observed, as predicted by former magnetostatic and
recent MHD models. <P />The second part of this review concerns the
role of emerging flux in the vicinity of filament channels. It has been
suggested that magnetic reconnection between the emerging flux and the
pre-existing coronal field can trigger filament eruptions and CMEs. For
a particular event, observed with Hinode/XRT, we observe signatures of
such a reconnection, but no eruption of the filament. We present a 3D
numerical simulation of emerging flux in the vicinity of a flux rope
which was performed to reproduce this event and we briefly discuss,
based on the simulation results, why the filament did not erupt.
---------------------------------------------------------
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: Simulations of the CME-Flare Relationship
Authors: Kliem, B.; Török, T.; Forbes, T. G.
2008AGUFMSH23B1648K Altcode:
Observations of coronal mass ejections (CMEs) and solar flares have
revealed a high correlation between the acceleration of the ejecta
and the plasma heating and particle acceleration signified by the
soft and hard X-ray emissions of the associated flare. The latter are
generally thought to result from magnetic reconnection. This finding
has stimulated the discussion of the CME-flare relationship, but at
the same time it has made it difficult to find a conclusive answer as
to whether magnetic reconnection or an ideal MHD instability is the
prime cause of the eruptions. Numerical simulations of unstable flux
ropes will be presented that are in very satisfactory quantitative
agreement with erupting filaments, both, confined to the corona and
ejective (i.e., developing into a CME). Some of these simulations
indeed show a high degree of synchronization between the initial
exponential acceleration of the flux rope, due to the ideal MHD
instability, and the development of reconnection flows. However,
others show a very delayed onset of reconnection, even after the
flux rope's acceleration peak. In addition, the reconnection flows
generally lag behind the motions driven by the ideal instability as
the flux rope rise velocity nears the saturation phase. Comparison of
the simulation results with observations suggests that the ideal MHD
process is the primary driver of the coupled CME-flare phenomenon. The
strong differences in the degree of synchronization, which the simulated
systems show in the main rise phase of the eruption, are related to
the magnetic topology prior to the eruption. Given the observational
result of a high correlation between CME and flare development (Zhang
& Dere 2006), these simulations yield constraints on the topology
and lead us to conclude that a seed for a reconnecting current sheet
must typically be present already at the onset of the eruption.
---------------------------------------------------------
Title: Magnetofrictional Extrapolations of Current-Carrying Flux Ropes
Authors: Valori, G.; Kliem, B.; Toeroek, T.
2008ESPM...12.2.90V Altcode:
The quiescent solar corona is regularly modified by very fast ejections
of coronal material and magnetic field (CME) that occur preferably
above active regions. Most CME models require the formation of twisted
magnetic field structures (flux ropes) before or during such events. <P
/>Unfortunately, the coronal magnetic field is not directly measurable
at present, and therefore it is difficult to verify the validity
of different CME models. <P />In order to remove this obstacle, the
extrapolation of photospheric magnetic field measurements can be used
to reconstruct the missing coronal information. As an application of
our magneto-frictional code, we present an extrapolation of a measured
magnetogram where a flux rope is found. <P />In such applications it
is necessary to estimate how well our extrapolation code can reproduce
all aspects of highly nonlinear structures such as flux ropes. This is
of course possible only using test fields. <P />The Titov and Demoulin
force-free equilibrium (Titov and Demoulin, Astr. and Astrophys. 351,
707, (1999), hereafter TD) models a semi-circular, 3D current-carrying
flux rope by means of a current ring embedded in a potential field. The
parameters of the TD model can be adjusted to create both stable and
kink- and torus-unstable configurations. <P />Its solar relevance
was confirmed by the quantitative reproduction of some specific CME
features (see e.g., Toeroek and Kliem, Astroph. J. Lett. 630 L97
(2005)). <P />Therefore, the TD solution is by far the most realistic
analytical equilibrium available to date for the modeling of solar
active regions. <P />Employing the TD equilibrium as a test-field,
we show that the magnetofrictional extrapolation code can reproduce
the energy and the twist of the magnetic field within a percent
accuracy. <P />This information is essential for the reconstruction
of coronal fields involved in eruptions because the twist is,
together with the height profile of the overlying potential field,
the most important stability parameter -- at least as long as the
TD equilibrium is a good model of the considered active region. <P
/>Perfectly reproduced are also X-type magnetic topology features,
sometimes referred to as Hyperbolic Flux Tubes, which are regarded to be
essential to the physics of CMEs and flares because they are preferred
locations for the formation of current sheets. <P />On the other hand,
we also show how the scale-height of the potential field that is used
as initial condition in the extrapolation influences the quality of the
reconstructed field: different initial conditions reproduce correctly
the twist and the topology, but less accurately the height and the
shape of the flux rope. <P />Consequently, care must be taken when
comparing the shapes of soft X-ray and EUV loops, especially those in
the nearly potential field overlying filaments, with the field lines
obtained from the extrapolation of the corresponding magnetogram.
