explanation blue bibcodes open ADS page with paths to full text
Author name code: kliem
ADS astronomy entries on 2022-09-14
author:"Kliem, Bernhard"
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Title: Torus instability in magnetic flux ropes
Authors: Kliem, Bernhard
2022cosp...44.2410K Altcode:
The torus instability is a lateral kink of an arched (toroidal)
current channel carrying a net current, which is in force-free
equilibrium provided by an external poloidal ("strapping") field. The
instability leads to expansion of the current channel and associated
magnetic flux rope in major radius (rise) as well as in minor radius
(cavity formation). Thus, it is a prime candidate for a standard model
of the coronal mass ejection (CME) component of solar eruptions. In
this presentation, I will introduce the instability, discussing the
differences between the MHD case with ambient magnetized plasma and
the vacuum case relevant in the tokamak device. Magnetic reconnection
is a key ingredient of the instability in the former case. Next, I
will review recent investigations of the parametric dependencies of
the threshold, the decay index of the strapping field. These aim to
substantiate the support for the occurrence of the instability from
solar observations, which typically indicate a threshold in the range
between 1 and 2. Finally, the height profile of the decay index holds
one of the keys for understanding fast vs. slow CMEs and, eventually,
the confinement of eruptions.
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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: Helicity shedding by flux rope ejection
Authors: Kliem, B.; Seehafer, N.
2022A&A...659A..49K Altcode: 2021arXiv211205833K
Context. We quantitatively address the conjecture that magnetic helicity
must be shed from the Sun by eruptions launching coronal mass ejections
in order to limit its accumulation in each hemisphere. <BR /> Aims:
By varying the ratio of guide and strapping field and the flux rope
twist in a parametric simulation study of flux rope ejection from
approximately marginally stable force-free equilibria, different
ratios of self- and mutual helicity are set and the onset of the
torus or helical kink instability is obtained. <BR /> Methods: The
helicity shed is found to vary over a broad range from a minor to a
major part of the initial helicity, with self helicity being largely
or completely shed and mutual helicity, which makes up the larger
part of the initial helicity, being shed only partly. Torus-unstable
configurations with subcritical twist and without a guide field
shed up to about two-thirds of the initial helicity, while a highly
twisted, kink-unstable configuration sheds only about one-quarter. The
parametric study also yields stable force-free flux rope equilibria
up to a total flux-normalized helicity of 0.25, with a ratio of self-
to total helicity of 0.32 and a ratio of flux rope to external poloidal
flux of 0.94. <BR /> Results: These results numerically demonstrate the
conjecture of helicity shedding by coronal mass ejections and provide
a first account of its parametric dependence. Both self- and mutual
helicity are shed significantly; this reduces the total initial helicity
by a fraction of ∼0.4 − 0.65 for typical source region parameters.
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Title: Fast plasmoid-mediated reconnection in a solar flare
Authors: Yan, Xiaoli; Xue, Zhike; Jiang, Chaowei; Priest, E. R.;
Kliem, Bernhard; Yang, Liheng; Wang, Jincheng; Kong, Defang; Song,
Yongliang; Feng, Xueshang; Liu, Zhong
2022NatCo..13..640Y Altcode:
Magnetic reconnection is a multi-faceted process of energy conversion
in astrophysical, space and laboratory plasmas that operates at
microscopic scales but has macroscopic drivers and consequences. Solar
flares present a key laboratory for its study, leaving imprints of the
microscopic physics in radiation spectra and allowing the macroscopic
evolution to be imaged, yet a full observational characterization
remains elusive. Here we combine high resolution imaging and spectral
observations of a confined solar flare at multiple wavelengths with
data-constrained magnetohydrodynamic modeling to study the dynamics
of the flare plasma from the current sheet to the plasmoid scale. The
analysis suggests that the flare resulted from the interaction of a
twisted magnetic flux rope surrounding a filament with nearby magnetic
loops whose feet are anchored in chromospheric fibrils. Bright
cusp-shaped structures represent the region around a reconnecting
separator or quasi-separator (hyperbolic flux tube). The fast
reconnection, which is relevant for other astrophysical environments,
revealed plasmoids in the current sheet and separatrices and associated
unresolved turbulent motions.
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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: Partial Eruption, Confinement, and Twist Buildup and Release
of a Double-decker Filament
Authors: Chen, Jialin; Su, Yingna; Liu, Rui; Kliem, Bernhard; Zhang,
Qingmin; Ji, Haisheng; Liu, Tie
2021ApJ...923..142C Altcode: 2021arXiv211113174C
We investigate the failed partial eruption of a filament system in NOAA
AR 12104 on 2014 July 5, using multiwavelength EUV, magnetogram, and Hα
observations, as well as magnetic field modeling. The filament system
consists of two almost co-spatial segments with different end points,
both resembling a C shape. Following an ejection and a precursor flare
related to flux cancellation, only the upper segment rises and then
displays a prominent twisted structure, while rolling over toward its
footpoints. The lower segment remains undisturbed, indicating that the
system possesses a double-decker structure. The erupted segment ends up
with a reverse-C shape, with material draining toward its footpoints,
while losing its twist. Using the flux rope insertion method, we
construct a model of the source region that qualitatively reproduces
key elements of the observed evolution. At the eruption onset, the model
consists of a flux rope atop a flux bundle with negligible twist, which
is consistent with the observational interpretation that the filament
possesses a double-decker structure. The flux rope reaches the critical
height of the torus instability during its initial relaxation, while the
lower flux bundle remains in stable equilibrium. The eruption terminates
when the flux rope reaches a dome-shaped quasi-separatrix layer that
is reminiscent of a magnetic fan surface, although no magnetic null is
found. The flux rope is destroyed by reconnection with the confining
overlying flux above the dome, transferring its twist in the process.
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Title: Pre-eruption Splitting of the Double-decker Structure in a
Solar Filament
Authors: Pan, Hanya; Liu, Rui; Gou, Tingyu; Kliem, Bernhard; Su,
Yingna; Chen, Jun; Wang, Yuming
2021ApJ...909...32P Altcode: 2021arXiv210103296P
Solar filaments often erupt partially. Although how they split remains
elusive, the splitting process has the potential of revealing the
filament structure and eruption mechanism. Here we investigate the
pre-eruption splitting of an apparently single filament and its
subsequent partial eruption on 2012 September 27. The evolution is
characterized by three stages with distinct dynamics. During the
quasi-static stage, the splitting proceeds gradually for about 1.5
hr, with the upper branch rising at a few kilometers per second and
displaying swirling motions about its axis. During the precursor
stage that lasts for about 10 minutes, the upper branch rises at
tens of kilometers per second, with a pair of conjugated dimming
regions starting to develop at its footpoints; with the swirling
motions turning chaotic, the axis of the upper branch whips southward,
which drives an arc-shaped extreme-ultraviolet front propagating in a
similar direction. During the eruption stage, the upper branch erupts
with the onset of a C3.7-class two-ribbon flare, while the lower
branch remains stable. Judging from the well-separated footpoints
of the upper branch from those of the lower one, we suggest that the
pre-eruption filament processes a double-decker structure composed of
two distinct flux bundles, whose formation is associated with gradual
magnetic flux cancellations and converging photospheric flows around
the polarity inversion line.
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Title: Nonequilibrium Flux Rope Formation by Confined Flares Preceding
a Solar Coronal Mass Ejection
Authors: Kliem, Bernhard; Lee, Jeongwoo; Liu, Rui; White, Stephen M.;
Liu, Chang; Masuda, Satoshi
2021ApJ...909...91K Altcode: 2021arXiv210102181K
We present evidence that a magnetic flux rope was formed before
a coronal mass ejection (CME) and its associated long-duration
flare during a pair of preceding confined eruptions and
associated impulsive flares in a compound event in NOAA Active
Region 12371. Extreme-ultraviolet images and the extrapolated
nonlinear force-free field show that the first two (impulsive)
flares, SOL2015-06-21T01:42, result from the confined eruption
of highly sheared low-lying flux, presumably a seed flux rope. The
eruption spawns a vertical current sheet, where magnetic reconnection
creates flare ribbons and loops, a nonthermal microwave source, and a
sigmoidal hot channel that can only be interpreted as a magnetic flux
rope. Until the subsequent long-duration flare, SOL2015-06-21T02:36,
the sigmoid's elbows expand, while its center remains stationary,
suggesting nonequilibrium but not yet instability. The "flare
reconnection" during the confined eruptions acts like "tether-cutting
reconnection" whose flux feeding of the rope leads to instability. The
subsequent full eruption is seen as an accelerated rise of the entire
hot channel, seamlessly evolving into the fast halo CME. Both the
confined and ejective eruptions are consistent with the onset of
the torus instability in the dipped decay index profile that results
from the region's two-scale magnetic structure. We suggest that the
formation or enhancement of a nonequilibrium but stable flux rope by
confined eruptions is a generic process occurring prior to many CMEs.
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Title: Laboratory Study of the Torus Instability Threshold in
Solar-relevant, Line-tied Magnetic Flux Ropes
Authors: Alt, Andrew; Myers, Clayton E.; Ji, Hantao; Jara-Almonte,
Jonathan; Yoo, Jongsoo; Bose, Sayak; Goodman, Aaron; Yamada, Masaaki;
Kliem, Bernhard; Savcheva, Antonia
2021ApJ...908...41A Altcode: 2020arXiv201010607A
Coronal mass ejections (CMEs) occur when long-lived magnetic flux
ropes (MFRs) anchored to the solar surface destabilize and erupt
away from the Sun. This destabilization is often described in
terms of an ideal magnetohydrodynamic instability called the torus
instability. It occurs when the external magnetic field decreases
sufficiently fast such that its decay index, ${n}_{}=-z\,\partial
(\mathrm{ln}{B}_{})/\partial z$ , is larger than a critical value, $n\gt
{n}_{\mathrm{cr}}^{}$ , where ${n}_{\mathrm{cr}}^{}=1.5$ for a full,
large aspect ratio torus. However, when this is applied to solar MFRs,
a range of conflicting values for ${n}_{\mathrm{cr}}^{}$ is found in
the literature. To investigate this discrepancy, we have conducted
laboratory experiments on arched, line-tied flux ropes and applied
a theoretical model of the torus instability. Our model describes
an MFR as a partial torus with foot points anchored in a conducting
surface and numerically calculates various magnetic forces on it. This
calculation yields better predictions of ${n}_{\mathrm{cr}}^{}$ that
take into account the specific parameters of the MFR. We describe a
systematic methodology to properly translate laboratory results to their
solar counterparts, provided that the MFRs have a sufficiently small
edge safety factor or, equivalently, a large enough twist. After this
translation, our model predicts that ${n}_{\mathrm{cr}}^{}$ in solar
conditions falls near ${n}_{\mathrm{cr}}^{\mathrm{solar}}\sim 0.9$
and within a larger range of ${n}_{\mathrm{cr}}^{\mathrm{solar}}\sim
(0.7,1.2)$ , depending on the parameters. The methodology of
translating laboratory MFRs to their solar counterparts enables
quantitative investigations of CME initiation through laboratory
experiments. These experiments allow for new physics insights that
are required for better predictions of space weather events but are
difficult to obtain otherwise.
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Title: A Statistical Study of Flare Ribbons in Confined Precursor
Eruptions
Authors: Kliem, Bernhard
2021cosp...43E1774K Altcode:
We present a statistical study of flare ribbons in confined eruptions
which precede a major ejective eruption (CME), using the complete sample
of CMEs associated with > M5.0 flares in 2011--2015 and source
distance from Sun center of < 50 deg (32 events, from Baumgartner
et al. 2018). Ribbons of precursor events within 12 hr from the onset
of the CME-associated (eruptive) flare are compared with the ribbons
of the main event to assess a potential contribution of the precursor
events to the buildup of a flux rope prior to CME onset. It is found
that 26 CMEs (81%) have one or several precursors with bright ribbons
that cover a part of the ribbons in the main event, hence, clearly
contribute to the buildup of the flux rope erupting in the CME. Two
further events develop such ribbons during an enhancement that is
part of the slow-rise phase of the GOES light curve, i.e., prior to
the onset of the main exponential rise of the soft X-ray flux. Two
CMEs (6%) only have one or several precursors with weak, very short,
or very transient ribbons that cover a part of the ribbons in the
main event. These precursors provide at least a minor contribution
to the buildup of the flux rope erupting in the CME. Precursor events
were absent, or did not produce ribbons overlapping those of the main
event, for the remaining two CMEs. Of the four events lacking separate
precursor events with relevant ribbons, three are characterized by
a large ribbon separation from their onset, suggesting a high-lying
flux rope, and one is triggered by the emergence of parasitic flux. In
conclusion, a significant role of flare reconnection for the buildup
of a flux rope in precursor events or during the slow-rise phase is
indicated for 88% of the considered major ejective eruptions. This
implies that the formation of a flux rope progresses significantly
already prior to the onset of the main acceleration phase in these
events. Since all studied precursor events produce shorter ribbons
than the main event, the forming flux rope is likely not yet fully
coherent along the whole length of the eventually erupting flux.
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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.
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Title: Decoding the Pre-Eruptive Magnetic Field Configurations of
Coronal Mass Ejections
Authors: Patsourakos, S.; Vourlidas, A.; Török, T.; Kliem, B.;
Antiochos, S. K.; Archontis, V.; Aulanier, G.; Cheng, X.; Chintzoglou,
G.; Georgoulis, M. K.; Green, L. M.; Leake, J. E.; Moore, R.; Nindos,
A.; Syntelis, P.; Yardley, S. L.; Yurchyshyn, V.; Zhang, J.
2020SSRv..216..131P Altcode: 2020arXiv201010186P
A clear understanding of the nature of the pre-eruptive magnetic
field configurations of Coronal Mass Ejections (CMEs) is required
for understanding and eventually predicting solar eruptions. Only
two, but seemingly disparate, magnetic configurations are considered
viable; namely, sheared magnetic arcades (SMA) and magnetic flux ropes
(MFR). They can form via three physical mechanisms (flux emergence,
flux cancellation, helicity condensation). Whether the CME culprit
is an SMA or an MFR, however, has been strongly debated for thirty
years. We formed an International Space Science Institute (ISSI) team to
address and resolve this issue and report the outcome here. We review
the status of the field across modeling and observations, identify
the open and closed issues, compile lists of SMA and MFR observables
to be tested against observations and outline research activities
to close the gaps in our current understanding. We propose that the
combination of multi-viewpoint multi-thermal coronal observations
and multi-height vector magnetic field measurements is the optimal
approach for resolving the issue conclusively. We demonstrate the
approach using MHD simulations and synthetic coronal images.
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Title: Coronal Mini-jets in an Activated Solar Tornado-like Prominence
Authors: Chen, Huadong; Zhang, Jun; De Pontieu, Bart; Ma, Suli; Kliem,
Bernhard; Priest, Eric
2020ApJ...899...19C Altcode: 2020arXiv200608252C
High-resolution observations from the Interface Region Imaging
Spectrometer reveal the existence of a particular type of small
solar jet, which arose singly or in clusters from a tornado-like
prominence suspended in the corona. In this study, we perform a detailed
statistical analysis of 43 selected mini-jets in the tornado event. Our
results show that the mini-jets typically have (1) a projected length
of 1.0-6.0 Mm, (2) a width of 0.2-1.0 Mm, (3) a lifetime of 10-50 s,
(4) a velocity of 100-350 km s<SUP>-1</SUP>, and (5) an acceleration
of 3-20 km s<SUP>-2</SUP>. Based on spectral diagnostics and EM-Loci
analysis, these jets seem to be multithermal small-scale plasma
ejections with an estimated average electron density of ∼2.4 ×
10<SUP>10</SUP> cm<SUP>-3</SUP> and an approximate mean temperature of
∼2.6 × 10<SUP>5</SUP> K. Their mean kinetic energy density, thermal
energy density, and dissipated magnetic field strength are roughly
estimated to be ∼9 erg cm<SUP>-3</SUP>, 3 erg cm<SUP>-3</SUP>, and
16 G, respectively. The accelerations of the mini-jets, the UV and EUV
brightenings at the footpoints of some mini-jets, and the activation
of the host prominence suggest that the tornado mini-jets are probably
created by fine-scale external or internal magnetic reconnections (a)
between the prominence field and the enveloping or background field or
(b) between twisted or braided flux tubes within the prominence. The
observations provide insight into the geometry of such reconnection
events in the corona and have implications for the structure of the
prominence magnetic field and the instability that is responsible for
the eruption of prominences and coronal mass ejections.
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Title: sagemath/sage: 9.1
Authors: Stein, William; Chapoton, Frédéric; Demeyer, Jeroen;
Köppe, Matthias; Krenn, Daniel; Rüth, Julian; Nathanncohen; Braun,
Volker; Delecroix, Vincent; Dcoudert; Palmieri, John H.; Pasechnik,
Dima; Bray, E. M.; Cheuberg; Stephan, Ralf; Bradshaw, Robert; Thiéry,
Nicolas M.; Grinberg, Darij; Jm58660; Hansen, Mike; Gourgoulhon, Eric;
King, Simon; Bruin, Peter; Hackl, Benjamin; Roed314; Kliem; Cremona,
John; Rubey, Martin; Mezzarobba, Marc; Yi
2020zndo...4066866S Altcode:
Mirror of the Sage source tree -- please do not submit PRs here --
everything must be submitted via https://trac.sagemath.org/
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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.
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Title: Extreme-ultraviolet Late Phase of Solar Flares
Authors: Chen, Jun; Liu, Rui; Liu, Kai; Awasthi, Arun Kumar; Zhang,
Peijin; Wang, Yuming; Kliem, Bernhard
2020ApJ...890..158C Altcode: 2020arXiv200106929C
A second peak in the extreme ultraviolet sometimes appears during the
gradual phase of solar flares, which is known as the EUV late phase
(ELP). Stereotypically ELP is associated with two separated sets of
flaring loops with distinct sizes, and it has been debated whether
ELP is caused by additional heating or extended plasma cooling in
the longer loop system. Here we carry out a survey of 55 M-and-above
GOES-class flares with ELP during 2010-2014. Based on the flare-ribbon
morphology, these flares are categorized as circular-ribbon (19 events),
two-ribbon (23 events), and complex-ribbon (13 events) flares. Among
them, 22 events (40%) are associated with coronal mass ejections,
while the rest are confined. An extreme ELP, with the late-phase peak
exceeding the main-phase peak, is found in 48% of two-ribbon flares, 37%
of circular-ribbon flares, and 31% of complex-ribbon flares, suggesting
that additional heating is more likely present during ELP in two-ribbon
than in circular-ribbon flares. Overall, cooling may be the dominant
factor causing the delay of the ELP peak relative to the main-phase
peak, because the loop system responsible for the ELP emission is
generally larger than, and well separated from, that responsible for
the main-phase emission. All but one of the circular-ribbon flares can
be well explained by a composite "dome-plate" quasi-separatrix layer
(QSL). Only half of these show a magnetic null point, with its fan and
spine embedded in the dome and plate, respectively. The dome-plate
QSL, therefore, is a general and robust structure characterizing
circular-ribbon flares.
