mackay

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Author name code: mackay
ADS astronomy entries on 2022-09-14
author:"Mackay, Duncan H."

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Title: Mini-filament eruption, QSL reconnection, and
reconnection-driven outflows: IRIS and AIA/HMI/SDO observations
and modelling
Authors: Madjarska, Maria S.; Mackay, Duncan H.; Galsgaard, Klaus;
Xie, Haixia; Wiegelmann, Thomas
2022cosp...44.2533M Altcode:
We will present unique observations of a mini-filament eruption
associated with cancelling magnetic fluxes of a small-scale loop system
known as a coronal bright point. The event is uniquely recorded in
both the imaging and spectroscopic data taken with the Interface
Region Imaging Spectrograph (IRIS). The study aims at providing
a better understanding of the physical processes driving these
ubiquitous small-scale eruptions. We also analysed images taken in the
extreme-ultraviolet channels of the Atmospheric Imaging Assembly (AIA)
and line-of-sight magnetic-field data from the Helioseismic Magnetic
Imager (HMI) onboard the Solar Dynamics Observatory. As the observations
can only give an inkling about the possible physical processes at play,
we also employed a non-linear force-free field (NLFFF) relaxation
approach based on the HMI magnetogram time series. Furthermore, we
computed the squashing factor, Q, in different horizontal planes of
the NLFFF model. This allowed us to further investigate the evolution
of the magnetic-field structures involved in the eruption process.

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Title: Eruptions from coronal bright points: A spectroscopic
view by IRIS of a mini-filament eruption, QSL reconnection, and
reconnection-driven outflows
Authors: Madjarska, Maria S.; Mackay, Duncan H.; Galsgaard, Klaus;
Wiegelmann, Thomas; Xie, Haixia
2022A&A...660A..45M Altcode: 2022arXiv220200370M
Context. Our study investigates a mini-filament eruption associated with
cancelling magnetic fluxes. The eruption originates from a small-scale
loop complex commonly known as a coronal bright point (CBP). The
event is uniquely recorded in both the imaging and spectroscopic data
taken with the Interface Region Imaging Spectrograph (IRIS). 
Aims: The investigation aims to gain a better understanding of the
physical processes driving these ubiquitous small-scale eruptions. Methods: We analysed IRIS spectroscopic and slit-jaw imaging
observations as well as images taken in the extreme-ultraviolet
channels of the Atmospheric Imaging Assembly (AIA) and line-of-sight
magnetic-field data from the Helioseismic Magnetic Imager (HMI) on
board the Solar Dynamics Observatory. As the observations can only
indicate the possible physical processes at play, we also employed a
non-linear force-free field (NLFFF) relaxation approach based on the
HMI magnetogram time series. This allowed us to further investigate
the evolution of the magnetic-field structures involved in the eruption
process. Results: We identified a strong small-scale brightening
as a micro-flare in a CBP, recorded in emission from chromospheric to
flaring plasmas. The mini-eruption is manifested via the ejection of hot
(CBP loops) and cool (mini-filament) plasma recorded in both the imaging
and spectroscopic data. The micro-flare is preceded by the appearance
of an elongated bright feature in the IRIS slit-jaw 1400 Å images,
located above the polarity inversion line. The micro-flare starts
with an IRIS pixel size brightening and propagates bi-directionally
along the elongated feature. We detected, in both the spectral and
imaging IRIS data and AIA data, strong flows along and at the edges of
the elongated feature; we believe that these represent reconnection
outflows. Both edges of the elongated feature that wrap around the
edges of the erupting MF evolve into a J-type shape, creating a
sigmoid appearance. A quasi-separatrix layer (QSL) is identified in
the vicinity of the polarity inversion line by computing the squashing
factor, Q, in different horizontal planes of the NLFFF model. 
Conclusions: This CBP spectro-imaging study provides further evidence
that CBPs represent downscaled active regions and, as such, they may
make a significant contribution to the mass and energy balance of
the solar atmosphere. They are the sources of all range of typical
active-region features, including magnetic reconnection along QSLs,
(mini-)filament eruptions, (micro-)flaring, reconnection outflows,
etc. The QSL reconnection site has the same spectral appearance
as the so-called explosive events identified by strong blue- and
red-shifted emission, thus providing an answer to an outstanding
question regarding the true nature of this spectral phenomenon. Movies associated to Figs. A.1 and A.2 are available at <A
href="https://www.aanda.org/10.1051/0004-6361/202142439/olm">https://www.aanda.org</A>

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Title: A Comparison of Sparse and Non-sparse Techniques for
Electric-Field Inversion from Normal-Component Magnetograms
Authors: Mackay, Duncan H.; Yeates, Anthony R.
2021SoPh..296..178M Altcode:
An important element of 3D data-driven simulations of solar magnetic
fields is the determination of the horizontal electric field at
the solar photosphere. This electric field is used to drive the 3D
simulations and inject energy and helicity into the solar corona. One
outstanding problem is the localisation of the horizontal electric field
such that it is consistent with Ohm's law. Yeates (Astrophys. J.836(1),
131, 2017) put forward a new "sparse" technique for computing the
horizontal electric field from normal-component magnetograms that
minimises the number of non-zero values. This aims to produce a better
representation of Ohm's law compared to previously used "non-sparse"
techniques. To test this new approach we apply it to active region
(AR) 10977, along with the previously developed non-sparse technique of
Mackay, Green, and van Ballegooijen (Astrophys. J.729(2), 97, 2011). A
detailed comparison of the two techniques with coronal observations
is used to determine which is the most successful. Results show that
the non-sparse technique of Mackay, Green, and van Ballegooijen (2011)
produces the best representation for the formation and structure of the
sigmoid above AR 10977. In contrast, the Yeates (2017) approach injects
strong horizontal fields between spatially separated, evolving magnetic
polarities. This injection produces highly twisted unphysical field
lines with significantly higher magnetic energy and helicity. It is also
demonstrated that the Yeates (2017) approach produces significantly
different results that can be inconsistent with the observations
depending on whether the horizontal electric field is solved directly
or indirectly through the magnetic vector potential. In contrast, the
Mackay, Green, and van Ballegooijen (2011) method produces consistent
results using either approach. The sparse technique of Yeates (2017)
has significant pitfalls when applied to spatially resolved solar data,
where future studies need to investigate why these problems arise.

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Title: Determining the source and eruption dynamics of a stealth
CME using NLFFF modelling and MHD simulations
Authors: Yardley, S. L.; Pagano, P.; Mackay, D. H.; Upton, L. A.
2021A&A...652A.160Y Altcode: 2021arXiv210614800Y
Context. Coronal mass ejections (CMEs) that exhibit weak or no eruption
signatures in the low corona, known as stealth CMEs, are problematic
as upon arrival at Earth they can lead to geomagnetic disturbances
that were not predicted by space weather forecasters. Aims: We
investigate the origin and eruption of a stealth event that occurred
on 2015 January 3 that was responsible for a strong geomagnetic storm
upon its arrival at Earth. Methods: To simulate the coronal
magnetic field and plasma parameters of the eruption we use a coupled
approach. This approach combines an evolutionary nonlinear force-free
field model of the global corona with a MHD simulation. Results:
The combined simulation approach accurately reproduces the stealth
event and suggests that sympathetic eruptions occur. In the combined
simulation we found that three flux ropes form and then erupt. The
first two flux ropes, which are connected to a large AR complex
behind the east limb, erupt first producing two near-simultaneous
CMEs. These CMEs are closely followed by a third, weaker flux rope
eruption in the simulation that originated between the periphery
of AR 12252 and the southern polar coronal hole. The third eruption
coincides with a faint coronal dimming, which appears in the SDO/AIA
211 Å observations, that is attributed as the source responsible
for the stealth event and later the geomagnetic disturbance at 1
AU. The incorrect interpretation of the stealth event being linked
to the occurrence of a single partial halo CME observed by LASCO/C2
is mainly due to the lack of STEREO observations being available at
the time of the CMEs. The simulation also shows that the LASCO CME
is not a single event but rather two near-simultaneous CMEs. 
Conclusions: These results show the significance of the coupled
data-driven simulation approach in interpreting the eruption
and that an operational L5 mission is crucial for space weather
forecasting. Movie associated with Fig. 4 is available at <A
href="https://www.aanda.org/10.1051/0004-6361/202141142/olm">https://www.aanda.org</A>

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Title: Identifying Non-potential Energy Hot Spots In A Global
Coronal Simulation
Authors: Corchado Albelo, M. F.; Gibson, S. E.; Linker, J.; Mackay,
D. H.; Dalmasse, K.; Malanushenko, A.
2021AAS...23832803C Altcode:
Observing the global coronal magnetic field remains a difficult task;
limiting our understanding of the evolution of global phenomena in these
external layers of the solar atmosphere. Therefore, we rely on models to
get the solar exterior global field. While models can extrapolate the
magnetic field from surface flux and vector magnetogram observations,
e.g. by assuming a current-free corona, other techniques are used
to simulate the current-carrying field via magnetohydrodynamic (MHD)
evolution or surface flux transport of large scale field, and inserting
current-carrying small scale field structures like twisted flux ropes
into the corona. These current-carrying fields are of interest for
studying solar energetic eruptions like coronal mass ejections and
flares because they provide the energy reservoir needed to drive these
events. Previous studies suggest that ground-based infrared polarimetric
measurements of Fe XIII (1074.7 nm) line correlate with the energy
of the current-carrying field. In this study we generated synthetic
polarimetric observations from a fully-resolved magnetohydrodynamics
model of the August 21, 2017 eclipse. The synthetic observations
were used as input to a diagnostic we developed to identify regions
where the modeling team inserted twisted flux ropes. The diagnostic
evaluated linearly and circularly polarized synthetic observations
of the corona as a means to identify the current-carrying magnetic
energy density. We found that the diagnostic does identify the
distribution of flux ropes in the corona. Thus, our findings motivate
the implementation of polarimetric measurements to identify "hot spots"
in which we can insert flux ropes and a degree of the twist/shear in
the current-carrying field.

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Title: Finding a second Sun - How can we detect solar-like magnetic
cycles with Zeeman-Doppler-Imaging (ZDI)?
Authors: Lehmann, Lisa Theres; Hussain, Gaitee A. J.; Vidotto, Aline
A.; Jardine, Moira M.; Mackay, Duncan H.
2021csss.confE..68L Altcode:
Spectropolarimetric surveys have now been running for long enough
to reveal solar-like magnetic activity cycles, e.g., for 61 Cyg A
(Boko Saikia et al. 2018). Our work examines if a solar-like cycle
can be observed with ZDI, given that this technique only detects the
large-scale field for very slowly-rotating and low-activity stars
like our Sun. We aim to determine the best strategy to detect stellar
cycles, and which parameters are most sensitive to cycle changes.This
poster presents our paper: Lehmann, L. T., et al., 2021, MNRAS, 500,
1243 (https://ui.adsabs.harvard.edu/abs/2021MNRAS.500.1243L)

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Title: Identifying solar-like magnetic cycles with
Zeeman-Doppler-Imaging
Authors: Lehmann, L. T.; Hussain, G. A. J.; Vidotto, A. A.; Jardine,
M. M.; Mackay, D. H.
2021MNRAS.500.1243L Altcode: 2020MNRAS.500.1243L; 2020arXiv201010214L; 2020MNRAS.tmp.3102L
We are reaching the point where spectropolarimetric surveys have run
for long enough to reveal solar-like magnetic activity cycles. In this
paper, we investigate what would be the best strategy to identify
solar-like magnetic cycles and ask which large-scale magnetic field
parameters best follow a solar-type magnetic cycle and are observable
with the Zeeman-Doppler-Imaging (ZDI) technique. We approach these
questions using the 3D non-potential flux transport simulations
of Yeates &amp; Mackay (2012) modelling the solar vector magnetic
field over 15 yr (centred on solar cycle 23). The flux emergence
profile was extracted from solar synoptic maps and used as input for a
photospheric flux transport model in combination with a non-potential
coronal evolution model. We synthesize spectropolarimetric data from
the simulated maps and reconstruct them using ZDI. The ZDI observed
solar cycle is set into the context of other cool star observations
and we present observable trends of the magnetic field topology with
time, sunspot number, and S-index. We find that the axisymmetric energy
fraction is the best parameter of the ZDI detectable large-scale field
to trace solar-like cycles. Neither the surface averaged large-scale
field or the total magnetic energy is appropriate. ZDI seems also to be
able to recover the increase of the toroidal energy with S-index. We
see further that ZDI might unveil hints of the dynamo modes that
are operating and of the global properties of the small-scale flux
emergence like active latitudes.

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Title: Simulating the Coronal Evolution of Bipolar Active Regions
to Investigate the Formation of Flux Ropes
Authors: Yardley, S. L.; Mackay, D. H.; Green, L. M.
2021SoPh..296...10Y Altcode: 2020arXiv201207708Y
The coronal magnetic field evolution of 20 bipolar active regions (ARs)
is simulated from their emergence to decay using the time-dependent
nonlinear force-free field method of Mackay, Green, and van Ballegooijen
(Astrophys. J. 729, 97, 2011). A time sequence of cleaned photospheric
line-of-sight magnetograms, which covers the entire evolution of each
AR, is used to drive the simulation. A comparison of the simulated
coronal magnetic field with the 171 and 193 Å observations obtained
by the Solar Dynamics Observatory (SDO)/Atmospheric Imaging Assembly
(AIA), is made for each AR by manual inspection. The results show
that it is possible to reproduce the evolution of the main coronal
features such as small- and large-scale coronal loops, filaments and
sheared structures for 80% of the ARs. Varying the boundary and initial
conditions, along with the addition of physical effects such as Ohmic
diffusion, hyperdiffusion and a horizontal magnetic field injection
at the photosphere, improves the match between the observations and
simulated coronal evolution by 20%. The simulations were able to
reproduce the build-up to eruption for 50% of the observed eruptions
associated with the ARs. The mean unsigned time difference between the
eruptions occurring in the observations compared to the time of eruption
onset in the simulations was found to be ≈5 hrs. The simulations were
particularly successful in capturing the build-up to eruption for all
four eruptions that originated from the internal polarity inversion line
of the ARs. The technique was less successful in reproducing the onset
of eruptions that originated from the periphery of ARs and large-scale
coronal structures. For these cases global, rather than local, nonlinear
force-free field models must be used. While the technique has shown
some success, eruptions that occur in quick succession are difficult
to reproduce by this method and future iterations of the model need
to address this.

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Title: Links between prominence/filament magnetic field and plasma:
What can 3D WPFS models teach us?
Authors: Gunár, Stanislav; Schmieder, Brigitte; Aulanier, Guillaume;
Heinzel, Petr; Mackay, Duncan; Dudik, Jaroslav
2021cosp...43E1769G Altcode:
The magnetic field constitutes the skeleton and the driving force of
prominences/filaments. It supports the dense prominence plasma against
gravity and insulates it from the hot, coronal environment. The
magnetic field is also responsible for the prominence stability,
evolution and eruptions which affect the heliosphere and ultimately
the Earth. However, a strong imbalance exists between the numerous
efforts in detailed modelling of prominence magnetic field and its
understanding from observations. That is due to the complex nature of
the direct (and indirect) observations of solar magnetic fields which
are challenging at the best of times and even more so in prominences
or filaments. The direct observations of the prominence magnetic
field require high-precision spectro-polarimetric measurements and
realistic assumptions about the plasma structure which allow us to
infer the field configuration from its effect on the polarized light
emergent from the observed structures. The indirect observations rely
on the perceived location, shape and dynamics of the prominence or
filament plasma, often using moving small-scale plasma structures
as tracers guided by the field lines. Both methods thus rely on the
presence of observable plasma in the magnetic field configuration,
and on the radiation which carries the information about the in-situ
conditions to the observer. No prominence/filament magnetic field
measurements are made without these two additional components,
which are sometimes an afterthought in the magnetic field models. We
have developed 3D Whole-Prominence Fine Structure (WPFS) models to
illuminate the links between the prominence magnetic field, its plasma
distributed among numerous fine structures and the radiation which
carries the information about the prominence physical conditions to
the observer. What can we learn from these 3D models? For example,
we can see that a small change of the magnetic field configuration
can have a large effect on the perceived structure of prominences and
filaments visible in the H-alpha line. Consequently, this means that
significant changes observed in prominences or filaments do not need
to suggest that equally large changes in the underlying magnetic field
configuration had to occur. In another example, we see that seemingly
incomparable differences in the morphological look of prominences (long
horizontal fine structures versus small blobs of plasma arranged into
more-less vertical features) may not need to imply the existence of
radically different magnetic field configurations. Rather, they might
simply be manifestations of projection effects that can differ greatly
depending on the viewing angle under which we observe the naturally
three-dimensional prominences/filaments.

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Title: Eruptions from coronal hole bright points: Observations and
non-potential modelling
Authors: Madjarska, Maria S.; Galsgaard, Klaus; Mackay, Duncan H.;
Koleva, Kostadinka; Dechev, Momchil
2020A&A...643A..19M Altcode: 2020arXiv200904628M
Context. We report on the third part of a series of studies on eruptions
associated with small-scale loop complexes named coronal bright points
(CBPs). Aims: A single case study of a CBP in an equatorial
coronal hole with an exceptionally large size is investigated to
expand on our understanding of the formation of mini-filaments, their
destabilisation, and the origin of the eruption triggering the formation
of jet-like features recorded in extreme ultraviolet (EUV) and X-ray
emission. We aim to explore the nature of the so-called micro-flares
in CBPs associated with jets in coronal holes and mini coronal mass
ejections in the quiet Sun. Methods: Co-observations from the
Atmospheric Imaging Assembly (AIA) and Helioseismic Magnetic Imager
(HMI) on board the Solar Dynamics Observatory as well as GONG Hα
images are used together with a non-linear force free field (NLFFF)
relaxation approach, where the latter is based on a time series of
HMI line-of-sight magnetograms. Results: A mini-filament (MF)
that formed beneath the CBP arcade about 3-4 h before the eruption is
seen in the Hα and EUV AIA images to lift up and erupt triggering the
formation of an X-ray jet. No significant photospheric magnetic flux
concentration displacement (convergence) is observed and neither is
magnetic flux cancellation between the two main magnetic polarities
forming the CBP in the time period leading to MF lift-off. The
CBP micro-flare is associated with three flare kernels that formed
shortly after the MF lift-off. No observational signature is found
for magnetic reconnection beneath the erupting MF. The applied NLFFF
modelling successfully reproduces both the CBP loop complex as well
as the magnetic flux rope that hosts the MF during the build-up to
the eruption. Conclusions: The applied NLFFF modelling is
able to clearly show that an initial potential field can be evolved
into a non-potential magnetic field configuration that contains
free magnetic energy in the region that observationally hosts the
eruption. The comparison of the magnetic field structure shows that the
magnetic NLFFF model contains many of the features that can explain
the different observational signatures found in the evolution and
eruption of the CBP. In the future, it may eventually indicate the
location of destabilisation that results in the eruptions of flux
ropes. Movies associated to Figs. 9 and B.2 are available at <A
href="https://www.aanda.org/10.1051/0004-6361/202038287/olm">https://www.aanda.org</A>

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Title: Investigation of the Middle Corona with SWAP and a Data-Driven
Non-Potential Coronal Magnetic Field Model
Authors: Meyer, Karen A.; Mackay, Duncan H.; Talpeanu, Dana-Camelia;
Upton, Lisa A.; West, Matthew J.
2020SoPh..295..101M Altcode: 2020arXiv200702668M
The large field-of-view of the Sun Watcher using Active Pixel System
detector and Image Processing (SWAP) instrument onboard the PRoject for
Onboard Autonomy 2 (PROBA2) spacecraft provides a unique opportunity
to study extended coronal structures observed in the EUV in conjunction
with global coronal magnetic field simulations. A global non-potential
magnetic field model is used to simulate the evolution of the global
corona from 1 September 2014 to 31 March 2015, driven by newly emerging
bipolar active regions determined from Helioseismic and Magnetic
Imager (HMI) magnetograms. We compare the large-scale structure of
the simulated magnetic field with structures seen off-limb in SWAP EUV
observations. In particular, we investigate how successful the model
is in reproducing regions of closed and open structures, the scale of
structures, and compare the evolution of a coronal fan observed over
several rotations. The model is found to accurately reproduce observed
large-scale, off-limb structures. When discrepancies do arise they
mainly occur off the east solar limb due to active regions emerging on
the far side of the Sun, which cannot be incorporated into the model
until they are observed on the Earth-facing side. When such "late"
active region emergences are incorporated into the model, we find that
the simulated corona self-corrects within a few days, so that simulated
structures off the west limb more closely match what is observed. Where
the model is less successful, we consider how this may be addressed,
through model developments or additional observational products.

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Title: Hydrogen non-equilibrium ionisation effects in coronal mass
ejections
Authors: Pagano, P.; Bemporad, A.; Mackay, D. H.
2020A&A...637A..49P Altcode: 2020arXiv200312337P
Context. A new generation of coronagraphs used to study solar wind and
coronal mass ejections (CMEs) are being developed and launched. These
coronagraphs will heavily rely on multi-channel observations where
visible light (VL) and UV-EUV (ultraviolet-extreme ultraviolet)
observations provide new plasma diagnostics. One of these instruments,
Metis on board ESA-Solar Orbiter, will simultaneously observe VL and
the UV Lyman-α line. The number of neutral hydrogen atoms (a small
fraction of coronal protons) is a key parameter for deriving plasma
properties, such as the temperature from the observed Lyman-α line
intensity. However, these measurements are significantly affected
if non-equilibrium ionisation effects occur, which can be relevant
during CMEs. Aims: The aim of this work is to determine if
non-equilibrium ionisation effects are relevant in CMEs and, in
particular, when and in which regions of the CME plasma ionisation
equilibrium can be assumed for data analysis. Methods:
We used a magneto-hydrodynamic (MHD) simulation of a magnetic flux
rope ejection to generate a CME. From this, we then reconstructed the
ionisation state of hydrogen atoms in the CME by evaluating both the
advection of neutral and ionised hydrogen atoms and the ionisation
and recombination rates in the MHD simulation. Results: We
find that the equilibrium ionisation assumption mostly holds in the
core of the CME, which is represented by a magnetic flux rope. In
contrast, non-equilibrium ionisation effects are significant at the
CME front, where we find about 100 times more neutral hydrogen atoms
than prescribed by ionisation equilibrium conditions. We find this to
be the case even if this neutral hydrogen excess might be difficult
to identify due to projection effects. Conclusions: This work
provides key information for the development of a new generation of
diagnostic techniques that aim to combine visible light and Lyman-α
line emissions. The results show that non-equilibrium ionisation effects
need to be considered when we analyse CME fronts. Incorrectly assuming
equilibrium ionisation in these regions would lead to a systematic
underestimate of plasma temperatures.

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Title: Modelling and observations: Comparison of the magnetic field
properties in a prominence
Authors: Mackay, D. H.; Schmieder, B.; López Ariste, A.; Su, Y.
2020A&A...637A...3M Altcode:
Context. Direct magnetic field measurements in solar prominences occur
infrequently and are difficult to make and interpret. As a consequence,
alternative methods are needed to derive the main properties of the
magnetic field that supports the prominence mass. This is important for
our understanding of solar prominences, but also for understanding how
eruptive prominences may affect space weather. Aims: We present
the first direct comparison of the magnetic field strength derived
from spectro-polarimetric observations of a solar prominence, with
corresponding results from a theoretical flux rope model constructed
from on-disc normal component magnetograms. Methods: We first
used spectro-polarimetric observations of a prominence obtained with
the magnetograph THEMIS operating in the Canary Islands to derive the
magnetic field of the observed prominence by inverting the Stokes
parameters measured in the He D3 line. Next, we constructed two
data-constrained non-linear force-free field (NLFFF) models of the
same prominence. In one model we assumed a strongly twisted flux rope
solution, and in the other a weakly twisted flux rope solution. 
Results: The physical extent of the prominence at the limb (height
and length) is best reproduced with the strongly twisted flux rope
solution. The line-of-sight average of the magnetic field for the
strongly twisted solution results in a magnetic field that has a
magnitude of within a factor of 1-2 of the observed magnetic field
strength. For the peak field strength along the line of sight,
an agreement to within 20% of the observations is obtained for
the strongly twisted solution. The weakly twisted solution produces
significantly lower magnetic field strengths and gives a poor agreement
with the observations. Conclusions: The results of this first
comparison are promising. We found that the flux rope insertion method
of producing a NLFFF is able to deduce the overall properties of the
magnetic field in an observed prominence.

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Title: Measuring stellar magnetic helicity density
Authors: Lund, K.; Jardine, M.; Lehmann, L. T.; Mackay, D. H.; See,
V.; Vidotto, A. A.; Donati, J. -F.; Fares, R.; Folsom, C. P.; Jeffers,
S. V.; Marsden, S. C.; Morin, J.; Petit, P.
2020MNRAS.493.1003L Altcode: 2020arXiv200111749L; 2020MNRAS.tmp..292L
Helicity is a fundamental property of a magnetic field but to date
it has only been possible to observe its evolution in one star -
the Sun. In this paper, we provide a simple technique for mapping
the large-scale helicity density across the surface of any star using
only observable quantities: the poloidal and toroidal magnetic field
components (which can be determined from Zeeman-Doppler imaging) and
the stellar radius. We use a sample of 51 stars across a mass range of
0.1-1.34 M⊙ to show how the helicity density relates to
stellar mass, Rossby number, magnetic energy, and age. We find that the
large-scale helicity density increases with decreasing Rossby number
Ro, peaking at Ro ≃ 0.1, with a saturation or
decrease below that. For both fully and partially convective stars,
we find that the mean absolute helicity density scales with the mean
squared toroidal magnetic flux density according to the power law: |&lt;
{h }&gt; | ∝ &lt; {{{B}_{tor}}^2_{} &gt; }^{0.86 ± 0.04}. The scatter
in this relation is consistent with the variation across a solar cycle,
which we compute using simulations and observations across solar cycles
23 and 24, respectively. We find a significant decrease in helicity
density with age.

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Title: A New Space Weather Tool for Identifying Eruptive Active
Regions
Authors: Pagano, Paolo; Mackay, Duncan H.; Yardley, Stephanie L.
2019ApJ...886...81P Altcode: 2019arXiv191004226P
One of the main goals of solar physics is the timely identification of
eruptive active regions. Space missions such as Solar Orbiter or future
space weather forecasting missions would largely benefit from this
achievement. Our aim is to produce a relatively simple technique that
can provide real-time indications or predictions that an active region
will produce an eruption. We expand on the theoretical work of Pagano
et al. that was able to distinguish eruptive from non-eruptive active
regions. From this, we introduce a new operational metric that uses
a combination of observed line-of-sight magnetograms, 3D data-driven
simulations, and the projection of the 3D simulations forward in
time. Results show that the new metric correctly distinguishes
active regions as eruptive when observable signatures of eruption
have been identified and as non-eruptive when there are no observable
signatures of eruption. After successfully distinguishing eruptive from
non-eruptive active regions we illustrate how this metric may be used
in a “real-time” operational sense were three levels of warning are
categorized. These categories are: high risk (red), medium risk (amber),
and low risk (green) of eruption. Through considering individual
cases, we find that the separation into eruptive and non-eruptive
active regions is more robust the longer the time series of observed
magnetograms used to simulate the build up of magnetic stress and
free magnetic energy within the active region. Finally, we conclude
that this proof of concept study delivers promising results where the
ability to categorize the risk of an eruption is a major achievement.