---------------------------------------------------------
Title: Modelling CMEs close to the Sun
Authors: Török, T.
2008ESPM...12.3.32T Altcode:
It is now widely accepted that large-scale solar eruptive phenomena like
flares, eruptive prominences or filaments, and coronal mass ejections
(CMEs) are magnetically driven. They are different observational
manifestations of a more general process, namely a large-scale
disruption of the coronal magnetic field ("solar eruption" in the
following). It is also widely accepted that the energy necessary to
drive solar eruptions is stored in the low corona, in form of sheared
and twisted magnetic fields which are held in equilibrium prior to
eruption by the ambient coronal field. An eruption occurs if this
equilibrium is driven or perturbed such that it becomes unstable. In
spite of this general understanding, the detailed processes which
initiate and drive solar eruptions are not yet well understood. Several
mechanisms have been proposed in the last decades. In recent years, the
availability of 3D MHD simulations has helped to test the models and has
greatly increased our understanding of these processes. In this talk,
I will review current theoretical models and corresponding numerical
simulations of solar eruptions. I will outline their differences and
similarities and briefly discuss how current and future observations
can help us to constrain the models. The simulation results indicate
a flux rope instability or loss of equilibrium to be the canonical
driving mechanism of solar eruptions in their fast acceleration phase
close to the Sun, and they point towards a relatively large variety
of possible mechanisms that initiate that phase. As an example for
such an initiation mechanism, I will present new simulations which
show how the eruption of a pre-existing 3D coronal flux rope can be
triggered by magnetic flux emergence.
---------------------------------------------------------
Title: Simulations of the CME-Flare Relationship
Authors: Kliem, B.; Török, T.
2008ESPM...12.3.67K Altcode:
Observations of coronal mass ejections (CMEs) and solar flares have
revealed a high correlation between the acceleration of the ejecta
and the plasma heating and particle acceleration signified by the
soft and hard X-ray emissions of the associated flare. The latter are
generally thought to result from magnetic reconnection. This finding has
stimulated the discussion of the CME-flare relationship, but at the same
time it has made it difficult to find a conclusive answer as to whether
magnetic reconnection or an ideal MHD instability is the prime cause of
the eruptions. <P />Numerical simulations of unstable flux ropes will
be presented that are in very satisfactory quantitative agreement with
erupting filaments, both, confined to the corona and ejective (i.e.,
developing into a CME). Some of these simulations indeed show a high
degree of synchronization between the initial exponential acceleration
of the flux rope, due to the ideal MHD instability, and the development
of reconnection flows. However, others show a very delayed onset of
reconnection, even after the flux rope's acceleration peak. In addition,
the reconnection flows generally lag behind the motions driven by the
ideal instability as the flux rope rise velocity nears the saturation
phase. Both findings indicate that the ideal MHD process is the primary
driver of the coupled CME-flare phenomenon. <P />The strong differences
in the degree of synchronization, which the simulated systems show
in the main rise phase of the eruption, are related to the magnetic
topology prior to the eruption. Given the observational result of
a high correlation between CME and flare development (Zhang &
Dere 2006), these simulations yield constraints on the topology and
lead us to conclude that a seed for a reconnecting current sheet must
typically be present already at the onset of the eruption.
---------------------------------------------------------
Title: Magnetic field changes preceding filament eruptions and
coronal mass ejections
Authors: Schmieder, B.; Török, T.; Aulanier, G.
2008AIPC.1043..260S Altcode:
Solar filaments (or prominences) can be represented by twisted flux
ropes in a bipolar magnetic environment. In such models, the dipped
field lines of the flux rope carry the filament material and parasitic
polarities in the filament channel are responsible for the existence
of the lateral feet of filaments. Most filaments eventually erupt, in
many cases as part of a coronal mass ejection (CME). Such eruptions are
often preceded by detectable changes in the photospheric magnetic field
in the vicinity of the filament. We first review recent observations of
such changes due to large-scale flows or variations of the background
magnetic field, and we discuss their role in eruptions. We then focus
on emerging flux in the vicinity of filament channels. It has been
suggested that magnetic reconnection between the emerging flux and
the pre-existing coronal field can trigger filament eruptions and
CMEs. For a particular event, observed with Hinode/XRT, we observe
signatures of such reconnection, but no eruption of the filament. We
present a numerical simulation of this event and we briefly argue why
no eruption took place in this case.
