valori

explanation blue bibcodes open ADS page with paths to full text

Author name code: valori
ADS astronomy entries on 2022-09-14
author:Valori, Gherardo

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Title: The on-ground data reduction and calibration pipeline for
SO/PHI-HRT
Authors: Sinjan, J.; Calchetti, D.; Hirzberger, J.; Orozco Suárez,
D.; Albert, K.; Albelo Jorge, N.; Appourchaux, T.; Alvarez-Herrero,
A.; Blanco Rodríguez, J.; Gandorfer, A.; Germerott, D.; Guerrero,
L.; Gutierrez Marquez, P.; Kahil, F.; Kolleck, M.; Solanki, S. K.; del
Toro Iniesta, J. C.; Volkmer, R.; Woch, J.; Fiethe, B.; Gómez Cama,
J. M.; Pérez-Grande, I.; Sanchis Kilders, E.; Balaguer Jiménez,
M.; Bellot Rubio, L. R.; Carmona, M.; Deutsch, W.; Fernandez-Rico,
G.; Fernández-Medina, A.; García Parejo, P.; Gasent Blesa, J. L.;
Gizon, L.; Grauf, B.; Heerlein, K.; Korpi-Lagg, A.; Lange, T.; López
Jiménez, A.; Maue, T.; Meller, R.; Michalik, H.; Moreno Vacas, A.;
Müller, R.; Nakai, E.; Schmidt, W.; Schou, J.; Schühle, U.; Staub,
J.; Strecker, H.; Torralbo, I.; Valori, G.
2022arXiv220814904S Altcode:
The ESA/NASA Solar Orbiter space mission has been successfully launched
in February 2020. Onboard is the Polarimetric and Helioseismic Imager
(SO/PHI), which has two telescopes, a High Resolution Telescope
(HRT) and the Full Disc Telescope (FDT). The instrument is designed
to infer the photospheric magnetic field and line-of-sight velocity
through differential imaging of the polarised light emitted by the
Sun. It calculates the full Stokes vector at 6 wavelength positions
at the Fe I 617.3 nm absorption line. Due to telemetry constraints,
the instrument nominally processes these Stokes profiles onboard,
however when telemetry is available, the raw images are downlinked and
reduced on ground. Here the architecture of the on-ground pipeline
for HRT is presented, which also offers additional corrections not
currently available on board the instrument. The pipeline can reduce
raw images to the full Stokes vector with a polarimetric sensitivity
of $10^{-3}\cdot I_{c}$ or better.

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Title: Investigating of the nature of magnetic oscillations associated
with FIP effect
Authors: Murabito, Mariarita; Jafarzadeh, Shahin; Van Driel-Gesztelyi,
Lidia; Ermolli, Ilaria; Baker, Deborah; Brooks, David; Long, David;
Jess, David; Valori, Gherardo; Stangalini, Marco
2022cosp...44.2591M Altcode:
Observations of the photosphere, chromosphere, and corona combined with
magnetic field modeling of one of the biggest sunspots of the 24 solar
cycle, revealed that regions of high FIP bias plasma in the corona
were magnetically linked to the locations of the intrinsic magnetic
oscillations in the solar chromosphere. In order to characterize
the driver of the oscillations, we analyzed the relation between
the spatial distribution of the magnetic wave power and the overall
field geometry and plasma parameters obtained from the multi-height
spectropolarimetric non-local thermodynamic equilibrium (NLTE)
inversions. In correspondence with the locations where the magnetic
wave energy is observed at chromospheric heights, we found evidence
in support of locally excited acoustic waves that, after crossing the
equipartition layer located close to the umbra-penumbra boundary at
photospheric heights, are converted into magnetic-like waves. These
results indicate a direct connection between sunspot chromospheric
activity and observable changes in coronal plasma composition,
demonstrating the power of high resolution, multi-height studies of the
solar atmosphere that will become the gold standard in the era of DKIST.

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Title: Understanding the Correlation between Solar Coronal Abundances
and F10.7 Radio Emission
Authors: To, Andy S. H.; Baker, Deborah; Long, David; James, Alexander;
Brooks, David; van Driel-Gesztelyi, Lidia; Valori, Gherardo; Bastian,
Tim; Lomuscio, Samantha; Stansby, David
2022cosp...44.2592T Altcode:
Solar corona plasma composition, derived from full-Sun spectra, and
the F10.7 radio flux (2.8 GHz) have been shown to be highly correlated
(r = 0.88) during the recent weak solar cycle. However, this correlation
becomes nonlinear at times of increased solar magnetic activity. We used
co-temporal, high spatial resolution, radio (JVLA), and EUV (Hinode/EIS)
images of the Sun taken on the 3 and 7 April 2020 to understand the
underlying causes of the non-linearity of the FIP bias-F10.7 solar
index correlation. We then calculated differential emission measures
from AIA images, and paired them with the observed FIP bias to predict
the bremsstrahlung component of F10.7 radio emission. Results of this
study provide constraints on the amplitude of composition variability
related to solar cycle amplitude, and provide an alternative method
to calculate coronal composition.

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Title: Evolution of Plasma Composition in an Eruptive Flux Rope
Authors: Baker, Deborah; Demoulin, Pascal; Long, David; Janvier, Miho;
Green, Lucie; Brooks, David; van Driel-Gesztelyi, Lidia; Mihailescu,
Teodora; To, Andy S. H.; Yardley, Stephanie; Valori, Gherardo
2022cosp...44.1361B Altcode:
Magnetic flux ropes are bundles of twisted magnetic field enveloping a
central axis. They harbor free magnetic energy and can be progenitors
of coronal mass ejections (CMEs). However, identifying flux ropes on
the Sun can be challenging. One of the key coronal observables that
has been shown to indicate the presence of a flux rope is a peculiar
bright coronal structure called a sigmoid. In this work, we show Hinode
EUV Imaging Spectrometer observations of sigmoidal active region (AR)
10977. We analyze the coronal plasma composition in the AR and its
evolution as a sigmoid (flux rope) forms and erupts as a CME. Plasma
with photospheric composition was observed in coronal loops close to
the main polarity inversion line during episodes of significant flux
cancellation, suggestive of the injection of photospheric plasma into
these loops driven by photospheric flux cancellation. Concurrently,
the increasingly sheared core field contained plasma with coronal
composition. As flux cancellation decreased and a sigmoid/flux
rope formed, the plasma evolved to an intermediate composition in
between photospheric and typical AR coronal compositions. Finally,
the flux rope contained predominantly photospheric plasma during and
after a failed eruption preceding the CME. Hence, plasma composition
observations of AR 10977 strongly support models of flux rope formation
by photospheric flux cancellation forcing magnetic reconnection first
at the photospheric level then at the coronal level.

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Title: The identification of magnetic perturbations in the solar
atmosphere
Authors: Stangalini, Marco; Jafarzadeh, Shahin; Baker, Deborah; Jess,
David; Murabito, Mariarita; Valori, Gherardo
2022cosp...44.2590S Altcode:
Magneto-hydrodynamic (MHD) waves and, in particular, magnetic
perturbations associated with specific wave modes are thought to be
important mechanisms not only for the heating of the outer layers of
the Sun's atmosphere, but also for the elemental abundance anomaly
observed in the corona. High resolution spectropolarimetry is nowadays
progressively extending to the upper layers of the solar atmosphere,
and this provides invaluable insight into MHD wave processes up to
chromospheric heights. However, the identification of real magnetic
perturbations remains a difficult task due to a number of spurious
effects that can mimic the signals associated with them. In this
contribution we will show a novel approach to the identification
of real magnetic oscillations potentially linked to FIP and discuss
proxies to be used in statistical analyses.

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Title: The magnetic drivers of campfires seen by the Polarimetric
and Helioseismic Imager (PHI) on Solar Orbiter
Authors: Kahil, F.; Hirzberger, J.; Solanki, S. K.; Chitta, L. P.;
Peter, H.; Auchère, F.; Sinjan, J.; Orozco Suárez, D.; Albert,
K.; Albelo Jorge, N.; Appourchaux, T.; Alvarez-Herrero, A.; Blanco
Rodríguez, J.; Gandorfer, A.; Germerott, D.; Guerrero, L.; Gutiérrez
Márquez, P.; Kolleck, M.; del Toro Iniesta, J. C.; Volkmer, R.;
Woch, J.; Fiethe, B.; Gómez Cama, J. M.; Pérez-Grande, I.; Sanchis
Kilders, E.; Balaguer Jiménez, M.; Bellot Rubio, L. R.; Calchetti,
D.; Carmona, M.; Deutsch, W.; Fernández-Rico, G.; Fernández-Medina,
A.; García Parejo, P.; Gasent-Blesa, J. L.; Gizon, L.; Grauf, B.;
Heerlein, K.; Lagg, A.; Lange, T.; López Jiménez, A.; Maue, T.;
Meller, R.; Michalik, H.; Moreno Vacas, A.; Müller, R.; Nakai,
E.; Schmidt, W.; Schou, J.; Schühle, U.; Staub, J.; Strecker, H.;
Torralbo, I.; Valori, G.; Aznar Cuadrado, R.; Teriaca, L.; Berghmans,
D.; Verbeeck, C.; Kraaikamp, E.; Gissot, S.
2022A&A...660A.143K Altcode: 2022arXiv220213859K
Context. The Extreme Ultraviolet Imager (EUI) on board the Solar Orbiter
(SO) spacecraft observed small extreme ultraviolet (EUV) bursts,
termed campfires, that have been proposed to be brightenings near the
apexes of low-lying loops in the quiet-Sun atmosphere. The underlying
magnetic processes driving these campfires are not understood. Aims: During the cruise phase of SO and at a distance of 0.523
AU from the Sun, the Polarimetric and Helioseismic Imager on Solar
Orbiter (SO/PHI) observed a quiet-Sun region jointly with SO/EUI,
offering the possibility to investigate the surface magnetic field
dynamics underlying campfires at a spatial resolution of about 380
km. Methods: We used co-spatial and co-temporal data of the
quiet-Sun network at disc centre acquired with the High Resolution
Imager of SO/EUI at 17.4 nm (HRIEUV, cadence 2 s) and the
High Resolution Telescope of SO/PHI at 617.3 nm (HRT, cadence 2.5
min). Campfires that are within the SO/PHI−SO/EUI common field
of view were isolated and categorised according to the underlying
magnetic activity. Results: In 71% of the 38 isolated events,
campfires are confined between bipolar magnetic features, which seem to
exhibit signatures of magnetic flux cancellation. The flux cancellation
occurs either between the two main footpoints, or between one of the
footpoints of the loop housing the campfire and a nearby opposite
polarity patch. In one particularly clear-cut case, we detected the
emergence of a small-scale magnetic loop in the internetwork followed
soon afterwards by a campfire brightening adjacent to the location
of the linear polarisation signal in the photosphere, that is to
say near where the apex of the emerging loop lays. The rest of the
events were observed over small scattered magnetic features, which
could not be identified as magnetic footpoints of the campfire hosting
loops. Conclusions: The majority of campfires could be driven
by magnetic reconnection triggered at the footpoints, similar to the
physical processes occurring in the burst-like EUV events discussed
in the literature. About a quarter of all analysed campfires, however,
are not associated to such magnetic activity in the photosphere, which
implies that other heating mechanisms are energising these small-scale
EUV brightenings.

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Title: Evolution of Plasma Composition in an Eruptive Flux Rope
Authors: Baker, D.; Green, L. M.; Brooks, D. H.; Démoulin, P.;
van Driel-Gesztelyi, L.; Mihailescu, T.; To, A. S. H.; Long, D. M.;
Yardley, S. L.; Janvier, M.; Valori, G.
2022ApJ...924...17B Altcode: 2021arXiv211011714B
Magnetic flux ropes are bundles of twisted magnetic field enveloping a
central axis. They harbor free magnetic energy and can be progenitors
of coronal mass ejections (CMEs). However, identifying flux ropes on
the Sun can be challenging. One of the key coronal observables that
has been shown to indicate the presence of a flux rope is a peculiar
bright coronal structure called a sigmoid. In this work, we show Hinode
EUV Imaging Spectrometer observations of sigmoidal active region (AR)
10977. We analyze the coronal plasma composition in the AR and its
evolution as a sigmoid (flux rope) forms and erupts as a CME. Plasma
with photospheric composition was observed in coronal loops close to
the main polarity inversion line during episodes of significant flux
cancellation, suggestive of the injection of photospheric plasma into
these loops driven by photospheric flux cancellation. Concurrently,
the increasingly sheared core field contained plasma with coronal
composition. As flux cancellation decreased and a sigmoid/flux
rope formed, the plasma evolved to an intermediate composition in
between photospheric and typical AR coronal compositions. Finally,
the flux rope contained predominantly photospheric plasma during and
after a failed eruption preceding the CME. Hence, plasma composition
observations of AR 10977 strongly support models of flux rope formation
by photospheric flux cancellation forcing magnetic reconnection first
at the photospheric level then at the coronal level.

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Title: Disambiguation of Vector Magnetograms by Stereoscopic
Observations from the Solar Orbiter (SO)/Polarimetric and Helioseismic
Imager (PHI) and the Solar Dynamic Observatory (SDO)/Helioseismic
and Magnetic Imager (HMI)
Authors: Valori, Gherardo; Löschl, Philipp; Stansby, David; Pariat,
Etienne; Hirzberger, Johann; Chen, Feng
2022SoPh..297...12V Altcode: 2021arXiv211210650V
Spectropolarimetric reconstructions of the photospheric vector magnetic
field are intrinsically limited by the 180∘ ambiguity
in the orientation of the transverse component. The successful
launch and operation of Solar Orbiter have made the removal of
the 180∘ ambiguity possible using solely observations
obtained from two different vantage points. While the exploitation
of such a possibility is straightforward in principle, it is less so
in practice, and it is therefore important to assess the accuracy
and limitations as a function of both the spacecrafts' orbits and
measurement principles. In this work, we present a stereoscopic
disambiguation method (SDM) and discuss thorough testing of its
accuracy in applications to modeled active regions and quiet-Sun
observations. In the first series of tests, we employ magnetograms
extracted from three different numerical simulations as test fields
and model observations of the magnetograms from different angles and
distances. In these more idealized tests, SDM is proven to reach a 100%
disambiguation accuracy when applied to moderately-to-well resolved
fields. In such favorable conditions, the accuracy is almost independent
of the relative position of the spacecraft with the obvious exceptions
of configurations where the spacecraft are within a few degrees of
co-alignment or quadrature. Even in the case of disambiguation of
quiet-Sun magnetograms with significant under-resolved spatial scales,
SDM provides an accuracy between 82% and 98%, depending on the field
strength. The accuracy of SDM is found to be mostly sensitive to the
variable spatial resolution of Solar Orbiter in its highly elliptic
orbit, as well as to the intrinsic spatial scale of the observed
field. Additionally, we provide an example of the expected accuracy as
a function of time that can be used to optimally place remote-sensing
observing windows during Solar Orbiter observation planning. Finally,
as a more realistic test, we consider magnetograms that are obtained
using a radiative-transfer inversion code and the SO/PHI Software
siMulator (SOPHISM) applied to a 3D-simulation of a pore, and we
present a preliminary discussion of the effect of the viewing angle
on the observed field. In this more realistic test of the application
of SDM, the method is able to successfully remove the ambiguity in
strong-field areas.