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Title: Understanding the Plasma and Magnetic Field Evolution of a
Filament Using Observations and Nonlinear Force-free Field Modeling
Authors: Yardley, Stephanie L.; Savcheva, Antonia; Green, Lucie M.;
van Driel-Gesztelyi, Lidia; Long, David; Williams, David R.; Mackay,
Duncan H.
2019ApJ...887..240Y Altcode: 2019arXiv191101314Y
We present observations and magnetic field models of an intermediate
filament present on the Sun in 2012 August, associated with a polarity
inversion line that extends from AR 11541 in the east into the quiet
Sun at its western end. A combination of Solar Dynamics Observatory
(SDO)/Atmospheric Imaging Assembly, SDO/Helioseismic and Magnetic
Imager (HMI), and Global Oscillation Network Group Hα data allow
us to analyze the structure and evolution of the filament from 2012
August 4 23:00 UT to 2012 August 6 08:00 UT when the filament was in
equilibrium. By applying the flux rope insertion method, nonlinear
force-free field models of the filament are constructed using SDO/HMI
line-of-sight magnetograms as the boundary condition at the two times
given above. Guided by observed filament barbs, both modeled flux ropes
are split into three sections each with a different value of axial flux
to represent the nonuniform photospheric field distribution. The flux
in the eastern section of the rope increases by 4 × 1020
Mx between the two models, which is in good agreement with the amount
of flux canceled along the internal PIL of AR 11541, calculated to be
3.2 × 1020 Mx. This suggests that flux cancellation builds
flux into the filament’s magnetic structure. Additionally, the number
of field line dips increases between the two models in the locations
where flux cancellation, the formation of new filament threads, and
growth of the filament is observed. This suggests that flux cancellation
associated with magnetic reconnection forms concave-up magnetic field
that lifts plasma into the filament. During this time, the free magnetic
energy in the models increases by 0.2 × 1031 ergs.

---------------------------------------------------------
Title: A Prospective New Diagnostic Technique for Distinguishing
Eruptive and Noneruptive Active Regions
Authors: Pagano, Paolo; Mackay, Duncan H.; Yardley, Stephanie L.
2019ApJ...883..112P Altcode: 2019arXiv190809223P
Active regions are the source of the majority of magnetic flux rope
ejections that become coronal mass ejections (CMEs). To identify in
advance which active regions will produce an ejection is key for both
space weather prediction tools and future science missions such as
Solar Orbiter. The aim of this study is to develop a new technique
to identify which active regions are more likely to generate magnetic
flux rope ejections. The new technique will aim to (i) produce timely
space weather warnings and (ii) open the way to a qualified selection of
observational targets for space-borne instruments. We use a data-driven
nonlinear force-free field (NLFFF) model to describe the 3D evolution
of the magnetic field of a set of active regions. We determine a metric
to distinguish eruptive from noneruptive active regions based on the
Lorentz force. Furthermore, using a subset of the observed magnetograms,
we run a series of simulations to test whether the time evolution
of the metric can be predicted. The identified metric successfully
differentiates active regions observed to produce eruptions from the
noneruptive ones in our data sample. A meaningful prediction of the
metric can be made between 6 and 16 hr in advance. This initial study
presents an interesting first step in the prediction of CME onset
using only line-of-sight magnetogram observations combined with NLFFF
modeling. Future studies will address how to generalize the model such
that it can be used in a more operational sense and for a variety of
simulation approaches.

---------------------------------------------------------
Title: Nonlinear Force-free Field Modeling of Solar Coronal Jets in
Theoretical Configurations
Authors: Meyer, K. A.; Savcheva, A. S.; Mackay, D. H.; DeLuca, E. E.
2019ApJ...880...62M Altcode:
Coronal jets occur frequently on the Sun, and may contribute
significantly to the solar wind. With the suite of instruments
available now, we can observe these phenomena in greater detail
than ever before. Modeling and simulations can assist further with
understanding the dynamic processes involved, but previous studies
tended to consider only one mechanism (e.g., emergence or rotation)
for the origin of the jet. In this study we model a series of idealized
archetypal jet configurations and follow the evolution of the coronal
magnetic field. This is a step toward understanding these idealized
situations before considering their observational counterparts. Several
simple situations are set up for the evolution of the photospheric
magnetic field: a single parasitic polarity rotating or moving in a
circular path; as well as opposite polarity pairs involved in flyby
(shearing), cancellation or emergence; all in the presence of a uniform,
open background magnetic field. The coronal magnetic field is evolved in
time using a magnetofrictional relaxation method. While magnetofriction
cannot accurately reproduce the dynamics of an eruptive phase, the
structure of the coronal magnetic field, as well as the buildup of
electric currents and free magnetic energy are instructive. Certain
configurations and motions produce a flux rope and allow the significant
buildup of free energy, reminiscent of the progenitors of so-called
blowout jets, whereas other, simpler configurations are more comparable
to the standard jet model. The next stage is a comparison with observed
coronal jet structures and their corresponding photospheric evolution.

---------------------------------------------------------
Title: 3D Whole-Prominence Fine Structure Model as a Test Case for
Verification and Development of Magnetic Field Inversion Techniques
Authors: Gunár, S.; Mackay, D. H.; Štěpán, J.; Heinzel, P.;
Trujillo Bueno, J.
2019ASPC..526..159G Altcode:
We show the potential of a new 3D whole-prominence fine structure
model to serve as a well-controlled yet complex environment for testing
inversion techniques for the magnetic field inference. The realistic
3D magnetic field and plasma environment provided by the model can
be used for the direct synthesis of spectro-polarimetric data. Such
synthetic data can be analyzed by advanced inversion tools and their
results compared with the known properties provided by the model.

---------------------------------------------------------
Title: Observing the simulations: applying ZDI to 3D non-potential
magnetic field simulations
Authors: Lehmann, L. T.; Hussain, G. A. J.; Jardine, M. M.; Mackay,
D. H.; Vidotto, A. A.
2019MNRAS.483.5246L Altcode: 2018arXiv181103703L; 2018MNRAS.tmp.3287L
The large-scale magnetic fields of stars can be obtained with the
Zeeman Doppler Imaging (ZDI) technique, but their interpretation is
still challenging as the contribution of the small-scale field or
the reliability of the reconstructed field properties is still not
fully understood. To quantify this, we use 3D non-potential magnetic
field simulations for slowly rotating solar-like stars as inputs to
test the capabilities of ZDI. These simulations are based on a flux
transport model connected to a non-potential coronal evolution model
using the observed solar flux emergence pattern. We first compare
four field prescriptions regarding their reconstruction capabilities
and investigate the influence of the spatial resolution of the input
maps on the corresponding circularly polarized profiles. We then
generate circularly polarized spectra based on our high-resolution
simulations of three stellar models with different activity levels,
and reconstruct their large-scale magnetic fields using a non-potential
ZDI code assuming two different stellar inclination angles. Our results
show that the ZDI technique reconstructs the main features of slowly
rotating solar-like stars but with ∼ one order of magnitude less
magnetic energy. The large-scale field morphologies are recovered up
to harmonic modes ℓ ∼ 5, especially after averaging over several
maps for each stellar model. While ZDI is not able to reproduce the
input magnetic energy distributions across individual harmonic modes,
the fractional energies across the modes are generally within 20 per
cent agreement. The fraction of axisymmetric and toroidal field tends
to be overestimated for stars with solar flux emergence patterns for
more pole-on inclination angles.

---------------------------------------------------------
Title: Eruptions from quiet Sun coronal bright
points. II. Non-potential modelling
Authors: Galsgaard, Klaus; Madjarska, Maria S.; Mackay, Duncan H.;
Mou, Chaozhou
2019A&A...623A..78G Altcode: 2019arXiv190109875G
Context. Our recent observational study shows that the majority
of coronal bright points (CBPs) in the quiet Sun are sources of
one or more eruptions during their lifetime. Aims: Here, we
investigate the non-potential time-dependent structure of the magnetic
field of the CBP regions with special emphasis on the time-evolving
magnetic structure at the spatial locations where the eruptions are
initiated. Methods: The magnetic structure is evolved in time
using a non-linear force-free field (NLFFF) relaxation approach based
on a time series of helioseismic and magnetic imager (HMI) longitudinal
magnetograms. This results in a continuous time series of NLFFFs. The
time series is initiated with a potential field extrapolation based
on a magnetogram taken well before the time of the eruptions. This
initial field is then evolved in time in response to the observed
changes in the magnetic field distribution at the photosphere. The
local and global magnetic field structures from the time series of NLFFF
field solutions are analysed in the vicinity of the eruption sites at
the approximate times of the eruptions. Results: The analysis
shows that many of the CBP eruptions reported in a recent publication
contain a twisted flux tube located at the sites of eruptions. The
presence of flux ropes at these locations provides in many cases a
direct link between the magnetic field structure, their eruption,
and the observation of mini coronal mass ejections (mini-CMEs). It is
found that all repetitive eruptions are homologous. Conclusions:
The NLFFF simulations show that twisted magnetic field structures are
created at the locations hosting eruptions in CBPs. These twisted
structures are produced by footpoint motions imposed by changes in
the photospheric magnetic field observations. The true nature of the
micro-flares remains unknown. Further 3D data-driven magnetohydrodynamic
modelling is required to show how these twisted regions become unstable
and erupt. Movies associated to Figs. 1-5 are available at <A
href="https://www.aanda.org/10.1051/0004-6361/201834329/olm">https://www.aanda.org</A>

---------------------------------------------------------
Title: Active Region evolution prior to magnetic flux rope ejections
Authors: Pagano, P.; Mackay, D. H.
2019NCimC..42...34P Altcode:
Magnetic flux rope ejections from the Sun are the main progenitors
of Coronal Mass Ejections and thus driver of the Space Weather. To
study and understand these phenomena is key to tackle the challenges
of Space Weather and to do so we need tools to identify where and when
magnetic flux ropes are ejected. We run a non-linear force-free field
magnetofrictional simulation of the active region AR11261 over the two
days prior to an observed magnetic flux rope ejection. We analyse the
distribution of three quantities from the numerical model at the time
of the flux rope ejections and we verify that in this application they
highlight the location where the flux rope ejection originates.

---------------------------------------------------------
Title: Magnetic Helicity Condensation and the Solar Cycle
Authors: Mackay, Duncan H.; DeVore, C. Richard; Antiochos, Spiro K.;
Yeates, Anthony R.
2018ApJ...869...62M Altcode:
Solar filaments exhibit a global chirality pattern where
dextral/sinistral filaments, corresponding to negative/positive magnetic
helicity, are dominant in the northern/southern hemisphere. This pattern
is opposite to the sign of magnetic helicity injected by differential
rotation along east-west oriented polarity inversion lines, posing
a major conundrum for solar physics. A resolution of this problem is
offered by the magnetic helicity-condensation model of Antiochos. To
investigate the global consequences of helicity condensation for the
hemispheric chirality pattern, we apply a temporally and spatially
averaged statistical approximation of helicity condensation. Realistic
magnetic field configurations in both the rising and declining phases of
the solar cycle are simulated. For the helicity-condensation process,
we assume convective cells consisting of positive/negative vorticities
in the northern/southern hemisphere that inject negative/positive
helicity. The magnitude of the vorticity is varied as a free parameter,
corresponding to different rates of helicity injection. To reproduce the
observed percentages of dominant and minority filament chiralities, we
find that a vorticity of magnitude 2.5 × 10-6 s-1
is required. This rate, however, is insufficient to produce the
observed unimodal profile of chirality with latitude. To achieve this, a
vorticity of at least 5 × 10-6 s-1 is needed. Our
results place a lower limit on the small-scale helicity injection
required to dominate differential rotation and reproduce the observed
hemispheric pattern. Future studies should aim to establish whether the
helicity injection rate due to convective flows and/or flux emergence
across all latitudes of the Sun is consistent with our results.

---------------------------------------------------------
Title: The Role of Flux Cancellation in Eruptions from Bipolar ARs
Authors: Yardley, S. L.; Green, L. M.; van Driel-Gesztelyi, L.;
Williams, D. R.; Mackay, D. H.
2018ApJ...866....8Y Altcode: 2018arXiv180810635Y
The physical processes or trigger mechanisms that lead to the eruption
of coronal mass ejections (CMEs), the largest eruptive phenomenon in the
heliosphere, are still undetermined. Low-altitude magnetic reconnection
associated with flux cancellation appears to play an important role in
CME occurrence as it can form an eruptive configuration and reduce the
magnetic flux that contributes to the overlying, stabilizing field. We
conduct the first comprehensive study of 20 small bipolar ARs (ARs)
in order to probe the role of flux cancellation as an eruption trigger
mechanism. We categorize eruptions from the bipolar regions into three
types related to location, and find that the type of eruption produced
depends on the evolutionary stage of the AR. In addition, we find that
ARs that form eruptive structures by flux cancellation (low-altitude
reconnection) had, on average, lower flux cancellation rates than the AR
sample as a whole. Therefore, while flux cancellation plays a key role,
by itself it is insufficient for the production of an eruption. The
results provide supporting evidence that although flux cancellation
in a sheared arcade may be able to build an eruptive configuration,
a successful eruption depends upon the removal of sufficient overlying
and stabilizing field. Convergence of the bipole polarities also appears
to be present in regions that produce an eruption. These findings have
important implications for understanding the physical processes that
occur on our Sun in relation to CMEs and for space weather forecasting.

---------------------------------------------------------
Title: Global Non-Potential Magnetic Models of the Solar Corona
During the March 2015 Eclipse
Authors: Yeates, Anthony R.; Amari, Tahar; Contopoulos, Ioannis; Feng,
Xueshang; Mackay, Duncan H.; Mikić, Zoran; Wiegelmann, Thomas; Hutton,
Joseph; Lowder, Christopher A.; Morgan, Huw; Petrie, Gordon; Rachmeler,
Laurel A.; Upton, Lisa A.; Canou, Aurelien; Chopin, Pierre; Downs,
Cooper; Druckmüller, Miloslav; Linker, Jon A.; Seaton, Daniel B.;
Török, Tibor
2018SSRv..214...99Y Altcode: 2018arXiv180800785Y
Seven different models are applied to the same problem of simulating
the Sun's coronal magnetic field during the solar eclipse on 2015
March 20. All of the models are non-potential, allowing for free
magnetic energy, but the associated electric currents are developed
in significantly different ways. This is not a direct comparison
of the coronal modelling techniques, in that the different models
also use different photospheric boundary conditions, reflecting
the range of approaches currently used in the community. Despite
the significant differences, the results show broad agreement in the
overall magnetic topology. Among those models with significant volume
currents in much of the corona, there is general agreement that the
ratio of total to potential magnetic energy should be approximately
1.4. However, there are significant differences in the electric current
distributions; while static extrapolations are best able to reproduce
active regions, they are unable to recover sheared magnetic fields in
filament channels using currently available vector magnetogram data. By
contrast, time-evolving simulations can recover the filament channel
fields at the expense of not matching the observed vector magnetic
fields within active regions. We suggest that, at present, the best
approach may be a hybrid model using static extrapolations but with
additional energization informed by simplified evolution models. This
is demonstrated by one of the models.

---------------------------------------------------------
Title: Predicting the corona for the 21 August 2017 total solar
eclipse
Authors: Mikić; , Zoran; Downs, Cooper; Linker, Jon A.; Caplan, Ronald
M.; Mackay, Duncan H.; Upton, Lisa A.; Riley, Pete; Lionello, Roberto;
Török, Tibor; Titov, Viacheslav S.; Wijaya, Janvier; Druckmüller,
Miloslav; Pasachoff, Jay M.; Carlos, Wendy
2018NatAs...2..913M Altcode: 2018NatAs.tmp..120M
The total solar eclipse that occurred on 21 August 2017 across the
United States provided an opportunity to test a magnetohydrodynamic
model of the solar corona driven by measured magnetic fields. We used
a new heating model based on the dissipation of Alfvén waves, and
a new energization mechanism to twist the magnetic field in filament
channels. We predicted what the corona would look like one week before
the eclipse. Here, we describe how this prediction was accomplished,
and show that it compared favourably with observations of the
eclipse in white light and extreme ultraviolet. The model allows us to
understand the relationship of observed features, including streamers,
coronal holes, prominences, polar plumes and thin rays, to the magnetic
field. We show that the discrepancies between the model and observations
arise from limitations in our ability to observe the Sun's magnetic
field. Predictions of this kind provide opportunities to improve the
models, forging the path to improved space weather prediction.

---------------------------------------------------------
Title: Connecting the large- and the small-scale magnetic fields of
solar-like stars
Authors: Lehmann, L. T.; Jardine, M. M.; Mackay, D. H.; Vidotto, A. A.
2018MNRAS.478.4390L Altcode: 2018MNRAS.tmp.1193L; 2018arXiv180504420L
A key question in understanding the observed magnetic field topologies
of cool stars is the link between the small- and the large-scale
magnetic field and the influence of the stellar parameters on the
magnetic field topology. We examine various simulated stars to connect
the small scale with the observable large-scale field. The highly
resolved 3D simulations we used couple a flux transport model with a
non-potential coronal model using a magnetofrictional technique. The
surface magnetic field of these simulations is decomposed into spherical
harmonics which enables us to analyse the magnetic field topologies on
a wide range of length scales and to filter the large-scale magnetic
field for a direct comparison with the observations. We show that
the large-scale field of the self-consistent simulations fits the
observed solar-like stars and is mainly set up by the global dipolar
field and the large-scale properties of the flux pattern, e.g. the
averaged latitudinal position of the emerging small-scale field and its
global polarity pattern. The stellar parameter flux emergence rate,
differential rotation, and meridional flow affect the large-scale
magnetic field topology. An increased flux emergence rate increases the
magnetic flux in all field components and an increased differential
rotation increases the toroidal field fraction by decreasing the
poloidal field. The meridional flow affects the distribution of the
magnetic energy across the spherical harmonic modes.

---------------------------------------------------------
Title: Large-Scale Patterns of Filament Channels and Filaments
Authors: Mackay, Duncan
2018cosp...42E2112M Altcode:
In this review the properties and large-scale patterns of filament
channels and filaments will be considered. Initially, the global
formation locations of filament channels and filaments arediscussed,
along with their hemispheric pattern. Next, observations of the
formation of filament channels and filaments are described where two
opposing views are considered. Finally, the wide range of models that
have been constructed to consider the formation of filament channels
and filaments over long time-scales are described, along with the
origin of the hemispheric pattern of filaments.

---------------------------------------------------------
Title: 3D modelling of magnetic field and plasma structure of entire
prominences
Authors: Gunár, Stanislav; Anzer, Ulrich; Heinzel, Petr; Mackay,
Duncan
2018cosp...42E1315G Altcode:
The 3D Whole-Prominence Fine Structure (WPFS) model allows us for
the first time to simulate entire prominences/filaments including
their numerous fine structures. This model combines a 3D magnetic
field configuration of an entire prominence obtained from non-linear
force-free field simulations, with a detailed description of the
prominence plasma. The plasma is located in magnetic dips in hydrostatic
equilibrium and is distributed along hundreds of fine structures
within the 3D magnetic model. The prominence plasma has realistic
density and temperature distributions including the prominence-corona
transition region. This allows us to produce synthetic H-alpha images
of simulated prominences both in emission on the solar limb and in
absorption against the solar disk (viewed as filaments) using a single
model.Such 3D WPFS model provides us with consistent information about
the prominence magnetic field configuration, prominence fine structure
plasma and its radiative output. Moreover, we are able to follow the
evolution of modeled prominences caused by changes of the underlying
photospheric magnetic flux distribution. Thanks to these capabilities we
can study links between the photospheric flux distribution, prominence
magnetic field configuration, distribution and composition of the
prominence plasma and its observable signatures. These relationships
are important for interpretation of the observed imaging and
spectral/spectropolarimetric data and for inference of the properties
of the prominence magnetic field.

---------------------------------------------------------
Title: Can 3D whole-prominence fine structure models be used for
assessment of the prominence plasma mass and distribution prior to
the onset of CMEs?
Authors: Gunár, Stanislav; Schmieder, Brigitte; Aulanier, Guillaume;
Anzer, Ulrich; Heinzel, Petr; Mackay, Duncan; Dudik, Jaroslav
2018cosp...42E1316G Altcode:
Two complex 3D models of entire prominences including their numerous
fine structures were recently developed. The first 3D Whole-Prominence
Fine Structure (WPFS) model was developed by Gunár and Mackay. The
second 3D WPFS model was put forward by Gunár, Aulanier, Dudík,
Heinzel, and Schmieder. These 3D prominence models combine simulations
of the 3D magnetic field configuration of an entire prominence with a
detailed description of the prominence plasma. The plasma is located
in magnetic dips in hydrostatic equilibrium and is distributed
along hundreds of fine structures. The assumed prominence plasma
has realistic density and temperature distributions including the
prominence-corona transition region.These 3D WPFS models allow us
to study the distribution and the mass of the prominence plasma
contained in prominence magnetic field configurations. These can
be crucial during the onset and early evolution of CMEs. Moreover,
prominence plasma represents a bulk of the material ejected by CMEs
into the interplanetary space. Here, we investigate the potential of
using the 3D WPFS models for assessment of the role the prominence
plasma plays in the initiation and evolution of CMEs.

---------------------------------------------------------
Title: A new technique for observationally derived boundary conditions
for space weather
Authors: Pagano, Paolo; Mackay, Duncan Hendry; Yeates, Anthony Robinson
2018JSWSC...8A..26P Altcode: 2018arXiv180207516P
Context. In recent years, space weather research has focused on
developing modelling techniques to predict the arrival time and
properties of coronal mass ejections (CMEs) at the Earth. The aim of
this paper is to propose a new modelling technique suitable for the
next generation of Space Weather predictive tools that is both efficient
and accurate. The aim of the new approach is to provide interplanetary
space weather forecasting models with accurate time dependent boundary
conditions of erupting magnetic flux ropes in the upper solar
corona. Methods: To produce boundary conditions, we couple
two different modelling techniques, MHD simulations and a quasi-static
non-potential evolution model. Both are applied on a spatial domain that
covers the entire solar surface, although they extend over a different
radial distance. The non-potential model uses a time series of observed
synoptic magnetograms to drive the non-potential quasi-static evolution
of the coronal magnetic field. This allows us to follow the formation
and loss of equilibrium of magnetic flux ropes. Following this a MHD
simulation captures the dynamic evolution of the erupting flux rope,
when it is ejected into interplanetary space. Results.The present paper
focuses on the MHD simulations that follow the ejection of magnetic
flux ropes to 4 R⊙. We first propose a technique for
specifying the pre-eruptive plasma properties in the corona. Next,
time dependent MHD simulations describe the ejection of two magnetic
flux ropes, that produce time dependent boundary conditions for the
magnetic field and plasma at 4 R⊙ that in future may
be applied to interplanetary space weather prediction models. Conclusions: In the present paper, we show that the dual use
of quasi-static non-potential magnetic field simulations and full
time dependent MHD simulations can produce realistic inhomogeneous
boundary conditions for space weather forecasting tools. Before a fully
operational model can be produced there are a number of technical and
scientific challenges that still need to be addressed. Nevertheless,
we illustrate that coupling quasi-static and MHD simulations in this
way can significantly reduce the computational time required to produce
realistic space weather boundary conditions.

---------------------------------------------------------
Title: Photospheric Observations of Surface and Body Modes in Solar
Magnetic Pores
Authors: Keys, Peter H.; Morton, Richard J.; Jess, David B.; Verth,
Gary; Grant, Samuel D. T.; Mathioudakis, Mihalis; Mackay, Duncan H.;
Doyle, John G.; Christian, Damian J.; Keenan, Francis P.; Erdélyi,
Robertus
2018ApJ...857...28K Altcode: 2018arXiv180301859K
Over the past number of years, great strides have been made in
identifying the various low-order magnetohydrodynamic wave modes
observable in a number of magnetic structures found within the solar
atmosphere. However, one aspect of these modes that has remained
elusive, until now, is their designation as either surface or body
modes. This property has significant implications for how these modes
transfer energy from the waveguide to the surrounding plasma. Here, for
the first time to our knowledge, we present conclusive, direct evidence
of these wave characteristics in numerous pores that were observed to
support sausage modes. As well as outlining methods to detect these
modes in observations, we make estimates of the energies associated
with each mode. We find surface modes more frequently in the data,
as well as that surface modes appear to carry more energy than those
displaying signatures of body modes. We find frequencies in the range
of ∼2-12 mHz, with body modes as high as 11 mHz, but we do not find
surface modes above 10 mHz. It is expected that the techniques we have
applied will help researchers search for surface and body signatures
in other modes and in differing structures from those presented here.

---------------------------------------------------------
Title: Simulating the Coronal Evolution of AR 11437 Using SDO/HMI
Magnetograms
Authors: Yardley, Stephanie L.; Mackay, Duncan H.; Green, Lucie M.
2018ApJ...852...82Y Altcode: 2017arXiv171200396Y
The coronal magnetic field evolution of AR 11437 is simulated by
applying the magnetofrictional relaxation technique of Mackay et al. A
sequence of photospheric line-of-sight magnetograms produced by the
Solar Dynamics Observatory (SDO)/Helioseismic Magnetic Imager (HMI)
is used to drive the simulation and continuously evolve the coronal
magnetic field of the active region through a series of nonlinear
force-free equilibria. The simulation is started during the first
stages of the active region emergence so that its full evolution from
emergence to decay can be simulated. A comparison of the simulation
results with SDO/Atmospheric Imaging Assembly (AIA) observations
show that many aspects of the active region’s observed coronal
evolution are reproduced. In particular, it shows the presence of a
flux rope, which forms at the same location as sheared coronal loops
in the observations. The observations show that eruptions occurred
on 2012 March 17 at 05:09 UT and 10:45 UT and on 2012 March 20 at
14:31 UT. The simulation reproduces the first and third eruption,
with the simulated flux rope erupting roughly 1 and 10 hr before
the observed ejections, respectively. A parameter study is conducted
where the boundary and initial conditions are varied along with the
physical effects of Ohmic diffusion, hyperdiffusion, and an additional
injection of helicity. When comparing the simulations, the evolution
of the magnetic field, free magnetic energy, relative helicity and flux
rope eruption timings do not change significantly. This indicates that
the key element in reproducing the coronal evolution of AR 11437 is
the use of line-of-sight magnetograms to drive the evolution of the
coronal magnetic field.