---------------------------------------------------------
Title: Twist, Writhe and Rotation of Magnetic Flux Ropes in Filament
Eruptions and Coronal Mass Ejections
Authors: Török, T.; Berger, M. A.; Kliem, B.; Démoulin, P.; Linton,
M.; van Driel-Gesztelyi, L.
2008ESPM...12.3.54T Altcode:
We present the first quantitative analysis of the conversion of twist
into writhe in the course of ideal MHD instabilities in erupting coronal
magnetic flux ropes. For our analysis, we consider numerical simulations
of two instabilities which have been suggested as trigger and initial
driving mechanisms in filament eruptions and coronal mass ejections,
namely the helical kink instability and the torus instability. We
use two different coronal flux rope models as initial conditions
in the simulations, namely the cylindrical Gold-Hoyle equilibrium
and the toroidal Titov-Demoulin equilibrium. <P />For each model, we
perform a series of simulations with different amounts of initial flux
rope twist. In order to study both confined and ejective eruptions,
we additionally use different initial potential fields overlying
the flux rope in the simulations of the Titov-Demoulin model. <P
/>In all simulations, we measure the writhe of the flux rope and the
corresponding rotation of its axis in vertical projection by making use
of recently developed expressions which permit us to calculate writhe as
a single integral in space. We discuss the implications of our results
for filament eruptions, coronal mass ejections and magnetic clouds.
---------------------------------------------------------
Title: Observations and Modeling of the Early Acceleration Phase of
Erupting Filaments Involved in Coronal Mass Ejections
Authors: Schrijver, Carolus J.; Elmore, Christopher; Kliem, Bernhard;
Török, Tibor; Title, Alan M.
2008ApJ...674..586S Altcode: 2007arXiv0710.1609S
We examine the early phases of two near-limb filament destabilizations
involved in coronal mass ejections (CMEs) on 2005 June 16 and July
27, using high-resolution, high-cadence observations made with the
Transition Region and Coronal Explorer (TRACE), complemented by
coronagraphic observations by the Mauna Loa Solar Observatory (MLSO)
and the Solar and Heliospheric Observatory (SOHO). The filaments'
heights above the solar limb in their rapid-acceleration phases are
best characterized by a height dependence h(t) propto t<SUP>m</SUP>
with m near, or slightly above, 3 for both events. Such profiles are
incompatible with published results for breakout, MHD-instability,
and catastrophe models. We show numerical simulations of the
torus instability that approximate this height evolution in case a
substantial initial velocity perturbation is applied to the developing
instability. We argue that the sensitivity of magnetic instabilities
to initial and boundary conditions requires higher fidelity modeling of
all proposed mechanisms if observations of rise profiles are to be used
to differentiate between them. The observations show no significant
delays between the motions of the filament and of overlying loops:
the filaments seem to move as part of the overall coronal field until
several minutes after the onset of the rapid-acceleration phase.
---------------------------------------------------------
Title: Interaction between emerging flux and coronal hole -
observations and simulations
Authors: van Driel-Gesztelyi, Lidia; Baker, Deborah; Murray, Michelle;
Demoulin, Pascal; Attrill, Gemma; Matthews, Sarah A.; Mandrini,
Cristina H.; Toeroek, Tibor
2008cosp...37.3288V Altcode: 2008cosp.meet.3288V
Flux emergence in the vicinity of or inside a coronal hole (CH) is
expected to induce magnetic reconnection between the closed emerging
and open CH magnetic field lines, resulting in an evolution of the
CH as formerly closed field lines become topologically open, while at
the same time, open field lines close down. Through two case studies
we show observational signatures of this (interchange) reconnection
process and discuss its implications. First, using SOHO EIT and MDI
data, we study a small active region (AR10869) emerging in the close
vicinity of a low-latitude coronal hole in April 2006. The interfacing
magnetic polarities between the AR and the CH were opposite, favourable
for magnetic reconnection. We indeed observe the coupled formation of
bright closed loops between the CH and the AR and coronal dimming on
the far side of the AR, which we interpret as evidence of interchange
reconnection. This process effectively modifies the CH boundary
(making it retreat), while simultaneously displacing open field lines
to the far side of the AR. In order to study this process in detail,
we perform 2.5D MHD simulations, which qualitatively reproduce important
aspects of the observations. We expect to find upflows of plasma at the
location where previously closed field lines are opening up as well as
on the reconnecting side, but since we had no spectroscopic data for
this event, we can not verify this. Therefore we analyze Hinode/EIS
line-of-sight velocity maps of another low-latitude CH with a small AR
in its midst observed on 18 Oct. 2007. We find that while closed loops
of the bipole are dominated by downflows in the Fe XII, Fe XIII and
Fe XV lines, the strongest coronal plasma upflows are indeed located
around and particularly at the "far side" of the bipolar AR, i.e. having
the same polarity as the dominant polarity of the CH. The emerging
biplole and the series of interchange reconnections it induces create
a significant additional plasma upflow in the CH, thus we identify
this outflow must contribute to the acceleration of the fast solar wind.