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Title: Genesis and impulsive evolution of the fast CME associated
with the X8.2 flare on 2017 September 10
Authors: Veronig, A.; Podladchikova, T.; Dissauer, K.; Temmer, M.;
Seaton, D. B.; Long, D.; Guo, J.; Vrsnak, B.; Harra, L. K.; Kliem, B.
2019AGUFMSH13A..02V Altcode:
The X8.2 event of 2017 September 10 provides unique observations to
study the genesis, magnetic morphology, impulsive dynamics and shock
formation in a very fast coronal mass ejection (CME). As will be
discussed in this presentation, fundamental insight in the processes
of magnetic reconnection, CME acceleration and shock formation are
provided through EUV observations of the middle corona. <P />Combining
the large field-of-view and high-cadence imagery from GOES-16/SUVI
and SDO/AIA EUV, respectively, we identify a hot (T ≈ 10-15 MK)
bright rim around a quickly expanding cavity, embedded inside a much
larger CME shell (T ≈ 1-2 MK). The CME shell develops from a dense
set of large AR loops (>0.5Rs) and seamlessly evolves into the
CME front observed in LASCO C2. The strong lateral overexpansion
of the CME shell acts as a piston initiating the fast and globally
propagating EUV shock wave. The hot cavity rim is demonstrated to be
a manifestation of the dominantly poloidal flux and frozen-in plasma
added to the rising flux rope by magnetic reconnection in the current
sheet beneath. The same structure is later observed as the core of the
white-light CME, challenging the traditional interpretation of the CME
three-part morphology (Veronig et al. 2018). <P />The large amount of
added magnetic flux suggested by these observations can explain the
extreme accelerations of the radial and lateral expansion of the CME
shell and cavity, all reaching values up to 5-10 km s<SUP>-2</SUP>. The
acceleration peaks occur simultaneously with the first RHESSI 100-300
keV hard X-ray burst of the associated flare, further underlining the
importance of the reconnection process for the impulsive CME evolution
in the low and middle corona.
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Title: Sheared Magnetic Arcades and the Pre-eruptive Magnetic
Configuration of Coronal Mass Ejections: Diagnostics, Challenges
and Future Observables
Authors: Patsourakos, Spiros; Vourlidas, A.; Anthiochos, S. K.;
Archontis, V.; Aulanier, G.; Cheng, X.; Chintzoglou, G.; Georgoulis,
M. K.; Green, L. M.; Kliem, B.; Leake, J.; Moore, R. L.; Nindos, A.;
Syntelis, P.; Torok, T.; Yardley, S. L.; Yurchyshyn, V.; Zhang, J.
2019shin.confE.194P Altcode:
Our thinking about the pre-eruptive magnetic configuration of Coronal
Mass Ejections has been effectively dichotomized into two opposing
and often fiercely contested views: namely, sheared magnetic arcades
and magnetic flux ropes. Finding a solution to this issue will have
important implications for our understanding of CME initiation. We
first discuss the very value of embarking into the arcade vs. flux rope
dilemma and illustrate the corresponding challenges and difficulties to
address it. Next, we are compiling several observational diagnostics of
pre-eruptive sheared magnetic arcades stemming from theory/modeling,
discuss their merits, and highlight potential ambiguities that could
arise in their interpretation. We finally conclude with a discussion
of possible new observables, in the frame of upcoming or proposed
instrumentation, that could help to circumvent the issues we are
currently facing.
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Title: Genesis, magnetic morphology and impulsive evolution of
the coronal mass ejection associated with the X8.2 flare on 2017
September 10
Authors: Veronig, Astrid; Podladchikova, Tatiana; Dissauer, Karin;
Temmer, Manuela; Seaton, Daniel; Long, David; Guo, Jingnan; Vrsnak,
Bojan; Harra, Louise; Kliem, Bernhard
2019EGUGA..21.9243V Altcode:
The extreme X8.2 event of 2017 September 10 provides unique observations
to study the genesis, magnetic morphology, impulsive dynamics and
shock formation in a very fast coronal mass ejection (CME). Combining
GOES-16/SUVI and SDO/AIA EUV imagery, we identify a hot (T ≈ 10-15
MK) bright rim around a quickly expanding cavity, embedded inside a
much larger CME shell (T ≈ 1-2 MK). The CME shell develops from a
dense set of large AR loops (>0.5Rs) and seamlessly evolves into
the CME front observed in LASCO C2. The strong lateral overexpansion
of the CME shell acts as a piston initiating the fast EUV shock
wave. The hot cavity rim is demonstrated to be a manifestation of
the dominantly poloidal flux and frozen-in plasma added to the rising
flux rope by magnetic reconnection in the current sheet beneath. The
same structure is later observed as the core of the white-light CME,
challenging the traditional interpretation of the CME three-part
morphology. The large amount of added magnetic flux suggested by these
observations explains the extreme accelerations of the radial and
lateral expansion of the CME shell and cavity, all reaching values
up to 5-10 km s-2. The acceleration peaks occur simultaneously with
the first RHESSI 100-300 keV hard X-ray burst of the associated flare,
further underlining the importance of the reconnection process for the
impulsive CME evolution. Finally, the much higher radial propagation
speed of the flux rope in relation to the CME shell causes a distinct
deformation of the white-light CME front and shock.
---------------------------------------------------------
Title: The Birth of a Coronal Mass Ejection
Authors: Liu, Rui; Gou, Tingyu; Kliem, Bernhard; Wang, Yuming;
Veronig, Astrid
2019EGUGA..21.5194L Altcode:
The Sun's atmosphere is frequently disrupted by coronal mass ejections
(CMEs), coupled with flares and energetic particles. In the standard
picture, the coupling is explained by magnetic reconnection at
a vertical current sheet connecting the flare loops and the CME,
with the latter embedding a helical magnetic structure known as flux
rope. As it jumps upward due to instabilities or loss of equilibrium,
the flux rope stretches the overlying coronal loops so that oppositely
directed field is brought together underneath, creating the current
sheet. However, both the origin of flux ropes and their nascent paths
toward eruption remain elusive. Here we present an observation of how
a stellar-sized CME bubble evolves continuously from plasmoids, mini
flux ropes that are barely resolved, within half an hour. The eruption
initiates when plasmoids springing from a vertical current sheet
merge into a leading plasmoid occupying the upper tip of the current
sheet. Rising at increasing speed to stretch the overlying loops,
this leading plasmoid then expands impulsively into the CME bubble,
in tandem with hard X-ray bursts. This observation illuminates for
the first time a complete CME evolutionary path that has the capacity
to accommodate a wide variety of plasma phenomena by bridging the gap
between micro-scale dynamics and macro-scale activities.
---------------------------------------------------------
Title: The Birth of A Coronal Mass Ejection
Authors: Gou, Tingyu; Liu, Rui; Kliem, Bernhard; Wang, Yuming; Veronig,
Astrid M.
2019SciA....5.7004G Altcode: 2018arXiv181104707G
The Sun's atmosphere is frequently disrupted by coronal mass ejections
(CMEs), coupled with flares and energetic particles. The coupling
is usually attributed to magnetic reconnection at a vertical current
sheet connecting the flare and CME, with the latter embedding a helical
magnetic structure known as flux rope. However, both the origin of flux
ropes and their nascent paths toward eruption remain elusive. Here,
we present an observation of how a stellar-sized CME bubble evolves
continuously from plasmoids, mini flux ropes that are barely resolved,
within half an hour. The eruption initiates when plasmoids springing
from a vertical current sheet merge into a leading plasmoid, which
rises at increasing speeds and expands impulsively into the CME
bubble, producing hard x-ray bursts simultaneously. This observation
illuminates a complete CME evolutionary path capable of accommodating a
wide variety of plasma phenomena by bridging the gap between microscale
and macroscale dynamics.
---------------------------------------------------------
Title: Genesis and Impulsive Evolution of the 2017 September 10
Coronal Mass Ejection
Authors: Veronig, Astrid M.; Podladchikova, Tatiana; Dissauer, Karin;
Temmer, Manuela; Seaton, Daniel B.; Long, David; Guo, Jingnan; Vršnak,
Bojan; Harra, Louise; Kliem, Bernhard
2018ApJ...868..107V Altcode: 2018arXiv181009320V
The X8.2 event of 2017 September 10 provides unique observations
to study the genesis, magnetic morphology, and impulsive dynamics
of a very fast coronal mass ejection (CME). Combining GOES-16/SUVI
and SDO/AIA EUV imagery, we identify a hot (T ≈ 10-15 MK) bright
rim around a quickly expanding cavity, embedded inside a much larger
CME shell (T ≈ 1-2 MK). The CME shell develops from a dense set of
large AR loops (≳0.5R <SUB> s </SUB>) and seamlessly evolves into
the CME front observed in LASCO C2. The strong lateral overexpansion
of the CME shell acts as a piston initiating the fast EUV wave. The
hot cavity rim is demonstrated to be a manifestation of the dominantly
poloidal flux and frozen-in plasma added to the rising flux rope by
magnetic reconnection in the current sheet beneath. The same structure
is later observed as the core of the white-light CME, challenging the
traditional interpretation of the CME three-part morphology. The large
amount of added magnetic flux suggested by these observations explains
the extreme accelerations of the radial and lateral expansion of the CME
shell and cavity, all reaching values of 5-10 km s<SUP>-2</SUP>. The
acceleration peaks occur simultaneously with the first RHESSI 100-300
keV hard X-ray burst of the associated flare, further underlining
the importance of the reconnection process for the impulsive CME
evolution. Finally, the much higher radial propagation speed of the
flux rope in relation to the CME shell causes a distinct deformation
of the white-light CME front and shock.
---------------------------------------------------------
Title: Evolution of flux rope, CME and associated EUV wave in the
10-Sep-2018 X8.2 event
Authors: Podladchikova, Tatiana; Veronig, Astrid M.; Dissauer, Karin;
Temmer, Manuela; Seaton, Daniel B.; Long, David; Guo, Jingnan; Vršnak,
Bojan; Harra, Louise; Kliem, Bernhard
2018csc..confE..38P Altcode:
We combine the high-cadence and large field-of-view EUV imagery of
the Atmospheric Imaging Assembly (AIA) onboard SDO and the Solar
Ultraviolet Imager (SUVI) onboard GOES-16 to study the origin and
impulsive evolution of the fast CME that originated in the September
10th 2017 X8.2 event as well as the initiation of the associated EUV
wave. In the LASCO field-of-view, the CME reveals speeds >3000
km/s. In the low-to-mid corona, it shows a distinct bubble in the EUV
imagery that reveals a significant lateral overexpansion. In addition,
is also shows a distinct expanding cavity that is interpreted as
manifestation of the flux rope driving the eruption. We present a method
to automatically identify and segment the CME bubble in SUVI images and
to derive its radial and lateral evolution up to about 2 solar radii,
in terms of velocity and acceleration. These measurements are set into
context with the evolution of the embedded flux rope/cavity observed by
AIA. The observations show clear signatures of new poloidal flux added
to the flux rope by magnetic reconnection in the current sheet beneath
the eruptive structure, which is important for the high accelerations
observed in this event. The radial propagation of the CME shell revealed
a peak value of the acceleration of about 5.3 km/s2, whereas the lateral
expansion reached a peak value of 10.1 km/s2, which is the largest value
reported so far. The flux rope/cavity reveals a radial acceleration of
6.7 km/s2 and lateral acceleration of 5.3 km/s2. We note that at this
early evolution phase, the speed of the cavity/flux rope is higher
than that of the CME bubble (front). The EUV wave associated with
this eruption was observed by AIA, SUVI and STEREO-A EUVI, which had
a separation angle with Earth of 128°, and the common field of view
of the spacecraft was 52°. AIA and SUVI images above the solar limb
reveal the initiation of the EUV wave by the accelerating flanks of
the CME bubble, followed by detachment and propagation of the wave
with a speed of 1100 km/s. The EUV wave shows a global propagation
over the full hemisphere visible to Earth view as well as into the
STEREO-A field-of-view. We study the propagation and kinematics of
the direct as well as the various reflected and refracted EUV wave
components on the solar sphere, finding speeds in the range from 370
to 1010 km/s. Finally, we note that this EUV wave is also distinct as
it reveals propagation and transmission through the polar coronal holes.
---------------------------------------------------------
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: Unambiguous Evidence of Filament Splitting-Induced Partial
Eruptions
Authors: Cheng, Xin; Kliem, Bernhard
2018cosp...42E.619C Altcode:
Coronal mass ejections are usually believed to be produced by the
full eruption of a magnetic flux rope (MFR). However, it is recently
recognised that the MFR may only release part of flux during once
eruption, leading to a partial eruption. Here, we investigate
two partial eruption events, the first one appears as a failed
filament eruption, while the second manifests as a successful hot
channel eruption. Thanks to the high resolution, high cadence, and
multi-temperature capability of the Atmospheric Imaging Assembly, it is
found that both partial eruptions are a consequence of the splitting of
an intact MFR-filament system caused by the internal reconnection. For
the failed one, the internal reconnection, most likely induced by
the rising and writhing motions of the filament, initially appears as
EUV brightenings in the middle of the filament. Quickly, it evolves
to the current sheet reconnection in the wake of the eruption of the
upper filament. For the successful one, the internal reconnection
initiates prior to the eruption, which is also evidenced by obvious
heating in the middle of the filament. As the internal reconnection
proceeds to the current sheet reconnection, the filament is split into
two; the upper flux escapes as an erupting hot channel. As a result,
regardless of being failed or successful, both of them produce hard
X-ray sources and cusp-shaped loops below the erupting flux but above
the surviving flux, as well as two ribbons at two sides of the latter.
---------------------------------------------------------
Title: Magnetic Structure of a Composite Solar Microwave Burst
Authors: Lee, Jeongwoo; White, Stephen M.; Liu, Chang; Kliem, Bernhard;
Masuda, Satoshi
2018ApJ...856...70L Altcode:
A composite flare consisting of an impulsive flare SOL2015-06-21T01:42
(GOES class M2.0) and a more gradual, long-duration flare
SOL2015-06-21T02:36 (M2.6) from NOAA Active Region 12371, is studied
using observations with the Nobeyama Radioheliograph (NoRH) and the
Solar Dynamics Observatory (SDO). While composite flares are defined
by their characteristic time profiles, in this paper we present
imaging observations that demonstrate the spatial relationship of
the two flares and allow us to address the nature of the evolution
of a composite event. The NoRH maps show that the first flare is
confined not only in time, but also in space, as evidenced by the
stagnation of ribbon separation and the stationarity of the microwave
source. The NoRH also detected another microwave source during the
second flare, emerging from a different location where thermal plasma
is so depleted that accelerated electrons could survive longer against
Coulomb collisional loss. The AIA 131 Å images show that a sigmoidal
EUV hot channel developed after the first flare and erupted before the
second flare. We suggest that this eruption removed the high-lying flux
to let the separatrix dome underneath reconnect with neighboring flux
and the second microwave burst follow. This scenario explains how the
first microwave burst is related to the much-delayed second microwave
burst in this composite event.
---------------------------------------------------------
Title: Unambiguous Evidence of Filament Splitting-induced Partial
Eruptions
Authors: Cheng, X.; Kliem, B.; Ding, M. D.
2018ApJ...856...48C Altcode: 2018arXiv180204932C
Coronal mass ejections are often considered to result from the full
eruption of a magnetic flux rope (MFR). However, it is recognized
that, in some events, the MFR may release only part of its flux,
with the details of the implied splitting not completely established
due to limitations in observations. Here, we investigate two partial
eruption events including a confined and a successful one. Both
partial eruptions are a consequence of the vertical splitting of
a filament-hosting MFR involving internal reconnection. A loss of
equilibrium in the rising part of the magnetic flux is suggested
by the impulsive onset of both events and by the delayed onset of
reconnection in the confined event. The remaining part of the flux
might be line-tied to the photosphere in a bald patch (BP) separatrix
surface, and we confirm the existence of extended BP sections for
the successful eruption. The internal reconnection is signified by
brightenings in the body of one filament and between the rising and
remaining parts of both filaments. It evolves quickly into the standard
current sheet reconnection in the wake of the eruption. As a result,
regardless of being confined or successful, both eruptions produce
hard X-ray sources and flare loops below the erupting but above the
surviving flux, as well as a pair of flare ribbons enclosing the latter.
---------------------------------------------------------
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: Helical Kink Instability in a Confined Solar Eruption
Authors: Hassanin, Alshaimaa; Kliem, Bernhard
2016ApJ...832..106H Altcode: 2016arXiv160900673H
A model for strongly writhing confined solar eruptions suggests
an origin in the helical kink instability of a coronal flux rope
that remains stable against the torus instability. This model is
tested against the well observed filament eruption on 2002 May 27
in a parametric MHD simulation study that comprises all phases of
the event. Good agreement with the essential observed properties is
obtained. These include the confinement, terminal height, writhing,
distortion, and dissolution of the filament, and the flare loops. The
agreement is robust against variations in a representative range of
parameter space. Careful comparisons with the observation data constrain
the ratio of the external toroidal and poloidal field components
to {B}<SUB>{et</SUB>}/{B}<SUB>{ep</SUB>}≈ 1 and the initial flux
rope twist to {{Φ }}≈ 4π . Different from ejective eruptions,
two distinct phases of strong magnetic reconnection can occur. First,
the erupting flux is cut by reconnection with overlying flux in the
helical current sheet formed by the instability. If the resulting flux
bundles are linked as a consequence of the erupting rope’s strong
writhing, they subsequently reconnect in the vertical current sheet
between them. This reforms the overlying flux and a far less twisted
flux rope, offering a pathway to homologous eruptions.
---------------------------------------------------------
Title: Magnetic Structure of Solar Eruptions: Flux Rope Formation,
Instability, Confinement, and Ejection
Authors: Kliem, B.
2016AGUFMSH12B..01K Altcode:
Recent observations of sigmoids and filaments and the associated
modeling of a nonlinear force-free coronal field increasingly suggest
the formation of a magnetic flux rope prior to the onset of an eruption,
albeit the degree of coherence, amount of twist, and exact timing
of the rope are not yet well known. Torus instability of a flux
rope, equivalent to the well known catastrophe and flux imbalance
descriptions, appears to be the mechanism of destabilization in the
majority of eruptions. Although the three-halves onset condition for
the field's decay index is a good quantitative guideline, its parametric
dependence requires further study. The helical kink explains the onset
of some strongly writhing eruptions, and, in particular, provides a
pathway to confined eruptions, which often writhe strongly. However,
many confined eruptions do not indicate the occurrence of a helical
kink, and magnetic environments that allow us to explain them solely
in terms of the torus instability are not yet well established. This
overview will also stress that some of the parameters determining the
geoeffectiveness of eruptions - their speed and magnetic orientation -
may be largely set already in the low corona. Finally, the expansion
of unstable flux ropes can be impulsive, explaining the triggering of
EUV waves without the recently suggested formation of a blast wave,
which we do not expect in low-beta plasma.
---------------------------------------------------------
Title: Helical kink instability in the confined solar eruption on
2002 May 27
Authors: Hassanin, A.; Kliem, B.; Seehafer, N.