---------------------------------------------------------
Title: Quiescent Prominences in the Era of ALMA. II. Kinetic
Temperature Diagnostics
Authors: Gunár, Stanislav; Heinzel, Petr; Anzer, Ulrich; Mackay,
Duncan H.
2018ApJ...853...21G Altcode:
We provide the theoretical background for diagnostics of the thermal
properties of solar prominences observed by the Atacama Large
Millimeter/submillimeter Array (ALMA). To do this, we employ the 3D
Whole-Prominence Fine Structure (WPFS) model that produces synthetic
ALMA-like observations of a complex simulated prominence. We use
synthetic observations derived at two different submillimeter/millimeter
(SMM) wavelengths—one at a wavelength at which the simulated
prominence is completely optically thin and another at a wavelength at
which a significant portion of the simulated prominence is optically
thick—as if these were the actual ALMA observations. This allows
us to develop a technique for an analysis of the prominence plasma
thermal properties from such a pair of simultaneous high-resolution ALMA
observations. The 3D WPFS model also provides detailed information about
the distribution of the kinetic temperature and the optical thickness
along any line of sight. We can thus assess whether the measure of the
kinetic temperature derived from observations accurately represents
the actual kinetic temperature properties of the observed plasma. We
demonstrate here that in a given pixel the optical thickness at the
wavelength at which the prominence plasma is optically thick needs
to be above unity or even larger to achieve a sufficient accuracy of
the derived information about the kinetic temperature of the analyzed
plasma. Information about the optical thickness cannot be directly
discerned from observations at the SMM wavelengths alone. However,
we show that a criterion that can identify those pixels in which
the derived kinetic temperature values correspond well to the actual
thermal properties in which the observed prominence can be established.

---------------------------------------------------------
Title: Modeling the 21 August 2017 Total Solar Eclipse: Prediction
Results and New Techniques
Authors: Downs, C.; Mikic, Z.; Caplan, R. M.; Linker, J.; Lionello,
R.; Torok, T.; Titov, V. S.; Riley, P.; MacKay, D.; Upton, L.
2017AGUFMSH13B2475D Altcode:
As has been our tradition for past solar eclipses, we conducted a
high resolution magnetohydrodynamic (MHD) simulation of the corona
to predict the appearance of the 21 August 2017 solar eclipse. In
this presentation, we discuss our model setup and our forward
modeled predictions for the corona's appearance, including images
of polarized brightness and EUV/soft X-Ray emission. We show how
the combination of forward modeled observables and knowledge of the
underlying magnetic field from the model can be used to interpret
the structures seen during the eclipse. We also discuss two new
features added to this year's prediction. First, in an attempt to
improve the morphological shape of streamers in the low corona,
we energize the large-scale magnetic field by emerging shear and
canceling flux within filament channels. The handedness of the shear
is deduced from a magnetofrictional model, which is driven by the
evolving photospheric field produced by the Advective Flux Transport
model. Second, we apply our new wave-turbulence-driven (WTD) model for
coronal heating. This model has substantially fewer free parameters
than previous empirical heating models, but is inherently sensitive to
the 3D geometry and connectivity of the magnetic field--a key property
for modeling the thermal-magnetic structure of the corona. We examine
the effect of these considerations on forward modeled observables,
and present them in the context of our final 2017 eclipse prediction
(www.predsci.com/corona/aug2017eclipse). Research supported by NASA's
Heliophysics Supporting Research and Living With a Star Programs.

---------------------------------------------------------
Title: The Role of Small-Scale Processes in Solar Active Region Decay
Authors: Meyer, Karen; Mackay, Duncan
2017SPD....4810106M Altcode:
Active regions are locations of intense magnetic activity on the Sun,
whose evolution can result in highly energetic eruptive phenomena
such as solar flares and coronal mass ejections (CMEs). Therefore,
fast and accurate simulation of their evolution and decay is essential
in the prediction of Space Weather events. In this talk we present
initial results from our new model for the photospheric evolution
of active region magnetic fields. Observations show that small-scale
processes appear to play a role in the dispersal and decay of solar
active regions, for example through cancellation at the boundary
of sunspot outflows and erosion of flux by surrounding convective
cells. Our active region model is coupled to our existing model for
the evolution of small-scale photospheric magnetic features. Focusing
first on the active region decay phase, we consider the evolution of
its magnetic field due to both large-scale (e.g. differential rotation)
and small-scale processes, such as its interaction with surrounding
small-scale magnetic features and convective flows.This project is
funded by The Carnegie Trust for the Universities of Scotland, through
their Research Incentives Grant scheme.

---------------------------------------------------------
Title: New Techniques Used in Modeling the 2017 Total Solar Eclipse:
Energizing and Heating the Large-Scale Corona
Authors: Downs, Cooper; Mikic, Zoran; Linker, Jon A.; Caplan, Ronald
M.; Lionello, Roberto; Torok, Tibor; Titov, Viacheslav; Riley, Pete;
Mackay, Duncan; Upton, Lisa
2017SPD....4820802D Altcode:
Over the past two decades, our group has used a magnetohydrodynamic
(MHD) model of the corona to predict the appearance of total solar
eclipses. In this presentation we detail recent innovations and
new techniques applied to our prediction model for the August 21,
2017 total solar eclipse. First, we have developed a method for
capturing the large-scale energized fields typical of the corona,
namely the sheared/twisted fields built up through long-term processes
of differential rotation and flux-emergence/cancellation. Using
inferences of the location and chirality of filament channels (deduced
from a magnetofrictional model driven by the evolving photospheric
field produced by the Advective Flux Transport model), we tailor a
customized boundary electric field profile that will emerge shear along
the desired portions of polarity inversion lines (PILs) and cancel flux
to create long twisted flux systems low in the corona. This method
has the potential to improve the morphological shape of streamers in
the low solar corona. Second, we apply, for the first time in our
eclipse prediction simulations, a new wave-turbulence-dissipation
(WTD) based model for coronal heating. This model has substantially
fewer free parameters than previous empirical heating models, but is
inherently sensitive to the 3D geometry and connectivity of the coronal
field---a key property for modeling/predicting the thermal-magnetic
structure of the solar corona. Overall, we will examine the effect
of these considerations on white-light and EUV observables from the
simulations, and present them in the context of our final 2017 eclipse
prediction model.Research supported by NASA's Heliophysics Supporting
Research and Living With a Star Programs.

---------------------------------------------------------
Title: Prediction of the Solar Corona for the 2017 August 21 Total
Solar Eclipse
Authors: Mikic, Zoran; Downs, Cooper; Linker, Jon A.; Caplan, Ronald
M.; Lionello, Roberto; Torok, Tibor; Titov, Viacheslav; Riley, Pete;
Mackay, Duncan; Upton, Lisa
2017SPD....4820801M Altcode:
It has become our tradition to predict the structure of the corona
prior to eclipses, using a magnetohydrodynamic (MHD) model based on
measurements of photospheric magnetic fields on the Sun. We plan to
continue this tradition for the August 21, 2017 total solar eclipse that
will sweep across the United States. We will predict the structure of
the corona using SDO/HMI photospheric magnetic field data, including
images of polarization brightness, magnetic field line traces, and
images of simulated emission in EUV and X-rays. These images can be
compared directly with observations of the total eclipse, as well as
observations from SDO/AIA, Hinode/XRT, and STEREO/EUVI. This year we
will attempt to energize the magnetic field within filament channels
for a more realistic prediction, by constructing flux ropes at the
locations where filament channels are observed. The handedness of the
flux ropes will be deduced from a magnetofrictional model driven by the
evolving photospheric field produced by the Advective Flux Transport
model.Research supported by NASA's Heliophysics Supporting Research
and Living With a Star Programs.

---------------------------------------------------------
Title: Origin and Ion Charge State Evolution of Solar Wind Transients
during 4 - 7 August 2011
Authors: Rodkin, D.; Goryaev, F.; Pagano, P.; Gibb, G.; Slemzin, V.;
Shugay, Y.; Veselovsky, I.; Mackay, D. H.
2017SoPh..292...90R Altcode: 2016arXiv161005048R
We present a study of the complex event consisting of several solar wind
transients detected by the Advanced Composition Explorer (ACE) on 4 - 7
August 2011, which caused a geomagnetic storm with Dst =−110 nT. The
supposed coronal sources, three flares and coronal mass ejections
(CMEs), occurred on 2 - 4 August 2011 in active region (AR) 11261. To
investigate the solar origin and formation of these transients, we study
the kinematic and thermodynamic properties of the expanding coronal
structures using the Solar Dynamics Observatory/Atmospheric Imaging
Assembly (SDO/AIA) EUV images and differential emission measure (DEM)
diagnostics. The Helioseismic and Magnetic Imager (HMI) magnetic field
maps were used as the input data for the 3D magnetohydrodynamic (MHD)
model to describe the flux rope ejection (Pagano, Mackay, and Poedts,
2013b). We characterize the early phase of the flux rope ejection in the
corona, where the usual three-component CME structure formed. The flux
rope was ejected with a speed of about 200 kms−1 to the
height of 0.25 R⊙. The kinematics of the modeled CME front
agrees well with the Solar Terrestrial Relations Observatory (STEREO)
EUV measurements. Using the results of the plasma diagnostics and MHD
modeling, we calculate the ion charge ratios of carbon and oxygen as
well as the mean charge state of iron ions of the 2 August 2011 CME,
taking into account the processes of heating, cooling, expansion,
ionization, and recombination of the moving plasma in the corona up to
the frozen-in region. We estimate a probable heating rate of the CME
plasma in the low corona by matching the calculated ion composition
parameters of the CME with those measured in situ for the solar wind
transients. We also consider the similarities and discrepancies between
the results of the MHD simulation and the observations.

---------------------------------------------------------
Title: Origin and Ion Charge State Evolution of Solar Wind Transients
4 - 7 August 2011
Authors: Rodkin, Denis; Goryaev, Farid; Pagano, Paolo; Gibb, Gordon;
Slemzin, Vladimir; Shugay, Yulia; Veselovsky, Igor; Mackay, Duncan
2017EGUGA..19.6920R Altcode:
Identification of transients and their origins on the Sun is one of the
most important problems of the space weather forecasting. In our work,
we present a case study of the complex event consisting of several
solar wind transients detected by ACE on 4 - 7 August 2011, that caused
a geomagnetic storm with Dst= - 110 nT. The supposed coronal sources -
three flares and coronal mass ejections (CMEs) occurred on 2 - 4 August
2011 in the active region AR 11261. To investigate the solar origins and
formation of these transients, we studied kinematic and thermodynamic
properties of expanding coronal structures using the SDO/AIA EUV
images and the differential emission measure (DEM) diagnostics. The
Helioseismic and Magnetic Imager (HMI) magnetic field maps were used
as the input data for the 3D numerical model to describe the flux rope
ejection. We characterize the early phase of the flux rope ejection in
the corona, where the usual three-component CME structure formed. The
flux rope ejected with the speed about 200 km/s to the height of 0.25
Rsun. The kinematics of the modeled CME front well agrees with the
STEREO EUV measurements. Using the results of the plasma diagnostics and
MHD modeling, we calculated the ion charge ratios of carbon and oxygen
as well as the mean charge state of iron ions of the 2 August 2011
CME taking into account the processes of heating, cooling, expansion,
ionization and recombination of the moving plasma in the corona up to
the freeze-in region. We estimated a probable heating rate of the CME
plasma in the low corona by matching the calculated ion composition
parameters of the CME with that measured in-situ parameters of the solar
wind transients. We also consider the similarities and discrepancies
between the results of the MHD simulation and the observation of the
event. Our results show that analysis of the ion composition of CMEs
enables to disclose a relationship between parameters of the solar
wind transients and properties of their solar origins, which opens new
possibilities to validate and improve the solar wind forecasting models.

---------------------------------------------------------
Title: The energy budget of stellar magnetic fields: comparing
non-potential simulations and observations
Authors: Lehmann, L. T.; Jardine, M. M.; Vidotto, A. A.; Mackay,
D. H.; See, V.; Donati, J. -F.; Folsom, C. P.; Jeffers, S. V.; Marsden,
S. C.; Morin, J.; Petit, P.
2017MNRAS.466L..24L Altcode: 2016arXiv161008314L
The magnetic geometry of the surface magnetic fields of more than 55
cool stars have now been mapped using spectropolarimetry. In order to
better understand these observations, we compare the magnetic field
topology at different surface scale sizes of observed and simulated
cool stars. For ease of comparison between the high-resolution
non-potential magnetofrictional simulations and the relatively
low-resolution observations, we filter out the small-scale field in
the simulations using a spherical harmonics decomposition. We show
that the large-scale field topologies of the solar-based simulations
produce values of poloidal/toroidal fields and fractions of energy in
axisymmetric modes which are similar to the observations. These global
non-potential evolution model simulations capture key magnetic features
of the observed solar-like stars through the processes of surface
flux transport and magnetic flux emergence. They do not, however,
reproduce the magnetic field of M-dwarfs or stars with dominantly
toroidal field. Furthermore, we analyse the magnetic field topologies
of individual spherical harmonics for the simulations and discover
that the dipole is predominately poloidal, while the quadrupole shows
the highest fraction of toroidal fields. Magnetic field structures
smaller than a quadrupole display a fixed ratio between the poloidal
and toroidal magnetic energies.

---------------------------------------------------------
Title: Observations and Modelling of the Pre-flare Period of the 29
March 2014 X1 Flare
Authors: Woods, M. M.; Harra, L. K.; Matthews, S. A.; Mackay, D. H.;
Dacie, S.; Long, D. M.
2017SoPh..292...38W Altcode: 2017arXiv170106457W
On 29 March 2014, NOAA Active Region (AR) 12017 produced an X1
flare that was simultaneously observed by an unprecedented number
of observatories. We have investigated the pre-flare period of this
flare from 14:00 UT until 19:00 UT using joint observations made
by the Interface Region Imaging Spectrometer (IRIS) and the Hinode
Extreme Ultraviolet Imaging Spectrometer (EIS). Spectral lines
providing coverage of the solar atmosphere from the chromosphere to
the corona were analysed to investigate pre-flare activity within the
AR. The results of the investigation have revealed evidence of strongly
blue-shifted plasma flows, with velocities up to 200 kms−1,
being observed 40 minutes prior to flaring. These flows are located
along the filament present in the active region and are both spatially
discrete and transient. In order to constrain the possible explanations
for this activity, we undertake non-potential magnetic field modelling
of the active region. This modelling indicates the existence of a
weakly twisted flux rope along the polarity inversion line in the region
where a filament and the strong pre-flare flows are observed. We then
discuss how these observations relate to the current models of flare
triggering. We conclude that the most likely drivers of the observed
activity are internal reconnection in the flux rope, early onset of the
flare reconnection, or tether-cutting reconnection along the filament.

---------------------------------------------------------
Title: Combined Global NLFFF simulations and MHD simulations of flux
rope ejections
Authors: Pagano, P.; Mackay, D.; Yeates, A.
2017psio.confE.117P Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Quiescent Prominences in the Era of ALMA: Simulated
Observations Using the 3D Whole-prominence Fine Structure Model
Authors: Gunár, Stanislav; Heinzel, Petr; Mackay, Duncan H.; Anzer,
Ulrich
2016ApJ...833..141G Altcode:
We use the detailed 3D whole-prominence fine structure model to
produce the first simulated high-resolution ALMA observations
of a modeled quiescent solar prominence. The maps of synthetic
brightness temperature and optical thickness shown in the present
paper are produced using a visualization method for synthesis of the
submillimeter/millimeter radio continua. We have obtained the simulated
observations of both the prominence at the limb and the filament
on the disk at wavelengths covering a broad range that encompasses
the full potential of ALMA. We demonstrate here extent to which the
small-scale and large-scale prominence and filament structures will be
visible in the ALMA observations spanning both the optically thin and
thick regimes. We analyze the relationship between the brightness and
kinetic temperature of the prominence plasma. We also illustrate the
opportunities ALMA will provide for studying the thermal structure
of the prominence plasma from the cores of the cool prominence fine
structure to the prominence-corona transition region. In addition, we
show that detailed 3D modeling of entire prominences with their numerous
fine structures will be important for the correct interpretation of
future ALMA observations of prominences.

---------------------------------------------------------
Title: Modeling the Sun’s Small-scale Global Photospheric Magnetic
Field
Authors: Meyer, K. A.; Mackay, D. H.
2016ApJ...830..160M Altcode:
We present a new model for the Sun’s global photospheric magnetic
field during a deep minimum of activity, in which no active regions
emerge. The emergence and subsequent evolution of small-scale magnetic
features across the full solar surface is simulated, subject to the
influence of a global supergranular flow pattern. Visually, the
resulting simulated magnetograms reproduce the typical structure
and scale observed in quiet Sun magnetograms. Quantitatively, the
simulation quickly reaches a steady state, resulting in a mean field
and flux distribution that are in good agreement with those determined
from observations. A potential coronal magnetic field is extrapolated
from the simulated full Sun magnetograms to consider the implications
of such a quiet photospheric magnetic field on the corona and inner
heliosphere. The bulk of the coronal magnetic field closes very low
down, in short connections between small-scale features in the simulated
magnetic network. Just 0.1% of the photospheric magnetic flux is found
to be open at 2.5 R ⊙, around 10-100 times less than that
determined for typical Helioseismic and Magnetic Imager synoptic map
observations. If such conditions were to exist on the Sun, this would
lead to a significantly weaker interplanetary magnetic field than is
currently observed, and hence a much higher cosmic ray flux at Earth.

---------------------------------------------------------
Title: The Possible Impact of L5 Magnetograms on Non-potential Solar
Coronal Magnetic Field Simulations
Authors: Weinzierl, Marion; Mackay, Duncan H.; Yeates, Anthony R.;
Pevtsov, Alexei A.
2016ApJ...828..102W Altcode:
The proposed Carrington-L5 mission would bring instruments to the
L5 Lagrange point to provide us with crucial data for space weather
prediction. To assess the importance of including a magnetograph,
we consider the possible differences in non-potential solar coronal
magnetic field simulations when magnetograph observations are available
from the L5 point, compared with an L1-based field of view (FOV). A
timeseries of synoptic radial magnetic field maps is constructed to
capture the emergence of two active regions from the L5 FOV. These
regions are initially absent in the L1 magnetic field maps, but are
included once they rotate into the L1 FOV. Non-potential simulations
for these two sets of input data are compared in detail. Within the
bipolar active regions themselves, differences in the magnetic field
structure can exist between the two simulations once the active regions
are included in both. These differences tend to reduce within 5 days
of the active region being included in L1. The delayed emergence in L1
can, however, lead to significant persistent differences in long-range
connectivity between the active regions and the surrounding fields, and
also in the global magnetic energy. In particular, the open magnetic
flux and the location of open magnetic footpoints, are sensitive to
capturing the real-time of emergence. These results suggest that a
magnetograph at L5 could significantly improve predictions of the
non-potential corona, the interplanetary magnetic field, and of solar
wind source regions on the Sun.

---------------------------------------------------------
Title: Magnetic reconnection between a solar filament and nearby
coronal loops
Authors: Li, Leping; Zhang, Jun; Peter, Hardi; Priest, Eric; Chen,
Huadong; Guo, Lijia; Chen, Feng; Mackay, Duncan
2016NatPh..12..847L Altcode: 2016arXiv160503320L
Magnetic reconnection is difficult to observe directly but coronal
structures on the Sun often betray the magnetic field geometry and
its evolution. Here we report the observation of magnetic reconnection
between an erupting filament and its nearby coronal loops, resulting
in changes in the filament connection. X-type structures form when the
erupting filament encounters the loops. The filament becomes straight,
and bright current sheets form at the interfaces. Plasmoids appear
in these current sheets and propagate bi-directionally. The filament
disconnects from the current sheets, which gradually disperse and
disappear, then reconnects to the loops. This evolution suggests
successive magnetic reconnection events predicted by theory but rarely
detected with such clarity in observations. Our results confirm the
three-dimensional magnetic reconnection theory and have implications
for the evolution of dissipation regions and the release of magnetic
energy for reconnection in many magnetized plasma systems.

---------------------------------------------------------
Title: Properties of the prominence magnetic field and plasma
distributions as obtained from 3D whole-prominence fine structure
modeling
Authors: Gunár, S.; Mackay, D. H.
2016A&A...592A..60G Altcode:
 Aims: We analyze distributions of the magnetic field strength
and prominence plasma (temperature, pressure, plasma β, and mass) using
the 3D whole-prominence fine structure model. Methods: The model
combines a 3D magnetic field configuration of an entire prominence,
obtained from non-linear force-free field simulations, with a detailed
semi-empirically derived description of the prominence plasma. The
plasma is located in magnetic dips in hydrostatic equilibrium and is
distributed along multiple fine structures within the 3D magnetic
model. Results: We show that in the modeled prominence, the
variations of the magnetic field strength and its orientation are
insignificant on scales comparable to the smallest dimensions of the
observed prominence fine structures. We also show the ability of the
3D whole-prominence fine structure model to reveal the distribution
of the prominence plasma with respect to its temperature within the
prominence volume. This provides new insights into the composition
of the prominence-corona transition region. We further demonstrate
that the values of the plasma β are small throughout the majority
of the modeled prominences when realistic photospheric magnetic flux
distributions and prominence plasma parameters are assumed. While this
is generally true, we also find that in the region with the deepest
magnetic dips, the plasma β may increase towards unity. Finally,
we show that the mass of the modeled prominence plasma is in good
agreement with the mass of observed non-eruptive prominences.

---------------------------------------------------------
Title: Impact of an L5 Magnetograph on Nonpotential Solar Global
Magnetic Field Modeling
Authors: Mackay, Duncan H.; Yeates, Anthony R.; Bocquet,
Francois-Xavier
2016ApJ...825..131M Altcode:
We present the first theoretical study to consider what improvement
could be obtained in global nonpotential modeling of the solar corona
if magnetograph data were available from the L5 Lagrange point, in
addition to from the direction of Earth. To consider this, we first
carry out a “reference Sun” simulation over two solar cycles. An
important property of this simulation is that random bipole emergences
are allowed across the entire solar surface at any given time (such
as can occur on the Sun). Next, we construct two “limited data”
simulations, where bipoles are only included when they could be
seen from (I) an Earth-based magnetograph and (II) either Earth- or
L5-based magnetographs. The improvement in reproducing the reference
Sun simulation when an L5 view is available is quantified through
considering global quantities in the limited data simulations. These
include surface and polar flux, total magnetic energy, volume electric
current, open flux, and the number of flux ropes. Results show that
when an L5 observational viewpoint is included, the accuracy of the
global quantities in the limited data simulations can increase by
26%-40%. This clearly shows that a magnetograph at the L5 point could
significantly increase the accuracy of global nonpotential modeling
and with this the accuracy of future space weather forecasts.

---------------------------------------------------------
Title: Large-Scale Patterns of Filament Channels and Filaments
Authors: Mackay, Duncan
2016cosp...41E1213M Altcode:
In this review the properties and large-scale patterns of filament
channels and filaments will be considered. Initially, the global
formation locations of filament channels and filaments are discussed,
along with their hemispheric pattern. Next, observations of the
formation of filament channels and filaments are described where two
opposing views are considered. Finally, the wide range of models that
have been constructed to consider the formation of filament channels
and filaments over long time-scales are described, along with the
origin of the hemispheric pattern of filaments.

---------------------------------------------------------
Title: A New Technique for the Photospheric Driving of Non-potential
Solar Coronal Magnetic Field Simulations
Authors: Weinzierl, Marion; Yeates, Anthony R.; Mackay, Duncan H.;
Henney, Carl J.; Arge, C. Nick
2016ApJ...823...55W Altcode:
In this paper, we develop a new technique for driving global
non-potential simulations of the Sun’s coronal magnetic field solely
from sequences of radial magnetic maps of the solar photosphere. A
primary challenge to driving such global simulations is that the
required horizontal electric field cannot be uniquely determined from
such maps. We show that an “inductive” electric field solution
similar to that used by previous authors successfully reproduces
specific features of the coronal field evolution in both single and
multiple bipole simulations. For these cases, the true solution
is known because the electric field was generated from a surface
flux-transport model. The match for these cases is further improved by
including the non-inductive electric field contribution from surface
differential rotation. Then, using this reconstruction method for the
electric field, we show that a coronal non-potential simulation can be
successfully driven from a sequence of ADAPT maps of the photospheric
radial field, without including additional physical observations which
are not routinely available.

---------------------------------------------------------
Title: Stellar coronal response to differential rotation and flux
emergence
Authors: Gibb, G. P. S.; Mackay, D. H.; Jardine, M. M.; Yeates, A. R.
2016MNRAS.456.3624G Altcode: 2016arXiv160303419G
We perform a numerical parameter study to determine what effect
varying differential rotation and flux emergence has on a star's
non-potential coronal magnetic field. In particular we consider the
effects on the star's surface magnetic flux, open magnetic flux,
mean azimuthal field strength, coronal free magnetic energy, coronal
heating and flux rope eruptions. To do this, we apply a magnetic flux
transport model to describe the photospheric evolution, and couple
this to the non-potential coronal evolution using a magnetofrictional
technique. A flux emergence model is applied to add new magnetic flux
on to the photosphere and into the corona. The parameters of this flux
emergence model are derived from the solar flux emergence profile,
however the rate of emergence can be increased to represent higher flux
emergence rates than the Sun's. Overall we find that flux emergence has
a greater effect on the non-potential coronal properties compared to
differential rotation, with all the aforementioned properties increasing
with increasing flux emergence rate. Although differential rotation
has a lesser effect on the overall coronal properties compared to
flux emergence, varying differential rotation does alter the coronal
structure. As the differential rotation rate increases, the corona
becomes more open, and more non-potential.

---------------------------------------------------------
Title: Solar coronal magnetic fields derived using seismology
techniques applied to omnipresent sunspot waves
Authors: Jess, David B.; Reznikova, Veronika E.; Ryans, Robert S. I.;
Christian, Damian J.; Keys, Peter H.; Mathioudakis, Mihalis; Mackay,
Duncan H.; Krishna Prasad, S.; Banerjee, Dipankar; Grant, Samuel D. T.;
Yau, Sean; Diamond, Conor
2016NatPh..12..179J Altcode: 2016arXiv160506112J
Sunspots on the surface of the Sun are the observational signatures of
intense manifestations of tightly packed magnetic field lines, with
near-vertical field strengths exceeding 6,000 G in extreme cases. It
is well accepted that both the plasma density and the magnitude of the
magnetic field strength decrease rapidly away from the solar surface,
making high-cadence coronal measurements through traditional Zeeman and
Hanle effects difficult as the observational signatures are fraught
with low-amplitude signals that can become swamped with instrumental
noise. Magneto-hydrodynamic (MHD) techniques have previously been
applied to coronal structures, with single and spatially isolated
magnetic field strengths estimated as 9-55 G (refs ,,,). A drawback
with previous MHD approaches is that they rely on particular wave modes
alongside the detectability of harmonic overtones. Here we show, for
the first time, how omnipresent magneto-acoustic waves, originating
from within the underlying sunspot and propagating radially outwards,
allow the spatial variation of the local coronal magnetic field to be
mapped with high precision. We find coronal magnetic field strengths
of 32 +/- 5 G above the sunspot, which decrease rapidly to values of
approximately 1 G over a lateral distance of 7,000 km, consistent with
previous isolated and unresolved estimations. Our results demonstrate
a new, powerful technique that harnesses the omnipresent nature of
sunspot oscillations to provide magnetic field mapping capabilities
close to a magnetic source in the solar corona.