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Title: Investigating the origin of magnetic perturbations associated
with the FIP Effect
Authors: Murabito, M.; Stangalini, M.; Baker, D.; Valori, G.; Jess,
D. B.; Jafarzadeh, S.; Brooks, D. H.; Ermolli, I.; Giorgi, F.; Grant,
S. D. T.; Long, D. M.; van Driel-Gesztelyi, L.
2021A&A...656A..87M Altcode: 2021arXiv210811164M
Recently, magnetic oscillations were detected in the chromosphere
of a large sunspot and found to be linked to the coronal locations
where a first ionization potential (FIP) effect was observed. In
an attempt to shed light on the possible excitation mechanisms
of these localized waves, we further investigate the same data
by focusing on the relation between the spatial distribution of
the magnetic wave power and the overall field geometry and plasma
parameters obtained from multi-height spectropolarimetric non-local
thermodynamic equilibrium (NLTE) inversions of IBIS data. We find,
in correspondence with the locations where the magnetic wave energy
is observed at chromospheric heights, that the magnetic fields have
smaller scale heights, meaning faster expansions of the field lines,
which ultimately results in stronger vertical density stratification
and wave steepening. In addition, the acoustic spectrum of the
oscillations at the locations where magnetic perturbations are
observed is broader than that observed at other locations, which
suggests an additional forcing driver to the p-modes. Analysis of the
photospheric oscillations in the sunspot surroundings also reveals
a broader spectrum between the two opposite polarities of the active
region (the leading spot and the trailing opposite polarity plage),
and on the same side where magnetic perturbations are observed in
the umbra. We suggest that strong photospheric perturbations between
the two polarities are responsible for this broader spectrum of
oscillations, with respect to the p-mode spectrum, resulting in locally
excited acoustic waves that, after crossing the equipartition layer,
located close to the umbra-penumbra boundary at photopheric heights,
are converted into magnetic waves and steepen due to the strong
density gradient. Movie associated to Fig. 1 is available at <A
href="https://www.aanda.org/10.1051/0004-6361/202141504/olm">https://www.aanda.org</A>

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Title: Localized Acceleration of Energetic Particles by a Weak Shock
in the Solar Corona
Authors: Long, David M.; Reid, Hamish A. S.; Valori, Gherardo;
O'Kane, Jennifer
2021ApJ...921...61L Altcode: 2021arXiv210805068L
Globally propagating shocks in the solar corona have long been studied
to quantify their involvement in the acceleration of energetic
particles. However, this work has tended to focus on large events
associated with strong solar flares and fast coronal mass ejections
(CMEs), where the waves are sufficiently fast to easily accelerate
particles to high energies. Here we present observations of particle
acceleration associated with a global wave event which occurred on
2011 October 1. Using differential emission measure analysis, the
global shock wave was found to be incredibly weak, with an Alfvén
Mach number of ~1.008-1.013. Despite this, spatially resolved type III
radio emission was observed by the Nançay RadioHeliograph at distinct
locations near the shock front, suggesting localized acceleration
of energetic electrons. Further investigation using magnetic field
extrapolation identified a fan structure beneath a magnetic null located
above the source active region, with the erupting CME contained within
this topological feature. We propose that a reconfiguration of the
coronal magnetic field driven by the erupting CME enabled the weak
shock to accelerate particles along field lines initially contained
within the fan and subsequently opening into the heliosphere, producing
the observed type III emission. These results suggest that even weak
global shocks in the solar corona can accelerate energetic particles
via reconfiguration of the surrounding magnetic field.

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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
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.

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Title: Plasma Upflows Induced by Magnetic Reconnection Above an
Eruptive Flux Rope
Authors: Baker, Deborah; Mihailescu, Teodora; Démoulin, Pascal;
Green, Lucie M.; van Driel-Gesztelyi, Lidia; Valori, Gherardo; Brooks,
David H.; Long, David M.; Janvier, Miho
2021SoPh..296..103B Altcode: 2021arXiv210616137B
One of the major discoveries of Hinode's Extreme-ultraviolet
Imaging Spectrometer (EIS) is the presence of upflows at the edges
of active regions. As active regions are magnetically connected
to the large-scale field of the corona, these upflows are a likely
contributor to the global mass cycle in the corona. Here we examine
the driving mechanism(s) of the very strong upflows with velocities
in excess of 70 km s−1, known as blue-wing asymmetries,
observed during the eruption of a flux rope in AR 10977 (eruptive flare
SOL2007-12-07T04:50). We use Hinode/EIS spectroscopic observations
combined with magnetic-field modeling to investigate the possible
link between the magnetic topology of the active region and the strong
upflows. A Potential Field Source Surface (PFSS) extrapolation of the
large-scale field shows a quadrupolar configuration with a separator
lying above the flux rope. Field lines formed by induced reconnection
along the separator before and during the flux-rope eruption are
spatially linked to the strongest blue-wing asymmetries in the upflow
regions. The flows are driven by the pressure gradient created when
the dense and hot arcade loops of the active region reconnect with
the extended and tenuous loops overlying it. In view of the fact
that separator reconnection is a specific form of the more general
quasi-separatrix (QSL) reconnection, we conclude that the mechanism
driving the strongest upflows is, in fact, the same as the one driving
the persistent upflows of ≈10 - 20 km s−1 observed in
all active regions.

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Title: The Evolution of Plasma Composition during a Solar Flare
Authors: To, Andy S. H.; Long, David M.; Baker, Deborah; Brooks, David
H.; van Driel-Gesztelyi, Lidia; Laming, J. Martin; Valori, Gherardo
2021ApJ...911...86T Altcode: 2021arXiv210209985T
We analyze the coronal elemental abundances during a small flare using
Hinode/EIS observations. Compared to the preflare elemental abundances,
we observed a strong increase in coronal abundance of Ca XIV 193.84
Å, an emission line with low first ionization potential (FIP &lt;
10 eV), as quantified by the ratio Ca/Ar during the flare. This is in
contrast to the unchanged abundance ratio observed using Si X 258.38
Å/S X 264.23 Å. We propose two different mechanisms to explain
the different composition results. First, the small flare-induced
heating could have ionized S, but not the noble gas Ar, so that the
flare-driven Alfvén waves brought up Si, S, and Ca in tandem via
the ponderomotive force which acts on ions. Second, the location of
the flare in strong magnetic fields between two sunspots may suggest
fractionation occurred in the low chromosphere, where the background
gas is neutral H. In this region, high-FIP S could behave more like a
low-FIP than a high-FIP element. The physical interpretations proposed
generate new insights into the evolution of plasma abundances in the
solar atmosphere during flaring, and suggests that current models must
be updated to reflect dynamic rather than just static scenarios.

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Title: Matching Temporal Signatures of Solar Features to Their
Corresponding Solar-Wind Outflows
Authors: de Pablos, D.; Long, D. M.; Owen, C. J.; Valori, G.; Nicolaou,
G.; Harra, L. K.
2021SoPh..296...68D Altcode: 2021arXiv210309077D
The role of small-scale coronal eruptive phenomena in the generation
and heating of the solar wind remains an open question. Here, we
investigate the role played by coronal jets in forming the solar wind
by testing whether temporal variations associated with jetting in EUV
intensity can be identified in the outflowing solar-wind plasma. This
type of comparison is challenging due to inherent differences between
remote-sensing observations of the source and in-situ observations
of the outflowing plasma, as well as travel time and evolution
of the solar wind throughout the heliosphere. To overcome these,
we propose a novel algorithm combining signal filtering, two-step
solar-wind ballistic back-mapping, window shifting, and Empirical
Mode Decomposition. We first validate the method using synthetic data,
before applying it to measurements from the Solar Dynamics Observatory
and Wind spacecraft. The algorithm enables the direct comparison of
remote-sensing observations of eruptive phenomena in the corona to
in-situ measurements of solar-wind parameters, among other potential
uses. After application to these datasets, we find several time windows
where signatures of dynamics found in the corona are embedded in
the solar-wind stream, at a time significantly earlier than expected
from simple ballistic back-mapping, with the best-performing in-situ
parameter being the solar-wind mass flux.

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Title: Spectropolarimetric fluctuations in a sunspot chromosphere
Authors: Stangalini, M.; Baker, D.; Valori, G.; Jess, D. B.;
Jafarzadeh, S.; Murabito, M.; To, A. S. H.; Brooks, D. H.; Ermolli,
I.; Giorgi, F.; MacBride, C. D.
2021RSPTA.37900216S Altcode: 2020arXiv200905302S
The instrumental advances made in this new era of 4 m class solar
telescopes with unmatched spectropolarimetric accuracy and sensitivity
will enable the study of chromospheric magnetic fields and their
dynamics with unprecedented detail. In this regard, spectropolarimetric
diagnostics can provide invaluable insight into magneto-hydrodynamic
(MHD) wave processes. MHD waves and, in particular, Alfvénic
fluctuations associated with particular wave modes were recently
recognized as important mechanisms not only for the heating of the outer
layers of the Sun's atmosphere and the acceleration of the solar wind,
but also for the elemental abundance anomaly observed in the corona
of the Sun and other Sun-like stars (also known as first ionization
potential) effect. Here, we take advantage of state-of-the-art and
unique spectropolarimetric Interferometric BIdimensional Spectrometer
observations to investigate the relation between intensity and circular
polarization (CP) fluctuations in a sunspot chromosphere. Our results
show a clear link between the intensity and CP fluctuations in a patch
which corresponds to a narrow range of magnetic field inclinations. This
suggests the presence of Alfvénic perturbations in the sunspot. This article is part of the Theo Murphy meeting issue `High-resolution
wave dynamics in the lower solar atmosphere'.

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Title: The Magnetic Environment of a Stealth Coronal Mass Ejection
Authors: O'Kane, Jennifer; Mac Cormack, Cecilia; Mandrini, Cristina H.;
Démoulin, Pascal; Green, Lucie M.; Long, David M.; Valori, Gherardo
2021ApJ...908...89O Altcode: 2020arXiv201203757O
Interest in stealth coronal mass ejections (CMEs) is increasing due to
their relatively high occurrence rate and space weather impact. However,
typical CME signatures such as extreme-ultraviolet dimmings and
post-eruptive arcades are hard to identify and require extensive image
processing techniques. These weak observational signatures mean that
little is currently understood about the physics of these events. We
present an extensive study of the magnetic field configuration in which
the stealth CME of 2011 March 3 occurred. Three distinct episodes
of flare ribbon formation are observed in the stealth CME source
active region (AR). Two occurred prior to the eruption and suggest the
occurrence of magnetic reconnection that builds the structure that will
become eruptive. The third occurs in a time close to the eruption of
a cavity that is observed in STEREO-B 171 Å data; this subsequently
becomes part of the propagating CME observed in coronagraph data. We
use both local (Cartesian) and global (spherical) models of the coronal
magnetic field, which are complemented and verified by the observational
analysis. We find evidence of a coronal null point, with field lines
computed from its neighborhood connecting the stealth CME source region
to two ARs in the northern hemisphere. We conclude that reconnection
at the null point aids the eruption of the stealth CME by removing the
field that acted to stabilize the preeruptive structure. This stealth
CME, despite its weak signatures, has the main characteristics of
other CMEs, and its eruption is driven by similar mechanisms.

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Title: Flaring activity and related eruptions from active regions
Authors: Green, Lucie; Long, David; Valori, Gherardo; O'Kane, Jennifer;
James, Alexander
2021cosp...43E.992G Altcode:
The Sun produces major eruptions, known as coronal mass ejections,
from a range of heights in its atmosphere and across a range of
kinematic and spatial scales. From compact, fast active region
eruptions to high-altitude, slow stealth CMEs. These eruptions are
the most energetic phenomena in the Solar System and CMEs that reach
the Earth can create severe space events. Many studies have now
shown that magnetic flux ropes are a fundamental component of the
pre-eruption corona in some cases. In addition, a key role appears
to be played by magnetic reconnection that evolves the pre-eruption
corona from a sheared arcade to a flux rope configuration, which can
then be destabilised by an ideal MHD process or by further magnetic
reconnection. This talk will look CMEs originating in active regions
covering the spectrum of events from energetic CMEs to low-energy
stealth CMEs, and will ask whether there are common processes taking
place across this wide range and whether flux ropes are involved in
all cases. A long-term perspective will be given using observational
and modelling results. The emergence of flux that forms the active
region will be discussed along with the evolution of this flux via
photospheric flows. Finally, thought will be given on the role of the
ambient coronal field on the destabilisation of the CME structure.

---------------------------------------------------------
Title: Alfvénic Perturbations in a Sunspot Chromosphere Linked to
Fractionated Plasma in the Corona
Authors: Baker, Deborah; Stangalini, Marco; Valori, Gherardo; Brooks,
David H.; To, Andy S. H.; van Driel-Gesztelyi, Lidia; Démoulin,
Pascal; Stansby, David; Jess, David B.; Jafarzadeh, Shahin
2021ApJ...907...16B Altcode: 2020arXiv201204308B
In this study, we investigate the spatial distribution of highly
varying plasma composition around one of the largest sunspots of solar
cycle 24. Observations of the photosphere, chromosphere, and corona
are brought together with magnetic field modeling of the sunspot
in order to probe the conditions that regulate the degree of plasma
fractionation within loop populations of differing connectivities. We
find that, in the coronal magnetic field above the sunspot umbra,
the plasma has photospheric composition. Coronal loops rooted in the
penumbra contain fractionated plasma, with the highest levels observed
in the loops that connect within the active region. Tracing field
lines from regions of fractionated plasma in the corona to locations
of Alfvénic fluctuations detected in the chromosphere shows that they
are magnetically linked. These results indicate a connection between
sunspot chromospheric activity and observable changes in coronal
plasma composition.

---------------------------------------------------------
Title: Analysis of time-domain correlations between EUV and in-situ
observations of coronal jets
Authors: de Pablos, D.; Owen, C. J.; Long, D.; Harra, L. K.; Valori,
G.; Nicolaou, G.
2020AGUFMSH0290018D Altcode:
The role of small-scale coronal eruptive phenomena in the origin and
heating of the solar wind remains an open question. In this work we
attempt to determine the role played by coronal jets in forming the
solar wind. This is a challenging problem due to inherent differences
between remote-sensing observations of the source and in-situ
observations of the outflowing plasma, as well as its travel time and
evolution throughout the heliosphere. To overcome these challenges,
we propose the use of Empirical Mode Decomposition to enable direct
comparison of temporal signatures within remote sensing observations of
eruptive phenomena in the corona and in-situ measurements of the solar
wind. The technique is first validated using artificial data before
being applied to measurements from the Solar Dynamics Observatory and
Wind spacecraft. We discuss the potential reasons for discrepancies
between results from the artificial data and observations at 1 AU,
and their implications on the solar wind nature.

---------------------------------------------------------
Title: A new trigger mechanism for coronal mass ejections. The role
of confined flares and photospheric motions in the formation of hot
flux ropes
Authors: James, A. W.; Green, L. M.; van Driel-Gesztelyi, L.;
Valori, G.
2020A&A...644A.137J Altcode: 2020arXiv201011204J
Context. Many previous studies have shown that the magnetic precursor of
a coronal mass ejection (CME) takes the form of a magnetic flux rope,
and a subset of them have become known as "hot flux ropes" due to
their emission signatures in ∼10 MK plasma. Aims: We seek to
identify the processes by which these hot flux ropes form, with a view
of developing our understanding of CMEs and thereby improving space
weather forecasts. Methods: Extreme-ultraviolet observations
were used to identify five pre-eruptive hot flux ropes in the solar
corona and study how they evolved. Confined flares were observed in the
hours and days before each flux rope erupted, and these were used as
indicators of episodic bursts of magnetic reconnection by which each
flux rope formed. The evolution of the photospheric magnetic field
was observed during each formation period to identify the process(es)
that enabled magnetic reconnection to occur in the β &lt; 1 corona and
form the flux ropes. Results: The confined flares were found
to be homologous events and suggest flux rope formation times that
range from 18 hours to 5 days. Throughout these periods, fragments of
photospheric magnetic flux were observed to orbit around each other
in sunspots where the flux ropes had a footpoint. Active regions
with right-handed (left-handed) twisted magnetic flux exhibited
clockwise (anticlockwise) orbiting motions, and right-handed
(left-handed) flux ropes formed. Conclusions: We infer that
the orbital motions of photospheric magnetic flux fragments about
each other bring magnetic flux tubes together in the corona, enabling
component reconnection that forms a magnetic flux rope above a flaring
arcade. This represents a novel trigger mechanism for solar eruptions
and should be considered when predicting solar magnetic activity. Movies associated to Figs. 4, 8, 12, and 14 are available at <A
href="https://www.aanda.org/10.1051/0004-6361/202038781/olm">https://www.aanda.org</A>

---------------------------------------------------------
Title: Metis - Solar Orbiter Topical Team on "Modelling of CME
propagation/evolution in corona and solar wind in connection with
Space Weather"
Authors: Bemporad, A.; Banerjee, D.; Berlicki, A.; Biondo, R.; Boe,
B.; Calchetti, D.; Capuano, G.; De Leo, Y.; Del Moro, D.; Feng, L.;
Foldes, R.; Frassati, F.; Frazin, R. A.; Giovannelli, L.; Giunta,
A. S.; Heinzel, P.; Ippolito, A.; Janvier, M.; Jerse, G.; Kilpua,
K. E. J.; Laurenza, M.; Lloveras, D.; Magdalenic, J.; Mancuso, S.;
Messerotti, M.; Mierla, M.; Nandy, D.; Napoletano, G.; Nuevo, F.;
Pagano, P.; Pinto, R.; Plainaki, C.; Reale, F.; Romoli, M.; Rodriguez,
L.; Slemer, A.; Spadaro, D.; Susino, R.; Stangalini, M.; Vainio,
R. O.; Valori, G.; Vásquez, A. M.; West, M. J.
2020AGUFMSH0360027B Altcode:
Despite the current availability of multi-spacecraft observations of
Coronal Mass Ejections (CMEs) and their interplanetary counterpart
(ICMEs), at present we still don't understand which physical phenomena
are driving their expansion and propagation phases. This also limits
our understanding on how CMEs (observed with remote sensing data)
become ICMEs (observed in situ), how they interact with the background
solar wind, and how their final geo-effectiveness can be modified
during their interplanetary evolution. Such problems match some of
the scientific objectives of the Solar Orbiter Science Activity Plan
and of the Metis coronagraph. Thanks to its multi-channel capability,
Metis (acquiring images in the visible light and at the same time in
the UV HI Lyman-alpha emission) will really provide an unprecedented
view of CMEs and in particular of their thermodynamic evolution. At
closest approaches to the Sun (in the nominal mission), Metis will
acquire high spatial resolution and/or temporal cadence multi-channel
images of CMEs. Farther from the Sun, Metis will shed light on the
early Interplanetary propagation of CMEs. Later on (in the extended
mission) Metis will observe for the first time the CME/ICME propagation
out-of-ecliptic. These novelties will be combined with the unique
vantage point that will be offered by the Solar Orbiter spacecraft,
and supported with valuable data acquired by other on-board remote
sensing (e.g. SPICE, EUI, SoloHI) and in situ (e.g. EPD, MAG,
SWA, RPW) instruments. In this contribution we present the ongoing
activities of the Metis Topical Team on "CME/ICME propagation", (<A
href="http://metis.oato.inaf.it/topical_teams.html">http://metis.oato.inaf.it/topical_teams.html</A>),
an international working group recently established and gathering
scientists from different countries, experts of both in-situ and remote
sensing observations, as well as numerical simulations, and we summarize
the main science objectives discussed during the last months.