2016AN....337.1082H Altcode: 2016arXiv161101008H
This paper presents an improved MHD modeling of the confined filament
eruption in solar active region NOAA 9957 on 2002 May 27 by extending
the parametric studies of the event in \citet{Torok&Kliem2005} and
\citet{Hassanin&Kliem2016}. Here the initial flux rope equilibrium
is chosen to possess a small apex height identical to the observed
initial filament height, which implies a more realistic inclusion
of the photospheric line tying. The model matches the observations
as closely as in the preceding studies, with the closest agreement
again being obtained for an initial average flux rope twist of about
4π. Thus, the model for strongly writhing confined solar eruptions,
which assumes that a kink-unstable flux rope in the stability domain
of the torus instability exists at the onset of the eruption's main
acceleration phase, is further substantiated.
---------------------------------------------------------
Title: Observing the release of twist by magnetic reconnection in
a solar filament eruption
Authors: Xue, Zhike; Yan, Xiaoli; Cheng, Xin; Yang, Liheng; Su,
Yingna; Kliem, Bernhard; Zhang, Jun; Liu, Zhong; Bi, Yi; Xiang,
Yongyuan; Yang, Kai; Zhao, Li
2016NatCo...711837X Altcode:
Magnetic reconnection is a fundamental process of topology change
and energy release, taking place in plasmas on the Sun, in space, in
astrophysical objects and in the laboratory. However, observational
evidence has been relatively rare and typically only partial. Here we
present evidence of fast reconnection in a solar filament eruption using
high-resolution H-alpha images from the New Vacuum Solar Telescope,
supplemented by extreme ultraviolet observations. The reconnection is
seen to occur between a set of ambient chromospheric fibrils and the
filament itself. This allows for the relaxation of magnetic tension
in the filament by an untwisting motion, demonstrating a flux rope
structure. The topology change and untwisting are also found through
nonlinear force-free field modelling of the active region in combination
with magnetohydrodynamic simulation. These results demonstrate a new
role for reconnection in solar eruptions: the release of magnetic twist.
---------------------------------------------------------
Title: Structure, Stability, and Evolution of Magnetic Flux Ropes
from the Perspective of Magnetic Twist
Authors: Liu, Rui; Kliem, Bernhard; Titov, Viacheslav S.; Chen, Jun;
Wang, Yuming; Wang, Haimin; Liu, Chang; Xu, Yan; Wiegelmann, Thomas
2016ApJ...818..148L Altcode: 2015arXiv151202338L
We investigate the evolution of NOAA Active Region (AR) 11817 during
2013 August 10-12, when it developed a complex field configuration
and produced four confined, followed by two eruptive, flares. These
C-and-above flares are all associated with a magnetic flux rope (MFR)
located along the major polarity inversion line, where shearing and
converging photospheric flows are present. Aided by the nonlinear
force-free field modeling, we identify the MFR through mapping magnetic
connectivities and computing the twist number {{ T }}<SUB>w</SUB>
for each individual field line. The MFR is moderately twisted (|
{{ T }}<SUB>w</SUB>| \lt 2) and has a well-defined boundary of high
squashing factor Q. We found that the field line with the extremum |
{{ T }}<SUB>w</SUB>| is a reliable proxy of the rope axis, and that
the MFR's peak | {{ T }}<SUB>w</SUB>| temporarily increases within
half an hour before each flare while it decreases after the flare peak
for both confined and eruptive flares. This pre-flare increase in | {{
T }}<SUB>w</SUB>| has little effect on the AR's free magnetic energy or
any other parameters derived for the whole region, due to its moderate
amount and the MFR's relatively small volume, while its decrease after
flares is clearly associated with the stepwise decrease in the whole
region's free magnetic energy due to the flare. We suggest that {{ T
}}<SUB>w</SUB> may serve as a useful parameter in forewarning the onset
of eruption, and therefore, the consequent space weather effects. The
helical kink instability is identified as the prime candidate onset
mechanism for the considered flares.
---------------------------------------------------------
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
---------------------------------------------------------
Title: Triggering an Eruptive Flare by Emerging Flux in a Solar
Active-Region Complex
Authors: Louis, Rohan E.; Kliem, Bernhard; Ravindra, B.; Chintzoglou,
Georgios
2015SoPh..290.3641L Altcode: 2015arXiv150608035L; 2015SoPh..tmp...81L
A flare and fast coronal mass ejection originated between solar active
regions NOAA 11514 and 11515 on 2012 July 1 (SOL2012-07-01) in response
to flux emergence in front of the leading sunspot of the trailing
region 11515. Analyzing the evolution of the photospheric magnetic flux
and the coronal structure, we find that the flux emergence triggered
the eruption by interaction with overlying flux in a non-standard
way. The new flux neither had the opposite orientation nor a location
near the polarity inversion line, which are favorable for strong
reconnection with the arcade flux under which it emerged. Moreover,
its flux content remained significantly smaller than that of the arcade
(≈40 % ). However, a loop system rooted in the trailing active region
ran in part under the arcade between the active regions, passing over
the site of flux emergence. The reconnection with the emerging flux,
leading to a series of jet emissions into the loop system, caused a
strong but confined rise of the loop system. This lifted the arcade
between the two active regions, weakening its downward tension force and
thus destabilizing the considerably sheared flux under the arcade. The
complex event was also associated with supporting precursor activity in
an enhanced network near the active regions, acting on the large-scale
overlying flux, and with two simultaneous confined flares within the
active regions.
---------------------------------------------------------
Title: Structure, Stability and Evolution of a Magnetic Flux Rope
Authors: Liu, R.; Kliem, B.; Titov, V. S.; Chen, J.; Wang, Y.; Wang,
H.; Liu, C.; Xu, Y.; Wiegelmann, T.
2015AGUFMSH13D2457L Altcode:
We investigate the evolution of NOAA Active Region 11817 during
2013 August 10--12, when it developed a complex field configuration
and produced four confined, followed by two eruptive, flares. These
C-and-above flares are all associated with a magnetic flux rope (MFR)
located along the major polarity inversion line, where shearing and
converging photospheric flows are present. With the aid of nonlinear
force-free field modeling, we identify the MFR through mapping
magnetic connectivities and computing the twist number Tw for each
individual field line. The MFR is moderately twisted (Tw < 2)
and has a well-defined boundary of high squashing factor Q. Its axis
coincides with the field line with the peak Tw in the rope. We find
that the MFR's peak Tw temporarily increases within half an hour before
each flare while it decreases after the flare peak for both confined
and eruptive flares. This pre-flare increase in Tw has little effect
on the active region's free magnetic energy or any other parameters
derived for the whole region, due to its moderate amount and the MFR's
relatively small volume, while its decrease after flares is clearly
associated with the stepwise decrease in free magnetic energy due
to the flare. We suggest that Tw may serve as a useful parameter in
prewarning the onset of eruption, and therefore, the consequent space
weather effects. The helical kink instability is identified as the
prime candidate onset mechanism for the considered flares.
---------------------------------------------------------
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.
---------------------------------------------------------
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: Fast Magnetic Reconnection in the Solar Chromosphere Mediated
by the Plasmoid Instability
Authors: Ni, Lei; Kliem, Bernhard; Lin, Jun; Wu, Ning
2015ApJ...799...79N Altcode: 2015arXiv150906895N
Magnetic reconnection in the partially ionized solar chromosphere is
studied in 2.5 dimensional magnetohydrodynamic simulations including
radiative cooling and ambipolar diffusion. A Harris current sheet
with and without a guide field is considered. Characteristic values
of the parameters in the middle chromosphere imply a high magnetic
Reynolds number of ~10<SUP>6</SUP>-10<SUP>7</SUP> in the present
simulations. Fast magnetic reconnection then develops as a consequence
of the plasmoid instability without the need to invoke anomalous
resistivity enhancements. Multiple levels of the instability are
followed as it cascades to smaller scales, which approach the ion
inertial length. The reconnection rate, normalized to the asymptotic
values of magnetic field and Alfvén velocity in the inflow region,
reaches values in the range ~0.01-0.03 throughout the cascading plasmoid
formation and for zero as well as for strong guide field. The outflow
velocity reaches ≈40 km s<SUP>-1</SUP>. Slow-mode shocks extend
from the X-points, heating the plasmoids up to ~8 × 10<SUP>4</SUP>
K. In the case of zero guide field, the inclusion of both ambipolar
diffusion and radiative cooling causes a rapid thinning of the current
sheet (down to ~30 m) and early formation of secondary islands. Both of
these processes have very little effect on the plasmoid instability for
a strong guide field. The reconnection rates, temperature enhancements,
and upward outflow velocities from the vertical current sheet correspond
well to their characteristic values in chromospheric jets.
---------------------------------------------------------
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: Formation of a Double-decker Magnetic Flux Rope in the
Sigmoidal Solar Active Region 11520
Authors: Cheng, X.; Ding, M. D.; Zhang, J.; Sun, X. D.; Guo, Y.;
Wang, Y. M.; Kliem, B.; Deng, Y. Y.
2014ApJ...789...93C Altcode: 2014arXiv1405.4923C
In this paper, we address the formation of a magnetic flux rope
(MFR) that erupted on 2012 July 12 and caused a strong geomagnetic
storm event on July 15. Through analyzing the long-term evolution
of the associated active region observed by the Atmospheric Imaging
Assembly and the Helioseismic and Magnetic Imager on board the Solar
Dynamics Observatory, it is found that the twisted field of an MFR,
indicated by a continuous S-shaped sigmoid, is built up from two groups
of sheared arcades near the main polarity inversion line a half day
before the eruption. The temperature within the twisted field and
sheared arcades is higher than that of the ambient volume, suggesting
that magnetic reconnection most likely works there. The driver behind
the reconnection is attributed to shearing and converging motions
at magnetic footpoints with velocities in the range of 0.1-0.6 km
s<SUP>-1</SUP>. The rotation of the preceding sunspot also contributes
to the MFR buildup. Extrapolated three-dimensional non-linear force-free
field structures further reveal the locations of the reconnection to
be in a bald-patch region and in a hyperbolic flux tube. About 2 hr
before the eruption, indications of a second MFR in the form of an
S-shaped hot channel are seen. It lies above the original MFR that
continuously exists and includes a filament. The whole structure
thus makes up a stable double-decker MFR system for hours prior to
the eruption. Eventually, after entering the domain of instability,
the high-lying MFR impulsively erupts to generate a fast coronal mass
ejection and X-class flare; while the low-lying MFR remains behind
and continuously maintains the sigmoidicity of the active region.
---------------------------------------------------------
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.
---------------------------------------------------------
Title: Rapid CME Cavity Formation and Expansion
Authors: Kliem, Bernhard; Forbes, Terry G.; Patsourakos, Spiros;
Vourlidas, Angelos
2014AAS...22421206K Altcode:
A cavity is supposed to be a general feature of well-developed CMEs at
the stage they can be imaged by white-light coronagraphs (in the outer
corona and solar wind). The cavity is interpreted as the cross section
of the CME flux rope in the plane of sky. Preexisting cavities are
observed around some quiescent erupting prominences, but usually not in
active regions. Observations of CME cavities in the inner corona, where
most of them appear to form, have become possible only with the STEREO
and SDO missions. These reveal a very rapid formation and expansion of
"EUV cavities" in fast and impulsively commencing eruptions early in the
phase of main CME acceleration and impulsive flare rise. Different from
the white-light observations, the EUV cavity initially appears to be
larger than the CME flux rope. However, it evolves into the white-light
cavity subsequently. MHD simulations of flux rope eruptions conform to
this picture of initially larger cavity but subsequently approaching
cavity and flux rope size. The initial expansion of ambient flux can
be understood as a "reverse pinch effect", driven by decreasing flux
rope current as the rope rises.
---------------------------------------------------------
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: 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: Sunspot splitting triggering an eruptive flare
Authors: Louis, Rohan E.; Puschmann, Klaus G.; Kliem, Bernhard;
Balthasar, Horst; Denker, Carsten
2014A&A...562A.110L Altcode: 2013arXiv1311.5054L
<BR /> Aims: We investigate how the splitting of the leading sunspot
and associated flux emergence and cancellation in active region NOAA
11515 caused an eruptive M5.6 flare on 2012 July 2. <BR /> Methods:
Continuum intensity, line-of-sight magnetogram, and dopplergram data
of the Helioseismic and Magnetic Imager were employed to analyse
the photospheric evolution. Filtergrams in Hα and He I 10830 Å of
the Chromospheric Telescope at the Observatorio del Teide, Tenerife,
track the evolution of the flare. The corresponding coronal conditions
were derived from 171 Å and 304 Å images of the Atmospheric Imaging
Assembly. Local correlation tracking was utilized to determine
shear flows. <BR /> Results: Emerging flux formed a neutral line
ahead of the leading sunspot and new satellite spots. The sunspot
splitting caused a long-lasting flow towards this neutral line, where
a filament formed. Further flux emergence, partly of mixed polarity,
as well as episodes of flux cancellation occurred repeatedly at
the neutral line. Following a nearby C-class precursor flare with
signs of interaction with the filament, the filament erupted nearly
simultaneously with the onset of the M5.6 flare and evolved into a
coronal mass ejection. The sunspot stretched without forming a light
bridge, splitting unusually fast (within about a day, complete ≈6 h
after the eruption) in two nearly equal parts. The front part separated
strongly from the active region to approach the neighbouring active
region where all its coronal magnetic connections were rooted. It
also rotated rapidly (by 4.9° h<SUP>-1</SUP>) and caused significant
shear flows at its edge. <BR /> Conclusions: The eruption resulted
from a complex sequence of processes in the (sub-)photosphere and
corona. The persistent flows towards the neutral line likely caused
the formation of a flux rope that held the filament. These flows,
their associated flux cancellation, the emerging flux, and the
precursor flare all contributed to the destabilization of the flux
rope. We interpret the sunspot splitting as the separation of two
flux bundles differently rooted in the convection zone and only
temporarily joined in the spot. This explains the rotation as the
continued rise of the separating flux, and it implies that at least
this part of the sunspot was still connected to its roots deep in
the convection zone. <P />Movie available in electronic form at <A
href="http://www.aanda.org/10.1051/0004-6361/201321106/olm">http://www.aanda.org</A>
---------------------------------------------------------
Title: Fast magnetic reconnection in solar chromosphere with radiation
cooling and ambipolar diffusion
Authors: Ni, Lei; Lin, Jun; Kliem, Bernhard
2014cosp...40E2270N Altcode:
Magnetic reconnection in solar chromosphere with radiation cooling
and ambipolar diffusion is studied through 2.5-dimensional numerical
MHD simulations employing a force-free Harris current sheet. Although}
the magnetic Reynolds number is high ( 10 (5) -10 (6) ), fast magnetic
reconnection still can happen because of multiple levels of the
plasmoid instability occurring in the current sheet. As secondary
instabilities start to appear, slow-mode shocks are detected between
secondary plasmoids and secondary fragments of the current sheet. The
temperature apparently increases around the shock front. Our numerical
simulations also indicate that the radiation cooling can strongly
accelerate magnetic reconnection for the case with weak guide field. The
radiation cooling can make the secondary plasmoids appear earlier and
the maximum current density at the main X-point reach a much higher
value than in the case without radiation cooling. For the case with
strong guide field, the radiation cooling becomes less important during
reconnection process. By analyze our numerical results, we find that
the ambipolar diffusion around the middle of the chromosphere can also
increases the recoonection rate. However, the higher orders of plasmoid
instabilities dominate eventually. The upward out-flow velocity in
our simulations approaches the velocity of chromospheric jets, with
the maximum velocity reaching 40 km s(-1) .
---------------------------------------------------------
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: Observations of flux rope formation prior to coronal mass
ejections
Authors: Green, Lucie M.; Kliem, Bernhard
2014IAUS..300..209G Altcode: 2013arXiv1312.4388G
Understanding the magnetic configuration of the source regions
of coronal mass ejections (CMEs) is vital in order to determine
the trigger and driver of these events. Observations of four CME
productive active regions are presented here, which indicate that
the pre-eruption magnetic configuration is that of a magnetic flux
rope. The flux ropes are formed in the solar atmosphere by the process
known as flux cancellation and are stable for several hours before the
eruption. The observations also indicate that the magnetic structure
that erupts is not the entire flux rope as initially formed, raising
the question of whether the flux rope is able to undergo a partial
eruption or whether it undergoes a transition in specific flux rope
configuration shortly before the CME.
---------------------------------------------------------
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: Magnetohydrodynamic Modeling of the Solar Eruption on 2010
April 8
Authors: Kliem, B.; Su, Y. N.; van Ballegooijen, A. A.; DeLuca, E. E.
2013ApJ...779..129K Altcode: 2013arXiv1304.6981K
The structure of the coronal magnetic field prior to eruptive
processes and the conditions for the onset of eruption are important
issues that can be addressed through studying the magnetohydrodynamic
(MHD) stability and evolution of nonlinear force-free field (NLFFF)
models. This paper uses data-constrained NLFFF models of a solar active
region (AR) that erupted on 2010 April 8 as initial conditions in MHD
simulations. These models, constructed with the techniques of flux rope
insertion and magnetofrictional relaxation (MFR), include a stable,
an approximately marginally stable, and an unstable configuration. The
simulations confirm previous related results of MFR runs, particularly
that stable flux rope equilibria represent key features of the
observed pre-eruption coronal structure very well, and that there is
a limiting value of the axial flux in the rope for the existence of
stable NLFFF equilibria. The specific limiting value is located within a
tighter range, due to the sharper discrimination between stability and
instability by the MHD description. The MHD treatment of the eruptive
configuration yields a very good agreement with a number of observed
features, like the strongly inclined initial rise path and the close
temporal association between the coronal mass ejection and the onset of
flare reconnection. Minor differences occur in the velocity of flare
ribbon expansion and in the further evolution of the inclination;
these can be eliminated through refined simulations. We suggest that
the slingshot effect of horizontally bent flux in the source region
of eruptions can contribute significantly to the inclination of the
rise direction. Finally, we demonstrate that the onset criterion,
formulated in terms of a threshold value for the axial flux in the
rope, corresponds very well to the threshold of the torus instability
in the considered AR.
---------------------------------------------------------
Title: Observation of a Moreton Wave and Wave-Filament Interactions
Associated with the Renowned X9 Flare on 1990 May 24
Authors: Liu, Rui; Liu, Chang; Xu, Yan; Liu, Wei; Kliem, Bernhard;
Wang, Haimin
2013ApJ...773..166L Altcode:
Using Big Bear Solar Observatory film data recently digitized at
NJIT, we investigate a Moreton wave associated with an X9 flare
on 1990 May 24, as well as its interactions with four filaments
F1-F4 located close to the flaring region. The interaction yields
interesting insight into physical properties of both the wave and
the filaments. The first clear Moreton wavefront appears at the
flaring-region periphery at approximately the same time as the peak
of a microwave burst and the first of two γ-ray peaks. The wavefront
propagates at different speeds ranging from 1500-2600 km s<SUP>-1</SUP>
in different directions, reaching as far as 600 Mm away from the flaring
site. Sequential chromospheric brightenings are observed ahead of the
Moreton wavefront. A slower diffuse front at 300-600 km s<SUP>-1</SUP>
is observed to trail the fast Moreton wavefront about one minute after
the onset. The Moreton wave decelerates to ~550 km s<SUP>-1</SUP> as it
sweeps through F1. The wave passage results in F1's oscillation which
is featured by ~1 mHz signals with coherent Fourier phases over the
filament, the activation of F3 and F4 followed by gradual recovery,
but no disturbance in F2. Different height and magnetic environment
together may account for the distinct responses of the filaments to
the wave passage. The wavefront bulges at F4, whose spine is oriented
perpendicular to the upcoming wavefront. The deformation of the
wavefront is suggested to be due to both the forward inclination of
the wavefront and the enhancement of the local Alfvén speed within
the filament channel.