---------------------------------------------------------
Title: Numerical modelling of solar eruptions
Authors: Toeroek, Tibor
2008cosp...37.3194T Altcode: 2008cosp.meet.3194T
It is now widely accepted that large-scale solar eruptive phenomena like
flares, eruptive prominences or filaments, and coronal mass ejections
(CMEs) are magnetically driven. They are different observational
manifestations of a more general process, namely a large-scale
disruption of the coronal magnetic field ("solar eruption" in the
following). It is also widely accepted that the energy necessary to
drive solar eruptions is stored in the low corona, in form of sheared
and twisted magnetic fields which are held in equilibrium prior to
eruption by the ambient coronal field. An eruption occurs if this
equilibrium is driven or perturbed such that it becomes unstable. In
spite of this general understanding, the detailed processes which
initiate and drive solar eruptions are not yet well understood. Several
mechanisms have been proposed in the last decades. In recent years, the
availability of 3D MHD simulations has helped to test the models and has
greatly increased our understanding of these processes. In this talk,
I will review the main theoretical models and corresponding numerical
simulations of solar eruptions. I will outline their differences and
similarities and briefly discuss how current and future observations
can help us to constrain the models. The simulation results indicate a
flux rope instability or loss of equilibrium to be the canonical driving
mechanism of solar eruptions in their fast acceleration phase, and they
point towards a relatively large variety of possible mechanisms that
initiate that phase. These initiation mechanisms are strongly related
to the coupling between the photosphere and the corona. As an example,
I will present new simulations which show for the first time how the
eruption of a pre-existing 3D coronal flux rope can be triggered by
magnetic flux emergence.
---------------------------------------------------------
Title: What kinking filament eruptions tell us about the physical
nature of transient coronal sigmoids ?
Authors: van Driel-Gesztelyi, Lidia; Green, Lucie M.; Kliem, Bernhard;
Toeroek, Tibor; Attrill, Gemma
2008cosp...37.3289V Altcode: 2008cosp.meet.3289V
Soft X-ray images of the Sun have shown that some active regions contain
loops, or collections of loops, which appear forward or reverse 'S'
in shape. These features have been termed sigmoids. These structures
are of interest because their presence in an active region has been
linked to eruptive activity and the sense of sigmoid orientation is
taken to indicate the sense of shear and twist (or helicity) in the
magnetic field. Differing models have been put forward in order to
explain the physical nature of sigmoids and the role they play in an
eruption. We use multiwavelength observations (Yohkoh/SXT, TRACE,
SOHO/EIT and MDI, H-alpha) to investigate how transient sigmoids
are formed. We also investigate filament eruptions from these active
regions, which show a clear sign of rotation of their apex. We find
that for positive (negative) helicity the filament apex rotates
clockwise (counterclockwise), consistent with the flux rope taking on
a reverse (forward) S shape, which is opposite to that observed for
the sigmoid. These observations put constraints on sigmoid models,
excluding some of them. We conclude that transient sigmoids are
associated with the formation of current sheets and heating along
field lines under a dynamic flux rope.
---------------------------------------------------------
Title: A New Model for Propagating Parts of EIT Waves: A Current
Shell in a CME
Authors: Delannée, C.; Török, T.; Aulanier, G.; Hochedez, J. -F.
2008SoPh..247..123D Altcode:
EIT waves are observed in EUV as bright fronts. Some of these bright
fronts propagate across the solar disk. EIT waves are all associated
with a flare and a CME and are commonly interpreted as fast-mode
magnetosonic waves. Propagating EIT waves could also be the direct
signature of the gradual opening of magnetic field lines during a
CME. We quantitatively addressed this alternative interpretation. Using
two independent 3D MHD codes, we performed nondimensional numerical
simulations of a slowly rotating magnetic bipole, which progressively
result in the formation of a twisted magnetic flux tube and its fast
expansion, as during a CME. We analyse the origins, the development,
and the observability in EUV of the narrow electric currents sheets that
appear in the simulations. Both codes give similar results, which we
confront with two well-known SOHO/EIT observations of propagating EIT
waves (7 April and 12 May 1997), by scaling the vertical magnetic field
components of the simulated bipole to the line of sight magnetic field
observed by SOHO/MDI and the sign of helicity to the orientation of the
soft X-ray sigmoids observed by Yohkoh/SXT. A large-scale and narrow
current shell appears around the twisted flux tube in the dynamic phase
of its expansion. This current shell is formed by the return currents
of the system, which separate the twisted flux tube from the surrounding
fields. It intensifies as the flux tube accelerates and it is co-spatial
with weak plasma compression. The current density integrated over the
altitude has the shape of an ellipse, which expands and rotates when
viewed from above, reproducing the generic properties of propagating
EIT waves. The timing, orientation, and location of bright and faint
patches observed in the two EIT waves are remarkably well reproduced. We
conjecture that propagating EIT waves are the observational signature of
Joule heating in electric current shells, which separate expanding flux
tubes from their surrounding fields during CMEs or plasma compression
inside this current shell. We also conjecture that the bright edges
of halo CMEs show the plasma compression in these current shells.
