parnell

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Author name code: parnell
ADS astronomy entries on 2022-09-14
author:"Parnell, Clare E."

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Title: Critical Science Plan for the Daniel K. Inouye Solar Telescope
(DKIST)
Authors: Rast, Mark P.; Bello González, Nazaret; Bellot Rubio,
Luis; Cao, Wenda; Cauzzi, Gianna; Deluca, Edward; de Pontieu, Bart;
Fletcher, Lyndsay; Gibson, Sarah E.; Judge, Philip G.; Katsukawa,
Yukio; Kazachenko, Maria D.; Khomenko, Elena; Landi, Enrico; Martínez
Pillet, Valentín; Petrie, Gordon J. D.; Qiu, Jiong; Rachmeler,
Laurel A.; Rempel, Matthias; Schmidt, Wolfgang; Scullion, Eamon; Sun,
Xudong; Welsch, Brian T.; Andretta, Vincenzo; Antolin, Patrick; Ayres,
Thomas R.; Balasubramaniam, K. S.; Ballai, Istvan; Berger, Thomas E.;
Bradshaw, Stephen J.; Campbell, Ryan J.; Carlsson, Mats; Casini,
Roberto; Centeno, Rebecca; Cranmer, Steven R.; Criscuoli, Serena;
Deforest, Craig; Deng, Yuanyong; Erdélyi, Robertus; Fedun, Viktor;
Fischer, Catherine E.; González Manrique, Sergio J.; Hahn, Michael;
Harra, Louise; Henriques, Vasco M. J.; Hurlburt, Neal E.; Jaeggli,
Sarah; Jafarzadeh, Shahin; Jain, Rekha; Jefferies, Stuart M.; Keys,
Peter H.; Kowalski, Adam F.; Kuckein, Christoph; Kuhn, Jeffrey R.;
Kuridze, David; Liu, Jiajia; Liu, Wei; Longcope, Dana; Mathioudakis,
Mihalis; McAteer, R. T. James; McIntosh, Scott W.; McKenzie, David
E.; Miralles, Mari Paz; Morton, Richard J.; Muglach, Karin; Nelson,
Chris J.; Panesar, Navdeep K.; Parenti, Susanna; Parnell, Clare E.;
Poduval, Bala; Reardon, Kevin P.; Reep, Jeffrey W.; Schad, Thomas A.;
Schmit, Donald; Sharma, Rahul; Socas-Navarro, Hector; Srivastava,
Abhishek K.; Sterling, Alphonse C.; Suematsu, Yoshinori; Tarr, Lucas
A.; Tiwari, Sanjiv; Tritschler, Alexandra; Verth, Gary; Vourlidas,
Angelos; Wang, Haimin; Wang, Yi-Ming; NSO and DKIST Project; DKIST
Instrument Scientists; DKIST Science Working Group; DKIST Critical
Science Plan Community
2021SoPh..296...70R Altcode: 2020arXiv200808203R
The National Science Foundation's Daniel K. Inouye Solar Telescope
(DKIST) will revolutionize our ability to measure, understand,
and model the basic physical processes that control the structure
and dynamics of the Sun and its atmosphere. The first-light DKIST
images, released publicly on 29 January 2020, only hint at the
extraordinary capabilities that will accompany full commissioning of
the five facility instruments. With this Critical Science Plan (CSP)
we attempt to anticipate some of what those capabilities will enable,
providing a snapshot of some of the scientific pursuits that the DKIST
hopes to engage as start-of-operations nears. The work builds on the
combined contributions of the DKIST Science Working Group (SWG) and
CSP Community members, who generously shared their experiences, plans,
knowledge, and dreams. Discussion is primarily focused on those issues
to which DKIST will uniquely contribute.

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Title: A comparison of methods for finding magnetic nulls in
simulations and in situ observations of space plasmas
Authors: Olshevsky, V.; Pontin, D. I.; Williams, B.; Parnell, C. E.;
Fu, H. S.; Liu, Y.; Yao, S.; Khotyaintsev, Y. V.
2020A&A...644A.150O Altcode: 2021arXiv210102014O
Context. Magnetic nulls are ubiquitous in space plasmas, and are
of interest as sites of localised energy dissipation or magnetic
reconnection. As such, a number of methods have been proposed for
detecting nulls in both simulation data and in situ spacecraft data
from Earth's magnetosphere. The same methods can be applied to detect
stagnation points in flow fields. Aims: In this paper we describe
a systematic comparison of different methods for finding magnetic
nulls. The Poincaré index method, the first-order Taylor expansion
(FOTE) method, and the trilinear method are considered. Methods:
We define a magnetic field containing fourteen magnetic nulls whose
positions and types are known to arbitrary precision. Furthermore,
we applied the selected techniques in order to find and classify
those nulls. Two situations are considered: one in which the magnetic
field is discretised on a rectangular grid, and the second in which the
magnetic field is discretised along synthetic "spacecraft trajectories"
within the domain. Results: At present, FOTE and trilinear are
the most reliable methods for finding nulls in the spacecraft data
and in numerical simulations on Cartesian grids, respectively. The
Poincaré index method is suitable for simulations on both tetrahedral
and hexahedral meshes. Conclusions: The proposed magnetic field
configuration can be used for grading and benchmarking the new and
existing tools for finding magnetic nulls and flow stagnation points.

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Title: Analysing AIA Flare Observations using Convolutional Neural
Networks
Authors: Love, Teri; Neukirch, Thomas; Parnell, Clare E.
2020FrASS...7...34L Altcode: 2020arXiv200513287L
In order to efficiently analyse the vast amount of data generated
by solar space missions and ground-based instruments, modern machine
learning techniques such as decision trees, support vector machines
(SVMs) and neural networks can be very useful. In this paper we present
initial results from using a convolutional neural network (CNN) to
analyse observations from the Atmospheric Imaging Assembly (AIA) in
the 1600 ̊A wavelength. The data is pre-processed to locate flaring
regions where Hα flare ribbons are visible in the observations. The CNN
is created and trained to automatically analyse the shape and position
of the flare ribbons, by identifying whether a data set belongs into
one of four classes: two-ribbon flare, compact/circular ribbon flare,
limb flare or quiet Sun, with the final class acting as a control for
any data included in the training or test sets where flaring regions are
not present. The network created can classify flare ribbon observations
into any of the four classes with a final accuracy of 94%. Initial
results show that most of the flares are correctly classified with
the limb flare class being the only class where accuracy drops and
some observations are wrongly classified

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Title: Particle acceleration with anomalous pitch angle scattering
in 3D separator reconnection
Authors: Borissov, A.; Neukirch, T.; Kontar, E. P.; Threlfall, J.;
Parnell, C. E.
2020A&A...635A..63B Altcode: 2020arXiv200107548B
Context. Understanding how the release of stored magnetic energy
contributes to the generation of non-thermal high energy particles
during solar flares is an important open problem in solar physics. There
is a general consensus that magnetic reconnection plays a fundamental
role in the energy release and conversion processes taking place during
flares. A common approach for investigating how reconnection contributes
to particle acceleration is to use test particle calculations in
electromagnetic fields derived from numerical magnetohydrodynamic (MHD)
simulations of reconnecting magnetic fields. These MHD simulations use
anomalous resistivities that are orders of magnitude larger than the
Spitzer resistivity that is based on Coulomb collisions. The processes
leading to such an enhanced resistivity should also affect the test
particles, for example, through pitch angle scattering. This study
explores the effect of such a link between the level of resistivity
and its impact on particle orbits and builds on a previous study using
a 2D MHD simulation of magnetic reconnection. Aims: This paper
aims to extend the previous investigation to a 3D magnetic reconnection
configuration and to study the effect on test particle orbits. 
Methods: We carried out orbit calculations using a 3D MHD simulation of
reconnection in a magnetic field with a magnetic separator. The orbit
calculations use the relativistic guiding centre approximation but,
crucially, they also include pitch angle scattering using stochastic
differential equations. The effects of varying the resistivity and
the models for pitch angle scattering on particle orbit trajectories,
final positions, energy spectra, final pitch angle distribution,
and orbit duration are all studied in detail. Results: Pitch
angle scattering widens highly collimated beams of unscattered orbit
trajectories, allowing orbits to access previously unaccessible field
lines; this causes final positions to spread along other topological
structures which could not be accessed without scattering. Scattered
orbit energy spectra are found to be predominantly affected by the
level of anomalous resistivity, with the pitch angle scattering model
only playing a role in specific, isolated cases. This is in contrast
to the study involving a 2D MHD simulation of magnetic reconnection,
where pitch angle scattering had a more noticeable effect on the energy
spectra. Pitch scattering effects are found to play a crucial role in
determining the pitch angle and orbit duration distributions.

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Title: Determining whether the squashing factor, Q, would be a good
indicator of reconnection in a resistive MHD experiment devoid of
null points
Authors: Reid, J.; Parnell, C. E.; Hood, A. W.; Browning, P. K.
2020A&A...633A..92R Altcode:
The squashing factor of a magnetic field, Q, is commonly used as an
indicator of magnetic reconnection, but few studies seek to evaluate
how reliable it is in comparison with other possible reconnection
indicators. By using a full, self-consistent, three-dimensional,
resistive magnetohydrodynamic experiment of interacting magnetic strands
constituting a coronal loop, Q and several different quantities are
determined. Each is then compared with the necessary and sufficient
condition for reconnection, namely the integral along a field line
of the component of the electric field parallel to the magnetic
field. Among the reconnection indicators explored, we find the squashing
factor less successful when compared with alternatives, such as Ohmic
heating. In a reconnecting magnetic field devoid of null points, our
work suggests that Q, being a geometric measure of the magnetic field,
is not a reliable indicator of the onset or a diagnostic of the location
of magnetic reconnection in some configurations.