---------------------------------------------------------
Title: Numerical simulations of the CME on 2010 April 8
Authors: Su, Yingna; Kliem, Bernhard; van Ballegooijen, Adriaan;
Deluca, Edward
2013IAUS..294..575S Altcode:
We present 3D zero-beta ideal MHD simulations of the solar flare/CME
event that occurred in Active Region 11060 on 2010 April 8. The initial
magnetic configurations of the two simulations are stable nonlinear
force-free field and unstable magnetic field models constructed by Su
et al. (2011) using the flux rope insertion method. The MHD simulations
confirm that the stable model relaxes to a stable equilibrium, while
the unstable model erupts as a CME. Comparisons between observations
and MHD simulations of the CME are also presented.
---------------------------------------------------------
Title: An MHD Model of a Solar Eruption Starting from NLFFF Initial
Conditions
Authors: DeLuca, Edward E.; Su, Y.; Kliem, B.; Van Ballegooijen, A. A.
2013SPD....4410301D Altcode:
The structure of the coronal magnetic field prior to eruptive processes
and the conditions for the onset of eruption are important issues that
can be addressed through studying the magnetohydrodynamic stability
and evolution of nonlinear force-free field (NLFFF) models. This
talk uses data-constrained NLFFF models of a solar active region that
erupted on 2010 Apri 8 as initial conditions in MHD simulations. These
models, constructed with the techniques of flux rope insertion and
magnetofrictional relaxation, include a stable, an approximately
marginally stable, and an unstable configuration. The simulations
confirm previous related results of magnetofrictional relaxation runs,
in particular that stable flux rope equilibria represent key features
of the observed pre-eruption coronal structure very well and that there
is a limiting value of the axial flux in the rope for the existence
of stable NLFFF equilibria. The specific limiting value is located
within a tighter range, due to the sharper discrimination between
stability and instability by the MHD description. The MHD treatment of
the eruptive configuration yields very good agreement with a number of
observed features like the strongly inclined initial rise path and the
close temporal association between the coronal mass ejection and the
onset of flare reconnection. Minor differences occur in the velocity of
flare ribbon expansion and in the further evolution of the inclination;
these can be eliminated through refined simulations. We suggest that
the slingshot effect of horizontally bent flux in the source region
of eruptions can contribute significantly to the inclination of the
rise direction. Finally, we demonstrate that the onset criterion
formulated in terms of a threshold value for the axial flux in the
rope corresponds very well to the threshold of the torus instability
in the considered active region.
---------------------------------------------------------
Title: MHD Modeling of the Solar Eruption on 2010 April 8
Authors: Kliem, B.; Su, Y.; Van Ballegooijen, A. A.; DeLuca, E.
2012AGUFMSH51A2194K Altcode:
We present a numerical MHD study of the solar eruption on 2010 April
8, extending the previous modeling of the source region in Su et
al. (2011) which had employed the flux rope insertion method and
magnetofrictional relaxation. The threshold of the rope's axial flux
for the loss of equilibrium obtained in Su et al. is confirmed. We find
that the inserted flux rope partly splits for slightly subcritical axial
flux. Starting with slightly supercritical axial flux in the rope, the
MHD simulation yields a fast and strongly inclined eruption as observed
by the STEREO and SDO instruments. The causes of the inclination will
be explored. We also model photospheric changes that may have driven
the flux rope from a stable to the unstable configuration.
---------------------------------------------------------
Title: CME Initiation and Evolution in the Corona
Authors: Kliem, B.
2012AGUFMSH14A..01K Altcode:
Evidence for the existence of a flux rope in the source volume of
coronal mass ejections (CMEs) has been accumulating in recent years,
supporting models of CME initiation that involve the loss of a flux
rope equilibrium. Such models allow detailed, comparative studies of
the various interactions of the CME flux rope with the coronal field,
which will be reviewed. It has been demonstrated that the height profile
of the overlying field strongly influences the CME speed. Important
processes governing the magnetic orientation are the rotation of the
CME flux rope by the coronal shear field component, the writhing of
the rope by the helical kink mode, reconnection with complex ambient
field, and eventually the alignment with the heliospheric current
sheet. Interactions with coronal holes or nearby flux concentrations,
the guidance by a helmet streamer, reconnection with the ambient field,
and the curvature of the filament channel can cause considerable
deviations of the CME path from the radial direction.
---------------------------------------------------------
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.
---------------------------------------------------------
Title: LEMUR: Large European module for solar Ultraviolet
Research. European contribution to JAXA's Solar-C mission
Authors: Teriaca, Luca; Andretta, Vincenzo; Auchère, Frédéric;
Brown, Charles M.; Buchlin, Eric; Cauzzi, Gianna; Culhane, J. Len;
Curdt, Werner; Davila, Joseph M.; Del Zanna, Giulio; Doschek, George
A.; Fineschi, Silvano; Fludra, Andrzej; Gallagher, Peter T.; Green,
Lucie; Harra, Louise K.; Imada, Shinsuke; Innes, Davina; Kliem,
Bernhard; Korendyke, Clarence; Mariska, John T.; Martínez-Pillet,
Valentin; Parenti, Susanna; Patsourakos, Spiros; Peter, Hardi; Poletto,
Luca; Rutten, Robert J.; Schühle, Udo; Siemer, Martin; Shimizu,
Toshifumi; Socas-Navarro, Hector; Solanki, Sami K.; Spadaro, Daniele;
Trujillo-Bueno, Javier; Tsuneta, Saku; Dominguez, Santiago Vargas;
Vial, Jean-Claude; Walsh, Robert; Warren, Harry P.; Wiegelmann,
Thomas; Winter, Berend; Young, Peter
2012ExA....34..273T Altcode: 2011ExA...tmp..135T; 2011arXiv1109.4301T
The solar outer atmosphere is an extremely dynamic environment
characterized by the continuous interplay between the plasma and the
magnetic field that generates and permeates it. Such interactions play a
fundamental role in hugely diverse astrophysical systems, but occur at
scales that cannot be studied outside the solar system. Understanding
this complex system requires concerted, simultaneous solar observations
from the visible to the vacuum ultraviolet (VUV) and soft X-rays, at
high spatial resolution (between 0.1” and 0.3”), at high temporal
resolution (on the order of 10 s, i.e., the time scale of chromospheric
dynamics), with a wide temperature coverage (0.01 MK to 20 MK,
from the chromosphere to the flaring corona), and the capability of
measuring magnetic fields through spectropolarimetry at visible and
near-infrared wavelengths. Simultaneous spectroscopic measurements
sampling the entire temperature range are particularly important. These
requirements are fulfilled by the Japanese Solar-C mission (Plan B),
composed of a spacecraft in a geosynchronous orbit with a payload
providing a significant improvement of imaging and spectropolarimetric
capabilities in the UV, visible, and near-infrared with respect to
what is available today and foreseen in the near future. The Large
European Module for solar Ultraviolet Research (LEMUR), described
in this paper, is a large VUV telescope feeding a scientific payload
of high-resolution imaging spectrographs and cameras. LEMUR consists
of two major components: a VUV solar telescope with a 30 cm diameter
mirror and a focal length of 3.6 m, and a focal-plane package composed
of VUV spectrometers covering six carefully chosen wavelength ranges
between 170 Å and 1270 Å. The LEMUR slit covers 280” on the Sun with
0.14” per pixel sampling. In addition, LEMUR is capable of measuring
mass flows velocities (line shifts) down to 2 km s<SUP> - 1</SUP> or
better. LEMUR has been proposed to ESA as the European contribution
to the Solar C mission.
---------------------------------------------------------
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: Hinode Observations of an Eruption from a Sigmoidal Active
Region
Authors: Green, L. M.; Wallace, A. J.; Kliem, B.
2012ASPC..454..391G Altcode:
We analyse the evolution of a bipolar active region which produces an
eruption during its decay phase. The soft X-ray arcade develops high
shear over a time span of two days and transitions to sigmoidal shortly
before the eruption. We propose that the continuous sigmoidal soft X-ray
threads indicate that a flux rope has formed which is lying low in the
solar atmosphere with a bald patch separatrix surface topology. The
formation of the flux rope is driven by the photospheric evolution
which is dominated by fragmentation of the main polarities, motion due
to supergranular flows and cancellation at the polarity inversion line.
---------------------------------------------------------
Title: Energy Storage and Release in the Solar Atmosphere - Key
SDO Results
Authors: Kliem, Bernhard
2012cosp...39..937K Altcode: 2012cosp.meet..937K
The Solar Dynamics Observatory (SDO) has opened several new windows
in solar observing. Energy release processes can now be studied
in a very wide temperature range at high resolution, high cadence,
and full-Sun field of view. In particular, the Atmospheric Imaging
Assembly (AIA) simultaneously images plasmas from transition region
up to flare temperatures (11 MK). This has stimulated a series
of discoveries as well as verifications of key elements in energy
release models. Only some of these can be reviewed. On the largest
scales, the corona of a whole hemisphere was found to be magnetically
coupled and involved in a sequence of eruption events. At the heart of
such events lies a magnetic flux rope, and, for the first time, this
structure has been directly imaged in the impulsive phase, and found
to be very hot. The association of the rope with the flare current
sheet was also verified. SDO allows us to observe a key process in
reconnection physics, the breakup of current sheets into filamentary
structures. Rapidly forming and expanding cavities provide insight into
the genesis of coronal mass ejections and begin to reveal how coronal
EUV waves and chromospheric Moreton waves are triggered. Indications
for the triggering of two EUV wave fronts were found, possibly resolving
the long-standing puzzle of different kinematic properties seen in such
waves. At the smallest scales, the multi-temperature data support the
hypothesis of nanoflare heating in coronal loops. The EUV Variability
Experiment (EVE) data complete our view of the radiative flare energy,
allow very detailed diagnostics of flare plasmas, and have enabled
the discovery of a second, delayed peak in the EUV flare emission. A
very intriguing sun quake was recently discovered in Helioseismic and
Magnetic Imager (HMI) data of the first X-class flare of the current
solar cycle. This quake challenges current trigger models based on
particle precipitation, as it occurred remote from the strong hard
X-ray sources. The first series of vector magnetograms from HMI,
which is now released, is beginning to boost the studies of energy
storage to a new level. These high-cadence, uniform-quality data show
how the magnetic field and the current system evolve in space and time
from flux emergence, via flux shearing, twisting and cancellation,
to an X-class flare. Using nonlinear force-free field extrapolation,
the evolution of the free magnetic energy and relative helicity up to
the event can be quantified, and strong indications for the formation
of a flux rope low in the corona prior to the eruption are found.
---------------------------------------------------------
Title: CME development in the corona and interplanetary medium :
A multi-wavelength approach
Authors: Pick, M.; Kliem, B.
2012EAS....55..299P Altcode: 2014arXiv1407.2271P
This review focuses on the so called three-part CMEs which essentially
represent the standard picture of a CME eruption. It is shown how the
multi-wavelength observations obtained in the last decade, especially
those with high cadence, have validated the early models and contributed
to their evolution. These observations cover a broad spectral range
including the EUV, white-light, and radio domains.
---------------------------------------------------------
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: 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: Helicity transport in a simulated coronal mass ejection
Authors: Kliem, B.; Rust, S.; Seehafer, N.
2011IAUS..274..125K Altcode: 2010arXiv1012.2297K
It has been suggested that coronal mass ejections (CMEs) remove the
magnetic helicity of their coronal source region from the Sun. Such
removal is often regarded to be necessary due to the hemispheric sign
preference of the helicity, which inhibits a simple annihilation by
reconnection between volumes of opposite chirality. Here we monitor
the relative magnetic helicity contained in the coronal volume of
a simulated flux rope CME, as well as the upward flux of relative
helicity through horizontal planes in the simulation box. The unstable
and erupting flux rope carries away only a minor part of the initial
relative helicity; the major part remains in the volume. This is a
consequence of the requirement that the current through an expanding
loop must decrease if the magnetic energy of the configuration is to
decrease as the loop rises, to provide the kinetic energy of the CME.
---------------------------------------------------------
Title: Photospheric flux cancellation and associated flux rope
formation and eruption
Authors: Green, L. M.; Kliem, B.; Wallace, A. J.
2011A&A...526A...2G Altcode: 2010arXiv1011.1227G
<BR /> Aims: We study an evolving bipolar active region that exhibits
flux cancellation at the internal polarity inversion line, the formation
of a soft X-ray sigmoid along the inversion line and a coronal mass
ejection. The aim is to investigate the quantity of flux cancellation
that is involved in flux rope formation in the time period leading up
to the eruption. <BR /> Methods: The active region is studied using
its extreme ultraviolet and soft X-ray emissions as it evolves from
a sheared arcade to flux rope configuration. The evolution of the
photospheric magnetic field is described and used to estimate how much
flux is reconnected into the flux rope. <BR /> Results: About one third
of the active region flux cancels at the internal polarity inversion
line in the 2.5 days leading up to the eruption. In this period, the
coronal structure evolves from a weakly to a highly sheared arcade
and then to a sigmoid that crosses the inversion line in the inverse
direction. These properties suggest that a flux rope has formed prior
to the eruption. The amount of cancellation implies that up to 60% of
the active region flux could be in the body of the flux rope. We point
out that only part of the cancellation contributes to the flux in the
rope if the arcade is only weakly sheared, as in the first part of the
evolution. This reduces the estimated flux in the rope to ~30% or less
of the active region flux. We suggest that the remaining discrepancy
between our estimate and the limiting value of ~10% of the active region
flux, obtained previously by the flux rope insertion method, results
from the incomplete coherence of the flux rope, due to nonuniform
cancellation along the polarity inversion line. A hot linear feature
is observed in the active region which rises as part of the eruption
and then likely traces out the field lines close to the axis of the
flux rope. The flux cancellation and changing magnetic connections at
one end of this feature suggest that the flux rope reaches coherence
by reconnection immediately before and early in the impulsive phase
of the associated flare. The sigmoid is destroyed in the eruption but
reforms quickly, with the amount of cancellation involved being much
smaller than in the course of its original formation.
---------------------------------------------------------
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: Simulations of Overexpanding CME Cavities
Authors: Kliem, B.; Forbes, T.; Vourlidas, A.; Patsourakos, S.
2010AGUFMSH51A1661K Altcode:
Coronal mass ejection (CME) cavities seen in white-light coronagraphs
expand nearly self similarly in the outer corona and inner solar
wind. Little is known about their initial expansion in the inner
corona. A two-phase evolution, consisting of an initial overexpansion
up to a heliocentric front height of about 1.5 solar radii, followed by
nearly self-similar expansion, was recently discovered in STEREO/SECCHI
observations of a fast CME (Patsourakos et al. 2010). The overexpansion
is expressed as a decrease of the cavity aspect ratio (center height
by radius) by at least a factor of 2 during the rise phase of the
main CME acceleration. We present MHD simulations of erupting flux
ropes that show the initial overexpansion of a cavity in line with
the observed evolution. The contributions of ideal-MHD expansion and
of magnetic reconnection to the growth of the flux rope and cavity
in the simulations will be quantified to identify the primary cause
of the overexpansion. This assesses the diagnostic potential of the
overexpansion for the change of flux rope current and the role of
magnetic reconnection in the early evolution of CMEs.
---------------------------------------------------------
Title: Helicity Shedding in a Simulated CME
Authors: Seehafer, N.; Kliem, B.
2010AGUFMSH23B1855S Altcode:
It has been suggested that coronal mass ejections remove the magnetic
helicity of active regions from the Sun. Such removal is often regarded
to be necessary due to the hemispheric sign preference of the helicity,
which inhibits a simple annihilation by reconnection between volumes of
opposite chirality. We have monitored the relative magnetic helicity
contained in the coronal volume of a simulated flux rope CME, as
well as the upward flux of helicity through a horizontal plane in the
simulation box. The unstable and erupting flux rope carries away only
part of the initial helicity through the open upper boundary of the
box; the larger part remains in the volume. We offer a simple physical
explanation for this result. Since most active regions erupt only
once in their lifetime, our finding suggests that the major part of
the helicity which is transported into the corona by emerging active
regions is redistributed into the coronal field upon the dispersal of
the active regions and is eventually transported back into the solar
interior as the field submerges.
---------------------------------------------------------
Title: Toward understanding the early stages of an impulsively
accelerated coronal mass ejection. SECCHI observations
Authors: Patsourakos, S.; Vourlidas, A.; Kliem, B.
2010A&A...522A.100P Altcode: 2010arXiv1008.1171P
Context. The expanding magnetic flux in coronal mass ejections (CMEs)
often forms a cavity. Studies of CME cavities have so far been limited
to the pre-event configuration to evolved CMEs at great heights, and
to two-dimensional imaging data. <BR /> Aims: Quantitative analysis of
three-dimensional cavity evolution at CME onset can reveal information
that is relevant to the genesis of the eruption. <BR /> Methods:
A spherical model was simultaneously fit to Solar Terrestrial
Relations Observatory (STEREO) Extreme Ultraviolet Imager (EUVI)
and Inner Coronagraph (COR1) data of an impulsively accelerated CME
on 25 March 2008, which displays a well-defined extreme ultraviolet
(EUV) and white-light cavity of nearly circular shape already at low
heights h ≈ 0.2 R_⊙. The center height h(t) and radial expansion
r(t) of the cavity were obtained in the whole height range of the main
acceleration. We interpret them as the axis height and as a quantity
proportional to the minor radius of a flux rope. <BR /> Results:
The three-dimensional expansion of the CME exhibits two phases in
the course of its main upward acceleration. From the first h and r
data points, taken shortly after the onset of the main acceleration,
the erupting flux shows an overexpansion compared to its rise, as
expressed by the decrease in the aspect ratio from κ = h/r ≈ 3
to κ ≈ (1.5-2). This phase is approximately coincident with the
impulsive rise in the acceleration and is followed by a phase of very
gradual change in the aspect ratio (a nearly self-similar expansion)
toward κ ~ 2.5 at h ~ 10 R_⊙. The initial overexpansion of the CME
cavity can be caused by flux conservation around a rising flux rope
of decreasing axial current and by the addition of flux to a growing,
or by even newly formed, flux rope by magnetic reconnection. Further
analysis will be required to decide which of these contributions is
dominant. The data also suggest that the horizontal component of the
impulsive cavity expansion (parallel to the solar surface) triggers the
associated EUV wave, which subsequently detaches from the CME volume.
---------------------------------------------------------
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: Reconnection of a Kinking Flux Rope Triggering the Ejection of
a Microwave and Hard X-ray Source I. Observations and Interpretation
Authors: Karlický, M.; Kliem, B.