---------------------------------------------------------
Title: Transient Coronal Sigmoids and Rotating Erupting Flux Ropes
Authors: Green, L. M.; Kliem, B.; Török, T.; van Driel-Gesztelyi,
L.; Attrill, G. D. R.
2007SoPh..246..365G Altcode:
To determine the relationship between transient coronal (soft X-ray
or EUV) sigmoids and erupting flux ropes, we analyse four events
in which a transient sigmoid could be associated with a filament
whose apex rotates upon eruption and two further events in which
the two phenomena were spatially but not temporally coincident. We
find the helicity sign of the erupting field and the direction of
filament rotation to be consistent with the conversion of twist
into writhe under the ideal MHD constraint of helicity conservation,
thus supporting our assumption of flux rope topology for the rising
filament. For positive (negative) helicity the filament apex rotates
clockwise (counterclockwise), consistent with the flux rope taking on
a reverse (forward) S shape, which is opposite to that observed for
the sigmoid. This result is incompatible with two models for sigmoid
formation: one identifying sigmoids with upward arching kink-unstable
flux ropes and one identifying sigmoids with a current layer between
two oppositely sheared arcades. We find instead that the observations
agree well with the model by Titov and Démoulin (Astron. Astrophys.351,
707, 1999), which identifies transient sigmoids with steepened current
layers below rising flux ropes.
---------------------------------------------------------
Title: Numerical simulations of fast and slow coronal mass ejections
Authors: Török, T.; Kliem, B.
2007AN....328..743T Altcode: 2007arXiv0705.2100T
Solar coronal mass ejections (CMEs) show a large variety in their
kinematic properties. CMEs originating in active regions and accompanied
by strong flares are usually faster and accelerated more impulsively
than CMEs associated with filament eruptions outside active regions
and weak flares. It has been proposed more than two decades ago that
there are two separate types of CMEs, fast (impulsive) CMEs and slow
(gradual) CMEs. However, this concept may not be valid, since the large
data sets acquired in recent years do not show two distinct peaks
in the CME velocity distribution and reveal that both fast and slow
CMEs can be accompanied by both weak and strong flares. We present
numerical simulations which confirm our earlier analytical result
that a flux-rope CME model permits describing fast and slow CMEs in
a unified manner. We consider a force-free coronal magnetic flux rope
embedded in the potential field of model bipolar and quadrupolar active
regions. The eruption is driven by the torus instability which occurs
if the field overlying the flux rope decreases sufficiently rapidly
with height. The acceleration profile depends on the steepness of
this field decrease, corresponding to fast CMEs for rapid decrease,
as is typical of active regions, and to slow CMEs for gentle decrease,
as is typical of the quiet Sun. Complex (quadrupolar) active regions
lead to the fastest CMEs.
---------------------------------------------------------
Title: The Evolving Sigmoid: Evidence for Magnetic Flux Ropes in
the Corona Before, During, and after CMES
Authors: Gibson, S. E.; Fan, Y.; Török, T.; Kliem, B.
2007sdeh.book..131G Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The Evolving Sigmoid: Evidence for Magnetic Flux Ropes in
the Corona Before, During, and After CMES
Authors: Gibson, S. E.; Fan, Y.; Török, T.; Kliem, B.
2006SSRv..124..131G Altcode: 2007SSRv..tmp...52G
It is generally accepted that the energy that drives coronal mass
ejections (CMEs) is magnetic in origin. Sheared and twisted coronal
fields can store free magnetic energy which ultimately is released
in the CME. We explore the possibility of the specific magnetic
configuration of a magnetic flux rope of field lines that twist
about an axial field line. The flux rope model predicts coronal
observables, including heating along forward or inverse S-shaped,
or sigmoid, topological surfaces. Therefore, studying the observed
evolution of such sigmoids prior to, during, and after the CME gives
us crucial insight into the physics of coronal storage and release of
magnetic energy. In particular, we consider (1) soft-X-ray sigmoids,
both transient and persistent; (2) The formation of a current sheet
and cusp-shaped post-flare loops below the CME; (3) Reappearance of
sigmoids after CMEs; (4) Partially erupting filaments; (5) Magnetic
cloud observations of filament material.