---------------------------------------------------------
Title: Additivity of relative magnetic helicity in finite volumes
Authors: Valori, Gherardo; Démoulin, Pascal; Pariat, Etienne; Yeates,
Anthony; Moraitis, Kostas; Linan, Luis
2020A&A...643A..26V Altcode: 2020arXiv200800968V
Context. Relative magnetic helicity is conserved by magneto-hydrodynamic
evolution even in the presence of moderate resistivity. For that reason,
it is often invoked as the most relevant constraint on the dynamical
evolution of plasmas in complex systems, such as solar and stellar
dynamos, photospheric flux emergence, solar eruptions, and relaxation
processes in laboratory plasmas. However, such studies often indirectly
imply that relative magnetic helicity in a given spatial domain can be
algebraically split into the helicity contributions of the composing
subvolumes, in other words that it is an additive quantity. A limited
number of very specific applications have shown that this is not the
case. Aims: Progress in understanding the nonadditivity of
relative magnetic helicity requires removal of restrictive assumptions
in favor of a general formalism that can be used in both theoretical
investigations and numerical applications. Methods: We derive the
analytical gauge-invariant expression for the partition of relative
magnetic helicity between contiguous finite volumes, without any
assumptions on either the shape of the volumes and interface, or the
employed gauge. Results: We prove the nonadditivity of relative
magnetic helicity in finite volumes in the most general, gauge-invariant
formalism, and verify this numerically. We adopt more restrictive
assumptions to derive known specific approximations, which yields a
unified view of the additivity issue. As an example, the case of a
flux rope embedded in a potential field shows that the nonadditivity
term in the partition equation is, in general, non-negligible. Conclusions: The nonadditivity of relative magnetic helicity can
potentially be a serious impediment to the application of relative
helicity conservation as a constraint on the complex dynamics of
magnetized plasmas. The relative helicity partition formula can be
applied to numerical simulations to precisely quantify the effect of
nonadditivity on global helicity budgets of complex physical processes.

---------------------------------------------------------
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.
2020ApJ...902L..48T Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Study of the spatial association between an active region
jet and a nonthermal type~${\rm III}$ radio burst
Authors: Mulay, Sargam M.; Sharma, Rohit; Valori, Gherardo; Vásquez,
Alberto M.; Del Zanna, Giulio; Mason, Helen; Oberoi, Divya
2020arXiv200914581M Altcode:
We aim to investigate the spatial location of the source of an active
region (AR) jet and its relation with associated nonthermal type~III
radio emission. An emission measure (EM) method was used to study
the thermodynamic nature of the AR jet. The nonthermal type~{\rm III}
radio burst observed at meterwavelength was studied using the Murchison
Widefield Array (MWA) radio imaging and spectroscopic data. The local
configuration of the magnetic field and the connectivity of the source
region of the jet with open magnetic field structures was studied using
a nonlinear force-free field (NLFFF) extrapolation and potential field
source surface (PFSS) extrapolation respectively. The plane-of-sky
velocity of the AR jet was found to be $\sim$136~km/s. The EM analysis
confirmed the presence of low temperature 2~MK plasma for the spire,
whereas hot plasma, between 5-8 MK, was present at the footpoint region
which also showed the presence of Fe~{\sc xviii} emission. A lower limit
on the electron number density was found to be 1.4$\times$10$^{8}$
cm$^{-3}$ for the spire and 2.2$\times$10$^{8}$~cm$^{-3}$ for
the footpoint. A temporal and spatial correlation between the AR
jet and nonthermal type III burst confirmed the presence of open
magnetic fields. An NLFFF extrapolation showed that the photospheric
footpoints of the null point were anchored at the location of the
source brightening of the jet. The spatial location of the radio
sources suggests an association with the extrapolated closed and
open magnetic fields although strong propagation effects are also
present. The multi-scale analysis of the field at local, AR, and solar
scales confirms the interlink between different flux bundles involved
in the generation of the type III radio signal with flux transferred
from a small coronal hole to the periphery of the sunspot via null
point reconnection with an emerging structure.

---------------------------------------------------------
Title: The Magnetic Environment of a Stealth CME
Authors: O'Kane, J.; Mandrini, C.; Demoulin, P.; Green, L.; Valori,
G.; Long, D.
2020SPD....5121005O Altcode:
Interest in Stealth Coronal Mass Ejections (CMEs) is increasing due to
their relatively high occurrence rate and space weather impact. However,
typical CME signatures such as EUV dimmings and post-eruptive arcades
are hard to identify for stealth CMEs and require extensive image
processing techniques. These weak observational signatures mean little
is currently understood about the physics of these events. We present
an extensive study of the magnetic field configuration in which the
stealth CME of 3 March 2011 occurred. The magnetic field prior to the
eruption is evaluated using a Linear Force Free Field (LFFF) model
and a Potential Field Source Surface (PFSS) model, and complemented
by in-depth observational analysis. The models are verified using
observations of plasma emission structures in the stealth CME source
region and trans-equatorial loops. We find evidence of a high-altitude
null point in both the LFFF model and the PFSS model, with surrounding
field lines connecting two active regions on the solar disk. One of
these active regions in the Southern Hemisphere is shown to be the
source region of the stealth CME. Three distinct episodes of flare
ribbon formation are observed in AIA 304Å. Two occurred prior to
the eruption and suggest the occurrence of magnetic reconnection that
builds the eruptive structure. The third occurs at the same time as an
erupting cavity is observed in STEREO-B 171Å data; this subsequently
becomes part of the propagating CME observed in COR1. We conclude that
reconnection at the null point, driven by eruptive activity in the
complex northern active region, aids the eruption of the stealth CME
by removing field that acted to stabilise the pre-eruptive structure.

---------------------------------------------------------
Title: Modelado magnético de regiones activas solares: Una
comparación entre dos modelos libres de fuerzas
Authors: Nuevo, F. A.; Valori, G.; López Fuentes, M.; Mandrini,
C. H.; Vásquez, A. M.
2020BAAA...61B...7N Altcode:
The knowledge of the three-dimensional coronal magnetic field
($\mathbf{B}$) at high spatial resolution is key to better understand
the physical mechanisms that trigger active phenomena in the corona,
the main drivers of { space weather}. High resolution, direct coronal
measurements of $\mathbf{B}$ are not available, so we must rely on
suitable coronal field models based on the available photospheric field
measurements (magnetograms). For active region (AR) magnetic fields,
where { magnetic pressure is much larger than plasma pressure}, the
force-free regime ($\nabla\times\mathbf{B} = \alpha \mathbf{B}$) is
a valid approximation. Force-free field models for which the $\alpha$
parameter is a constant are called Linear (LFFF) and models for which
$\alpha$ is a function of { the} position are called non-linear
(NLFFF).In this work we test and compare specific numerical
implementations of both LFFF and NLFFF models applied to two ARs.

---------------------------------------------------------
Title: Can Subphotospheric Magnetic Reconnection Change the Elemental
Composition in the Solar Corona?
Authors: Baker, Deborah; van Driel-Gesztelyi, Lidia; Brooks, David H.;
Démoulin, Pascal; Valori, Gherardo; Long, David M.; Laming, J. Martin;
To, Andy S. H.; James, Alexander W.
2020ApJ...894...35B Altcode: 2020arXiv200303325B
Within the coronae of stars, abundances of those elements with low
first ionization potential (FIP) often differ from their photospheric
values. The coronae of the Sun and solar-type stars mostly show
enhancements of low-FIP elements (the FIP effect) while more active
stars such as M dwarfs have coronae generally characterized by the
inverse-FIP effect (I-FIP). Here we observe patches of I-FIP effect
solar plasma in AR 12673, a highly complex βγδ active region. We
argue that the umbrae of coalescing sunspots, and more specifically
strong light bridges within the umbrae, are preferential locations for
observing I-FIP effect plasma. Furthermore, the magnetic complexity
of the active region and major episodes of fast flux emergence also
lead to repetitive and intense flares. The induced evaporation of
the chromospheric plasma in flare ribbons crossing umbrae enables
the observation of four localized patches of I-FIP effect plasma in
the corona of AR 12673. These observations can be interpreted in the
context of the ponderomotive force fractionation model which predicts
that plasma with I-FIP effect composition is created by the refraction
of waves coming from below the chromosphere. We propose that the waves
generating the I-FIP effect plasma in solar active regions are generated
by subphotospheric reconnection of coalescing flux systems. Although
we only glimpse signatures of I-FIP effect fractionation produced by
this interaction in patches on the Sun, on highly active M stars it
may be the dominant process.

---------------------------------------------------------
Title: Energy and helicity fluxes in line-tied eruptive simulations
Authors: Linan, L.; Pariat, É.; Aulanier, G.; Moraitis, K.; Valori, G.
2020A&A...636A..41L Altcode: 2020arXiv200301698L
Context. Conservation properties of magnetic helicity and energy in
the quasi-ideal and low-β solar corona make these two quantities
relevant for the study of solar active regions and eruptions. 
Aims: Based on a decomposition of the magnetic field into potential
and nonpotential components, magnetic energy and relative helicity
can both also be decomposed into two quantities: potential and free
energies, and volume-threading and current-carrying helicities. In
this study, we perform a coupled analysis of their behaviors in a set
of parametric 3D magnetohydrodynamic (MHD) simulations of solar-like
eruptions. Methods: We present the general formulations for
the time-varying components of energy and helicity in resistive
MHD. We calculated them numerically with a specific gauge, and
compared their behaviors in the numerical simulations, which differ
from one another by their imposed boundary-driving motions. Thus,
we investigated the impact of different active regions surface flows
on the development of the energy and helicity-related quantities. Results: Despite general similarities in their overall behaviors,
helicities and energies display different evolutions that cannot be
explained in a unique framework. While the energy fluxes are similar
in all simulations, the physical mechanisms that govern the evolution
of the helicities are markedly distinct from one simulation to another:
the evolution of volume-threading helicity can be governed by boundary
fluxes or helicity transfer, depending on the simulation. 
Conclusions: The eruption takes place for the same value of the
ratio of the current-carrying helicity to the total helicity in all
simulations. However, our study highlights that this threshold can be
reached in different ways, with different helicity-related processes
dominating for different photospheric flows. This means that the
details of the pre-eruptive dynamics do not influence the eruption-onset
helicity-related threshold. Nevertheless, the helicity-flux dynamics
may be more or less efficient in changing the time required to reach
the onset of the eruption.

---------------------------------------------------------
Title: Study of the spatial association between an active region
jet and a nonthermal type III radio burst
Authors: Mulay, Sargam M.; Sharma, Rohit; Valori, Gherardo; Vásquez,
Alberto M.; Del Zanna, Giulio; Mason, Helen; Oberoi, Divya
2019A&A...632A.108M Altcode:
 Aims: We aim to investigate the spatial location of the source
of an active region (AR) jet and its relation with associated nonthermal
type III radio emission. Methods: An emission measure (EM) method
was used to study the thermodynamic nature of the AR jet. The nonthermal
type III radio burst observed at meterwavelength was studied using
the Murchison Widefield Array (MWA) radio imaging and spectroscopic
data. The local configuration of the magnetic field and the connectivity
of the source region of the jet with open magnetic field structures was
studied using a nonlinear force-free field (NLFFF) extrapolation and
potential field source surface (PFSS) extrapolation respectively. Results: The plane-of-sky velocity of the AR jet was found to be
∼136 km s-1. The EM analysis confirmed the presence of
low temperature 2 MK plasma for the spire, whereas hot plasma, between
5 and 8 MK, was present at the footpoint region which also showed the
presence of Fe XVIII emission. A lower limit on the electron number
density was found to be 1.4 × 108 cm-3 for the
spire and 2.2 × 108 cm-3 for the footpoint. A
temporal and spatial correlation between the AR jet and nonthermal
type III burst confirmed the presence of open magnetic fields. An NLFFF
extrapolation showed that the photospheric footpoints of the null point
were anchored at the location of the source brightening of the jet. The
spatial location of the radio sources suggests an association with the
extrapolated closed and open magnetic fields although strong propagation
effects are also present. Conclusions: The multi-scale analysis
of the field at local, AR, and solar scales confirms the interlink
between different flux bundles involved in the generation of the type
III radio signal with flux transferred from a small coronal hole to the
periphery of the sunspot via null point reconnection with an emerging
structure. The movie associated to Fig. 4 is available at <A
href="https://www.aanda.org/10.1051/0004-6361/201936369/olm">https://www.aanda.org</A>

---------------------------------------------------------
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.
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: Quantifying the Relationship between Moreton-Ramsey Waves
and “EIT Waves” Using Observations of Four Homologous Wave Events
Authors: Long, David M.; Jenkins, Jack; Valori, Gherardo
2019ApJ...882...90L Altcode: 2019arXiv190707963L
Freely propagating global waves in the solar atmosphere are commonly
observed using extreme ultraviolet passbands (EUV or “EIT waves”),
and less regularly in H-alpha (Moreton-Ramsey waves). Despite
decades of research, joint observations of EUV and Moreton-Ramsey
waves remain rare, complicating efforts to quantify the connection
between these phenomena. We present observations of four homologous
global waves originating from the same active region between 2014
March 28 and 30 and observed using both EUV and H-alpha data. Each
global EUV wave was observed by the Solar Dynamics Observatory,
with the associated Moreton-Ramsey waves identified using the Global
Oscillations Network Group network. All of the global waves exhibit
high initial velocity (e.g., 842-1388 km s-1 in the 193 Å
passband) and strong deceleration (e.g., -1437 to -782 m s-2
in the 193 Å passband) in each of the EUV passbands studied, with the
EUV wave kinematics exceeding those of the Moreton-Ramsey wave. The
density compression ratio of each global wave was estimated using both
differential emission measure and intensity variation techniques, with
both indicating that the observed waves were weakly shocked with a fast
magnetosonic Mach number slightly greater than one. This suggests that,
according to current models, the global coronal waves were not strong
enough to produce Moreton-Ramsey waves, indicating an alternative
explanation for these observations. Instead, we conclude that the
evolution of the global waves was restricted by the surrounding
coronal magnetic field, in each case producing a downward-angled
wavefront propagating toward the north solar pole, which perturbed
the chromosphere and was observed as a Moreton-Ramsey wave.