---------------------------------------------------------
Title: Towards a Data-Optimized Coronal Magnetic Field Model (DOC-FM):
Synthetic Test Beds and Multiwavelength Forward Modeling
Authors: Gibson, S. E.; Dalmasse, K.; Fan, Y.; Fineschi, S.; MacKay,
D.; Rempel, M.; White, S. M.
2015AGUFMSH54B..04G Altcode:
Understanding the physical state of the solar corona is key to
deciphering the origins of space weather as well as to realistically
representing the environment to be navigated by missions such as
Solar Orbiter and Solar Probe Plus. However, inverting solar coronal
observations to reconstruct this physical state -- and in particular
the three-dimensional coronal magnetic field - is complicated by
limited lines of sight and by projection effects. On the other hand,
the sensitivity of multiwavelength observations to different physical
mechanisms implies a potential for simultaneous probing of different
parts of the coronal plasma. In order to study this complementarity, and
to ultimately establish an optimal set of observations for constraining
the three-dimensional coronal magnetic field, we are developing a suite
of representative simulations to act as diagnostic test beds. We will
present three such test beds: a coronal active region, a quiescent
prominence, and a global corona. Each fully define the physical state
of density, temperature, and vector magnetic field in three dimensions
throughout the simulation domain. From these test beds, and using the
FORWARD SolarSoft IDL codes, we will create a broad range of synthetic
data. Radio observables will include intensity and circular polarization
(including gyroresonance effects) and Faraday rotation for a range of
frequencies. Infrared and visible forbidden line diagnostics of Zeeman
and saturated Hanle effects will yield full Stokes vector (I, Q, U,
V) synthetic data, and UV permitted line Hanle diagnostics will yield
intensity and linear polarization. In addition, we will synthesize
UV and SXR imager data, UV/EUV spectrometric data, and white light
brightness and polarized brightness. All of these synthetic data,
along with the "ground truth" physical state of the simulations from
which they are derived, will be made available to the community for
the purpose of testing coronal inversion techniques.

---------------------------------------------------------
Title: 3D Whole-prominence Fine Structure Modeling. II. Prominence
Evolution
Authors: Gunár, Stanislav; Mackay, Duncan H.
2015ApJ...812...93G Altcode:
We use the new three-dimensional (3D) whole-prominence fine structure
model to study the evolution of prominences and their fine structures
in response to changes in the underlying photospheric magnetic flux
distribution. The applied model combines a detailed 3D prominence
magnetic field configuration with a realistic description of the
prominence plasma distributed along multiple fine structures. In
addition, we utilize an approximate Hα visualization technique to
study the evolution of the visible cool prominence plasma both in
emission (prominence) and absorption (filament). We show that the
initial magnetic field configuration of the modeled prominence is
significantly disturbed by the changing position of a single polarity
of a magnetic bipole as the bipole is advected toward the main body
of the filament. This leads to the creation of a barb, which becomes
the dominant feature visible in the synthetic Hα images of both
the prominence and filament views. The evolution of the bipole also
creates conditions that lead to the disappearance and reappearance
of large portions of the main body. We also show that an arch-like
region containing a dark void (a bubble) can be naturally produced
in the synthetic prominence Hα images. While not visible in terms
of the magnetic field lines, it is due to a lack of Hα emission from
low-pressure, low-density plasma located in shallow magnetic dips lying
along the lines of sight intersecting the dark void. In addition, a
quasi-vertical small-scale feature consisting of short and deep dips,
piled one above the other, is produced.

---------------------------------------------------------
Title: Influence of Non-Potential Coronal Magnetic Topology on
Solar-Wind Models
Authors: Edwards, S. J.; Yeates, A. R.; Bocquet, F. -X.; Mackay, D. H.
2015SoPh..290.2791E Altcode: 2015arXiv151100427E
By comparing a magneto-frictional model of the low-coronal
magnetic-field to a potential-field source-surface model,
we investigate the possible impact of non-potential magnetic
structure on empirical solar-wind models. These empirical models
(such as Wang-Sheeley-Arge) estimate the distribution of solar-wind
speed solely from the magnetic-field structure in the low corona. Our
models are computed in a domain between the solar surface and 2.5 solar
radii, and they are extended to 0.1 AU using a Schatten current-sheet
model. The non-potential field has a more complex magnetic skeleton
and quasi-separatrix structures than the potential field, leading
to different sub-structure in the solar-wind speed proxies. It
contains twisted magnetic structures that can perturb the separatrix
surfaces traced down from the base of the heliospheric current sheet. A
significant difference between the models is the greater amount of open
magnetic flux in the non-potential model. Using existing empirical
formulae this leads to higher predicted wind speeds for two reasons:
partly because magnetic-flux tubes expand less rapidly with height,
but more importantly because more open-field lines are further from
coronal-hole boundaries.

---------------------------------------------------------
Title: Future capabilities of CME polarimetric 3D reconstructions
with the METIS instrument: A numerical test
Authors: Pagano, P.; Bemporad, A.; Mackay, D. H.
2015A&A...582A..72P Altcode: 2015arXiv150805276P
Context. Understanding the 3D structure of coronal mass ejections
(CMEs) is crucial for understanding the nature and origin of solar
eruptions. However, owing to the optical thinness of the solar corona we
can only observe the line of sight integrated emission. As a consequence
the resulting projection effects hide the true 3D structure of CMEs. To
derive information on the 3D structure of CMEs from white-light (total
and polarized brightness) images, the polarization ratio technique is
widely used. The soon-to-be-launched METIS coronagraph on board Solar
Orbiter will use this technique to produce new polarimetric images. Aims: This work considers the application of the polarization ratio
technique to synthetic CME observations from METIS. In particular we
determine the accuracy at which the position of the centre of mass,
direction and speed of propagation, and the column density of the CME
can be determined along the line of sight. Methods: We perform a
3D MHD simulation of a flux rope ejection where a CME is produced. From
the simulation we (i) synthesize the corresponding METIS white-light
(total and polarized brightness) images and (ii) apply the polarization
ratio technique to these synthesized images and compare the results with
the known density distribution from the MHD simulation. In addition,
we use recent results that consider how the position of a single blob
of plasma is measured depending on its projected position in the plane
of the sky. From this we can interpret the results of the polarization
ratio technique and give an estimation of the error associated with
derived parameters. Results: We find that the polarization ratio
technique reproduces with high accuracy the position of the centre
of mass along the line of sight. However, some errors are inherently
associated with this determination. The polarization ratio technique
also allows information to be derived on the real 3D direction of
propagation of the CME. The determination of this is of fundamental
importance for future space weather forecasting. In addition, we find
that the column density derived from white-light images is accurate
and we propose an improved technique where the combined use of the
polarization ratio technique and white-light images minimizes the
error in the estimation of column densities. Moreover, by applying the
comparison to a set of snapshots of the simulation we can also assess
the errors related to the trajectory and the expansion of the CME. Conclusions: Our method allows us to thoroughly test the performance
of the polarization ratio technique and allows a determination of the
errors associated with it, which means that it can be used to quantify
the results from the analysis of the forthcoming METIS observations in
white light (total and polarized brightness). Finally, we describe a
satellite observing configuration relative to the Earth that can allow
the technique to be efficiently used for space weather predictions. A movie attached to Fig. 15 is available in electronic form at <A
href="http://www.aanda.org/10.1051/0004-6361/201425462/olm">http://www.aanda.org</A>

---------------------------------------------------------
Title: High-resolution fine-structure synthetic imaging of an entire
prominence using 3D whole-prominence fine structure modelling
Authors: Gunar, Stanislav; Mackay, Duncan; Heinzel, Petr; Anzer, Ulrich
2015IAUGA..2251323G Altcode:
The newly developed 3D whole-prominence fine structure (WPFS)
model (Gunár &amp; Mackay 2015) allows us for the first time to
simulate entire prominences/filaments including their numerous fine
structures. This model combines a 3D magnetic field configuration of an
entire prominence obtained from non-linear force-free field simulations,
with a detailed description of the prominence plasma. The plasma is
located in magnetic dips in hydrostatic equilibrium and is distributed
along hundreds of fine structures within the 3D magnetic model. The
prominence plasma has realistic density and temperature distributions
including the prominence-corona transition region.To produce the
high-resolution synthetic H-alpha images of the WPFS model we use
a novel fast approximate radiative transfer visualization technique
(Heinzel et al. 2015). This allows us for the first time to produce
images of the prominences in emission on the solar limb and filaments in
absorption against the solar disk using a single model. The prominence
plasma and magnetic field are described in the WPFS model on scales
that allow us to produce synthetic images with resolution matching that
of the state-of-the-art observations, or indeed that of the upcoming
solar observatories, such as DKIST or Solar-C. Moreover, to complement
the prominence/filament synthetic images we have consistent information
about the magnetic field and plasma parameters everywhere in the modeled
prominences. This allows us to investigate the apparent puzzling nature
of the observed prominence and filament fine structures. We can also
study the connections between the local configuration of the magnetic
field and the observable structure of the finest prominence/filament
features. In addition, we are able to investigate the prominence
evolution. We can consistently study the influence of the varying
photospheric flux distribution on the prominence magnetic field
configuration and its effect on the observable prominence plasma.

---------------------------------------------------------
Title: 3D Whole-Prominence Fine Structure Modeling
Authors: Gunár, Stanislav; Mackay, Duncan H.
2015ApJ...803...64G Altcode:
We present the first 3D whole-prominence fine structure model. The
model combines a 3D magnetic field configuration of an entire prominence
obtained from nonlinear force-free field simulations, with a detailed
description of the prominence plasma. The plasma is located in magnetic
dips in hydrostatic equilibrium and is distributed along multiple
fine structures within the 3D magnetic model. Through the use of a
novel radiative transfer visualization technique for the Hα line such
plasma-loaded magnetic field model produces synthetic images of the
modeled prominence comparable with high-resolution observations. This
allows us for the first time to use a single technique to consistently
study, in both emission on the limb and absorption against the solar
disk, the fine structures of prominences/filaments produced by a
magnetic field model.

---------------------------------------------------------
Title: Numerical Simulations of a Flux Rope Ejection
Authors: Pagano, P.; Mackay, D. H.; Poedts, S.
2015JApA...36..123P Altcode: 2015JApA..tmp...19P
Coronal mass ejections (CMEs) are the most violent phenomena observed
on the Sun. One of the most successful models to explain CMEs is the
flux rope ejection model, where a magnetic flux rope is expelled from
the solar corona after a long phase along which the flux rope stays
in equilibrium while magnetic energy is being accumulated. However,
still many questions are outstanding on the detailed mechanism of the
ejection and observations continuously provide new data to interpret
and put in the context. Currently, extreme ultraviolet (EUV) images
from the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamic
Observatory (SDO) are providing new insights into the early phase
of CME evolution. In particular, observations show the ejection of
magnetic flux ropes from the solar corona and how they evolve into
CMEs. However, these observations are difficult to interpret in terms
of basic physical mechanisms and quantities, thus, we need to compare
equivalent quantities to test and improve our models. In our work,
we intend to bridge the gap between models and observations with our
model of flux rope ejection where we consistently describe the full
life span of a flux rope from its formation to ejection. This is done
by coupling the global non-linear force-free model (GNLFFF) built to
describe the slow low- β formation phase, with a full MHD simulation
run with the software MPI-AMRVAC, suitable to describe the fast MHD
evolution of the flux rope ejection that happens in a heterogeneous β
regime. We also explore the parameter space to identify the conditions
upon which the ejection is favoured (gravity stratification and
magnetic field intensity) and we produce synthesised AIA observations
(171 Å and 211 Å). To carry this out, we run 3D MHD simulation in
spherical coordinates where we include the role of thermal conduction
and radiative losses, both of which are important for determining the
temperature distribution of the solar corona during a CME. Our model
of flux rope ejection is successful in realistically describing the
entire life span of a flux rope and we also set some conditions for
the backgroud solar corona to favour the escape of the flux rope, so
that it turns into a CME. Furthermore, our MHD simulation reproduces
many of the features found in the AIA observations.

---------------------------------------------------------
Title: Formation and Large-Scale Patterns of Filament Channels
and Filaments
Authors: Mackay, Duncan H.
2015ASSL..415..355M Altcode:
The properties and large-scale patterns of filament channels and
filaments are considered. Initially, the global formation locations
of filament channels and filaments are discussed, along with their
hemispheric pattern. Next, observations of the formation of filament
channels and filaments are described where two opposing views are
considered. Finally, the wide range of models that have been constructed
to consider the formation of filament channels and filaments over long
time-scales are described, along with the origin of the hemispheric
pattern of filaments.

---------------------------------------------------------
Title: Stellar differential rotation and coronal time-scales
Authors: Gibb, G. P. S.; Jardine, M. M.; Mackay, D. H.
2014MNRAS.443.3251G Altcode: 2014arXiv1407.3388G
We investigate the time-scales of evolution of stellar coronae
in response to surface differential rotation and diffusion. To
quantify this, we study both the formation time and lifetime of a
magnetic flux rope in a decaying bipolar active region. We apply
a magnetic flux transport model to prescribe the evolution of the
stellar photospheric field, and use this to drive the evolution of the
coronal magnetic field via a magnetofrictional technique. Increasing
the differential rotation (i.e. decreasing the equator-pole lap time)
decreases the flux rope formation time. We find that the formation
time is dependent upon the lap time and the surface diffusion
time-scale through the relation τ_Form ∝ √{τ_Lapτ_Diff}. In
contrast, the lifetimes of flux ropes are proportional to the lap time
(τLife∝τLap). With this, flux ropes on stars
with a differential rotation of more than eight times the solar value
have a lifetime of less than 2 d. As a consequence, we propose that
features such as solar-like quiescent prominences may not be easily
observable on such stars, as the lifetimes of the flux ropes which host
the cool plasma are very short. We conclude that such high differential
rotation stars may have very dynamical coronae.

---------------------------------------------------------
Title: Simulating AIA observations of a flux rope ejection
Authors: Pagano, P.; Mackay, D. H.; Poedts, S.
2014A&A...568A.120P Altcode: 2014arXiv1407.8397P
Context. Coronal mass ejections (CMEs) are the most violent phenomena
observed on the Sun. Currently, extreme ultraviolet (EUV) images from
the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamic
Observatory (SDO) are providing new insights into the early phase of
CME evolution. In particular, observations now show the ejection of
magnetic flux ropes from the solar corona and how they evolve into
CMEs. While this is the case, these observations are difficult to
interpret in terms of basic physical mechanisms and quantities. To fully
understand CMEs we need to compare equivalent quantities derived from
both observations and theoretical models. This will aid in bridging the
gap between observations and models. Aims: To this end, we aim
to produce synthesised AIA observations from simulations of a flux rope
ejection. To carry this out we include the role of thermal conduction
and radiative losses, both of which are important for determining the
temperature distribution of the solar corona during a CME. 
Methods: We perform a simulation where a flux rope is ejected from
the solar corona. From the density and temperature of the plasma in
the simulation we synthesise AIA observations. The emission is then
integrated along the line of sight using the instrumental response
function of AIA. Results: We sythesise observations of AIA in
the channels at 304 Å, 171 Å, 335 Å, and 94 Å. The synthesised
observations show a number of features similar to actual observations
and in particular reproduce the general development of CMEs in the low
corona as observed by AIA. In particular we reproduce an erupting and
expanding arcade in the 304 Å and 171 Å channels with a high density
core. Conclusions: The ejection of a flux rope reproduces many
of the features found in the AIA observations. This work is therefore
a step forward in bridging the gap between observations and models, and
can lead to more direct interpretations of EUV observations in terms of
flux rope ejections. We plan to improve the model in future studies in
order to perform a more quantitative comparison. Movies associated
with Figs. 3, 9, and 10 are available in electronic form at <A
href="http://www.aanda.org/10.1051/0004-6361/201424019/olm">http://www.aanda.org</A>

---------------------------------------------------------
Title: The Sun's Magnetic Field During a Grand Minimum of Activity
Authors: Meyer, Karen; Mackay, Duncan
2014AAS...22411205M Altcode:
During a grand minimum of solar activity, no sunspots are observed
on the photosphere, but what might the Sun's magnetic field look
like? One possibility is that there would be no active regions or larger
scale magnetic activity. We have extended our photospheric model for
small-scale magnetic flux evolution to cover the full Sun. As an initial
study, we consider how the surface magnetic field of the Sun would look
if only smaller-scale magnetic features were allowed to emerge. We also
consider the resultant coronal and inner heliospheric magnetic fields,
and discuss potential consequences of such fields for Earth.

---------------------------------------------------------
Title: Dynamic properties of bright points in an active region
Authors: Keys, P. H.; Mathioudakis, M.; Jess, D. B.; Mackay, D. H.;
Keenan, F. P.
2014A&A...566A..99K Altcode: 2014arXiv1405.3923K
Context. Bright points (BPs) are small-scale, magnetic features
ubiquitous across the solar surface. Previously, we have observed and
noted their properties for quiet Sun regions. Here, we determine the
dynamic properties of BPs using simultaneous quiet Sun and active region
data. Aims: The aim of this paper is to compare the properties
of BPs in both active and quiet Sun regions and to determine any
difference in the dynamics and general properties of BPs as a result of
the varying magnetic activity within these two regions. Methods:
High spatial and temporal resolution G-band observations of active
region AR11372 were obtained with the Rapid Oscillations in the Solar
Atmosphere instrument at the Dunn Solar Telescope. Three subfields of
varying polarity and magnetic flux density were selected with the aid of
magnetograms obtained from the Helioseismic and Magnetic Imager on board
the Solar Dynamics Observatory. Bright points within these subfields
were subsequently tracked and analysed. Results: It is found that
BPs within active regions display attenuated velocity distributions
with an average horizontal velocity of ~0.6 km s-1,
compared to the quiet region which had an average velocity of 0.9 km
s-1. Active region BPs are also ~21% larger than quiet
region BPs and have longer average lifetimes (~132 s) than their
quiet region counterparts (88 s). No preferential flow directions are
observed within the active region subfields. The diffusion index (γ) is
estimated at ~1.2 for the three regions. Conclusions: We confirm
that the dynamic properties of BPs arise predominately from convective
motions. The presence of stronger field strengths within active regions
is the likely reason behind the varying properties observed. We believe
that larger amounts of magnetic flux will attenuate BP velocities by a
combination of restricting motion within the intergranular lanes and
by increasing the number of stagnation points produced by inhibited
convection. Larger BPs are found in regions of higher magnetic flux
density and we believe that lifetimes increase in active regions as
the magnetic flux stabilises the BPs.

---------------------------------------------------------
Title: Data-constrained Magnetofrcitional Simulation of a Flux Rope
Build-up in a Sigmoidal Active Region
Authors: Savcheva, Antonia Stefanova; Mackay, D.; Meyer, K.; Gibb,
G.; DeLuca, E.
2014shin.confE...3S Altcode:
We present a data-constrained magnetofrictional (MF) simulation of
the evolution over two days of the sigmoidal active region from 6-7
Dec 2007. The lower boundary condition is supplied by a series of
line-of-sight (LoS) namgnetograms from MDI, but for the first time the
initial condition is taken from a data-constrained non-linear force-free
(NLFFF) model of the active region early on Dec 6. The NLFFF model is
produced with the flux rope insertion method and is constrained by a LoS
magnetogram, filament path from STEREO, and coronal loops from XRT. The
initial condition is that of a sheared arcade and as time progresses
the photospheric evolution builds a flux rope, which becomes unstable
a few hours before the actual observed eruption. We show field lines
and current density distributions over time and compare them to XRT
images. We present the evolution of the free and potential energy and
relative helicity in the region. We compare our results to a previous
a simulation starting from a potential field as initial condition.

---------------------------------------------------------
Title: The solar cycle variation of topological structures in the
global solar corona
Authors: Platten, S. J.; Parnell, C. E.; Haynes, A. L.; Priest, E. R.;
Mackay, D. H.
2014A&A...565A..44P Altcode: 2014arXiv1406.5333P
Context. The complicated distribution of magnetic flux across the
solar photosphere results in a complex web of coronal magnetic field
structures. To understand this complexity, the magnetic skeleton
of the coronal field can be calculated. The skeleton highlights
the (separatrix) surfaces that divide the field into topologically
distinct regions, allowing open-field regions on the solar surface to be
located. Furthermore, separatrix surfaces and their intersections with
other separatrix surfaces (i.e., separators) are important likely energy
release sites. Aims: The aim of this paper is to investigate,
throughout the solar cycle, the nature of coronal magnetic-field
topologies that arise under the potential-field source-surface
approximation. In particular, we characterise the typical global fields
at solar maximum and minimum. Methods: Global magnetic fields are
extrapolated from observed Kitt Peak and SOLIS synoptic magnetograms,
from Carrington rotations 1645 to 2144, using the potential-field
source-surface model. This allows the variations in the coronal
skeleton to be studied over three solar cycles. Results: The
main building blocks which make up magnetic fields are identified and
classified according to the nature of their separatrix surfaces. The
magnetic skeleton reveals that, at solar maximum, the global coronal
field involves a multitude of topological structures at all latitudes
criss-crossing throughout the atmosphere. Many open-field regions
exist originating anywhere on the photosphere. At solar minimum, the
coronal topology is heavily influenced by the solar magnetic dipole. A
strong dipole results in a simple large-scale structure involving just
two large polar open-field regions, but, at short radial distances
between ± 60° latitude, the small-scale topology is complex. If the
solar magnetic dipole if weak, as in the recent minimum, then the
low-latitude quiet-sun magnetic fields may be globally significant
enough to create many disconnected open-field regions between ± 60°
latitude, in addition to the two polar open-field regions.

---------------------------------------------------------
Title: Global-scale Consequences of Magnetic-helicity Injection and
Condensation on the Sun
Authors: Mackay, Duncan H.; DeVore, C. Richard; Antiochos, Spiro K.
2014ApJ...784..164M Altcode:
In the recent paper of Antiochos, a new concept for the injection of
magnetic helicity into the solar corona by small-scale convective
motions and its condensation onto polarity inversion lines (PILs)
was developed. We investigate this concept through global simulations
of the Sun's photospheric and coronal magnetic fields, and compare the
results with the hemispheric pattern of solar filaments. Assuming that
the vorticity of the cells is predominantly counterclockwise/clockwise
in the northern/southern hemisphere, the convective motions inject
negative/positive helicity into each hemisphere. The simulations show
that: (1) on a north-south oriented PIL, both differential rotation
and convective motions inject the same sign of helicity, which matches
that required to reproduce the hemispheric pattern of filaments. (2) On
a high-latitude east-west oriented polar crown or subpolar crown PIL,
the vorticity of the cells has to be approximately 2-3 times greater
than the local differential-rotation gradient in order to overcome the
incorrect sign of helicity injection from differential rotation. (3)
In the declining phase of the cycle, as a bipole interacts with the
polar field, in some cases, helicity condensation can reverse the
effect of differential rotation along the east-west lead arm but not
in all cases. The results show that this newly developed concept of
magnetic helicity injection and condensation, in conjunction with
the mechanisms used in Yeates et al., is a viable explanation for the
hemispheric pattern of filaments. Future observational studies should
focus on examining the vorticity component within convective motions
to determine both its magnitude and latitudinal variation relative to
the differential-rotation gradient on the Sun.

---------------------------------------------------------
Title: Simulating the Formation of a Sigmoidal Flux Rope in AR10977
from SOHO/MDI Magnetograms
Authors: Gibb, G. P. S.; Mackay, D. H.; Green, L. M.; Meyer, K. A.
2014ApJ...782...71G Altcode:
The modeling technique of Mackay et al. is applied to simulate the
coronal magnetic field of NOAA active region AR10977 over a seven day
period (2007 December 2-10). The simulation is driven with a sequence
of line-of-sight component magnetograms from SOHO/MDI and evolves
the coronal magnetic field though a continuous series of non-linear
force-free states. Upon comparison with Hinode/XRT observations, results
show that the simulation reproduces many features of the active region's
evolution. In particular, it describes the formation of a flux rope
across the polarity inversion line during flux cancellation. The flux
rope forms at the same location as an observed X-ray sigmoid. After five
days of evolution, the free magnetic energy contained within the flux
rope was found to be 3.9 × 1030 erg. This value is more
than sufficient to account for the B1.4 GOES flare observed from the
active region on 2007 December 7. At the time of the observed eruption,
the flux rope was found to contain 20% of the active region flux. We
conclude that the modeling technique proposed in Mackay et al.—which
directly uses observed magnetograms to energize the coronal field—is
a viable method to simulate the evolution of the coronal magnetic field.

---------------------------------------------------------
Title: Where Do Solar Filaments Form?
Authors: Mackay, Duncan H.; Gaizauskas, Victor; Yeates, Anthony R.
2014IAUS..300..445M Altcode:
In the present study, we consider where large, stable solar filaments
form relative to underlying magnetic polarities. We find that 92% of
all large stable filaments form in magnetic configurations involving
the interaction of two or more bipoles. Only 7% form above the Polarity
Inversion Line (PIL) of a single bipole. This indicates that a key
element in the formation of large-scale stable filaments is the
convergence of magnetic flux, resulting in either flux cancellation
or coronal reconnection.

---------------------------------------------------------
Title: Magnetohydrodynamic study on the effect of the gravity
stratification on flux rope ejections
Authors: Pagano, Paolo; Mackay, Duncan H.; Poedts, Stefaan
2014IAUS..300..197P Altcode:
Coronal Mass Ejections (CMEs) are one of the most violent phenomena
found on the Sun. One model to explain their occurrence is the flux rope
ejection model where these magnetic structures firt form in the solar
corona then are ejected to produce a CME. We run simulations coupling
two models. The Global Non-Linear Force-Free Field (GNLFFF) evolution
model to follow the quasi-static formation of a flux rope and MHD
simulations for the production of a CME through the loss of equilibrium
and ejection of this flux rope in presence of solar gravity and density
stratification. Our realistic multi-beta simulations describe the CME
following the flux rope ejection and highlight the decisive role played
by the gravity stratification on the CME propagation speed.

---------------------------------------------------------
Title: Local and global coronal magnetic-field extrapolation
Authors: Yeates, Anthony; Mackay, Duncan
2014cosp...40E3714Y Altcode:
Extrapolation relies on assumptions, and coronal magnetic field
modelling based on photospheric magnetogram data is no exception. A
current research problem is how best to go beyond the current-free
assumption of traditional potential-field extrapolations. I will
argue that realistic modelling requires not just the input of a
photospheric magnetogram at a single time, but rather a time history
of such data. As evidence for this viewpoint, I will present results
from a global model that takes into account this time-dependence to
approximate a continuously evolving sequence of nonlinear force-free
fields. Not only does magnetic helicity accumulate in magnetic flux
ropes, which can eventually lose equilibrium and "erupt", but we find
that the local magnetic direction in these ropes can be sensitive to
the time history over multiple years. This allows for an observational
test of the model using observations of Halpha filaments.

---------------------------------------------------------
Title: Explaining the Hemispheric Pattern of Filament Chirality
Authors: Mackay, Duncan H.; Yeates, Anthony R.
2014IAUS..300..172M Altcode:
Solar filaments are known to exhibit a hemispheric pattern in
their chirality, where dextral/sinistral filaments dominate in
the northern/southern hemisphere. We show that this pattern may be
explained through data driven 3D global magnetic field simulations of
the Sun's large-scale magnetic field. Through a detailed comparison with
109 filaments over a 6 month period, the model correctly reproduces
the filament chirality and helicity with a 96% agreement. The data
driven simulation is extended to run over a full solar cycle, where
predictions are made for the spatial and temporal dependence of the
hemispheric pattern over the solar cycle.