2010SoPh..266...71K Altcode: 2010arXiv1007.2146K; 2010SoPh..tmp..139K
Imaging microwave observations of an eruptive, partially occulted solar
flare on 18 April 2001 suggest that the global structure of the event
can be described by the helical kink instability of a twisted magnetic
flux rope. This model is suggested by the inverse gamma shape of the
source exhibiting crossing legs of a rising flux loop and by evidence
that the legs interact at or near the crossing point. The interaction is
reflected by the location of peak brightness near the crossing point and
by the formation of superimposed compact nonthermal sources most likely
at or near the crossing point. These sources propagate upward along both
legs, merge into a single, bright source at the top of the structure,
and continue to rise at a velocity >1000 km s<SUP>−1</SUP>. The
compact sources trap accelerated electrons which radiate in the radio
and hard X-ray ranges. This suggests that they are plasmoids, although
their internal structure is not revealed by the data. They exhibit
variations of the radio brightness temperature at a characteristic time
scale of ∼ 40 s, anti-correlated to their area, which also support
their interpretation as plasmoids. Their propagation path differs
from the standard scenario of plasmoid formation and propagation in
the flare current sheet, suggesting the helical current sheet formed
by the instability instead.
---------------------------------------------------------
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: 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: Flux Rope Formation Preceding Coronal Mass Ejection Onset
Authors: Kliem, Bernhard; Green, L. M.
2009SPD....41.2120K Altcode:
We analyse the evolution of a sigmoidal (S shaped) active region toward
eruption, which includes a coronal mass ejection (CME) but leaves part
of the filament in place. The X-ray sigmoid is found to trace out three
different magnetic topologies in succession: a highly sheared arcade
of coronal loops in its long-lived phase, a bald-patch separatrix
surface (BPSS) in the hours before the CME, and the first flare loops
in its major transient intensity enhancement. The coronal evolution
is driven by photospheric changes which involve the convergence and
cancellation of flux elements under the sigmoid and filament. The data
yield unambiguous evidence for the existence of a BPSS, and hence a
flux rope, in the corona prior to the onset of the CME.
---------------------------------------------------------
Title: Evidence for Mixed Helicity in Erupting Filaments
Authors: Muglach, K.; Wang, Y. -M.; Kliem, B.
2009ApJ...703..976M Altcode: 2009arXiv0907.4446M
Erupting filaments are sometimes observed to undergo a rotation
about the vertical direction as they rise. This rotation of the
filament axis is generally interpreted as a conversion of twist into
writhe in a kink-unstable magnetic flux rope. Consistent with this
interpretation, the rotation is usually found to be clockwise (as viewed
from above) if the post-eruption arcade has right-handed helicity, but
counterclockwise if it has left-handed helicity. Here, we describe two
non-active-region filament events recorded with the Extreme-Ultraviolet
Imaging Telescope on the Solar and Heliospheric Observatory in which
the sense of rotation appears to be opposite to that expected from
the helicity of the post-event arcade. Based on these observations,
we suggest that the rotation of the filament axis is, in general,
determined by the net helicity of the erupting system, and that the
axially aligned core of the filament can have the opposite helicity sign
to the surrounding field. In most cases, the surrounding field provides
the main contribution to the net helicity. In the events reported here,
however, the helicity associated with the filament "barbs" is opposite
in sign to and dominates that of the overlying arcade.
---------------------------------------------------------
Title: Flux Rope Formation Preceding Coronal Mass Ejection Onset
Authors: Green, L. M.; Kliem, B.
2009ApJ...700L..83G Altcode: 2009arXiv0906.4794G
We analyze the evolution of a sigmoidal (S-shaped) active region toward
eruption, which includes a coronal mass ejection (CME) but leaves part
of the filament in place. The X-ray sigmoid is found to trace out three
different magnetic topologies in succession: a highly sheared arcade
of coronal loops in its long-lived phase, a bald-patch separatrix
surface (BPSS) in the hours before the CME, and the first flare loops
in its major transient intensity enhancement. The coronal evolution
is driven by photospheric changes which involve the convergence and
cancellation of flux elements under the sigmoid and filament. The data
yield unambiguous evidence for the existence of a BPSS, and hence a
flux rope, in the corona prior to the onset of the CME.
---------------------------------------------------------
Title: Endpoint Brightenings in Erupting Filaments
Authors: Wang, Y. -M.; Muglach, K.; Kliem, B.
2009ApJ...699..133W Altcode:
Two well known phenomena associated with erupting filaments are
the transient coronal holes that form on each side of the filament
channel and the bright post-event arcade with its expanding double
row of footpoints. Here we focus on a frequently overlooked signature
of filament eruptions: the spike- or fan-shaped brightenings that
appear to mark the far endpoints of the filament. From a sample of
non-active-region filament events observed with the Extreme-Ultraviolet
Imaging Telescope on the Solar and Heliospheric Observatory, we find
that these brightenings usually occur near the outer edges of the
transient holes, in contrast to the post-event arcades, which define
their inner edges. The endpoints are often multiple and are rooted
in and around strong network flux well outside the filament channel,
a result that is consistent with the axial field of the filament being
much stronger than the photospheric field inside the channel. The
extreme ultraviolet brightenings, which are most intense at the
time of maximum outward acceleration of the filament, can be used to
determine unambiguously the direction of the axial field component from
longitudinal magnetograms. Their location near the outer boundary of
the transient holes suggests that we are observing the footprints of
the current sheet formed at the leading edge of the erupting filament,
as distinct from the vertical current sheet behind the filament which
is the source of the post-event arcade.
---------------------------------------------------------
Title: Temperature Tomography of a Coronal Sigmoid Supporting the
Gradual Formation of a Flux Rope
Authors: Tripathi, Durgesh; Kliem, Bernhard; Mason, Helen E.; Young,
Peter R.; Green, Lucie M.
2009ApJ...698L..27T Altcode: 2009arXiv0904.4782T
Multiwavelength observations of a sigmoidal (S-shaped) solar coronal
source by the EUV Imaging Spectrometer and the X-Ray Telescope
aboard the Hinode spacecraft and by the EUV Imager aboard STEREO are
reported. The data reveal the coexistence of a pair of J-shaped hot
arcs at temperatures T>2 MK with an S-shaped structure at somewhat
lower temperatures (T ≈ 1-1.3 MK). The middle section of the S-shaped
structure runs along the polarity inversion line of the photospheric
field, bridging the gap between the arcs. Flux cancellation occurs
at the same location in the photosphere. The sigmoid forms in the
gradual decay phase of the active region, which does not experience
an eruption. These findings correspond to the expected signatures
of a flux rope forming, or being augmented, gradually by a topology
transformation inside a magnetic arcade. In such a transformation, the
plasma on newly formed helical field lines in the outer flux shell of
the rope (S-shaped in projection) is expected to enter a cooling phase
once the reconnection of their parent field line pairs (double-J shaped
in projection) is complete. Thus, the data support the conjecture that
flux ropes can exist in the corona prior to eruptive activity.
---------------------------------------------------------
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: Quadrature STEREO Observations Determine the Nature of
EUV Waves
Authors: Kliem, Bernhard; Patsourakos, S.; Vourlidas, A.; Ontiveros, V.
2009SPD....40.2603K Altcode:
One of the major discoveries of EIT on SOHO was the observation of
large-scale EUV intensity disturbances which travel over significant
fractions of the solar disk. These `EUV waves' are associated with
CME onsets and can be either an MHD wave triggered by the eruption or
the footprints of the associated CME, which currently is a subject of
intense debate. EUV waves are better observed when their source region
is close to disk center, whereas CME onsets and CMEs in general are
better observed off-limb. Therefore, simultaneous multi-viewpoint
observations of EUV waves are best suited to clarify the nature of
these transients and to determine their true relationship with CMEs. <P
/>We present here the first quadrature STEREO observations of an EUV
wave. The wave was observed on 2009 February 13 by both satellites,
which were at a separation of 90 degrees. The wave originated from
an erupting active region near disk center as seen from SC B and
propagated over almost the entire visible solar disk. For SC A the
active region was at the east limb and showed a small erupting bubble,
expanding impulsively in both radial and lateral directions and inducing
deflections of nearby and remote coronal structures. We present high
cadence EUVI and COR1 measurements of both the wave (SC B), and the
expanding EUV bubble (SC A), and of the resulting white-light CME (SC A;
COR1). These would allow to quantify for the first time the true sizes
and expansion characteristics of both the EUV wave and the associated
CME. <P />Finally, we search for wave-associated features in 3D MHD
simulations of CME onsets based on ideal MHD instabilities. These are
compared with the STEREO observations.
---------------------------------------------------------
Title: Intermittent Reconnection Downflow Enhancements In A Simulated
Flux Rope Eruption
Authors: Kliem, Bernhard; Linton, M. G.
2009SPD....40.2006K Altcode:
Supra-arcade downflows in X-ray and EUV flare emissions and
post-eruption inflows in coronagraph data have been interpreted to
be signatures of the downward reconnection outflow from a vertical
(flare) current sheet. These downflows show an intermittent occurrence
pattern, indicating that the reconnection is bursty in time or patchy
in space, or both. We present MHD simulations of such reconnection
in the realistic configuration of a vertical current sheet formed
beneath and driven by an erupting flux rope. The reconnection is
found to develop bursty outflows, both upward and downward, with the
upward outflows generally showing the stronger variablity. While the
reconnection starts early in the rise of the flux rope and its peak
upward outflow velocity is closely correlated with the rope's rise
velocity, the burstiness develops in a clear fashion only as the rope's
height has increased from the initial position by about an order of
magnitude, so that the current sheet has reached a sufficient vertical
extent. The reconnection downflow shows a series of enhancements,
each of them starting at a successively greater height from a newly
developed magnetic X line. The plasma temporarily accelerated downward
in such an enhancement soon turns into a gradual deceleration and then
eventually comes to rest on top of previously accelerated plasma. These
findings are consistent with the observations of intermittent downflows.
---------------------------------------------------------
Title: Endpoint Brightenings in Erupting Filaments
Authors: Muglach, Karin; Wang, Y.; Kliem, B.
2009SPD....40.2109M Altcode:
Two well-known phenomena associated with erupting filaments are
the transient coronal holes that form on each side of the filament
channel and the bright post-event arcade with its expanding double
row of footpoints. Here we focus on a frequently overlooked signature
of filament eruptions: the spike- or fan-shaped brightenings that
appear to mark the far endpoints of the filament. From a sample of
non-active-region filament events observed with the Extreme-Ultraviolet
Imaging Telescope (EIT) on the Solar and Heliospheric Observatory
(SOHO), we find that these brightenings usually occur near the outer
edges of the transient holes, in contrast to the post-event arcades,
which define their inner edges. The endpoints are often multiple
and are rooted in and around strong network flux well outside the
filament channel, a result that is consistent with the axial field of
the filament being much stronger than the photospheric field inside
the channel. The EUV brightenings, which are most intense at the
time of maximum outward acceleration of the filament, can be used to
determine unambiguously the direction of the axial field component from
longitudinal magnetograms. Their location near the outer boundary of
the transient holes suggests that we are observing the footprints of
the current sheet formed at the leading edge of the erupting filament,
as distinct from the vertical current sheet behind the filament which
is the source of the post-event arcade.
---------------------------------------------------------
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: Coronal Magnetic Field Extrapolation from Photospheric
Measurements Applied to an Active Region
Authors: Fuhrmann, M.; Kliem, B.; Valori, G.; Seehafer, N.
2008ESPM...12.3.37F Altcode:
We outline an MHD relaxation method that permits to extrapolate
photospheric vector magnetograms into coronal nonlinear force-free
fields. The method is applied to a magnetogram taken before an eruptive
event in NOAA AR 7792 on 25 October 1994. The event produced a coronal
mass ejection (CME) and eruptive flare with a prominent sigmoidal
soft X-ray source. Multiwavelength observations as well as theoretical
modeling indicate the importance of twisted magnetic configurations in
solar active regions(ARs) in the initiation of such events. Manoharan
et al. (1996) proposed a model for this event that included the merging
of two slightly twisted flux bundles near the polarity inversion line
into a flux rope of larger total twist and an overlying flux system much
closer to a potential-field state. For the extrapolation we use a vector
magnetogram taken at the Mees Solar Observatory 16 hours before the
actual event. The magnetic field extrapolation is able to recover main
parts of the structures suggested in the model by Manoharan et al. We
find the overlying nearly potential flux and part of the sigmoidal
field, i.e., one of the suggested weakly twisted flux bundles, in the
location observed. This supports the notion that sigmoids are coronal
manifestations of twisted magnetic flux tubes which start expanding in
eruptive events and may exist even before the onset of such events. We
tentatively attribute the incomplete reconstruction of the sigmoidal
field structure to the strong evolution of the photospheric field at
the suggested location of flux tube merging between the time of the
magnetogram and the eruption, as indicated by a magnetogram on the
following day.
---------------------------------------------------------
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: 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: 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: 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: 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: Force-free magnetic fields in the solar atmosphere
Authors: Seehafer, N.; Fuhrmann, M.; Valori, G.; Kliem, B.
2007AN....328.1166S Altcode:
Reliable measurements of the solar magnetic field are restricted to the
level of the photosphere. For about half a century attempts have been
made to calculate the field in the layers above the photosphere, i.e. in
the chromosphere and in the corona, from the measured photospheric
field. The procedure is known as magnetic field extrapolation. In
the superphotospheric parts of active regions the magnetic field is
approximately force-free, i.e. electric currents are aligned with the
magnetic field. The practical application to solar active regions has
been largely confined to constant-α or linear force-free fields, with
a spatially constant ratio, α, between the electric current and the
magnetic field. We review results obtained from extrapolations with
constant-α force-free fields, in particular on magnetic topologies
favourable for flares and on magnetic and current helicities. Presently,
different methods are being developed to calculate non-constant-α or
nonlinear force-free fields from photospheric vector magnetograms. We
also briefly discuss these methods and present a comparison of a linear
and a nonlinear force-free magnetic field extrapolation applied to
the same photospheric boundary data.
---------------------------------------------------------
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: Magnetofrictional Extrapolations of Low and Lou's Force-Free
Equilibria
Authors: Valori, G.; Kliem, B.; Fuhrmann, M.
2007SoPh..245..263V Altcode:
We present a careful investigation of the magnetofrictional relaxation
and extrapolation technique applied to the reconstruction of two test
fields. These fields are taken from the family of nonlinear force-free
magnetic equilibria constructed by Low and Lou (Astrophys. J.352,
343, 1990), which have emerged as standard tests for extrapolation
techniques in recent years. For the practically relevant case that
only the field values in the bottom plane of the considered volume
(vector magnetogram) are used as input information (i.e., not including
the knowledge about the test field at the side and top boundaries),
the test field is reconstructed to a higher accuracy than obtained
previously. Detailed diagnostics of the reconstruction accuracy
show that the implementation of fourth-order spatial discretization
was essential to reach this accuracy for the given test fields
and to achieve near machine precision in satisfying the solenoidal
condition. Different variants of boundary conditions are tested,
which all yield comparable accuracy. In its present implementation,
the technique yields a scaling of computing time with total number of
grid points only slightly below N<SUP>5/3</SUP>, which is too steep
for applications to large (≥1024<SUP>2</SUP>) magnetograms, except
on supercomputers. Directions for improvement are outlined.
---------------------------------------------------------
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: Testing non-linear force-free coronal magnetic field
extrapolations with the Titov-Démoulin equilibrium
Authors: Wiegelmann, T.; Inhester, B.; Kliem, B.; Valori, G.;
Neukirch, T.
2006A&A...453..737W Altcode: 2006astro.ph.12650W
Context.As the coronal magnetic field can usually not be measured
directly, it has to be extrapolated from photospheric measurements
into the corona. <BR />Aims.We test the quality of a non-linear
force-free coronal magnetic field extrapolation code with the help of
a known analytical solution.<BR />Methods. The non-linear force-free
equations are numerically solved with the help of an optimization
principle. The method minimizes an integral over the force-free
and solenoidal condition. As boundary condition we use either the
magnetic field components on all six sides of the computational box
in Case I or only on the bottom boundary in Case II. We check the
quality of the reconstruction by computing how well force-freeness
and divergence-freeness are fulfilled and by comparing the numerical
solution with the analytical solution. The comparison is done with
magnetic field line plots and several quantitative measures, like the
vector correlation, Cauchy Schwarz, normalized vector error, mean vector
error and magnetic energy. <BR />Results.For Case I the reconstructed
magnetic field shows good agreement with the original magnetic field
topology, whereas in Case II there are considerable deviations from
the exact solution. This is corroborated by the quantitative measures,
which are significantly better for Case I.<BR />Conclusions. Despite
the strong nonlinearity of the considered force-free equilibrium, the
optimization method of extrapolation is able to reconstruct it; however,
the quality of reconstruction depends significantly on the consistency
of the input data, which is given only if the known solution is provided
also at the lateral and top boundaries, and on the presence or absence
of flux concentrations near the boundaries of the magnetogram.
---------------------------------------------------------
Title: Relativistic electron scattering by electrostatic upper hybrid
waves in the radiation belt
Authors: Shklyar, D. R.; Kliem, B.
2006JGRA..111.6204S Altcode: 2006JGRA..11106204S
Observations of particle precipitation into the Earth's atmosphere
performed on low-altitude satellites have excited substantial interest
in relativistic electron precipitation bursts from the outer zone
of the radiation belt. As underlying mechanism of such bursts, we
suggest particle scattering into the loss cone due to higher-order
cyclotron resonance interaction between relativistic electrons and
intense narrow-band upper hybrid waves, which are frequently observed
outside the plasmapause. The case of a single wave and the case of a
wide wave number spectrum are considered, and approximate expressions
for the relativistic particle diffusion coefficients in phase space are
calculated for conditions of an inhomogeneous plasma. It is found that
relativistic electrons have a preference over lower-energy electrons
from the viewpoint of the number of cyclotron resonances that a
particle crosses during each bounce period. Owing to the predominantly
longitudinal direction of the upper hybrid wave group velocity, the
resonant wave-particle interaction can take place over many electron
bounce periods, which facilitates the particle scattering into the loss
cone. The theory developed in the present paper accounts for two main
features of the relativistic electron precipitation bursts, namely,
a strong energy dependence in the electron precipitation process and
the small-scale burst structure, which is primarily attributed to the
localization and strong inhomogeneity of the growth region of the upper
hybrid modes responsible for the scattering. The existence of intrinsic
temporal structure of the precipitation on timescales comparable to
the bounce period is also consistent with the theory due to the high
efficiency of the scattering process.
---------------------------------------------------------
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: Extrapolation of a nonlinear force-free field containing a
highly twisted magnetic loop
Authors: Valori, G.; Kliem, B.; Keppens, R.
2005A&A...433..335V Altcode:
The stress-and-relax method for the extrapolation of nonlinear
force-free coronal magnetic fields from photospheric vector
magnetograms is formulated and implemented in a manner analogous to
the evolutionary extrapolation method. The technique is applied to a
numerically constructed force-free equilibrium that has a simple bipolar
structure of the normal field component in the bottom (magnetogram)
plane but contains a highly twisted loop and a shear (current) layer,
with a smooth but strong variation of the force-free parameter α in
the magnetogram. A standard linear force-free extrapolation of this
magnetogram, using the so-called α_best value, is found to fail
in reproducing the twisted loop (or flux rope) and the shear layer;
it yields a loop pair instead and the shear is not concentrated in a
layer. With the nonlinear extrapolation technique, the given equilibrium
is readily reconstructed to a high degree of accuracy if the magnetogram
is sufficiently resolved. A parametric study quantifies the requirements
on the resolution for a successful nonlinear extrapolation. Permitting
magnetic reconnection by a controlled use of resistivity improved the
extrapolation at a resolution comparable to the smallest structures
in the magnetogram.
