Author name code: thalmann
ADS astronomy entries on 2022-09-14
author:"Thalmann, Julia K."
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Title: The Importance of Method Redundancy in Studying Pre-Eruption
Evolution in Solar Active Regions
Authors: Georgoulis, Manolis K.; Pariat, Etienne; Liu, Yang; Thalmann,
Julia K.
Bibcode: 2022cosp...44.1358G
Altcode:
In a recent synergistic work stemming from a prior International
Space Science Institute (ISSI) Working Group, the evolution of
magnetic helicity in an intensely eruptive solar active region was
studied using several different helicity calculation methods. This
was the first time all these methods were tested on real solar data,
without the possibility of a ground truth. Focusing on the pre-eruption
evolution prior to an eruptive X-class flare (SOL2006-12-13T02:14X3.4)
in NOAA active region (AR) 10930, we reveal a more complex picture than
what any single method might convey. Through imperfect but overall
converging calculations from different methods, we find artifacts
that could mislead conclusions. More importantly, we find evidence of
competing physical tendencies in the active region whose omission could
lead to counterintuitive, hence misleading, again, conclusions. While
for the Sun we have the capability to use different data and methods
for related purposes, this is not the case for other eruptive stars,
which is a fact calling for robust modeling approaches, relying
on scarce and indirect observations of stellar magnetic fields and
CME properties. Confluence of any data available and modeling could
offer the redundancy needed to critically assess partial findings
and reconcile them into a physically consistent picture of stellar
eruptions, quite possibly with qualitative / quantitative similarities
and differences from the eruptions of our own Sun.
Title: Magnetic helicity and energy budget around large confined
and eruptive solar flares
Authors: Gupta, Manu; Veronig, Astrid; Thalmann, Julia K.
Bibcode: 2022cosp...44.2424G
Altcode:
In order to better understand the underlying process and prerequisites
for solar activity, it is essential to study the time evolution of
the coronal magnetic field of solar active regions (ARs), which is
associated to flare activity and leads to large coronal mass ejections
(CMEs). We investigate the coronal magnetic energy and helicity budgets
of ten solar ARs around the times of large flares. In particular,
we are interested in a possible relation of the derived quantities
to the particular type of flares that the AR produces, i.e., whether
they are associated with a CME or are confined. Using an optimization
approach, we employed time series of 3D nonlinear force-free magnetic
field models for each target AR, covering a time span of several hours
around the time of occurrence of large solar flares (GOES class M1.0
and larger). We subsequently computed the 3D magnetic vector potentials
associated to the model 3D coronal magnetic field using a finite-volume
method. This allows us to correspondingly compute the coronal magnetic
energy and helicity budgets (so-called extensive quantities), as well as
related intensive proxies, such as the relative contribution of free
magnetic energy (the energy ratio), the fraction of non-potential
(current-carrying) helicity, and the normalized current-carrying
helicity. The extensive quantities of flare-productive ARs cover a
broad range of magnitudes, with no apparent relation to the potential
of an AR to produce a CME-associated flare. In contrast, we find
the intensive proxies (the energy ratio, the helicity ratio, and
the normalized current-carrying helicity) to be distinctly different
for ARs that produce CME-associated large flares compared to those
which produce confined flares. Thus, for the majority of ARs in our
sample, characteristic pre-flare levels of the intensive proxies allow
statements regarding the likelihood of subsequent CME-productivity.
Title: Probing the coronal magnetic field with physics informed
neural networks
Authors: Jarolim, Robert; Podladchikova, Tatiana; Veronig, Astrid;
Thalmann, Julia K.
Bibcode: 2022cosp...44.2463J
Altcode:
While the photospheric magnetic field of our Sun is routinely
measured, its extent into the upper solar atmosphere (the corona)
remains elusive. In this study, we present a novel approach for coronal
magnetic field extrapolation using physics informed neural networks. The
neural network is optimized to match observations of the photospheric
magnetic field vector at the bottom-boundary, while simultaneously
satisfying the force-free and divergence-free equations in the entire
simulation volume. We demonstrate that our method can account for
noisy data and deviates from the physical model where the force-free
magnetic field assumption cannot be satisfied. We utilize meta-learning
concepts to simulate the evolution of the active region 11158. Our
simulation of 5 days of observations at full cadence, requires less
than 13 hours of total computation time. The derived evolution of the
free magnetic energy and helicity in the active region, shows that
our model captures flare signatures, and that the depletion of free
magnetic energy spatially aligns with the observed EUV emission. Our
method provides the ability to perform magnetic field extrapolations
in quasi real-time, which can be used for space weather monitoring,
studying pre-eruptive structures and as initial condition for MHD
simulations. The flexibility in terms of data and the possibility of
extending the underlying physical model, offers great potential for
the field of magnetic field simulations.
Title: The effect of spatial sampling on magnetic field modeling
and helicity computation
Authors: Thalmann, J. K.; Gupta, M.; Veronig, A. M.
Bibcode: 2022A&A...662A...3T
Altcode: 2022arXiv220409267T
Context. Nonlinear force-free (NLFF) modeling is regularly used to
indirectly infer the 3D geometry of the coronal magnetic field,
which is not otherwise accessible on a regular basis by means of
direct measurements.
Aims: We study the effect of binning in
time-series NLFF modeling of individual active regions (ARs) in order to
quantify the effect of a different underlying spatial sampling on the
quality of modeling as well as on the derived physical parameters.
Methods: We apply an optimization method to sequences of Solar
Dynamics Observatory (SDO) Helioseismic and Magnetic Imager (HMI) vector
magnetogram data at three different plate scales for three solar active
regions to obtain nine NLFF model time series. From the NLFF models,
we deduce active-region magnetic fluxes, electric currents, magnetic
energies, and relative helicities, and analyze those with respect
to the underlying spatial sampling. We calculate various metrics to
quantify the quality of the derived NLFF models and apply a Helmholtz
decomposition to characterize solenoidal errors.
Results: At
a given spatial sampling, the quality of NLFF modeling is different
for different ARs, and the quality varies along the individual model
time series. For a given AR, modeling at a certain spatial sampling is
not necessarily of superior quality compared to that performed with a
different plate scale. Generally, the NLFF model quality tends to be
higher for larger pixel sizes with the solenoidal quality being the
ultimate cause for systematic variations in model-deduced physical
quantities.
Conclusions: Optimization-based modeling using
SDO/HMI vector data binned to larger pixel sizes yields variations
in magnetic energy and helicity estimates of ≲30% on overall,
given that concise checks ensure the physical plausibility and high
solenoidal quality of the tested model. Spatial-sampling-induced
differences are relatively small compared to those arising from other
sources of uncertainty, including the effects of applying different
data calibration methods, those of using vector data from different
instruments, or those arising from application of different NLFF
methods to identical input data.
Title: The 2019 International Women's Day Event: A Two-step Solar
Flare with Multiple Eruptive Signatures and Low Earth Impact
Authors: Dumbovic, Mateja; Veronig, Astrid; Podladchikova, Tatiana;
Thalmann, Julia; Chikunova, Galina; Dissauer, Karin; Magdalenic,
Jasmina; Temmer, Manuela; Guo, Jingnan; Samara, Evangelia
Bibcode: 2021AGUFMSH32A..08D
Altcode:
We present a detailed analysis of an eruptive event that occurred on
early 2019 March 8 in active region AR 12734, to which we refer as the
International Women's day event. The event under study is intriguing in
several aspects: 1) low-coronal eruptive signatures come in ''pairs'' (a
double-peak flare, two coronal dimmings, and two EUV waves); 2) although
the event is characterized by a complete chain of eruptive signatures,
the corresponding coronagraphic signatures are weak; 3) although
the source region of the eruption is located close to the center of
the solar disc and the eruption is thus presumably Earth-directed,
heliospheric signatures are very weak with little Earth-impact. We
analyze a number of multi-spacecraft and multi-instrument (both
remote-sensing and in situ) observations, including Soft X-ray,
(extreme-) ultraviolet (E)UV), radio and white-light emission, as well
as plasma, magnetic field and particle measurements. We employ 3D NLFF
modeling to investigate the coronal magnetic field configuration in and
around the active region, the GCS model to make a 3D reconstruction of
the CME geometry and the 3D MHD numerical model EUHFORIA to model the
background state of the heliosphere. Our results indicate two subsequent
eruptions of two systems of sheared and twisted magnetic fields,
which merge already in the upper corona and start to evolve further
out as a single entity. The large-scale magnetic field significantly
influences both, the early and the interplanetary evolution of the
structure. During the first eruption the stability of the overlying
field was disrupted which enabled the second eruption. We find that
during the propagation in the interplanetary space the large-scale
magnetic field, i.e. , the location of heliospheric current sheet
between the AR and the Earth likely influences propagation and the
evolution of the erupted structure(s).
Title: Magnetic Helicity Estimations in Models and Observations of
the Solar Magnetic Field. IV. Application to Solar Observations
Authors: Thalmann, J. K.; Georgoulis, M. K.; Liu, Y.; Pariat, E.;
Valori, G.; Anfinogentov, S.; Chen, F.; Guo, Y.; Moraitis, K.; Yang,
S.; Mastrano, Alpha; ISSI Team on Magnetic Helicity
Bibcode: 2021ApJ...922...41T
Altcode: 2021arXiv210808525T
In this ISSI-supported series of studies on magnetic helicity in the
Sun, we systematically implement different magnetic helicity calculation
methods on high-quality solar magnetogram observations. We apply
finite-volume, discrete flux tube (in particular, connectivity-based)
and flux-integration methods to data from Hinode's Solar Optical
Telescope. The target is NOAA Active Region 10930 during a 1.5-day
interval in 2006 December that included a major eruptive flare
(SOL2006-12-13T02:14X3.4). Finite-volume and connectivity-based methods
yield instantaneous budgets of the coronal magnetic helicity, while
the flux-integration methods allow an estimate of the accumulated
helicity injected through the photosphere. The objectives of our work
are twofold: a cross-validation of methods, as well as an interpretation
of the complex events leading to the eruption. To the first objective,
we find (i) strong agreement among the finite-volume methods, (ii)
a moderate agreement between the connectivity-based and finite-volume
methods, (iii) an excellent agreement between the flux-integration
methods, and (iv) an overall agreement between finite-volume- and
flux-integration-based estimates regarding the predominant sign and
magnitude of the helicity. To the second objective, we are confident
that the photospheric helicity flux significantly contributed to the
coronal helicity budget and that a right-handed structure erupted from
a predominantly left-handed corona during the X-class flare. Overall,
we find that the use of different methods to estimate the (accumulated)
coronal helicity may be necessary in order to draw a complete picture
of an active region corona, given the careful handling of identified
data (preparation) issues, which otherwise would mislead the event
analysis and interpretation.
Title: Magnetic helicity and energy budget around large confined
and eruptive solar flares
Authors: Gupta, M.; Thalmann, J. K.; Veronig, A. M.
Bibcode: 2021A&A...653A..69G
Altcode: 2021arXiv210608781G
Context. In order to better understand the underlying processes
and prerequisites for solar activity, it is essential to study the
time evolution of the coronal magnetic field of solar active regions
(ARs) associated with flare activity.
Aims: We investigate the
coronal magnetic energy and helicity budgets of ten solar ARs around
the times of large flares. In particular, we are interested in a
possible relation of the derived quantities to the particular type of
the flares that the AR produces, namely, whether they are associated
with a CME or whether they are confined (i.e., not accompanied by a
CME).
Methods: Using an optimization approach, we employed time
series of 3D nonlinear force-free magnetic field models of ten ARs,
covering a time span of several hours around the time of occurrence
of large solar flares (GOES class M1.0 and larger). We subsequently
computed the 3D magnetic vector potentials associated to the model 3D
coronal magnetic field using a finite-volume method. This allows us
to correspondingly compute the coronal magnetic energy and helicity
budgets, as well as related (intensive) quantities such as the
relative contribution of free magnetic energy, EF/E (energy
ratio), the fraction of non-potential (current-carrying) helicity,
|HJ|/|HV| (helicity ratio), and the normalized
current-carrying helicity, |HJ|/ϕ'2.
Results: The total energy and helicity budgets of flare-productive
ARs (extensive parameters) cover a broad range of magnitudes, with
no obvious relation to the eruptive potential of the individual
ARs, that is, whether or not a CME is produced in association with
the flare. The intensive eruptivity proxies, EF/E and
|HJ|/|HV|, and |HJ|/ϕ'2,
however, seem to be distinctly different for ARs that produce
CME-associated large flares compared to those which produce confined
flares. For the majority of ARs in our sample, we are able to identify
characteristic pre-flare magnitudes of the intensive quantities
that are clearly associated with subsequent CME-productivity.
Conclusions: If the corona of an AR exhibits characteristic values of
⟨|HJ|/|HV|⟩ > 0.1, ⟨EF/E⟩
> 0.2, and ⟨|HJ|/ϕ'2⟩ > 0.005, then
the AR is likely to produce large CME-associated flares. Conversely,
confined large flares tend to originate from ARs that exhibit
coronal values of ⟨|HJ|/|HV|⟩ ≲ 0.1,
⟨EF/E⟩ ≲ 0.1, and ⟨|HJ|/ϕ'2⟩
≲ 0.002.
Title: 2019 International Women's Day event. Two-step solar flare
with multiple eruptive signatures and low Earth impact
Authors: Dumbović, M.; Veronig, A. M.; Podladchikova, T.; Thalmann,
J. K.; Chikunova, G.; Dissauer, K.; Magdalenić, J.; Temmer, M.; Guo,
J.; Samara, E.
Bibcode: 2021A&A...652A.159D
Altcode: 2021arXiv210615417D
Context. We present a detailed analysis of an eruptive event that
occurred on 2019 March 8 in the active region AR 12734, which we
refer as the International Women's Day event. The event under study
is intriguing based on several aspects: (1) low-coronal eruptive
signatures come in `pairs', namely, there is a double-peaked flare,
two coronal dimmings, and two extreme ultraviolet (EUV) waves; (2)
although the event is characterized by a complete chain of eruptive
signatures, the corresponding coronagraphic signatures are weak;
and (3) although the source region of the eruption is located close
to the center of the solar disc and the eruption is thus presumably
Earth-directed, heliospheric signatures are very weak with very weak
Earth impact.
Aims: In order to understand the initiation and
evolution of this particular event, we performed a comprehensive
analysis using a combined observational-modeling approach.
Methods: We analyzed a number of multi-spacecraft and multi-instrument
(both remote-sensing and in situ) observations, including soft X-ray,
EUV, radio and white-light emission, as well as plasma, magnetic field,
and particle measurements. We employed 3D nonlinear force-free modeling
to investigate the coronal magnetic field configuration in and around
the active region, the graduated cylindrical shell model to make a 3D
reconstruction of the CME geometry, and the 3D magnetohydrodynamical
numerical model EUropean Heliospheric FORecasting Information Asset
to model the background state of the heliosphere.
Results:
Our results reveal a two-stage C1.3 flare, associated with two
EUV waves that occur in close succession and two-stage coronal
dimmings that evolve co-temporally with the flare and type II and
III radio bursts. Despite its small GOES class, a clear drop in
magnetic free energy and helicity is observed during the flare. White
light observations do not unambiguously indicate two separate CMEs,
but rather a single entity most likely composed of two sheared and
twisted structures corresponding to the two eruptions observed in the
low corona. The corresponding interplanetary signatures are that of
a small flux rope swith indications of strong interactions with the
ambient plasma, which result in a negligible geomagnetic impact.
Conclusions: Our results indicate two subsequent eruptions of
two systems of sheared and twisted magnetic fields, which already
begin to merge in the upper corona and start to evolve further out
as a single entity. The large-scale magnetic field significantly
influences both the early and the interplanetary evolution of the
structure. During the first eruption, the stability of the overlying
field was disrupted, enabling the second eruption. We find that during
the propagation in the interplanetary space the large-scale magnetic
field, that is, the location of heliospheric current sheet between the
AR and the Earth, is likely to influence propagation, along with the
evolution of the erupted structure(s).