---------------------------------------------------------
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.
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 &amp; 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: Relative magnetic field line helicity
Authors: Moraitis, K.; Pariat, E.; Valori, G.; Dalmasse, K.
2019A&A...624A..51M Altcode: 2019arXiv190210410M
Context. Magnetic helicity is an important quantity in studies
of magnetized plasmas as it provides a measure of the geometrical
complexity of the magnetic field in a given volume. A more detailed
description of the spatial distribution of magnetic helicity is given
by the field line helicity, which expresses the amount of helicity
associated to individual field lines rather than in the full analysed
volume. Aims: Magnetic helicity is not a gauge-invariant quantity
in general, unless it is computed with respect to a reference field,
yielding the so-called relative magnetic helicity. The field line
helicity corresponding to the relative magnetic helicity has only been
examined under specific conditions so far. This work aims to define
the field line helicity corresponding to relative magnetic helicity
in the most general way. In addition to its general form, we provide
the expression for the relative magnetic field line helicity in a few
commonly used gauges, and reproduce known results as a limit of our
general formulation. Methods: By starting from the definition
of relative magnetic helicity, we derived the corresponding field line
helicity, and we noted the assumptions on which it is based. 
Results: We checked that the developed quantity reproduces relative
magnetic helicity by using three different numerical simulations. For
these cases we also show the morphology of field line helicity in
the volume, and on the photospheric plane. As an application to solar
situations, we compared the morphology of field line helicity on the
photosphere with that of the connectivity-based helicity flux density
in two reconstructions of an active region's magnetic field. We
discuss how the derived relative magnetic field line helicity has
a wide range of applications, notably in solar physics and magnetic
reconnection studies.

---------------------------------------------------------
Title: Transient Inverse-FIP Plasma Composition Evolution within a
Solar Flare
Authors: Baker, Deborah; van Driel-Gesztelyi, Lidia; Brooks, David
H.; Valori, Gherardo; James, Alexander W.; Laming, J. Martin; Long,
David M.; Démoulin, Pascal; Green, Lucie M.; Matthews, Sarah A.;
Oláh, Katalin; Kővári, Zsolt
2019ApJ...875...35B Altcode: 2019arXiv190206948B
Understanding elemental abundance variations in the solar corona
provides an insight into how matter and energy flow from the
chromosphere into the heliosphere. Observed variations depend on the
first ionization potential (FIP) of the main elements of the Sun’s
atmosphere. High-FIP elements (&gt;10 eV) maintain photospheric
abundances in the corona, whereas low-FIP elements have enhanced
abundances. Conversely, inverse FIP (IFIP) refers to the enhancement of
high-FIP or depletion of low-FIP elements. We use spatially resolved
spectroscopic observations, specifically the Ar XIV/Ca XIV intensity
ratio, from Hinode’s Extreme-ultraviolet Imaging Spectrometer to
investigate the distribution and evolution of plasma composition
within two confined flares in a newly emerging, highly sheared
active region. During the decay phase of the first flare, patches
above the flare ribbons evolve from the FIP to the IFIP effect, while
the flaring loop tops show a stronger FIP effect. The patch and loop
compositions then evolve toward the preflare basal state. We propose
an explanation of how flaring in strands of highly sheared emerging
magnetic fields can lead to flare-modulated IFIP plasma composition
over coalescing umbrae which are crossed by flare ribbons. Subsurface
reconnection between the coalescing umbrae leads to the depletion of
low-FIP elements as a result of an increased wave flux from below. This
material is evaporated when the flare ribbons cross the umbrae. Our
results are consistent with the ponderomotive fractionation model for
the creation of IFIP-biased plasma.

---------------------------------------------------------
Title: Modeling the Effect of Mass-draining on Prominence Eruptions
Authors: Jenkins, Jack M.; Hopwood, Matthew; Démoulin, Pascal; Valori,
Gherardo; Aulanier, Guillaume; Long, David M.; van Driel-Gesztelyi,
Lidia
2019ApJ...873...49J Altcode: 2019arXiv190110970J
Quiescent solar prominences are observed within the solar atmosphere
for up to several solar rotations. Their eruption is commonly preceded
by a slow increase in height that can last from hours to days. This
increase in the prominence height is believed to be due to their host
magnetic flux rope transitioning through a series of neighboring
quasi-equilibria before the main loss of equilibrium that drives
the eruption. Recent work suggests that the removal of prominence
mass from a stable, quiescent flux rope is one possible cause for
this change in height. However, these conclusions are drawn from
observations and are subject to interpretation. Here, we present a
simple model to quantify the effect of “mass-draining” during the
pre-eruptive height evolution of a solar flux rope. The flux rope is
modeled as a line current suspended within a background potential
magnetic field. We first show that the inclusion of mass, up to
1012 kg, can modify the height at which the line current
experiences loss of equilibrium by up to 14%. Next, we show that
the rapid removal of mass prior to the loss of equilibrium can allow
the height of the flux rope to increase sharply and without an upper
bound as it approaches its loss-of-equilibrium point. This indicates
that the critical height for the loss of equilibrium can occur at a
range of heights depending explicitly on the amount and evolution of
mass within the flux rope. Finally, we demonstrate that for the same
amount of drained mass, the effect on the height of the flux rope is
up to two orders of magnitude larger for quiescent prominences than
for active region prominences.

---------------------------------------------------------
Title: An Observationally Constrained Model of a Flux Rope that
Formed in the Solar Corona
Authors: James, Alexander W.; Valori, Gherardo; Green, Lucie M.; Liu,
Yang; Cheung, Mark C. M.; Guo, Yang; van Driel-Gesztelyi, Lidia
2018csc..confE...9J Altcode:
Coronal mass ejections (CMEs) are large-scale eruptions of plasma
from the coronae of stars, and it is important to study the plasma
processes involved in their initiation. This first requires us to
understand the pre-eruptive configuration of CMEs. To this end, we used
extreme-ultraviolet (EUV) observations from SDO/AIA to conclude that a
magnetic flux rope formed high-up in the solar corona above NOAA Active
Region 11504 before it erupted on 2012 June 14. Then, we used data from
SDO/HMI and our knowledge of the EUV observations to model the coronal
magnetic field of the active region one hour prior to eruption using a
nonlinear force-free field extrapolation. The extrapolation revealed
a flux rope that matches the EUV observations remarkably well, with
its axis 120 Mm above the photosphere. The erupting structure was not
observed to kink, but the decay index near the apex of the axis of
the extrapolated flux rope is comparable to typical critical values
required for the onset of the torus instability. Therefore, we suggest
that the torus instability drove the eruption of the flux rope.

---------------------------------------------------------
Title: Multi-wavelength observations of 4 homologous global coronal
waves
Authors: Long, David; Lawless, Julia; Valori, Gherardo; Jenkins, Jack
2018csc..confE..15L Altcode:
Global coronal waves (commonly called "EIT waves") were first observed
by SOHO/EIT in 1997 and are now considered to be large-scale shock
fronts initially driven by the rapid expansion of an erupting coronal
mass ejection in the low corona. I will present observations of four
homologous global waves which erupted from the same active region over
the course of three days in March 2014. Each global EUV wave was well
observed by SDO/AIA and was associated with a H-alpha Moreton-Ramsey
wave observed at high cadence by the GONG network. These observations
provide the opportunity to directly relate global waves in EUV and
H-alpha observations with high cadence and answer a fundamental
question about the relationship between these phenomena which has
persisted since they were first observed.

---------------------------------------------------------
Title: Time Variations of the Nonpotential and Volume-threading
Magnetic Helicities
Authors: Linan, L.; Pariat, É.; Moraitis, K.; Valori, G.; Leake, J.
2018ApJ...865...52L Altcode: 2018arXiv180903765L
Relative magnetic helicity is a gauge-invariant quantity suitable
for the study of the magnetic helicity content of heliospheric
plasmas. Relative magnetic helicity can be decomposed uniquely
into two gauge-invariant quantities, the magnetic helicity
of the nonpotential component of the field and a complementary
volume-threading helicity. Recent analysis of numerical experiments
simulating the generation of solar eruptions have shown that the
ratio of the nonpotential helicity to the total relative helicity is
a clear marker of the eruptivity of the magnetic system, and that the
high value of that quantity could be a sufficient condition for the
onset of the instability generating the eruptions. The present study
introduces the first analytical examination of the time variations
of these nonpotential and volume-threading helicities. The validity
of the analytical formulae derived are confirmed with analysis of 3D
magnetohydrodynamics (MHD) simulations of solar coronal dynamics. Both
the analytical investigation and the numerical application show that,
unlike magnetic helicity, the nonpotential and the volume-threading
helicities are not conserved quantities, even in the ideal MHD regime. A
term corresponding to the transformation between the nonpotential and
volume-threading helicities frequently dominates their dynamics. This
finding has an important consequence for their estimation in the
solar corona: unlike with relative helicity, their volume coronal
evolution cannot be ascertained by the flux of these quantities
through the volume’s boundaries. Only techniques extrapolating the
3D coronal field will enable both the proper study of the nonpotential
and volume-threading helicities and the observational analysis of
helicity-based solar-eruptivity proxies.

---------------------------------------------------------
Title: Threshold of Non-potential Magnetic Helicity Ratios at the
Onset of Solar Eruptions
Authors: Zuccarello, F. P.; Pariat, E.; Valori, G.; Linan, L.
2018ApJ...863...41Z Altcode: 2018arXiv180700532Z
The relative magnetic helicity is a quantity that is often used to
describe the level of entanglement of non-isolated magnetic fields,
such as the magnetic field of solar active regions. The aim of this
paper is to investigate how different kinds of photospheric boundary
flows accumulate relative magnetic helicity in the corona and if and
how well magnetic-helicity-related quantities identify the onset
of an eruption. We use a series of three-dimensional, parametric
magnetohydrodynamic simulations of the formation and eruption of
magnetic flux ropes. All the simulations are performed on the same
grid, using the same parameters, but they are characterized by different
driving photospheric flows, i.e., shearing, convergence, stretching, and
peripheral- and central- dispersion flows. For each of the simulations,
the instant of the onset of the eruption is carefully identified
by using a series of relaxation runs. We find that magnetic energy
and total relative helicity are mostly injected when shearing flows
are applied at the boundary, while the magnetic energy and helicity
associated with the coronal electric currents increase regardless of
the kind of photospheric flows. We also find that, at the onset of
the eruptions, the ratio between the non-potential magnetic helicity
and the total relative magnetic helicity has the same value for all
the simulations, suggesting the existence of a threshold in this
quantity. Such a threshold is not observed for other quantities as,
for example, those related to the magnetic energy.

---------------------------------------------------------
Title: Computation of Relative Magnetic Helicity in Spherical
Coordinates
Authors: Moraitis, Kostas; Pariat, Étienne; Savcheva, Antonia;
Valori, Gherardo
2018SoPh..293...92M Altcode: 2018arXiv180603011M
Magnetic helicity is a quantity of great importance in solar studies
because it is conserved in ideal magnetohydrodynamics. While many
methods for computing magnetic helicity in Cartesian finite volumes
exist, in spherical coordinates, the natural coordinate system
for solar applications, helicity is only treated approximately. We
present here a method for properly computing the relative magnetic
helicity in spherical geometry. The volumes considered are finite,
of shell or wedge shape, and the three-dimensional magnetic field is
considered to be fully known throughout the studied domain. Testing of
the method with well-known, semi-analytic, force-free magnetic-field
models reveals that it has excellent accuracy. Further application to
a set of nonlinear force-free reconstructions of the magnetic field of
solar active regions and comparison with an approximate method used
in the past indicates that the proposed method can be significantly
more accurate, thus making our method a promising tool in helicity
studies that employ spherical geometry. Additionally, we determine
and discuss the applicability range of the approximate method.

---------------------------------------------------------
Title: An Observationally Constrained Model of a Flux Rope that
Formed in the Solar Corona
Authors: James, Alexander W.; Valori, Gherardo; Green, Lucie M.; Liu,
Yang; Cheung, Mark C. M.; Guo, Yang; van Driel-Gesztelyi, Lidia
2018ApJ...855L..16J Altcode: 2018arXiv180207965J
Coronal mass ejections (CMEs) are large-scale eruptions of plasma
from the coronae of stars. Understanding the plasma processes involved
in CME initiation has applications for space weather forecasting and
laboratory plasma experiments. James et al. used extreme-ultraviolet
(EUV) observations to conclude that a magnetic flux rope formed in
the solar corona above NOAA Active Region 11504 before it erupted on
2012 June 14 (SOL2012-06-14). In this work, we use data from the Solar
Dynamics Observatory (SDO) to model the coronal magnetic field of the
active region one hour prior to eruption using a nonlinear force-free
field extrapolation, and find a flux rope reaching a maximum height
of 150 Mm above the photosphere. Estimations of the average twist of
the strongly asymmetric extrapolated flux rope are between 1.35 and
1.88 turns, depending on the choice of axis, although the erupting
structure was not observed to kink. The decay index near the apex
of the axis of the extrapolated flux rope is comparable to typical
critical values required for the onset of the torus instability,
so we suggest that the torus instability drove the eruption.

---------------------------------------------------------
Title: Studying the Transfer of Magnetic Helicity in Solar Active
Regions with the Connectivity-based Helicity Flux Density Method
Authors: Dalmasse, K.; Pariat, É.; Valori, G.; Jing, J.; Démoulin, P.
2018ApJ...852..141D Altcode: 2017arXiv171204691D
In the solar corona, magnetic helicity slowly and continuously
accumulates in response to plasma flows tangential to the photosphere
and magnetic flux emergence through it. Analyzing this transfer of
magnetic helicity is key for identifying its role in the dynamics of
active regions (ARs). The connectivity-based helicity flux density
method was recently developed for studying the 2D and 3D transfer
of magnetic helicity in ARs. The method takes into account the 3D
nature of magnetic helicity by explicitly using knowledge of the
magnetic field connectivity, which allows it to faithfully track the
photospheric flux of magnetic helicity. Because the magnetic field is
not measured in the solar corona, modeled 3D solutions obtained from
force-free magnetic field extrapolations must be used to derive the
magnetic connectivity. Different extrapolation methods can lead to
markedly different 3D magnetic field connectivities, thus questioning
the reliability of the connectivity-based approach in observational
applications. We address these concerns by applying this method to the
isolated and internally complex AR 11158 with different magnetic field
extrapolation models. We show that the connectivity-based calculations
are robust to different extrapolation methods, in particular with
regard to identifying regions of opposite magnetic helicity flux. We
conclude that the connectivity-based approach can be reliably used in
observational analyses and is a promising tool for studying the transfer
of magnetic helicity in ARs and relating it to their flaring activity.

---------------------------------------------------------
Title: Non-thermal distributions and energy transport in the solar
flares
Authors: Matthews, Sarah; del Zanna, Guilio; Calcines, Ariadna;
Mason, Helen; Mathioudakis, Mihalis; Culhane, Len; Harra, Louise;
van Driel-Gesztelyi, Lidia; Green, Lucie; Long, David; Baker, Deb;
Valori, Gherardo
2017arXiv171200773M Altcode:
Determining the energy transport mechanisms in flares remains a central
goal in solar flares physics that is still not adequately answered
by the 'standard flare model'. In particular, the relative roles of
particles and/or waves as transport mechanisms, the contributions of low
energy protons and ions to the overall flare budget, and the limits of
low energy non-thermal electron distribution are questions that still
cannot be adequately reconciled with current instrumentation. In this
'White Paper' submitted in response to the call for inputs to the Next
Generation Solar Physics Mission review process initiated by JAXA,
NASA and ESA in 2016, we outline the open questions in this area and
possible instrumentation that could provide the required observations
to help answer these and other flare-related questions.

---------------------------------------------------------
Title: Measuring the magnetic field of a trans-equatorial loop system
using coronal seismology (Corrigendum)
Authors: Long, D. M.; Valori, G.; Pérez-Suárez, D.; Morton, R. J.;
Vásquez, A. M.
2017A&A...607C...3L Altcode:
No abstract at ADS

---------------------------------------------------------
Title: The 2013 February 17 Sunquake in the Context of the Active
Region's Magnetic Field Configuration
Authors: Green, L. M.; Valori, G.; Zuccarello, F. P.; Zharkov, S.;
Matthews, S. A.; Guglielmino, S. L.
2017ApJ...849...40G Altcode: 2017arXiv170904874G
Sunquakes are created by the hydrodynamic response of the lower
atmosphere to a sudden deposition of energy and momentum. In this study,
we investigate a sunquake that occurred in NOAA active region 11675
on 2013 February 17. Observations of the corona, chromosphere, and
photosphere are brought together for the first time with a nonlinear
force-free model of the active region’s magnetic field in order to
probe the magnetic environment in which the sunquake was initiated. We
find that the sunquake was associated with the destabilization of a
flux rope and an associated M-class GOES flare. Active region 11675
was in its emergence phase at the time of the sunquake and photospheric
motions caused by the emergence heavily modified the flux rope and its
associated quasi-separatrix layers, eventually triggering the flux
rope’s instability. The flux rope was surrounded by an extended
envelope of field lines rooted in a small area at the approximate
position of the sunquake. We argue that the configuration of the
envelope, by interacting with the expanding flux rope, created a
“magnetic lens” that may have focussed energy on one particular
location of the photosphere, creating the necessary conditions for
the initiation of the sunquake.