---------------------------------------------------------
Title: The Influence of the Magnetic Field on Running Penumbral
Waves in the Solar Chromosphere
Authors: Jess, D. B.; Reznikova, V. E.; Van Doorsselaere, T.; Keys,
P. H.; Mackay, D. H.
2013ApJ...779..168J Altcode: 2013arXiv1310.7939J
We use images of high spatial and temporal resolution,
obtained using both ground- and space-based instrumentation, to
investigate the role magnetic field inclination angles play in
the propagation characteristics of running penumbral waves in the
solar chromosphere. Analysis of a near-circular sunspot, close to
the center of the solar disk, reveals a smooth rise in oscillatory
period as a function of distance from the umbral barycenter. However,
in one directional quadrant, corresponding to the north direction,
a pronounced kink in the period-distance diagram is found. Utilizing a
combination of the inversion of magnetic Stokes vectors and force-free
field extrapolations, we attribute this behavior to the cut-off
frequency imposed by the magnetic field geometry in this location. A
rapid, localized inclination of the magnetic field lines in the north
direction results in a faster increase in the dominant periodicity
due to an accelerated reduction in the cut-off frequency. For the
first time, we reveal how the spatial distribution of dominant wave
periods, obtained with one of the highest resolution solar instruments
currently available, directly reflects the magnetic geometry of the
underlying sunspot, thus opening up a wealth of possibilities in future
magnetohydrodynamic seismology studies. In addition, the intrinsic
relationships we find between the underlying magnetic field geometries
connecting the photosphere to the chromosphere, and the characteristics
of running penumbral waves observed in the upper chromosphere, directly
supports the interpretation that running penumbral wave phenomena are
the chromospheric signature of upwardly propagating magneto-acoustic
waves generated in the photosphere.

---------------------------------------------------------
Title: Effect of gravitational stratification on the propagation of
a CME
Authors: Pagano, P.; Mackay, D. H.; Poedts, S.
2013A&A...560A..38P Altcode: 2013arXiv1310.6960P
Context. Coronal mass ejections (CMEs) are the most violent phenomenon
found on the Sun. One model that explains their occurrence is the
flux rope ejection model. A magnetic flux rope is ejected from the
solar corona and reaches the interplanetary space where it interacts
with the pre-existing magnetic fields and plasma. Both gravity and
the stratification of the corona affect the early evolution of the
flux rope. Aims: Our aim is to study the role of gravitational
stratification on the propagation of CMEs. In particular, we assess how
it influences the speed and shape of CMEs and under what conditions
the flux rope ejection becomes a CME or when it is quenched. 
Methods: We ran a set of MHD simulations that adopt an eruptive
initial magnetic configuration that has already been shown to be
suitable for a flux rope ejection. We varied the temperature of
the backgroud corona and the intensity of the initial magnetic
field to tune the gravitational stratification and the amount of
ejected magnetic flux. We used an automatic technique to track the
expansion and the propagation of the magnetic flux rope in the MHD
simulations. From the analysis of the parameter space, we evaluate the
role of gravitational stratification on the CME speed and expansion. Results: Our study shows that gravitational stratification plays a
significant role in determining whether the flux rope ejection will
turn into a full CME or whether the magnetic flux rope will stop in
the corona. The CME speed is affected by the background corona where
it travels faster when the corona is colder and when the initial
magnetic field is more intense. The fastest CME we reproduce in
our parameter space travels at ~850 km s-1. Moreover,
the background gravitational stratification plays a role in the side
expansion of the CME, and we find that when the background temperature
is higher, the resulting shape of the CME is flattened more. 
Conclusions: Our study shows that although the initiation mechanisms
of the CME are purely magnetic, the background coronal plasma plays
a key role in the CME propagation, and full MHD models should be
applied when one focuses especially on the production of a CME from
a flux rope ejection. Movies are available in electronic form at
<A href="http://www.aanda.org">http://www.aanda.org</A>

---------------------------------------------------------
Title: Solar Magnetic Carpet III: Coronal Modelling of Synthetic
Magnetograms
Authors: Meyer, K. A.; Mackay, D. H.; van Ballegooijen, A. A.; Parnell,
C. E.
2013SoPh..286..357M Altcode: 2013arXiv1303.1342M
This article is the third in a series working towards the construction
of a realistic, evolving, non-linear force-free coronal-field model
for the solar magnetic carpet. Here, we present preliminary results of
3D time-dependent simulations of the small-scale coronal field of the
magnetic carpet. Four simulations are considered, each with the same
evolving photospheric boundary condition: a 48-hour time series of
synthetic magnetograms produced from the model of Meyer et al. (Solar
Phys.272, 29, 2011). Three simulations include a uniform, overlying
coronal magnetic field of differing strength, the fourth simulation
includes no overlying field. The build-up, storage, and dissipation of
magnetic energy within the simulations is studied. In particular, we
study their dependence upon the evolution of the photospheric magnetic
field and the strength of the overlying coronal field. We also consider
where energy is stored and dissipated within the coronal field. The
free magnetic energy built up is found to be more than sufficient to
power small-scale, transient phenomena such as nanoflares and X-ray
bright points, with the bulk of the free energy found to be stored low
down, between 0.5 - 0.8 Mm. The energy dissipated is currently found
to be too small to account for the heating of the entire quiet-Sun
corona. However, the form and location of energy-dissipation regions
qualitatively agree with what is observed on small scales on the
Sun. Future MHD modelling using the same synthetic magnetograms may
lead to a higher energy release.

---------------------------------------------------------
Title: A Non-Linear Force-Free Field Model for the Solar Magnetic
Carpet
Authors: Meyer, Karen; Mackay, D.; van Ballegooijen, A.; Parnell, C.
2013SPD....4430201M Altcode:
The magnetic carpet is defined to be the small-scale photospheric
magnetic field of the quiet-Sun. Recent high resolution, high cadence
observations have shown that although small-scale, the magnetic carpet
is far from 'quiet', it is continually evolving in a complex and
dynamic manner. I will present a two-component model for the dynamic
evolution of the Sun's magnetic carpet. The first component is a 2D
model for the photospheric evolution of the small-scale solar magnetic
field, that reproduces many observed parameters. The basic evolution of
magnetic elements within the model is governed by a supergranular flow
profile. In addition, magnetic elements may evolve through the processes
of emergence, cancellation, coalescence and fragmentation. The synthetic
magnetograms produced by the 2D model are then applied as photospheric
boundary data to drive the continuous evolution of a 3D non-linear
force-free coronal field. We studied the resultant complex, small-scale
coronal magnetic field, in particular the energetics of the field.

---------------------------------------------------------
Title: Filament Chirality over an Entire Cycle Determined with an
Automated Detection Module -- a Neat Surprise!
Authors: Martens, Petrus C.; Yeates, A. R.; Mackay, D.; Pillai, K. G.
2013SPD....4410104M Altcode:
Using metadata produced by automated solar feature detection modules
developed for SDO (Martens et al. 2012) we have discovered some trends
in filament chirality and filament-sigmoid relations that are new
and in part contradict the current consensus. Automated detection
of solar features has the advantage over manual detection of having
the detection criteria applied consistently, and in being able to
deal with enormous amounts of data, like the 1 Terabyte per day that
SDO produces. Here we use the filament detection module developed by
Bernasconi, which has metadata from 2000 on, and the sigmoid sniffer,
which has been producing metadata from AIA 94 A images since October
2011. The most interesting result we find is that the hemispheric
chirality preference for filaments (dextral in the north, and v.v.),
studied in detail for a three year period by Pevtsov et al. (2003)
seems to disappear during parts of the decline of cycle 23 and during
the extended solar minimum that followed. Moreover the hemispheric
chirality rule seems to be much less pronounced during the onset
of cycle 24. For sigmoids we find the expected correlation between
chirality and handedness (S or Z) shape but not as strong as expected.

---------------------------------------------------------
Title: Puzzling nature of the fine structure of quiescent prominences
and filaments
Authors: Gunár, Stanislav; Heinzel, Petr; Anzer, Ulrich; Mackay,
Duncan H.
2013JPhCS.440a2035G Altcode:
Even after more than 160 years of observations and modelling of solar
prominences their true nature contains many open questions. In this
work we argue that current 2D prominence fine structure models can
help us to understand the puzzling connection between quasi-vertical
fine structures often seen in quiescent prominences observed on the
solar limb and horizontally aligned dark fibrils representing the fine
structures of prominences observed in absorption against the solar disk
(filaments).

---------------------------------------------------------
Title: Magnetohydrodynamic simulations of the ejection of a magnetic
flux rope
Authors: Pagano, P.; Mackay, D. H.; Poedts, S.
2013A&A...554A..77P Altcode:
Context. Coronal mass ejections (CME's) are one of the most violent
phenomena found on the Sun. One model to explain their occurrence is
the flux rope ejection model. In this model, magnetic flux ropes form
slowly over time periods of days to weeks. They then lose equilibrium
and are ejected from the solar corona over a few hours. The contrasting
time scales of formation and ejection pose a serious problem for
numerical simulations. Aims: We simulate the whole life span
of a flux rope from slow formation to rapid ejection and investigate
whether magnetic flux ropes formed from a continuous magnetic field
distribution, during a quasi-static evolution, can erupt to produce a
CME. Methods: To model the full life span of magnetic flux ropes
we couple two models. The global non-linear force-free field (GNLFFF)
evolution model is used to follow the quasi-static formation of a flux
rope. The MHD code ARMVAC is used to simulate the production of a CME
through the loss of equilibrium and ejection of this flux rope. Results: We show that the two distinct models may be successfully
coupled and that the flux rope is ejected out of our simulation box,
where the outer boundary is placed at 2.5 R⊙. The plasma
expelled during the flux rope ejection travels outward at a speed of
100 km s-1, which is consistent with the observed speed of
CMEs in the low corona. Conclusions: Our work shows that flux
ropes formed in the GNLFFF can lead to the ejection of a mass loaded
magnetic flux rope in full MHD simulations. Coupling the two distinct
models opens up a new avenue of research to investigate phenomena where
different phases of their evolution occur on drastically different
time scales. Movies are available in electronic form at <A
href="http://www.aanda.org">http://www.aanda.org</A>

---------------------------------------------------------
Title: The Storage and Dissipation of Magnetic Energy in the Quiet
Sun Corona Determined from SDO/HMI Magnetograms
Authors: Meyer, K. A.; Sabol, J.; Mackay, D. H.; van Ballegooijen,
A. A.
2013ApJ...770L..18M Altcode:
In recent years, higher cadence, higher resolution observations
have revealed the quiet-Sun photosphere to be complex and rapidly
evolving. Since magnetic fields anchored in the photosphere extend
up into the solar corona, it is expected that the small-scale
coronal magnetic field exhibits similar complexity. For the first
time, the quiet-Sun coronal magnetic field is continuously evolved
through a series of non-potential, quasi-static equilibria, deduced
from magnetograms observed by the Helioseismic and Magnetic Imager
on board the Solar Dynamics Observatory, where the photospheric
boundary condition which drives the coronal evolution exactly
reproduces the observed magnetograms. The build-up, storage, and
dissipation of magnetic energy within the simulations is studied. We
find that the free magnetic energy built up and stored within the
field is sufficient to explain small-scale, impulsive events such
as nanoflares. On comparing with coronal images of the same region,
the energy storage and dissipation visually reproduces many of the
observed features. The results indicate that the complex small-scale
magnetic evolution of a large number of magnetic features is a key
element in explaining the nature of the solar corona.

---------------------------------------------------------
Title: Non-linear force-free magnetic dip models of quiescent
prominence fine structures
Authors: Gunár, S.; Mackay, D. H.; Anzer, U.; Heinzel, P.
2013A&A...551A...3G Altcode:
 Aims: We use 3D non-linear force-free magnetic field modeling
of prominence/filament magnetic fields to develop the first 2D models
of individual prominence fine structures based on the 3D configuration
of the magnetic field of the whole prominence. Methods: We use
an iterative technique to fill the magnetic dips produced by the 3D
modeling with realistic prominence plasma in hydrostatic equilibrium
and with a temperature structure that contains the prominence-corona
transition region. With this well-defined plasma structure the radiative
transfer can be treated in detail in 2D and the resulting synthetic
emission can be compared with prominence/filament observations. Results: Newly developed non-linear force-free magnetic dip models
are able to produce synthetic hydrogen Lyman spectra in a qualitative
agreement with a range of quiescent prominence observations. Moreover,
the plasma structure of these models agrees with the gravity induced
prominence fine structure models which have already been shown to
produce synthetic spectra in good qualitative agreement with several
observed prominences. Conclusions: We describe in detail the
iterative technique which can be used to produce realistic plasma
models of prominence fine structures located in prominence magnetic
field configurations containing dips, obtained using any kind of
magnetic field modeling.

---------------------------------------------------------
Title: SWIFF: Space weather integrated forecasting framework
Authors: Lapenta, Giovanni; Pierrard, Viviane; Keppens, Rony; Markidis,
Stefano; Poedts, Stefaan; Šebek, Ondřej; Trávníček, Pavel M.;
Henri, Pierre; Califano, Francesco; Pegoraro, Francesco; Faganello,
Matteo; Olshevsky, Vyacheslav; Restante, Anna Lisa; Nordlund, Åke;
Trier Frederiksen, Jacob; Mackay, Duncan H.; Parnell, Clare E.;
Bemporad, Alessandro; Susino, Roberto; Borremans, Kris
2013JSWSC...3A..05L Altcode:
SWIFF is a project funded by the Seventh Framework Programme of the
European Commission to study the mathematical-physics models that
form the basis for space weather forecasting. The phenomena of space
weather span a tremendous scale of densities and temperature with
scales ranging 10 orders of magnitude in space and time. Additionally
even in local regions there are concurrent processes developing at
the electron, ion and global scales strongly interacting with each
other. The fundamental challenge in modelling space weather is the
need to address multiple physics and multiple scales. Here we present
our approach to take existing expertise in fluid and kinetic models to
produce an integrated mathematical approach and software infrastructure
that allows fluid and kinetic processes to be modelled together. SWIFF
aims also at using this new infrastructure to model specific coupled
processes at the Solar Corona, in the interplanetary space and in the
interaction at the Earth magnetosphere.

---------------------------------------------------------
Title: The Sun's Global Photospheric and Coronal Magnetic Fields:
Observations and Models
Authors: Mackay, Duncan H.; Yeates, Anthony R.
2012LRSP....9....6M Altcode: 2012arXiv1211.6545M
In this review, our present day understanding of the Sun's global
photospheric and coronal magnetic fields is discussed from both
observational and theoretical viewpoints. Firstly, the large-scale
properties of photospheric magnetic fields are described, along
with recent advances in photospheric magnetic flux transport
models. Following this, the wide variety of theoretical models used
to simulate global coronal magnetic fields are described. From this,
the combined application of both magnetic flux transport simulations
and coronal modeling techniques to describe the phenomena of coronal
holes, the Sun's open magnetic flux and the hemispheric pattern of solar
filaments is discussed. Finally, recent advances in non-eruptive global
MHD models are described. While the review focuses mainly on solar
magnetic fields, recent advances in measuring and modeling stellar
magnetic fields are described where appropriate. In the final section
key areas of future research are identified.

---------------------------------------------------------
Title: Chirality of High-latitude Filaments over Solar Cycle 23
Authors: Yeates, A. R.; Mackay, D. H.
2012ApJ...753L..34Y Altcode: 2012arXiv1206.2327Y
A non-potential quasi-static evolution model coupling the Sun's
photospheric and coronal magnetic fields is applied to the problem
of filament chirality at high latitudes. For the first time, we
run a continuous 15 year simulation, using bipolar active regions
determined from US National Solar Observatory, Kitt Peak magnetograms
between 1996 and 2011. Using this simulation, we are able to address
the outstanding question of whether magnetic helicity transport from
active latitudes can overcome the effect of differential rotation at
higher latitudes. Acting alone, differential rotation would produce
high-latitude filaments with opposite chirality to the majority type
in each hemisphere. We find that differential rotation can indeed
lead to opposite chirality at high latitudes, but only for around 5
years of the solar cycle following the polar field reversal. At other
times, including the rising phase, transport of magnetic helicity from
lower latitudes overcomes the effect of in situ differential rotation,
producing the majority chirality even on the polar crowns at polar field
reversal. These simulation predictions will allow for future testing of
the non-potential coronal model. The results indicate the importance
of long-term memory and helicity transport from active latitudes when
modeling the structure and topology of the coronal magnetic field at
higher latitudes.

---------------------------------------------------------
Title: The Sun's global magnetic field
Authors: Mackay, D. H.
2012RSPTA.370.3151M Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Solar Magnetic Carpet II: Coronal Interactions of Small-Scale
Magnetic Fields
Authors: Meyer, K. A.; Mackay, D. H.; van Ballegooijen, A. A.
2012SoPh..278..149M Altcode: 2012arXiv1211.3924M
This paper is the second in a series of studies working towards
constructing a realistic, evolving, non-potential coronal model for
the solar magnetic carpet. In the present study, the interaction of
two magnetic elements is considered. Our objectives are to study
magnetic energy build-up, storage and dissipation as a result of
emergence, cancellation, and flyby of these magnetic elements. In
the future these interactions will be the basic building blocks of
more complicated simulations involving hundreds of elements. Each
interaction is simulated in the presence of an overlying uniform
magnetic field, which lies at various orientations with respect to the
evolving magnetic elements. For these three small-scale interactions,
the free energy stored in the field at the end of the simulation ranges
from 0.2 - 2.1×1026 ergs, whilst the total energy dissipated
ranges from 1.3 - 6.3×1026 ergs. For all cases, a stronger
overlying field results in higher energy storage and dissipation. For
the cancellation and emergence simulations, motion perpendicular
to the overlying field results in the highest values. For the flyby
simulations, motion parallel to the overlying field gives the highest
values. In all cases, the free energy built up is sufficient to explain
small-scale phenomena such as X-ray bright points or nanoflares. In
addition, if scaled for the correct number of magnetic elements for
the volume considered, the energy continually dissipated provides a
significant fraction of the quiet Sun coronal heating budget.

---------------------------------------------------------
Title: Using Kepler transit observations to measure stellar spot
belt migration rates
Authors: Llama, J.; Jardine, M.; Mackay, D. H.; Fares, R.
2012MNRAS.422L..72L Altcode: 2012arXiv1202.3785L; 2012MNRAS.tmpL.421L
Planetary transits provide a unique opportunity to investigate the
surface distributions of star spots. Our aim is to determine if, with
continuous observation (such as the data that will be provided by the
Kepler mission), we can in addition measure the rate of drift of the
spot belts. We begin by simulating magnetic cycles suitable for the Sun
and more active stars, incorporating both flux emergence and surface
transport. This provides the radial magnetic field distribution on the
stellar surface as a function of time. We then model the transit of
a planet whose orbital axis is misaligned with the stellar rotation
axis. Such a planet could occult spots at a range of latitudes. This
allows us to complete the forward modelling of the shape of the transit
light curve. We then attempt the inverse problem of recovering spot
locations from the transit alone. From this we determine if transit
light curves can be used to measure spot belt locations as a function of
time. We find that for low-activity stars such as the Sun, the 3.5-year
Kepler window is insufficient to determine this drift rate. For more
active stars, it may be difficult to distinguish subtle differences
in the nature of flux emergence, such as the degree of overlap of the
'butterfly wings'. The rate and direction of drift of the spot belts
can however be determined for these stars. This would provide a critical
test of dynamo theory.

---------------------------------------------------------
Title: The structure and evolution of global solar magnetic fields
Authors: Mackay, Duncan H.
2011IAUS..273..290M Altcode:
This review will discuss both observational and theoretical aspects
of the Sun's global magnetic field. First recent observations will be
described, along with the main physical processes leading to the time
evolution and structure of the global field. Following this, recent
theoretical models of both the global surface and coronal magnetic field
will be presented. The application of these models to the structure of
the corona, formation of solar filaments, the onset of CMEs and finally
the origin and variation of the Sun's open flux will be discussed.

---------------------------------------------------------
Title: Solar Magnetic Carpet I: Simulation of Synthetic Magnetograms
Authors: Meyer, K. A.; Mackay, D. H.; van Ballegooijen, A. A.; Parnell,
C. E.
2011SoPh..272...29M Altcode: 2011SoPh..tmp..294M; 2011SoPh..tmp..198M; 2011SoPh..tmp..319M;
2011SoPh..tmp..267M; 2011arXiv1108.1080M
This paper describes a new 2D model for the photospheric evolution
of the magnetic carpet. It is the first in a series of papers
working towards constructing a realistic 3D non-potential model
for the interaction of small-scale solar magnetic fields. In the
model, the basic evolution of the magnetic elements is governed by a
supergranular flow profile. In addition, magnetic elements may evolve
through the processes of emergence, cancellation, coalescence and
fragmentation. Model parameters for the emergence of bipoles are based
upon the results of observational studies. Using this model, several
simulations are considered, where the range of flux with which bipoles
may emerge is varied. In all cases the model quickly reaches a steady
state where the rates of emergence and cancellation balance. Analysis
of the resulting magnetic field shows that we reproduce observed
quantities such as the flux distribution, mean field, cancellation
rates, photospheric recycle time and a magnetic network. As expected,
the simulation matches observations more closely when a larger, and
consequently more realistic, range of emerging flux values is allowed
(4×1016 - 1019 Mx). The model best reproduces
the current observed properties of the magnetic carpet when we take
the minimum absolute flux for emerging bipoles to be 4×1016
Mx. In future, this 2D model will be used as an evolving photospheric
boundary condition for 3D non-potential modeling.

---------------------------------------------------------
Title: Magnetic helicity evolution inside a hexagonal convective cell
Authors: Smyrli, Aimilia; Mackay, Duncan; Zuccarello, Francesca
2011IAUS..274..192S Altcode:
Magnetic helicity has received considerable attention in the area of
fluid dynamics. Recently, this quantity is attracting the interest
of solar physicists and much research has been carried out related
to magnetic helicity generation and transport through different solar
layers, starting from the interior and the convection zone, towards the
photosphere, the corona and finally into the heliosphere. Taking into
account the global importance of supergranular cells in convection
theories, we study the motion of magnetic features into such a
geometrical element simplified as hexagonal cell and we analyse the
results in terms of the accumulated magnetic helicity. We compute the
emergence of a bipole inside the hexagonal cell and its motion from
the centre of the cell towards its sides and its vertices, where the
magnetic elements are considered to be sinking down. Multiple bipoles
are also considered and phenomena such as cancellation, coalescence and
fragmentation are also investigated. We find that the most important
process for the accumulation of magnetic helicity is the shear motion
between the polarities. The magnetic helicity accumulation changes
its trend when one polarity reaches the side of the hexagon, and later
the vertex. It has zero value when there is no shear motion inside the
hexagonal cell, and it is constant when there is no shear between the
two polarities during their motion along the cell sides.

---------------------------------------------------------
Title: Modeling the Dispersal of an Active Region: Quantifying Energy
Input into the Corona
Authors: Mackay, Duncan H.; Green, L. M.; van Ballegooijen, Aad
2011ApJ...729...97M Altcode: 2011arXiv1102.5296M
In this paper, a new technique for modeling nonlinear force-free fields
directly from line-of-sight magnetogram observations is presented. The
technique uses sequences of magnetograms directly as lower boundary
conditions to drive the evolution of coronal magnetic fields between
successive force-free equilibria over long periods of time. It is
illustrated by applying it to SOHO: MDI observations of a decaying
active region, NOAA AR 8005. The active region is modeled during a
four-day period around its central meridian passage. Over this time,
the dispersal of the active region is dominated by random motions
due to small-scale convective cells. Through studying the buildup of
magnetic energy in the model, it is found that such small-scale motions
may inject anywhere from (2.5-3) × 1025 erg s-1
of free magnetic energy into the coronal field. Most of this energy
is stored within the center of the active region in the low corona,
below 30 Mm. After four days, the buildup of free energy is 10%
that of the corresponding potential field. This energy buildup is
sufficient to explain the radiative losses at coronal temperatures
within the active region. Small-scale convective motions therefore
play an integral part in the energy balance of the corona. This new
technique has wide ranging applications with the new high-resolution,
high-cadence observations from the SDO:HMI and SDO:AIA instruments.

---------------------------------------------------------
Title: Trend of photospheric magnetic helicity flux in active regions
generating halo coronal mass ejections
Authors: Smyrli, A.; Zuccarello, F.; Romano, P.; Zuccarello, F. P.;
Guglielmino, S. L.; Spadaro, D.; Hood, A. W.; Mackay, D.
2010A&A...521A..56S Altcode:
Context. Coronal mass ejections (CMEs) are very energetic events (~
1032 erg) initiated in the solar atmosphere, resulting
in the expulsion of magnetized plasma clouds that propagate into
interplanetary space. It has been proposed that CMEs can play an
important role in shedding magnetic helicity, avoiding its endless
accumulation in the corona. Aims: The aim of this work is to
investigate the behavior of magnetic helicity accumulation in sites
where the initiation of CMEs occurred to determine whether and how
changes in magnetic helicity accumulation are temporally correlated
with CME occurrence. Methods: We used MDI/SOHO line-of-sight
magnetograms to calculate magnetic flux evolution and magnetic
helicity injection in 10 active regions that gave rise to halo CMEs
observed during the period 2000 February to 2003 June. Results:
The magnetic helicity injection does not have a unique trend in the
events analyzed: in 40% of the cases it shows a large sudden and abrupt
change that is temporally correlated with a CME occurrence, while in
the other cases it shows a steady monotonic trend, with a slight change
in magnetic helicity at CME occurrence. Conclusions: The results
obtained from the sample of events that we have analyzed indicate that
major changes in magnetic helicity flux are observed in active regions
characterized by emergence of new magnetic flux and/or generating halo
CMEs associated with X-class flares or filament eruptions. In some of
the analyzed cases the changes in magnetic helicity flux follow the
CME events and can be attributed to a process of restoring a torque
balance between the subphotospheric and the coronal domain of the
flux tubes. Appendix is only available in electronic form at <A
href="http://www.aanda.org">http://www.aanda.org</A>

---------------------------------------------------------
Title: A nonpotential model for the Sun's open magnetic flux
Authors: Yeates, A. R.; Mackay, D. H.; van Ballegooijen, A. A.;
Constable, J. A.
2010JGRA..115.9112Y Altcode: 2010JGRA..11509112Y; 2010arXiv1006.4011Y
Measurements of the interplanetary magnetic field (IMF) over several
solar cycles do not agree with computed values of open magnetic flux
from potential field extrapolations. The discrepancy becomes greater
around solar maximum in each cycle when the IMF can be twice as strong
as predicted by the potential field model. Here we demonstrate that this
discrepancy may be resolved by allowing for electric currents in the
low corona (below 2.5R$\odot$). We present a quasi-static
numerical model of the large-scale coronal magnetic evolution, which
systematically produces these currents through flux emergence and
shearing by surface motions. The open flux is increased by 75%-85%
at solar maximum, but only 25% at solar minimum, bringing it in line
with estimates from IMF measurements. The additional open flux in the
nonpotential model arises through inflation of the magnetic field by
electric currents, with superimposed fluctuations due to coronal mass
ejections. The latter are modeled by the self-consistent ejection of
twisted magnetic flux ropes.