Movies are available at https://www.aanda.org
Title: Interpretable Solar Flare Forecasting with Deep Learning
Authors: Jarolim, Robert; Podladchikova, Tatiana; Veronig, Astrid;
Thalmann, Julia K.; Hofinger, -Markus; Narnhofer, -Dominik; Pock,
-Thomas; Schopper, Tobias
Bibcode: 2021cosp...43E1036J
Altcode:
Solar flares and coronal mass ejections (CMEs) are the main drivers
for severe space weather disturbances on Earth and other planets. While
the geo-effects of CMEs give us a lead time of about 1 to 4 days, the
effects of flare induced enhanced radiation and flare-accelerated solar
energetic particles (SEPs) are very immediate, approximately 8 and 20
minutes, respectively. Thus, predictions of solar flare occurrence at
least several hours ahead are of high importance for the mitigation
of severe space weather effects. Observations and simulations of solar
flares suggest that the structure and evolution of the active region's
magnetic field is a key component for energetic eruptions. However,
the main changes are assumed to happen in the coronal fields, whereas
current measurements are mostly restricted to the photospheric magnetic
field. We present an automatic flare prediction deep learning algorithm
based on the HMI photospheric line-of-sight magnetic field and its
temporal evolution together with the coronal evolution as observed by
multi-wavelengths EUV filtergrams from the AIA instrument onboard the
Solar Dynamics Observatory. As input to our deep learning model we use
the magnetograms and EUV filtergrams with a cadence of 10 minutes over
a 40 minutes time interval from pre-identified active regions. The
neural network predicts X, M and C class flares up to 3 hours ahead,
hereby the network assigns probabilities for the flare occurrence to
consecutive time frames of 20 minutes. From this setup the network
learns independently to identify features in the imaging data based
on the dynamic evolution of the coronal structure and the photospheric
magnetic field evolution, which may hint at flare occurrence in the near
future. In order to overcome the "black box problem" of machine-learning
algorithms, and thus to allow for physical interpretation of the network
findings, we employ an attention mechanism at multiple resolution
scales, which enables the network to focus on relevant regions within
the spatio-temporal domain. This allows us to extract the emphasized
regions, which reveal the neural network interpretation of the flare
onset conditions. Our novel approach combines the performance of neural
network predictions with the benefit of a direct interpretation of
the relevant physical features.
Title: Estimating the magnetic flux within an eruptive flux rope
Authors: Temmer, Manuela; Rodriguez, Luciano; Dissauer, Karin; Veronig,
Astrid; Tschernitz, Johannes; Thalmann, Julia K.; Hinterreiter, Jürgen
Bibcode: 2021cosp...43E1741T
Altcode:
Erupting magnetic flux ropes develop into coronal mass ejections (CMEs)
as they evolve and finally propagate into interplanetary space. Those
large scale eruptions are observed to be frequently related to dynamic
surface phenomena such as coronal waves and dimming regions. The better
we are able to estimate initial CME parameters such as kinematics,
geometry, and magnetic properties, the more precisely we can feed
state-of-the-art CME propagation models and with that improve CME
forecasting. In that respect, we report on a well-observed flare-CME
event from 1 October 2011 focusing on the dynamic evolution of the
CME and its embedded magnetic field. Using combined STEREO and SDO
observations together with nonlinear force-free (NLFF) modeling we
derive separately the flare reconnection and dimming flux. We find
that already before the start of the impulsive flare phase magnetic
reconnection was ongoing, that added magnetic flux to the flux rope
before its final eruption. As the dimming evolves over a longer time
span than the flaring phase, we find that the dimming flux increases by
more than 25% after the end of the flare. This indicates that magnetic
flux is still added to the flux rope after eruption and that the derived
flare reconnection flux is most probably a lower limit for estimating
the magnetic flux within the flux rope.
Title: Homologous Flaring Activity over a Sunspot Light Bridge in
an Emerging Active Region
Authors: Louis, Rohan Eugene; Thalmann, Julia K.
Bibcode: 2021ApJ...907L...4L
Altcode: 2020arXiv201207454L
Sunspot light bridges are known to exhibit a variety of dynamic and
persistent phenomena such as surges, small-scale jets, etc., in the
chromosphere and transition region. While it has generally been proposed
that magnetic reconnection is responsible for this small-scale dynamism,
persistent flaring activity lasting several hours from the same spatial
location on a sunspot light bridge has rarely been reported. We combine
observations from the Atmospheric Imaging Assembly and the Helioseismic
Magnetic Imager on board the Solar Dynamics Observatory to investigate
homologous flaring activity over a small sunspot light bridge in
an emerging flux region. The homologous flares all produced broad,
collimated jets including a B6.4 class flare. The jets rise at a speed
of about 200 km s-1, reach projected heights of about 98
Mm, and emerge from the same spatial location for nearly 14 hrs, after
which they cease completely. A nonlinear force-free extrapolation of
the photospheric magnetic field shows a low-lying flux rope connecting
the light bridge to a remote opposite-polarity network. The persistent
flares occur as a result of the rapid horizontal motion of the leading
sunspot that causes the relatively vertical magnetic fields in the
adjacent umbra to reconnect with the low-lying flux rope in the light
bridge. Our results indicate that the flaring ceases once the flux
rope has lost sufficient twist through repeated reconnections.
Title: Deducing the reliability of relative helicities from nonlinear
force-free coronal models
Authors: Thalmann, J. K.; Sun, X.; Moraitis, K.; Gupta, M.
Bibcode: 2020A&A...643A.153T
Altcode: 2020arXiv200905287T
Aims: We study the relative helicity of active region (AR) NOAA
12673 during a ten-hour time interval centered around a preceding X2.2
flare (SOL2017-09-06T08:57) and also including an eruptive X9.3 flare
that occurred three hours later (SOL2017-09-06T11:53). In particular,
we aim for a reliable estimate of the normalized self-helicity of
the current-carrying magnetic field, the so-called helicity ratio,
|HJ|/|H𝒱|, a promising candidate to quantity
the eruptive potential of solar ARs.
Methods: Using Solar
Dynamics Observatory Helioseismic and Magnetic Imager vector magnetic
field data as an input, we employ nonlinear force-free (NLFF) coronal
magnetic field models using an optimization approach. The corresponding
relative helicity, and related quantities, are computed using a
finite-volume method. From multiple time series of NLFF models based
on different choices of free model parameters, we are able to assess
the spread of |HJ|/|H𝒱|, and to estimate
its uncertainty.
Results: In comparison to earlier works,
which identified the non-solenoidal contribution to the total magnetic
energy, Ediv/E, as selection criterion regarding the required
solenoidal quality of magnetic field models for subsequent relative
helicity analysis, we propose to use in addition the non-solenoidal
contribution to the free magnetic energy, |Emix|/EJ,
s. As a recipe for a reliable estimate of the relative magnetic
helicity (and related quantities), we recommend to employ multiple NLFF
models based on different combinations of free model parameters, to
retain only those that exhibit smallest values of both Ediv/E
and |Emix|/EJ, s at a certain time instant,
to subsequently compute mean estimates, and to use the spread of the
individually contributing values as an indication for the uncertainty.
Title: Erratum: "On the Reliability of Magnetic Energy and Helicity
Computations Based on Nonlinear Force-free Coronal Magnetic Field
Models" (2019, ApJL, 880, L6)
Authors: Thalmann, Julia K.; Linan, L.; Pariat, E.; Valori, G.
Bibcode: 2020ApJ...902L..48T
Altcode:
No abstract at ADS
Title: Magnetic Helicity Budget of Solar Active Regions Prolific of
Eruptive and Confined Flares
Authors: Thalmann, Julia K.; Moraitis, K.; Linan, L.; Pariat, E.;
Valori, G.; Dalmasse, K.
Bibcode: 2019ApJ...887...64T
Altcode: 2019arXiv191006563T
We compare the coronal magnetic energy and helicity of two solar active
regions (ARs), prolific in major eruptive (AR 11158) and confined
(AR 12192) flaring, and analyze the potential of deduced proxies
to forecast upcoming flares. Based on nonlinear force-free (NLFF)
coronal magnetic field models with a high degree of solenoidality,
and applying three different computational methods to investigate
the coronal magnetic helicity, we are able to draw conclusions
with a high level of confidence. Based on real observations of two
solar ARs we checked trends regarding the potential eruptivity of
the active-region corona, as suggested earlier in works that were
based on numerical simulations, or solar observations. Our results
support that the ratio of current-carrying to total helicity, |
{H}{{J}}| /| {H}{ \mathcal V }| , shows a strong
ability to indicate the eruptive potential of a solar AR. However, |
{H}{{J}}| /| {H}{ \mathcal V }| does not seem to
be indicative for the magnitude or type of an upcoming flare (confined
or eruptive). Interpreted in the context of earlier observational
studies, our findings furthermore support that the total relative
helicity normalized to the magnetic flux at the NLFF model’s lower
boundary, {H}{ \mathcal V }/{φ }2, represents
no indicator for the eruptivity.
Title: A Hot Cusp-shaped Confined Solar Flare
Authors: Hernandez-Perez, Aaron; Su, Yang; Thalmann, Julia; Veronig,
Astrid M.; Dickson, Ewan C.; Dissauer, Karin; Joshi, Bhuwan; Chandra,
Ramesh
Bibcode: 2019ApJ...887L..28H
Altcode: 2019arXiv191110859H
We analyze a confined flare that developed a hot cusp-like structure
high in the corona (H ∼ 66 Mm). A growing cusp-shaped flare arcade
is a typical feature in the standard model of eruptive flares, caused
by magnetic reconnection at progressively larger coronal heights. In
contrast, we observe a static hot cusp during a confined flare. Despite
an initial vertical temperature distribution similar to that in eruptive
flares, we observe a distinctly different evolution during the late
(decay) phase, in the form of prolonged hot emission. The distinct
cusp shape, rooted at locations of nonthermal precursor activity, was
likely caused by a magnetic field arcade that kinked near the top. Our
observations indicate that the prolonged heating was a result of slow
local reconnection and an increased thermal pressure near the kinked
apexes due to continuous plasma upflows.
Title: Observations of a Footpoint Drift of an Erupting Flux Rope
Authors: Zemanová, Alena; Dudík, Jaroslav; Aulanier, Guillaume;
Thalmann, Julia K.; Gömöry, Peter
Bibcode: 2019ApJ...883...96Z
Altcode: 2019arXiv190802082Z
We analyze the imaging observations of an M-class eruptive flare of 2015
November 4. The pre-eruptive Hα filament was modeled by the nonlinear
force-free field model, which showed that it consisted of two helical
systems. Tether-cutting reconnection involving these two systems led
to the formation of a hot sigmoidal loop structure rooted in a small
hook that formed at the end of the flare ribbon. Subsequently, the hot
loops started to slip away from the small hook until it disappeared. The
loops continued slipping and the ribbon elongated itself by several
tens of arcseconds. A new and larger hook then appeared at the end of
the elongated ribbon with hot and twisted loops rooted there. After
the eruption of these hot loops, the ribbon hook expanded and later
contracted. We interpret these observations in the framework of
the recent three-dimensional (3D) extensions to the standard solar
flare model predicting the drift of the flux rope footpoints. The hot
sigmoidal loop is interpreted as the flux rope, whose footpoints drift
during the eruption. While the deformation and drift of the new hook can
be described by the model, the displacement of the flux rope footpoint
from the filament to that of the erupting flux rope indicate that the
hook evolution can be more complex than those captured by the model.
Title: On the Reliability of Magnetic Energy and Helicity Computations
Based on Nonlinear Force-free Coronal Magnetic Field Models
Authors: Thalmann, Julia K.; Linan, L.; Pariat, E.; Valori, G.
Bibcode: 2019ApJ...880L...6T
Altcode: 2019arXiv190701179T
We demonstrate the sensitivity of magnetic energy and helicity
computations regarding the quality of the underlying coronal magnetic
field model. We apply the method of Wiegelmann & Inhester to a
series of Solar Dynamics Observatory/Helioseismic and Magnetic Imager
vector magnetograms, and discuss nonlinear force-free (NLFF) solutions
based on two different sets of the free model parameters. The two
time series differ from each other concerning their force-free and
solenoidal quality. Both force- and divergence-freeness are required
for a consistent NLFF solution. Full satisfaction of the solenoidal
property is inherent in the definition of relative magnetic helicity
in order to ensure gauge independence. We apply two different magnetic
helicity computation methods to both NLFF time series and find that
the output is highly dependent on the level to which the NLFF magnetic
fields satisfy the divergence-free condition, with the computed magnetic
energy being less sensitive than the relative helicity. Proxies for
the nonpotentiality and eruptivity derived from both quantities are
also shown to depend strongly on the solenoidal property of the NLFF
fields. As a reference for future applications, we provide quantitative
thresholds for the force- and divergence-freeness, for the assurance
of reliable computation of magnetic energy and helicity, and of their
related eruptivity proxies.
Title: Invisibility of Solar Active Region Umbra-to-Umbra Coronal
Loops: New Evidence that Magnetoconvection Drives Solar-Stellar
Coronal Heating
Authors: Moore, Ronald L.; Tiwari, Sanjiv; Thalmann, Julia; Panesar,
Navdeep; Winebarger, Amy
Bibcode: 2019AAS...23410603M
Altcode:
How magnetic energy is injected and released in the solar corona,
keeping it heated to several million degrees, remains elusive. The
corona is shaped by the magnetic field that fills it and the heating
of the corona generally increases with increasing strength of the
field. For each of two bipolar solar active regions having one or
more sunspots in each of the two main opposite-polarity domains of
magnetic flux, from comparison of a nonlinear force-free model of the
active region's three-dimensional coronal magnetic field to observed
extreme-ultraviolet coronal loops, we find that (1) umbra-to-umbra
loops, despite being rooted in the strongest magnetic flux at both ends,
are invisible, and (2) the brightest loops have one foot in a sunspot
umbra or penumbra and the other foot in another sunspot's penumbra or
in unipolar or mixed-polarity plage. The invisibility of umbra-to-umbra
loops is new evidence that magnetoconvetion drives solar-stellar coronal
heating: evidently, the strong umbral field at both ends quenches the
magnetoconvection and hence the heating. Broadly, our results indicate
that depending on the field strength in both feet, the photospheric feet
of a coronal loop on any convective star can either engender or quench
coronal heating in the body of the loop. This work was supported
by funding from the Heliophysics Division of NASA's Science Mission
Directorate, from NASA's Postdoctoral Program, and from the Austrian
Science Fund. The results have been published in The Astrophysical
Journal Letters (Tiwari, S. K., Thalmann, J. K., Panesar, N. K., Moore,
R. L., & Winebarger, A. R. 2017, ApJ Letters, 843:L20).
Title: Pre-eruption Processes: Heating, Particle Acceleration, and the
Formation of a Hot Channel before the 2012 October 20 M9.0 Limb Flare
Authors: Hernandez-Perez, Aaron; Su, Yang; Veronig, Astrid M.;
Thalmann, Julia; Gömöry, Peter; Joshi, Bhuwan
Bibcode: 2019ApJ...874..122H
Altcode: 2019arXiv190208436H
We report a detailed study of the pre-eruption activities that led to
the occurrence of an M9.0 flare/CME event on 2012 October 20 in NOAA
AR 11598. This includes the study of the preceding confined C2.4 flare
that occurred on the same AR ∼25 minutes earlier. We observed that the
M9.0 flare occurred as a consequence of two distinct triggering events
well separated in time. The first triggering episode occurred as early
as ∼20 minutes before the onset of the M9.0 flare, evidenced by the
destabilization and rise of a pre-existing filament to a new position of
equilibrium at a higher coronal altitude during the decay phase of the
C2.4 flare. This brought the system to a magnetic configuration where
the establishment of the second triggering event was favorable. The
second triggering episode occurred ∼17 minutes later, during the
early phase of the M9.0 flare, evidenced by the further rise of the
filament and successful ejection. The second trigger is followed by a
flare precursor phase, characterized by nonthermal emission and the
sequential formation of a hot channel as shown by the SDO/AIA DEM
(differential emission measure) maps, the RHESSI X-ray images and
spectra. These observations are suggestive of magnetic reconnection
and particle acceleration that can explain the precursor phase and can
be directly related to the formation of the hot channel. We discuss
the triggering mechanisms, their implications during the early and
precursor phases and highlight the importance of early activities and
preceding small confined flares to understand the initiation of large
eruptive flares.
Title: Which factors of an active region determine whether a strong
flare will be CME associated or not?
Authors: Baumgartner, Christian; Thalmann, Julia K.; Veronig, Astrid M.
Bibcode: 2018csc..confE..10B
Altcode:
We study how the magnetic field determines whether a strong flare
launched from an active region (AR) will be eruptive or confined,
i.e. associated with a coronal mass ejection (CME) or not. To this aim,
we selected all large flares that were observed by the SDO HMI and
AIA instruments during the period 2011 to 2015 within 50° from the
disk center. In total, our data set comprises 44 flares of GOES class
>M5.0. Out of these, 12 events were confined (7 M and 5 X-flares) and
32 were eruptive (18 M- and 14 X-flares). We used 3D potential magnetic
field models to study their location within the host AR (using the flare
distance from the flux-weighted AR center, d_{FC}) and the strength
of the overlying coronal field (via decay index n). We also present a
first systematic study of the orientation of the coronal magnetic field
changing with height, using the orientation φ of the flare-relevant
polarity inversion line as a measure. We analyzed all quantities with
respect to the size of the underlying active-region dipole field,
defined by the distance between the flux-weighted opposite-polarity
centers, d_{PC}. We find that flares originating from the periphery
of an AR dipole field (d_{FC} / d_{PC} > 0.5) are predominantly
eruptive. Flares originating from underneath the AR dipole field (d_{FC}
/ d_{PC} < 0.5) tend to be eruptive when they are launched from
a compact AR and confined when launched from an extended AR (d_{PC}
> 60 Mm). In confined events, the flare-relevant field adjusts its
orientation quickly to that of the underlying dipole field with height
(δ φ > 40° between the surface and the apex of the active-region
dipole field), in contrast to eruptive events where it changes more
slowly. The critical height for torus instability discriminates best
between confined (h_{crit} > 40 Mm) and eruptive flares (h_{crit}
< 40 Mm). It discriminates better than δ φ, implying that the decay
of the confining field plays a stronger role in the eruptive/confined
character of a flare than its orientation at different heights.