---------------------------------------------------------
Title: Flux Ropes and CMEs: The Kink and Torus Instabilities,
Catastrophe, and Magnetic Reconnection
Authors: Kliem, Bernhard; Toeroek, T.
2006SPD....37.0820K Altcode: 2006BAAS...38..234K
Prior to eruption, the coronal magnetic field evolves along an
equilibrium sequence due to slow photospheric changes. Configurations
containing a flux rope can erupt when the rope passes the threshold
of an instability in the sequence or when the rope is driven beyond
an end point of the sequence in parameter space (catastrophe). The
kink instability of a flux rope yields quantitative agreement with
characteristic properties of many CMEs during their onset and early
evolution: development of helical shape and exponential-to-linear rise
profiles. The large-scale evolution of kinking flux ropes is governed
by the torus (expansion) instability (TI). This instability yields a
unified description of fast and slow CMEs, the preferred occurrence of
very fast CMEs in quadrupolar active regions, and an indication why
the minor flux rope radius expands overproportionally in the course
of the eruption, creating or deepening the cavity seen in three-part
CMEs. If an eruption is triggered by a flux rope catastrophe, we expect
its evolution to possess characteristics similar to the TI-driven
case. The magnetic reconnection that commences in the wake of a rising
unstable flux rope is an integral part of the eruption and proceeds
in a highly dynamic and complex manner, forming many intermittent X-
and O-type structures along a vertical current sheet.
---------------------------------------------------------
Title: Torus Instability
Authors: Kliem, B.; Török, T.
2006PhRvL..96y5002K Altcode: 2006physics...5217K
The expansion instability of a toroidal current ring in low-beta
magnetized plasma is investigated. Qualitative agreement is obtained
with experiments on spheromak expansion and with essential properties
of solar coronal mass ejections, unifying the two apparently disparate
classes of fast and slow coronal mass ejections.
---------------------------------------------------------
Title: Confined and Ejective Eruptions of Kink-unstable Flux Ropes
Authors: Török, T.; Kliem, B.
2005ApJ...630L..97T Altcode: 2005astro.ph..7662T
The ideal helical kink instability of a force-free coronal magnetic
flux rope, anchored in the photosphere, is studied as a model for
solar eruptions. Using the flux rope model of Titov and Démoulin
as the initial condition in MHD simulations, both the development of
helical shape and the rise profile of a confined (or failed) filament
eruption (on 2002 May 27) are reproduced in very good agreement
with the observations. By modifying the model such that the magnetic
field decreases more rapidly with height above the flux rope, a full
(or ejective) eruption of the rope is obtained in very good agreement
with the developing helical shape and the exponential-to-linear rise
profile of a fast coronal mass ejection (CME) on 2001 May 15. This
confirms that the helical kink instability of a twisted magnetic flux
rope can be the mechanism of the initiation and the initial driver
of solar eruptions. The agreement of the simulations with properties
that are characteristic of many eruptions suggests that they are often
triggered by the kink instability. The decrease of the overlying field
with height is a main factor in deciding whether the instability leads
to a confined event or to a CME.
---------------------------------------------------------
Title: Eruption of a Kink-unstable Filament in NOAA Active Region
10696
Authors: Williams, David R.; Török, Tibor; Démoulin, Pascal;
van Driel-Gesztelyi, Lidia; Kliem, Bernhard
2005ApJ...628L.163W Altcode: 2005astro.ph..7661W
We present rapid-cadence Transition Region and Coronal Explorer (TRACE)
observations that show evidence of a filament eruption from NOAA active
region 10696, accompanied by an X2.5 flare, on 2004 November 10. The
eruptive filament, which manifests as a fast coronal mass ejection
some minutes later, rises as a kinking structure with an apparently
exponential growth of height within TRACE's field of view. We compare
the characteristics of this filament eruption with MHD numerical
simulations of a kink-unstable magnetic flux rope, finding excellent
qualitative agreement. We suggest that while tether weakening by
breakout-like quadrupolar reconnection may be the release mechanism
for the previously confined flux rope, the driver of the expansion is
most likely the MHD helical kink instability.
---------------------------------------------------------
Title: The Kink Instability in Solar Eruptions
Authors: Török, T.; Kliem, B.
2004ESASP.575...56T Altcode: 2004soho...15...56T
No abstract at ADS
---------------------------------------------------------
Title: The kink instability of a coronal magnetic loop as a trigger
mechanism for solar eruptions
Authors: Török, T.; Kliem, B.