---------------------------------------------------------
Title: Flux rope, hyperbolic flux tube, and late extreme ultraviolet
phases in a non-eruptive circular-ribbon flare
Authors: Masson, Sophie; Pariat, Étienne; Valori, Gherardo; Deng,
Na; Liu, Chang; Wang, Haimin; Reid, Hamish
2017A&A...604A..76M Altcode: 2017arXiv170401450M
Context. The dynamics of ultraviolet (UV) emissions during solar flares
provides constraints on the physical mechanisms involved in the trigger
and the evolution of flares. In particular it provides some information
on the location of the reconnection sites and the associated magnetic
fluxes. In this respect, confined flares are far less understood
than eruptive flares generating coronal mass ejections. Aims:
We present a detailed study of a confined circular flare dynamics
associated with three UV late phases in order to understand more
precisely which topological elements are present and how they constrain
the dynamics of the flare. Methods: We perform a non-linear
force-free field extrapolation of the confined flare observed with the
Helioseismic and Magnetic Imager (HMI) and Atmospheric Imaging Assembly
(AIA) instruments on board Solar Dynamics Observatory (SDO). From the
3D magnetic field we compute the squashing factor and we analyse its
distribution. Conjointly, we analyse the AIA extreme ultraviolet (EUV)
light curves and images in order to identify the post-flare loops,
and their temporal and thermal evolution. By combining the two analyses
we are able to propose a detailed scenario that explains the dynamics
of the flare. Results: Our topological analysis shows that in
addition to a null-point topology with the fan separatrix, the spine
lines and its surrounding quasi-separatix layer (QSL) halo (typical
for a circular flare), a flux rope and its hyperbolic flux tube (HFT)
are enclosed below the null. By comparing the magnetic field topology
and the EUV post-flare loops we obtain an almost perfect match between
the footpoints of the separatrices and the EUV 1600 Å ribbons and
between the HFT field line footpoints and bright spots observed inside
the circular ribbons. We show, for the first time in a confined flare,
that magnetic reconnection occurred initially at the HFT below the flux
rope. Reconnection at the null point between the flux rope and the
overlying field is only initiated in a second phase. In addition, we
showed that the EUV late phase observed after the main flare episode
is caused by the cooling loops of different length which have all
reconnected at the null point during the impulsive phase. 
Conclusions: Our analysis shows in one example that flux ropes are
present in null-point topology not only for eruptive and jet events,
but also for confined flares. This allows us to conjecture on the
analogies between conditions that govern the generation of jets,
confined flares or eruptive flares. A movie is available at <A
href="http://www.aanda.org/10.1051/0004-6361/201629654/olm">http://www.aanda.org</A>

---------------------------------------------------------
Title: Studying the transfer of magnetic helicity in solar active
regions
Authors: Dalmasse, Kevin; Valori, Gherardo; Jing, Ju; Pariat, Etienne;
Demoulin, Pascal
2017SPD....4811206D Altcode:
Analyzing the transfer of magnetic helicity in active regions is a
key component for understanding the nature of its coronal storage
and release and for identifying its role in the coronal dynamics
of active regions. We recently developed a method for studying the
photospheric flux of magnetic helicity in both 2D and 3D. The method
takes into account the 3D nature of magnetic helicity by explicitly
using knowledge of the magnetic field connectivity. Since the coronal
magnetic field in active regions is not measured, we rely on the
non-unique 3D solution obtained from force-free coronal magnetic
field extrapolations to derive the magnetic field connectivity. In
this poster, we apply the method to the complex and highly-flaring
active region NOAA 11158 using the magnetic field connectivity derived
from different force-free extrapolation models and implementations. We
show that the calculations of photospheric flux of magnetic helicity
are robust to different extrapolation methods and assumptions, in
particular with regards to identifying regions of opposite magnetic
helicity flux. Finally, we discuss the implications of our results
for tracking the transfer of magnetic helicity in active regions and
relate it to their flaring activity.

---------------------------------------------------------
Title: Studying the transfer of magnetic helicity in solar active
regions
Authors: Dalmasse, Kévin; Jing, J.; Pariat, E.; Valori, G.;
Démoulin, P.
2017shin.confE.160D Altcode:
Analyzing the transfer of magnetic helicity in active regions is a
key component for understanding the nature of its coronal storage
and release and for identifying its role in the coronal dynamics
of active regions. We recently developed a method for studying the
photospheric flux of magnetic helicity in both 2D and 3D. The method
takes into account the 3D nature of magnetic helicity by explicitly
using knowledge of the magnetic field connectivity. Since the coronal
magnetic field in active regions is not measured, we rely on the
approximate 3D solution obtained from force-free coronal magnetic
field extrapolations to derive the magnetic field connectivity. In
this poster, we apply the method to the complex and highly-flaring
active region NOAA 11158 using the magnetic field connectivity derived
from different force-free extrapolation models and implementations. We
show that the calculations of photospheric flux of magnetic helicity
are robust to different extrapolation methods and assumptions, in
particular with regards to identifying regions of opposite magnetic
helicity flux. Finally, we discuss the implications of our results
for tracking the transfer of magnetic helicity in active regions and
relate it to their flaring activity.

---------------------------------------------------------
Title: Measuring the magnetic field of a trans-equatorial loop system
using coronal seismology
Authors: Long, D. M.; Valori, G.; Pérez-Suárez, D.; Morton, R. J.;
Vásquez, A. M.
2017A&A...603A.101L Altcode: 2017arXiv170310020L
Context. EIT waves are freely-propagating global pulses in the low
corona which are strongly associated with the initial evolution of
coronal mass ejections (CMEs). They are thought to be large-amplitude,
fast-mode magnetohydrodynamic waves initially driven by the rapid
expansion of a CME in the low corona. Aims: An EIT wave was
observed on 6 July 2012 to impact an adjacent trans-equatorial loop
system which then exhibited a decaying oscillation as it returned to
rest. Observations of the loop oscillations were used to estimate the
magnetic field strength of the loop system by studying the decaying
oscillation of the loop, measuring the propagation of ubiquitous
transverse waves in the loop and extrapolating the magnetic field
from observed magnetograms. Methods: Observations from the
Atmospheric Imaging Assembly onboard the Solar Dynamics Observatory
(SDO/AIA) and the Coronal Multi-channel Polarimeter (CoMP) were used
to study the event. An Empirical Mode Decomposition analysis was used
to characterise the oscillation of the loop system in CoMP Doppler
velocity and line width and in AIA intensity. Results: The
loop system was shown to oscillate in the 2nd harmonic mode rather
than at the fundamental frequency, with the seismological analysis
returning an estimated magnetic field strength of ≈ 5.5 ± 1.5
G. This compares to the magnetic field strength estimates of ≈1-9
G and ≈3-9 G found using the measurements of transverse wave
propagation and magnetic field extrapolation respectively. A movie associated to Figs. 1 and 2 is available at <A
href="http://www.aanda.org/10.1051/0004-6361/201730413/olm">http://www.aanda.org</A>

---------------------------------------------------------
Title: On-Disc Observations of Flux Rope Formation Prior to Its
Eruption
Authors: James, A. W.; Green, L. M.; Palmerio, E.; Valori, G.; Reid,
H. A. S.; Baker, D.; Brooks, D. H.; van Driel-Gesztelyi, L.; Kilpua,
E. K. J.
2017SoPh..292...71J Altcode: 2017arXiv170310837J
Coronal mass ejections (CMEs) are one of the primary manifestations of
solar activity and can drive severe space weather effects. Therefore,
it is vital to work towards being able to predict their
occurrence. However, many aspects of CME formation and eruption
remain unclear, including whether magnetic flux ropes are present
before the onset of eruption and the key mechanisms that cause CMEs
to occur. In this work, the pre-eruptive coronal configuration of
an active region that produced an interplanetary CME with a clear
magnetic flux rope structure at 1 AU is studied. A forward-S sigmoid
appears in extreme-ultraviolet (EUV) data two hours before the onset
of the eruption (SOL2012-06-14), which is interpreted as a signature
of a right-handed flux rope that formed prior to the eruption. Flare
ribbons and EUV dimmings are used to infer the locations of the flux
rope footpoints. These locations, together with observations of the
global magnetic flux distribution, indicate that an interaction between
newly emerged magnetic flux and pre-existing sunspot field in the days
prior to the eruption may have enabled the coronal flux rope to form
via tether-cutting-like reconnection. Composition analysis suggests
that the flux rope had a coronal plasma composition, supporting our
interpretation that the flux rope formed via magnetic reconnection in
the corona. Once formed, the flux rope remained stable for two hours
before erupting as a CME.

---------------------------------------------------------
Title: Analysis and modelling of recurrent solar flares observed
with Hinode/EIS on March 9, 2012
Authors: Polito, V.; Del Zanna, G.; Valori, G.; Pariat, E.; Mason,
H. E.; Dudík, J.; Janvier, M.
2017A&A...601A..39P Altcode: 2016arXiv161203504P
Three homologous C-class flares and one last M-class flare were observed
by both the Solar Dynamics Observatory (SDO) and the Hinode EUV Imaging
Spectrometer (EIS) in the AR 11429 on March 9, 2012. All the recurrent
flares occurred within a short interval of time (less than 4 h),
showed very similar plasma morphology and were all confined, until the
last one when a large-scale eruption occurred. The C-class flares are
characterized by the appearance, at approximatively the same locations,
of two bright and compact footpoint sources of ≈3-10 MK evaporating
plasma, and a semi-circular ribbon. During all the flares, the
continuous brightening of a spine-like hot plasma (≈10 MK) structure
is also observed. Spectroscopic observations with Hinode/EIS are used to
measure and compare the blueshift velocities in the Fe xxiii emission
line and the electron number density at the flare footpoints for each
flare. Similar velocities, of the order of 150-200 km s-1,
are observed during the C2.0 and C4.7 confined flares, in agreement
with the values reported by other authors in the study of the last M1.8
class flare. On the other hand, lower electron number densities and
temperatures tend to be observed in flares with lower peak soft X-ray
flux. In order to investigate the homologous nature of the flares, we
performed a non-linear force-free field (NLFFF) extrapolation of the 3D
magnetic field configuration in the corona. The NLFFF extrapolation and
the Quasi-Separatrix Layers (QSLs) provide the magnetic field context
which explains the location of the kernels, spine-like hot plasma and
semi-circular brightenings observed in the (non-eruptive) flares. Given
the absence of a coronal null point, we argue that the homologous
flares were all generated by the continuous recurrence of bald patch
reconnection. The movie associated to Fig. 2 is available at <A
href="http://www.aanda.org/10.1051/0004-6361/201629703/olm">http://www.aanda.org</A>

---------------------------------------------------------
Title: Relative magnetic helicity as a diagnostic of solar eruptivity
Authors: Pariat, E.; Leake, J. E.; Valori, G.; Linton, M. G.;
Zuccarello, F. P.; Dalmasse, K.
2017A&A...601A.125P Altcode: 2017arXiv170310562P
Context. The discovery of clear criteria that can deterministically
describe the eruptive state of a solar active region would lead
to major improvements on space weather predictions. Aims:
Using series of numerical simulations of the emergence of a magnetic
flux rope in a magnetized coronal, leading either to eruptions or to
stable configurations, we test several global scalar quantities for
the ability to discriminate between the eruptive and the non-eruptive
simulations. Methods: From the magnetic field generated by the
three-dimensional magnetohydrodynamical simulations, we compute and
analyze the evolution of the magnetic flux, of the magnetic energy
and its decomposition into potential and free energies, and of the
relative magnetic helicity and its decomposition. Results:
Unlike the magnetic flux and magnetic energies, magnetic helicities
are able to markedly distinguish the eruptive from the non-eruptive
simulations. We find that the ratio of the magnetic helicity of the
current-carrying magnetic field to the total relative helicity presents
the highest values for the eruptive simulations, in the pre-eruptive
phase only. We observe that the eruptive simulations do not possess the
highest value of total magnetic helicity. Conclusions: In the
framework of our numerical study, the magnetic energies and the total
relative helicity do not correspond to good eruptivity proxies. Our
study highlights that the ratio of magnetic helicities diagnoses very
clearly the eruptive potential of our parametric simulations. Our study
shows that magnetic-helicity-based quantities may be very efficient
for the prediction of solar eruptions.

---------------------------------------------------------
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.
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
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: Determining the Intrinsic CME Flux Rope Type Using
Remote-sensing Solar Disk Observations
Authors: Palmerio, E.; Kilpua, E. K. J.; James, A. W.; Green, L. M.;
Pomoell, J.; Isavnin, A.; Valori, G.
2017SoPh..292...39P Altcode: 2017arXiv170108595P
A key aim in space weather research is to be able to use remote-sensing
observations of the solar atmosphere to extend the lead time of
predicting the geoeffectiveness of a coronal mass ejection (CME). In
order to achieve this, the magnetic structure of the CME as it
leaves the Sun must be known. In this article we address this issue
by developing a method to determine the intrinsic flux rope type of
a CME solely from solar disk observations. We use several well-known
proxies for the magnetic helicity sign, the axis orientation, and the
axial magnetic field direction to predict the magnetic structure of
the interplanetary flux rope. We present two case studies: the 2 June
2011 and the 14 June 2012 CMEs. Both of these events erupted from an
active region, and despite having clear in situ counterparts, their
eruption characteristics were relatively complex. The first event was
associated with an active region filament that erupted in two stages,
while for the other event the eruption originated from a relatively high
coronal altitude and the source region did not feature a filament. Our
magnetic helicity sign proxies include the analysis of magnetic
tongues, soft X-ray and/or extreme-ultraviolet sigmoids, coronal
arcade skew, filament emission and absorption threads, and filament
rotation. Since the inclination of the post-eruption arcades was not
clear, we use the tilt of the polarity inversion line to determine the
flux rope axis orientation and coronal dimmings to determine the flux
rope footpoints, and therefore, the direction of the axial magnetic
field. The comparison of the estimated intrinsic flux rope structure
to in situ observations at the Lagrangian point L1 indicated a good
agreement with the predictions. Our results highlight the flux rope
type determination techniques that are particularly useful for active
region eruptions, where most geoeffective CMEs originate.

---------------------------------------------------------
Title: &lt;p&gt;Prediction of In-Situ Magnetic Structure of Flux
Ropes from Coronal Observations.
Authors: Palmerio, E.; Kilpua, E.; James, A.; Green, L.; Pomoell,
J.; Isavnin, A.; Valori, G.; Lumme, E.
2016AGUFMSH14A..03P Altcode:
Coronal mass ejections (CMEs) are believed to be the main drivers
of intense magnetic storms and various space weather phenomena at
Earth. The most important parameter that defines the ability of a
CME to drive geomagnetic storms is the north-south magnetic field
component. One of the most significant problems in current long-term
space weather forecasts is that there is no method to directly
measure the magnetic structure of CMEs before they are observed in
situ. In recent years, CMEs have been successfully modeled as unstable
expanding flux ropes originating from low-corona, force-free flux
equilibria (either containing or forming a flux rope in the wake of the
instability). Due to their influence on the coronal plasma environment,
the magnetic structure of CME flux ropes can be indirectly estimated
based on the properties of the source active region and characteristics
of the nearby structures, such as filament details, coronal EUV arcades
and X-ray sigmoids. We present here a study of two CME flux ropes,
aiming at determining their magnetic properties (magnetic helicity
sign, flux rope tilt, and direction of the flux rope axial field)
when launched from the Sun by using a synthesis of indirect proxies
based on multi-wavelength remote sensing observations. In addition,
we employ a data-driven magnetofrictional method that models the CME
initiation in the corona to determine the magnetic structure in the
two case studies. Finally, the predictions given by the observational
synthesis and coronal modeling are compared with the structure detected
in situ at Earth.

---------------------------------------------------------
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
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: Measuring the magnetic field of a trans-equatorial loop system
using coronal seismology
Authors: Long, David M.; Pérez-Suárez; D.; Valori; G.
2016usc..confE..25L Altcode:
First observed by SOHO/EIT, "EIT waves" are strongly associated with the
initial evolution of coronal mass ejections (CMEs) and after almost 20
years of investigation a consensus is being reached which interprets
them as freely-propagating waves produced by the rapid expansion of
a CME in the low corona. An "EIT wave" was observed on 6 July 2012
to erupt from active region AR11514 into a particularly structured
corona that included multiple adjacent active regions as well as an
adjacent trans-equatorial loop system anchored at the boundary of a
nearby coronal hole. The eruption was well observed by SDO/AIA and
CoMP, allowing the effects of the "EIT wave" on the trans-equatorial
loop system to be studied in detail. In particular, it was possible
to characterise the oscillation of the loop system using Doppler
velocity measurements from CoMP. These Doppler measurements were used
to estimate the magnetic field strength of the trans-equatorial loop
system via coronal seismology. It was then possible to compare these
inferred magnetic field values with both extrapolated magnetic field
values from a Potential Field Source Surface extrapolation as well
as the direct measurements of magnetic field provided by CoMP. These
results show that the magnetic field strength of loop systems in the
solar corona may be estimated using loop seismology.