Title: Magnetic reconnection fluxes in solar flares and their
implications for solar and stellar superflares
Authors: Veronig, Astrid; Tschernitz, Johannes; Thalmann, Julia K.;
Hinterreiter, Jürgen; Pötzi, Werner
Bibcode: 2018cosp...42E3538V
Altcode:
We study the energy release process of a set of 51 solar flares
which span almost four orders of magnitude in flare energy, from GOES
class B3 to X17. 19 events of our sample are eruptive, i.e. have a
CME associated, and 32 are confined (no CME associated). We use Hα
filtergrams from Kanzelhöhe Observatory together with SDO HMI and SOHO
MDI magnetograms to derive magnetic reconnection fluxes and reconnection
rates. We find that the flare reconnection flux is strongly correlated
with the peak of the GOES 1-8 Å soft X-ray flux (r=0.9, in log-log
space), both for confined and eruptive flares. In the largest events,
up to ≈50% of the total magnetic flux of the host active region
(AR) is involved in the flare magnetic reconnection. Based on these
findings, we extrapolate the properties of the largest flares that may
be launched from our present day's Sun. A complex solar AR that hosts
a magnetic flux of 2\cdot 10^{23} {Mx}, which is supported by the
largest active-region magnetic fluxes directly measured, is capable
of producing an X80 flare (corresponding to a bolometric energy of
about 7 \cdot 10^{32} ergs). Using a magnetic flux estimate of 6\cdot
10^{23} {Mx} for the largest solar AR observed, we find that flares
of GOES class ≈X500 could be produced (E_{bol} ≈ 3 \cdot 10^{33}
ergs). Our results lie on the lower end of the energies of superflares
on solar-type stars recently detected in Kepler data. Furthermore, they
suggest that the present day's Sun is capable of producing flares and
related space weather events more than an order of magnitude stronger
than observed in the past.
Title: Which factors of an active region determine whether a flare
will be eruptive or confined?
Authors: Veronig, Astrid; Thalmann, Julia K.; Baumgartner, Christian
Bibcode: 2018cosp...42E3539V
Altcode:
We study how the magnetic field determines whether a strong flare
launched from an active region (AR) will be eruptive or confined. To
this aim, we analyzed 44 flares above GOES class M5.0 that occurred
during 2011-2015. We used 3D potential magnetic field models to study
their location within the host AR (using the flare distance from the
flux-weighted AR center, d_{{FC}}) and the strength of the overlying
coronal field (via decay index n). We also present a first systematic
study of the orientation of the coronal magnetic field changing
with height, using the orientation φ of the flare-relevant polarity
inversion line as a measure. We analyzed all quantities with respect to
the size of the underlying active-region dipole field, characterized
by the distance between the flux-weighted opposite-polarity centers,
d_{{PC}}. We find that flares originating from the periphery
of an active-region dipole field (d_{{FC}}/d_{{PC}}>0.5) are
predominantly eruptive. Flares originating from underneath the AR
dipole field (d_{{FC}}/d_{{PC}}<0.5) tend to be eruptive when they
are launched from a compact AR (d_{{PC}}≤60 Mm) and confined when
launched from an extended AR. In confined events the flare-relevant
field adjusts its orientation quickly to that of the underlying dipole
field with height (Δφ≳40° between the surface and the apex of
the active-region dipole field), in contrast to eruptive events where
it changes more slowly with height. The critical height for torus
instability, h_{{crit}}=h(n=1.5), discriminates best between confined
(h_{{crit}}≳40 Mm) and eruptive flares (h_{{crit}}≲40 Mm). It
discriminates better than Δφ, implying that the decay of the confining
field plays a stronger role than its orientation at different heights.
Title: Characteristics of ribbon evolution and reconnection electric
fields in Hα two-ribbon flares
Authors: Hinterreiter, Jürgen; Veronig, Astrid; Thalmann, Julia;
Tschernitz, Johannes; Pötzi, Werner
Bibcode: 2018EGUGA..20.9819H
Altcode:
We perform a statistical study of magnetic reconnection related
parameters in Hα two-ribbon flares. 50 flare events, including 19
eruptive flares (i.e. associated to a coronal mass ejection) and 31
confined flares (i.e. CME-less) are analyzed, which are distributed
over a wide range of GOES classes (from B3 to X17). The maximum ribbon
separation, ribbon-separation velocity, mean magnetic-field strength,
and reconnection electric field (i.e., local reconnection rate) are
derived from Hα filtergrams obtained at Kanzelhöhe Observatory in
combination with co-registered SOHO MDI and SDO HMI magnetograms. We
find that the ribbon separation of eruptive flares correlates with the
GOES flux and is statistically larger than that of confined flares,
whereas no dependence was found for the maximum ribbon-separation
velocity and the GOES flux. The local reconnection rate strongly
correlates with the GOES flux. In addition, eruptive flares with a
stronger peak reconnection electric field tend to be accompanied by
faster CMEs. The estimated reconnection-related proxies for confined
and eruptive events, however, appear in the form of two distinct
but largely overlapping populations. This suggests that there is no
significant difference in the underlying reconnection process.
Title: Combining remote-sensing image data with in-situ measurements
supported by modeling for Earth-affecting CME events
Authors: Temmer, Manuela; Thalmann, Julia; Dissauer, Karin; Veronig,
Astrid; Tschernitz, Johannes; Hinterreiter, Jürgen; Rodriguez, Luciano
Bibcode: 2018EGUGA..20.3999T
Altcode:
We analyze the well observed flare-CME event from October 1, 2011
and cover the complete chain of action - from the Sun to Earth. We
study in detail the solar surface and atmosphere (SDO and ground-based
instruments) associated to the flare/CME and also track the off-limb CME
signatures in interplanetary space (STEREO-SoHO). This is complemented
by surface magnetic field information and 3D coronal magnetic field
modeling. From in-situ measurements (Wind), we extract the corresponding
ICME characteristics. Results show that the flare reconnection flux is
most probably a lower limit for estimating the magnetic flux within the
flux rope as 1) magnetic reconnection processes were already ongoing
before the start of the impulsive flare phase and 2) the dimming flux
increased by more than 25% after the end of the flare, indicating that
magnetic flux was still added to the flux rope after eruption. When
comparing this to the in-situ axial magnetic flux of the magnetic cloud,
we find that it is reduced by at least 75%, referring to substantial
erosion in interplanetary space. Careful inspection of on-disk features
associated with CMEs are essential for interpreting such scenarios.
Title: Statistical Properties of Ribbon Evolution and Reconnection
Electric Fields in Eruptive and Confined Flares
Authors: Hinterreiter, J.; Veronig, A. M.; Thalmann, J. K.; Tschernitz,
J.; Pötzi, W.
Bibcode: 2018SoPh..293...38H
Altcode: 2018arXiv180103370H
A statistical study of the chromospheric ribbon evolution in Hα
two-ribbon flares was performed. The data set consists of 50 confined
(62%) and eruptive (38%) flares that occurred from June 2000 to
June 2015. The flares were selected homogeneously over the Hα and
Geostationary Operational Environmental Satellite (GOES) classes, with
an emphasis on including powerful confined flares and weak eruptive
flares. Hα filtergrams from the Kanzelhöhe Observatory in combination
with Michelson Doppler Imager (MDI) and Helioseismic and Magnetic
Imager (HMI) magnetograms were used to derive the ribbon separation,
the ribbon-separation velocity, the magnetic-field strength, and
the reconnection electric field. We find that eruptive flares reveal
statistically larger ribbon separation and higher ribbon-separation
velocities than confined flares. In addition, the ribbon separation
of eruptive flares correlates with the GOES SXR flux, whereas no clear
dependence was found for confined flares. The maximum ribbon-separation
velocity is not correlated with the GOES flux, but eruptive flares
reveal on average a higher ribbon-separation velocity (by ≈ 10 km
s−1). The local reconnection electric field of confined
(c c =0.50 ±0.02 ) and eruptive (c c =0.77 ±0.03 ) flares correlates
with the GOES flux, indicating that more powerful flares involve
stronger reconnection electric fields. In addition, eruptive flares
with higher electric-field strengths tend to be accompanied by faster
coronal mass ejections.
Title: On the Factors Determining the Eruptive Character of Solar
Flares
Authors: Baumgartner, Christian; Thalmann, Julia K.; Veronig, Astrid M.
Bibcode: 2018ApJ...853..105B
Altcode: 2017arXiv171205106B
We investigated how the magnetic field in solar active regions (ARs)
controls flare activity, i.e., whether a confined or eruptive flare
occurs. We analyzed 44 flares of GOES class M5.0 and larger that
occurred during 2011-2015. We used 3D potential magnetic field models to
study their location (using the flare distance from the flux-weighted
AR center d FC) and the strength of the magnetic field in
the corona above (via decay index n and flux ratio). We also present a
first systematic study of the orientation of the coronal magnetic field,
using the orientation φ of the flare-relevant polarity inversion line
as a measure. We analyzed all quantities with respect to the size of
the underlying dipole field, characterized by the distance between
the opposite-polarity centers, d PC. Flares originating
from underneath the AR dipole (d FC/d PC <
0.5) tend to be eruptive if launched from compact ARs (d PC
≤ 60 Mm) and confined if launched from extended ARs. Flares ejected
from the periphery of ARs (d FC/d PC > 0.5)
are predominantly eruptive. In confined events, the flare-relevant field
adjusts its orientation quickly to that of the underlying dipole with
height (Δφ ≳ 40° until the apex of the dipole field), in contrast
to eruptive events where it changes more slowly with height. The
critical height for torus instability, h crit = h(n = 1.5),
discriminates best between confined (h crit ≳ 40 Mm)
and eruptive flares (h crit ≲ 40 Mm). It discriminates
better than Δφ, implying that the decay of the confining field plays
a stronger role than its orientation at different heights.
Title: Reconnection Fluxes in Eruptive and Confined Flares and
Implications for Superflares on the Sun
Authors: Tschernitz, Johannes; Veronig, Astrid M.; Thalmann, Julia K.;
Hinterreiter, Jürgen; Pötzi, Werner
Bibcode: 2018ApJ...853...41T
Altcode: 2017arXiv171204701T
We study the energy release process of a set of 51 flares (32 confined,
19 eruptive) ranging from GOES class B3 to X17. We use Hα filtergrams
from Kanzelhöhe Observatory together with Solar Dynamics Observatory
HMI and Solar and Heliospheric Observatory MDI magnetograms to derive
magnetic reconnection fluxes and rates. The flare reconnection flux
is strongly correlated with the peak of the GOES 1-8 Å soft X-ray
flux (c = 0.92, in log-log space) for both confined and eruptive
flares. Confined flares of a certain GOES class exhibit smaller ribbon
areas but larger magnetic flux densities in the flare ribbons (by a
factor of 2). In the largest events, up to ≈50% of the magnetic flux
of the active region (AR) causing the flare is involved in the flare
magnetic reconnection. These findings allow us to extrapolate toward the
largest solar flares possible. A complex solar AR hosting a magnetic
flux of 2 × 1023 Mx, which is in line with the largest
AR fluxes directly measured, is capable of producing an X80 flare,
which corresponds to a bolometric energy of about 7 × 1032
erg. Using a magnetic flux estimate of 6 × 1023 Mx for
the largest solar AR observed, we find that flares of GOES class
≈X500 could be produced (E bol ≈ 3 × 1033
erg). These estimates suggest that the present day’s Sun is capable
of producing flares and related space weather events that may be more
than an order of magnitude stronger than have been observed to date.
Title: Invisibility of Solar Active Region Umbra-to-Umbra Coronal
Loops: New Evidence that Magnetoconvection Drives Solar-Stellar
Coronal Heating
Authors: Tiwari, S. K.; Thalmann, J. K.; Panesar, N. K.; Moore, R. L.;
Winebarger, A. R.
Bibcode: 2017AGUFMSH43A2789T
Altcode:
Coronal heating generally increases with increasing magnetic field
strength: the EUV/X-ray corona in active regions is 10-100 times more
luminous and 2-4 times hotter than that in quiet regions and coronal
holes, which are heated to only about 1.5 MK, and have fields that are
10-100 times weaker than that in active regions. From a comparison of
a nonlinear force-free model of the three-dimensional active region
coronal field to observed extreme-ultraviolet loops, we find that (1)
umbra-to-umbra coronal loops, despite being rooted in the strongest
magnetic flux, are invisible, and (2) the brightest loops have one
foot in an umbra or penumbra and the other foot in another sunspot's
penumbra or in unipolar or mixed-polarity plage. The invisibility of
umbra-to-umbra loops is new evidence that magnetoconvection drives
solar-stellar coronal heating: evidently, the strong umbral field at
both ends quenches the magnetoconvection and hence the heating. Our
results from EUV observations and nonlinear force-free modeling of
coronal magnetic field imply that, for any coronal loop on the Sun or
on any other convective star, as long as the field can be braided by
convection in at least one loop foot, the stronger the field in the
loop, the stronger the coronal heating.
Title: Observational and Model Analysis of a Two-ribbon Flare Possibly
Induced by a Neighboring Blowout Jet
Authors: Joshi, Bhuwan; Thalmann, Julia K.; Mitra, Prabir K.; Chandra,
Ramesh; Veronig, Astrid M.
Bibcode: 2017ApJ...851...29J
Altcode: 2017arXiv171008099J
In this paper, we present unique observations of a blowout coronal jet
that possibly triggered a two-ribbon confined C1.2 flare in bipolar
solar active region NOAA 12615 on 2016 December 5. The jet activity
initiates at chromospheric/transition region heights with a small
brightening that eventually increases in volume, with well-developed
standard morphological jet features, viz., base and spire. The spire
widens up with a collimated eruption of cool and hot plasma components,
observed in the 304 and 94 Å channels of AIA, respectively. The speed
of the plasma ejection, which forms the jet’s spire, was higher
for the hot component (∼200 km s-1) than the cooler one
(∼130 km s-1). The NLFF model of coronal fields at the
pre- and post-jet phases successfully reveals openings of previously
closed magnetic field lines with a rather inclined/low-lying jet
structure. The peak phase of the jet emission is followed by the
development of a two-ribbon flare that shows coronal loop emission in
HXRs up to ∼25 keV energy. The coronal magnetic fields rooted at the
location of EUV flare ribbons, derived from the NLFF model, demonstrate
the pre-flare phase to exhibit an “X-type” configuration, while
the magnetic fields at the post-flare phase are more or less oriented
parallel. Comparisons of multi-wavelength measurements with the magnetic
field extrapolations suggest that the jet activity likely triggered
the two-ribbon flare by perturbing the field in the interior of the
active region.
Title: Generation Mechanisms of Quasi-parallel and Quasi-circular
Flare Ribbons in a Confined Flare
Authors: Hernandez-Perez, Aaron; Thalmann, Julia K.; Veronig, Astrid
M.; Su, Yang; Gömöry, Peter; Dickson, Ewan C.
Bibcode: 2017ApJ...847..124H
Altcode: 2017arXiv170808612H
We analyze a confined multiple-ribbon M2.1 flare (SOL2015-01-29T11:42)
that originated from a fan-spine coronal magnetic field configuration,
within active region NOAA 12268. The observed ribbons form in
two steps. First, two primary ribbons form at the main flare site,
followed by the formation of secondary ribbons at remote locations. We
observe a number of plasma flows at extreme-ultraviolet temperatures
during the early phase of the flare (as early as 15 minutes before
the onset) propagating toward the formation site of the secondary
ribbons. The secondary ribbon formation is co-temporal with the
arrival of the pre-flare generated plasma flows. The primary ribbons
are co-spatial with Ramaty High Energy Spectroscopic Imager (RHESSI)
hard X-ray sources, whereas no enhanced X-ray emission is detected at
the secondary ribbon sites. The (E)UV emission, associated with the
secondary ribbons, peaks ∼1 minute after the last RHESSI hard X-ray
enhancement. A nonlinear force-free model of the coronal magnetic field
reveals that the secondary flare ribbons are not directly connected to
the primary ribbons, but to regions nearby. Detailed analysis suggests
that the secondary brightenings are produced due to dissipation of
kinetic energy of the plasma flows (heating due to compression), and
not due to non-thermal particles accelerated by magnetic reconnection,
as is the case for the primary ribbons.