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Title: Coronal energy release by MHD avalanches: Heating mechanisms
Authors: Reid, J.; Cargill, P. J.; Hood, A. W.; Parnell, C. E.; Arber,
T. D.
2020A&A...633A.158R Altcode:
The plasma heating associated with an avalanche involving three
twisted magnetic threads within a coronal loop is investigated using
three-dimensional magnetohydrodynamic simulations. The avalanche
is triggered by the kink instability of one thread, with the others
being engulfed as a consequence. The heating as a function of both
time and location along the strands is evaluated. It is shown to be
bursty at all times but to have no preferred spatial location. While
there appears to be a level of "background" heating, this is shown
to be comprised of individual, small heating events. A comparison
between viscous and resistive (Ohmic) heating demonstrates that the
strongest heating events are largely associated with the Ohmic heating
that arises when the current exceeds a critical value. Viscous heating
is largely (but not entirely) associated with smaller events. Ohmic
heating dominates viscous heating only at the time of the initial kink
instability. It is also demonstrated that a variety of viscous models
lead to similar heating rates, suggesting that the system adjusts to
dissipate the same amount of energy.

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Title: Coronal energy release by MHD avalanches: continuous driving
Authors: Reid, J.; Hood, A. W.; Parnell, C. E.; Browning, P. K.;
Cargill, P. J.
2018A&A...615A..84R Altcode:
Previous work has confirmed the concept of a magnetohydrodynamic (MHD)
avalanche in pre-stressed threads within a coronal loop. We undertook
a series of full, three-dimensional MHD simulations in order to create
three threads by twisting the magnetic field through boundary motions
until an instability ensues. We find that, following the original
instability, one unstable thread can disrupt its neighbours with
continued driving. A "bursty" heating profile results, with a series
of ongoing energy releases, but no evident steady state. For the first
time using full MHD, we show that avalanches are a viable mechanism
for the storing and release of magnetic energy in the solar corona,
as a result of photospheric motions.

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Title: MHS Models of Current Layers in the Solar Atmosphere
Authors: Parnell, Clare E.
2018GMS...235..219P Altcode:
No abstract at ADS

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Title: Effects of Anomalous Resistivity on Particle Acceleration
due to Pitch Angle Scattering.
Authors: Borissov, A.; Kontar, E.; Neukirch, T.; Threlfall, J. W.;
Stevenson, J.; Parnell, C. E.
2017AGUFMSH41B2765B Altcode:
The mechanisms for generation of non-thermal accelerated particles in
flares is one of the outstanding problems in solar physics. The energy
for powering solar flares fundamentally comes from the coronal magnetic
field and its release involves magnetic reconnection. One direct way
of accelerating charged particles is due to the parallel electric field
generated during magnetic reconnection. To achieve a sufficiently rapid
release of energy an anomalous resistivity, several orders of magnitude
larger than the Spitzer resistivity in the corona, is often invoked,
particularly when performing mganetohydrodynamic (MHD) simulations
of solar flares. Since resistivity is fundamentally connected to
particle scattering, an enhanced anomalous resistivity relative to the
Spitzer resistivity should result in an enhanced scattering frequency
relative to the Coulomb scattering rate. We present results of test
particle simulations that attempt to account for these phenomena by
introducing pitch angle scattering at a rate dependent on the ratio of
the anomalous to Spitzer resistivity in the context of MHD simulations
of magnetic reconnection. We find that test particle trajectories and
durations are significantly modified by the presence of resistivity
dependent pitch angle scattering, with particle energy spectra also
being affected in some cases

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Title: Particle Acceleration Due to Coronal Non-null Magnetic
Reconnection
Authors: Threlfall, James; Neukirch, Thomas; Parnell, Clare Elizabeth
2017SoPh..292...45T Altcode:
Various topological features, for example magnetic null points and
separators, have been inferred as likely sites of magnetic reconnection
and particle acceleration in the solar atmosphere. In fact, magnetic
reconnection is not constrained to solely take place at or near such
topological features and may also take place in the absence of such
features. Studies of particle acceleration using non-topological
reconnection experiments embedded in the solar atmosphere are
uncommon. We aim to investigate and characterise particle behaviour in a
model of magnetic reconnection which causes an arcade of solar coronal
magnetic field to twist and form an erupting flux rope, crucially in
the absence of any common topological features where reconnection
is often thought to occur. We use a numerical scheme that evolves
the gyro-averaged orbit equations of single electrons and protons in
time and space, and simulate the gyromotion of particles in a fully
analytical global field model. We observe and discuss how the magnetic
and electric fields of the model and the initial conditions of each
orbit may lead to acceleration of protons and electrons up to 2 MeV
in energy (depending on model parameters). We describe the morphology
of time-dependent acceleration and impact sites for each particle
species and compare our findings to those recovered by topologically
based studies of three-dimensional (3D) reconnection and particle
acceleration. We also broadly compare aspects of our findings to general
observational features typically seen during two-ribbon flare events.

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Title: Particle dynamics in a non-flaring solar active region model
Authors: Threlfall, J.; Bourdin, Ph. -A.; Neukirch, T.; Parnell, C. E.
2016A&A...587A...4T Altcode: 2015arXiv151004211T
 Aims: The aim of this work is to investigate and characterise
particle behaviour in an (observationally-driven) 3D magnetohydrodynamic
(MHD) model of the solar atmosphere above a slowly evolving, non-flaring
active region. Methods: We use a relativistic guiding-centre
particle code to investigate the behaviour of selected particle orbits,
distributed throughout a single snapshot of the 3D MHD simulation. Results: Two distinct particle acceleration behaviours are recovered,
which affect both electrons and protons: (I) direct acceleration
along field lines and (II) tangential drifting of guiding centres with
respect to local magnetic field. However, up to 40% of all particles
actually experience a form of (high energy) particle trap, because of
changes in the direction of the electric field and unrelated to the
strength of the magnetic field; such particles are included in the first
category. Additionally, category (I) electron and proton orbits undergo
surprisingly strong acceleration to non-thermal energies (≲42 MeV),
because of the strength and extent of super-Dreicer electric fields
created by the MHD simulation. Reducing the electric field strength
of the MHD model does not significantly affect the efficiency of the
(electric field-based) trapping mechanism, but does reduce the peak
energies gained by orbits. We discuss the implications for future
experiments, which aim to simulate non-flaring active region heating
and reconnection.

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Title: Particle acceleration at reconnecting separator current layers
Authors: Threlfall, J.; Stevenson, J. E. H.; Parnell, C. E.;
Neukirch, T.
2016A&A...585A..95T Altcode: 2015arXiv151004215T
 Aims: The aim of this work is to investigate and characterise
particle behaviour in a 3D magnetohydrodynamic (MHD) model of a
reconnecting magnetic separator. Methods: We use a relativistic
guiding-centre test-particle code to investigate electron and
proton acceleration in snapshots from 3D MHD separator reconnection
experiments, and compare the results with findings from an analytical
separator reconnection model studied in a previous investigation. Results: The behaviour and acceleration of large distributions of
particles are examined in detail for both analytical and numerical
separator reconnection models. Accelerated particle orbit trajectories
are shown to follow the separator before leaving the system along the
separatrix surface of one of the nulls (determined by particle species)
in the system of both models. A sufficiently localised electric field
about the separator causes the orbits to appear to follow the spine
bounding the separatrix surface field lines instead. We analyse and
discuss the locations and spread of accelerated particle orbit final
positions, which are seen to change over time in the numerical separator
reconnection model. We deduce a simple relationship between the final
energy range of particle orbits and the model dimensions, and discuss
its implications for observed magnetic separators in the solar corona.

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Title: A Comparison of Global Magnetic Field Skeletons and
Active-Region Upflows
Authors: Edwards, S. J.; Parnell, C. E.; Harra, L. K.; Culhane, J. L.;
Brooks, D. H.
2016SoPh..291..117E Altcode: 2015SoPh..tmp..161E
Plasma upflows have been detected in active regions using Doppler
velocity maps. The origin and nature of these upflows is not well known
with many of their characteristics determined from the examination
of single events. In particular, some studies suggest these upflows
occur along open field lines and, hence, are linked to sources of
the solar wind. To investigate the relationship these upflows may
have with the solar wind, and to probe what may be driving them, this
paper considers seven active regions observed on the solar disc using
the Extreme ultraviolet Imaging Spectrometer aboard Hinode between
August 2011 and September 2012. Plasma upflows are observed in all
these active regions. The locations of these upflows are compared
to the global potential magnetic field extrapolated from the Solar
Dynamics Observatory, Helioseismic and Magnetic Imager daily synoptic
magnetogram taken on the day the upflows were observed. The structure
of the magnetic field is determined by constructing its magnetic
skeleton in order to help identify open-field regions and also sites
where magnetic reconnection at global features is likely to occur. As
a further comparison, measurements of the temperature, density and
composition of the plasma are taken from regions with active-region
upflows. In most cases the locations of the upflows in the active
regions do not correspond to areas of open field, as predicted by
a global coronal potential-field model, and therefore these upflows
are not always sources of the slow solar wind. The locations of the
upflows are, in general, intersected by separatrix surfaces associated
with null points located high in the corona; these could be important
sites of reconnection with global consequences.

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Title: Spontaneous reconnection at a separator current layer:
2. Nature of the waves and flows
Authors: Stevenson, J. E. H.; Parnell, C. E.
2015JGRA..12010353S Altcode: 2015arXiv150907743S
Sudden destabilizations of the magnetic field, such as those caused
by spontaneous reconnection, will produce waves and/or flows. Here
we investigate the nature of the plasma motions resulting from
spontaneous reconnection at a 3-D separator. In order to clearly see
these perturbations, we start from a magnetohydrostatic equilibrium
containing two oppositely signed null points joined by a generic
separator along which lies a twisted current layer. The nature of
the magnetic reconnection initiated in this equilibrium as a result
of an anomalous diffusivity is discussed in detail in Stevenson and
Parnell (2015). The resulting sudden loss of force balance inevitably
generates waves that propagate away from the diffusion region carrying
the dissipated current. In their wake a twisting stagnation flow,
in planes perpendicular to the separator, feeds flux back into the
original diffusion site (the separator) in order to try to regain
equilibrium. This flow drives a phase of slow weak impulsive bursty
reconnection that follows on after the initial fast-reconnection phase.