---------------------------------------------------------
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: Pulsating Microwave Emission from the Star AD Leo
Authors: Zaitsev, V. V.; Kislyakov, A. G.; Stepanov, A. V.; Kliem,
B.; Furst, E.
2004AstL...30..319Z Altcode:
We have performed a spectral analysis of the quasi-periodic
low-frequency modulation of microwave emission from a flare on the
star AD Leo. We used the observations of the May 19, 1997 flare in
the frequency range 4.5-5.1 GHz with a total duration of the burst
phase of about 50 s obtained in Effelsberg with a time resolution
of 1 ms. The time profile of the radio emission was analyzed by
using the Wigner-Ville transformation, which yielded the dynamic
spectrum of low-frequency pulsations with a satisfactory frequency-time
resolution. In addition to the noise component, two regular components
were found to be present in the low-frequency modulation spectrum
of the stellar radio emission: a quasi-periodic component whose
frequency smoothly decreased during the flare from ~2 to ~0.2 Hz and
a periodic sequence of pulses with a repetition rate of about 2 Hz,
which was approximately constant during the flare. We consider the
possibility of the combined effect of MHD and LCR oscillations of the
radio source on the particle acceleration in the stellar atmosphere
and give estimates of the source's parameters that follow from an
analysis of the low-frequency modulation spectra.
---------------------------------------------------------
Title: SUMER Observations of Heating and Cooling of Coronal Loops
Authors: Curdt, W.; Wang, T. J.; Dwivedi, B. N.; Kliem, B.; Dammasch,
I. E.
2004ESASP.547..333C Altcode: 2004soho...13..333C
Hot-loop transient events observed by SUMER in hot EUV lines are known
to trigger loop oscillations, as reported by Kliem et al. [1] and Wang
et al. [2], [3]. Apart from the inference of physical parameters in the
solar corona, these observations also carry the signatures of heating
and cooling of coronal loops. We present the light curves for various
highly-ionized ions which were simultaneously observed during and after
the trigger. Even though the majority of SUMER events occur on sub-flare
level, it is clear that the heating is impulsive and drives the plasma
to a very high temperature of up to 10 MK within minutes. During the
cooling phase, however, we find the plasma in gradually decreasing
ionization stages which implies that the entire loop system involved
in such events is basically in the isothermal state. Such events may
also help in our understanding of mass supply and energy transport in
the corona.
---------------------------------------------------------
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: Flare/CME Relationship
Authors: Kliem, B.
2004cosp...35.4097K Altcode: 2004cosp.meet.4097K
In recent years evidence has accumulated showing that flares and CMEs
are different observational manifestations of a single process --
the destabilization and reorganization of magnetic fields at active
region spatial scales. Neupert et al. (2001) and Zhang et al. (2001)
have clearly shown the connection between the two in a couple of
events. I will present a further well-observed example showing the same
connection, the 2002 April 21 solar X flare. Combined data from the
TRACE, SUMER, RHESSI, NoRH, UVCS, and LASCO instruments show erupting
core flux, associated with nonthermal and thermal flare emissions
and evolving into one of the fastest CMEs ever observed. Although the
observations are very detailed, they still do not seem to permit a firm
conclusion regarding the destabilization mechanism, but they point to
an instability of a complex flux rope structure, with some elements
of the tether cutting and magnetic breakout models possibly being
included. The evolution of unstable magnetic flux from the impulsive
rise phase of flare emissions to a fully developed CME typically
happens in the inner and middle corona, a region too sparsely sampled by
current instrumentation. It is therefore still largely ambiguous which
height-time characteristic should be fitted to the data and whether a
distinct acceleration phase of the ejecta occurs during the impulsive
flare phase. Guidance by theoretical models is needed. I will briefly
discuss a few height-time characteristics suggested in the literature,
including the one implied by a recently proposed destabilization
mechanism which is based on the kink instability of a flux rope.
---------------------------------------------------------
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: Hot coronal loop oscillations observed with SUMER: Examples
and statistics
Authors: Wang, T. J.; Solanki, S. K.; Curdt, W.; Innes, D. E.;
Dammasch, I. E.; Kliem, B.
2003A&A...406.1105W Altcode:
We give an extensive overview of Doppler shift oscillations in
hot active region loops obtained with SUMER. The oscillations have
been detected in loops sampled 50-100 arcsec off the limb of the
Sun in ultraviolet lines, mainly Fe Xix and Fe Xxi, with formation
temperature greater than 6 MK. The spectra were recorded along a
300 arcsec slit placed at a fixed position in the corona above the
active regions. Oscillations are usually seen along an extended
section of the slit and often appear to be from several different
portions of the loops (or from different loops). Different portions
are sometimes in phase, sometimes out of phase and sometimes show
phase shifts along the slit. We measure physical parameters of 54
Doppler shift oscillations in 27 flare-like events and give geometric
parameters of the associated hot loops when soft X-ray (SXR) images
are available. The oscillations have periods in the range 7-31 min,
with decay times 5.7-36.8 min, and show an initial large Doppler
shift pulse with peak velocities up to 200 km s<SUP>-1</SUP>. The
oscillation periods are on average a factor of three longer than the
TRACE transverse loop oscillations. The damping times and velocity
amplitude are roughly the same, but the derived displacement amplitude
is four or five times larger than the transverse oscillation amplitude
measured in TRACE images. Unlike TRACE oscillations, only a small
fraction of them are triggered by large flares, and they often recur
2-3 times within a couple of hours. All recurring events show initial
shifts of the same sign. These data provide the following evidence to
support the conclusion that these oscillations are slow magnetoacoustic
standing waves in hot loops: (1) the phase speeds derived from observed
periods and loop lengths roughly agree with the sound speed; (2)
the intensity fluctuation lags the Doppler shifts by 1/4 period; (3)
The scaling of the dissipation time of slow waves with period agrees
with the observed scaling for 49 cases. They seem to be triggered
by micro- or subflares near a footpoint, as revealed in one example
with SXR image observations. However other mechanisms cannot as yet
be ruled out. Some oscillations showed phase propagation along the
slit in one or both directions with apparent speeds in the range of
8-102 km s<SUP>-1</SUP>, together with distinctly different intensity
and line width distributions along the slit. These features can be
explained by the excitation of the oscillation at a footpoint of an
inhomogeneous coronal loop, e.g. a loop with fine structure. <P />Table
\ref{osctab} and Appendices A and B are only available in electronic
form at http://www.edpsciences.org
---------------------------------------------------------
Title: The Initial Phase of the 2002 April 21 Solar X Flare: Aspects
of Agreement and of Disagreement with Solar Flare Models
Authors: Kliem, Bernhard
2003ANS...324R..11K Altcode: 2003ANS...324..B06K
No abstract at ADS
---------------------------------------------------------
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: Recent Progress in Understanding Energy Conversion and Particle
Acceleration in the Solar Corona
Authors: Kliem, B.; MacKinnon, A.; Trottet, G.; et al.
2003LNP...612..263K Altcode: 2003ecpa.conf..263K
We report on results of the working group sessions at the CESRA 2001
workshop on "Energy Conversion and Particle Acceleration in the
Solar Corona" which was focused on radio observations and related
modeling. Progress reached in the following areas is summarized:
(1) diagnostics of coronal magnetic fields and the morphology of
the field in flares and filament eruptions; (2) evidence of magnetic
reconnection and MHD turbulence in radio emissions; (3) acceleration
site, propagation, and trapping of radio-emitting energetic particles
in flares; (4) the sites of particle acceleration in long duration
events, as evidenced by the 2000 July 14 ("Bastille Day") flare; (5)
radio imaging of CMEs and filament eruptions; (6) the relationship
of coronal and interplanetary shock waves to flares, CMEs, and other
coronal waves; and (7) the origin of solar energetic particles.
---------------------------------------------------------
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: Doppler oscillations in hot coronal loops
Authors: Curdt, W.; Wang, T. J.; Innes, D. E.; Solanki, S. K.;
Dammasch, I. E.; Kliem, B.; Ofman, L.
2002ESASP.506..581C Altcode: 2002ESPM...10..581C; 2002svco.conf..581C
Recently, a new kind of damped oscillations of hot coronal loops was
revealed by the Solar Ultraviolet Measurements of Emitted Radiation
(SUMER) spectrometer on SOHO. Such events seem to be a common feature
observed in active region loops, seen very often when these lines
brighten. The oscillations always have an impulsive trigger and
are strongly damped while they cool down. However, in lines formed
at coronal temperatures of ≍2 MK never any signature of these
oscillations has been observed. In this study, we present the main
properties of Doppler oscillations derived from a statistical study
of 17 flare-like events, and a comparison with TRACE transverse loop
oscillations. We also discuss the oscillation modes and their damping
mechanism.
---------------------------------------------------------
Title: Drifting decimetric pulsation structures in the initial phase
of solar flares
Authors: Karlický, M.; Kliem, B.; Mészárosová, H.; Jiřička, K.
2002ESASP.506..653K Altcode: 2002svco.conf..653K; 2002ESPM...10..653K
Two new cases of slowly negatively drifting pulsation structures in the
decimetric wavelength range are reported. It is shown that the August
18, 1998 flare started with a slowly drifting pulsation structure
associated with a plasmoid ejection observed by Yohkoh/SXT. This
drifting pulsation structure is an example of a narrow-band one,
which even shows intensity enhancements at its high- and low-frequency
edges. Using the cross-correlation method no significant relation
between the radio emission of this drifting structure and the hard
X-rays was found. On the other hand, the second drifting structure
observed during the hard X-ray decay phase of the March 24, 2000
flare shows a clear separation from the large-scale coronal shock
wave released by the flare (a partly simultaneous type II radio burst
occurred at much lower frequencies). The slowly drifting structures
are analyzed and possible underlying physical processes are discussed
assuming the plasmoid ejection model of eruptive solar flares.
---------------------------------------------------------
Title: Correlated Dynamics of Hot and Cool Plasmas in the Main Phase
of a Solar Flare
Authors: Kliem, B.; Dammasch, I. E.; Curdt, W.; Wilhelm, K.
2002ApJ...568L..61K Altcode:
We report far-ultraviolet observations of a solar limb flare obtained
by the Solar Ultraviolet Measurements of Emitted Radiation (SUMER)
spectrometer. At a fixed pointing of the slit above the limb, spectra
were simultaneously obtained in several emission lines that covered
a wide temperature range from ~10<SUP>4</SUP> to ~10<SUP>7</SUP>
K. The temporal evolution of the spectra revealed, for the first
time, a high degree of correlation between the dynamical behavior
of hot (T~10<SUP>7</SUP> K) and cool (T~10<SUP>4</SUP> K) coronal
material during the main phase of a flare. We note that the data
did not show any indication of the presence of a prominence. Hot and
cool plasmas brightened at nearly the same location. Their Doppler
shifts, which were opposite to each other, reached peak values
simultaneously. Thereafter, the two components showed anticorrelated,
rapidly damped, and oscillatory Doppler shifts and a very similar
decay of the line widths, but with the cool plasma reaching maximum
brightness before the hot plasma. This behavior points to an active
role for cool plasma in the dynamics of this flare, different from
the usual picture of passive cooling after the impulsive phase. We
suggest a model in which the localized cooling of coronal plasma by the
thermal instability triggers magnetic reconnection through the resulting
enhanced resistivity, the combined processes leading to the correlated
dynamics of hot and cool plasmas in a loop-loop interaction geometry.
---------------------------------------------------------
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: Correlated Dynamics of Hot and Cool Plasmas in Two Solar Flares
Authors: Kliem, B.; Dammasch, I. E.; Curdt, W.; Wilhelm, K.
2002mwoc.conf..271K Altcode:
We report far-ultraviolet observations of a solar limb flare by
the Solar Ultraviolet Measurements of Emitted Radiation (SUMER)
spectrometer. At a fixed pointing of the slit above the limb, spectra
were simultaneously obtained in several emission lines that covered a
wide temperature range of ≅ 10<SUP>4</SUP> -- 10<SUP>7</SUP> K. The
temporal evolution of the spectra revealed a high degree of correlation
between the dynamical behavior of hot (T ~10<SUP>7</SUP> K) and cool (T
~10<SUP>4</SUP> K) material during the main flare phase. Hot and cool
plasma brightened at nearly the same place, with cool plasma reaching
maximum brightness before the hot plasma. The opposite line-of-sight
velocities reached their peak values simultaneously. A correlated,
rapidly damped oscillatory motion followed, while the excess (turbulent)
line widths decayed in a similar manner. This behavior points to an
active role of cool plasma in the dynamics of this flare, different
from the usual picture of passive cooling after the impulsive phase. An
interpretation is suggested in terms of magnetic reconnection which
is triggered by rapid localized cooling due to the thermal instability
in a loop-loop interaction geometry.
---------------------------------------------------------
Title: Microwave plasma emission of a flare on AD Leo
Authors: Stepanov, A. V.; Kliem, B.; Zaitsev, V. V.; Fürst, E.;
Jessner, A.; Krüger, A.; Hildebrandt, J.; Schmitt, J. H. M. M.
2001A&A...374.1072S Altcode: 2001astro.ph..6369S
An intense radio flare on the dMe star AD Leo, observed with the
Effelsberg radio telescope and spectrally resolved in a band of 480
MHz centred at 4.85 GHz is analysed. A lower limit of the brightness
temperature of the totally right handed polarized emission is estimated
as T_b ~ 5*E<SUP>10</SUP> K (with values T_bga3 *E<SUP>13</SUP>
K considered to be more probable), which requires a coherent radio
emission process. In the interpretation we favour fundamental plasma
radiation by mildly relativistic electrons trapped in a hot and
dense coronal loop above electron cyclotron maser emission. This
leads to densities and magnetic field strengths in the radio source
of n ~ 2*E<SUP>11</SUP> cm<SUP>-3</SUP> and B ~ 800 G. Quasi-periodic
pulsations during the decay phase of the event suggest a loop radius
of r ~ 7*E<SUP>8</SUP> cm. A filamentary corona is implied in which
the dense radio source is embedded in hot thin plasma with temperature
T>=2*E<SUP>7</SUP> K and density n_ext<=10<SUP>-2</SUP>n. Runaway
acceleration by sub-Dreicer electric fields in a magnetic loop is
found to supply a sufficient number of energetic electrons.
---------------------------------------------------------
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: Spectroscopic Signatures of a Flare Observed by SUMER
Onboard SOHO
Authors: Dammasch, I. E.; Curdt, W.; Kliem, B.; Dwivedi, B. N.;
Wilhelm, K.
2001IAUS..203..264D Altcode:
On 06 November 1999, SUMER (the Solar Ultraviolet Measurements of
Emitted Radiation spectrometer on SOHO) observed a post-flare site
above active region NOAA 8758 which was approaching the north-east
limb. SUMER recorded a time series taken with a constant slit position
and several spectral windows covering a wide temperature range (10 000 -
10 000 000 K), preceded and followed by contextual raster scans. During
this operation, a flare of size C4.6 occurred in the observed region,
also observed by the GOES 8 X-ray flux monitor, the SOHO/EIT imager
and the YOHKOH/SXT instrument. All data sets have been coaligned. The
temporal evolution seen in SUMER spectra is presented and compared
with the other observations.
---------------------------------------------------------
Title: Dynamic three-dimensional spontaneous reconnection in a
sheared current sheet
Authors: Kliem, B.; Schumacher, J.
2001sps..proc..264K Altcode:
We present one-fluid MHD simulations of threedimensional spontaneous
magnetic reconnection, triggered by the localized occurrence of
anomalous resistivity in a current sheet. The resistivity is coupled to
the dynamics of the fluid through a threshold criterion based on the
electron-ion drift velocity. A sheared current sheet with a uniform
guide field component is considered, which is relevant, e.g., to the
reconnection between newly emerging and preexisting flux on the Sun. A
guide field component (Bz) of intermediate strength (comparable to the
antiparallel component Bx) leads to efficient coupling and expansion of
the reconnection process along the z direction and enables an evolution
towards a macroscopic reconnection regime also in the present case,
where the initial perturbation is of microscopic scale in all three
dimensions. However, the reconnection rate decreases rapidly for
increasing Bz/Bx >∼ 2. The splitting of the central X line
(so-called secondary tearing) results in the spontaneous formation
of interlinked flux tubes, which occur in this system in place of
plasmoids. The interaction between the interlinked flux tubes leads
to high and variable electric fields but also hinders the complete
reconnection of the flux systems, at least temporarily.
---------------------------------------------------------
Title: Solar flare radio pulsations as a signature of dynamic
magnetic reconnection
Authors: Kliem, B.; Karlický, M.; Benz, A. O.
2000A&A...360..715K Altcode: 2000astro.ph..6324K
Decimetric radio observations of the impulsive solar flare on October 5,
1992, 09:25 UT show a long series of quasi-periodic pulsations deeply
modulating a continuum in the 0.6-2 GHz range that is slowly drifting
toward lower frequencies. We propose a model in which the pulsations
of the radio flux are caused by quasi-periodic particle acceleration
episodes that result from a dynamic phase of magnetic reconnection in
a large-scale current sheet. The reconnection is dominated by repeated
formation and subsequent coalescence of magnetic islands (known as
"secondary tearing" or "impulsive bursty" regime of reconnection),
while a continuously growing plasmoid is fed by newly coalescing
islands. Such a model, involving a current sheet and a growing plasmoid,
is consistent with the Yohkoh observations of the same flare (Ohyama
& Shibata ?). We present two-dimensional MHD simulations of dynamic
magnetic reconnection that support the model. Within the framework of
the proposed interpretation, the radio observations reveal details of
plasmoid formation in flares.
---------------------------------------------------------
Title: Three-dimensional spontaneous magnetic reconnection in neutral
current sheets
Authors: Schumacher, Jörg; Kliem, Bernhard; Seehafer, Norbert
2000PhPl....7..108S Altcode: 1999astro.ph.12443S
Magnetic reconnection in an antiparallel uniform Harris current sheet
equilibrium, which is initially perturbed by a region of enhanced
resistivity limited in all three dimensions, is investigated through
compressible magnetohydrodynamic simulations. Variable resistivity,
coupled to the dynamics of the plasma by an electron-ion drift
velocity criterion, is used during the evolution. A phase of magnetic
reconnection amplifying with time and leading to eruptive energy release
is triggered only if the initial perturbation is strongly elongated
in the direction of current flow or if the threshold for the onset of
anomalous resistivity is significantly lower than in the corresponding
two-dimensional case. A Petschek-like configuration is then built up
for ~10<SUP>2</SUP> Alfvén times, but remains localized in the third
dimension. Subsequently, a change of topology to an O-line at the center
of the system (“secondary tearing”) occurs. This leads to enhanced and
time-variable reconnection, to a second pair of outflow jets directed
along the O-line, and to expansion of the reconnection process into
the third dimension. High parallel current density components are
created mainly near the region of enhanced resistivity.
---------------------------------------------------------
Title: Second-Harmonic Plasma Radiation of Magnetically Trapped
Electrons in Stellar Coronae
Authors: Stepanov, A. V.; Kliem, B.; Krüger, A.; Hildebrandt, J.;
Garaimov, V. I.