Title: The Causes of Quasi-homologous CMEs
Authors: Liu, Lijuan; Wang, Yuming; Liu, Rui; Zhou, Zhenjun; Temmer,
M.; Thalmann, J. K.; Liu, Jiajia; Liu, Kai; Shen, Chenglong; Zhang,
Quanhao; Veronig, A. M.
Bibcode: 2017ApJ...844..141L
Altcode: 2017arXiv170608878L
In this paper, we identified the magnetic source locations of 142
quasi-homologous (QH) coronal mass ejections (CMEs), of which 121
are from solar cycle (SC) 23 and 21 from SC 24. Among those CMEs, 63%
originated from the same source location as their predecessor (defined
as S-type), while 37% originated from a different location within the
same active region as their predecessor (defined as D-type). Their
distinctly different waiting time distributions, peaking around 7.5 and
1.5 hr for S- and D-type CMEs, suggest that they might involve different
physical mechanisms with different characteristic timescales. Through
detailed analysis based on nonlinear force-free coronal magnetic field
modeling of two exemplary cases, we propose that the S-type QH CMES
might involve a recurring energy release process from the same source
location (by magnetic free energy replenishment), whereas the D-type
QH CMEs can happen when a flux tube system is disturbed by a nearby CME.
Title: Erratum: “The Confined X-class Flares of Solar Active Region
2192” (2015,
ApJL, 801, L23)
Authors: Thalmann, J. K.; Su, Y.; Temmer, M.; Veronig, A. M.
Bibcode: 2017ApJ...844L..27T
Altcode:
No abstract at ADS
Title: On Flare-CME Characteristics from Sun to Earth Combining
Remote-Sensing Image Data with In Situ Measurements Supported
by Modeling
Authors: Temmer, Manuela; Thalmann, Julia K.; Dissauer, Karin;
Veronig, Astrid M.; Tschernitz, Johannes; Hinterreiter, Jürgen;
Rodriguez, Luciano
Bibcode: 2017SoPh..292...93T
Altcode: 2017arXiv170300694T
We analyze the well-observed flare and coronal mass ejection (CME)
from 1 October 2011 (SOL2011-10-01T09:18) covering the complete chain of
effects - from Sun to Earth - to better understand the dynamic evolution
of the CME and its embedded magnetic field. We study in detail the
solar surface and atmosphere associated with the flare and CME using the
Solar Dynamics Observatory (SDO) and ground-based instruments. We also
track the CME signature off-limb with combined extreme ultraviolet
(EUV) and white-light data from the Solar Terrestrial Relations
Observatory (STEREO). By applying the graduated cylindrical shell
(GCS) reconstruction method and total mass to stereoscopic STEREO-SOHO
(Solar and Heliospheric Observatory) coronagraph data, we track
the temporal and spatial evolution of the CME in the interplanetary
space and derive its geometry and 3D mass. We combine the GCS and
Lundquist model results to derive the axial flux and helicity of
the magnetic cloud (MC) from in situ measurements from Wind. This is
compared to nonlinear force-free (NLFF) model results, as well as to
the reconnected magnetic flux derived from the flare ribbons (flare
reconnection flux) and the magnetic flux encompassed by the associated
dimming (dimming flux). We find that magnetic reconnection processes
were already ongoing before the start of the impulsive flare phase,
adding magnetic flux to the flux rope before its final eruption. The
dimming flux increases by more than 25% after the end of the flare,
indicating that magnetic flux is still added to the flux rope after
eruption. Hence, the derived flare reconnection flux is most probably a
lower limit for estimating the magnetic flux within the flux rope. We
find that the magnetic helicity and axial magnetic flux are lower in
the interplanetary space by ∼ 50% and 75%, respectively, possibly
indicating an erosion process. A CME mass increase of 10% is observed
over a range of ∼4 -20 R⊙. The temporal evolution of
the CME-associated core-dimming regions supports the scenario that
fast outflows might supply additional mass to the rear part of the CME.
Title: New Evidence that Magnetoconvection Drives Solar-Stellar
Coronal Heating
Authors: Tiwari, Sanjiv K.; Thalmann, Julia K.; Panesar, Navdeep K.;
Moore, Ronald L.; Winebarger, Amy R.
Bibcode: 2017ApJ...843L..20T
Altcode: 2017arXiv170608035T
How magnetic energy is injected and released in the solar
corona, keeping it heated to several million degrees, remains
elusive. Coronal heating generally increases with increasing magnetic
field strength. From a comparison of a nonlinear force-free model
of the three-dimensional active region coronal field to observed
extreme-ultraviolet loops, we find that (1) umbra-to-umbra coronal
loops, despite being rooted in the strongest magnetic flux, are
invisible, and (2) the brightest loops have one foot in an umbra or
penumbra and the other foot in another sunspot’s penumbra or in
unipolar or mixed-polarity plage. The invisibility of umbra-to-umbra
loops is new evidence that magnetoconvection drives solar-stellar
coronal heating: evidently, the strong umbral field at both ends
quenches the magnetoconvection and hence the heating. Broadly, our
results indicate that depending on the field strength in both feet,
the photospheric feet of a coronal loop on any convective star can
either engender or quench coronal heating in the loop’s body.
Title: Magnetic Helicity Estimations in Models and Observations of
the Solar Magnetic Field. III. Twist Number Method
Authors: Guo, Y.; Pariat, E.; Valori, G.; Anfinogentov, S.; Chen,
F.; Georgoulis, M. K.; Liu, Y.; Moraitis, K.; Thalmann, J. K.; Yang, S.
Bibcode: 2017ApJ...840...40G
Altcode: 2017arXiv170402096G
We study the writhe, twist, and magnetic helicity of different
magnetic flux ropes, based on models of the solar coronal magnetic
field structure. These include an analytical force-free Titov-Démoulin
equilibrium solution, non-force-free magnetohydrodynamic simulations,
and nonlinear force-free magnetic field models. The geometrical
boundary of the magnetic flux rope is determined by the quasi-separatrix
layer and the bottom surface, and the axis curve of the flux rope is
determined by its overall orientation. The twist is computed by the
Berger-Prior formula, which is suitable for arbitrary geometry and
both force-free and non-force-free models. The magnetic helicity is
estimated by the twist multiplied by the square of the axial magnetic
flux. We compare the obtained values with those derived by a finite
volume helicity estimation method. We find that the magnetic helicity
obtained with the twist method agrees with the helicity carried by the
purely current-carrying part of the field within uncertainties for
most test cases. It is also found that the current-carrying part of
the model field is relatively significant at the very location of the
magnetic flux rope. This qualitatively explains the agreement between
the magnetic helicity computed by the twist method and the helicity
contributed purely by the current-carrying magnetic field.
Title: Magnetic helicity estimations in models and observations of
the solar magnetic field
Authors: Valori, Gherardo; Pariat, Etienne; Anfinogentov, Sergey;
Chen, Feng; Georgoulis, Manolis; Guo, Yang; Liu, Yang; Moraitis,
Kostas; Thalmann, Julia K.; Yang, Shangbin
Bibcode: 2017EGUGA..19.3692V
Altcode:
Magnetic helicity, as one of the few conserved quantities
in magneto-hydrodynamics, is often invoked as the principle
driving the generation and structuring of magnetic fields in a
variety of environments, from dynamo models in stars and planets,
to post-disruption reconfigurations of tokamak's plasmas. Most
particularly magnetic helicity has raised the interest of solar
physicists, since helicity is suspected to represent a key quantity for
the understanding of solar flares and the generation of coronal mass
ejections. In recent years, several methods of estimation of magnetic
helicity have been proposed and already applied to observations and
numerical simulations. However, no systematic comparison of accuracy,
mutual consistency, and reliability of such methods has ever been
performed. We present the results of the first benchmark of several
finite-volume methods in estimating magnetic helicity in 3D test
models. In addition to finite volume methods, two additional methods
are also included that estimate magnetic helicity based either on the
field line's twist, or on the field's values on one boundary and an
inferred minimal volume connectivity. The employed model tests range
from solutions of the force-free equations to 3D magneto-hydrodynamical
numerical simulations. Almost all methods are found to produce the same
value of magnetic helicity within few percent in all tests. However,
methods show differences in the sensitivity to numerical resolution and
to errors in the solenoidal property of input fields. Our benchmark of
finite volume methods allows to determine the reliability and precision
of estimations of magnetic helicity in practical cases. As a next step,
finite volume methods are used to test estimation methods that are
based on the flux of helicity through one boundary, in particular
for applications to observation-based models of coronal magnetic
fields. The ultimate goal is to assess if and how can helicity be
meaningfully used as a diagnostic of the evolution of magnetic fields
in the solar atmosphere.
Title: Advances in solar flare science through modeling of the
magnetic field in the solar atmosphere (Arne Richter Award for
Outstanding ECSs Lecture)
Authors: Thalmann, Julia K.
Bibcode: 2017EGUGA..19.2310T
Altcode:
Ever since we know of the phenomenon of solar flares and coronal mass
ejections, we try to unravel the secrets of the underlying physical
processes. The magnetic field in the Sun's atmosphere is the driver
of any solar activity. Therefore, the combined study of the surface
(photosphere) magnetic field and the magnetic field in the atmosphere
above (the chromosphere and corona) is essential. At present, direct
measurements of the solar magnetic field are regularly available only
for the solar surface, so that we have to rely on models to reconstruct
the magnetic field in the corona. Corresponding model-based research
on the magnetic field within flaring active regions is inevitable
for the understanding of the key physical processes of flares and
possibly associated mass ejections, as well as their time evolution. I
will focus on recent advances in the understanding of the magnetic
processes in solar flares based on quasi-static force-free coronal
magnetic field modeling. In particular, I will discuss aspects
such as the structure (topology) of the coronal magnetic field, its
flare-induced reconfiguration, as well as the associated modifications
to the inherent magnetic energy and helicity. I will also discuss the
potential and limitations of studies trying to cover the complete chain
of action, i.e., to relate the (magnetic) properties of solar flares
to that of the associated disturbances measured in-situ at Earth,
as induced by flare-associated coronal mass ejections after passage
of the interplanetary space separating Sun and Earth. Finally, I will
discuss future prospects regarding model-based research of the coronal
magnetic field in the course of flares, including possible implications
for improved future flare forecasting attempts.
Title: Magnetic reconnection rates in solar flares and implications
for "superflares"
Authors: Veronig, Astrid; Tschernitz, Johannes; Hinterreiter, Jürgen;
Thalmann, Julia
Bibcode: 2017EGUGA..19.4751V
Altcode:
We present a statistical study of magnetic reconnection rates and
fluxes to study the energy release process in solar flares. Our data
set covers 50 events, including 19 eruptive flares (i.e. flares
associated with a coronal mass ejection) and 31 confined flares
(i.e. not associated with a coronal mass ejection). The events under
study are distributed over a wide range of GOES classes, from B to
>X10. Magnetic reconnection rates and fluxes are derived from the
flare ribbon evolution studied in Halpha filtergrams from Kanzelhöhe
Observatory and co-registered photospheric line-of-sight magnetic
field maps from HMI/SDO and MDI/SOHO. We find a distinct correlation
between the total flare reconnection flux with the GOES peak flux for
both eruptive and confined flares. In the largest events, the flare
reconnection fluxes may reach up to >30% of the total active region
magnetic flux. The implications of the distinct correlations obtained
are discussed with respect to the recently detected superflares on
solar-like stars and the largest flares expected on the Sun.
Title: Flare-CME characteristics from Sun to Earth combining
observations and modeling
Authors: Temmer, Manuela; Thalmann, Julia K.; Dissauer, Karin;
Veronig, Astrid M.; Tschernitz, Johannes; Hinterreiter, Jürgen;
Rodriguez, Luciano
Bibcode: 2017EGUGA..19.1942T
Altcode:
We analyze the well observed flare-CME event from October 1, 2011
(SOL2011-10-01T09:18) covering the complete chain of action - from
Sun to Earth - for a better understanding of the dynamic evolution
of the CME and its embedded magnetic field. We study in detail the
solar surface and atmosphere from SDO and ground-based instruments
associated to the flare-CME and also track the CME signature offlimb
from combined EUV and white-light data with STEREO. By applying 3D
reconstruction techniques (GCS, total mass) to stereoscopic STEREO-SoHO
coronagraph data, we track the temporal and spatial evolution of the
CME in interplanetary space and derive its geometry and 3D-mass. We
combine the GCS and Lundquist model results to derive the axial flux
and helicity of the MC from in situ measurements (Wind). This is
compared to nonlinear force-free (NLFF) model results as well as to
the reconnected magnetic flux derived from the flare ribbons (flare
reconnection flux) and the magnetic flux encompassed by the associated
dimming (dimming flux). We find that magnetic reconnection processes
were already ongoing before the start of the impulsive flare phase,
adding magnetic flux to the flux rope before its final eruption. The
dimming flux increases by more than 25% after the end of the flare,
indicating that magnetic flux is still added to the flux rope after
eruption. Hence, the derived flare reconnection flux is most probably a
lower limit for estimating the magnetic flux within the flux rope. We
obtain that the magnetic helicity and axial magnetic flux are reduced
in interplanetary space by ∼50% and 75%, respectively, possibly
indicating to an erosion process. A mass increase of 10% for the CME
is observed over the distance range from about 4-20 Rs. The temporal
evolution of the CME associated core dimming regions supports the
scenario that fast outflows might supply additional mass to the rear
part of the CME.
Title: Arcade Implosion Caused by a Filament Eruption in a Flare
Authors: Wang, Juntao; Simões, P. J. A.; Fletcher, L.; Thalmann,
J. K.; Hudson, H. S.; Hannah, I. G.
Bibcode: 2016ApJ...833..221W
Altcode: 2016arXiv161005931W
Coronal implosions—the convergence motion of plasmas and entrained
magnetic field in the corona due to a reduction in magnetic
pressure—can help to locate and track sites of magnetic energy
release or redistribution during solar flares and eruptions. We report
here on the analysis of a well-observed implosion in the form of an
arcade contraction associated with a filament eruption, during the
C3.5 flare SOL2013-06-19T07:29. A sequence of events including the
magnetic flux-rope instability and distortion, followed by a filament
eruption and arcade implosion, lead us to conclude that the implosion
arises from the transfer of magnetic energy from beneath the arcade
as part of the global magnetic instability, rather than due to local
magnetic energy dissipation in the flare. The observed net contraction
of the imploding loops, which is found also in nonlinear force-free
field extrapolations, reflects a permanent reduction of magnetic
energy underneath the arcade. This event shows that, in addition to
resulting in the expansion or eruption of an overlying field, flux-rope
instability can also simultaneously implode an unopened field due to
magnetic energy transfer. It demonstrates the “partial opening of
the field” scenario, which is one of the ways in 3D to produce a
magnetic eruption without violating the Aly-Sturrock hypothesis. In
the framework of this observation, we also propose a unification of
three main concepts for active region magnetic evolution, namely the
metastable eruption model, the implosion conjecture, and the standard
“CSHKP” flare model.
Title: Magnetic Helicity Estimations in Models and Observations of
the Solar Magnetic Field. Part I: Finite Volume Methods
Authors: Valori, Gherardo; Pariat, Etienne; Anfinogentov, Sergey;
Chen, Feng; Georgoulis, Manolis K.; Guo, Yang; Liu, Yang; Moraitis,
Kostas; Thalmann, Julia K.; Yang, Shangbin
Bibcode: 2016SSRv..201..147V
Altcode: 2016SSRv..tmp...68V; 2016arXiv161002193V
Magnetic helicity is a conserved quantity of ideal magneto-hydrodynamics
characterized by an inverse turbulent cascade. Accordingly, it
is often invoked as one of the basic physical quantities driving
the generation and structuring of magnetic fields in a variety of
astrophysical and laboratory plasmas. We provide here the first
systematic comparison of six existing methods for the estimation of
the helicity of magnetic fields known in a finite volume. All such
methods are reviewed, benchmarked, and compared with each other,
and specifically tested for accuracy and sensitivity to errors. To
that purpose, we consider four groups of numerical tests, ranging
from solutions of the three-dimensional, force-free equilibrium, to
magneto-hydrodynamical numerical simulations. Almost all methods are
found to produce the same value of magnetic helicity within few percent
in all tests. In the more solar-relevant and realistic of the tests
employed here, the simulation of an eruptive flux rope, the spread
in the computed values obtained by all but one method is only 3 %,
indicating the reliability and mutual consistency of such methods in
appropriate parameter ranges. However, methods show differences in the
sensitivity to numerical resolution and to errors in the solenoidal
property of the input fields. In addition to finite volume methods,
we also briefly discuss a method that estimates helicity from the
field lines' twist, and one that exploits the field's value at one
boundary and a coronal minimal connectivity instead of a pre-defined
three-dimensional magnetic-field solution.