2004PADEU..14..165T Altcode:
The kink instability of twisted magnetic flux tubes in the solar corona
is regarded as a possible initiation process of solar eruptions. We
study the stability properties and the dynamic evolution of such
coronal magnetic loops using 3D numerical simulations within the
framework of ideal MHD. The analytical force-free coronal loop
model by Titov and Demoulin (1999) is used as initial condition in
the simulations. The loop model is found to be kink-unstable if a
critical twist is exceeded. The growing kink perturbation leads to the
formation of current sheets, which steepen exponentially and define
the locations of plasma heating. Due to the twist in the magnetic
field, the heated structures are S shaped - in very good agreement
with soft X-ray observations of solar eruptions. The model, however,
does not yet show a successful eruption, rather the kink instability
starts to saturate. We present an improvement of the model which is
promising with regard to eruption: a modification of the equilibrium
so that the magnetic field surrounding the loop decreases more rapidly
with height above the photosphere. Furthermore, we briefly discuss
how the simulation results can be related to observations of solar
eruptive phenomena.
---------------------------------------------------------
Title: Ideal kink instability of a magnetic loop equilibrium
Authors: Török, T.; Kliem, B.; Titov, V. S.
2004A&A...413L..27T Altcode: 2003astro.ph.11198T
The force-free coronal loop model by \cite{Tit:Dem-99} is found
to be unstable with respect to the ideal kink mode, which suggests
this instability as a mechanism for the initiation of flares. The
long-wavelength (m = 1) mode grows for average twists Φ⪆3.5π (at a
loop aspect ratio of ≈5). The threshold of instability increases with
increasing major loop radius, primarily because the aspect ratio then
also increases. Numerically obtained equilibria at subcritical twist
are very close to the approximate analytical equilibrium; they do not
show indications of sigmoidal shape. The growth of kink perturbations
is eventually slowed down by the surrounding potential field, which
varies only slowly with radius in the model. With this field a global
eruption is not obtained in the ideal MHD limit. Kink perturbations with
a rising loop apex lead to the formation of a vertical current sheet
below the apex, which does not occur in the cylindrical approximation.
---------------------------------------------------------
Title: Formation of current sheets and sigmoidal structure by the
kink instability of a magnetic loop
Authors: Kliem, B.; Titov, V. S.; Török, T.
2004A&A...413L..23K Altcode: 2003astro.ph.11199K
We study dynamical consequences of the kink instability of a
twisted coronal flux rope, using the force-free coronal loop
model by \cite{Tit:Dem-99} as the initial condition in ideal-MHD
simulations. When a critical value of the twist is exceeded, the
long-wavelength (m = 1) kink mode develops. Analogous to the well-known
cylindrical approximation, a helical current sheet is then formed at the
interface with the surrounding medium. In contrast to the cylindrical
case, upward-kinking loops form a second, vertical current sheet below
the loop apex at the position of the hyperbolic flux tube (generalized X
line) in the model. The current density is steepened in both sheets and
eventually exceeds the current density in the loop (although the kink
perturbation starts to saturate in our simulations without leading to a
global eruption). The projection of the field lines that pass through
the vertical current sheet shows an S shape whose sense agrees with
the typical sense of transient sigmoidal (forward or reverse S-shaped)
structures that brighten in soft X rays prior to coronal eruptions. The
upward-kinked loop has the opposite S shape, leading to the conclusion
that such sigmoids do not generally show the erupting loops themselves
but indicate the formation of the vertical current sheet below them
that is the central element of the standard flare model.
---------------------------------------------------------
Title: The kink instability of a coronal magnetic loop as a trigger
mechanism for solar eruptions
Authors: Török, T.; Kliem, B.; Titov, V. S.
2004cosp...35.3327T Altcode: 2004cosp.meet.3327T
MHD instabilities of twisted magnetic flux tubes in the solar corona
are regarded as a possible initiation process of solar eruptions. We
study the stability properties and the dynamic evolution of coronal
magnetic loops using 3D numerical simulations within the framework of
ideal MHD. The analytical force-free coronal loop model by Titov and
Démoulin (1999) is used as initial condition in the simulations. The
loop model is found to be kink-unstable if a critical twist is
exceeded. The growing kink perturbation leads to the formation of
current sheets, which steepen exponentially and define the locations
of plasma heating. Due to the twist in the magnetic field, the heated
structures are S shaped -- in very good agreement with sigmoidal soft
X-ray structures that brighten in solar eruptions. The model, however,
does not yet show a successful eruption, rather the kink instability
starts to saturate. We present an improvement of the model which is
promising with regard to eruption: a modification of the equilibrium so
that the magnetic field surrounding the loop decreases more rapidly with
height above the photosphere. Furthermore, we discuss how the simulation
results can be related to observations of solar eruptive phenomena.