1999ApJ...524..961S Altcode:
Plasma radiation near the second harmonic of the plasma frequency
driven by the loss-cone instability of magnetically trapped
energetic electrons in stellar coronae is considered. Growth rates
of longitudinal waves near the upper hybrid frequency are determined
for warm background plasma and sufficiently high plasma densities,
ω<SUB>p</SUB>>ω<SUB>c</SUB>, where the electrostatic instability
prevails over the electromagnetic cyclotron maser instability, with
particular attention given to the intermediate magnetic field condition,
1<ω<SUP>2</SUP><SUB>p</SUB>/ω<SUP>2</SUP><SUB>c</SUB><~5. The
plasma turbulence level and the brightness temperature of the
second-harmonic plasma radiation arising from the coalescence of
upper hybrid waves are estimated. The brightness temperature can
reach ~10<SUP>14</SUP> K for spontaneous conversion of the waves and
~10<SUP>16</SUP> K for induced conversion. The radiation pattern of
the second-harmonic plasma emission is also calculated; it shows a
prevalence of the extraordinary mode. Analyzing the problem of the
escape of radiation from stellar coronae, it is found that the escape
window is wider for the o-mode because the x-mode radiation is strongly
absorbed by the warm background plasma at the low harmonic gyrolevels,
and thus the observed radiation can be polarized in the ordinary sense
in the intermediate magnetic field case. Because of the high temperature
of the plasma in the coronae of X-ray-emitting stars, the characteristic
length scale of the wave conversion and the efficiency of the plasma
radiation mechanism can be much higher than on the Sun. The results
are discussed in the context of nonthermal quiescent and flare radio
emission from active stars.
---------------------------------------------------------
Title: The evolution of solar bipolar jets observed in the UV by
SUMER on SOHO.
Authors: Mendoza-Torres, J. E.; Wilhelm, K.; Innes, D. E.; Curd, W.;
Kliem, B.; Brekke, P.
1998larm.confE..17M Altcode:
Observations of bipolar jets made in the UV by SUMER onboard SOHO
are presented. The observations were carried out in the interval
74.9-79.0 nm which contains some chromospheric and transition region
lines. The theory predicts that under the conditions of the solar
chromosphere the reconnection of magnetic lines accelerates plasma
into two oppositely directed jets. In the chromosphere, as well as in
other solar layers, different phenomena, where plasma acceleration
take place, are observed. Of them the UV jets, observed by SUMER,
represent one of the most clear evidences of the link between magnetic
reconnection and plasma acceleration. The jets are clearly revealed
by a considerably intensity increase and large doppler shifts of the
line wings. It should be noted that the enhancement of only one wing
also occurs and that the central component of the line also undergoes
velocity shifts and amplitude enhancement. The behavior of the wings
found in this work is consistent with results obtained for other events
(Innes et al. 1997) namely: 1)The wings are observed during few minutes
2) The doppler velocities of the line wings (red and blue) are similar
to each other and typical values are of the order of 100 km/sec, 3) In
general, the blue component is more long-lived than the red one, 4)the
red component is, in general, more intense than the blue one. In this
work, an analysis of the velocities of the three spectral components
(the central and the wings) as a function of the location on the slit,
used for the observations, is also done. The ratios of two OV (density
sensitive) and two OIV (temperature sensitive) lines, whose wavelengths
are in the observed range, are also analyzed. An important result is
that these parameters show gradients along the slit. An analysis of
the spatial distributions and the correlations between them is done. A
discussion of these results, in relation with the appearance and the
evolution of the jets, will be done.
---------------------------------------------------------
Title: Wavelet Analysis of Solar Flare Hard X-Rays
Authors: Aschwanden, Markus J.; Kliem, Bernhard; Schwarz, Udo; Kurths,
Jürgen; Dennis, Brian R.; Schwartz, Richard A.
1998ApJ...505..941A Altcode:
We apply a multiresolution analysis to hard X-ray (HXR) time profiles
f(t) of solar flares. This method is based on a wavelet transform
(with triangle-shaped wavelets), which yields a dynamic decomposition
of the power at different timescales T, the scalogram P(T, t). For
stationary processes, time-averaged power coefficients, the scalegram
S(T), can be calculated. We develop an algorithm to transform these
(multiresolution) scalegrams S(T) into a standard distribution function
of physical timescales, N(T). We analyze 647 solar flares observed with
the Compton Gamma Ray Observatory (CGRO), recorded at energies >=25
keV with a time resolution of 64 ms over 4 minutes in each flare. The
main findings of our wavelet analysis are: <P />1. In strong flares,
the shortest detected timescales are found in the range T<SUB>min</SUB>
~ 0.1-0.7 s. These minimum timescales are found to correlate with
the flare loop size r (measured from Yohkoh images in 46 flares),
according to the relation T<SUB>min</SUB>(r) ~ 0.5(r/10<SUP>9</SUP>
cm) s. Moreover, these minimum timescales are subject to a cutoff,
T<SUB>min</SUB>(n<SUB>e</SUB>) >~ T<SUB>Defl</SUB>(n<SUB>e</SUB>),
which corresponds to the electron collisional deflection time at the
loss-cone site of the flare loops (inferred from energy-dependent
time delays in CGRO data). <P />2. In smoothly varying flares, the
shortest detected timescales are found in the range T<SUB>min</SUB> ~
0.5-5 s. Because these smoothly varying flares exhibit also large trap
delays, the lack of detected fine structure is likely to be caused
by the convolution with trapping times. <P />3. In weak flares, the
shortest detected timescales cover a large range, T<SUB>min</SUB> ~
0.5-50 s, mostly affected by Poisson noise. <P />4. The scalegrams S(T)
show a power-law behavior with slopes of β<SUB>max</SUB> ~ 1.5-3.2
(for strong flares) over the timescale range of [T<SUB>min</SUB>,
T<SUB>peak</SUB>]. Dominant peaks in the timescale distribution N(T)
are found in the range T<SUB>peak</SUB> ~ 0.5-10<SUP>2</SUP> s, often
coinciding with the upper cutoff of N(T). These observational results
indicate that the fastest significant HXR time structures detected
with wavelets (in strong flares) are related to physical parameters
of propagation and collision processes. If the minimum timescale
T<SUB>min</SUB> is associated with an Alfvénic crossing time through
elementary acceleration cells, we obtain sizes of r<SUB>acc</SUB>
~ 75-750 km, which have a scale-invariant ratio r<SUB>acc</SUB>/r ~
0.03 to flare loops and are consistent with cell sizes inferred from
the frequency bandwidth of decimetric millisecond spikes.
---------------------------------------------------------
Title: Plasma jets in the solar atmosphere observed in EUV emission
lines by SUMER on SOHO
Authors: Wilhelm, K.; Innes, E. E.; Curdt, W.; Kliem, B.; Brekke, P.
1998ESASP.421..103W Altcode: 1998sjcp.conf..103W
No abstract at ADS
---------------------------------------------------------
Title: Coronal magnetic-field changes of a strong, very fast
developing compact solar active region and related processes
Authors: Shibasaki, K.; Kosugi, T.; Bogod, V.; Garaimov, V.; Gelfreikh,
G.; Stepanov, A. V.; Kliem, B.; Hildebrandt, J.; Kruger, A.; Hofmann,
A.; Urpo, S.
1998cee..workE..49S Altcode:
We have studied the development of the large active region NOAA AR
7321, which suddenly appeared near the central meridian of the Sun,
during the first days of its remarkably fast evolution. To analyse the
structural and evolutional features of the magnetosphere of the active
region spatially resolved spectral-polarization observations of the
large Radioheliograph at Nobeyama (Japan), the RATAN-600 telescope at
Zelenchuk (Russia), and mm-wave data of the Metsahovi Radio Research
Station (Finland) have been used and compared with model calculations
of the radio emission. Our main goals concern the evolution of the
magnetic scale height above the sunspots centre deduced from the
gyromagnetic emission and its relation to the flare activity, as well
as the nature of the energy release responsible for the source of
long-duration coronal mm-wave radiation.
---------------------------------------------------------
Title: On the SORS project of CORONAS I and F
Authors: Fomichev, V. V.; Oraevsky, V. N.; Pulinets, S. A.; Prutensky,
I. S.; Gorgutsa, R. V.; Klos, Z.; Kiraga, A.; Rothkaehl, H.; Kruger,
A.; Hildebrandt, J.; Aurass, H.; Klassen, A.; Mann, G.; Kliem, B.;
Estel, C.
1998cee..workE..48F Altcode:
The scientific experiment SOLAR RADIO SPECTROMETER (abbreviated: SORS)
of the CORONAS-I satellite project is described and prospects to the
planned mission of CORONAS-F are given. The astrophysical goal of these
experiments is the investigation of solar radio emission in a wide
frequency band detecting low-frequency solar flare emission outside the
terrestrial radio window and its physical interpretation. The principal
capacity of SORS could be demonstrated by observation of dynamic spectra
of type III bursts during a relatively short operational period aboard
CORONAS-I restricted by low solar activity. Using the experience of
CORONAS-I it can be anticipated that a new SORS instrument on CORONAS-F
will deliver a broader material for the exploration of coronal and
heliospheric plasma processes.
---------------------------------------------------------
Title: Test particle acceleration in 2D current sheets undergoing
dynamical phases of reconnection
Authors: Kliem, B.; Schumacher, J.; Shklyar, D. R.
1998AdSpR..21..563K Altcode:
Calculations of collision-free test particle orbits in a dynamic,
filamentary current sheet are presented. The electro-magnetic fields
are obtained from a two-dimensional MHD simulation; thus this study is
restricted to energization in perpendicular electric fields. Starting
from a multiple X-line initial perturbation, the current sheet undergoes
complex dynamics, comprising a tearing-like phase, multiple island
coalescence and plasmoid formation, and finally the acceleration and
ejection of the plasmoid. The dynamic, multiple X-line configuration
possessses two important properties supporting strong particle
acceleration: efficient particle confinement which permits multiple or
long-lasting acceleration phases, and appropriate asymmetries of the
average electric field about the O-lines, which permits acceleration
of the majority of test particles. Dynamic current sheets may thus be
responsible for the efficient bulk acceleration of particles observed
in the impulsive phase of solar flares. For parameters of the lower
solar corona, sub-second acceleration times and maximum energies in
the 10^7-10^8 eV range are obtained.
---------------------------------------------------------
Title: On the source height of decimetre-wave burst emission
(abstract)
Authors: Krüger, A.; Kliem, B.; Hildebrandt, J.; Karlický, M.;
Zlobec, P.
1998PAICz..88...93K Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Test Particle Acceleration in 2d Current Sheets Undergoing
Dynamical Phases of Reconnection
Authors: Kliem, B.; Schumacher, J.; Shklyar, D. R.
1998paac.conf..563K Altcode:
No abstract at ADS
---------------------------------------------------------
Title: An Attempt to Classify Solar Microwave-Bursts by Source
Localization Characteristics and Dynamics of Flare-Energy Release
Authors: Kruger, A.; Kliem, B.; Hildebrandt, J.; Nefedev, V. P.;
Agalakov, B. V.; Smolkov, G. Ya.
1998IAUS..188..205K Altcode:
An overview of spatially resolved observations of solar radio bursts
obtained by the Siberian Solar Radio Telescope at 5.8 GHz during the
last ten years reveals the occurrence of different classes of burst
emission defined by their source characteristics. Four major classes
of bursts according to the source position relative to sunspots, the
source size and structure, and the source height, could be tentatively
distinguished and compared with burst spectral characteristics as well
as with soft X-ray emission oberved by YOHKOH. Possible consequences
of these findings on the understanding of the underlying flare process
are briefly discussed.
---------------------------------------------------------
Title: On the double plasma resonance effect in solar and stellar
coronae
Authors: Kliem, B.; Stepanov, A.; Kruger, A.
1998cee..workE..50K Altcode:
The double plasma resonance (DPR) implies the equality of a plasma
eigen mode near the upper hybrid frequency and a harmonic of the
electron gyrofrequency. Under the DPR-condition, the growth rate of
a plasma instability can exceed the growth rate of the nonresonant
case by more than an order of magnitude. This effect has been used to
interpret some fine structures of solar radio bursts, in particular the
Zebra pattern (Rosenberg, 1972; Zheleznyakov & Zlotnik, 1975). We
present calculations on the DPR of the upper hybrid mode instability
in a two-component plasma (warm background and energetic loss cone
component) that are based on the rigorous dispersion equation derived
by Karpman et al. (1975) and take the background plasma temperature
into account. We find that the DPR under the intermediate magnetic
field condition, where the upper hybrid frequency coincides with one
of the low harmonics of the electron gyrofrequency, requires high
temperature ratios, T_h / T_0 ga 100. Such ratios are not expected
to occur in the coronae of red dwarfs and close binaries like RS CVn
and Algol where the background component is already rather hot, T_0
= 10^7-10^8K. Hence, the generation of Zebra patterns in the radio
emission of these objects appears improbable. The DPR effect is more
important for the solar corona under the weak magnetic field condition,
omega_p ga 10 omega_c and less hot background plasma in a flare loop.
---------------------------------------------------------
Title: Logistic Avalanche Processes, Elementary Time Structures,
Frequency Distributions, and Wavelet Analysis of Solar Flares
Authors: Aschwanden, M. J.; Dennis, B. R.; Schwartz, R. A.; Benz,
A. O.; Kliem, B.; Schwarz, U.; Kurths, J.
1998cee..workE..10A Altcode:
Do elementary time scales exist in solar flares that could provide
us a clue on the spatial fragmentation of the primary energy
release process? Or is there a continuous distribution, composed
of a hierarchy of unresolved time structures? We present results
of two recent studies on these fundamental questions. We applied a
multi-resolution analysis (using triangle-shaped wavelet transforms)
to 647 solar flares observed with the COMPTON Gamma Ray Observatory
(CGRO) at hard X-ray energies of >= 25 keV with a time resolution
of 64 ms. From the wavelet scalegrams we infer a distribution of time
scales N(T) for each flare and find a cutoff for the shortest detected
time scales T<SUB>min</SUB> that is independent of the Poisson noise for
strong flares. These shortest time scales T<SUB>min</SUB> are found to
correlate with the flare loop radius r (Fig.1), i.e. T<SUB>min</SUB> =
0.5 (r/10^9 cm)s (measured with Yohkoh) and the collisional deflection
time, T<SUB>min</SUB> > ~t<SUP>Defl</SUP>(n_e), (determined from the
electron density n_e of trapped electrons by measuring energy-dependent
time delays detected with CGRO). From these observations we infer
spatial sizes of r<SUB>acc</SUB> = 75-750 km for elementary acceleration
cells. In a second study we determined the frequency distributions
of elementary time structures in over 600 flares, based on some
10^4 hard X-ray pulses at 25 and 50 keV, 4000 radio type III bursts,
4000 decimetric quasi-periodic broadband pulsation events, and 10^4
decimetric millisecond spike events. All elementary time structures
have a quasi-Gaussian shape and can be modeled with the logistic
equation, which describes the exponential growth phase and nonlinear
saturation (caused by the limited amount of available free energy) of a
general instability. We derive a theoretical description of frequency
distributions in terms of this logistic avalanche model and find that
the power-law slope of observed frequency distributions provides a
powerful diagnostic on coherent versus incoherent instabilities.
---------------------------------------------------------
Title: Multiresolution analysis of solar mm-wave bursts
Authors: Schwarz, U.; Kurths, J.; Kliem, B.; Kruger, A.; Urpo, S.
1998A&AS..127..309S Altcode:
Two methods of multi-scale time series analysis are applied to solar
mm-wavelength flux time profiles in order to assess the diagnostic
power of these tools for the exploration of nonlinear energy release
processes. Both the multiresolution analysis (MRA), a method based
on the wavelet transform, and the structure function analysis (SFA)
permit the treatment of non-stationary time series. In addition,
the MRA offers a local decomposition of the scaling behavior of the
flux variations. Our main emphasis is directed at a decomposition of
the contributions of the different time scales to the overall flux
profile. The methods yield consistent values of the “spectral index"
which describes the scalings contained in the time series. We find
that time profiles of bursts are qualitatively analogous to fractional
Brownian motion (fBm), possessing long-range temporal correlations. Such
correlations are not found in quiet Sun observations. The MRA of six
solar mm-wave bursts shows that the radio flux is always composed of
contributions from a broad range of time scales. Also during the main
phase of bursts, which appears to be structurally analogous to the pre-
and post-burst phases at a resolution limit of 1 s, flux fluctuations
are enhanced in a broad range of time scales. This suggests that the
mm-wave bursts are composed of unresolved elements, just as the pre-
and post-burst time profiles. The underlying energy release thus appears
to be fragmentary. These results are discussed in terms of the avalanche
model and plasma physical models for solar energy release events.
---------------------------------------------------------
Title: Plasma radiation of power-law electrons in magnetic loops:
application to solar decimeter-wave continua
Authors: Zaitsev, V. V.; Kruger, A.; Hildebrandt, J.; Kliem, B.
1997A&A...328..390Z Altcode:
It is shown that flare-produced fast electrons with a power-law
energy distribution trapped in magnetic loops are capable to produce
plasma waves at the upper hybrid frequency due to a loss-cone
instability. Conditions for instability are obtained in dependence
of the parameters of the power-law distribution and the magnetic-loop
parameters. The growth rate of the plasma waves has been calculated and
their energy density has been estimated in the frame of quasi-linear
saturation. This instability has been considered as the cause of
the solar decimetric continuum which exhibits a strong temporal and
spatial correlation with regions of flare-energy release and sources
of microwave bursts. The strong absorption of the first harmonic in
the decimeter range and also the peculiarities of the conversion of
the plasma waves into electromagnetic waves yield a preference of
the generation of the decimetric continuum at the second harmonic of
the plasma frequency. In this case the polarization of the radiation
corresponds to the ordinary wave mode in a wide cone of propagation
angles around the direction perpendicular to the magnetic field.
---------------------------------------------------------
Title: Electrostatic Wave Instability and Plasma Radiation from
Coronal Loops
Authors: Stepanov, A. V.; Kliem, B.; Krüger, A.; Hildebrandt, J.
1997SoPh..176..147S Altcode:
Polarization properties of solar and stellar radio emission require,
in some cases, emission below the third or fourth coronal electron gyro
level, ω < 3,ω_c; 4, ω_c. In the context of plasma radiation, the
source parameters should be such that the `intermediate magnetic field
condition' 1 < ω<SUB>p</SUB><SUP>2</SUP>/ω<SUB>c</SUB><SUP>2</SUP>
< 3 is satisfied. Supposing this condition, we investigate the
generation of electrostatic waves in a `warm' background plasma
with a high-energy component of magnetically trapped electrons. We
invoke the conversion of upper-hybrid waves and Bernstein waves into
electromagnetic radiation as being responsible for intense radio
emission from a coronal magnetic loop. Moreover, `odd-half' harmonic
emissions in the solar radio spectrum as well as the o-mode polarization
at the second harmonic of the plasma frequency are natural consequence
of this proposed model.
---------------------------------------------------------
Title: Particle Acceleration by Magnetic Reconnection in Filamentary
Current Sheets
Authors: Kliem, B.