---------------------------------------------------------
Title: Modelling stellar coronae from surface magnetograms: the role
of missing magnetic flux
Authors: Johnstone, C.; Jardine, M.; Mackay, D. H.
2010MNRAS.404..101J Altcode: 2010arXiv1001.2526J; 2010MNRAS.tmp..379J
Recent advances in spectropolarimetry have allowed the reconstruction of
stellar coronal magnetic fields. This uses Zeeman-Doppler magnetograms
(ZDI) of the surface magnetic field as a lower boundary condition. The
ZDI maps, however, suffer from the absence of information about the
magnetic field over regions of the surface due to the presence of dark
starspots and portions of the surface out of view due to a tilt in
the rotation axis. They also suffer from finite resolution which leads
to small-scale field structures being neglected. This paper explores
the effects of this loss of information on the extrapolated coronal
fields. For this, we use simulated stellar surface magnetic maps for
two hypothetical stars. Using the potential field approximation,
the coronal fields and emission measures are calculated. This is
repeated for the cases of missing information due to, (i) starspots,
(ii) a large area of the stellar surface out of view and (iii) a
finite resolution. The largest effect on the magnetic field structure
arises when a significant portion of the stellar surface remains out
of view. This changes the nature of the field lines that connect to
this obscured hemisphere. None the less, the field structure in the
visible hemisphere is reliably reproduced. Thus, the calculation of the
locations and surface filling factors of accretion funnels is reasonably
well reproduced for the observed hemisphere. The decrease with height of
the magnetic pressure, which is important in calculating disc truncation
radii for accreting stars, is also largely unaffected in the equatorial
plane. The fraction of surface flux that is open and therefore able to
supply angular momentum loss in a wind, however, is often overestimated
in the presence of missing flux. The magnitude and rotational modulation
of the calculated emission measures is consistently decreased by the
loss of magnetic flux in dark starspots. For very inactive stars,
this may make it impossible to recover a magnetic cycle in the coronal
emission. Finite resolution has little effect on those quantities,
such as the emission measure and the average coronal electron density,
that can currently be observed.

---------------------------------------------------------
Title: Physics of Solar Prominences: II—Magnetic Structure and
Dynamics
Authors: Mackay, D. H.; Karpen, J. T.; Ballester, J. L.; Schmieder,
B.; Aulanier, G.
2010SSRv..151..333M Altcode: 2010SSRv..tmp...32M; 2010arXiv1001.1635M
Observations and models of solar prominences are reviewed. We focus on
non-eruptive prominences, and describe recent progress in four areas of
prominence research: (1) magnetic structure deduced from observations
and models, (2) the dynamics of prominence plasmas (formation and
flows), (3) Magneto-hydrodynamic (MHD) waves in prominences and (4)
the formation and large-scale patterns of the filament channels in
which prominences are located. Finally, several outstanding issues in
prominence research are discussed, along with observations and models
required to resolve them.

---------------------------------------------------------
Title: Comparison of a Global Magnetic Evolution Model with
Observations of Coronal Mass Ejections
Authors: Yeates, A. R.; Attrill, G. D. R.; Nandy, Dibyendu; Mackay,
D. H.; Martens, P. C. H.; van Ballegooijen, A. A.
2010ApJ...709.1238Y Altcode: 2009arXiv0912.3347Y
The relative importance of different initiation mechanisms for coronal
mass ejections (CMEs) on the Sun is uncertain. One possible mechanism is
the loss of equilibrium of coronal magnetic flux ropes formed gradually
by large-scale surface motions. In this paper, the locations of flux
rope ejections in a recently developed quasi-static global evolution
model are compared with observed CME source locations over a 4.5 month
period in 1999. Using extreme ultraviolet data, the low-coronal source
locations are determined unambiguously for 98 out of 330 CMEs. An
alternative method of determining the source locations using recorded
Hα events was found to be too inaccurate. Despite the incomplete
observations, positive correlation (with coefficient up to 0.49) is
found between the distributions of observed and simulated ejections,
but only when binned into periods of 1 month or longer. This binning
timescale corresponds to the time interval at which magnetogram data are
assimilated into the coronal simulations, and the correlation arises
primarily from the large-scale surface magnetic field distribution;
only a weak dependence is found on the magnetic helicity imparted to the
emerging active regions. The simulations are limited in two main ways:
they produce fewer ejections, and they do not reproduce the strong
clustering of observed CME sources into active regions. Due to this
clustering, the horizontal gradient of radial photospheric magnetic
field is better correlated with the observed CME source distribution
(coefficient 0.67). Our results suggest that while the gradual formation
of magnetic flux ropes over weeks can account for many observed CMEs,
especially at higher latitudes, there exists a second class of CMEs (at
least half) for which dynamic active region flux emergence on shorter
timescales must be the dominant factor. Improving our understanding
of CME initiation in future will require both more comprehensive
observations of CME source regions and more detailed magnetic field
simulations.

---------------------------------------------------------
Title: Trend of photospheric helicity flux in active regions
generating halo CMEs
Authors: Smyrli, Aimilia; Zuccarello, Francesco; Zuccarello, Francesca;
Romano, Paolo; Guglielmino, Salvatore Luigi; Spadaro, Daniele; Hood,
Alan; Mackay, Duncan
2010cosp...38.1860S Altcode: 2010cosp.meet.1860S
Coronal Mass Ejections (CMEs) are very energetic events initiated
in the solar atmosphere, re-sulting in the expulsion of magnetized
plasma clouds that propagate into interplanetary space. It has been
proposed that CMEs can play an important role in shedding magnetic
helicity, avoiding its endless accumulation in the corona. We therefore
investigated the behavior of magnetic helicity accumulation in sites
where the initiation of CMEs occurred, in order to de-termine whether
and how changes in magnetic helicity accumulation are temporally
correlated with CME occurrence. After identifying the active
regions (AR) where the CMEs were ini-tiated by means of a double
cross-check based on the flaring-eruptive activity and the use of
SOHO/EIT difference images, we used MDI magnetograms to calculate
magnetic flux evolu-tion, magnetic helicity injection rate and
magnetic helicity injection in 10 active regions that gave rise to
12 halo CMEs observed during the period February 2000 -June 2003. No
unique behavior in magnetic helicity injection accompanying halo CME
occurrence is found. In fact, in some cases there is an abrupt change
in helicity injection timely correlated with the CME event, while
in some others no significant variation is recorded. However, our
analysis show that the most significant changes in magnetic flux and
magnetic helicity injection are associated with impulsive CMEs rather
than gradual CMEs. Moreover, the most significant changes in mag-netic
helicity are observed when X-class flares or eruptive filaments occur,
while the occurrence of flares of class C or M seems not to affect
significantly the magnetic helicity accumulation.

---------------------------------------------------------
Title: A Non-Linear Force-Free Field Model for the Evolving Magnetic
Structure of Solar Filaments
Authors: Mackay, Duncan H.; van Ballegooijen, A. A.
2009SoPh..260..321M Altcode:
In this paper the effect of a small magnetic element approaching
the main body of a solar filament is considered through non-linear
force-free field modeling. The filament is represented by a series of
magnetic dips. Once the dips are calculated, a simple hydrostatic
atmosphere model is applied to determine which structures have
sufficient column mass depth to be visible in Hα. Two orientations
of the bipole are considered, either parallel or anti-parallel to
the overlying arcade. The magnetic polarity that lies closest to the
filament is then advected towards the filament. Initially for both the
dominant and minority polarity advected elements, right/left bearing
barbs are produced for dextral/sinsitral filaments. The production
of barbs due to dominant polarity elements is a new feature. In later
stages the filament breaks into two dipped sections and takes a highly
irregular, non-symmetrical form with multiple pillars. The two sections
are connected by field lines with double dips even though the twist
of the field is less than one turn. Reconnection is not found to play
a key role in the break up of the filament. The non-linear force-free
fields produce very different results to extrapolated linear-force free
fields. For the cases considered here the linear force-free field does
not produce the break up of the filament nor the production of barbs
as a result of dominant polarity elements.

---------------------------------------------------------
Title: Solar Cycle Variations of Coronal Null Points: Implications
for the Magnetic Breakout Model of Coronal Mass Ejections
Authors: Cook, G. R.; Mackay, D. H.; Nandy, Dibyendu
2009ApJ...704.1021C Altcode:
In this paper, we investigate the solar cycle variation of coronal null
points and magnetic breakout configurations in spherical geometry, using
a combination of magnetic flux transport and potential field source
surface models. Within the simulations, a total of 2843 coronal null
points and breakout configurations are found over two solar cycles. It
is found that the number of coronal nulls present at any time varies
cyclically throughout the solar cycle, in phase with the flux emergence
rate. At cycle maximum, peak values of 15-17 coronal nulls per day are
found. No significant variation in the number of nulls is found from
the rising to the declining phase. This indicates that the magnetic
breakout model is applicable throughout both phases of the solar
cycle. In addition, it is shown that when the simulations are used
to construct synoptic data sets, such as those produced by Kitt Peak,
the number of coronal nulls drops by a factor of 1/6. The vast majority
of the coronal nulls are found to lie above the active latitudes and
are the result of the complex nature of the underlying active region
fields. Only 8% of the coronal nulls are found to be connected to the
global dipole. Another interesting feature is that 18% of coronal nulls
are found to lie above the equator due to cross-equatorial interactions
between bipoles lying in the northern and southern hemispheres. As
the majority of coronal nulls form above active latitudes, their
average radial extent is found to be in the low corona below 1.25 R
sun (175, 000 km above the photosphere). Through considering
the underlying photospheric flux, it is found that 71% of coronal
nulls are produced though quadrupolar flux distributions resulting
from bipoles in the same hemisphere interacting. When the number
of coronal nulls present in each rotation is compared to the number
of bipoles emerging, a wide scatter is found. The ratio of coronal
nulls to emerging bipoles is found to be approximately 1/3. Overall,
the spatio-temporal evolution of coronal nulls is found to follow the
typical solar butterfly diagram and is in qualitative agreement with
the observed time dependence of coronal mass ejection source-region
locations.

---------------------------------------------------------
Title: Non-potential Enhancement of the Sun's Open Magnetic Flux
Authors: Yeates, Anthony R.; Mackay, D. H.; van Ballegooijen, A. A.
2009shin.confE.182Y Altcode:
Measurements of the interplanetary magnetic field (IMF) over several
solar cycles do not agree with expected values of open magnetic flux
from potential field models. The discrepancy becomes greater around
solar maximum in each cycle, when the IMF can be twice as strong as
predicted by the models. It has previously been suggested that the
increased rate of coronal mass ejections around solar maximum could
be responsible for enhancing the open flux. We test this idea by
removing the potential field assumption in simulations of the coronal
magnetic field evolution over the solar cycle. The simulations use
magneto-frictional relaxation in response to flux emergence and surface
flux transport, and allow for the development of coronal currents
and for the ejection of twisted magnetic flux ropes. Preliminary
results show that the non-potential fields have significantly more
open flux than potential extrapolations, in agreement with IMF
observations. We find that the additional open flux arises from two
separate effects: temporary enhancement due to coronal mass ejections,
and a background enhancement due to the non-potential nature of the
force-free equilibria. (Supported by NASA/LWS grant NNG05GK32G, and
NASA contract NNM07AB07C to SAO.)

---------------------------------------------------------
Title: Initiation of Coronal Mass Ejections in a Global Evolution
Model
Authors: Yeates, A. R.; Mackay, D. H.
2009ApJ...699.1024Y Altcode: 2009arXiv0904.4419Y
Loss of equilibrium of magnetic flux ropes is a leading candidate
for the origin of solar coronal mass ejections (CMEs). The aim of
this paper is to explore to what extent this mechanism can account
for the initiation of CMEs in the global context. A simplified MHD
model for the global coronal magnetic field evolution in response
to flux emergence and shearing by large-scale surface motions is
described and motivated. Using automated algorithms for detecting flux
ropes and ejections in the global magnetic model, the effects of key
simulation parameters on the formation of flux ropes and the number
of ejections are considered, over a 177 day period in 1999. These
key parameters include the magnitude and sign of magnetic helicity
emerging in active regions, and coronal diffusion. The number of flux
ropes found in the simulation at any one time fluctuates between about
28 and 48, sustained by the emergence of new bipolar regions, but with
no systematic dependence on the helicity of these regions. However,
the emerging helicity does affect the rate of flux rope ejections,
which doubles from 0.67 per day if the bipoles emerge untwisted to 1.28
per day in the run with greatest emerging twist. The number of ejections
in the simulation is also increased by 20%-30% by choosing the majority
sign of emerging bipole helicity in each hemisphere, or by halving the
turbulent diffusivity in the corona. For reasonable parameter choices,
the model produces approximately 50% of the observed CME rate. This
indicates that the formation and loss of equilibrium of flux ropes may
be a key element in explaining a significant fraction of observed CMEs.

---------------------------------------------------------
Title: A Global Magnetic Field Evolution Model for the Solar Corona
Authors: Yeates, Anthony R.; Mackay, D. H.; van Ballegooijen, A. A.
2009SPD....40.3708Y Altcode:
We have developed new simulations of the global magnetic field
evolution in the solar corona. Using a coupled surface flux transport
and magnetofrictional model, we can follow, for the first time, the
build-up of magnetic helicity and shear on a global scale over many
solar rotations. The evolution is driven by surface motions and by
flux emergence, with properties of new active regions determined from
synoptic normal-component photospheric magnetograms from NSO/Kitt
Peak. As a first application we compare the model to observations
of sheared magnetic fields in filaments (aka. prominences), over
a 6-month period. We have unprecedented success in reproducing the
chirality (axial magnetic field direction) of filaments. Depending on
the sign of helicity in newly-emerging regions, the correct chirality
is simulated for up to 96% of filaments tested. On the basis of these
simulations, an explanation for the observed hemispheric pattern of
filament chirality is put forward, including why exceptions occur
for filaments in certain locations. When too much axial magnetic flux
builds up in filament channels, magnetic flux ropes lose equilibrium
and are ejected from the simulation. Using automated techniques for
detecting flux ropes and their ejection in the global simulations,
we find that the number of ejections depends on both the magnitude
and sign of the emerging helicity. For reasonable parameter choices,
loss of equilibrium of magnetic flux ropes formed by quasi-static flux
cancellation produces about 50% of the observed CME rate.

---------------------------------------------------------
Title: A Prominence Puzzle Explained?
Authors: Yeates, A. R.; Mackay, D. H.; van Ballegooijen, A. A.
2009AIPC.1094..216Y Altcode: 2009csss...15..216Y
Long-standing observations reveal a global organisation of the magnetic
field direction in solar prominences (aka filaments), large clouds of
cool dense plasma suspended in the Sun's hot corona. However, theorists
have thus far been unable to explain the origin of this hemispheric
pattern. In particular, simple shearing by large-scale surface motions
would appear to lead to the wrong magnetic field direction. To explain
the observations, we have developed a new model of the global magnetic
field evolution in the solar corona over six months. For the first time
our model can follow the build-up of magnetic helicity and shear on a
global scale, driven by flux emergence and surface motions. The model
is successful in predicting the correct magnetic field direction in the
vast majority of prominences tested, and has enabled us to determine
the key physical mechanisms behind the mysterious hemispheric pattern.

---------------------------------------------------------
Title: Evolution of Current Helicity in Full-Sun Simulations
Authors: Yeates, A. R.; Mackay, D. H.; van Ballegooijen, A. A.
2009AIPC.1094..772Y Altcode: 2009csss...15..772Y
The density of current helicity quantifies the location of twisted and
sheared non-potential structures in a magnetic field. We simulate the
continuous evolution over many solar rotations of the magnetic field in
the Sun's global corona, in response to flux emergence and shearing by
photospheric motions. The latitudinal distribution of current helicity
in our simulation develops a clear statistical pattern, matching the
observed hemispheric sign at active latitudes. Also in agreement
with observations there is significant scatter and intermixing of
both signs of helicity, and we find local values of current helicity
density that are much higher than those predicted by linear force-free
extrapolations. Forthcoming full-disk vector magnetograms from Solar
Dynamics Observatory will provide an ideal opportunity to test our
theoretical results.

---------------------------------------------------------
Title: Modelling the Global Solar Corona: III. Origin of the
Hemispheric Pattern of Filaments
Authors: Yeates, A. R.; Mackay, D. H.
2009SoPh..254...77Y Altcode: 2008arXiv0810.0517Y; 2008SoPh..tmp..179Y
We consider the physical origin of the hemispheric pattern of
filament chirality on the Sun. Our 3D simulations of the coronal
field evolution over a period of six months, based on photospheric
magnetic measurements, were previously shown to be highly successful at
reproducing observed filament chiralities. In this paper we identify and
describe the physical mechanisms responsible for this success. The key
mechanisms are found to be (1) differential rotation of north - south
polarity inversion lines, (2) the shape of bipolar active regions, and
(3) evolution of skew over a period of many days. As on the real Sun,
the hemispheric pattern in our simulations holds in a statistical
sense. Exceptions arise naturally for filaments in certain locations
relative to bipolar active regions or from interactions among a number
of active regions.

---------------------------------------------------------
Title: Evolution and Distribution of Current Helicity in Full-Sun
Simulations
Authors: Yeates, A. R.; Mackay, D. H.; van Ballegooijen, A. A.
2008ApJ...680L.165Y Altcode: 2008arXiv0805.1883Y
Current helicity quantifies the location of twisted and sheared
nonpotential structures in a magnetic field. We simulate the evolution
of magnetic fields in the solar atmosphere in response to flux
emergence and shearing by photospheric motions. In our global-scale
simulation over many solar rotations, the latitudinal distribution
of current helicity develops a clear statistical pattern, matching
the observed hemispheric sign at active latitudes. In agreement with
observations, there is significant scatter and intermixing of both
signs of helicity, where we find local values of current helicity
density that are much higher than those predicted by linear force-free
extrapolations. Forthcoming full-disk vector magnetograms from the
Solar Dynamics Observatory will provide an ideal opportunity to test
our theoretical results on the evolution and distribution of current
helicity, both globally and in single active regions.

---------------------------------------------------------
Title: Where Do Solar Filaments Form?: Consequences for Theoretical
Models
Authors: Mackay, Duncan H.; Gaizauskas, Victor; Yeates, Anthony R.
2008SoPh..248...51M Altcode: 2008SoPh..tmp...25M
This paper examines the locations where large, stable solar filaments
form relative to magnetic bipoles lying underneath them. The
study extends the earlier work of F. Tang to include two additional
classification categories for stable filaments and to consider their
population during four distinct phases of the solar cycle. With this
new classification scheme, results show that over 92% of filaments
form in flux distributions that are nonbipolar in nature where the
filament lies either fully (79%) or partially (13%) above a polarity
inversion line (PIL) external to any single bipole. Filaments that
form within a single bipole (traditionally called Type A) are not as
common as previously thought. These results are a significant departure
from those of F. Tang. Consistency with the earlier work is shown when
our data are regrouped to conform to the two-category classification
scheme for filaments adopted by F. Tang. We also demonstrate that only
filaments that form along the external PIL lying between two bipoles
(62% of the full sample, traditionally called Type B) show any form of
solar cycle dependence, where their number significantly increases with
magnetic activity over the solar cycle. Finally, current observations
and theoretical models for the formation of filaments are discussed
in the context of the present results. We conclude that key elements
in the formation of the majority of filaments considered within this
study must be the convergence of magnetic flux resulting in either
flux cancellation or coronal reconnection.

---------------------------------------------------------
Title: Twisted solar active region magnetic fields as drivers of
space weather: Observational and theoretical investigations
Authors: Nandy, Dibyendu; Mackay, Duncan H.; Canfield, Richard C.;
Martens, P. C. H.
2008JASTP..70..605N Altcode:
The properties and dynamics of magnetic fields on the Sun's photosphere
and outer layers--notably those within solar active regions--govern
the eruptive activity of the Sun. These photospheric magnetic
fields also act as the evolving lower boundary of the Sun-Earth
coupled system. Quantifying the physical attributes of these magnetic
fields and exploring the mechanisms underlying their influence on the
near-Earth space environment are of vital importance for forecasting
and mitigating adverse space weather effects. In this context, we
discuss here a novel technique for measuring twist in the magnetic
field lines of solar active regions that does not invoke the force-free
field assumption. Twist in solar active regions can play an important
role in flaring activity and the initiation of CMEs via the kink
instability mechanism; we outline a procedure for determining this
solar active region eruptive potential. We also discuss how twist in
active region magnetic fields can be used as inputs in simulations of
the coronal and heliospheric fields; specifically, we explore through
simulations, the formation, evolution and ejection of magnetic flux
ropes that originate in twisted magnetic structures. The results and
ideas presented here are relevant for exploring the role of twisted
solar active region magnetic fields and flux ropes as drivers of space
weather in the Sun-Earth system.

---------------------------------------------------------
Title: Modelling the Global Solar Corona II: Coronal Evolution and
Filament Chirality Comparison
Authors: Yeates, A. R.; Mackay, D. H.; van Ballegooijen, A. A.
2008SoPh..247..103Y Altcode: 2007arXiv0711.2887Y
This paper considers the hemispheric pattern of solar filaments using
newly developed simulations of the real photospheric and 3D coronal
magnetic fields over a six-month period, on a global scale. The
magnetic field direction in the simulation is compared directly with
the chirality of observed filaments, at their observed locations. In
our model the coronal field evolves through a continuous sequence
of nonlinear force-free equilibria, in response to the changing
photospheric boundary conditions and the emergence of new magnetic
flux. In total 119 magnetic bipoles with properties matching observed
active regions are inserted. These bipoles emerge twisted and inject
magnetic helicity into the solar atmosphere. When we choose the sign of
this active-region helicity to match that observed in each hemisphere,
the model produces the correct chirality for up to 96% of filaments,
including exceptions to the hemispheric pattern. If the emerging
bipoles have zero helicity, or helicity of the opposite sign, then
this percentage is much reduced. In addition, the simulation produces
a higher proportion of filaments with the correct chirality after
longer times. This indicates that a key element in the evolution
of the coronal field is its long-term memory, and the build-up and
transport of helicity from low to high latitudes over many months. It
highlights the importance of continuous evolution of the coronal field,
rather than independent extrapolations at different times. This has
significant consequences for future modelling such as that related to
the origin and development of coronal mass ejections.

---------------------------------------------------------
Title: Exploring the Physical Basis of Solar Cycle Predictions:
Flux Transport Dynamics and Persistence of Memory in Advection-
versus Diffusion-dominated Solar Convection Zones
Authors: Yeates, Anthony R.; Nandy, Dibyendu; Mackay, Duncan H.
2008ApJ...673..544Y Altcode: 2007arXiv0709.1046Y
The predictability, or lack thereof, of the solar cycle is governed by
numerous separate physical processes that act in unison in the interior
of the Sun. Magnetic flux transport and the finite time delay that it
introduces, specifically in the so-called Babcock-Leighton models of
the solar cycle with spatially segregated source regions for the α-
and Ω-effects, play a crucial rule in this predictability. Through
dynamo simulations with such a model, we study the physical basis
of solar cycle predictions by examining two contrasting regimes, one
dominated by diffusive magnetic flux transport in the solar convection
zone, the other dominated by advective flux transport by meridional
circulation. Our analysis shows that diffusion plays an important
role in flux transport, even when the solar cycle period is governed
by the meridional flow speed. We further examine the persistence of
memory of past cycles in the advection- and diffusion-dominated regimes
through stochastically forced dynamo simulations. We find that in the
advection-dominated regime this memory persists for up to three cycles,
whereas in the diffusion-dominated regime this memory persists for
mainly one cycle. This indicates that solar cycle predictions based
on these two different regimes would have to rely on fundamentally
different inputs, which may be the cause of conflicting predictions. Our
simulations also show that the observed solar cycle amplitude-period
relationship arises more naturally in the diffusion-dominated regime,
thereby supporting those dynamo models in which diffusive flux transport
plays a dominant role in the solar convection zone.

---------------------------------------------------------
Title: Yohkoh SXT Full-Resolution Observations of Sigmoids: Structure,
Formation, and Eruption
Authors: Canfield, Richard C.; Kazachenko, Maria D.; Acton, Loren W.;
Mackay, D. H.; Son, Ji; Freeman, Tanya L.
2007ApJ...671L..81C Altcode:
We study the structure of 107 bright sigmoids using full-resolution
(2.5" pixels) images from the Yohkoh Soft X-Ray Telescope (SXT)
obtained between 1991 December and 2001 December. We find that none of
these sigmoids are made of single loops of S or inverse-S shape; all
comprise a pattern of multiple loops. We also find that all S-shaped
sigmoids are made of right-bearing loops and all inverse-S-shaped
sigmoids of left-bearing loops, without exception. We co-align the SXT
images with Kitt Peak magnetograms to determine the magnetic field
directions in each sigmoid. We use a potential-field source surface
model to determine the direction of the overlying magnetic field. We
find that sigmoids for which the relative orientation of these two
fields has a parallel component outnumber antiparallel ones by more than
an order of magnitude. We find that the number of sigmoids per active
region varies with the solar cycle in a manner that is consistent with
this finding. Finally, those few sigmoids that are antiparallel erupt
roughly twice as often as those that are parallel. We briefly discuss
the implications of these results in terms of formation and eruption
mechanisms of flux tubes and sigmoids.

---------------------------------------------------------
Title: Formation of polar starspots through meridional circulation
Authors: Holzwarth, V.; Mackay, D. H.; Jardine, M.
2007AN....328.1108H Altcode:
To explain the observed intermingling of polarities in the magnetic
field distributions of rapidly rotating stars, surface magnetic flux
transport models demand the presence of fast meridional flows. We
combine simulations of the pre-eruptive and post-eruptive magnetic
flux transport in cool stars to investigate the influence of a fast
meridional circulation on the latitudinal eruption pattern of magnetic
flux tubes and on the polar magnetic field properties. Magnetic flux
tubes rising through the convection zone experience an enhanced
latitude-dependent poleward deflection through meridional flows,
which renders the wings of stellar butterfly diagrams convex. The
larger amount of magnetic flux emerging at higher latitudes supports
the intermingling of opposite polarities of polar magnetic fields and
yields magnetic flux densities in the polar regions about 20% higher
than in the case disregarding the pre-eruptive deflection. Taking the
pre-eruptive evolution of magnetic flux into account therefore eases the
need for the fast meridional flows predicted by previous investigations.

---------------------------------------------------------
Title: Modelling the Global Solar Corona: Filament Chirality
Observations and Surface Simulations
Authors: Yeates, A. R.; Mackay, D. H.; van Ballegooijen, A. A.
2007SoPh..245...87Y Altcode: 2007arXiv0707.3256Y
The hemispheric pattern of solar filaments is considered in the context
of the global magnetic field of the solar corona. In recent work
Mackay and van Ballegooijen have shown how, for a pair of interacting
magnetic bipoles, the observed chirality pattern could be explained
by the dominant range of bipole tilt angles and helicity in each
hemisphere. This study aims to test this earlier result through
a direct comparison between theory and observations, using newly
developed simulations of the actual surface and 3D coronal magnetic
fields over a 6-month period, on a global scale. We consider two key
components: (1) observations of filament chirality for the sample of
255 filaments and (2) our new simulations of the large-scale surface
magnetic field. Based on a flux-transport model, these will be used as
the lower boundary condition for the future 3D coronal simulations. Our
technique differs significantly from those of other authors, where the
coronal field is either assumed to be purely potential or has to be
reset back to potential every 27 days for the photospheric field to
remain accurate. In our case we ensure accuracy by the insertion of
newly emerging bipolar active regions, based on observed photospheric
synoptic magnetograms. The large-scale surface field is shown to
remain accurate over the 6-month period, without any resetting. This
new technique will enable future simulations to consider the long-term
buildup and transport of helicity and shear in the coronal magnetic
field over many months or years.