---------------------------------------------------------
Title: Prediction of in-situ magnetic structure of flux ropes from
coronal observations
Authors: Palmerio, Erika; Kilpua, Emilia K. J.; Pomoell, Jens;
James, Alexander; Green, Lucie M.; Isavnin, Alexey; Valori, Gherardo;
Lumme, Erkka
2016usc..confE..33P Altcode:
Coronal Mass Ejections (CMEs) are built at the Sun as nearly force-free
(J x B = 0) magnetic flux ropes. It is well-established that CMEs
are the main drivers of intense magnetic storms and various space
weather phenomena at Earth. The most important parameter that defines
the ability of a CME to drive geomagnetic storms is the north-south
magnetic field component. One of the most significant problems in
current long-term space weather forecasts is that there is no method
to directly measure the magnetic structure of CMEs before they are
observed in situ. However, due to their influence on the coronal
plasma environment, the magnetic structure of CME flux ropes can be
indirectly estimated based on the properties of the source active
region and characteristics of the nearby structures, such as filament
details, coronal EUV arcades and X-ray sigmoids. We present here a
study of two CME flux ropes, aiming at determining their magnetic
properties (magnetic helicity sign, flux rope tilt, and direction
of the flux rope axial field) when launched from the Sun by using a
synthesis of indirect proxies based on multi-wavelength remote sensing
observations. In addition, we employ a data-driven magnetofrictional
method that models the CME initiation in the corona to determine the
magnetic structure in the two case studies. Finally, the predictions
given by the observational synthesis and coronal modeling are compared
with the structure detected in situ at Earth.

---------------------------------------------------------
Title: Magneto-frictional Modeling of Coronal Nonlinear Force-free
Fields. I. Testing with Analytic Solutions
Authors: Guo, Y.; Xia, C.; Keppens, R.; Valori, G.
2016ApJ...828...82G Altcode:
We report our implementation of the magneto-frictional method in the
Message Passing Interface Adaptive Mesh Refinement Versatile Advection
Code (MPI-AMRVAC). The method aims at applications where local adaptive
mesh refinement (AMR) is essential to make follow-up dynamical modeling
affordable. We quantify its performance in both domain-decomposed
uniform grids and block-adaptive AMR computations, using all frequently
employed force-free, divergence-free, and other vector comparison
metrics. As test cases, we revisit the semi-analytic solution of Low
and Lou in both Cartesian and spherical geometries, along with the
topologically challenging Titov-Démoulin model. We compare different
combinations of spatial and temporal discretizations, and find that the
fourth-order central difference with a local Lax-Friedrichs dissipation
term in a single-step marching scheme is an optimal combination. The
initial condition is provided by the potential field, which is the
potential field source surface model in spherical geometry. Various
boundary conditions are adopted, ranging from fully prescribed cases
where all boundaries are assigned with the semi-analytic models,
to solar-like cases where only the magnetic field at the bottom is
known. Our results demonstrate that all the metrics compare favorably
to previous works in both Cartesian and spherical coordinates. Cases
with several AMR levels perform in accordance with their effective
resolutions. The magneto-frictional method in MPI-AMRVAC allows us
to model a region of interest with high spatial resolution and large
field of view simultaneously, as required by observation-constrained
extrapolations using vector data provided with modern instruments. The
applications of the magneto-frictional method to observations are
shown in an accompanying paper.

---------------------------------------------------------
Title: Flux Cancellation and the Evolution of the Eruptive Filament
of 2011 June 7
Authors: Yardley, S. L.; Green, L. M.; Williams, D. R.; van
Driel-Gesztelyi, L.; Valori, G.; Dacie, S.
2016ApJ...827..151Y Altcode: 2016arXiv160608264Y
We investigate whether flux cancellation is responsible for the
formation of a very massive filament resulting in the spectacular
eruption on 2011 June 7. We analyze and quantify the amount of flux
cancellation that occurs in NOAA AR 11226 and its two neighboring active
regions (ARs 11227 &amp; 11233) using line-of-sight magnetograms from
the Heliospheric Magnetic Imager. During a 3.6 day period building
up to the eruption of the filament, 1.7 × 1021 Mx, 21%
of AR 11226's maximum magnetic flux, was canceled along the polarity
inversion line (PIL) where the filament formed. If the flux cancellation
continued at the same rate up until the eruption then up to 2.8 ×
1021 Mx (34% of the AR flux) may have been built into the
magnetic configuration that contains the filament plasma. The large flux
cancellation rate is due to an unusual motion of the positive-polarity
sunspot, which splits, with the largest section moving rapidly toward
the PIL. This motion compresses the negative polarity and leads to
the formation of an orphan penumbra where one end of the filament is
rooted. Dense plasma threads above the orphan penumbra build into the
filament, extending its length, and presumably injecting material into
it. We conclude that the exceptionally strong flux cancellation in
AR 11226 played a significant role in the formation of its unusually
massive filament. In addition, the presence and coherent evolution of
bald patches in the vector magnetic field along the PIL suggest that
the magnetic field configuration supporting the filament material is
that of a flux rope.

---------------------------------------------------------
Title: Photospheric Vector Magnetic Field Evolution of NOAA Active
Region 11504 and the Ensuing CME
Authors: James, Alexander; Green, Lucie; Valori, Gherardo; van
Driel-Gesztelyi, Lidia; Baker, Deborah; Brooks, David; Palmerio, Erika
2016SPD....4730305J Altcode:
Coronal mass ejections (CMEs) are eruptions of billions of tonnes of
plasma from the Sun that drive the most severe space weather effects
we observe. In order to be able to produce forecasts of space weather
with lead times of the order of days, accurate predictions of the
occurrence of CMEs must be developed. The eruptive active-region
studied in this work (NOAA 11504) is complex, featuring fragmentation
of penumbral magnetic field in the days prior to eruption, as well as
rotation of the leading sunspot. SDO/HMI vector photospheric magnetic
field measurements are utilised alongside SDO/AIA multi-wavelength
extreme ultra-violet (EUV) observations to study the dynamics of the
photospheric and coronal structures, as well as Hinode/EIS spectroscopic
measurements, including elemental composition data. The EUV data show
flare ribbons as well as coronal dimmings, which are used to infer
the orientation of the erupting flux rope. This flux rope orientation
is then compared to in situ measurements of the flux rope. The vector
magnetic field data is used to determine the possible contributions
the field fragmentation and sunspot rotation may have made to the
formation of the flux rope and the triggering of the CME.

---------------------------------------------------------
Title: Measuring the magnetic field of a trans-equatorial loop system
using coronal seismology
Authors: Long, David; Perez-Suarez, David; Valori, Gherardo
2016SPD....47.0319L Altcode:
First observed by SOHO/EIT, "EIT waves" are strongly associated with the
initial evolution of coronal mass ejections (CMEs) and after almost 20
years of investigation a consensus is being reached which interprets
them as freely-propagating waves produced by the rapid expansion of
a CME in the low corona. An "EIT wave" was observed on 6 July 2012
to erupt from active region AR11514 into a particularly structured
corona that included multiple adjacent active regions as well as an
adjacent trans-equatorial loop system anchored at the boundary of a
nearby coronal hole. The eruption was well observed by SDO/AIA and
CoMP, allowing the effects of the "EIT wave" on the trans-equatorial
loop system to be studied in detail. In particular, it was possible
to characterise the oscillation of the loop system using Doppler
velocity measurements from CoMP. These Doppler measurements were used
to estimate the magnetic field strength of the trans-equatorial loop
system via coronal seismology. It was then possible to compare these
inferred magnetic field values with extrapolated magnetic field values
derived using a Potential Field Source Surface extrapolation as well
as the direct measurements of magnetic field provided by CoMP. These
results show that the magnetic field strength of loop systems in the
solar corona may be estimated using loop seismology.

---------------------------------------------------------
Title: Anomalous transmission of a coronal "EIT wave" through a
nearby coronal hole
Authors: Long, David; Perez-Suarez, David; Valori, Gherardo
2016SPD....47.0320L Altcode:
Observations of reflection at coronal hole boundaries and transmission
through the coronal hole suggest that "EIT waves" may be interpreted
as freely--propagating wave--pulses initially driven by the rapid
expansion of a coronal mass ejection (CME) in the low corona. An
"EIT wave" observed on 2012 July 07 is seen to impact an adjacent
coronal hole. However, rather than reappearing at the far edge of
the coronal hole as with previous observations, the "EIT wave" was
subsequently observed to reappear ~360 Mm away in the quiet Sun. The
non-typical evolution of the "EIT wave" is examined using a combination
of observations of the eruption from SDO/AIA and STEREO-A/EUVI as well
as extrapolations of the global magnetic field. The observed "jump"
in position of the "EIT wave" is shown to be due to the wave pulse
traveling along hot coronal loops connecting the edge of the coronal
hole with the quiet Sun.

---------------------------------------------------------
Title: Tracking the magnetic structure of flux ropes from eruption
to in-situ detection
Authors: Palmerio, Erika; Kilpua, Emilia; Green, Lucie; James,
Alexander; Pomoell, Jens; Valori, Gherardo
2016EGUGA..18.1641P Altcode:
Coronal Mass Ejections (CMEs) are spectacular explosions from the
Sun where huge amounts of plasma and magnetic flux are ejected into
the heliosphere. CMEs are built at the Sun as a force-free (J ×B =
0) magnetic flux rope. It is well-established that CMEs are the main
drivers of intense magnetic storms and various space weather effects
at the Earth. One of the most significant problems for improving the
long lead-time space weather predictions is that there is no method to
directly measure the structure of CME magnetic fields, neither in the
onset process nor during the subsequent propagation from the solar
surface to the Earth. The magnetic properties of the CME flux rope
(magnetic helicity sign, the flux rope tilt and the direction of the
flux rope axial field) can be estimated based on the properties of the
source active region and characteristics of the related structures, such
as filament details, coronal EUV arcades and X-ray sigmoids. We present
here a study of two CME flux ropes. We compare their magnetic structure
using the synthesis of these indirect proxies based on multi-wavelength
remote sensing observations with the structure detected in-situ near
the orbit of the Earth.

---------------------------------------------------------
Title: Restricted propagation of an “EIT wave” in the low solar
corona
Authors: Long, David M.; Pérez-Suárez, David; Valori, Gherardo
2016IAUS..320...98L Altcode:
We present observations of an “EIT wave” associated with an X-class
flare from 2012 July 6, the propagation of which was severely restricted
by the magnetic structure of the solar corona surrounding the erupting
active region. The “EIT wave” was observed by both SDO and STEREO-A,
allowing a three-dimensional examination of how the propagation of
the disturbance was affected both by a neighbouring coronal hole
and a trans-equatorial loop system. In addition, the eruption was
observed at the limb by the ground-based CoMP instrument, allowing the
Doppler motion associated with the eruption and resulting coronal loop
oscillation to be investigated in detail. This combination of data-sets
provides a unique insight into the three-dimensional evolution of the
“EIT wave” and its effects on the surrounding corona.

---------------------------------------------------------
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.
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: Testing magnetic helicity conservation in a solar-like
active event
Authors: Pariat, E.; Valori, G.; Démoulin, P.; Dalmasse, K.
2015A&A...580A.128P Altcode: 2015arXiv150609013P
Context. Magnetic helicity has the remarkable property of being a
conserved quantity of ideal magnetohydrodynamics (MHD). Therefore, it
could be used as an effective tracer of the magnetic field evolution
of magnetized plasmas. Aims: Theoretical estimations indicate
that magnetic helicity is also essentially conserved with non-ideal MHD
processes, for example, magnetic reconnection. This conjecture has been
barely tested, however, either experimentally or numerically. Thanks
to recent advances in magnetic helicity estimation methods, it is
now possible to numerically test its dissipation level in general
three-dimensional datasets. Methods: We first revisit the
general formulation of the temporal variation of relative magnetic
helicity on a fully bounded volume when no hypothesis on the gauge is
made. We introduce a method for precisely estimating its dissipation
independently of which type of non-ideal MHD processes occurs. For
a solar-like eruptive-event simulation, using different gauges, we
compare an estimate of the relative magnetic helicity computed in a
finite volume with its time-integrated flux through the boundaries. We
thus test the conservation and dissipation of helicity. Results:
We provide an upper bound of the real dissipation of magnetic helicity:
It is quasi-null during the quasi-ideal MHD phase. Even with magnetic
reconnection, the relative dissipation of magnetic helicity is also very
low (&lt;2.2%), in particular compared to the relative dissipation
of magnetic energy (&gt;30 times higher). We finally illustrate
how the helicity-flux terms involving velocity components are gauge
dependent, which limits their physical meaning. Conclusions:
Our study paves the way for more extended and diverse tests of the
magnetic helicity conservation properties. Our study confirms the
central role of helicity in the study of MHD plasmas. For instance,
the conservation of helicity can be used to track the evolution of
solar magnetic fields from when they form in the solar interior
until their detection as magnetic clouds in the interplanetary
space. Appendix A is available in electronic form at <A
href="http://www.aanda.org/10.1051/0004-6361/201525811/olm">http://www.aanda.org</A>

---------------------------------------------------------
Title: Time Evolution of Force-Free Parameter and Free Magnetic
Energy in Active Region NOAA 10365
Authors: Valori, G.; Romano, P.; Malanushenko, A.; Ermolli, I.;
Giorgi, F.; Steed, K.; van Driel-Gesztelyi, L.; Zuccarello, F.;
Malherbe, J. -M.
2015SoPh..290..491V Altcode:
We describe the variation of the accumulated coronal helicity derived
from the magnetic helicity flux through the photosphere in active region
(AR) NOAA 10365, where several large flares and coronal mass ejections
(CMEs) occurred. We used SOHO/MDI full-disk line-of-sight magnetograms
to measure the helicity flux, and the integral of GOES X-ray flux as a
proxy of the coronal energy variations due to flares or CMEs. Using the
linear force-free field model, we transformed the accumulated helicity
flux into a time sequence of the force-free parameter α accounting for
flares or CMEs via the proxy derived from GOES observations. This method
can be used to derive the value of α at different times during the
AR evolution, and is a partial alternative to the commonly used match
of field lines with EUV loops. By combining the accumulated helicity
obtained from the observations with the linear force-free theory, we
describe the main phases of the emergence process of the AR, and relate
them temporally with the occurrence of flares or CMEs. Additionally,
a comparison with the loop-matching method of fixing alpha at each time
independently shows that the proposed method may be helpful in avoiding
unrealistic or undetermined values of alpha that may originate from
an insufficient quality of the image used to identify coronal loops
at a given time. For the relative intensity of the considered events,
the linear force-free field theory implies that there is a direct
correlation between the released energy on the one hand and the product
of the coronal helicity with the variation of α due to the event on
the other. Therefore, the higher the value of the accumulated coronal
helicity, the smaller the force-free parameter variation required to
produce the same decrease in the free energy during the CMEs.

---------------------------------------------------------
Title: Coronal Magnetic Reconnection Driven by CME Expansion—the
2011 June 7 Event
Authors: van Driel-Gesztelyi, L.; Baker, D.; Török, T.; Pariat, E.;
Green, L. M.; Williams, D. R.; Carlyle, J.; Valori, G.; Démoulin,
P.; Kliem, B.; Long, D. M.; Matthews, S. A.; Malherbe, J. -M.
2014ApJ...788...85V Altcode: 2014arXiv1406.3153V
Coronal mass ejections (CMEs) erupt and expand in a magnetically
structured solar corona. Various indirect observational pieces of
evidence have shown that the magnetic field of CMEs reconnects with
surrounding magnetic fields, forming, e.g., dimming regions distant
from the CME source regions. Analyzing Solar Dynamics Observatory
(SDO) observations of the eruption from AR 11226 on 2011 June 7, we
present the first direct evidence of coronal magnetic reconnection
between the fields of two adjacent active regions during a CME. The
observations are presented jointly with a data-constrained numerical
simulation, demonstrating the formation/intensification of current
sheets along a hyperbolic flux tube at the interface between the CME
and the neighboring AR 11227. Reconnection resulted in the formation of
new magnetic connections between the erupting magnetic structure from
AR 11226 and the neighboring active region AR 11227 about 200 Mm from
the eruption site. The onset of reconnection first becomes apparent
in the SDO/AIA images when filament plasma, originally contained
within the erupting flux rope, is redirected toward remote areas in
AR 11227, tracing the change of large-scale magnetic connectivity. The
location of the coronal reconnection region becomes bright and directly
observable at SDO/AIA wavelengths, owing to the presence of down-flowing
cool, dense (1010 cm-3) filament plasma in its
vicinity. The high-density plasma around the reconnection region is
heated to coronal temperatures, presumably by slow-mode shocks and
Coulomb collisions. These results provide the first direct observational
evidence that CMEs reconnect with surrounding magnetic structures,
leading to a large-scale reconfiguration of the coronal magnetic field.