Title: Erratum: “Evolution of Magnetic Field and Energy in A
Major Eruptive Active Region Based on SDO/HMI Observation” (2012, ApJ,
748, 77)
Authors: Sun, Xudong; Hoeksema, J. Todd; Liu, Yang; Wiegelmann,
Thomas; Hayashi, Keiji; Chen, Qingrong; Thalmann, Julia
Bibcode: 2016ApJ...828...65S
Altcode:
No abstract at ADS
Title: Temporal and Spatial Relationship of Flare Signatures and
the Force-free Coronal Magnetic Field
Authors: Thalmann, J. K.; Veronig, A.; Su, Y.
Bibcode: 2016ApJ...826..143T
Altcode: 2016arXiv160503703T
We investigate the plasma and magnetic environment of active
region NOAA 11261 on 2011 August 2 around a GOES M1.4 flare/CME
(SOL2011-08-02T06:19). We compare coronal emission at the (extreme)
ultraviolet and X-ray wavelengths, using SDO AIA and RHESSI
images, in order to identify the relative timing and locations of
reconnection-related sources. We trace flare ribbon signatures at
ultraviolet wavelengths in order to pin down the intersection
of previously reconnected flaring loops in the lower solar
atmosphere. These locations are used to calculate field lines from
three-dimensional (3D) nonlinear force-free magnetic field models,
established on the basis of SDO HMI photospheric vector magnetic
field maps. Using this procedure, we analyze the quasi-static time
evolution of the coronal model magnetic field previously involved
in magnetic reconnection. This allows us, for the first time, to
estimate the elevation speed of the current sheet’s lower tip during
an on-disk observed flare as a few kilometers per second. A comparison
to post-flare loops observed later above the limb in STEREO EUVI images
supports this velocity estimate. Furthermore, we provide evidence for
an implosion of parts of the flaring coronal model magnetic field,
and identify the corresponding coronal sub-volumes associated with
the loss of magnetic energy. Finally, we spatially relate the build
up of magnetic energy in the 3D models to highly sheared fields,
established due to the dynamic relative motions of polarity patches
within the active region.
Title: Suppression of heating of coronal loops rooted in opposite
polarity sunspot umbrae
Authors: Tiwari, Sanjiv K.; Thalmann, Julia; Moore, Ronald; Panesar,
Navdeep; Winebarger, Amy
Bibcode: 2016shin.confE..61T
Altcode:
EUV observations of active region (AR) coronae reveal the presence
of loops at different temperatures. To understand the mechanisms that
result in hotter or cooler loops, we study a typical bipolar AR, near
solar disk center, which has moderate overall magnetic twist and at
least one fully developed sunspot of each polarity. From AIA 193 and
94 Å images we identify many clearly discernible coronal loops that
connect plage or a sunspot of one polarity to an opposite-polarity
plage region. The AIA 94 Å images show dim regions in the umbrae of
the sunspots. To see which coronal loops are rooted in a dim umbral
area, we performed a non-linear force-free field (NLFFF) modeling
using photospheric vector magnetic field measurements obtained with
the Heliosesmic Magnetic Imager (HMI) onboard SDO. The NLFFF model,
validated by comparison of calculated model field lines with observed
loops in AIA 193 and 94 Å, specifies the photospheric roots of the
model field lines. Some model coronal magnetic field lines arch from
the dim umbral area of the positive-polarity sunspot to the dim umbral
area of a negative-polarity sunspot. Because these coronal loops are
not visible in any of the coronal EUV and X-ray images of the AR, we
conclude they are the coolest loops in the AR. This result suggests
that the loops connecting opposite polarity umbrae are the least heated
because the field in umbrae is so strong that the convective braiding
of the field is strongly suppressed.
Title: Suppression of heating of coronal loops rooted in opposite
polarity sunspot umbrae
Authors: Tiwari, Sanjiv K.; Thalmann, Julia K.; Moore, Ronald L.;
Panesar, Navdeep; Winebarger, Amy R.
Bibcode: 2016SPD....47.0336T
Altcode:
EUV observations of active region (AR) coronae reveal the presence
of loops at different temperatures. To understand the mechanisms that
result in hotter or cooler loops, we study a typical bipolar AR, near
solar disk center, which has moderate overall magnetic twist and at
least one fully developed sunspot of each polarity. From AIA 193 and
94 A images we identify many clearly discernible coronal loops that
connect plage or a sunspot of one polarity to an opposite-polarity
plage region. The AIA 94 A images show dim regions in the umbrae of
the spots. To see which coronal loops are rooted in a dim umbral area,
we performed a non-linear force-free field (NLFFF) modeling using
photospheric vector magnetic field measurements obtained with the
HMI onboard SDO. After validation of the NLFFF model by comparison of
calculated model field lines and observed loops in AIA 193 and 94, we
specify the photospheric roots of the model field lines. The model field
then shows the coronal magnetic loops that arch from the dim umbral
areas of the opposite polarity sunspots. Because these coronal loops
are not visible in any of the coronal EUV and X-ray images of the AR,
we conclude they are the coolest loops in the AR. This result suggests
that the loops connecting opposite polarity umbrae are the least
heated because the field in umbrae is so strong that the convective
braiding of the field is strongly suppressed.We hypothesize that the
convective freedom at the feet of a coronal loop, together with the
strength of the field in the body of the loop, determines the strength
of the heating. In particular, we expect the hottest coronal loops
to have one foot in an umbra and the other foot in opposite-polarity
penumbra or plage (coronal moss), the areas of strong field in which
convection is not as strongly suppressed as in umbra. Many transient,
outstandingly bright, loops in the AIA 94 movie of the AR do have this
expected rooting pattern. We will also present another example of AR
in which we find a similar rooting pattern of coronal loops.
Title: Exceptions to the rule: the X-flares of AR 2192 Lacking
Coronal Mass Ejections
Authors: Thalmann, J. K.; Su, Y.; Temmer, M.; Veronig, A. M.
Bibcode: 2016ASPC..504..203T
Altcode:
NOAA Active region (AR) 2192, that was present on the Sun in October
2014, was the largest region which occurred since November 1990
(see Figure 1). The huge size accompanied by a very high activity
level, was quite unexpected as it appeared during the unusually weak
solar cycle 24. Nevertheless, the AR turned out to be one of the most
prolific flaring ARs of cycle 24. It produced in total 6 X, 29 M, 79
C flares during its disk passage from October 18-29, 2014 (see Figure
2). Surprisingly, all flares greater than GOES class M5 and X were
confined, i.e. had no coronal mass ejections (CME) associated. All
the flare events had some obvious similarity in morphology, as they
were located in the core of the AR and revealed only minor separation
motion away from the neutral line but a large initial separation of
the conjugate flare ribbons. In the paper by Thalmann et al. (2015)
we describe the series of flares and give details about the confined
X1.6 flare event from October 22, 2014 as well as the single eruptive
M4.0 flare event from October 24, 2014. The study of the X1.6 flare
revealed a large initial separation of flare ribbons together with
recurrent flare brightenings, which were related to two episodes of
enhanced hard X-ray emission as derived from RHESSI observations. This
suggests that magnetic field structures connected to specific regions
were repeatedly involved in the process of reconnection and energy
release. Opposite to the central location of the sequence of confined
events within the AR, a single eruptive (M4.0) event occurred on
the outskirt of the AR in the vicinity of open magnetic fields. Our
investigations revealed a predominantly north-south oriented magnetic
system of arcade fields overlying the AR that could have preserved
the magnetic arcade to erupt, and consequently kept the energy release
trapped in a localized volume of magnetic field high up in the corona
(as supported by the absence of a lateral motion of the flare ribbons
and the recurrent brightenings within them). We conclude that the
background magnetic field configuration is an essential parameter
for deriving the "eruptiveness" of flare events. Sun et al. (2015)
supports this conclusion and derived for this AR a quite slow
decay of the strength of the overlying magnetic field (decay index;
see Török & Kliem 2005). Interestingly, our magnetic field
modellings revealed no flux rope inherent to the AR, indicating that
further investigations are needed. In a recent paper by Veronig $
Polanec (2015), who investigated in more detail the X-flares using
also ground-based observations in Hα from Kanzelhöhe Observatory
(Pötzi et al. 2015), it was shown that such confined events could be
explained by the emerging-flux model, where newly emerging small flux
tubes reconnect with pre-existing large coronal loops.
Title: Chromospheric evaporation flows and density changes deduced
from Hinode/EIS during an M1.6 flare
Authors: Gömöry, P.; Veronig, A. M.; Su, Y.; Temmer, M.; Thalmann,
J. K.
Bibcode: 2016A&A...588A...6G
Altcode: 2016arXiv160202145G
Aims: We study the response of the solar atmosphere during a GOES
M1.6 flare using spectroscopic and imaging observations. In particular,
we examine the evolution of the mass flows and electron density together
with the energy input derived from hard X-ray (HXR) in the context of
chromospheric evaporation.
Methods: We analyzed high-cadence
sit-and-stare observations acquired with the Hinode/EIS spectrometer
in the Fe xiii 202.044 Å (log T = 6.2) and Fe xvi 262.980 Å (log T =
6.4) spectral lines to derive temporal variations of the line intensity,
Doppler shifts, and electron density during the flare. We combined these
data with HXR measurements acquired with RHESSI to derive the energy
input to the lower atmosphere by flare-accelerated electrons.
Results: During the flare impulsive phase, we observe no significant
flows in the cooler Fe xiii line but strong upflows, up to 80-150 km
s-1, in the hotter Fe xvi line. The largest Doppler shifts
observed in the Fe xvi line were co-temporal with the sharp intensity
peak. The electron density obtained from a Fe xiii line pair ratio
exhibited fast increase (within two minutes) from the pre-flare level
of 5.01 × 109 cm-3 to 3.16 × 1010
cm-3 during the flare peak. The nonthermal energy flux
density deposited from the coronal acceleration site to the lower
atmospheric layers during the flare peak was found to be 1.34 ×
1010 erg s-1 cm-2 for a low-energy
cut-off that was estimated to be 16 keV. During the decline flare phase,
we found a secondary intensity and density peak of lower amplitude
that was preceded by upflows of ~15 km s-1 that were
detected in both lines. The flare was also accompanied by a filament
eruption that was partly captured by the EIS observations. We derived
Doppler velocities of 250-300 km s-1 for the upflowing
filament material.
Conclusions: The spectroscopic results
for the flare peak are consistent with the scenario of explosive
chromospheric evaporation, although a comparatively low value of the
nonthermal energy flux density was determined for this phase of the
flare. This outcome is discussed in the context of recent hydrodynamic
simulations. It provides observational evidence that the response
of the atmospheric plasma strongly depends on the properties of the
electron beams responsible for the heating, in particular the steepness
of the energy distribution. The secondary peak of line intensity and
electron density detected during the decline phase is interpreted as a
signature of flare loops being filled by expanding hot material that
is due to chromospheric evaporation. A movie is available at http://www.aanda.org
Title: Space Weather and confined CME events
Authors: Thalmann, Julia; Temmer, Manuela; Veronig, Astrid; Su, Yang
Bibcode: 2016EGUGA..18.7517T
Altcode:
The unusually large NOAA active region (AR) 2192, observed in October
and November 2014, was outstanding in its productivity of major flares
(GOES class M5 and larger). During the time when the AR faced Earth,
major Space Weather events would have been expected. However, none of
the X-flares was associated to a coronal mass ejection. Observational
evidence for the confinement of the flare are large initial separation
of the flare ribbons, together with an almost absent growth in ribbon
separation. The low dynamic of the ribbons also suggests a reconnection
site high up in the corona. From NLFF modeling we show that the
arcade overlying the AR had a predominantly north-south oriented
magnetic system, which served as a strong, also lateral, confinement
for the flares at the core of the active region. From the magnetic
field modeling we derived the decay of the constraining background,
and it was found that the overlying field was only slowly decaying
with height. We conclude that observational data of the solar surface,
especially of flare ribbon dynamics as well as magnetic field models
support Space Weather predictions.
Title: The exceptional aspects of the confined X-class flares of
solar active region 2192
Authors: Thalmann, Julia K.; Su, Yang; Temmer, Manuela; Veronig,
Astrid M.
Bibcode: 2016IAUS..320...60T
Altcode: 2016arXiv160503712T
During late October 2014, active region NOAA 2192 caused an unusual high
level of solar activity, within an otherwise weak solar cycle. While
crossing the solar disk, during a period of 11 days, it was the source
of 114 flares of GOES class C1.0 and larger, including 29 M- and 6
X-flares. Surprisingly, none of the major flares (GOES class M5.0
and larger) was accompanied by a coronal mass ejection, contrary to
statistical tendencies found in the past. From modeling the coronal
magnetic field of NOAA 2192 and its surrounding, we suspect that the
cause of the confined character of the flares is the strong surrounding
and overlying large-scale magnetic field. Furthermore, we find evidence
for multiple magnetic reconnection processes within a single flare,
during which electrons were accelerated to unusual high energies.
Title: The Influence of Spatial resolution on Nonlinear Force-free
Modeling
Authors: DeRosa, M. L.; Wheatland, M. S.; Leka, K. D.; Barnes, G.;
Amari, T.; Canou, A.; Gilchrist, S. A.; Thalmann, J. K.; Valori,
G.; Wiegelmann, T.; Schrijver, C. J.; Malanushenko, A.; Sun, X.;
Régnier, S.
Bibcode: 2015ApJ...811..107D
Altcode: 2015arXiv150805455D
The nonlinear force-free field (NLFFF) model is often used to
describe the solar coronal magnetic field, however a series of
earlier studies revealed difficulties in the numerical solution of the
model in application to photospheric boundary data. We investigate
the sensitivity of the modeling to the spatial resolution of the
boundary data, by applying multiple codes that numerically solve the
NLFFF model to a sequence of vector magnetogram data at different
resolutions, prepared from a single Hinode/Solar Optical Telescope
Spectro-Polarimeter scan of NOAA Active Region 10978 on 2007 December
13. We analyze the resulting energies and relative magnetic helicities,
employ a Helmholtz decomposition to characterize divergence errors, and
quantify changes made by the codes to the vector magnetogram boundary
data in order to be compatible with the force-free model. This study
shows that NLFFF modeling results depend quantitatively on the spatial
resolution of the input boundary data, and that using more highly
resolved boundary data yields more self-consistent results. The
free energies of the resulting solutions generally trend higher
with increasing resolution, while relative magnetic helicity values
vary significantly between resolutions for all methods. All methods
require changing the horizontal components, and for some methods also
the vertical components, of the vector magnetogram boundary field in
excess of nominal uncertainties in the data. The solutions produced
by the various methods are significantly different at each resolution
level. We continue to recommend verifying agreement between the modeled
field lines and corresponding coronal loop images before any NLFFF
model is used in a scientific setting.
Title: Two-fluid 2.5D code for simulations of small scale magnetic
fields in the lower solar atmosphere
Authors: Piantschitsch, Isabell; Amerstorfer, Ute; Thalmann, Julia
Katharina; Hanslmeier, Arnold; Lemmerer, Birgit
Bibcode: 2015IAUGA..2250036P
Altcode:
Our aim is to investigate magnetic reconnection as a result of the
time evolution of magnetic flux tubes in the solar chromosphere. A
new numerical two-fluid code was developed, which will perform a
2.5D simulation of the dynamics from the upper convection zone up
to the transition region. The code is based on the Total Variation
Diminishing Lax-Friedrichs method and includes the effects of
ion-neutral collisions, ionisation/recombination, thermal/resistive
diffusivity as well as collisional/resistive heating. What is innovative
about our newly developed code is the inclusion of a two-fluid model
in combination with the use of analytically constructed vertically
open magnetic flux tubes, which are used as initial conditions for
our simulation. First magnetohydrodynamic (MHD) tests have already
shown good agreement with known results of numerical MHD test problems
like e.g. the Orszag-Tang vortex test, the Current Sheet test or the
Spherical Blast Wave test. Furthermore, the single-fluid approach will
also be applied to the initial conditions, in order to compare the
different rates of magnetic reconnection in both codes, the two-fluid
code and the single-fluid one.