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Title: Spontaneous reconnection at a separator current layer:
1. Nature of the reconnection
Authors: Stevenson, J. E. H.; Parnell, C. E.
2015JGRA..12010334S Altcode: 2015arXiv150907729S
Magnetic separators, which lie on the boundary between four
topologically distinct flux domains, are prime locations in
three-dimensional magnetic fields for reconnection, especially in the
magnetosphere between the planetary and interplanetary magnetic fields
and also in the solar atmosphere. Little is known about the details
of separator reconnection, and so the aim of this paper, which is the
first of two, is to study the properties of magnetic reconnection at
a single separator. Three-dimensional, resistive magnetohydrodynamic
numerical experiments are run to study separator reconnection starting
from a magnetohydrostatic equilibrium which contains a twisted current
layer along a single separator linking a pair of opposite-polarity
null points. The resulting reconnection occurs in two phases. The
first is short involving rapid reconnection in which the current at
the separator is reduced by a factor of around 2.3. Most (75%) of the
magnetic energy is converted during this phase, via Ohmic dissipation,
directly into internal energy, with just 0.1% going into kinetic
energy. During this phase the reconnection occurs along most of the
separator away from its ends (the nulls) but in an asymmetric manner
which changes both spatially and temporally over time. The second phase
is much longer and involves slow impulsive bursty reconnection. Again,
Ohmic heating dominates over viscous damping. Here the reconnection
occurs in small localized bursts at random anywhere along the separator.

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Title: The appearance, motion, and disappearance of three-dimensional
magnetic null points
Authors: Murphy, Nicholas A.; Parnell, Clare E.; Haynes, Andrew L.
2015PhPl...22j2117M Altcode: 2015arXiv150905915M
While theoretical models and simulations of magnetic reconnection
often assume symmetry such that the magnetic null point when present
is co-located with a flow stagnation point, the introduction of
asymmetry typically leads to non-ideal flows across the null point. To
understand this behavior, we present exact expressions for the motion
of three-dimensional linear null points. The most general expression
shows that linear null points move in the direction along which the
magnetic field and its time derivative are antiparallel. Null point
motion in resistive magnetohydrodynamics results from advection by the
bulk plasma flow and resistive diffusion of the magnetic field, which
allows non-ideal flows across topological boundaries. Null point motion
is described intrinsically by parameters evaluated locally; however,
global dynamics help set the local conditions at the null point. During
a bifurcation of a degenerate null point into a null-null pair or the
reverse, the instantaneous velocity of separation or convergence of
the null-null pair will typically be infinite along the null space of
the Jacobian matrix of the magnetic field, but with finite components
in the directions orthogonal to the null space. Not all bifurcating
null-null pairs are connected by a separator. Furthermore, except under
special circumstances, there will not exist a straight line separator
connecting a bifurcating null-null pair. The motion of separators cannot
be described using solely local parameters because the identification
of a particular field line as a separator may change as a result of
non-ideal behavior elsewhere along the field line.

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Title: The appearance, motion, and disappearance of 3D magnetic
null points
Authors: Murphy, Nicholas A.; Parnell, Clare; Haynes, Andrew
2015shin.confE...7M Altcode:
While theoretical models and simulations of magnetic reconnection
often assume symmetry such that the magnetic null point when present
is co-located with a flow stagnation point, the introduction of
asymmetry typically leads to non-ideal flows across the null point. To
understand this behavior, we present exact expressions for the motion
of three-dimensional linear null points. The most general expression
shows that linear null points move in the direction along which the
vector field and its time derivative are antiparallel. Null point
motion in resistive magnetohydrodynamics results from advection by
the bulk plasma flow and resistive diffusion of the magnetic field,
which allows non-ideal flows across topological boundaries. Null point
motion is described intrinsically by parameters evaluated locally;
however, global dynamics help set the local conditions at the null
point. Linear null points typically appear and disappear in pairs
during the bifurcation of a degenerate null point that has a linear
structure with a one-dimensional null space. During a bifurcation,
the instanteous velocity of separation or convergence of the null-null
pair will typically be infinite along the null space but with finite
components in the directions orthogonal to the null space. Except under
special circumstances, there will not exist a straight line separator
connecting a bifurcating null-null pair. The motion of separators cannot
be described using solely local parameters because the identification
of a particular field line as a separator may change as a result of
non-ideal behavior elsewhere along the field line.

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Title: Null Point Distribution in Global Coronal Potential Field
Extrapolations
Authors: Edwards, S. J.; Parnell, C. E.
2015SoPh..290.2055E Altcode: 2015SoPh..tmp...93E
Magnetic null points are points in space where the magnetic
field is zero. Thus, they can be important sites for magnetic
reconnection by virtue of the fact that they are weak points in the
magnetic field and also because they are associated with topological
structures, such as separators, which lie on the boundary between four
topologically distinct flux domains and therefore are also locations
where reconnection occurs. The number and distribution of nulls in a
magnetic field acts as a measure of the complexity of the field.

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Title: Extreme ultraviolet imaging of three-dimensional magnetic
reconnection in a solar eruption
Authors: Sun, J. Q.; Cheng, X.; Ding, M. D.; Guo, Y.; Priest, E. R.;
Parnell, C. E.; Edwards, S. J.; Zhang, J.; Chen, P. F.; Fang, C.
2015NatCo...6.7598S Altcode: 2015NatCo...6E7598S; 2015arXiv150608255S
Magnetic reconnection, a change of magnetic field connectivity, is
a fundamental physical process in which magnetic energy is released
explosively, and it is responsible for various eruptive phenomena in the
universe. However, this process is difficult to observe directly. Here,
the magnetic topology associated with a solar reconnection event is
studied in three dimensions using the combined perspectives of two
spacecraft. The sequence of extreme ultraviolet images clearly shows
that two groups of oppositely directed and non-coplanar magnetic loops
gradually approach each other, forming a separator or quasi-separator
and then reconnecting. The plasma near the reconnection site is
subsequently heated from ~1 to &gt;=5 MK. Shortly afterwards, warm flare
loops (~3 MK) appear underneath the hot plasma. Other observational
signatures of reconnection, including plasma inflows and downflows, are
unambiguously revealed and quantitatively measured. These observations
provide direct evidence of magnetic reconnection in a three-dimensional
configuration and reveal its origin.

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Title: Is magnetic topology important for heating the solar
atmosphere?
Authors: Parnell, Clare E.; Stevenson, Julie E. H.; Threlfall, James;
Edwards, Sarah J.
2015RSPTA.37340264P Altcode: 2015arXiv150505701P
Magnetic fields permeate the entire solar atmosphere weaving an
extremely complex pattern on both local and global scales. In order
to understand the nature of this tangled web of magnetic fields, its
magnetic skeleton, which forms the boundaries between topologically
distinct flux domains, may be determined. The magnetic skeleton consists
of null points, separatrix surfaces, spines and separators. The
skeleton is often used to clearly visualize key elements of the
magnetic configuration, but parts of the skeleton are also locations
where currents and waves may collect and dissipate. In this review,
the nature of the magnetic skeleton on both global and local scales,
over solar cycle time scales, is explained. The behaviour of wave
pulses in the vicinity of both nulls and separators is discussed and
so too is the formation of current layers and reconnection at the
same features. Each of these processes leads to heating of the solar
atmosphere, but collectively do they provide enough heat, spread over
a wide enough area, to explain the energy losses throughout the solar
atmosphere? Here, we consider this question for the three different
solar regions: active regions, open-field regions and the quiet Sun. We
find that the heating of active regions and open-field regions is highly
unlikely to be due to reconnection or wave dissipation at topological
features, but it is possible that these may play a role in the heating
of the quiet Sun. In active regions, the absence of a complex topology
may play an important role in allowing large energies to build up
and then, subsequently, be explosively released in the form of a solar
flare. Additionally, knowledge of the intricate boundaries of open-field
regions (which the magnetic skeleton provides) could be very important
in determining the main acceleration mechanism(s) of the solar wind.

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Title: Particle acceleration at a reconnecting magnetic separator
Authors: Threlfall, J.; Neukirch, T.; Parnell, C. E.; Eradat Oskoui, S.
2015A&A...574A...7T Altcode: 2014arXiv1410.6465T
Context. While the exact acceleration mechanism of energetic particles
during solar flares is (as yet) unknown, magnetic reconnection plays
a key role both in the release of stored magnetic energy of the
solar corona and the magnetic restructuring during a flare. Recent
work has shown that special field lines, called separators, are
common sites of reconnection in 3D numerical experiments. To date, 3D
separator reconnection sites have received little attention as particle
accelerators. Aims: We investigate the effectiveness of separator
reconnection as a particle acceleration mechanism for electrons and
protons. Methods: We study the particle acceleration using a
relativistic guiding-centre particle code in a time-dependent kinematic
model of magnetic reconnection at a separator. Results: The
effect upon particle behaviour of initial position, pitch angle,
and initial kinetic energy are examined in detail, both for specific
(single) particle examples and for large distributions of initial
conditions. The separator reconnection model contains several free
parameters, and we study the effect of changing these parameters upon
particle acceleration, in particular in view of the final particle
energy ranges that agree with observed energy spectra.

---------------------------------------------------------
Title: The nature of separator current layers in MHS
equilibria. I. Current parallel to the separator
Authors: Stevenson, J. E. H.; Parnell, C. E.; Priest, E. R.; Haynes,
A. L.
2015A&A...573A..44S Altcode: 2014arXiv1410.8691S
Context. Separators, which are in many ways the three-dimensional
equivalent to two-dimensional nulls, are important sites for magnetic
reconnection. Magnetic reconnection occurs in strong current layers
which have very short length scales. Aims: The aim of this
work is to explore the nature of current layers around separators. A
separator is a special field line which lies along the intersection
of two separatrix surfaces and forms the boundary between four
topologically distinct flux domains. In particular, here the current
layer about a separator that joins two 3D nulls and lies along the
intersection of their separatrix surfaces is investigated. 
Methods: A magnetic configuration containing a single separator
embedded in a uniform plasma with a uniform electric current parallel
to the separator is considered. This initial magnetic setup, which
is not in equilibrium, relaxes in a non-resistive manner to form
an equilibrium. The relaxation is achieved using the 3D MHD code,
Lare3d, with resistivity set to zero. A series of experiments with
varying initial current are run to investigate the characteristics of
the resulting current layers present in the final (quasi-)equilibrium
states. Results: In each experiment, the separator collapses
and a current layer forms along it. The dimensions and strength of
the current layer increase with initial current. It is found that
separator current layers formed from current parallel to the separator
are twisted. Also the collapse of the separator is a process that
evolves like an infinite-time singularity where the length, width and
peak current in the layer grow slowly whilst the depth of the current
layer decreases.