---------------------------------------------------------
Title: Constraining magnetic flux emergence from a timeseries of
helicitigrams
Authors: Dalmasse, Kévin; Pariat, Etienne; Green, Lucie M.; Aulanier,
Guillaume; Demoulin, Pascal; Valori, Gherardo
2014cosp...40E.612D Altcode:
Magnetic helicity quantifies how globally twisted and/or sheared is
the magnetic field in a volume. Observational studies have reported
the injection of large amounts of magnetic helicity associated with
the emergence of magnetic flux into the solar atmosphere. Because
magnetic helicity is conserved in the convection zone, the injection of
magnetic helicity into the solar corona reflects the helicity content
of emerging magnetic flux tubes. Mapping the photospheric injection
of magnetic helicity thus seems to be a key tool for constraining the
parameters of the emerging flux tubes in numerical case-studies of
observed active regions. We recently developed a method to compute the
distribution of magnetic helicity flux. Contrary to previous proxies,
this method takes into account the 3D nature of magnetic helicity, and
is thus, better-suited to study the distribution of helicity flux. After
introducing this method, we will present the results of its application
to the NOAA AR 11158. We will show that, the distribution of helicity
flux is complex, with patterns of real mixed signals of helicity flux
related to the specific topology of the active region's magnetic
field. Finally, we will discuss the implications of our results on
the evolution and dynamics of this active region.

---------------------------------------------------------
Title: Magnetic reconnection driven by filament eruption in the 7
June 2011 event
Authors: van Driel-Gesztelyi, L.; Baker, D.; Török, T.; Pariat, E.;
Green, L. M.; Williams, D. R.; Carlyle, J.; Valori, G.; Démoulin,
P.; Matthews, S. A.; Kliem, B.; Malherbe, J. -M.
2014IAUS..300..502V Altcode:
During an unusually massive filament eruption on 7 June 2011,
SDO/AIA imaged for the first time significant EUV emission around a
magnetic reconnection region in the solar corona. The reconnection
occurred between magnetic fields of the laterally expanding CME
and a neighbouring active region. A pre-existing quasi-separatrix
layer was activated in the process. This scenario is supported by
data-constrained numerical simulations of the eruption. Observations
show that dense cool filament plasma was re-directed and heated in
situ, producing coronal-temperature emission around the reconnection
region. These results provide the first direct observational evidence,
supported by MHD simulations and magnetic modelling, that a large-scale
re-configuration of the coronal magnetic field takes place during
solar eruptions via the process of magnetic reconnection.

---------------------------------------------------------
Title: Initiation of Coronal Mass Ejections by Sunspot Rotation
Authors: Valori, G.; Török, T.; Temmer, M.; Veronig, A. M.; van
Driel-Gesztelyi, L.; Vršnak, B.
2014IAUS..300..201V Altcode:
We report observations of a filament eruption, two-ribbon flare, and
coronal mass ejection (CME) that occurred in Active Region NOAA 10898
on 6 July 2006. The filament was located South of a strong sunspot that
dominated the region. In the evolution leading up to the eruption, and
for some time after it, a counter-clockwise rotation of the sunspot of
about 30 degrees was observed. We suggest that the rotation triggered
the eruption by progressively expanding the magnetic field above the
filament. To test this scenario, we study the effect of twisting
the initially potential field overlying a pre-existing flux rope,
using three-dimensional zero-β MHD simulations. We consider a magnetic
configuration whose photospheric flux distribution and coronal structure
is guided by the observations and a potential field extrapolation. We
find that the twisting leads to the expansion of the overlying field. As
a consequence of the progressively reduced magnetic tension, the flux
rope quasi-statically adapts to the changed environmental field, rising
slowly. Once the tension is sufficiently reduced, a distinct second
phase of evolution occurs where the flux rope enters an unstable regime
characterized by a strong acceleration. Our simulation thus suggests
a new mechanism for the triggering of eruptions in the vicinity of
rotating sunspots.

---------------------------------------------------------
Title: Initiation of Coronal Mass Ejections by Sunspot Rotation
Authors: Török, T.; Temmer, M.; Valori, G.; Veronig, A. M.; van
Driel-Gesztelyi, L.; Vršnak, B.
2013SoPh..286..453T Altcode: 2014arXiv1401.2922T
We study a filament eruption, two-ribbon flare, and coronal mass
ejection (CME) that occurred in NOAA Active Region 10898 on 6 July
2006. The filament was located South of a strong sunspot that dominated
the region. In the evolution leading up to the eruption, and for some
time after it, a counter-clockwise rotation of the sunspot of about
30 degrees was observed. We suggest that the rotation triggered the
eruption by progressively expanding the magnetic field above the
filament. To test this scenario, we study the effect of twisting
the initially potential field overlying a pre-existing flux-rope,
using three-dimensional zero-β MHD simulations. We first consider
a relatively simple and symmetric system, and then study a more
complex and asymmetric magnetic configuration, whose photospheric-flux
distribution and coronal structure are guided by the observations and a
potential field extrapolation. In both cases, we find that the twisting
leads to the expansion of the overlying field. As a consequence of the
progressively reduced magnetic tension, the flux-rope quasi-statically
adapts to the changed environmental field, rising slowly. Once the
tension is sufficiently reduced, a distinct second phase of evolution
occurs where the flux-rope enters an unstable regime characterised by
a strong acceleration. Our simulations thus suggest a new mechanism
for the triggering of eruptions in the vicinity of rotating sunspots.

---------------------------------------------------------
Title: First observational application of a connectivity-based
helicity flux density
Authors: Dalmasse, K.; Pariat, E.; Valori, G.; Démoulin, P.; Green,
L. M.
2013A&A...555L...6D Altcode: 2013arXiv1307.2838D
Context. Measuring the magnetic helicity distribution in the solar
corona can help in understanding the trigger of solar eruptive
events because magnetic helicity is believed to play a key role in
solar activity due to its conservation property. Aims: A new
method for computing the photospheric distribution of the helicity
flux was recently developed. This method takes into account the
magnetic field connectivity whereas previous methods were based
on photospheric signatures only. This novel method maps the true
injection of magnetic helicity in active regions. We applied this
method for the first time to an observed active region, NOAA 11158,
which was the source of intense flaring activity. Methods: We
used high-resolution vector magnetograms from the SDO/HMI instrument
to compute the photospheric flux transport velocities and to perform
a nonlinear force-free magnetic field extrapolation. We determined
and compared the magnetic helicity flux distribution using a purely
photospheric as well as a connectivity-based method. Results:
While the new connectivity-based method confirms the mixed pattern
of the helicity flux in NOAA 11158, it also reveals a different, and
more correct, distribution of the helicity injection. This distribution
can be important for explaining the likelihood of an eruption from the
active region. Conclusions: The connectivity-based approach is
a robust method for computing the magnetic helicity flux, which can
be used to study the link between magnetic helicity and eruptivity of
observed active regions.

---------------------------------------------------------
Title: Accuracy of magnetic energy computations
Authors: Valori, G.; Démoulin, P.; Pariat, E.; Masson, S.
2013A&A...553A..38V Altcode: 2013arXiv1303.6773V
Context. For magnetically driven events, the magnetic energy of
the system is the prime energy reservoir that fuels the dynamical
evolution. In the solar context, the free energy (i.e., the energy in
excess of the potential field energy) is one of the main indicators
used in space weather forecasts to predict the eruptivity of active
regions. A trustworthy estimation of the magnetic energy is therefore
needed in three-dimensional (3D) models of the solar atmosphere, e.g.,
in coronal fields reconstructions or numerical simulations. Aims:
The expression of the energy of a system as the sum of its potential
energy and its free energy (Thomson's theorem) is strictly valid when
the magnetic field is exactly solenoidal. For numerical realizations on
a discrete grid, this property may be only approximately fulfilled. We
show that the imperfect solenoidality induces terms in the energy that
can lead to misinterpreting the amount of free energy present in a
magnetic configuration. Methods: We consider a decomposition
of the energy in solenoidal and nonsolenoidal parts which allows
the unambiguous estimation of the nonsolenoidal contribution to the
energy. We apply this decomposition to six typical cases broadly used
in solar physics. We quantify to what extent the Thomson theorem is
not satisfied when approximately solenoidal fields are used. 
Results: The quantified errors on energy vary from negligible to
significant errors, depending on the extent of the nonsolenoidal
component of the field. We identify the main source of errors and
analyze the implications of adding a variable amount of divergence to
various solenoidal fields. Finally, we present pathological unphysical
situations where the estimated free energy would appear to be negative,
as found in some previous works, and we identify the source of this
error to be the presence of a finite divergence. Conclusions:
We provide a method of quantifying the effect of a finite divergence in
numerical fields, together with detailed diagnostics of its sources. We
also compare the efficiency of two divergence-cleaning techniques. These
results are applicable to a broad range of numerical realizations of
magnetic fields. Appendices are available in electronic form at
<A href="http://www.aanda.org">http://www.aanda.org</A>

---------------------------------------------------------
Title: Comparing Values of the Relative Magnetic Helicity in Finite
Volumes
Authors: Valori, G.; Démoulin, P.; Pariat, E.
2012SoPh..278..347V Altcode: 2012SoPh..tmp..271G
Relative magnetic helicity, as a conserved quantity of ideal
magnetohydrodynamics, has been highlighted as an important quantity to
study in plasma physics. Due to its nonlocal nature, its estimation
is not straightforward in both observational and numerical data. In
this study we derive expressions for the practical computation of the
gauge-independent relative magnetic helicity in three-dimensional
finite domains. The derived expressions are easy to implement and
rapid to compute. They are derived in Cartesian coordinates, but can
be easily written in other coordinate systems. We apply our method to
a numerical model of a force-free equilibrium containing a flux rope,
and compare the results with those obtained employing known half-space
equations. We find that our method requires a much smaller volume
than half-space expressions to derive the full helicity content. We
also prove that values of relative magnetic helicity of different
magnetic fields can be compared with each other in the same sense as
free-energy values can. Therefore, relative magnetic helicity can
be meaningfully and directly compared between different datasets,
such as those from different active regions, but also within the
same dataset at different times. Typical applications of our formulae
include the helicity computation in three-dimensional models of the
solar atmosphere, e.g., coronal-field reconstructions by force-free
extrapolation and discretized magnetic fields of numerical simulations.

---------------------------------------------------------
Title: Nonlinear Force-Free Extrapolation of Emerging Flux with a
Global Twist and Serpentine Fine Structures
Authors: Valori, G.; Green, L. M.; Démoulin, P.; Vargas Domínguez,
S.; van Driel-Gesztelyi, L.; Wallace, A.; Baker, D.; Fuhrmann, M.
2012SoPh..278...73V Altcode:
We study the flux emergence process in NOAA active region 11024, between
29 June and 7 July 2009, by means of multi-wavelength observations
and nonlinear force-free extrapolation. The main aim is to extend
previous investigations by combining, as much as possible, high spatial
resolution observations to test our present understanding of small-scale
(undulatory) flux emergence, whilst putting these small-scale events
in the context of the global evolution of the active region. The
combination of these techniques allows us to follow the whole process,
from the first appearance of the bipolar axial field on the east limb,
until the buoyancy instability could set in and raise the main body
of the twisted flux tube through the photosphere, forming magnetic
tongues and signatures of serpentine field, until the simplification
of the magnetic structure into a main bipole by the time the active
region reaches the west limb. At the crucial time of the main emergence
phase high spatial resolution spectropolarimetric measurements of the
photospheric field are employed to reconstruct the three-dimensional
structure of the nonlinear force-free coronal field, which is then
used to test the current understanding of flux emergence processes. In
particular, knowledge of the coronal connectivity confirms the identity
of the magnetic tongues as seen in their photospheric signatures,
and it exemplifies how the twisted flux, which is emerging on small
scales in the form of a sea-serpent, is subsequently rearranged by
reconnection into the large-scale field of the active region. In
this way, the multi-wavelength observations combined with a nonlinear
force-free extrapolation provide a coherent picture of the emergence
process of small-scale magnetic bipoles, which subsequently reconnect
to form a large-scale structure in the corona.

---------------------------------------------------------
Title: A comparison of preprocessing methods for solar force-free
magnetic field extrapolation
Authors: Fuhrmann, M.; Seehafer, N.; Valori, G.; Wiegelmann, T.
2011A&A...526A..70F Altcode: 2010arXiv1010.6015F
Context. Extrapolations of solar photospheric vector magnetograms into
three-dimensional magnetic fields in the chromosphere and corona are
usually done under the assumption that the fields are force-free. This
condition is violated in the photosphere itself and a thin layer in
the lower atmosphere above. The field calculations can be improved
by preprocessing the photospheric magnetograms. The intention here
is to remove a non-force-free component from the data. Aims:
We compare two preprocessing methods presently in use, namely the
methods of Wiegelmann et al. (2006, Sol. Phys., 233, 215) and Fuhrmann
et al. (2007, A&amp;A, 476, 349). Methods: The two preprocessing
methods were applied to a vector magnetogram of the recently observed
active region NOAA AR 10 953. We examine the changes in the magnetogram
effected by the two preprocessing algorithms. Furthermore, the original
magnetogram and the two preprocessed magnetograms were each used as
input data for nonlinear force-free field extrapolations by means of two
different methods, and we analyze the resulting fields. Results:
Both preprocessing methods managed to significantly decrease the
magnetic forces and magnetic torques that act through the magnetogram
area and that can cause incompatibilities with the assumption of
force-freeness in the solution domain. The force and torque decrease is
stronger for the Fuhrmann et al. method. Both methods also reduced the
amount of small-scale irregularities in the observed photospheric field,
which can sharply worsen the quality of the solutions. For the chosen
parameter set, the Wiegelmann et al. method led to greater changes
in strong-field areas, leaving weak-field areas mostly unchanged,
and thus providing an approximation of the magnetic field vector in
the chromosphere, while the Fuhrmann et al. method weakly changed
the whole magnetogram, thereby better preserving patterns present
in the original magnetogram. Both preprocessing methods raised the
magnetic energy content of the extrapolated fields to values above the
minimum energy, corresponding to the potential field. Also, the fields
calculated from the preprocessed magnetograms fulfill the solenoidal
condition better than those calculated without preprocessing.

---------------------------------------------------------
Title: Testing magnetofrictional extrapolation with the
Titov-Démoulin model of solar active regions
Authors: Valori, G.; Kliem, B.; Török, T.; Titov, V. S.
2010A&A...519A..44V Altcode: 2010arXiv1005.0254V
We examine the nonlinear magnetofrictional extrapolation scheme
using the solar active region model by Titov and Démoulin as test
field. This model consists of an arched, line-tied current channel
held in force-free equilibrium by the potential field of a bipolar
flux distribution in the bottom boundary. A modified version with a
parabolic current density profile is employed here. We find that the
equilibrium is reconstructed with very high accuracy in a representative
range of parameter space, using only the vector field in the bottom
boundary as input. Structural features formed in the interface
between the flux rope and the surrounding arcade - “hyperbolic
flux tube” and “bald patch separatrix surface” - are reliably
reproduced, as are the flux rope twist and the energy and helicity of
the configuration. This demonstrates that force-free fields containing
these basic structural elements of solar active regions can be obtained
by extrapolation. The influence of the chosen initial condition on
the accuracy of reconstruction is also addressed, confirming that the
initial field that best matches the external potential field of the
model quite naturally leads to the best reconstruction. Extrapolating
the magnetogram of a Titov-Démoulin equilibrium in the unstable
range of parameter space yields a sequence of two opposing evolutionary
phases, which clearly indicate the unstable nature of the configuration:
a partial buildup of the flux rope with rising free energy is followed
by destruction of the rope, losing most of the free energy.