---------------------------------------------------------
Title: Modeling the Hemispheric Pattern of Solar Filaments
Authors: Mackay, D. H.; van Ballegooijen, A. A.
2007ASPC..368..343M Altcode:
New results in modeling the hemispheric pattern of solar filaments
are presented. The simulations consider what type of chirality forms
along the Polarity Inversion Line (PIL) lying in between two magnetic
bipoles as they interact. The results demonstrate not only the origin
of the dominant hemispheric pattern, but also why exceptions to
it occur. The dominant hemispheric pattern may be attributed to the
dominant range of bipole tilt angles and helicities in each hemisphere
(\cite{dunc-1989SoPh..124...81W,dunc-1995ApJ...440L.109P}). Exceptions
to the hemispheric pattern occur in cases of no initial helicity
or for helicity of the minority type in each hemisphere, when large
positive bipole tilt angles are used. As the simulations show a clear
dependence of the chirality on observational quantities, this may be
used to check the validity of the results.

---------------------------------------------------------
Title: Models of the Large-Scale Corona: Formation, Evolution and
Lift-Off of Magnetic Flux Ropes
Authors: Mackay, D. H.; van Ballegooijen, A. A.
2007ASPC..368..251M Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Model for the Coupled Evolution of Subsurface and Coronal
Magnetic Fields in Solar Active Regions
Authors: van Ballegooijen, A. A.; Mackay, D. H.
2007ApJ...659.1713V Altcode:
According to Babcock's theory of the solar dynamo, bipolar active
regions are Ω-shaped loops emerging from a toroidal field located near
the base of the convection zone. In this paper, a mean field model for
the evolution of a twisted Ω-loop is developed. The model describes the
coupled evolution of the magnetic field in the convection zone and the
corona after the loop has fully emerged into the solar atmosphere. Such
a coupled evolution is required to fully understand what happens to the
coronal and subsurface fields as magnetic flux cancels at polarity
inversion lines on the photosphere. The jump conditions for the
magnetic field at the photosphere are derived from the magnetic stress
balance between the convection zone and corona. The model reproduces
the observed spreading of active region magnetic flux over the solar
surface. At polarity inversion lines, magnetic flux submerges below the
photosphere, but the component of magnetic field along the inversion
line cannot submerge, because the field in the upper convection zone is
nearly radial. Therefore, magnetic shear builds up in the corona above
the inversion line, which eventually leads to a loss of equilibrium of
the coronal fields and the “lift-off” of a coronal flux rope. Fields
that submerge are transported back to the base of the convection zone,
leading to the repair of the toroidal flux rope. Following Martens
and Zwaan, interactions between bipoles are also considered.

---------------------------------------------------------
Title: Modeling magnetic flux ropes in the solar atmosphere
Authors: van Ballegooijen, A. A.; Deluca, E. E.; Squires, K.; Mackay,
D. H.
2007JASTP..69...24V Altcode: 2007JATP...69...24V
Coronal flux ropes are highly sheared or twisted magnetic fields
overlying polarity inversion lines on the solar photosphere. The
formation of such flux ropes is briefly discussed. A coronal flux
rope can be stable for many days and then suddenly lose equilibrium
and erupt, producing a coronal mass ejection (CME). To understand
what causes such eruptions, we need to determine the 3D magnetic
structure of observed active regions prior to CMEs. This requires
constructing nonlinear force free field models of active regions based
on observed photospheric vector fields, Hα filaments, or coronal loop
structures. We describe a new method for constructing models containing
flux ropes.

---------------------------------------------------------
Title: The impact of meridional circulation on stellar butterfly
diagrams and polar caps
Authors: Holzwarth, V.; Mackay, D. H.; Jardine, M.
2006MNRAS.369.1703H Altcode: 2006astro.ph..4102H; 2006MNRAS.tmp..558H
Observations of rapidly rotating solar-like stars show a significant
mixture of opposite-polarity magnetic fields within their polar
regions. To explain these observations, models describing the surface
transport of magnetic flux demand the presence of fast meridional
flows. Here, we link subsurface and surface magnetic flux transport
simulations to investigate (i) the impact of meridional circulations
with peak velocities of &lt;=125ms-1 on the latitudinal
eruption pattern of magnetic flux tubes and (ii) the influence of the
resulting butterfly diagrams on polar magnetic field properties. Prior
to their eruption, magnetic flux tubes with low field strengths and
initial cross-sections below ~300km experience an enhanced poleward
deflection through meridional flows (assumed to be polewards at the top
of the convection zone and equatorwards at the bottom). In particular,
flux tubes which originate between low and intermediate latitudes
within the convective overshoot region are strongly affected. This
latitude-dependent poleward deflection of erupting magnetic flux renders
the wings of stellar butterfly diagrams distinctively convex. The
subsequent evolution of the surface magnetic field shows that the
increased number of newly emerging bipoles at higher latitudes
promotes the intermingling of opposite polarities of polar magnetic
fields. The associated magnetic flux densities are about 20 per cent
higher than in the case disregarding the pre-eruptive deflection,
which eases the necessity for fast meridional flows predicted by
previous investigations. In order to reproduce the observed polar
field properties, the rate of the meridional circulation has to be
of the order of 100ms-1, and the latitudinal range from
which magnetic flux tubes originate at the base of the convective zone
(&lt;~50°) must be larger than in the solar case (&lt;~35°).

---------------------------------------------------------
Title: Models of the Large-Scale Corona. II. Magnetic Connectivity
and Open Flux Variation
Authors: Mackay, D. H.; van Ballegooijen, A. A.
2006ApJ...642.1193M Altcode:
In this paper the changing connectivity of the coronal magnetic
field during the formation and ejection of magnetic flux ropes
is considered. Using recent simulations of the coronal field,
it is shown that reconnection may occur both above and below the
flux ropes. Those occurring above slowly strip away coronal arcades
overlying the flux ropes and allow the flux ropes to be ejected. In
contrast, those below help to push the flux ropes out. It is found
that the reconnection occurring below each flux rope may result in
significant skew being maintained within the coronal field above the
PIL after the flux rope is ejected. In addition, after the eruption, as
the coronal field closes down, the large-scale transport of open flux
across the bipoles takes place through the process of “interchange
reconnection.” As a result, new photospheric domains of open flux
are created within the centers of the bipoles, where field lines were
previously closed. The net open flux in the simulation may be split
into two distinct contributions. The first contribution is due to the
nonpotential equilibrium coronal fields of the bipoles. The second
contribution is a temporary enhancement to this during the ejection of
the flux ropes, where previously closed field lines become open. It is
shown that the nonpotential equilibrium contribution to the open flux
is significantly higher than that due to a potential field deduced
from the same photospheric boundary conditions. These results suggest
that the nonpotential nature of coronal magnetic fields may affect the
variation of the Sun's open flux during periods of high solar activity
and should be considered in future simulations.

---------------------------------------------------------
Title: Models of the Large-Scale Corona. I. Formation, Evolution,
and Liftoff of Magnetic Flux Ropes
Authors: Mackay, D. H.; van Ballegooijen, A. A.
2006ApJ...641..577M Altcode:
The response of the large-scale coronal magnetic field to transport of
magnetic flux in the photosphere is investigated. In order to follow the
evolution on long timescales, the coronal plasma velocity is assumed
to be proportional to the Lorentz force (magnetofriction), causing
the coronal field to evolve through a series of nonlinear force-free
states. Magnetofrictional simulations are used to study the formation
and evolution of coronal flux ropes, highly sheared and/or twisted
fields located above polarity inversion lines on the photosphere. As in
our earlier studies, the three-dimensional numerical model includes the
effects of the solar differential rotation and small-scale convective
flows; the latter are described in terms of surface diffusion. The
model is extended to include the effects of coronal magnetic diffusion,
which limits the degree of twist of coronal flux ropes, and the solar
wind, which opens up the field at large height. The interaction of two
bipolar magnetic regions is considered. A key element in the formation
of flux ropes is the reconnection of magnetic fields associated with
photospheric flux cancellation at the polarity inversion lines. Flux
ropes are shown to form both above the external inversion line between
bipoles (representing type B filaments) and above the internal inversion
line of each bipole in a sigmoid shape. It is found that once a flux
rope has formed, the coronal field may diverge from equilibrium with
the ejection of the flux rope. After the flux rope is ejected, the
coronal field once again relaxes down to an equilibrium. This ability
to follow the evolution of the coronal fields through eruptions is
essential for future full-Sun simulations in which multiple bipoles
are evolved for many months or years.

---------------------------------------------------------
Title: Simulated X-ray cycles in rapidly rotating solar-like stars
Authors: McIvor, T.; Jardine, M.; Mackay, D.; Holzwarth, V.
2006MNRAS.367..592M Altcode: 2006MNRAS.tmp..259M
It is generally accepted that the presence of a hot magnetic
corona provides the source of X-ray emission in cool stars. With
this connection one could expect to see the variation of magnetic
flux in the activity cycle of a star mirrored by a similar variation
in the stars X-ray emission. Using magnetic maps produced from flux
emergence and transport simulations and assuming a potential field for
the corona, we can extrapolate the coronal magnetic field and hence
calculate the variation of the X-ray emission. We consider three
types of activity cycle that successfully reproduce the pattern of
intermingled magnetic flux at high latitudes, a feature observed with
Zeeman-Doppler imaging. The three different cycles take the form of
(1) an enhanced butterfly pattern where flux emergence is extended
to a latitude of 70°, (2) an extended emergence profile as before
but with an overlap of 4 yr in the butterfly diagram and (3) where no
butterfly diagram is used. The cyclic variation in the X-ray emission
is around two orders of magnitude for cases (1) and (3), but less than
one order of magnitude for case (2). For all three cases, the rotational
modulation of the X-ray emission is greatest at cycle minimum, but the
emission measure weighted density varies little over the cycle. For
cases (1) and (2) the fraction of the total flux that is open (along
which a wind can escape) varies little over the cycle, but for case
(3) this is three times larger at cycle minimum than at maximum. Our
results clearly show that although magnetic cycles may exist for stars
they are not necessarily observable in the X-ray emission.

---------------------------------------------------------
Title: Role of Large-scale Magnetic Fields and Material Flows in
the Formation of Filaments and Filament Channels
Authors: Mackay, D. H.
2005ASPC..346..177M Altcode:
In this review paper we discuss the role of large-scale magnetic
fields and material flows in the formation of solar filaments
and filament channels. Particular attention is paid to the type of
magnetic configurations within which filaments form. Observations of
the formation of filaments are described along with the wide variety
of theoretical models and mechanisms which have been put forward to
explain filament formation. Simulations are presented which consider
the origin of the large-scale hemispheric pattern that exists for the
axial magnetic fields within filaments. The role that mass motions
play in filament formation is introduced from both observational and
theoretical viewpoints. A summary concludes with recommendations for
future observing programs which would advance our understanding of
the formation mechanism of filaments.

---------------------------------------------------------
Title: New Results in Modeling the Hemispheric Pattern of Solar
Filaments
Authors: Mackay, D. H.; van Ballegooijen, A. A.
2005ApJ...621L..77M Altcode:
New results in modeling the hemispheric pattern of solar filaments
through magnetic flux transport and magnetofrictional simulations are
presented. The simulations consider for the first time what type of
chirality forms along the polarity inversion line lying in between two
magnetic bipoles as they interact. Such interactions are important for
filament formation, as observations by F. Tang show that the majority of
filaments form in between bipolar regions rather than within a single
magnetic bipole. The simulations also include additional physics of
coronal diffusion and a radial outflow velocity at the source surface,
which was not included in previous studies. The results clearly
demonstrate for the first time not only the origin of the dominant
hemispheric pattern but also why exceptions to it may occur. The
dominant hemispheric pattern may be attributed to the dominant range
of bipole tilt angles and helicities in each hemisphere. Exceptions to
the hemispheric pattern are found to only occur in cases of no initial
helicity or for helicity of the minority type in each hemisphere when
large positive bipole tilt angles (α&gt;20deg) are used. As
the simulations show a clear dependence of the hemispheric pattern
and its exceptions on observational quantities, this may be used to
check the validity of the results. Future programs to consider this
are put forward.

---------------------------------------------------------
Title: Activity cycles and polar caps on solar-like stars
Authors: Jardine, M.; Mackay, D. H.; Hussain, G. A. J.; McIvor, T.
2005ESASP.560..107J Altcode: 2005csss...13..107J
No abstract at ADS

---------------------------------------------------------
Title: Polar caps on active stars: magnetic flux emergence and
transport
Authors: Mackay, D. H.; Jardine, M.; Collier Cameron, A.; Donati,
J. -F.; Hussain, G. A. J.
2004MNRAS.354..737M Altcode:
In recent years, Zeeman Doppler imaging of rapidly rotating solar-like
stars has shown high-latitude magnetic flux patterns of intermingled
magnetic polarities. Such high-latitude intermingling of positive
and negative flux is inconsistent with our present understanding of
how magnetic flux emerges on the Sun and is transported poleward. To
determine how these patterns may arise, magnetic flux transport
simulations are carried out. These simulations follow the evolution of
the radial magnetic field at the surface of the star as new magnetic
bipoles emerge and are advected poleward by the surface effects of
differential rotation, meridional flow and supergranular diffusion. To
produce intermingling of flux at high latitudes, key parameters such
as the emergence profiles and transport coefficients are varied
from presently used solar values. In doing so, it is found that,
in order to explain the high-latitude intermingling, at least two of
these parameters must be changed. First, the emergence profile must
be extended to higher latitudes (λ= 50°-70°), and secondly the
value of the meridional flow must be increased by around a factor
of 10 (~100 m s-1). The results show that the observed
intermingling of high-latitude flux can only occur through a flux
emergence and transport process that is significantly different from
that which occurs on the Sun. Observable features produced by both of
these changes are considered, and the significance of the simulations
to future observing programmes discussed. Finally the emergence profile
and transport coefficients that best fit the observations of the young
active star AB Dor (period = 0.514 d) are put forward.

---------------------------------------------------------
Title: A Possible Solar Cycle Dependence to the Hemispheric Pattern
of Filaments?
Authors: Mackay, D. H.
2003csss...12..595M Altcode:
The origin of the hemispheric pattern of filaments is considered
for both the rising and declining phases of the solar cycle. This is
carried out by using a magnetic flux transport model which considers
how bipoles interact with surrounding polar fields. For the rising
phase of the cycle a range of initial twists and tilt angles of the
bipoles consistent with observations can be found which reproduces the
observed hemispheric pattern. For the declining phase no such range
can be found. To determine whether there is a cycle variation to the
hemispheric pattern new detailed observations are required.

---------------------------------------------------------
Title: Helicity as a Component of Filament Formation
Authors: Mackay, D. H.; Gaizauskas, V.
2003SoPh..216..121M Altcode:
In this paper we seek the origin of the axial component of the magnetic
field in filaments by adapting theory to observations. A previous paper
(Mackay, Gaizauskas, and van Ballegooijen, 2000) showed that surface
flows acting on potential magnetic fields for 27 days - the maximum
time between the emergence of magnetic flux and the formation of large
filaments between the resulting activity complexes - cannot explain the
chirality or inverse polarity nature of the observed filaments. We show
that the inclusion of initial helicity, for which there is observational
evidence, in the flux transport model results in sufficiently strong
dextral fields of inverse polarity to account for the existence and
length of an observed filament within the allotted time. The simulations
even produce a large length of dextral chirality when just small
amounts of helicity are included in the initial configuration. The
modeling suggests that the axial field component in filaments can
result from a combination of surface (flux transport) and sub-surface
(helicity) effects acting together. Here surface effects convert the
large-scale helicity emerging in active regions into a smaller-scale
magnetic-field component parallel to the polarity inversion line so
as to form a magnetic configuration suitable for a filament.

---------------------------------------------------------
Title: Magnetic Flux Transport Simulations of Solar Surface Magnetic
Distributions During a Grand Minimum
Authors: Mackay, D. H.
2003SoPh..213..173M Altcode:
It is well known that magnetic activity on the Sun modulates from one
cycle to the next. The most striking occurrence of this is called a
grand minimum where magnetic activity all but disappears. The latest
grand minimum occurred between the years 1645 and 1715 and is called
the Maunder minimum. In this paper magnetic flux transport simulations
are used to consider what type of surface magnetic field configurations
may be produced both during and after a grand minimum depending on how
the grand minimum occurs. It is shown that the surface configurations
during and after a grand minimum strongly depend on the phase of
the cycle in which the grand minimum starts and whether it lasts for
an odd or even number of cycles. If the grand minimum starts around
cycle minimum then a significant amount of large-scale magnetic flux
may persist on the Sun at high latitudes during the grand minimum. In
contrast, if it starts at cycle maximum during the grand minimum it is
possible for there to be essentially zero large-scale magnetic flux
over the entire surface of the Sun. It is shown that for a single
grand minimum event the reversal of the polar fields at the presently
observed time in the solar cycle is only reproduced if the event starts
at cycle minimum and extends over an even number of cycles. In contrast,
if the grand minimum runs for an odd number of cycles it is possible
for there to be no reversal of the polar fields or for the reversals
to occur at times inconsistent with our present understanding of the
solar cycle. Consequences of the assumptions made in the modelling are
discussed and the significance of the simulations for direct modelling
of events such as the Maunder minimum are considered.

---------------------------------------------------------
Title: Statistical Flux Tube Properties of 3D Magnetic Carpet Fields
Authors: Close, R. M.; Parnell, C. E.; Mackay, D. H.; Priest, E. R.
2003SoPh..212..251C Altcode:
The quiet-Sun photosphere consists of numerous magnetic flux fragments
of both polarities that evolve with granular and supergranular flow
fields. These concentrations give rise to a web of intermingled magnetic
flux tubes which characterise the coronal magnetic field. Here, the
nature of these flux tubes is studied. The photosphere is taken to be
the source plane and each photospheric fragment is represented by a
series of point sources. By analysing the potential field produced by
these sources, it is found that the distribution of flux tube lengths
obtained by (i) integrating forward from positive sources and (ii)
tracing back from negative sources is highly dependent on the total
flux imbalance within the region of interest. It is established that
the relation between the footpoint separation of a flux tube and its
height cannot be assumed to be linear. Where there is a significant
imbalance of flux within a region, it is found that fragments of the
dominant polarity will have noticeably more connections, on average,
than the minority polarity fragments. Despite this difference, the
flux from a single fragment of either polarity is typically divided
such that (i) 60-70% connects to one opposite-polarity fragment,
(ii) 25-30% goes to a further 1 to 2 opposite-polarity fragments,
and (iii) any remaining flux may connect to as many as another 50 or
more other opposite-polarity fragments. This is true regardless of any
flux imbalance within the region. It is found that fragments connect
preferentially to their nearest neighbours, with, on average, around
60-70% of flux closing down within 10 Mm of a typical fragment. Only
50% of the flux in a quiet region extends higher than 2.5 Mm above the
solar surface and 5-10% extends higher than 25 Mm. The fragments that
contribute to the field above this height cover a range of sizes,
with even the smallest of fragments contributing to the field at
heights of over 50 Mm.

---------------------------------------------------------
Title: The Skew of High-Latitude X-ray Arcades in the Declining
Phase of Cycle 22
Authors: Mc Allister, A. H.; Mackay, D. H.; Martin, S. F.
2002SoPh..211..155M Altcode:
The chirality of high-latitude coronal arcades in the declining phase
of cycle 22 has been studied. It is found that the observed skew of
the high-latitude arcades is opposite to the dominant arcade skew at
lower latitudes. This new result which applies only to the declining
phase of the solar cycle is consistent with differential rotation and
the simulations of Mackay and van Ballegooijen (2001). Limitations of
the present study are discussed along with its consequences for the
global pattern of filaments in each hemisphere. The results suggest
that, for the declining phase, the previously observed global pattern
may be more complex with latitudinal variations. Future observing
programs required to clarify the issue are discussed.

---------------------------------------------------------
Title: Inferring X-ray coronal structures from Zeeman-Doppler images
Authors: Jardine, M.; Wood, K.; Collier Cameron, A.; Donati, J. -F.;
Mackay, D. H.
2002MNRAS.336.1364J Altcode: 2002astro.ph..7522J
We have modelled the X-ray emission from the young rapid rotator AB
Doradus (Prot= 0.514 d) using as a basis Zeeman-Doppler
maps of the surface magnetic field. This allows us to reconcile the
apparently conflicting observations of a high X-ray emission measure and
coronal density with a low rotational modulation in the X-ray band. The
technique is to extrapolate the coronal field from the surface maps
by assuming the field to be potential. We then determine the coronal
density for an isothermal corona by solving hydrostatic equilibrium
along each field line and scaling the surface plasma pressure with the
surface magnetic pressure. We set the density to zero along those field
lines that are open and those where at any point along their length
the plasma pressure exceeds the magnetic pressure. We then calculate
the optically thin X-ray emission measure and rotational modulation
for models with a range of coronal densities. Although the corona can
be very extended, much of the emission comes from high-latitude regions
close to the stellar surface. Since these are always in view as the star
rotates, there is little rotational modulation. We find that emission
measures in the observed range 1052.8-1053.3
cm-3 can be reproduced with densities in the range
109-1010.7 cm-3 for coronae at
temperatures of 106-107 K.

---------------------------------------------------------
Title: The Evolution of the Sun's Open Magnetic Flux - II. Full
Solar Cycle Simulations
Authors: Mackay, D. H.; Priest, E. R.; Lockwood, M.
2002SoPh..209..287M Altcode:
In this paper the origin and evolution of the Sun's open magnetic flux
is considered by conducting magnetic flux transport simulations over
many solar cycles. The simulations include the effects of differential
rotation, meridional flow and supergranular diffusion on the radial
magnetic field at the surface of the Sun as new magnetic bipoles
emerge and are transported poleward. In each cycle the emergence
of roughly 2100 bipoles is considered. The net open flux produced
by the surface distribution is calculated by constructing potential
coronal fields with a source surface from the surface distribution
at regular intervals. In the simulations the net open magnetic flux
closely follows the total dipole component at the source surface and
evolves independently from the surface flux. The behaviour of the open
flux is highly dependent on meridional flow and many observed features
are reproduced by the model. However, when meridional flow is present
at observed values the maximum value of the open flux occurs at cycle
minimum when the polar caps it helps produce are the strongest. This
is inconsistent with observations by Lockwood, Stamper and Wild (1999)
and Wang, Sheeley, and Lean (2000) who find the open flux peaking
1-2 years after cycle maximum. Only in unrealistic simulations where
meridional flow is much smaller than diffusion does a maximum in open
flux consistent with observations occur. It is therefore deduced that
there is no realistic parameter range of the flux transport variables
that can produce the correct magnitude variation in open flux under
the present approximations. As a result the present standard model
does not contain the correct physics to describe the evolution of the
Sun's open magnetic flux over an entire solar cycle. Future possible
improvements in modeling are suggested.

---------------------------------------------------------
Title: Evolution of a density enhancement in a stratified atmosphere
with uniform vertical magnetic field
Authors: Mackay, D. H.; Galsgaard, K.
2002ESASP.505..485M Altcode: 2002solm.conf..485M; 2002IAUCo.188..485M
The evolution of a density enhancement under the effect of gravity in
a stratified atmosphere is considered. The atmosphere is threaded with
an initially uniform vertical magnetic field. The magnetic field plays
an important role in the evolution of the density enhancement and if
strong enough results in the density enhancement rebounding a number
of times. Both upward and downward velocities of the enhancement
are obtained with speeds much less than the free fall speed. The
enhancement can remain in the corona at least 11 times longer than
a free-fall particle. The relevance of the simulations to the solar
atmosphere is then discussed.

---------------------------------------------------------
Title: Solar cycle variation of the temperature structure within
the cores of coronal streamers
Authors: Culhane, J. L.; Foley, C. R.; Patsourakos, S.; Mackay, D.
2002ESASP.508..371C Altcode: 2002soho...11..371C
We use the Coronal Diagnostic Spectrometer onboard the Solar and
Heliospheric Observatory (SOHO) to analyze conditions in coronal
streamer structures observed close to solar minimum (1996, July 8) and
near maximum (1999, August 5). From emission line intensities (Fe IX-XV
ions), the line ratio method gives the radial temperature behaviour. The
solar minimum peak values were about 1.4 MK at 1.3 R0, while
near solar maximum values were consistent with Yohkoh observations at
the last maximum, displaying an asymptotic temperature of around 2.2
MK above 1.2 R0. We discuss the observations in relation to
possible mechanisms for energy deposition in large coronal structures.

---------------------------------------------------------
Title: The Evolution of the Sun's Open Magnetic Flux - I. A Single
Bipole
Authors: Mackay, D. H.; Priest, E. R.; Lockwood, M.
2002SoPh..207..291M Altcode:
In this paper the origin and evolution of the Sun's open magnetic flux
are considered for single magnetic bipoles as they are transported
across the Sun. The effects of magnetic flux transport on the radial
field at the surface of the Sun are modeled numerically by developing
earlier work by Wang, Sheeley, and Lean (2000). The paper considers how
the initial tilt of the bipole axis (α) and its latitude of emergence
affect the variation and magnitude of the surface and open magnetic
flux. The amount of open magnetic flux is estimated by constructing
potential coronal fields. It is found that the open flux may evolve
independently from the surface field for certain ranges of the tilt
angle. For a given tilt angle, the lower the latitude of emergence,
the higher the magnitude of the surface and open flux at the end of the
simulation. In addition, three types of behavior are found for the open
flux depending on the initial tilt angle of the bipole axis. When the
tilt is such that αge2° the open flux is independent of the surface
flux and initially increases before decaying away. In contrast, for
tilt angles in the range −16°&lt;α&lt;2° the open flux follows
the surface flux and continually decays. Finally, for αle−16° the
open flux first decays and then increases in magnitude towards a second
maximum before decaying away. This behavior of the open flux can be
explained in terms of two competing effects produced by differential
rotation. Firstly, differential rotation may increase or decrease the
open flux by rotating the centers of each polarity of the bipole at
different rates when the axis has tilt. Secondly, it decreases the
open flux by increasing the length of the polarity inversion line
where flux cancellation occurs. The results suggest that, in order
to reproduce a realistic model of the Sun's open magnetic flux over
a solar cycle, it is important to have accurate input data on the
latitude of emergence of bipoles along with the variation of their
tilt angles as the cycle progresses.

---------------------------------------------------------
Title: What are the Origins of Quiescent Coronal Soft X-Rays?
Authors: Foley, C. R.; Culhane, J. L.; Patsourakos, S.; Yurow, R.;
Moroney, C.; Mackay, D.
2002mwoc.conf..341F Altcode:
We have examined the evolution and modulation of the Sun's atmosphere
from the photosphere up to the outer corona through the decline and
rise of solar cycles 22, and 23 respectfully. For this we have used
Yohkoh soft X-ray telescope (SXT) images, Kitt peak magnetograms and EUV
spectra provided by the Coronal Diagnostic Spectrometer (CDS). We find
as Hara (1996, 1997) found, that there is a modulation of the coronal
brightness which varies annually in the high latitude activity zones,
and that this is linked to the presence and disappearance of active
regions on the sun's disk. We interpret our results with regards to the
emergence and diffusion of magnetic flux. We find that the appearance
of high latitude activity zones may be explained simply by the decay
of diffused active region flux, We also find evidence for a positive
temperature gradient within the corona from the emission profiles in
the different lines.