1997ICRC....1..149K Altcode: 1997ICRC...25a.149K
No abstract at ADS
---------------------------------------------------------
Title: Coronal Energy Release and Magnetic Fields at Low Solar
Activity
Authors: Krüger, A.; Hildebrandt, J.; Kliem, B.; Hofmann, A.; Nefedev,
V. P.; Agalakov, A. B.; Smolkov, G. Ya.
1997ESASP.404..469K Altcode: 1997cswn.conf..469K
No abstract at ADS
---------------------------------------------------------
Title: Transient fast reconnection in dynamic current sheets with
anomalous resistivity
Authors: Schumacher, J.; Kliem, B.
1997AdSpR..19.1797S Altcode:
Two-dimensional compressible magnetohydrodynamic simulations of current
sheet dynamics under the influence of multiple anomalous resistivity
areas and slight asymmetries are presented. Following induced tearing
and multiple coalescence, a plasmoid is formed and accelerated. Dominant
X-points drive the dynamical evolution and lead to transient occurrence
of a Petschek-like reconnection geometry. The dependence of current
density extrema, plasmoid bulk velocity and maximum reconnection rate
on the Lundquist number is examined.
---------------------------------------------------------
Title: On the Origin of Decimetric-Wave Continuum of Solar Flares
Authors: Zaitsev, V. V.; Krüger, A.; Hildebrandt, J.; Kliem, B.
1997pre4.conf..453Z Altcode:
It is shown that flare-produced fast electrons with a power-law energy
distribution trapped in magnetic loops are capable to produce plasma
waves at the upper hybrid frequency due to a loss-cone instability. This
instability has been considered as the cause of the solar decimetric
continuum which exhibits a strong temporal and spatial correlation with
regions of flare-energy release and sources of microwave bursts. The
strong absorption of the first harmonic in the decimeter range and
also the peculiarities of the conversion of the plasma waves into
electromagnetic waves yield a preference of the generation of the
decimetric continuum at the second harmonic of the plasma frequency. In
this case the polarization of the radiation corresponds to the ordinary
wave mode in a wide cone of propagation angles around the direction
perpendicular to the magnetic field.
---------------------------------------------------------
Title: Plasma Radiation and Polarization of Stellar Radio Emission
Authors: Stepanov, A. V.; Hildebrandt, J.; Kliem, B.; Kruger, A.
1996ASPC...93..333S Altcode: 1996ress.conf..333S
No abstract at ADS
---------------------------------------------------------
Title: VLA Observations of Decimetric Microbuzsts in the Solar Corona
Authors: Kliem, B.; Bastian, T. S.
1996ASPC...93..372K Altcode: 1996ress.conf..372K
No abstract at ADS
---------------------------------------------------------
Title: MHD Simulations of Coronal Current Sheet Dynamics
Authors: Kliem, B.; Schumacher, J.
1996mpsa.conf..305K Altcode: 1996IAUCo.153..305K
No abstract at ADS
---------------------------------------------------------
Title: Book-Review - Computational Plasma Physics - with Applications
to Fusion and Astrophysics
Authors: Tajima, T.; Kliem, B.
1995AN....316..195T Altcode:
No abstract at ADS
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Title: VLA observations of decimetric microbursts in the solar corona.
Authors: Kliem, B.
1995AGAb...11..118K Altcode:
No abstract at ADS
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Title: Coupled Magnetohydrodynamic and Kinetic Development of Current
Sheets in the Solar Corona
Authors: Kliem, B.
1995LNP...444...93K Altcode: 1995cmer.conf...93K
This review considers (1) the stabilization of current sheets by shear
flows, (2) the dynamical behaviour of coronal current sheets due to
the interaction of magnetohydrodynamic (i.e., tearing and coalescence)
and kinetic currentdriven instabilities, (3) the resulting spatial
and temporal scales of energy release, (4) the acceleration of
particles in such an environment. The dynamical current sheet model
naturally explains fragmentary energy release, and it may lead to a
better understanding of particle acceleration and of the cross-field
propagation of the flare disturbance.
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Title: MHD simulations of coronal current sheet dynamics.
Authors: Schumacher, J.; Kliem, B.
1995AGAb...11..322S Altcode:
No abstract at ADS
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Title: Microwave Burst Timescales and Solar Flare Acceleration
Processes
Authors: Krueger, A.; Kliem, B.; Hildebrandt, J.; Zaitsev, V. V.
1994ApJS...90..683K Altcode: 1994IAUCo.142..683K
The occurrence of quite different timescales in solar microwave bursts
is considered and possible ways of their physical interpretation are
discussed. An interesting feature is the existence of hierarchic time
structures, an example of which is provided by the solar event of 1991
March 23. Plasma parameter sets to be invoked for the interpretation
of timescales are estimated on the base of different models such as the
collisionless conduction front model. The twisted magnetic loop model,
the coalescence model, and the electric circuit model. With emphasis
on the interpretation of burst fine structures with timescales of a
few seconds the coalescence model has been favored. On the other hand,
the simultaneous occurrence of a large range of different timescales
appears well suited to be described by the circuit model.
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Title: Particle Orbits, Trapping, and Acceleration in a Filamentary
Current Sheet Model
Authors: Kliem, Bernhard
1994ApJS...90..719K Altcode: 1994IAUCo.142..719K
Test particle orbits in the two-dimensional Fadeev equilibrium
with a perpendicular electric field added are analyzed to show that
impulsive bursty reconnection, which has been proposed as a model
for fragmentary energy release in solar flares, may account also for
particle acceleration to (near) relativistic energies within a fraction
of a second. The convective electric field connected with magnetic
island dynamics can play an important role in the acceleration process.
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Title: Particle acceleration in a filamentary current sheet model.
Authors: Kliem, Bernhard
1993ppcn.conf..223K Altcode:
Considering the observational indications that the energy release
and particle acceleration in solar flares occur in a fragmentary
manner as an important clue to the underlying plasma processes, the
behaviour of test particles in a filamentary current sheet, which
had earlier been proposed as a model for fragmentary energy release,
is investigated. For simplicity, an analytic current sheet model with
antiparallel external fields is used and a perpendicular electric
field is added. By considering test particle orbits in detail it
is demonstrated that the perpendicular electric field can accelerate
particles efficiently and that the multiple X-line configuration permits
higher particle energies than a single X-line configuration for similar
plasma parameters. Taking typical plasma parameters of impulsive flare
regions in the lower solar corona, many of the observed particle
charcteristics can be reproduced; on the other hand, the relevance
of the considered process to particle acceleration in galactic and
extragalactic plasmas requires further study.
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Title: U-shaped multiband type II radio burst from a behind-limb
solar flare
Authors: Kliem, B.; Kruger, A.; Urbars, H. W.; Fomichev, R. V.;
Gorgutsa, V. V.; Chertok, I. M.
1993AZh....70..188K Altcode:
Spectral characteristics of an unusual type II radio burst from a flare
site far behind (130 deg W) the solar limb are analyzed. The type II
burst consists of three or four simultaneous analogous U-shaped bands
with a slow variation of the frequency drift sign. It is proposed
that the form of the dynamic spectrum corresponds to the generation
of type II burst emission from the interaction of a blast shock wave
with a dense large-scale arch structure of a loop-shaped coronal
transient. Analysis of different variants of an interpretation of the
spectral band structure points to the possibility of the generation
of type II burst emission at higher harmonics (n is geater than 2)
of the local plasma frequency.
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Title: Solar Coronal Magnetic Fields
Authors: Hildebrandt, J.; Kliem, B.; Kruger, A.
1993IAUS..157...59H Altcode:
No abstract at ADS
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Title: A U-shaped, multiband, type II radio burst from a solar flare
beyond the limb
Authors: Kliem, B.; Kruger, A.; Urbarz, H. W.; Gorgutsa, R. V.;
Fomichev, V. V.; Chertok, I. M.
1993ARep...37...99K Altcode: 1993SvA....37...99K
No abstract at ADS
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Title: Fiber Bursts in Type-Iv DM Radio Continua as a Signature of
Coronal Current Sheet Dynamics
Authors: Aurass, H.; Kliem, B.
1992SoPh..141..371A Altcode:
In the late phase of some complex flare events which produce type IV
radio emission, a narrow-band decimetric component with generally a
high abundance of spectral fine structures is observed. We identify
this late dm continuum as a hypothetical `switch-off' signature of the
equivalent two-ribbon flare current system. The quasi-periodic and
highly dynamic fiber-burst pattern studied in the February 5, 1986
example is understood as radio evidence for a final reactivation of
a cyclic sequence of instabilities in the driven reconnecting current
sheet below the rising prominence.
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Title: Third Plasma Harmonic Radiation in Type-II Bursts
Authors: Kliem, B.; Krueger, A.; Treumann, R. A.
1992SoPh..140..149K Altcode:
We present observational evidence for simultaneous fundamental,
second and third harmonic radio emission during an excessively strong
type II burst on February 16, 1984. This burst was emitted from an
active region behind the limb allowing for fair resolution of the wave
bands. If interpreted as a triple harmonic system, three different,
nearly equally probable mechanisms for higher harmonic emission are
qualitatively discussed. These are a four-wave process which involves
very strong Langmuir waves, a decay process first proposed by Cairns
(1987) for higher harmonic emission near the Earth's bow shock, and time
evolution of the emitted frequency during Langmuir wave collapse. In
sufficiently strong coronal shock waves, both of the former mechanisms
may be more efficient than under solar wind conditions. In the third
mechanism, Langmuir wave collapse may be driven by strong electron
beams as are expected to exist in quasiparallel shocks where electron
reflection may be strongest. We discuss the differences between the
signatures of these mechanisms.
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Title: Particle orbits and acceleration in a filamentary current
sheet model
Authors: Kliem, B.
1992sws..coll..469K Altcode:
Test particle orbits in the 2D Fadeev equilibrium with a perpendicular
electric field added are analyzed to show that impulsive bursty
reconnection, which has been proposed as a model for fragmentary energy
release in solar flares, may account also for particle acceleration
to (near) relativistic energies within a fraction of a second. The
convective electric field connected with magnetic island dynamics can
play an important role in the acceleration process.
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Title: Erratum: Microwave Characteristics of a behind-the-Limb
Proton Flare
Authors: Krüger, A.; Hildebrandt, J.; Kliem, B.; Aurass, H.;
Kurths, J.
1991SoPh..135..423K Altcode:
No abstract at ADS
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Title: Microwave characteristics of a behind-the-limb proton flare
Authors: Krüger, A.; Hildebrandt, J.; Kliem, B.; Aurass, H.; Kurths,
J.; Fomichev, V. V.; Chertok, I. M.; Křivský, L.
1991SoPh..134..171K Altcode:
Behind-the-limb flares provide a unique opportunity for the study
of vertical source structures of microwave bursts and dynamic
flare processes. Based on complex observational data related to the
outstanding solar proton event on 16 February, 1984, the development
of burst emission at a height z ≥ 200000 km above the photosphere
has been investigated. A comparison with the associated X-ray emission
measured aboard various spacecraft yields a time lag of about ≤ 1
min between the onset of the unocculted impulsive HXR-emission and
the onsets of the X-ray and microwave emissions occulted by the solar
limb. The lag corresponds to a range of speeds of the propagation of
the flare volume of about 3000-5000 km s<SUP>−1</SUP>. Considering
competing transport agents that could account for such expansion of
the source volume, a qualitative model of shock-wave activation of
loops successively reaching into larger coronal heights is proposed.
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Title: Third plasma harmonic radiation in type 2 bursts
Authors: Kliem, B.; Krueger, A.; Treumann, R. A.
1991STIN...9212982K Altcode:
Some observational evidence for simultaneous fundamental, second,
and third harmonic radio emission during an excessively strong type II
burst on February 16, 1984 is presented. This burst was emitted from an
active region behind the limb allowing for good resolution of the wave
bands. Three different nearly equally probable mechanisms for higher
harmonic emission are qualitatively discussed, however, preferred
choice is given either to a four wave process which involves very
strong Langmuir waves and a decay process for higher harmonic emission
near the Earth's bow shock or Langmuir wave collapse. In sufficiently
strong coronal shock waves both mechanisms may even be more efficient
than under solar wind conditions. Langmuir wave collapse may be driven
by strong electron beams. Observations like the present one should open
additional diagnostic opportunities and should stimulate further search
for higher than second harmonic emission in strong solar radio bursts.
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Title: Plasma Heating by Current Sheets in Solar Active Regions
Authors: Kliem, B.; Seehafer, N.
1991mcch.conf..564K Altcode:
No abstract at ADS
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Title: Fragmentary energy release due to tearing and coalescence in
coronal current sheets
Authors: Kliem, B.
1990AN....311..399K Altcode:
No abstract at ADS
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Title: Book-Review - Laboratory and Space Plasmas
Authors: Kikuchi, H.; Kliem, B.
1990AN....311..423K Altcode:
No abstract at ADS
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Title: Plasma heating by current sheets in solar active regions.
Authors: Kliem, B.; Seehafer, N.
1990AGAb....5...40K Altcode:
No abstract at ADS
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Title: A survey of the peculiar radio emission of the solar
behind-limb event on 16th February 1984
Authors: Chertok, I. M.; Fomichev, V. V.; Gorgutsa, R. V.; Markeev,
A. K.; Podstrigach, T. S.; Aurass, H.; Hildebrandt, J.; Kliem, B.;
Krüger, A.; Kurths, J.; Karlický, M.; Tlamicha, A.; Urbarz, H. W.;
Zlobec, P.
1990AN....311...55C Altcode:
This paper presents radio emission measurements, taken from different
European stations, of the February 16th, 1984 solar behind-limb burst,
which started in the active region AR 4408 at a longitude of about 130
deg W. The event was related to a very strong relativistic particle
emission, the presence of cm-wave radiation from altitudes above
200,000 km above the photosphere, and an unusual U-shaped type II burst
pattern consisting of four simultaneous emission bands. The spectrum
in the cm-dm range exhibited different stages of evolution related
to the burst onset, the burst maximum, and the postburst increase. It
appears possible that, at dm-waves during the burst maximum, a partial
overlapping of plasma radiation with gyrosynchroton radiation governing
the cm-wave range took place.
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Title: Higher Harmonic Plasma Radiation in Solar Type-II Radio Bursts
Authors: Fomichev, V. V.; Chertok, I. M.; Gorgutsa, R. V.; Markeev,
A. K.; Kliem, B.; Aurass, H.; Kruger, A.; Kurts, J.; Urbarz, H.
1990IAUS..142..517F Altcode:
No abstract at ADS
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Title: Source heights of radio emission derived from behind-limb
flares.
Authors: Krüger, A.; Hildebrandt, J.; Kliem, B.
1990AGAb....5...36K Altcode:
No abstract at ADS
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Title: Book-Review - Instabilities in Space and Laboratory Plasmas
Authors: Melrose, D. B.; Kliem, B.
1990AN....311...40M Altcode:
No abstract at ADS
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Title: Book-Review - Instabilities in Space and Laboratory Plasmas
Authors: Melrose, D. B.; Kliem, B.
1989AN....310..434M Altcode:
No abstract at ADS
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Title: Time Scales of Solar Microwave Bursts and Scenarios of Flare
Energy Release
Authors: Krueger, A.; Kliem, B.; Hildebrandt, J.
1989ESASP.285..169K Altcode: 1989rsp..conf..169K
Based on observational evidence that characteristic time scales of
different solar microwave burst types are distributed over a wide range
(0.001 to 10.000 sec), different mechanisms of energy release are
considered to account for the impulsive flux increase (time scale less
than 1000 sec). The coalescence instability is found to be a promising
candidate to combine sufficiently short time scales with substantial
energy release.
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Title: Dynamic Development of Coronal Current Sheets
Authors: Kliem, B.
1989ESASP.285..117K Altcode: 1989rsp..conf..117K
The possible interplay of tearing, coalescence, and current-driven
kinetic instabilities in current sheets is considered on the
basis of order of magnitude estimates of instability thresholds
and scalings. Arguments are given that favor the development of
internal dynamics for current sheets with stationary inflow and
appropriate combination of parameters (the impulsive bursty regime of
reconnection). The model relates newly emerging flux to flare energy
release and provides the basis for an understanding of moving type 4
radio bursts associated with coronal mass ejections.
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Title: Examination of solar radio burst onsets in view of different
flare scenarios and preflare structures.
Authors: Krueger, A.; Aurass, H.; Kliem, B.; Urpo, S.
1987PAICz..66..245K Altcode: 1987eram....1..245K
The time profiles of the solar microwave emission exhibit various
phenomena reflecting the evolution of magnetic flux tubes before and
during the onset of flare events. Different scenarios are possible
to describe the processes of energy release in a flux tube and the
interaction of a number of tubes during the preflare stage and the
early flare development. Multi-peak structures at quite different time
scales displayed by flux records at mm-, cm-, and dm-waves are examined.
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Title: Book-Review - Plasma Astrophysics
Authors: Melrose, D. B.; Kliem, B.
1987AN....308...87M Altcode:
No abstract at ADS
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Title: Radiostrahlung durch Temperaturanisotropieinstabilität
elektronenzyklotronharmonischer Wellen in der Sonnenkorona
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Title: Radiostrahlung durch Temperaturanisotropieinstabilität
elektronenzyklotronharmonischer Wellen in der Sonnenkorona Title:
Radio emission by temperature anisotropy instability of electron
cyclotron harmonic waves in the solar corona;
Authors: Kliem, Bernhard
1987PhDT.......184K Altcode:
No abstract at ADS
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Title: Book-Review - Radiotelescopes - ED.2
Authors: Christiansen, W. N.; Hogbom, J. A.; Kliem, B.
1986AN....307..384C Altcode:
No abstract at ADS
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Title: A new plasma emission model for bright, slowly moving type
IV solar radio bursts
Authors: Kliem, B.
1986CoSka..15..709K Altcode:
As a model of bright, slowly moving type IV bursts, coalescence
of oblique Bernstein modes into predominantly o-mode waves is
proposed. Bernstein mode instability is supposed in an adiabatically
compressed plasma within the current sheet associated with the
underlying traveling coronal disturbance.
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Title: Small time-scale features of impulsive solar nm-bursts
Authors: Krüger, A.; Kliem, B.; Urpo, S.
1986CoSka..15..139K Altcode:
High time-resolution observations of the Metsähhovi Radio Research
Station of the University of Technology of Helsinki are investigated
with respect to an analysis of the impulsive phase of solar microwave
burst radiation. Results of an earlier statistical study (Urpo et al.,
1986) are used to discuss the steepness of the time profiles of the
radio flux (which was observed at 36.8 and 22.2 GHz with an angular
resolution of 2.4 and 4.0 arc min, respectively) as well as their
temporal fine structures, in the frames of current physical models.
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Title: Brightness temperature of radiation from the coupling of
oblique Bernstein modes in coronal plasma
Authors: Kliem, B.
1986CoSka..15..703K Altcode:
The probability for coalescence of electrostatic Bernstein modes
propagating obliquely to an ambient magnetic field and having random
phases is evaluated in a kinetic weak turbulence description. Using
knowledge of dispersion from linear theory and of the saturation
spectrum of the waves in a temperature anisotropy instability from
numerical simulation (Gitomer et al., 1972), the brightness temperature
of the emitted high-frequency electro-magnetic waves is calculated.