---------------------------------------------------------
Title: 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.
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
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: 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.
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: Coronal Magnetic Field Extrapolation from Photospheric
Measurements Applied to an Active Region
Authors: Fuhrmann, M.; Kliem, B.; Valori, G.; Seehafer, N.
2008ESPM...12.3.37F Altcode:
We outline an MHD relaxation method that permits to extrapolate
photospheric vector magnetograms into coronal nonlinear force-free
fields. The method is applied to a magnetogram taken before an eruptive
event in NOAA AR 7792 on 25 October 1994. The event produced a coronal
mass ejection (CME) and eruptive flare with a prominent sigmoidal
soft X-ray source. Multiwavelength observations as well as theoretical
modeling indicate the importance of twisted magnetic configurations in
solar active regions(ARs) in the initiation of such events. Manoharan
et al. (1996) proposed a model for this event that included the merging
of two slightly twisted flux bundles near the polarity inversion line
into a flux rope of larger total twist and an overlying flux system much
closer to a potential-field state. For the extrapolation we use a vector
magnetogram taken at the Mees Solar Observatory 16 hours before the
actual event. The magnetic field extrapolation is able to recover main
parts of the structures suggested in the model by Manoharan et al. We
find the overlying nearly potential flux and part of the sigmoidal
field, i.e., one of the suggested weakly twisted flux bundles, in the
location observed. This supports the notion that sigmoids are coronal
manifestations of twisted magnetic flux tubes which start expanding in
eruptive events and may exist even before the onset of such events. We
tentatively attribute the incomplete reconstruction of the sigmoidal
field structure to the strong evolution of the photospheric field at
the suggested location of flux tube merging between the time of the
magnetogram and the eruption, as indicated by a magnetogram on the
following day.

---------------------------------------------------------
Title: Magnetofrictional Extrapolations of Current-Carrying Flux Ropes
Authors: Valori, G.; Kliem, B.; Toeroek, T.
2008ESPM...12.2.90V Altcode:
The quiescent solar corona is regularly modified by very fast ejections
of coronal material and magnetic field (CME) that occur preferably
above active regions. Most CME models require the formation of twisted
magnetic field structures (flux ropes) before or during such events. Unfortunately, the coronal magnetic field is not directly measurable
at present, and therefore it is difficult to verify the validity
of different CME models. In order to remove this obstacle, the
extrapolation of photospheric magnetic field measurements can be used
to reconstruct the missing coronal information. As an application of
our magneto-frictional code, we present an extrapolation of a measured
magnetogram where a flux rope is found. In such applications it
is necessary to estimate how well our extrapolation code can reproduce
all aspects of highly nonlinear structures such as flux ropes. This is
of course possible only using test fields. The Titov and Demoulin
force-free equilibrium (Titov and Demoulin, Astr. and Astrophys. 351,
707, (1999), hereafter TD) models a semi-circular, 3D current-carrying
flux rope by means of a current ring embedded in a potential field. The
parameters of the TD model can be adjusted to create both stable and
kink- and torus-unstable configurations. Its solar relevance
was confirmed by the quantitative reproduction of some specific CME
features (see e.g., Toeroek and Kliem, Astroph. J. Lett. 630 L97
(2005)). Therefore, the TD solution is by far the most realistic
analytical equilibrium available to date for the modeling of solar
active regions. Employing the TD equilibrium as a test-field,
we show that the magnetofrictional extrapolation code can reproduce
the energy and the twist of the magnetic field within a percent
accuracy. This information is essential for the reconstruction
of coronal fields involved in eruptions because the twist is,
together with the height profile of the overlying potential field,
the most important stability parameter -- at least as long as the
TD equilibrium is a good model of the considered active region. Perfectly reproduced are also X-type magnetic topology features,
sometimes referred to as Hyperbolic Flux Tubes, which are regarded to be
essential to the physics of CMEs and flares because they are preferred
locations for the formation of current sheets. On the other hand,
we also show how the scale-height of the potential field that is used
as initial condition in the extrapolation influences the quality of the
reconstructed field: different initial conditions reproduce correctly
the twist and the topology, but less accurately the height and the
shape of the flux rope. Consequently, care must be taken when
comparing the shapes of soft X-ray and EUV loops, especially those in
the nearly potential field overlying filaments, with the field lines
obtained from the extrapolation of the corresponding magnetogram.

---------------------------------------------------------
Title: 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.
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.
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: Nonlinear Force-Free Modeling of Coronal Magnetic
Fields. II. Modeling a Filament Arcade and Simulated Chromospheric
and Photospheric Vector Fields
Authors: Metcalf, Thomas R.; De Rosa, Marc L.; Schrijver, Carolus J.;
Barnes, Graham; van Ballegooijen, Adriaan A.; Wiegelmann, Thomas;
Wheatland, Michael S.; Valori, Gherardo; McTtiernan, James M.
2008SoPh..247..269M Altcode: 2008SoPh..tmp...17M
We compare a variety of nonlinear force-free field (NLFFF) extrapolation
algorithms, including optimization, magneto-frictional, and Grad -
Rubin-like codes, applied to a solar-like reference model. The model
used to test the algorithms includes realistic photospheric Lorentz
forces and a complex field including a weakly twisted, right helical
flux bundle. The codes were applied to both forced "photospheric" and
more force-free "chromospheric" vector magnetic field boundary data
derived from the model. When applied to the chromospheric boundary data,
the codes are able to recover the presence of the flux bundle and the
field's free energy, though some details of the field connectivity are
lost. When the codes are applied to the forced photospheric boundary
data, the reference model field is not well recovered, indicating
that the combination of Lorentz forces and small spatial scale
structure at the photosphere severely impact the extrapolation of the
field. Preprocessing of the forced photospheric boundary does improve
the extrapolations considerably for the layers above the chromosphere,
but the extrapolations are sensitive to the details of the numerical
codes and neither the field connectivity nor the free magnetic energy in
the full volume are well recovered. The magnetic virial theorem gives
a rapid measure of the total magnetic energy without extrapolation
though, like the NLFFF codes, it is sensitive to the Lorentz forces in
the coronal volume. Both the magnetic virial theorem and the Wiegelmann
extrapolation, when applied to the preprocessed photospheric boundary,
give a magnetic energy which is nearly equivalent to the value derived
from the chromospheric boundary, but both underestimate the free
energy above the photosphere by at least a factor of two. We discuss
the interpretation of the preprocessed field in this context. When
applying the NLFFF codes to solar data, the problems associated with
Lorentz forces present in the low solar atmosphere must be recognized:
the various codes will not necessarily converge to the correct, or
even the same, solution.

---------------------------------------------------------
Title: Force-free magnetic fields in the solar atmosphere
Authors: Seehafer, N.; Fuhrmann, M.; Valori, G.; Kliem, B.
2007AN....328.1166S Altcode:
Reliable measurements of the solar magnetic field are restricted to the
level of the photosphere. For about half a century attempts have been
made to calculate the field in the layers above the photosphere, i.e. in
the chromosphere and in the corona, from the measured photospheric
field. The procedure is known as magnetic field extrapolation. In
the superphotospheric parts of active regions the magnetic field is
approximately force-free, i.e. electric currents are aligned with the
magnetic field. The practical application to solar active regions has
been largely confined to constant-α or linear force-free fields, with
a spatially constant ratio, α, between the electric current and the
magnetic field. We review results obtained from extrapolations with
constant-α force-free fields, in particular on magnetic topologies
favourable for flares and on magnetic and current helicities. Presently,
different methods are being developed to calculate non-constant-α or
nonlinear force-free fields from photospheric vector magnetograms. We
also briefly discuss these methods and present a comparison of a linear
and a nonlinear force-free magnetic field extrapolation applied to
the same photospheric boundary data.

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Title: Preprocessing of solar vector magnetograms for force-free
magnetic field extrapolation
Authors: Fuhrmann, M.; Seehafer, N.; Valori, G.
2007A&A...476..349F Altcode:
Context: Reliable measurements of the solar magnetic field are
restricted to the phoptosphere. As an alternative to measurements,
the field in the higher layers of the atmosphere is calculated from
the measured photospheric field, mostly under the assumption that
it is force-free. However, the magnetic field in the photosphere
is not force-free. Moreover, most methods for the extrapolation of
the photospheric magnetic field into the higher layers prescribe the
magnetic vector on the whole boundary of the considered volume, which
overdetermines the force-free field. Finally, the extrapolation methods
are very sensitive to small-scale noise in the magnetograph data,
which, however, if sufficienly resolved numerically, should affect the
solution only in a thin boundary layer close to the photosphere. Aims: A new method for the preprocessing of solar photospheric vector
magnetograms has been developed that, by improving their compatibility
with the condition of force-freeness and removing small-scale noise,
makes them more suitable for extrapolations into three-dimensional
nonlinear force-free magnetic fields in the chromosphere and corona. Methods: A functional of the photospheric field values is minimized
whereby the total magnetic force and the total magnetic torque on the
considered volume above the photosphere, as well as a quantity measuring
the degree of small-scale noise in the photospheric boundary data,
are simultaneously made small. For the minimization, the method of
simulated annealing is used and the smoothing of noisy magnetograph
data is attained by windowed median averaging. Results: The
method was applied to a magnetogram derived from a known nonlinear
force-free test field to which an artificial noise had been added. The
algorithm recovered all main structures of the magnetogram and removed
small-scale noise. The main test was to extrapolate from the noisy
photospheric vector magnetogram before and after the preprocessing. The
preprocessing was found to significantly improve the agreement of the
extrapolated with the exact field.

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Title: Magnetofrictional Extrapolations of Low and Lou's Force-Free
Equilibria
Authors: Valori, G.; Kliem, B.; Fuhrmann, M.
2007SoPh..245..263V Altcode:
We present a careful investigation of the magnetofrictional relaxation
and extrapolation technique applied to the reconstruction of two test
fields. These fields are taken from the family of nonlinear force-free
magnetic equilibria constructed by Low and Lou (Astrophys. J.352,
343, 1990), which have emerged as standard tests for extrapolation
techniques in recent years. For the practically relevant case that
only the field values in the bottom plane of the considered volume
(vector magnetogram) are used as input information (i.e., not including
the knowledge about the test field at the side and top boundaries),
the test field is reconstructed to a higher accuracy than obtained
previously. Detailed diagnostics of the reconstruction accuracy
show that the implementation of fourth-order spatial discretization
was essential to reach this accuracy for the given test fields
and to achieve near machine precision in satisfying the solenoidal
condition. Different variants of boundary conditions are tested,
which all yield comparable accuracy. In its present implementation,
the technique yields a scaling of computing time with total number of
grid points only slightly below N5/3, which is too steep
for applications to large (≥10242) magnetograms, except
on supercomputers. Directions for improvement are outlined.

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Title: Non-linear Force-free Modeling Of Coronal Magnetic Fields
Authors: Metcalf, Thomas R.; De Rosa, M. L.; Schrijver, C. J.; Barnes,
G.; van Ballegooijen, A.; Wiegelmann, T.; Wheatland, M. S.; Valori,
G.; McTiernan, J. M.
2007AAS...210.9102M Altcode: 2007BAAS...39..204M
We compare a variety of nonlinear force-free field (NLFFF)
extrapolation algorithms, including optimization, magneto-frictional,
and Grad-Rubin-like codes, applied to a solar-like reference
model. The model used to test the algorithms includes realistic
photospheric Lorentz forces and a complex field including a weakly
twisted, right helical flux bundle. The codes were applied to both
forced "photospheric” and more force-free "chromospheric” vector
magnetic field boundary data derived from the model. When applied to
the chromospheric boundary data, the codes are able to recover
the presence of the flux bundle and the field's free energy, though
some details of the field connectivity are lost. When the codes are
applied to the forced photospheric boundary data, the reference
model field is not well recovered, indicating that the Lorentz
forces on the photosphere severely impact the extrapolation of the
field. Preprocessing of the photospheric boundary does improve the
extrapolations considerably, although the results depend sensitively
on the details of the numerical codes. When applying the NLFFF codes
to solar data, the problems associated with Lorentz forces present in
the low solar atmosphere must be recognized: the various codes will
not necessarily converge to the correct, or even the same, solution.

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Title: Testing non-linear force-free coronal magnetic field
extrapolations with the Titov-Démoulin equilibrium
Authors: Wiegelmann, T.; Inhester, B.; Kliem, B.; Valori, G.;
Neukirch, T.
2006A&A...453..737W Altcode: 2006astro.ph.12650W
Context.As the coronal magnetic field can usually not be measured
directly, it has to be extrapolated from photospheric measurements
into the corona. Aims.We test the quality of a non-linear
force-free coronal magnetic field extrapolation code with the help of
a known analytical solution. Methods. The non-linear force-free
equations are numerically solved with the help of an optimization
principle. The method minimizes an integral over the force-free
and solenoidal condition. As boundary condition we use either the
magnetic field components on all six sides of the computational box
in Case I or only on the bottom boundary in Case II. We check the
quality of the reconstruction by computing how well force-freeness
and divergence-freeness are fulfilled and by comparing the numerical
solution with the analytical solution. The comparison is done with
magnetic field line plots and several quantitative measures, like the
vector correlation, Cauchy Schwarz, normalized vector error, mean vector
error and magnetic energy. Results.For Case I the reconstructed
magnetic field shows good agreement with the original magnetic field
topology, whereas in Case II there are considerable deviations from
the exact solution. This is corroborated by the quantitative measures,
which are significantly better for Case I. Conclusions. Despite
the strong nonlinearity of the considered force-free equilibrium, the
optimization method of extrapolation is able to reconstruct it; however,
the quality of reconstruction depends significantly on the consistency
of the input data, which is given only if the known solution is provided
also at the lateral and top boundaries, and on the presence or absence
of flux concentrations near the boundaries of the magnetogram.

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Title: Nonlinear Force-Free Modeling of Coronal Magnetic Fields Part
I: A Quantitative Comparison of Methods
Authors: Schrijver, Carolus J.; De Rosa, Marc L.; Metcalf, Thomas R.;
Liu, Yang; McTiernan, Jim; Régnier, Stéphane; Valori, Gherardo;
Wheatland, Michael S.; Wiegelmann, Thomas
2006SoPh..235..161S Altcode:
We compare six algorithms for the computation of nonlinear force-free
(NLFF) magnetic fields (including optimization, magnetofrictional,
Grad-Rubin based, and Green's function-based methods) by evaluating
their performance in blind tests on analytical force-free-field models
for which boundary conditions are specified either for the entire
surface area of a cubic volume or for an extended lower boundary
only. Figures of merit are used to compare the input vector field to
the resulting model fields. Based on these merit functions, we argue
that all algorithms yield NLFF fields that agree best with the input
field in the lower central region of the volume, where the field and
electrical currents are strongest and the effects of boundary conditions
weakest. The NLFF vector fields in the outer domains of the volume
depend sensitively on the details of the specified boundary conditions;
best agreement is found if the field outside of the model volume is
incorporated as part of the model boundary, either as potential field
boundaries on the side and top surfaces, or as a potential field in
a skirt around the main volume of interest. For input field (B) and
modeled field (b), the best method included in our study yields an
average relative vector error En = « |B−b|»/« |B|» of
only 0.02 when all sides are specified and 0.14 for the case where only
the lower boundary is specified, while the total energy in the magnetic
field is approximated to within 2%. The models converge towards the
central, strong input field at speeds that differ by a factor of one
million per iteration step. The fastest-converging, best-performing
model for these analytical test cases is the Wheatland, Sturrock, and
Roumeliotis (2000) optimization algorithm as implemented by Wiegelmann
(2004).

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Title: Extrapolation of a nonlinear force-free field containing a
highly twisted magnetic loop
Authors: Valori, G.; Kliem, B.; Keppens, R.
2005A&A...433..335V Altcode:
The stress-and-relax method for the extrapolation of nonlinear
force-free coronal magnetic fields from photospheric vector
magnetograms is formulated and implemented in a manner analogous to
the evolutionary extrapolation method. The technique is applied to a
numerically constructed force-free equilibrium that has a simple bipolar
structure of the normal field component in the bottom (magnetogram)
plane but contains a highly twisted loop and a shear (current) layer,
with a smooth but strong variation of the force-free parameter α in
the magnetogram. A standard linear force-free extrapolation of this
magnetogram, using the so-called α_best value, is found to fail
in reproducing the twisted loop (or flux rope) and the shear layer;
it yields a loop pair instead and the shear is not concentrated in a
layer. With the nonlinear extrapolation technique, the given equilibrium
is readily reconstructed to a high degree of accuracy if the magnetogram
is sufficiently resolved. A parametric study quantifies the requirements
on the resolution for a successful nonlinear extrapolation. Permitting
magnetic reconnection by a controlled use of resistivity improved the
extrapolation at a resolution comparable to the smallest structures
in the magnetogram.

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Title: Fluid and kinetic aspects of collisionless magnetic
reconnection
Authors: Valori, Gherardo
2001PhDT........99V Altcode:
No abstract at ADS