---------------------------------------------------------
Title: The Distribution and Behaviour of Photospheric Magnetic
Features
Authors: Parnell, C. E.; Lamb, D. A.; DeForest, C. E.
2014AGUFMSH34A..05P Altcode:
Over the past two decades enormous amounts of data on the magnetic
fields of the solar photosphere have been produced by both ground-based
(Kitt Peak &amp; SOLIS), as well as space-based instruments (MDI,
Hinode &amp; HMI). In order to study the behaviour and distribution of
photospheric magnetic features, efficient automated detection routines
need to be utilised to identify and track magnetic features. In this
talk, I will discuss the pros and cons of different automated magnetic
feature identification and tracking routines with a special focus on the
requirements of these codes to deal with the large data sets produced by
HMI. By patching together results from Hinode and MDI (high-res &amp;
full-disk), the fluxes of magnetic features were found to follow a
power-law over 5 orders of magnitude. At the strong flux tail of this
distribution, the power law was found to fall off at solar minimum,
but was maintained over all fluxes during solar maximum. However,
the point of deflection in the power-law distribution occurs at a
patching point between instruments and so questions remain over the
reasons for the deflection. The feature fluxes determined from the
superb high-resolution HMI data covers almost all of the 5 orders of
magnitude. Considering both solar mimimum and solar maximum HMI data
sets, we investigate whether the power-law over 5 orders of magnitude in
flux still holds. Furthermore, we investigate the behaviour of magnetic
features in order to probe the nature of their origin. In particular, we
analyse small-scale flux emergence events using HMI data to investigate
the existence of a small-scale dynamo just below the solar photosphere.

---------------------------------------------------------
Title: Particle Acceleration at a Reconnecting Magnetic Separator
Authors: Threlfall, J. W.; Neukirch, T.; Parnell, C. E.; Stevenson, J.
2014AGUFMSH23A4153T Altcode:
We present first results of test particle orbit calculations in
two different environments which model separator reconnection in
three dimensions. The test particle (electron and proton) orbits are
calculated using the relativistic guiding-centre approximation. We
compare and contrast test particle orbits in a time-dependent
(analytical) electro-magnetic field configuration with those based
upon the results of large-scale (numerical) 3D MHD simulations of a
reconnecting magnetic separator. We will discuss how the test-particle
orbits and the energy gain depend on the initial conditions, and how
observations (for example, of solar flares) may be used to constrain
model parameters.

---------------------------------------------------------
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: Determining the location of open field regions in active
regions and their potential as source regions of the slow solar wind.
Authors: Harra, Louise K.; Culhane, J. Leonard; Parnell, Clare;
Brooks, David; Platten, Sarah
2014cosp...40E1158H Altcode:
One of the significant discoveries from the Hinode EUV Imaging
Spectrometer (EIS) instrument is the observation of persistent upflows
at edges of active regions. These had been observed in the pre-Hinode
era with TRACE imaging, and with SOHO. However, with Hinode these
upflows are now observed regularly spectroscopically, and are seen
in some form in every active region observed. These tend to occur
in regions of low intensity. Although the upflows are always seen,
it is far from clear whether they form part of the plasma that flows
out into the heliosphere. In this work, we study six active regions in
various time periods, with different characteristics - not all are in
the same hemisphere, some are located next to coronal holes, others
are not. All of them show upflowing plasma. Our aim is to study each
active region and compare their physical characteristics. We will then
carry out modelling to determine where the truly open magnetic field
is, and correlate this with the observations. We want to attempt to
distinguish between open and closed field regions. If this can be done
in a consistent and fast way, this would prove to be extremely valuable
in understanding the source of the slow solar wind. Indeed in terms of
a practical use, it could be used as a tool for choosing regions to
observe for the future Solar Orbiter mission - the purpose of which
is to understand the source of the solar wind from its creation on
the Sun through its propagation into the heliosphere

---------------------------------------------------------
Title: Coronal heating and nanoflares: current sheet formation
and heating
Authors: Bowness, R.; Hood, A. W.; Parnell, C. E.
2013A&A...560A..89B Altcode:
 Aims: Solar photospheric footpoint motions can produce strong,
localised currents in the corona. A detailed understanding of the
formation process and the resulting heating is important in modelling
nanoflares, as a mechanism for heating the solar corona. Methods:
A 3D MHD simulation is described in which an initially straight
magnetic field is sheared in two directions. Grid resolutions up to
5123 were used and two boundary drivers were considered;
one where the boundaries are continuously driven and one where
the driving is switched off once a current layer is formed. 
Results: For both drivers a twisted current layer is formed. After a
long time we see that, when the boundary driving has been switched off,
the system relaxes towards a lower energy equilibrium. For the driver
which continuously shears the magnetic field we see a repeating cycle
of strong current structures forming, fragmenting and decreasing in
magnitude and then building up again. Realistic coronal temperatures
are obtained.

---------------------------------------------------------
Title: Solar Magnetic Tracking. IV. The Death of Magnetic Features
Authors: Lamb, D. A.; Howard, T. A.; DeForest, C. E.; Parnell, C. E.;
Welsch, B. T.
2013ApJ...774..127L Altcode: 2013arXiv1307.4019L
The removal of magnetic flux from the quiet-Sun photosphere is
important for maintaining the statistical steady state of the magnetic
field there, for determining the magnetic flux budget of the Sun,
and for estimating the rate of energy injected into the upper solar
atmosphere. Magnetic feature death is a measurable proxy for the
removal of detectable flux, either by cancellation (submerging or
rising loops, or reconnection in the photosphere) or by dispersal
of flux. We used the SWAMIS feature tracking code to understand how
nearly 2 × 104 magnetic features die in an hour-long
sequence of Hinode/SOT/NFI magnetograms of a region of the quiet
Sun. Of the feature deaths that remove visible magnetic flux from the
photosphere, the vast majority do so by a process that merely disperses
the previously detected flux so that it is too small and too weak
to be detected, rather than completely eliminating it. The behavior
of the ensemble average of these dispersals is not consistent with
a model of simple planar diffusion, suggesting that the dispersal is
constrained by the evolving photospheric velocity field. We introduce
the concept of the partial lifetime of magnetic features, and show
that the partial lifetime due to Cancellation of magnetic flux, 22 hr,
is three times slower than previous measurements of the flux turnover
time. This indicates that prior feature-based estimates of the flux
replacement time may be too short, in contrast with the tendency
for this quantity to decrease as resolution and instrumentation have
improved. This suggests that dispersal of flux to smaller scales is
more important for the replacement of magnetic fields in the quiet
Sun than observed bipolar cancellation. We conclude that processes
on spatial scales smaller than those visible to Hinode dominate the
processes of flux emergence and cancellation, and therefore also the
quantity of magnetic flux that threads the photosphere.

---------------------------------------------------------
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: The Emergence, Motion, and Disappearance of Magnetic Null
Points
Authors: Murphy, Nicholas A.; Parnell, C.; Haynes, A. L.; Pontin, D.
2013SPD....44..103M Altcode:
Magnetic reconnection frequently occurs at and around magnetic nulls:
locations where the magnetic field strength equals zero. While
theoretical models and simulations of magnetic reconnection often
assume that the magnetic field null is co-located with a flow
stagnation point, the introduction of asymmetry typically leads to
flow across the magnetic null. We derive an exact expression for
the three dimensional motion of a magnetic null point in a smoothly
varying magnetic field. We define xn as the position of
a null, U≡dxn/dt as the null's velocity, and M as the
Jacobian matrix of the magnetic field at the null. By using Faraday's
law and evaluating the convective derivative of the magnetic field at
xn with velocity U, the velocity of the null is given by
U=M-1▽×E. This expression is independent of Ohm's law. For
resistive magnetohydrodynamics with uniform resistivity η, this
reduces to U=V(xn)-ηM-1▽2B. This
indicates that any difference between the plasma flow velocity at
the null and the velocity of the null itself is due to resistive
diffusion of the magnetic field. Null points must diffuse in and out
of existence. Null-null pairs first appear (or disappear) as a single
degenerate null with singular M, and then instantaneously move apart
(or together) infinitely fast. However, the motion of separators cannot
be described using solely local parameters because the identification
of a particular magnetic field line as a separator may change due to
non-ideal behavior at another location.

---------------------------------------------------------
Title: Magnetohydrodynamics dynamical relaxation of coronal magnetic
fields. IV. 3D tilted nulls
Authors: Fuentes-Fernández, J.; Parnell, C. E.
2013A&A...554A.145F Altcode: 2013arXiv1309.3019F
Context. There are various types of reconnection that may take place at
3D magnetic null points. Each different reconnection scenario must be
associated with a particular type of current layer. Aims: A range
of current layers may form because the topology of 3D nulls permits
currents to form by either twisting the field about the spine of the
null or by folding the fan and spine into each other. Additionally,
the initial geometry of the field can lead to variations in the
currents that are accumulated. Here, we study current accumulations
in so-called 3D "tilted" nulls formed by a folding of the spine and
fan. A non-zero component of current parallel to the fan is required
such that the null's fan plane and spine are not perpendicular. Our
aims are to provide valid magnetohydrostatic equilibria and to describe
the current accumulations in various cases involving finite plasma
pressure. Methods: To create our equilibrium current structures
we use a full, non-resistive, magnetohydrodynamic (MHD) code so that
no reconnection is allowed. A series of experiments are performed in
which a perturbed 3D tilted null relaxes towards an equilibrium via
real, viscous damping forces. Changes to the initial plasma pressure
and to magnetic parameters are investigated systematically. 
Results: An initially tilted fan is associated with a non-zero Lorentz
force that drives the fan and spine to collapse towards each other,
in a similar manner to the collapse of a 2D X-point. In the final
equilibrium state for an initially radial null with only the current
perpendicular to the spine, the current concentrates along the tilt
axis of the fan and in a layer about the null point with a sharp
peak at the null itself. The continued growth of this peak indicates
that the system is in an asymptotic regime involving an infinite time
singularity at the null. When the initial tilt disturbance (current
perpendicular to the spine) is combined with a spiral-type disturbance
(current parallel to the spine), the final current density concentrates
in three regions: one on the fan along its tilt axis and two around
the spine, above and below the fan. The increased area of current
accumulation leads to a weakening of the singularity formed at the
null. The 3D spine-fan collapse with generic current studied here
provides the ideal setup for non-steady reconnection studies.