Title: The exceptional aspects of the confined X-Flares of Solar
Active Region 2192
Authors: Thalmann, Julia K.; Su, Yang; Temmer, Manuela; Veronig, Astrid
Bibcode: 2015IAUGA..2215645T
Altcode:
Active region NOAA 2192 showed an outstanding productivity
of major (GOES class M5 and larger) two-ribbon flares lacking
eruptive events. None of the X-flares was associated to a coronal
mass ejection. The major confined flares on 2014 October 22 and 24
originated from the active-region core and were prohibited to develop
an associated mass ejection due to the confinement of the overlying
strong magnetic field. In contrast, the single eruptive M-flare on
October 24 originated from the outer parts of the active region, in the
neighborhood of open large-scale fields, which allowed for the observed
mass ejection. Analysis of the spacial and temporal characteristics
of the major confined flares revealed exceptional aspects, including a
large initial separation of the confined flares' ribbons and an almost
absent growth in ribbon separation, suggesting a reconnection site
high up in the corona. Furthermore, detailed analysis of a confined
X-flare on October 22 provides evidence that magnetic field structures
were repeatedly involved in magnetic reconnection, that a large number
of electrons was accelerated to non-thermal energies but that only a
small fraction out of these accelerated electrons was accelerated to
high energies. We conclude the latter due to the unusual steepness
of the associated power law spectrum. Finally, we demonstrate that
a considerable portion of the magnetic energy released during the
X-flare was consumed by the non-thermal flare energy.
Title: Evidence of suppressed heating of coronal loops rooted in
opposite polarity sunspot umbrae
Authors: Tiwari, Sanjiv K.; Thalmann, Julia K.; Winebarger, Amy R.;
Panesar, Navdeep K.; Moore, Ronald
Bibcode: 2015TESS....120404T
Altcode:
Observations of active region (AR) coronae in different EUV wavelengths
reveal the presence of various loops at different temperatures. To
understand the mechanisms that result in hotter or cooler loops, we
study a typical bipolar AR, near solar disk center, which has moderate
overall magnetic twist and at least one fully developed sunspot of
each polarity. From AIA 193 and 94 A images we identify many clearly
discernible coronal loops that connect opposite-polarity plage or
a sunspot to a opposite-polarity plage region. The AIA 94 A images
show dim regions in the umbrae of the spots. To see which coronal
loops are rooted in a dim umbral area, we performed a non-linear
force-free field (NLFFF) modeling using photospheric vector magnetic
field measurements obtained with the Heliosesmic Magnetic Imager (HMI)
onboard SDO. After validation of the NLFFF model by comparison of
calculated model field lines and observed loops in AIA 193 and 94 A,
we specify the photospheric roots of the model field lines. The model
field then shows the coronal magnetic loops that arch from the dim
umbral area of the positive-polarity sunspot to the dim umbral area of a
negative-polarity sunspot. Because these coronal loops are not visible
in any of the coronal EUV and X-ray images of the AR, we conclude they
are the coolest loops in the AR. This result suggests that the loops
connecting opposite polarity umbrae are the least heated because the
field in umbrae is so strong that the convective braiding of the field
is strongly suppressed.From this result, we further hypothesize that
the convective freedom at the feet of a coronal loop, together with the
strength of the field in the body of the loop, determines the strength
of the heating. In particular, we expect the hottest coronal loops
to have one foot in an umbra and the other foot in opposite-polarity
penumbra or plage (coronal moss), the areas of strong field in which
convection is not as strongly suppressed as in umbrae. Many transient,
outstandingly bright, loops in the AIA 94 A movie of the AR do have
this expected rooting pattern.
Title: The Confined X-class Flares of Solar Active Region 2192
Authors: Thalmann, J. K.; Su, Y.; Temmer, M.; Veronig, A. M.
Bibcode: 2015ApJ...801L..23T
Altcode: 2015arXiv150205157T
The unusually large active region (AR) NOAA 2192, observed in 2014
October, was outstanding in its productivity of major two-ribbon flares
without coronal mass ejections. On a large scale, a predominantly
north-south oriented magnetic system of arcade fields served as a strong
top and lateral confinement for a series of large two-ribbon flares
originating from the core of the AR. The large initial separation of
the flare ribbons, together with an almost absent growth in ribbon
separation, suggests a confined reconnection site high up in the
corona. Based on a detailed analysis of the confined X1.6 flare on
October 22, we show how exceptional the flaring of this AR was. We
provide evidence for repeated energy release, indicating that the
same magnetic field structures were repeatedly involved in magnetic
reconnection. We find that a large number of electrons was accelerated
to non-thermal energies, revealing a steep power-law spectrum, but
that only a small fraction was accelerated to high energies. The total
non-thermal energy in electrons derived (on the order of 1025
J) is considerably higher than that in eruptive flares of class X1,
and corresponds to about 10% of the excess magnetic energy present in
the active-region corona.
Title: The magnetic field in the solar atmosphere
Authors: Wiegelmann, Thomas; Thalmann, Julia K.; Solanki, Sami K.
Bibcode: 2014A&ARv..22...78W
Altcode: 2014arXiv1410.4214W
This publication provides an overview of magnetic fields in the solar
atmosphere with the focus lying on the corona. The solar magnetic field
couples the solar interior with the visible surface of the Sun and with
its atmosphere. It is also responsible for all solar activity in its
numerous manifestations. Thus, dynamic phenomena such as coronal mass
ejections and flares are magnetically driven. In addition, the field
also plays a crucial role in heating the solar chromosphere and corona
as well as in accelerating the solar wind. Our main emphasis is the
magnetic field in the upper solar atmosphere so that photospheric and
chromospheric magnetic structures are mainly discussed where relevant
for higher solar layers. Also, the discussion of the solar atmosphere
and activity is limited to those topics of direct relevance to the
magnetic field. After giving a brief overview about the solar magnetic
field in general and its global structure, we discuss in more detail
the magnetic field in active regions, the quiet Sun and coronal holes.
Title: Force-free Field Modeling of Twist and Braiding-induced
Magnetic Energy in an Active-region Corona
Authors: Thalmann, J. K.; Tiwari, S. K.; Wiegelmann, T.
Bibcode: 2014ApJ...780..102T
Altcode: 2013arXiv1311.3413T
The theoretical concept that braided magnetic field lines in the solar
corona may dissipate a sufficient amount of energy to account for the
brightening observed in the active-region (AR) corona has only recently
been substantiated by high-resolution observations. From the analysis
of coronal images obtained with the High Resolution Coronal Imager,
first observational evidence of the braiding of magnetic field lines
was reported by Cirtain et al. (hereafter CG13). We present nonlinear
force-free reconstructions of the associated coronal magnetic field
based on Solar Dynamics Observatory/Helioseismic and Magnetic Imager
vector magnetograms. We deliver estimates of the free magnetic energy
associated with a braided coronal structure. Our model results suggest
(~100 times) more free energy at the braiding site than analytically
estimated by CG13, strengthening the possibility of the AR corona
being heated by field line braiding. We were able to appropriately
assess the coronal free energy by using vector field measurements and
we attribute the lower energy estimate of CG13 to the underestimated
(by a factor of 10) azimuthal field strength. We also quantify the
increase in the overall twist of a flare-related flux rope that was
noted by CG13. From our models we find that the overall twist of the
flux rope increased by about half a turn within 12 minutes. Unlike
another method to which we compare our results, we evaluate the
winding of the flux rope's constituent field lines around each other
purely based on their modeled coronal three-dimensional field line
geometry. To our knowledge, this is done for the first time here.
Title: Two-Fluid 2.5D MHD-Code for Simulations in the Solar Atmosphere
Authors: Piantschitsch, I.; Amerstorfer, U.; Thalmann, J.; Utz, D.;
Hanslmeier, A.; Bárta, M.; Thonhofer, S.; Lemmerer, B.
Bibcode: 2014CEAB...38...59P
Altcode:
We investigate magnetic reconnection due to the evolution of magnetic
flux tubes in the solar chromosphere. We developed a new numerical
two-fluid magnetohydrodynamic (MHD) code which will perform a 2.5D
simulation of the dynamics from the upper convection zone up to the
transition region. Our code is based on the Total Variation Diminishing
Lax-Friedrichs scheme and makes use of an alternating-direction implicit
method, in order to accommodate the two spatial dimensions. Since we
apply a two-fluid model for our simulations, the effects of ion-neutral
collisions, ionization/recombination, thermal/resistive diffusivity
and collisional/resistive heating are included in the code. As initial
conditions for the code we use analytically constructed vertically open
magnetic flux tubes within a realistic stratified atmosphere. Initial
MHD tests have already shown good agreement with known results of
numerical MHD test problems like e.g. the Orszag-Tang vortex test.
Title: Twisting solar coronal jet launched at the boundary of an
active region
Authors: Schmieder, B.; Guo, Y.; Moreno-Insertis, F.; Aulanier, G.;
Yelles Chaouche, L.; Nishizuka, N.; Harra, L. K.; Thalmann, J. K.;
Vargas Dominguez, S.; Liu, Y.
Bibcode: 2013A&A...559A...1S
Altcode: 2013arXiv1309.6514S
Aims: A broad jet was observed in a weak magnetic field area
at the edge of active region NOAA 11106 that also produced other
nearby recurring and narrow jets. The peculiar shape and magnetic
environment of the broad jet raised the question of whether it was
created by the same physical processes of previously studied jets
with reconnection occurring high in the corona.
Methods:
We carried out a multi-wavelength analysis using the EUV images
from the Atmospheric Imaging Assembly (AIA) and magnetic fields
from the Helioseismic and Magnetic Imager (HMI) both on-board the
Solar Dynamics Observatory, which we coupled to a high-resolution,
nonlinear force-free field extrapolation. Local correlation tracking
was used to identify the photospheric motions that triggered the jet,
and time-slices were extracted along and across the jet to unveil its
complex nature. A topological analysis of the extrapolated field was
performed and was related to the observed features.
Results:
The jet consisted of many different threads that expanded in around 10
minutes to about 100 Mm in length, with the bright features in later
threads moving faster than in the early ones, reaching a maximum speed
of about 200 km s-1. Time-slice analysis revealed a striped
pattern of dark and bright strands propagating along the jet, along with
apparent damped oscillations across the jet. This is suggestive of a
(un)twisting motion in the jet, possibly an Alfvén wave. Bald patches
in field lines, low-altitude flux ropes, diverging flow patterns, and a
null point were identified at the basis of the jet.
Conclusions:
Unlike classical λ or Eiffel-tower-shaped jets that appear to be caused
by reconnection in current sheets containing null points, reconnection
in regions containing bald patches seems to be crucial in triggering
the present jet. There is no observational evidence that the flux
ropes detected in the topological analysis were actually being ejected
themselves, as occurs in the violent phase of blowout jets; instead,
the jet itself may have gained the twist of the flux rope(s) through
reconnection. This event may represent a class of jets different from
the classical quiescent or blowout jets, but to reach that conclusion,
more observational and theoretical work is necessary.
Title: Comparison of Force-free Coronal Magnetic Field Modeling
Using Vector Fields from Hinode and Solar Dynamics Observatory
Authors: Thalmann, J. K.; Tiwari, S. K.; Wiegelmann, T.
Bibcode: 2013ApJ...769...59T
Altcode: 2013arXiv1304.3619T
Photospheric magnetic vector maps from two different instruments
are used to model the nonlinear force-free coronal magnetic field
above an active region. We use vector maps inferred from polarization
measurements of the Solar Dynamics Observatory/Helioseismic and Magnetic
Imager (HMI) and the Solar Optical Telescope's Spectropolarimeter (SP)
on board Hinode. Besides basing our model calculations on HMI data,
we use both SP data of original resolution and scaled down to the
resolution of HMI. This allows us to compare the model results based
on data from different instruments and to investigate how a binning
of high-resolution data affects the model outcome. The resulting
three-dimensional magnetic fields are compared in terms of magnetic
energy content and magnetic topology. We find stronger magnetic fields
in the SP data, translating into a higher total magnetic energy
of the SP models. The net Lorentz forces of the HMI and SP lower
boundaries verify their force-free compatibility. We find substantial
differences in the absolute estimates of the magnetic field energy but
similar relative estimates, e.g., the fraction of excess energy and
of the flux shared by distinct areas. The location and extension of
neighboring connectivity domains differ and the SP model fields tend
to be higher and more vertical. Hence, conclusions about the magnetic
connectivity based on force-free field models are to be drawn with
caution. We find that the deviations of the model solution when based
on the lower-resolution SP data are small compared to the differences
of the solutions based on data from different instruments.
Title: Force-free coronal magnetic field modeling using vector fields
from Hinode and SDO
Authors: Thalmann, Julia K.; Tiwari, Sanjiv K.; Wiegelmann, Thomas
Bibcode: 2013EGUGA..15.1368T
Altcode:
Given the lack of routine direct measurements of the magnetic
field in the solar corona, force-free reconstruction methods are
a promising tool for the diagnostics of the magnetic structure
there. Routine photospheric magnetic field measurements which monitor
the temporal evolution of an active region and contain information on
the non-potentiality of the field above are used as an input. Based on
the assumption that magnetic forces dominate the solar atmosphere, these
models allow estimates of the total and free magnetic energy content and
the structure of the magnetic field above active regions. The outcome
of force-free field modeling strongly depends on the vector magnetic
field data used as boundary condition. We compare the model results
based on simultaneously observed vector maps from the Helioseismic and
Magnetic Imager (HMI) on board Solar Dynamics Observatory and from the
Solar Optical Telescope Spectropolarimeter (SP) on board Hinode. We
find substantial differences in the absolute estimates of the magnetic
field energy but very similar relative estimates, e.g., the fraction
of energy to be set free during an eruption or the fraction of flux
linking distinct areas within an active region. Our study reveals that
only relative estimates of coronal physical quantities from force-free
models might be save and conclusions about the magnetic field topology
might be drawn with caution.
Title: On the Comparison of Nonlinear Force-free Models Based on
Vector-magnetograms from Different Instruments
Authors: Thalmann, J. K.; Wiegelmann, T.; Tiwari, S. K.; Sun, X.
Bibcode: 2012AGUFMSH41C2120T
Altcode:
We investigate the three-dimensional structure of the magnetic field in
the outer solar atmosphere with the help of photospheric magnetic vector
maps based on measurements from the Helioseismic and Magnetic Imager
(HMI) on board the Solar Dynamics Observatory and of the Solar Optical
Telescope Spectral-polarimeter (SP) on board the Hinode spacecraft. HMI
and SP magnetic vector maps of NOAA AR 11382 on 21-22 December 2011
are used as lower boundary condition for nonlinear force-free field
reconstructions. We compare the resulting three-dimensional coronal
magnetic field models in terms of the energy content, the magnetic
pressure, the vertical distribution of the magnetic field and
associated electric current density, as well as the magnetic field
line configuration and compare the latter to the loops visible in
coronal images from the SDO Atmospheric Imaging Assembly.
Title: How Should One Optimize Nonlinear Force-Free Coronal Magnetic
Field Extrapolations from SDO/HMI Vector Magnetograms?
Authors: Wiegelmann, T.; Thalmann, J. K.; Inhester, B.; Tadesse, T.;
Sun, X.; Hoeksema, J. T.
Bibcode: 2012SoPh..281...37W
Altcode: 2012SoPh..tmp...67W; 2012arXiv1202.3601W
The Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics
Observatory (SDO) provides photospheric vector magnetograms with
a high spatial and temporal resolution. Our intention is to model
the coronal magnetic field above active regions with the help of
a nonlinear force-free extrapolation code. Our code is based on an
optimization principle and has been tested extensively with semianalytic
and numeric equilibria and applied to vector magnetograms from Hinode
and ground-based observations. Recently we implemented a new version
which takes into account measurement errors in photospheric vector
magnetograms. Photospheric field measurements are often affected by
measurement errors and finite nonmagnetic forces inconsistent for use
as a boundary for a force-free field in the corona. To deal with these
uncertainties, we developed two improvements: i) preprocessing of the
surface measurements to make them compatible with a force-free field,
and ii) new code which keeps a balance between the force-free constraint
and deviation from the photospheric field measurements. Both methods
contain free parameters, which must be optimized for use with data from
SDO/HMI. In this work we describe the corresponding analysis method
and evaluate the force-free equilibria by how well force-freeness and
solenoidal conditions are fulfilled, by the angle between magnetic
field and electric current, and by comparing projections of magnetic
field lines with coronal images from the Atmospheric Imaging Assembly
(SDO/AIA). We also compute the available free magnetic energy and
discuss the potential influence of control parameters.
Title: Nonlinear Force-free Field Modeling of a Solar Active Region
Using SDO/HMI and SOLIS/VSM Data
Authors: Thalmann, J. K.; Pietarila, A.; Sun, X.; Wiegelmann, T.
Bibcode: 2012AJ....144...33T
Altcode: 2012arXiv1206.1141T
We use SDO/HMI and SOLIS/VSM photospheric magnetic field measurements
to model the force-free coronal field above a solar active region,
assuming magnetic forces dominate. We take measurement uncertainties
caused by, e.g., noise and the particular inversion technique, into
account. After searching for the optimum modeling parameters for the
particular data sets, we compare the resulting nonlinear force-free
model fields. We show the degree of agreement of the coronal field
reconstructions from the different data sources by comparing the
relative free energy content, the vertical distribution of the magnetic
pressure, and the vertically integrated current density. Though the
longitudinal and transverse magnetic flux measured by the VSM and
HMI is clearly different, we find considerable similarities in the
modeled fields. This indicates the robustness of the algorithm we use
to calculate the nonlinear force-free fields against differences and
deficiencies of the photospheric vector maps used as an input. We also
depict how much the absolute values of the total force-free, virial,
and the free magnetic energy differ and how the orientation of the
longitudinal and transverse components of the HMI- and VSM-based model
volumes compare to each other.