---------------------------------------------------------
Title: Solar cycle variation of the temperature structure within
the cores of coronal streamers
Authors: Foley, C. R.; Patsourakos, S.; Culhane, J. L.; MacKay, D.
2002A&A...381.1049F Altcode:
We use the Coronal Diagnostic Spectrometer onboard the Solar and
Heliospheric Observatory (SOHO) to analyze conditions in coronal
streamer structures observed close to solar minimum (1996 July 8)
and near maximum (1999 August 5). We measured the intensities of
emission lines from Fe IX-XV ions and found the most intense emission
to be from Fe XI at solar minimum and from Fe XV at solar maximum. We
then used the line ratio method with transitions in selected ions to
extract the radial temperature behavior in the structures. The solar
minimum peak values were about 1.4 MK at 1.3 Rsun, while
values derived close to solar maximum were consistent with the Yohkoh
observations at the last maximum, displaying an apparently asymptotic
temperature of around 2.2 MK above 1.2 Rsun. We discuss the
observations in relation to possible mechanisms for energy deposition
in large coronal structures at different phases of the solar cycle.

---------------------------------------------------------
Title: A Possible Solar Cycle Dependence to the Hemispheric Pattern
of Filament Magnetic Fields?
Authors: Mackay, D. H.; van Ballegooijen, A. A.
2001ApJ...560..445M Altcode:
The origin of the observed hemispheric pattern of filament magnetic
fields is considered. Using a magnetic flux transport model, we simulate
the interactions of magnetic bipoles with each other and with polar
magnetic fields in the rising and declining phases of the solar activity
cycle. In contrast to previous studies, the nonpotential character of
the initial coronal fields is taken into account, and the dependence
of the hemispheric pattern on the initial tilt and helicity of the
bipoles is considered. For the rising phase of the cycle, a range
of initial bipole twists and tilt angles can be found that reproduce
the observed hemispheric pattern. However, for the declining phase no
such range can be found: the predicted fields on the return arms at
the rear of switchbacks are consistent with filament observations,
but those on the high-latitude east-west arms are not. It is argued
that existing observations of the hemispheric pattern are weighted
toward the rising phase of the solar activity cycle and may give us a
biased view of the Sun. New observations of filament magnetic fields
are needed to determine whether there is a cycle dependence of the
observed hemispheric pattern.

---------------------------------------------------------
Title: Evolution of Solar Filament Channels Observed during a Major
Poleward Surge of Photospheric Magnetic Flux
Authors: Gaizauskas, V.; Mackay, D. H.; Harvey, K. L.
2001ApJ...558..888G Altcode:
We describe the evolution of a solar filament channel marked by
extremes: a length near one solar radius, and a duration of a year. Its
genesis centers on an episode of flux emergence so powerful that it
launched a surge of photospheric magnetic flux almost to the northern
polar cap. This extraordinary injection of new flux at the solar
surface occurred in midterm of the longest lived activity complex
of cycle 21 (~20 rotations). The new flux emerged just north of the
equator as a pair of adjacent activity complexes-a “supercluster”
of sunspots-remote from other active regions in a longitude band
spanning ~90°. Channels quickly formed along separate polarity
inversion lines in this large-scale quadrupolar configuration. None
of the initial channels survived more than two solar rotations; none
merged to form a greater whole. As individual bipoles within and
between the activity complexes expanded, fragmented, and cancelled,
only flux at the outermost edges of the adjacent complexes survived,
thanks to the remoteness of other strong concentrations of magnetic
flux. The result, after three solar rotations, was a simplified bipolar
pattern of poleward-streaming flux subject to global processes of flux
transport that sustained and extended it for up to a year. The long
and long-lived filament channel formed in the shape of a “switchback”
along the polarity inversion between the converging streams of opposite
polarity flux, continuing along the polarity inversion between the
migrating flux and the flux in the polar cap. Our observations reveal
large-scale swirled patterns of chromospheric fibrils from which we
infer that substantial negative helicity was built up across both
adjacent activity complexes during their emergence. The patterns were
still detectable in the migrating flux after the source regions had
disappeared. Convergence of opposite polarity fluxes with negative
helicity leads naturally to dextral filaments and filament channels,
consistent with the chirality rule for the northern hemisphere found
by Martin, Bilimoria, &amp; Tracadas. We measured the chiralities of
10 filament channels associated with the initial massive emergence
of magnetic flux and its subsequent surge poleward. Implications of
our findings on models for forming filaments and filament channels
are discussed.

---------------------------------------------------------
Title: Evolution of a Density Enhancement in a Stratified Atmosphere
With Uniform Vertical Magnetic Field
Authors: Mackay, D. H.; Galsgaard, K.
2001SoPh..198..289M Altcode:
In this paper the evolution of a density enhancement under the effect of
gravity in a stratified atmosphere is considered in a 2D simulation. The
stratified atmosphere is chosen with a high-density photosphere,
transition region and low-density corona where the enhancement is added
in non-equilibrium to the corona. The atmosphere is also threaded with
an initially uniform vertical magnetic field. If sufficiently strong,
the magnetic field plays an important role in the evolution of the
density enhancement as it tries to gain equilibrium. It not only
enables the density enhancement to maintain its shape as it falls,
but if strong enough results in the density enhancement rebounding a
number of times. Therefore both upward and downward velocities of the
enhancement are obtained. In all cases the density enhancement is found
to fall with speeds much less than the free-fall speed and can remain
in the corona at least 11 times longer than a free-fall particle. The
relevance of the simulations to the solar atmosphere is then discussed.

---------------------------------------------------------
Title: The Temperature of The Extended Solar Corona
Authors: Foley, C. R.; Culhane, J. L.; Mackay, D.
2001IAUS..203..505F Altcode:
We use the Coronal Diagnostic Spectrometer instrument on board the
Solar and Heliospheric Observatory to analyse coronal helmet streamer
structures observed close to the solar minimum / maximum on the 1996
July 8 / 1999 July 4-5th. The radial variation of peak electron
temperature is extracted out to 2 solar radii. These are found to
agree well with Yohkoh observations close to the solar maximum, but
are found to be reduced by around half a million close to the solar
minimum. Extrapolations of the photospheric field observations of MDI
are used to aid interpreted with regard to the energy depostion in
the low corona and solar wind.

---------------------------------------------------------
Title: Theory of Solar Chromospheric and Coronal Magnetic Fields
Authors: van Ballegooijen, A. A.; Mackay, D. H.
2001ASPC..248..105V Altcode: 2001mfah.conf..105V
No abstract at ADS

---------------------------------------------------------
Title: Comparison of Theory and Observations of the Chirality of
Filaments within a Dispersing Activity Complex
Authors: Mackay, D. H.; Gaizauskas, V.; van Ballegooijen, A. A.
2000ApJ...544.1122M Altcode:
We investigate the origin of the hemispheric pattern of filaments and
filament channels by comparing theoretical predictions with observations
of the chirality of filament channels within a dispersing activity
complex. Our aim is to determine how the chirality of each specific
channel arises so that general principles underlying the hemispheric
pattern can be recognized. We simulate the field lines representing
the filaments in the activity complex by applying a model of global
flux transport to an initial magnetic configuration. The model
combines the surface effects of differential rotation, meridional
flows, and supergranular diffusion along with a magnetofrictional
relaxation method in the overlying corona. The simulations are run
with and without injecting axial magnetic fields at polarity inversion
lines in the dispersing activity complex for four successive solar
rotations. When the initial magnetic configuration, based on synoptic
magnetic maps, is set to a potential field at the beginning of each
rotation, the simulations poorly predict the chirality of the filament
channels and filaments. The cases that predict the correct chirality
correspond to an initial polarity inversion line, which is north-south
the wrong chirality arises when the initial polarity inversion lines
lie east-west. Results improve when field-line connectivities at low
latitudes are retained and allowed to propagate to higher latitudes
without resetting the field to a potential configuration between
each rotation. When axial flux emergence exceeding 1×1019
Mx day-1 is included at the location of each filament, an
excellent agreement is obtained between the theory and observations. In
additon to predicting the correct chirality in all cases, axial flux
emergence allows more readily the production of inverse-polarity
dipped field lines needed to support filamentary mass. An origin
for the hemispheric pattern as a result of the combined effects of
flux transport, axial flux emergence, and magnetic helicity is then
discussed.

---------------------------------------------------------
Title: A Method to Determine the Heating Mechanisms of the Solar
Corona
Authors: Priest, E. R.; Foley, C. R.; Heyvaerts, J.; Arber, T. D.;
Mackay, D.; Culhane, J. L.; Acton, L. W.
2000ApJ...539.1002P Altcode:
One of the paradigms about coronal heating has been the belief that the
mean or summit temperature of a coronal loop is completely insensitive
to the nature of the heating mechanisms. However, we point out that
the temperature profile along a coronal loop is highly sensitive to
the form of the heating. For example, when a steady state heating
is balanced by thermal conduction, a uniform heating function makes
the heat flux a linear function of distance along the loop, while
T7/2 increases quadratically from the coronal footpoints;
when the heating is concentrated near the coronal base, the heat flux
is small and the T7/2 profile is flat above the base;
when the heat is focused near the summit of a loop, the heat flux
is constant and T7/2 is a linear function of distance
below the summit. It is therefore important to determine how the
heat deposition from particular heating mechanisms varies spatially
within coronal structures such as loops or arcades and to compare it
to high-quality measurements of the temperature profiles. We propose
a new two-part approach to try and solve the coronal heating problem,
namely, first of all to use observed temperature profiles to deduce the
form of the heating, and second to use that heating form to deduce the
likely heating mechanism. In particular, we apply this philosophy to
a preliminary analysis of Yohkoh observations of the large-scale solar
corona. This gives strong evidence against heating concentrated near the
loop base for such loops and suggests that heating uniformly distributed
along the loop is slightly more likely than heating concentrated at
the summit. The implication is that large-scale loops are heated in
situ throughout their length, rather than being a steady response
to low-lying heating near their feet or at their summits. Unless
waves can be shown to produce a heating close enough to uniform, the
evidence is therefore at present for these large loops more in favor
of turbulent reconnection at many small randomly distributed current
sheets, which is likely to be able to do so. In addition, we suggest
that the decline in coronal intensity by a factor of 100 from solar
maximum to solar minimum is a natural consequence of the observed
ratio of magnetic field strength in active regions and the quiet Sun;
the altitude of the maximum temperature in coronal holes may represent
the dissipation height of Alfvén waves by turbulent phase mixing;
and the difference in maximum temperature in closed and open regimes
may be understood in terms of the roles of the conductive flux there.

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Title: Mean Field Model for the Formation of Filament Channels on
the Sun
Authors: van Ballegooijen, A. A.; Priest, E. R.; Mackay, D. H.
2000ApJ...539..983V Altcode:
The coronal magnetic field is subject to random footpoint motions that
cause small-scale twisting and braiding of field lines. We present a
mean field theory describing the effects of such small-scale twists on
the large-scale coronal field. This theory assumes that the coronal
field is force free, with electric currents flowing parallel or
antiparallel to magnetic field lines. Random footpoint motions are
described in terms of diffusion of the mean magnetic field at the
photosphere. The appropriate mean field equations are derived, and
a numerical method for solving these equations in three dimensions
is presented. Preliminary results obtained with this method are
also presented. In particular the formation of filament channels is
studied. Filament channels are regions where the coronal magnetic field
is strongly aligned with the underlying polarity inversion line in
the photosphere. It is found that magnetic flux cancellation plays an
important role in the formation of such channels. Various models of the
coronal field are presented, including some in which the axial field is
assumed to originate from below the photosphere. The models reproduce
many of the observed features of filament channels, but the observed
hemisphere pattern of dextral and sinistral channels remains a mystery.

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Title: How Accurately Can We Determine the Coronal Heating Mechanism
in the Large-Scale Solar Corona?
Authors: Mackay, D. H.; Galsgaard, K.; Priest, E. R.; Foley, C. R.
2000SoPh..193...93M Altcode:
In recent papers by Priest et al., the nature of the coronal heating
mechanism in the large-scale solar corona was considered. The authors
compared observations of the temperature profile along large coronal
loops with simple theoretical models and found that uniform heating
along the loop gave the best fit to the observed data. This then led
them to speculate that turbulent reconnection is a likely method
to heat the large-scale solar corona. Here we reconsider their
data and their suggestion about the nature of the coronal heating
mechanism. Two distinct models are compared with the observations
of temperature profiles. This is done to determine the most likely
form of heating under different theoretical constraints. From this,
more accurate judgments on the nature of the coronal heating mechanism
are made. It is found that, due to the size of the error estimates in
the observed temperatures, it is extremely difficult to distinguish
between some of the different heat forms. In the initial comparison
the limited range of observed temperatures (T&gt;1.5 MK) in the
data sets suggests that heat deposited in the upper portions of the
loop, fits the data more accurately than heat deposited in the lower
portions. However if a fuller model temperature range (T&lt;1.0 MK)
is used results in contridiction to this are found. In light of this
several improvements are required from the observations in order to
produce theoretically meaningful results. This gives serious bounds
on the accuracy of the observations of the large-scale solar corona
in future satellite missions such a Solar-B or Stereo.

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Title: Dipped Magnetic Field Configurations Associated with Solar
Filaments
Authors: Mackay, D. H.; Longbottom, A. W.
1999ESASP.448..451M Altcode: 1999mfsp.conf..451M; 1999ESPM....9..451M
No abstract at ADS

---------------------------------------------------------
Title: On the Comparison of Filament Chirality and Axial Magnetic
Fields Deduced from a Flux Transport Model
Authors: Mackay, D. H.; Gaizauskas, V.; van Ballegooijen, A. A.
1999ESASP.448..507M Altcode: 1999mfsp.conf..507M; 1999ESPM....9..507M
No abstract at ADS

---------------------------------------------------------
Title: On the location of energy release and temperature profiles
along coronal loops
Authors: Galsgaard, K.; Mackay, D. H.; Priest, E. R.; Nordlund, Å.
1999SoPh..189...95G Altcode:
Several mechanisms have been suggested to contribute to the heating
of the solar corona, each of which deposits energy along coronal
loops in a characteristic way. To compare the theoretical models
with observations one has to derive observable quantities from the
models. One such parameter is the temperature profile along a loop. Here
numerical experiments of flux braiding are used to provide the spatial
distribution of energy deposition along a loop. It is found that
braiding produces a heat distribution along the loop which has slight
peaks near the footpoints and summit and whose magnitude depends on
the driving time. Using different examples of the heat deposition,
the temperature profiles along the loop are determined assuming a
steady state. Along with this, different methods for providing average
temperature profiles from the time-series have been investigated. These
give summit temperatures within approximately 10% of each other. The
distribution of the heating has a significant impact on both the summit
temperature and the temperature distribution along the loop. In each
case the ratio between the heat deposited and radiation provides a
scaling for the summit temperature.

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Title: Dipped Magnetic Field Configurations Associated with Filaments
and Barbs
Authors: Mackay, D. H.; Longbottom, A. W.; Priest, E. R.
1999SoPh..185...87M Altcode:
In this paper, three-dimensional linear force-free field configurations
that can be associated with filaments are considered. It is assumed
that the field configurations are suitable to represent filaments if
they contain magnetic dips. With the photospheric flux distribution
chosen to be an arcade with a dextral/sinistral axial component, it is
found that dipped configurations exist only for large values of alpha
(where, ∇×B=αB). The dips always lie above the polarity inversion
line in the centre of the channel between the flux regions. When the
dips are viewed from above to a depth of 1 Mm they resemble closely
the shape of filaments viewed in absorption on the solar disk. As the
magnitude of alpha increases, the horizontal and vertical extent of
the dips also increases, giving active-region filaments for low values
of alpha and quiescient filaments for high values of alpha. Dextral
filaments only form for negative values of alpha and sinistral
filaments for positive values of alpha. The portion of the field line
that is dipped is always of inverse polarity and the magnitude of the
field in the dipped region increases with height, both of which are
consistent with Leroy, Bommier, and Sahal-Bréchot (1983). Overlying
the region of dips there are arcades of normal polarity which have the
correct left-bearing/right-bearing orientation for dextral/sinistral
filaments. When the hypothesis of barbs occurring in dipped field lines
is used, barbs that branch out of the main axis and to the right/left
for dextral/sinistral filaments can be formed around minority polarity
elements on either side of the polarity inversion line. No barbs are
found around normal polarity elements. The model reproduces many of
the observed features of filament channels, filaments and their barbs.

---------------------------------------------------------
Title: Role of Helicity in the Formation of Intermediate Filaments
Authors: Mackay, D. H.; Priest, E. R.; Gaizauskas, V.; van
Ballegooijen, A. A.
1998SoPh..180..299M Altcode:
In the last few years new observations have shown that solar filaments
and filament channels have a surprising hemispheric pattern. To explain
this pattern, a new theory for filament channel and filament formation
is put forward. The theory describes the formation of a specific type of
filament, namely the `intermediate filament' which forms either between
active regions or at the boundary of an active region. It describes the
formation in terms of the emergence of a sheared activity complex. The
complex then interacts with remnant flux and, after convergence and
flux cancellation, the filament forms in the channel. A key feature
of the model is the net magnetic helicity of the complex. With the
correct sign a filament channel can form, but with the opposite sign
no filament channel forms after convergence. It is shown how the
hemispheric pattern of helicity in emerging flux regions produces the
observed hemispheric pattern for filaments.

---------------------------------------------------------
Title: Force-Free Models of a Filament Channel in Which a Filament
Forms
Authors: Mackay, D. H.; Gaizauskas, V.; Priest, E. R.
1998ASPC..150..286M Altcode: 1998npsp.conf..286M; 1998IAUCo.167..286M
No abstract at ADS

---------------------------------------------------------
Title: Preliminary Results for Coronal Magnetic Fields as Suggested
by MDI Magnetograms
Authors: Walsh, R. W.; Ireland, J.; Mackay, D. H.; Galsgaard, K.;
Longbottom, A. W.
1998ASPC..155..371W Altcode: 1998sasp.conf..371W
No abstract at ADS

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Title: The Skew of Polar Crown X-ray Arcades
Authors: McAllister, A. H.; Hundhausen, A. J.; Mackay, D.; Priest, E.
1998ASPC..150..430M Altcode: 1998npsp.conf..430M; 1998IAUCo.167..430M
No abstract at ADS

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Title: Force-free and Potential Models of a Filament Channel in
Which a Filament Forms
Authors: Mackay, D. H.; Gaizauskas, V.; Rickard, G. J.; Priest, E. R.
1997ApJ...486..534M Altcode:
Few examples of the creation of a filament channel or filament have
ever been documented. In a recent paper, Gaizauskas and coworkers
observed the early stages of creation of such a channel and then the
formation of a filament in it. The filament channel was born when
a new activity complex emerged near an old, decaying bipolar active
region. The filament itself then formed after convergence of flux in
the channel. In this paper, force-free models are constructed
for two phases of the channel's development. For the early days,
the models show that the formation of the filament channel seen in
Hα is due to the emerging activity complex. The field lines that
give the best comparison to the fibril observations are low-lying and
have a strong horizontal component. Later, when the activity complex
has matured and a filament has formed between it and the adjacent
decaying bipolar region, the models give a good representation of the
path of the filament in the channel. It is found that the presence of
flat or dipped field lines and of converging flux are necessary but
not sufficient conditions for filament formation. Furthermore, the
magnetic field lines of the filament itself form a narrow, vertical,
sheetlike flux-tube corridor that is flat and low-lying. It connects
one particular magnetic source to a sink and is bounded by separatrix
surfaces that separate the filament from the old remnant region and
most of the newly emerged flux.

---------------------------------------------------------
Title: The Skew of Polar Crown X-ray Arcades
Authors: McAllister, A. H.; Mackay, D.; Hundhausen, A. J.; Priest, E.
1997SPD....28.0255M Altcode: 1997BAAS...29..903M
A one-to-one relationship between the chirality of filament channels
and the skew (relative orientation) of the overlying coronal arcades,
as seen with the Yohkoh Soft X-ray Telescope (SXT) was found by Martin
and McAllister [1997]. The basis of the relationship is a sample of
over 30 mid-and low-latitude filaments during a 6 month period in
early 1992. This relationship can be coupled with the predictions
for the axial component of polar crown filaments based on the work
of Leroy et al. [1983] to predict the skew of polar crown arcades in
the recent cycle 22. Thus the axial component of the filament fields
along the southern polar crown is predicted to point to the west and a
similar component in the corona will lead to right skewed arcades. As
has been pointed out in the past, this orientation is inconsistent
with the action of photospheric differential rotation on an east-west
arcade [Ballegooijen and Martens, 1990]. In this poster we report
on the results of a recent survey of the SXT images over the whole
declining phase of cycle 22 (Oct. 1991 to June 1995). These results
are not in general agreement with those expected based on the past
filament observations. They show highly left skewed polar arcades
rather than the predicted right skew. The observations are, however,
in general agreement with the effects of differential rotation and
with recent numerical simulations of polar crown structures, which we
will also briefly present. This posses an unexpected and challenging
problem and we will discuss some possible ways of reconciling the
different observational results. Leroy, Bommier, and Sahal-Brechot,
The Magnetic Field in Prominences of the Polar Crown, Solar Physics,
83, 135-142, 1983. Martin and McAllister, The Chirality of X-ray
Coronal Arcades Overlying Quiescent Filaments, Astrophys. Journ.,
submitted, 1997. Ballegooijen and Martens, Magnetic Fields in Quiescent
Prominences, ApJ, 361, 283-289, 1990.

---------------------------------------------------------
Title: Basic magnetic field configurations for filament channels
and filaments
Authors: Mackay, D. H.; Priest, E. R.
1997A&AT...13..111M Altcode:
Recent observations of Martin et al. have revealed two new magnetic
and structural classes for solar filaments and filament channels. The
magnetic classes are called sinistral and dextral while the structural
classes are left-bearing and right-bearing. A potential model of the
magnetic field in a filament channel consistent with the observations
is developed, including the magnetic sources of network flux on both
sides of the channel and concentrations of flux along the channel. A
particular filament channel is also modelled by a set of discrete
magnetic sources and sinks approximating the observed flux of the
channel. In addition, the bending of a filament as it passes between
opposite polarity sources is modelled.

---------------------------------------------------------
Title: Basic magnetic field configurations for solar filament channels
and filaments
Authors: Mackay, Duncan H.
1997PhDT.......376M Altcode:
The three-dimensional magnetic structure of solar filament channels
and filaments is considered. A simple analytical potential model
of a filament channel is setup with line sources representing the
overlying arcades and point sources the flux of the filament. A possible
explanation of the distinct upper and lower bounds of a filament is
given. A more detailed numerical force-free model with discrete flux
sources is then developed and the effect of magnetic shear on the
separatrix surface explored. Dextral channels are shown to exist for a
wider range of negative values of the force-free alpha and sinistral
channels for positive values of alpha. Potential models of a variety
of coronal structures are then considered. The bending of a filament
is modelled and a method of determining the horizontal component of
a filament's magnetic field is proposed. Next, the observed opposite
skew of arcades lying above switchbacks of polarity inversion lines
is shown to be produced by a local flux imbalance at the corner
of the switchback. Then, the magnetic structure of a particular
filament in a filament channel is modelled using observations from
a photospheric magnetogram. It is shown that dips in the filaments
magnetic field could result from opposite polarity fragments lying
below the filament. Finally, the formation of a specific filament
channel and filament is modelled. The formation of the channel is
shown to be due to the emergence of new flux in a sheared state. It
is shown that convergence and reconnections between the new flux and
old remnant flux is required for the filament to form. The field lines
that represent the filament form a thin vertical sheet of flux. The
changing angle of inclination of the sheet gives the appearance of
twist. The method of formation is then generalised to other cases and
it is shown that a hemispheric pattern consistent with the results of
Martin et al. (1995) can be obtained.

---------------------------------------------------------
Title: A Potential-Field Model for Dextral and Sinistral Filament
Channels
Authors: Mackay, D. H.; Priest, E. R.
1996SoPh..167..281M Altcode:
Recent observations of Martin, Bilimoria, and Tracadas (1995) have
revealed two new magnetic and structural classes for solar filaments
and filament channels. The magnetic classes are called sinistral
and dextral, while the structural classes are left-bearing and
right-bearing. Dextral filaments dominate in the northern hemisphere
and sinistral in the southern. A model consistent with the observations
is developed with magnetic sources that represent the network flux
on both sides of the channel and extra concentrations of flux that
produce the strong field component along the channel. We suggest that
it is the imbalance of flux locations along the channel that creates
the field of a filament channel. The resulting separatrix surfaces
have distinct upper and lower boundaries that may produce the upper
boundary of the filament cavity or filament and the lower boundary
of the filament. The model is applied to a specific filament channel,
with discrete sources and sinks that represent the flux observed in a
photospheric magnetogram. The resulting three-dimensional field lines
near the filament location are low-lying and possess dips.

---------------------------------------------------------
Title: A Model for Dextral and Sinistral Prominences
Authors: Priest, E. R.; van Ballegooijen, A. A.; Mackay, D. H.
1996ApJ...460..530P Altcode:
In a recent paper Martin and coworkers have discussed several striking
facts about the structure of solar prominences and the filament
channels in which they lie. They form two classes, called dextral and
sinistral. In a dextral (sinistral) prominence, an observer viewing a
prominence or filament channel from the positive-polarity side would
see the magnetic field point to the right (left) along the axis of the
filament channel, whereas an observer viewing from above would see the
prominence feet bear off the axis to the right (left). Furthermore,
dextral prominences dominate the northern hemisphere and sinistral
the southern hemisphere, regardless of the cycle. Fibrils in the
filament channels do not cross the prominence but usually stream from
or to plagettes parallel to the prominence axis. These pioneering
observations suggest that there is a coherent organizational principle
orchestrating the global nature of prominences, and they have led us
to reexamine the standard paradigms of contemporary prominence theory,
such as that (1) prominences form in a sheared force-free arcade, (2)
formation is by radiative instability, (3) the prominence material is
static, and (4) eruption occurs when the shear or twist is too great. We
propose a new model which accounts for the above new observational
features in a natural way, replaces many of the above paradigms, and
explains the previously puzzling feet of a prominence. It is a dynamic
model in which a prominence is maintained by the continual input of mass
and magnetic flux. The correct global dextral and sinistral patterns
for high-latitude east-west prominences (such as those in the polar
crown) are created by an organizational principle that includes the
combined effects of differential rotation on subphotospheric flux,
its subsequent emergence by magnetic buoyancy, and its rearrangement
by flux reconnection to form a filament channel with magnetic flux
oriented along its axis. Continual emergence and reconnection creates
a prominence as a flux tube along the filament channel axis and
filled with cool plasma which is lifted up from the photosphere and
chromosphere by the reconnection process. Prominences at low latitudes
are in this model formed in a similar way, except that it is a general
subphotospheric flow (rather than differential rotation) which acts and
so may produce either dextral or sinistral structures, depending on the
sense of the flow. The effect of neighboring plagettes in avoiding the
prominence and making it snake its way along the filament channel is
modeled. It is suggested that feet are short-lived structures caused
by the interaction of nearby magnetic fragments with the prominence
field and may represent either the addition or the extraction of mass
from the prominence.