---------------------------------------------------------
Title: The Emergence, Motion, and Disappearance of Magnetic Null
Points
Authors: Murphy, Nicholas A.; Parnell, Clare; Haynes, Andrew L.;
Pontin, David
2013shin.confE.118M Altcode:
Magnetic reconnection frequently occurs at and around magnetic
nulls: locations where the magnetic field strength equals zero. While
theoretical models and simulations of laminar, non-turbulent magnetic
reconnection often assume that the magnetic field null is co-located
with a flow stagnation point, the introduction of asymmetry typically
leads to flows across the magnetic null. We derive an exact expression
for the three dimensional motion of a magnetic null point in a
smoothly varying magnetic field by using Faraday's law and evaluating
the convective derivative of the magnetic field at the null using the
null's velocity. In resistive magnetohydrodynamics, any difference
between the plasma flow velocity at the null and the velocity of
the null itself must be due to resistive diffusion of the magnetic
field. Null points must diffuse in and out of existence. Null-null
pairs first appear (disappear) as a single degenerate null with
a singular Jacobian matrix, and then instantaneously move apart
(together) infinitely fast. However, the motion of separators cannot
be described using solely local parameters because the identification
of a particular magnetic field line as a separator may change due to
non-ideal behavior at another location.

---------------------------------------------------------
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: Nonlinear wave propagation and reconnection at magnetic
X-points in the Hall MHD regime
Authors: Threlfall, J.; Parnell, C. E.; De Moortel, I.; McClements,
K. G.; Arber, T. D.
2012A&A...544A..24T Altcode: 2012arXiv1202.3648T
Context. The highly dynamical, complex nature of the solar atmosphere
naturally implies the presence of waves in a topologically varied
magnetic environment. Here, the interaction of waves with topological
features such as null points is inevitable and potentially important
for energetics. The low resistivity of the solar coronal plasma implies
that non-magnetohydrodynamic (MHD) effects should be considered in
studies of magnetic energy release in this environment. Aims:
This paper investigates the role of the Hall term in the propagation and
dissipation of waves, their interaction with 2D magnetic X-points and
the nature of the resulting reconnection. Methods: A Lagrangian
remap shock-capturing code (Lare2d) was used to study the evolution of
an initial fast magnetoacoustic wave annulus for a range of values of
the ion skin depth (δi) in resistive Hall MHD. A magnetic
null-point finding algorithm was also used to locate and track the
evolution of the multiple null-points that are formed in the system. Results: Depending on the ratio of ion skin depth to system size,
our model demonstrates that Hall effects can play a key role in the
wave-null interaction. In particular, the initial fast-wave pulse now
consists of whistler and ion-cyclotron components; the dispersive nature
of the whistler wave leads to (i) earlier interaction with the null;
(ii) the creation of multiple additional, transient nulls and, hence,
an increased number of energy release sites. In the Hall regime, the
relevant timescales (such as the onset of reconnection and the period
of the oscillatory relaxation) of the system are reduced significantly,
and the reconnection rate is enhanced.

---------------------------------------------------------
Title: Magnetohydrodynamics dynamical relaxation of coronal magnetic
fields. III. 3D spiral nulls
Authors: Fuentes-Fernández, J.; Parnell, C. E.
2012A&A...544A..77F Altcode: 2012arXiv1206.5527F
Context. The majority of studies on stressed 3D magnetic null points
consider magnetic reconnection driven by an external perturbation, but
the formation of a genuine current sheet equilibrium remains poorly
understood. This problem has been considered more extensively in two
dimensions, but lacks a generalization into 3D fields. Aims: 3D
magnetic nulls are more complex than 2D nulls and the field can take a
greater range of magnetic geometries local to the null. Here, we focus
on one type and consider the dynamical non-resistive relaxation of 3D
spiral nulls with initial spine-aligned current. We aim to provide a
valid magnetohydrostatic equilibrium, and describe the electric current
accumulations in various cases, involving a finite plasma pressure. Methods: A full MHD code was used, with the resistivity set to zero
so that reconnection is not allowed, to run a series of experiments in
which a perturbed spiral 3D null point was allowed to relax towards an
equilibrium via real, viscous damping forces. Changes to the initial
plasma pressure and other magnetic parameters were systematically
investigated. Results: For the axisymmetric case, the evolution
of the field and the plasma is such that it concentrates the current
density into two cone-shaped regions along the spine, thus concentrating
the twist of the magnetic field around the spine, leaving a radial
configuration in the fan plane. The plasma pressure redistributes to
maintain the current density accumulations. However, it is found that
changes in the initial plasma pressure do not significantly modify
the final state. In the cases where the initial magnetic field is not
axisymmetric, an infinite-time singularity of current perpendicular
to the fan is found at the location of the null.

---------------------------------------------------------
Title: The onset of impulsive bursty reconnection at a two-dimensional
current layer
Authors: Fuentes-Fernández, J.; Parnell, C. E.; Priest, E. R.
2012PhPl...19g2901F Altcode: 2012arXiv1205.2120F
The sudden reconnection of a non-force free 2D current layer,
embedded in a low-beta plasma, triggered by the onset of an anomalous
resistivity, is studied in detail. The resulting behaviour consists
of two main phases. First, a transient reconnection phase, in which
the current in the layer is rapidly dispersed and some flux is
reconnected. This dispersal of current launches a family of small
amplitude magnetic and plasma perturbations, which propagate away
from the null at the local fast and slow magnetosonic speeds. The
vast majority of the magnetic energy released in this phase goes into
internal energy of the plasma, and only a tiny amount is converted
into kinetic energy. In the wake of the outwards propagating pulses,
an imbalance of Lorentz and pressure forces creates a stagnation flow
which drives a regime of impulsive bursty reconnection, in which fast
reconnection is turned on and off in a turbulent manner as the current
density exceeds and falls below a critical value. During this phase,
the null current density is continuously built up above a certain
critical level, then dissipated very rapidly, and built up again,
in a stochastic manner. Interestingly, the magnetic energy converted
during this quasi-steady phase is greater than that converted during
the initial transient reconnection phase. Again essentially all
the energy converted during this phase goes directly to internal
energy. These results are of potential importance for solar flares
and coronal heating, and set a conceptually important reference for
future 3D studies.

---------------------------------------------------------
Title: A contemporary view of coronal heating
Authors: Parnell, C. E.; De Moortel, I.
2012RSPTA.370.3217P Altcode: 2012arXiv1206.6097P
Determining the heating mechanism (or mechanisms) that causes the outer
atmosphere of the Sun, and many other stars, to reach temperatures
orders of magnitude higher than their surface temperatures has long
been a key problem. For decades the problem has been known as the
coronal heating problem, but it is now clear that `coronal heating'
cannot be treated or explained in isolation and that the heating of the
whole solar atmosphere must be studied as a highly coupled system. The
magnetic field of the star is known to play a key role, but, despite
significant advancements in solar telescopes, computing power and
much greater understanding of theoretical mechanisms, the question
of which mechanism or mechanisms are the dominant supplier of energy
to the chromosphere and corona is still open. Following substantial
recent progress, we consider the most likely contenders and discuss
the key factors that have made, and still make, determining the actual
(coronal) heating mechanism (or mechanisms) so difficult.

---------------------------------------------------------
Title: Astrophysical processes on the Sun
Authors: Parnell, C. E.
2012RSPTA.370.3043P Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Consequences of spontaneous reconnection at a two-dimensional
non-force-free current layer
Authors: Fuentes-Fernández, J.; Parnell, C. E.; Hood, A. W.; Priest,
E. R.; Longcope, D. W.
2012PhPl...19b2901F Altcode: 2012arXiv1202.0161F
Magnetic neutral points, where the magnitude of the magnetic field
vanishes locally, are potential locations for energy conversion in the
solar corona. The fact that the magnetic field is identically zero at
these points suggests that for the study of current sheet formation and
of any subsequent resistive dissipation phase, a finite beta plasma
should be considered, rather than neglecting the plasma pressure as
has often been the case in the past. The rapid dissipation of a finite
current layer in non-force-free equilibrium is investigated numerically,
after the sudden onset of an anomalous resistivity. The aim of this
study is to determine how the energy is redistributed during the initial
diffusion phase, and what is the nature of the outward transmission of
information and energy. The resistivity rapidly diffuses the current
at the null point. The presence of a plasma pressure allows the vast
majority of the free energy to be transferred into internal energy. Most
of the converted energy is used in direct heating of the surrounding
plasma, and only about 3% is converted into kinetic energy, causing a
perturbation in the magnetic field and the plasma which propagates away
from the null at the local fast magnetoacoustic speed. The propagating
pulses show a complex structure due to the highly non-uniform initial
state. It is shown that this perturbation carries no net current as it
propagates away from the null. The fact that, under the assumptions
taken in this paper, most of the magnetic energy released in the
reconnection converts internal energy of the plasma, may be highly
important for the chromospheric and coronal heating problem.