Title: Evolution of Magnetic Field and Energy in a Major Eruptive
Active Region Based on SDO/HMI Observation
Authors: Sun, Xudong; Hoeksema, J. Todd; Liu, Yang; Wiegelmann,
Thomas; Hayashi, Keiji; Chen, Qingrong; Thalmann, Julia
Bibcode: 2012ApJ...748...77S
Altcode: 2012arXiv1201.3404S
We report the evolution of the magnetic field and its energy in NOAA
active region 11158 over five days based on a vector magnetogram series
from the Helioseismic and Magnetic Imager (HMI) on board the Solar
Dynamic Observatory (SDO). Fast flux emergence and strong shearing
motion led to a quadrupolar sunspot complex that produced several
major eruptions, including the first X-class flare of Solar Cycle
24. Extrapolated nonlinear force-free coronal fields show substantial
electric current and free energy increase during early flux emergence
near a low-lying sigmoidal filament with a sheared kilogauss field
in the filament channel. The computed magnetic free energy reaches a
maximum of ~2.6 × 1032 erg, about 50% of which is stored
below 6 Mm. It decreases by ~0.3 × 1032 erg within 1 hr
of the X-class flare, which is likely an underestimation of the actual
energy loss. During the flare, the photospheric field changed rapidly:
the horizontal field was enhanced by 28% in the core region, becoming
more inclined and more parallel to the polarity inversion line. Such
change is consistent with the conjectured coronal field "implosion" and
is supported by the coronal loop retraction observed by the Atmospheric
Imaging Assembly (AIA). The extrapolated field becomes more "compact"
after the flare, with shorter loops in the core region, probably because
of reconnection. The coronal field becomes slightly more sheared in the
lowest layer, relaxes faster with height, and is overall less energetic.
Title: Nonlinear Force-Free Extrapolation of Vector Magnetograms
into the Corona
Authors: Thalmann, J. K.; Wiegelmann, T.; Sun, X.; Hoeksema, J. T.;
Liu, Y.; Tadesse, T.
Bibcode: 2011AGUFMSH33C..05T
Altcode:
To investigate the structure and evolution of the coronal magnetic
field, we extrapolate measurements of the photospheric magnetic
field vector into the corona based on the force-free assumption. A
complication of this approach is that the measured photospheric
magnetic field is not force-free and that one has to apply a
preprocessing routine in order to achieve boundary conditions suitable
for the force-free modelling. Furthermore the nonlinear force-free
extrapolation code takes errors in the photospheric field data into
account which occur due to noise, incomplete inversions or ambiguity
removing techniques. Within this work we compare extrapolations from
SDO/HMI and SOLIS vector magnetograms and explain how to find optimum
parameters for handling the data of a particular instrument. The
resulting coronal magnetic field lines are quantitatively compared
with coronal EUV-images from SDO/AIA.
Title: Evolution of Magnetic Field and Energy in A Major Eruptive
Active Region Based on SDO/HMI Observation
Authors: Sun, Xudong; Hoeksema, Todd; Liu, Yang; Wiegelmann, Thomas;
Hayashi, Keiji; Chen, Qingrong; Thalmann, Julia
Bibcode: 2011sdmi.confE..63S
Altcode:
We report the evolution of magnetic field and its energy in NOAA
AR 11158 based on a vector magnetogram series from the Helioseismic
and Magnetic Imager (HMI). Fast flux emergence and strong shearing
motion created a quadrupolar sunspot complex that produced several
major eruptions, including the first X-class flare of solar cycle
24. Extrapolated non-linear force-free coronal field shows substantial
electric current and free energy increase during early flux emergence
along a newly-formed, low-lying filament with a typical 1000 G field
strength and 0.45 Mm^(-1) alpha-parameter at its center. The computed
magnetic free energy reaches a maximum of 2.62E32 erg, about 50%
stored below 6 Mm. This free energy decreases by 0.33E32 erg within
1 hour of the X-class flare, which is likely an underestimation of
the actual energy loss. During the flare, photospheric field changed
rapidly: the horizontal field was enhanced by 28% in the AR core
region. Such change is consistent with the conjectured coronal field
"implosion", and is in line with both the reconnection signatures
and the coronal loop retraction observed by the Atmospheric Image
Assembly (AIA). Extrapolation indicates that the coronal field relaxes
more rapidly with height after the flare and becomes overall less
energetic. These preliminary results demonstrate the capability to
quantitatively study the AR field topology and energetics using SDO
data- although difficulties still abound.
Title: Estimating the Relative Helicity of Coronal Magnetic Fields
Authors: Thalmann, J. K.; Inhester, B.; Wiegelmann, T.
Bibcode: 2011SoPh..272..243T
Altcode:
To quantify changes of the solar coronal field connectivity during
eruptive events, one can use magnetic helicity, which is a measure of
the shear or twist of a current-carrying (non-potential) field. To
find a physically meaningful quantity, a relative measure, giving
the helicity of a current-carrying field with respect to a reference
(potential) field, is often evaluated. This requires a knowledge of the
three-dimensional vector potential. We present a method to calculate
the vector potential for a solenoidal magnetic field as the sum of a
Laplacian part and a current-carrying part. The only requirements are
the divergence freeness of the Laplacian and current-carrying magnetic
field and the sameness of their normal field component on the bounding
surface of the considered volume.
Title: Monitoring free magnetic energy in erupting active regions
Authors: Wiegelmann, Thomas; Thalmann, Julia; Jing, Ju; Wang, Haimin
Bibcode: 2010cosp...38.2960W
Altcode: 2010cosp.meet.2960W
In solar eruptions, like flares and coronal mass ejections, free
magnetic energy stored in the solar corona is converted into kinetic
energy. Unfortunately the coronal magnetic field cannot be measured
directly. We can, however, reconstruct the coronal magnetic field
from measurements of the photospheric magnetic field vector under
the reasonable assumption of a force-free coronal plasma. With
a procedure dubbed preprocessing we derive force-free consistent
boundary conditions, which are extrapolated into the solar corona
with a nonlinear force-free extrapolation code. The resulting 3D
coronal magnetic field allows us to derive the magnetic topology and
to computed the magnetic energy as well as an upper limited of the
free energy available for driving eruptive phenomena. We apply our
code to measurements from several ground based vector magnetographs,
e.g. the Solar Flare Telescope, SOLIS and the Big Bear Solar
Observatory. Within our studies we find a clear relationship between
the stored magnetic energy and the strength of eruptions. In most cases
not the entire free energy is converted to kinetic energy, but only a
fraction. Consequently, the post-flare magnetic field configuration
is usually not entirely current free, but significantly closer to a
potential field as before the flare.
Title: Evolution of coronal magnetic fields
Authors: Thalmann, Julia Katharina
Bibcode: 2010PhDT.......222T
Altcode:
No abstract at ADS
Title: Nonlinear Force-Free Magnetic Field Modeling of AR 10953:
A Critical Assessment
Authors: De Rosa, Marc L.; Schrijver, C. J.; Barnes, G.; Leka, K. D.;
Lites, B. W.; Aschwanden, M. J.; Amari, T.; Canou, A.; McTiernan,
J. M.; Régnier, S.; Thalmann, J. K.; Valori, G.; Wheatland, M. S.;
Wiegelmann, T.; Cheung, M. C. M.; Conlon, P. A.; Fuhrmann, M.;
Inhester, B.; Tadesse, T.
Bibcode: 2009SPD....40.3102D
Altcode:
Nonlinear force-free field (NLFFF) modeling seeks to provide accurate
representations of the structure of the magnetic field above solar
active regions, from which estimates of physical quantities of interest
(e.g., free energy and helicity) can be made. However, the suite of
NLFFF algorithms have failed to arrive at consistent solutions when
applied to (thus far, two) cases using the highest-available-resolution
vector magnetogram data from Hinode/SOT-SP (in the region of the
modeling area of interest) and line-of-sight magnetograms from
SOHO/MDI (where vector data were not available). One issue is that
NLFFF models require consistent, force-free vector magnetic boundary
data, and vector magnetogram data sampling the photosphere do not
satisfy this requirement. Consequently, several problems have arisen
that are believed to affect such modeling efforts. We use AR 10953
to illustrate these problems, namely: (1) some of the far-reaching,
current-carrying connections are exterior to the observational field
of view, (2) the solution algorithms do not (yet) incorporate the
measurement uncertainties in the vector magnetogram data, and/or (3)
a better way is needed to account for the Lorentz forces within the
layer between the photosphere and coronal base. In light of these
issues, we conclude that it remains difficult to derive useful and
significant estimates of physical quantities from NLFFF models.
Title: A Critical Assessment of Nonlinear Force-Free Field Modeling
of the Solar Corona for Active Region 10953
Authors: De Rosa, Marc L.; Schrijver, Carolus J.; Barnes, Graham;
Leka, K. D.; Lites, Bruce W.; Aschwanden, Markus J.; Amari, Tahar;
Canou, Aurélien; McTiernan, James M.; Régnier, Stéphane; Thalmann,
Julia K.; Valori, Gherardo; Wheatland, Michael S.; Wiegelmann, Thomas;
Cheung, Mark C. M.; Conlon, Paul A.; Fuhrmann, Marcel; Inhester,
Bernd; Tadesse, Tilaye
Bibcode: 2009ApJ...696.1780D
Altcode: 2009arXiv0902.1007D
Nonlinear force-free field (NLFFF) models are thought to be viable
tools for investigating the structure, dynamics, and evolution of
the coronae of solar active regions. In a series of NLFFF modeling
studies, we have found that NLFFF models are successful in application
to analytic test cases, and relatively successful when applied
to numerically constructed Sun-like test cases, but they are less
successful in application to real solar data. Different NLFFF models
have been found to have markedly different field line configurations
and to provide widely varying estimates of the magnetic free energy in
the coronal volume, when applied to solar data. NLFFF models require
consistent, force-free vector magnetic boundary data. However,
vector magnetogram observations sampling the photosphere, which is
dynamic and contains significant Lorentz and buoyancy forces, do not
satisfy this requirement, thus creating several major problems for
force-free coronal modeling efforts. In this paper, we discuss NLFFF
modeling of NOAA Active Region 10953 using Hinode/SOT-SP, Hinode/XRT,
STEREO/SECCHI-EUVI, and SOHO/MDI observations, and in the process
illustrate three such issues we judge to be critical to the success of
NLFFF modeling: (1) vector magnetic field data covering larger areas
are needed so that more electric currents associated with the full
active regions of interest are measured, (2) the modeling algorithms
need a way to accommodate the various uncertainties in the boundary
data, and (3) a more realistic physical model is needed to approximate
the photosphere-to-corona interface in order to better transform the
forced photospheric magnetograms into adequate approximations of nearly
force-free fields at the base of the corona. We make recommendations
for future modeling efforts to overcome these as yet unsolved problems.
Title: Magnetic Field Extrapolation of Flaring Active Regions
Authors: Thalmann, J. K.; Wiegelmann, T.
Bibcode: 2009CEAB...33..131T
Altcode:
The solar corona is structured by magnetic fields. As direct
measurements of the coronal magnetic field are not routinely available,
it is extrapolated from photospheric vector magnetograms. When magnetic
flux emerges from below the solar surface and expands into the corona,
the coronal magnetic field is destabilized, leading to explosive
phenomena like flares or coronal mass ejections. Our aim is to get
insights in the coronal magnetic field structure in active regions and
to study its temporal evolution. We are in particular interested to
investigate the magnetic configuration of active regions in the course
of flares. Therefore, we study the temporal evolution of the flaring
active regions NOAA 10540 and NOAA 10960 as observed in January 2004 and
June 2007, respectively. We are in particular interested in the free
magnetic energy available to power the flares associated with it. To
investigate AR 10540 we used photospheric vector magnetograms measured
with the Solar Flare Telescope VectorMagnetograph and for AR 10960 we
used data provided by the Synoptic Optical Long-term Investigations of
the Sun VectorSpectroMagnetograph. We extrapolated these measurements
into the corona with the help of a nonlinear force-free field model
based on a well-tested multigrid-like optimization code with which
we were able to estimate the energy content of the 3D coronal fields,
as well as an upper limit for its free magnetic energy.
Title: Nonlinear Force-Free Magnetic Field Modeling of the Solar
Corona: A Critical Assessment
Authors: De Rosa, M. L.; Schrijver, C. J.; Barnes, G.; Leka, K. D.;
Lites, B. W.; Aschwanden, M. J.; McTiernan, J. M.; Régnier, S.;
Thalmann, J.; Valori, G.; Wheatland, M. S.; Wiegelmann, T.; Cheung,
M.; Conlon, P. A.; Fuhrmann, M.; Inhester, B.; Tadesse, T.
Bibcode: 2008AGUFMSH41A1604D
Altcode:
Nonlinear force-free field (NLFFF) modeling promises to provide accurate
representations of the structure of the magnetic field above solar
active regions, from which estimates of physical quantities of interest
(e.g., free energy and helicity) can be made. However, the suite of
NLFFF algorithms have so far failed to arrive at consistent solutions
when applied to cases using the highest-available-resolution vector
magnetogram data from Hinode/SOT-SP (in the region of the modeling
area of interest) and line-of-sight magnetograms from SOHO/MDI (where
vector data were not been available). It is our view that the lack of
robust results indicates an endemic problem with the NLFFF modeling
process, and that this process will likely continue to fail until (1)
more of the far-reaching, current-carrying connections are within the
observational field of view, (2) the solution algorithms incorporate
the measurement uncertainties in the vector magnetogram data, and/or
(3) a better way is found to account for the Lorentz forces within
the layer between the photosphere and coronal base. In light of these
issues, we conclude that it remains difficult to derive useful and
significant estimates of physical quantities from NLFFF models.
Title: Evolution of two Flaring Active Regions With CME Association
Authors: Thalmann, J. K.; Wiegelmann, T.
Bibcode: 2008AGUFMSH23B1642T
Altcode:
We study the coronal magnetic field structure of two active regions, one
during solar activity minimum (June 2007) and another one during a more
active time (January 2004). The temporal evolution was explored with the
help of nonlinear force-free coronal magnetic field extrapolations of
SOLIS/VSM and NAOJ/SFT photospheric vector magnetograms. We study the
active region NOAA 10960 observed on 2007 June 7 with three SOLIS/VSM
snapshots taken during a small C1.0 flare of time cadence 10 minutes
and six snapshots during a quiet period. The total magnetic energy in
the active region was approximately 3 × 1025 J. Before the flare the
free magnetic energy was about 5~% of the potential field energy. A part
of this excess energy was released during the flare, producing almost
a potential configuration at the beginning of the quiet period. The
return to an almost potential structure can be assigned to a CME as
recorded by the SoHO/LASCO instrument on 2007 June 07 around 10 minutes
after the flare peaked, so that whatever magnetic helicity was bodily
removed from the structure. This was compared with active region 10540
observed on 2004 January 18 -- 21, which was analyzed with the help
of vector magnetograph data from the Solar Flare Telescope in Japan
of time cadence of about 1 day. The free energy was Efree≈ 66~%
of the total energy which was sufficiently high to power a M6.1 flare
on January 20, which was associated with a CME 20 minutes later. The
activity of AR 10540 was significantly higher than for AR 10960,
as was the total magnetic energy. Furthermore, we found the common
feature that magnetic energy accumulates before the flare/CME and a
significant part of the excess energy is released during the eruption.
Title: First nonlinear force-free field extrapolations of SOLIS/VSM
data
Authors: Thalmann, J. K.; Wiegelmann, T.; Raouafi, N. -E.
Bibcode: 2008A&A...488L..71T
Altcode: 2008arXiv0809.1428T
Aims: We study the coronal magnetic field structure inside active
regions and its temporal evolution. We attempt to compare the magnetic
configuration of an active region in a very quiet period with that
for the same region during a flare.
Methods: Probably for
the first time, we use vector magnetograph data from the Synoptic
Optical Long-term Investigations of the Sun survey (SOLIS) to model
the coronal magnetic field as a sequence of nonlinear force-free
equilibria. We study the active region NOAA 10960 observed on 2007
June 7 with three snapshots taken during a small C1.0 flare of time
cadence 10 min and six snapshots during a quiet period.
Results:
The total magnetic energy in the active region was approximately 3 ×
1025 J. Before the flare the free magnetic energy was about
5% of the potential field energy. A part of this excess energy was
released during the flare, producing almost a potential configuration
at the beginning of the quiet period.