---------------------------------------------------------
Title: Instabilität magnetischer Flußröhren in solaren Eruptionen
---------------------------------------------------------
Title: Instabilität magnetischer Flußröhren in solaren Eruptionen
Authors: Török, Tibor
2004PhDT.......150T Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The evolution of twisting coronal magnetic flux tubes
Authors: Török, T.; Kliem, B.
2003A&A...406.1043T Altcode:
We simulate the twisting of an initially potential coronal flux
tube by photospheric vortex motions, centred at two photospheric
flux concentrations, using the compressible zero-beta ideal MHD
equations. A twisted flux tube is formed, surrounded by much less
twisted and sheared outer flux. Under the action of continuous slow
driving, the flux tube starts to evolve quasi-statically along a
sequence of force-free equilibria, which rise slowly with increasing
twist and possess helical shape. The flux bundle that extends from
the location of peak photospheric current density (slightly displaced
from the vortex centre) shows a sigmoidal shape in agreement with
observations of sigmoidal soft X-ray loops. There exists a critical
twist, above which no equilibrium can be found in the simulation
and the flux tube ascends rapidly. Then either stable equilibrium
ceases to exist or the character of the sequence changes such that
neighbouring stable equilibria rise by enormous amounts for only modest
additions of twist. A comparison with the scalings of the rise of flux
in axisymmetric geometry by \cite{Stur:al-95} suggests the former. Both
cases would be observed as an eruption. The critical end-to-end twist,
for a particular set of parameters describing the initial potential
field, is found to lie in the range 2.5pi <Phi<SUB>c</SUB><2.75pi
. There are some indications for the growth of helical perturbations
at supercritical twist. Depending on the radial profiles of the
photospheric flux concentration and vortex velocity, the outer part
or all of the twisted flux expands from the central field line of the
flux tube. This effect is particularly efficient in the dynamic phase,
provided the density is modeled realistically, falling off sufficiently
rapidly with height. It is expected to lead to the formation of a
cavity in which the twisted flux tube is embedded, analogous to the
typical structure of coronal mass ejections.
---------------------------------------------------------
Title: Formation of Current Sheets and Sigmoidal Structure by the
Ideal Kink Instability of a Magnetic Loop
Authors: Kliem, Bernhard; Török, Tibor; Titov, Viatcheslav S.
2003ANS...324...73K Altcode: 2003ANS...324..I16K
No abstract at ADS
---------------------------------------------------------
Title: The evolution of coronal magnetic flux tubes twisted by
photospheric vortex motions
Authors: Török, T.; Kliem, B.
2002ESASP.506..781T Altcode: 2002svco.conf..781T; 2002ESPM...10..781T
We simulate the twisting of initially potential coronal magnetic
flux by slow photospheric vortex motions using the compressional,
zero-beta ideal MHD equations. The twisted flux tube starts to
evolve quasi-statically along a sequence of force-free equilibria,
rising slowly and possessing helical shape. As a critical amount of
twist is reached, the evolution becomes dynamic and the tube rises
and expands rapidly. No neighbouring equilibrium can be found in the
simulation domain at this stage of the evolution, as confirmed by
relaxation runs. Hence, above the critical twist either the flux tube
becomes unstable or neighbouring equilibria rise by enormous amounts
for only small additional twist. Both cases would be observed as an
eruption. The critical end-to-end twist of the flux tube is found to
lie in the range 2.5π < Φ<SUB>c</SUB> < 3.0π.
---------------------------------------------------------
Title: The evolution of coronal magnetic flux tubes subject to
footpoint twisting motions
Authors: Kliem, B.; Török, T.
2002ocnd.confE..25K Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The evolution of coronal magnetic flux tubes under the
influence of footpoint twisting motions
Authors: Török, T.; Kliem, B.
2001sps..proc..364T Altcode:
We present first results of an MHD simulation study of the quasi-static
evolution of coronal magnetic flux tubes under the influence of
vortex motions at their footpoints. An initial vacuum field of
two sub-photospheric dipoles is subjected to slow vortex motions
at the photospheric level, centred at the projected positions of
the dipoles. The flux tube connecting the vortices becomes strongly
twisted with time. This leads to an ascending and also laterally
expanding motion of the upper parts of the tube, which takes an S-type,
or sigmoidal, shape. Most of the field lines emanating at the sides
of the vortices lean sidewards (to let the central flux tube ascend
freely). If the amount of twist increases, the flux tube tends to take
uniform width, i.e., field strength, along its upper parts. Decreasing
the separation of the flux tube footpoints leads to the formation of
a central current sheet between them.
---------------------------------------------------------
Title: Technical dictionary of spectroscopy and spectral
analysis. English-French-German-Russian
Authors: Moritz, Heinrich; Toeroek, Tibor
1971tdss.book.....M Altcode:
No abstract at ADS