---------------------------------------------------------
Title: Magnetohydrodynamics dynamical relaxation of coronal magnetic
fields. II. 2D magnetic X-points
Authors: Fuentes-Fernández, J.; Parnell, C. E.; Hood, A. W.
2011A&A...536A..32F Altcode: 2011arXiv1110.5253F
Context. Magnetic neutral points are potential locations for energy
conversion in the solar corona. 2D X-points have been widely studied
in the past, but only a few of those studies have taken finite plasma
beta effects into consideration, and none of them look at the dynamical
evolution of the system. At the moment there exists no description of
the formation of a non-force-free equilibrium around a two-dimensional
X-point. Aims: Our aim is to provide a valid magnetohydrostatic
equilibrium from the collapse of a 2D X-point in the presence of a
finite plasma pressure, in which the current density is not simply
concentrated in an infinitesimally thin, one-dimensional current sheet,
as found in force-free solutions. In particular, we wish to determine
if a finite pressure current sheet will still involve a singular
current, and if so, what is the nature of the singularity. 
Methods: We use a full MHD code, with the resistivity set to zero,
so that reconnection is not allowed, to run a series of experiments
in which an X-point is perturbed and then is allowed to relax towards
an equilibrium, via real, viscous damping forces. Changes to the
magnitude of the perturbation and the initial plasma pressure are
investigated systematically. Results: The final state found
in our experiments is a "quasi-static" equilibrium where the viscous
relaxation has completely ended, but the peak current density at the
null increases very slowly following an asymptotic regime towards
an infinite time singularity. Using a high grid resolution allows
us to resolve the current structures in this state both in width and
length. In comparison with the well known pressureless studies, the
system does not evolve towards a thin current sheet, but concentrates
the current at the null and the separatrices. The growth rate of the
singularity is found to be tD, with 0 &lt; D &lt; 1. This
rate depends directly on the initial plasma pressure, and decreases
as the pressure is increased. At the end of our study, we present an
analytical description of the system in a quasi-static non-singular
equilibrium at a given time, in which a finite thick current layer
has formed at the null. The dynamical evolution of the system and
the dependence of the final state on the initial plasma and magnetic
quantities is discussed, as are the energetic consequences.

---------------------------------------------------------
Title: 3D Magnetic Reconnection
Authors: Parnell, Clare E.; Maclean, Rhona C.; Haynes, Andrew L.;
Galsgaard, Klaus
2011IAUS..271..227P Altcode:
Magnetic reconnection is an important process that is prevalent in a
wide range of astrophysical bodies. It is the mechanism that permits
magnetic fields to relax to a lower energy state through the global
restructuring of the magnetic field and is thus associated with a range
of dynamic phenomena such as solar flares and CMEs. The characteristics
of three-dimensional reconnection are reviewed revealing how much
more diverse it is than reconnection in two dimensions. For instance,
three-dimensional reconnection can occur both in the vicinity of null
points, as well as in the absence of them. It occurs continuously and
continually throughout a diffusion volume, as opposed to at a single
point, as it does in two dimensions. This means that in three-dimensions
field lines do not reconnect in pairs of lines making the visualisation
and interpretation of three-dimensional reconnection difficult. By considering particular numerical 3D magnetohydrodynamic models of
reconnection, we consider how magnetic reconnection can lead to complex
magnetic topologies and current sheet formation. Indeed, it has been
found that even simple interactions, such as the emergence of a flux
tube, can naturally give rise to `turbulent-like' reconnection regions.

---------------------------------------------------------
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: Small-Scale Flux Emergence Observed Using Hinode/SOT
Authors: Thornton, L. M.; Parnell, C. E.
2011SoPh..269...13T Altcode: 2010SoPh..tmp..220T
The aim of this paper is to determine the flux emergence rate due to
small-scale magnetic features in the quiet Sun using high-resolution
Hinode SOT NFI data. Small-scale magnetic features are identified in
the data using two different feature identification methods (clumping
and downhill); then three methods are applied to detect flux emergence
events. The distribution of the intranetwork peak emerged fluxes
is determined. When combined with previous emergence results, from
ephemeral regions to sunspots, the distribution of all fluxes are found
to follow a power-law distribution which spans nearly seven orders of
magnitude in flux (1016 - 1023 Mx) and 18 orders
of magnitude in frequency. The power-law fit to all these data is of
the form dN/dΨ = n_0/Ψ0Ψ/Ψ0-2.7
where Ψ0=1016 Mx and is used to predict a global
flux emergence rate of ≈ 450 Mx cm−2 day−1
from all features with fluxes of 1016 Mx or more. Since
the slope of all emerged fluxes is less than −2, this implies that
most of the new flux that is fed into the solar atmosphere is from
small-scale emerging events. This suggests that the rate of flux
emergence is independent of the solar cycle and is equivalent to a
global rate of flux emergence of more than a few times 1025
Mx day−1. The single power-law distribution over all
emerged fluxes implies a scale-free dynamo, therefore indicating that
a turbulent dynamo may act throughout the convection zone. Moreover,
from the slope of the emerging flux distribution the (turbulent?) dynamo
producing small-scale features produces considerably more flux than
the active-region dynamo at the tachocline.

---------------------------------------------------------
Title: Three Dimensional Magnetic Reconnection at Null Points and
Separators
Authors: Parnell, Clare E.; Haynes, Andrew L.; Maclean, Rhona C.
2011sswh.book..147P Altcode:
No abstract at ADS

---------------------------------------------------------
Title: The Detection of Numerous Magnetic Separators in a
Three-Dimensional Magnetohydrodynamic Model of Solar Emerging Flux
Authors: Parnell, C. E.; Maclean, R. C.; Haynes, A. L.
2010ApJ...725L.214P Altcode:
Magnetic separators in three-dimensional (3D) magnetic fields
are believed to be often associated with locations of magnetic
reconnection. In this preliminary study, we investigate this
relationship using data from a numerical resistive 3D MHD experiment of
a solar flux emergence event. For the first time separators are detected
in complex magnetic fields resulting from a 3D resistive MHD model of
flux emergence. Two snapshots of the model, taken from different stages
of its evolution, are analyzed. Numerous separators are found in both
snapshots, and their properties, including their geometry, length,
relationship to the magnetic null points, and integrated parallel
electric field are studied. The separators reside at the junctions
between the emerging flux, the overlying field, and two other flux
domains that are newly formed by reconnection. The long separators,
which connect clusters of nulls that lie either side of the emerging
flux, pass through spatially localized regions of high parallel electric
field and correspond to local maxima in integrated parallel electric
field. These factors indicate that strong magnetic reconnection takes
place along many of the separators, and that separators play a key
role during the interaction of emerging and overlying flux.

---------------------------------------------------------
Title: Formation, Evolution, and Associated Flows for Flux Transfer
Events: 3D Nature of Reconnection
Authors: Loring, B.; Karimabadi, H.; Raeder, J.; Vu, H.; Omelchenko,
Y. A.; Parnell, C.; Haynes, A.; Daughton, W. S.; Roytershteyn, V.;
Dorelli, J.
2010AGUFMSM43B..05L Altcode:
In this presentation, we examine the formation and evolution of
flux transfer events (FTEs) in our 3D global hybrid simulation of
the magnetosphere during southward IMF with no dipole tilt. Some
of the questions that we address include: (1) Do FTEs form during
periods of southward IMF under steady IMF conditions? (2) What is the
trigger mechanism for FTEs and does vortex formation cause FTEs? (3)
What is the structure and plasma composition of FTEs? (4) What are
the flows associated with FTEs? (5) How does the structure of FTEs
evolve in time? (6) How are FTEs in a 3D global hybrid simulation
different from FTEs in an equivalent global MHD simulation? 3D
reconnection at the dayside magnetopause is found to be quite complex
and identification of separatrices in the turbulent environment of
magnetosheath is challenging. Preliminary results show evidence of
separator reconnection. We compare and contrast the results with FTE
formation in 2D global hybrid and 2D global full PIC simulation. The 2D
global full PIC simulations show formation of elongated electron layers
which become unstable to formation of secondary islands (FTEs). This
is consistent with our previous work on 2D Harris sheet. 2D global
full PIC and 2D global hybrid simulations show qualitatively similar
results. However, there are significant differences in structure, size,
evolution, and plasma composition within the FTE in 2D and 3D global
hybrid simulations. We will discuss the cause of these differences
and their implications for spacecraft studies of FTEs.

---------------------------------------------------------
Title: Solar Magnetic Tracking. III. Apparent Unipolar Flux Emergence
in High-resolution Observations
Authors: Lamb, D. A.; DeForest, C. E.; Hagenaar, H. J.; Parnell,
C. E.; Welsch, B. T.
2010ApJ...720.1405L Altcode:
Understanding the behavior of weak magnetic fields near the detection
limit of current instrumentation is important for determining the
flux budget of the solar photosphere at small spatial scales. Using
0farcs3-resolution magnetograms from the Solar Optical Telescope's
Narrowband Filter Imager (NFI) on the Hinode spacecraft, we confirm
that the previously reported apparent unipolar magnetic flux emergence
seen in intermediate-resolution magnetograms is indeed the coalescence
of previously existing flux. We demonstrate that similar but smaller
events seen in NFI magnetograms are also likely to correspond to
the coalescence of previously existing weak fields. The uncoalesced
flux, detectable only in the ensemble average of hundreds of these
events, accounts for 50% of the total flux within 3 Mm of the detected
features. The spatial scale at which apparent unipolar emergence can
be directly observed as coalescence remains unknown. The polarity of
the coalescing flux is more balanced than would be expected given the
imbalance of the data set, however without further study we cannot
speculate whether this implies that the flux in the apparent unipolar
emergence events is produced by a granulation-scale dynamo or is
recycled from existing field.

---------------------------------------------------------
Title: Sunspots and starspots, by John H. Thomas and Nigel O. Weiss
Authors: Parnell, Clare
2010GApFD.104..453P Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Magnetic reconnection in the solar atmosphere: from proposal
to paradigm
Authors: Cargill, Peter; Parnell, Clare; Browning, Philippa; de
Moortel, Ineke; Hood, Alan
2010A&G....51c..31C Altcode:
MEETING REPORT On 13 November 2009, the RAS hosted a discussion meeting
to commemorate the formal retirement of Prof. Eric Priest. Here Peter
Cargill, Clare Parnell, Philippa Browning, Ineke de Moortel and Alan
Hood examine how magnetic reconnection has evolved over the past
50 years from an important but controversial proposal, to a general
paradigm.