Conclusions: During the
investigated period, the coronal magnetic energy was only a few percent
higher than that of the potential field and consequently only a small
C1.0 flare occurred. This was compared with an earlier investigated
active region 10540, where the free magnetic energy was about 60% higher
than that of the potential field producing two M-class flares. However,
the free magnetic energy accumulates before and is released during
the flare which appears to be the case for both large and small flares.
Title: Preprocessing of Hinode/SOT Vector Magnetograms for Nonlinear
Force-Free Coronal Magnetic Field Modeling
Authors: Wiegelmann, T.; Thalmann, J. K.; Schrijver, C. J.; De Rosa,
M. L.; Metcalf, T. R.
Bibcode: 2008ASPC..397..198W
Altcode: 2008arXiv0801.2884W
The solar magnetic field is key to understanding the physical processes
in the solar atmosphere. Nonlinear force-free codes have been shown
to be useful in extrapolating the coronal field from underlying vector
boundary data (for an overview see Schrijver et al. (2006)). However,
we can only measure the magnetic field vector routinely with high
accuracy in the photosphere with, e.g., Hinode/SOT, and unfortunately
these data do not fulfill the force-free consistency condition as
defined by Aly (1989). We must therefore apply some transformations
to these data before nonlinear force-free extrapolation codes can be
legitimately applied. To this end, we have developed a minimization
procedure that uses the measured photospheric field vectors as input
to approximate a more chromospheric like field (The method was dubbed
preprocessing. See Wiegelmann et al. (2006) for details). The procedure
includes force-free consistency integrals and spatial smoothing. The
method has been intensively tested with model active regions (see
Metcalf et al. 2008) and been applied to several ground based vector
magnetogram data before. Here we apply the preprocessing program to
photospheric magnetic field measurements with the Hinode/SOT instrument.
Title: Evolution of the flaring active region NOAA 10540 as a sequence
of nonlinear force-free field extrapolations
Authors: Thalmann, J. K.; Wiegelmann, T.
Bibcode: 2008A&A...484..495T
Altcode:
Context: The solar corona is structured by magnetic fields. As direct
measurements of the coronal magnetic field are not routinely available,
it is extrapolated from photospheric vector magnetograms. When
magnetic flux emerges from below the solar surface and expands into
the corona, the coronal magnetic field is destabilized, leading to
explosive phenomena like flares or coronal mass ejections.
Aims:
We study the temporal evolution of the flaring active region NOAA
10540 and are in particular interested in the free magnetic energy
available to power the flares associated with it.
Methods: We
extrapolated photospheric vector magnetograms measured with the Solar
Flare Telescope, located in Tokyo, into the corona with the help of a
nonlinear force-free field model. This coronal magnetic field model is
based on a well-tested multigrid-like optimization code with which we
were able to estimate the energy content of the 3D coronal field, as
well as an upper limit for its free magnetic energy. Furthermore, the
evolution of the energy density with height and time was studied.
Results: The coronal magnetic field energy in active region 10540
increases slowly during the three days before an M6.1 flare and drops
significantly after it. We estimated the energy that was set free
during this event as ∝1025 J. A sequence of nonlinear
force-free extrapolations of the coronal magnetic field shows a build
up of magnetic energy before a flare and release of energy during the
flare. The drop in magnetic energy of the active region is sufficient
to power an M6.1 flare.
Title: Non-Linear Force-Free Field Modeling of a Solar Active Region
Around the Time of a Major Flare and Coronal Mass Ejection
Authors: De Rosa, M. L.; Schrijver, C. J.; Metcalf, T. R.; Barnes,
G.; Lites, B.; Tarbell, T.; McTiernan, J.; Valori, G.; Wiegelmann,
T.; Wheatland, M.; Amari, T.; Aulanier, G.; Démoulin, P.; Fuhrmann,
M.; Kusano, K.; Régnier, S.; Thalmann, J.
Bibcode: 2008AGUSMSP31A..06D
Altcode:
Solar flares and coronal mass ejections are associated with rapid
changes in coronal magnetic field connectivity and are powered by
the partial dissipation of electrical currents that run through
the solar corona. A critical unanswered question is whether the
currents involved are induced by the advection along the photosphere
of pre-existing atmospheric magnetic flux, or whether these currents
are associated with newly emergent flux. We address this problem by
applying nonlinear force-free field (NLFFF) modeling to the highest
resolution and quality vector-magnetographic data observed by the
recently launched Hinode satellite on NOAA Active Region 10930 around
the time of a powerful X3.4 flare in December 2006. We compute 14
NLFFF models using 4 different codes having a variety of boundary
conditions. We find that the model fields differ markedly in geometry,
energy content, and force-freeness. We do find agreement of the best-fit
model field with the observed coronal configuration, and argue (1)
that strong electrical currents emerge together with magnetic flux
preceding the flare, (2) that these currents are carried in an ensemble
of thin strands, (3) that the global pattern of these currents and
of field lines are compatible with a large-scale twisted flux rope
topology, and (4) that the ~1032~erg change in energy associated with
the coronal electrical currents suffices to power the flare and its
associated coronal mass ejection. We discuss the relative merits of
these models in a general critique of our present abilities to model
the coronal magnetic field based on surface vector field measurements.
Title: Nonlinear Force-free Field Modeling of a Solar Active Region
around the Time of a Major Flare and Coronal Mass Ejection
Authors: Schrijver, C. J.; DeRosa, M. L.; Metcalf, T.; Barnes, G.;
Lites, B.; Tarbell, T.; McTiernan, J.; Valori, G.; Wiegelmann, T.;
Wheatland, M. S.; Amari, T.; Aulanier, G.; Démoulin, P.; Fuhrmann,
M.; Kusano, K.; Régnier, S.; Thalmann, J. K.
Bibcode: 2008ApJ...675.1637S
Altcode: 2007arXiv0712.0023S
Solar flares and coronal mass ejections are associated with rapid
changes in field connectivity and are powered by the partial dissipation
of electrical currents in the solar atmosphere. A critical unanswered
question is whether the currents involved are induced by the motion of
preexisting atmospheric magnetic flux subject to surface plasma flows or
whether these currents are associated with the emergence of flux from
within the solar convective zone. We address this problem by applying
state-of-the-art nonlinear force-free field (NLFFF) modeling to the
highest resolution and quality vector-magnetographic data observed
by the recently launched Hinode satellite on NOAA AR 10930 around
the time of a powerful X3.4 flare. We compute 14 NLFFF models with
four different codes and a variety of boundary conditions. We find
that the model fields differ markedly in geometry, energy content,
and force-freeness. We discuss the relative merits of these models in
a general critique of present abilities to model the coronal magnetic
field based on surface vector field measurements. For our application
in particular, we find a fair agreement of the best-fit model field
with the observed coronal configuration, and argue (1) that strong
electrical currents emerge together with magnetic flux preceding the
flare, (2) that these currents are carried in an ensemble of thin
strands, (3) that the global pattern of these currents and of field
lines are compatible with a large-scale twisted flux rope topology,
and (4) that the ~1032 erg change in energy associated with
the coronal electrical currents suffices to power the flare and its
associated coronal mass ejection.
Title: Can We Improve the Preprocessing of Photospheric Vector
Magnetograms by the Inclusion of Chromospheric Observations?
Authors: Wiegelmann, T.; Thalmann, J. K.; Schrijver, C. J.; De Rosa,
M. L.; Metcalf, T. R.
Bibcode: 2008SoPh..247..249W
Altcode: 2008arXiv0801.2707W; 2008SoPh..tmp...27W
The solar magnetic field is key to understanding the physical processes
in the solar atmosphere. Nonlinear force-free codes have been shown to
be useful in extrapolating the coronal field upward from underlying
vector boundary data. However, we can only measure the magnetic
field vector routinely with high accuracy in the photosphere, and
unfortunately these data do not fulfill the force-free condition. We
must therefore apply some transformations to these data before nonlinear
force-free extrapolation codes can be self-consistently applied. To
this end, we have developed a minimization procedure that yields a more
chromosphere-like field, using the measured photospheric field vectors
as input. The procedure includes force-free consistency integrals,
spatial smoothing, and - newly included in the version presented here
- an improved match to the field direction as inferred from fibrils
as can be observed in, for example, chromospheric Hα images. We test
the procedure using a model active-region field that included buoyancy
forces at the photospheric level. The proposed preprocessing method
allows us to approximate the chromospheric vector field to within a few
degrees and the free energy in the coronal field to within one percent.
Title: Nonlinear force-free field models
Authors: Wiegelmann, Thomas; Thalmann, Julia; Inhester, Bernd
Bibcode: 2008cosp...37.3462W
Altcode: 2008cosp.meet.3462W
The photospheric magnetic field vector is routinely measured with high
accuracy from ground based and space born instruments. We use these
measurements to prescribe suitable boundary conditions for modelling
the coronal magnetic field. Because of the low-beta plasma the magnetic
field is in lowest order assumed to be force-free in the corona and
upper chromosphere, but not in the high-beta photosphere. We developed
a program package which contains a preprocessing program and a nonlinear
force-free coronal magnetic extrapolation code. Both programs are based
on optimization principles. The preprocessing routine uses the measured
photospheric vector magnetogram as input and approximates the magnetic
field vector in the force-free upper chromosphere. These data are used
as boundary condition for a nonlinear force-free extrapolation of the
coronal magnetic field. We applied our method to study the temporal
evolution of a flaring active region as a sequence of nonlinear
force-free equilibria. We found that magnetic energy was build up
before the occurance of a flare and released after it. Furthermore,
the 3D-magnetic field model allows us to trace the temporal evolution
of the energy flows in the flaring region.
Title: Nonlinear Force-Free Field Extrapolation of NOAA AR 0696
Authors: Thalmann, J. K.; Wiegelmann, T.
Bibcode: 2007AGUFMSH13A1095T
Altcode:
We investigate the 3D coronal magnetic field structure of NOAA AR 0696
in the period of November 09-11, 2004, before and after an X2.5 flare
(occurring around 02:13 UT on November 10, 2004). The coronal magnetic
field dominates the structure of the solar corona and consequently plays
a key role for the understanding of the initiation of flares. The most
accurate presently available method to derive the coronal magnetic
field is currently the nonlinear force-free field extrapolation
from measurements of the photospheric magnetic field vector. These
vector-magnetograms were processed from stokes I, Q, U, and V
measurements of the Big Bear Solar Observatory and extrapolated into
the corona with the nonlinear force-free optimization code developed by
Wiegelmann (2004). We analyze the corresponding time series of coronal
equilibria regarding topology changes of the 3D coronal magnetic field
during the flare. Furthermore, quantities such as the temporal evolution
of the magnetic energy and helicity are computed.
Title: Can we Improve the Preprocessing of Photospheric
Vectormagnetograms by the Inclusion of Chromospheric Observations?
Authors: Wiegelmann, T.; Thalmann, J. K.; Schrijver, C. J.; De Rosa,
M. L.; Metcalf, T. R.
Bibcode: 2007AGUFMSH51C..02W
Altcode:
The solar magnetic field is key to understanding the physical
processes in the solar atmosphere. Unfortunately, we can measure
the magnetic field vector routinely with high accuracy only in the
photosphere with, e.g., Hinode/SOT and in future with SDO/HMI. These
measurements are extrapolated into the corona under the assumption
that the field is force-free. That condition is not fulfilled in the
photosphere, but is in the chromosphere and corona. In order to make
the observed boundary data consistent with the force-free assumption,
we therefore have to apply some transformations before nonlinear
force-free extrapolation codes can be legitimately applied. We develop
a minimization procedure that uses the measured photospheric field
vectors as input to approximate a more chromospheric-like field. The
procedure includes force-free consistency integrals, spatial smoothing,
and - newly included in the version presented here - an improved match
to the field direction as inferred from fibrils as can be observed in,
e.g., chromospheric H-alpha images. We test the procedure using a model
active-region field that included buoyancy forces at the photospheric
level. We apply the combined preprocessing and nonlinear force-free
extrapolation method to compute the coronal magnetic field in an active
region measured with the Hinode/SOT instrument.
Title: Large amplitude oscillatory motion along a solar filament
Authors: Vršnak, B.; Veronig, A. M.; Thalmann, J. K.; Žic, T.
Bibcode: 2007A&A...471..295V
Altcode: 2007arXiv0707.1752V
Context: Large amplitude oscillations of solar filaments is a phenomenon
that has been known for more than half a century. Recently, a new mode
of oscillations, characterized by periodical plasma motions along
the filament axis, was discovered.
Aims: We analyze such an
event, recorded on 23 January 2002 in Big Bear Solar Observatory Hα
filtergrams, to infer the triggering mechanism and the nature of the
restoring force.
Methods: Motion along the filament axis of a
distinct buldge-like feature was traced, to quantify the kinematics of
the oscillatory motion. The data were fitted by a damped sine function
to estimate the basic parameters of the oscillations. To identify the
triggering mechanism, morphological changes in the vicinity of the
filament were analyzed.
Results: The observed oscillations of the
plasma along the filament were characterized by an initial displacement
of 24 Mm, an initial velocity amplitude of 51 km s-1,
a period of 50 min, and a damping time of 115 min. We interpret
the trigger in terms of poloidal magnetic flux injection by magnetic
reconnection at one of the filament legs. The restoring force is caused
by the magnetic pressure gradient along the filament axis. The period of
oscillations, derived from the linearized equation of motion (harmonic
oscillator) can be expressed as P=π√{2}L/v_Aϕ≈4.4L/v_Aϕ, where
v_Aϕ =Bϕ0/√μ_0ρ represents the Alfvén speed based
on the equilibrium poloidal field Bϕ0.
Conclusions:
Combination of our measurements with some previous observations of
the same kind of oscillations shows good agreement with the proposed
interpretation. Movie to Fig. 1 is only available in electronic
form at http://www.aanda.org
Title: Analysis of the Flare Wave Associated with the 3B/X3.8 Flare
of January 17, 2005
Authors: Thalmann, J. K.; Veronig, A. M.; Temmer, M.; Vršnak, B.;
Hanslmeier, A.
Bibcode: 2007CEAB...31..187T
Altcode:
The flare wave associated with the 3B/X3.8 flare and coronal mass
ejection (CME) of January 17, 2005 are studied using imaging data
in the Hα and EUV spectral channels. Due to the high-cadence Hα
observations from Kanzelhöhe Solar Observatory (KSO), a distinct
Moreton wave can be identified in ∼40 Hα frames over a period
of ∼7 minutes. The associated coronal EIT wave is identifiable in
only one EUV frame and appears close to the simultaneously observed
Moreton wave front, indicating that they are closely associated
phenomena. Beside the morphology of the wave across the solar disc
(covering an angular extend of ∼130°), the evolution in different
directions is studied to analyse the influence of a coronal hole (CH)
on the wave propagation. The Moreton wave shows a decelerating character
which can be interpreted in terms of a freely propagating fast-mode MHD
shock. The parts of the wave front moving towards the CH show a lower
initial and mean speed, and a greater amount of deceleration than the
segments moving into the undisturbed direction. This is interpreted
as the tendency of high Alfvén velocity regions to influence the
propagation of wave packets.
Title: Interaction of a Moreton/EIT Wave and a Coronal Hole
Authors: Veronig, Astrid M.; Temmer, Manuela; Vršnak, Bojan; Thalmann,
Julia K.
Bibcode: 2006ApJ...647.1466V
Altcode: 2006astro.ph..4613V
We report high-cadence Hα observations of a distinct Moreton wave
observed at Kanzelhöhe Solar Observatory associated with the 3B/X3.8
flare and coronal mass ejection (CME) event of 2005 January 17. The
Moreton wave can be identified in about 40 Hα frames over a period of
7 minutes. The EIT wave is observed in only one frame, but the derived
propagation distance is close to that of the simultaneously measured
Moreton wave fronts, indicating that they are closely associated
phenomena. The large angular extent of the Moreton wave allows us to
study the wave kinematics in different propagation directions with
respect to the location of a polar coronal hole (CH). In particular, we
find that the wave segment whose propagation direction is perpendicular
to the CH boundary (``frontal encounter'') is stopped by the CH, which
is in accordance with observations reported from EIT waves. However,
we also find that at a tongue-shaped edge of the coronal hole, where
the front orientation is perpendicular to the CH boundary (the wave
``slides along'' the boundary), the wave signatures can be found up
to 100 Mm inside the CH. These findings are briefly discussed in the
frame of recent modeling results.
Title: Wave Phenomena Associated with the X3.8 Flare/cme of
17-JAN-2005
Authors: Temmer, M.; Veronig, A.; Vršnak, B.; Thalmann, J.;
Hanslmeier, A.
Bibcode: 2005ESASP.600E.144T
Altcode: 2005ESPM...11..144T; 2005dysu.confE.144T
No abstract at ADS