---------------------------------------------------------
Title: Magnetohydrodynamics dynamical relaxation of coronal magnetic
fields . I. Parallel untwisted magnetic fields in 2D
Authors: Fuentes-Fernández, J.; Parnell, C. E.; Hood, A. W.
2010A&A...514A..90F Altcode: 2011arXiv1110.5258F
Context. For the last thirty years, most of the studies on the
relaxation of stressed magnetic fields in the solar environment have
only considered the Lorentz force, neglecting plasma contributions,
and therefore, limiting every equilibrium to that of a force-free
field. Aims: Here we begin a study of the non-resistive evolution
of finite beta plasmas and their relaxation to magnetohydrostatic
states, where magnetic forces are balanced by plasma-pressure
gradients, by using a simple 2D scenario involving a hydromagnetic
disturbance to a uniform magnetic field. The final equilibrium state
is predicted as a function of the initial disturbances, with aims to
demonstrate what happens to the plasma during the relaxation process
and to see what effects it has on the final equilibrium state. 
Methods: A set of numerical experiments are run using a full MHD code,
with the relaxation driven by magnetoacoustic waves damped by viscous
effects. The numerical results are compared with analytical calculations
made within the linear regime, in which the whole process must remain
adiabatic. Particular attention is paid to the thermodynamic behaviour
of the plasma during the relaxation. Results: The analytical
predictions for the final non force-free equilibrium depend only on
the initial perturbations and the total pressure of the system. It is
found that these predictions hold surprisingly well even for amplitudes
of the perturbation far outside the linear regime. Conclusions:
Including the effects of a finite plasma beta in relaxation experiments
leads to significant differences from the force-free case.

---------------------------------------------------------
Title: Structure of magnetic separators and separator reconnection
Authors: Parnell, C. E.; Haynes, A. L.; Galsgaard, K.
2010JGRA..115.2102P Altcode: 2010JGRA..11502102P
Magnetic separators are important locations of three-dimensional
magnetic reconnection. They are field lines that lie along the edges
of four flux domains and represent the intersection of two separatrix
surfaces. Since the intersection of two surfaces produces an X-type
structure, when viewed along the line of intersection, the global
three-dimensional topology of the magnetic field around a separator
is hyperbolic. It is therefore usually assumed that the projection
of the magnetic field lines themselves onto a two-dimensional plane
perpendicular to a separator is also hyperbolic in nature. In this
paper, we use the results of a three-dimensional MHD experiment
of separator reconnection to show that, in fact, the projection
of the magnetic field lines in a cut perpendicular to a separator
may be either hyperbolic or elliptic and that the structure of the
magnetic field projection may change in space, along the separator,
as well as in time, during the life of the separator. Furthermore,
in our experiment, we find that there are both spatial and temporal
variations in the parallel component of current (and electric field)
along the separator, with all high parallel current regions (which
are associated with reconnection) occurring between counterrotating
flow regions. Importantly, reconnection occurs not only at locations
where the structure of the projected perpendicular magnetic field is
hyperbolic but also where it is elliptic.

---------------------------------------------------------
Title: Three-Dimensional Magnetic Reconnection
Authors: Parnell, C. E.; Haynes, A. L.
2010ASSP...19..261P Altcode: 2010mcia.conf..261P; 2009arXiv0903.0274P
The importance of magnetic reconnection as an energy release mechanism
in many solar, stellar, magnetospheric and astrophysical phenomena
has long been recognised. Reconnection is the only mechanism by which
magnetic fields can globally restructure, enabling them to access a
lower energy state. Over the past decade, there have been some major
advances in our understanding of three-dimensional reconnection. In
particular, the key characteristics of 3D magnetohydrodynamic (MHD)
reconnection have been determined. For instance, 3D reconnection (1)
occurs with or without nulls, (2) occurs continuously and continually
throughout a diffusion region and (3) is driven by counter rotating
flows. Furthermore, analysis of resistive 3D MHD magnetic experiments
have revealed some intriguing effects relating to where and how
reconnection occurs. To illustrate these new features, a series
of constant-resistivity experiments, involving the interaction of
two opposite-polarity magnetic sources in an overlying field, are
considered. Such a simple interaction represents a typical building
block of the Sun's magnetic atmosphere. By following the evolution of
the magnetic topology, we are able to explain where, how and at what
rate the reconnection occurs. Remarkably, there can be up to five energy
release sites at any one time (compared to one in the potential case)
and the duration of the interaction increases (more than doubles) as the
resistivity decreases (by a factor of 16). The decreased resistivity
also leads to a higher peak ohmic dissipation and more energy being
released in total, as a result of a greater injection of Poynting flux.

---------------------------------------------------------
Title: Interaction of twisted curved flux tubes
Authors: Selwa, Malgorzata; Parnell, Clare; Priest, Eric
2010cosp...38.1947S Altcode: 2010cosp.meet.1947S
Most solar eruptions are initiated from sigmoidal structures. We
perform 3D MHD numerical experiments of the interaction of force-free
dipolar flux tubes. The magnetic configuration is initialized as either
a potential or a force-free dipole with a constant density. Next we
perturb the dipoles by twisting or rotating them leading to reconnection
in a resistive MHD regime. We compare the connectivity, energetics
and topological features in both models, vary the contact angle of
the dipoles and check if the initial configuration (sigmoidal or not)
affects flares and eruption initiation leading to faster and stronger
reconnection.

---------------------------------------------------------
Title: Is Null-Point Reconnection Important for Solar Flux Emergence?
Authors: Maclean, R. C.; Parnell, C. E.; Galsgaard, K.
2009SoPh..260..299M Altcode: 2009arXiv0910.0368M
The role of null-point reconnection in a three-dimensional
numerical magnetohydrodynamic (MHD) model of solar emerging flux
is investigated. The model consists of a twisted magnetic flux
tube rising through a stratified convection zone and atmosphere to
interact and reconnect with a horizontal overlying magnetic field
in the atmosphere. Null points appear as the reconnection begins and
persist throughout the rest of the emergence, where they can be found
mostly in the model photosphere and transition region, forming two
loose clusters on either side of the emerging flux tube. Up to 26
nulls are present at any one time, and tracking in time shows that
there is a total of 305 overall, despite the initial simplicity of
the magnetic field configuration. We find evidence for the reality
of the nulls in terms of their methods of creation and destruction,
their balance of signs, their long lifetimes, and their geometrical
stability. We then show that due to the low parallel electric fields
associated with the nulls, null-point reconnection is not the main
type of magnetic reconnection involved in the interaction of the newly
emerged flux with the overlying field. However, the large number of
nulls implies that the topological structure of the magnetic field must
be very complex and the importance of reconnection along separators
or separatrix surfaces for flux emergence cannot be ruled out.

---------------------------------------------------------
Title: How skeletons turn into quasi-separatrix layers in source
models
Authors: Restante, A. L.; Aulanier, G.; Parnell, C. E.
2009A&A...508..433R Altcode:
Context: In situations where there are no magnetic null points located
above a reference photospheric plane, and when the photospheric
magnetic field is modeled by discrete flux concentrations, the
magnetic connectivity is defined by the magnetic skeleton of the
configuration. For a continuous distribution of non-zero photospheric
flux, the connectivity is defined by quasi-separatrix layers
(QSLs). Both the magnetic skeleton and QSLs can account for current
sheet formation and dissipation. Observationally, though, only some
portions of the skeleton are found to be related to flare ribbons,
which are generally associated with QSL footpoints. Aims:
In potential magnetic source models, a transition from the skeleton
to QSLs has been shown to occur when the sources are displaced below
the photospheric plane. The objective of this paper is to understand
the topological and geometrical nature of this transition, and to
derive rules to predict which parts of a given skeleton will give
rise to QSLs. Methods: We consider magnetic configurations,
derived from potential magnetic sources, which possess no coronal null
points. We have calculated their skeletons, composed of null points,
spine field lines and separatrix (fan) surfaces. Choosing a reference
photospheric plane above the sources, we have calculated their QSL
footprints. Results: As already known, the latter mostly match
with subphotospheric spine field lines since, above these lines, field
lines tend to diverge as a result of approaching a null and lying either
side of the separatrix surface extending out of from this null. However,
many non-spine related QSL footprints are also found, which we call
branches. They correspond to the intersection with the photosphere of
portions of fan field lines which “branch” away from the sources and
result in QSLs due to the inclination of the coronal field lines. 
Conclusions: Our findings allow a better geometrical understanding of
the relations between QSLs and skeletons. We show that in the absence
of coronal null points, spines, as well as specific portions of fans
as calculated in standard potential source models, are good predictors
for the location of QSL footprints and of flare ribbons.

---------------------------------------------------------
Title: The Density of Coronal Null Points from Hinode and MDI
Authors: Longcope, D.; Parnell, C.; DeForest, C.
2009ASPC..415..178L Altcode: 2009arXiv0901.0865L
Magnetic null points can be located numerically in a potential field
extrapolation or their average density can be estimated from the
Fourier spectrum of a magnetogram. We use both methods to compute the
null point density from a quiet Sun magnetogram made with Hinode's NFI
and from magnetograms from SOHO's MDI in both its high-resolution and
low-resolution modes. All estimates of the super-chromospheric column
density (z&gt;1.5 Mm) agree with one another and with the previous
measurements: 3×10-3 null points per square Mm of solar
surface.

---------------------------------------------------------
Title: A Power-Law Distribution of Solar Magnetic Fields Over More
Than Five Decades in Flux
Authors: Parnell, C. E.; DeForest, C. E.; Hagenaar, H. J.; Johnston,
B. A.; Lamb, D. A.; Welsch, B. T.
2009ApJ...698...75P Altcode:
Solar flares, coronal mass ejections, and indeed phenomena on all
scales observed on the Sun, are inextricably linked with the Sun's
magnetic field. The solar surface is covered with magnetic features
observed on many spatial scales, which evolve on differing timescales:
the largest features, sunspots, follow an 11-year cycle; the smallest
seem to follow no cycle. Here, we analyze magnetograms from Solar and
Heliospheric Observatory (SOHO)/Michelson Doppler Imager (full disk
and high resolution) and Hinode/Solar Optical Telescope to determine
the fluxes of all currently observable surface magnetic features. We
show that by using a "clumping" algorithm, which counts a single
"flux massif" as one feature, all feature fluxes, regardless of flux
strength, follow the same distribution—a power law with slope -1.85
± 0.14—between 2 × 1017 and 1023 Mx. A power
law suggests that the mechanisms creating surface magnetic features
are scale-free. This implies that either all surface magnetic features
are generated by the same mechanism, or that they are dominated by
surface processes (such as fragmentation, coalescence, and cancellation)
in a way which leads to a scale-free distribution.

---------------------------------------------------------
Title: A Power-law Distribution of Solar Magnetic Fields Over More
Than Five Decades in Flux
Authors: Parnell, Clare; DeForest, C. E.; Hagenaar, H. J.; Johnston,
B. A.; Lamb, D. A.; Welsch, B. T.
2009SPD....40.0603P Altcode:
The surface of the Sun is covered with magnetic features observed
on many spatial scales, which evolve on differing time scales: the
largest features, sunspots, follow an 11 year cycle; the smallest
apparently follow no cycle. Magnetograms from SoHO/MDI (full disk and
high-resolution) and Hinode/SOT are analysed to determine the fluxes
of all currently observable surface magnetic features. To identify
features we use a 'clumping' algorithm, which defines a single feature
as a group of contiguous, same-sign pixels, each of which exceeds an
absolute flux cutoff. We show that, using this feature identification
method, all feature fluxes, regardless of flux strength, follow the
same distribution - a power-law with slope -1.85±0.14 - between 2x
1017 and 1023 Mx. This result implies that the
processes that determine the spatial structure of surface magnetic
features are scale-free. Hence, suggesting that either all surface
magnetic features are generated by the same mechanism, or that their
spatial structure is dominated by processes in the interior or at the
surface (e.g., fragmentation, coalescence and cancellation) that produce
a scale-free distribution. We will discuss the likelihood of these two
mechanisms for generating the powerlaw distribution of feature fluxes.

---------------------------------------------------------
Title: The Number of Magnetic Null Points in the Quiet Sun Corona
Authors: Longcope, D. W.; Parnell, C. E.
2009SoPh..254...51L Altcode: 2008SoPh..tmp..185L; 2008arXiv0811.0097L
The coronal magnetic field above a particular photospheric region
will vanish at a certain number of points, called null points. These
points can be found directly in a potential field extrapolation
or their density can be estimated from the Fourier spectrum of the
magnetogram. The spectral estimate, in which the extrapolated field
is assumed to be random and homogeneous with Gaussian statistics,
is found here to be relatively accurate for quiet Sun magnetograms
from SOHO's MDI. The majority of null points occur at low altitudes,
and their distribution is dictated by high wavenumbers in the Fourier
spectrum. This portion of the spectrum is affected by Poisson noise,
and as many as five-sixths of null points identified from a direct
extrapolation can be attributed to noise. The null distribution above
1500 km is found to depend on wavelengths that are reliably measured
by MDI in either its low-resolution or high-resolution mode. After
correcting the spectrum to remove white noise and compensate for
the modulation transfer function we find that a potential field
extrapolation contains, on average, one magnetic null point, with
altitude greater than 1.5 Mm, above every 322 Mm2 patch of
quiet Sun. Analysis of 562 quiet Sun magnetograms spanning the two
latest solar minima shows that the null point density is relatively
constant with roughly 10% day-to-day variation. At heights above 1.5 Mm,
the null point density decreases approximately as the inverse cube of
height. The photospheric field in the quiet Sun is well approximated as
that from discrete elements with mean flux «|φ|»=1.0×1019
Mx distributed randomly with density n=0.007 Mm−2.

---------------------------------------------------------
Title: Quiet-Sun: A Comparison of MDI and SOT Fluxes
Authors: Parnell, C. E.; Deforest, C. E.; Hagenaar, H. J.; Lamb,
D. A.; Welsch, B. T.
2008ASPC..397...31P Altcode:
The SOT-NFI on Hinode has both higher resolution and better sensitivity
than MDI on SOHO. Line-of-sight magnetograms of the quiet Sun taken
simultaneously by both MDI and SOT are investigated to show how the
observed flux differs between the two instruments. We find that: (i)
the total unsigned flux observed by SOT is approximately 50% greater
than that observed by MDI and (ii) the total signed flux remains
approximately constant. Thus, the extra flux observed by SOT is made
up of equal amounts of positive and negative flux. By comparing the
observed flux distributions from MDI and SOT we find that the extra flux
is contained in features with fluxes less than the smallest observed
by MDI. Indeed, the smallest features in SOT have just ≥ 10^{16} Mx,
a factor of thirty less than the smallest observed by MDI. The
distributions of feature fluxes observed by the two instruments are
also compared. We find that by using a `clumping' algorithm, which
counts a single `flux massif' as one feature, the fluxes in MDI and
SOT follow the same distribution - a power-law - between 2× 10^{17}
and 10^{20} Mx. Thus, the mechanism producing network and intranetwork
features appears to be the same. Furthermore, the power-law index of
this distribution is found to be -1.85. This value is neither the
Kolomogrov -5/3 slope of hydrodynamic turbulence nor the Krichenen
-2 slope of magneto-hydrodynamic turbulence, although both of these
numbers may be within the error bars of our analysis.

---------------------------------------------------------
Title: A new view of quiet-Sun topology from Hinode/SOT
Authors: Régnier, S.; Parnell, C. E.; Haynes, A. L.
2008A&A...484L..47R Altcode: 2008arXiv0805.1602R
Context: With the recent launch of the Hinode satellite our view of
the nature and evolution of quiet-Sun regions has been improved. In
light of the new high resolution observations, we revisit the study
of the quiet Sun's topological nature. Aims: Topology is a tool
to explain the complexity of the magnetic field, the occurrence of
reconnection processes, and the heating of the corona. This Letter
aims to give new insights to these different topics. Methods:
Using a high-resolution Hinode/SOT observation of the line-of-sight
magnetic field on the photosphere, we calculate the three dimensional
magnetic field in the region above assuming a potential field. From
the 3D field, we determine the existence of null points in the magnetic
configuration. Results: From this model of a continuous field, we
find that the distribution of null points with height is significantly
different from that reported in previous studies. In particular, the
null points are mainly located above the bottom boundary layer in the
photosphere (54%) and in the chromosphere (44%) with only a few null
points in the corona (2%). The density of null points (expressed as
the ratio of the number of null points to the number of photospheric
magnetic fragments) in the solar atmosphere is estimated to be between
3% and 8% depending on the method used to identify the number of
magnetic fragments in the observed photosphere. Conclusions: This
study reveals that the heating of the corona by magnetic reconnection
at coronal null points is unlikely. Our findings do not rule out the
heating of the corona at other topological features. We also report
the topological complexity of the chromosphere as strongly suggested
by recent observations from Hinode/SOT.

---------------------------------------------------------
Title: The Small-Scale Field Measured With Hinode/SOT and Feature
Tracking: Where is the mixed- polarity flux?
Authors: Deforest, C. E.; Lamb, D. A.; Berger, T.; Hagenaar, H.;
Parnell, C.; Welsch, B.
2008AGUSMSP51D..01D Altcode:
We report on the results of the first feature tracking study of
the solar magnetic field with Hinode/SOT. We processed a SOT Na-D
line-of-sight magnetogram sequence with five different magnetic
tracking codes. The SOT data allow us to probe the evolving magnetic
field on the granular scale for hours at a time, something that was
not possible with either ground-based observations (which are limited
to short periods of good seeing) or prior space-based observations
(which are limited to arcsecond spatial scales). We find that the field
is much less mixed than previously supposed: while Hinode resolves
small-scale structure within features that, to SOHO/MDI, would
appear as monolithic flux concentrations, this substructure has but
a single sign. Furthermore, the average distance between identifiable
flux concentrations of opposite sign remains nearly unchanged at the
higher resolution, a result that is quite surprising in light of the
common picture of a sea of strong mixed-polarity flux concentrations
dotting the inter-granular lanes. We discuss possible mechanisms for
this surprising result, and implications for the small-scale dynamo.

---------------------------------------------------------
Title: Recursive Reconnection and Magnetic Skeletons
Authors: Parnell, C. E.; Haynes, A. L.; Galsgaard, K.
2008ApJ...675.1656P Altcode:
By considering a simple driven model involving the resistive 3D MHD
interaction of magnetic sources, it is shown that it is essential
to know the magnetic skeleton to determine (1) the locations of
reconnection, (2) type of reconnection, (3) the rate of reconnection,
and (4) how much reconnection is occurring. In the model, two
opposite-polarity magnetic fragments interact in an overlying magnetic
field with reconnection, first closing and then opening the magnetic
field from the sources. There are two main reconnection phases: the
first has one reconnection site at which the flux is closed, and the
second has three sites. The latter is a hybrid case involving both
closing and reopening reconnection processes. Each reconnection site
coincides with its own separator, and hence all reconnection is via
separator reconnection. All the separators connect the same two nulls
and thus mark the intersection between the same four types of flux
domain. In the hybrid state, the two competing reconnection processes
(which open and close flux connecting the same two source pairs)
run simultaneously, leading to recursive reconnection. That is, the
same flux may be closed and then reopened not just once, but many
times. This leads to two interesting consequences: (1) the global
reconnection rate is enhanced and (2) heating occurs for a longer
period and over a wider area than in the single-separator case.

---------------------------------------------------------
Title: Solar Magnetic Tracking. II. The Apparent Unipolar Origin of
Quiet-Sun Flux
Authors: Lamb, D. A.; DeForest, C. E.; Hagenaar, H. J.; Parnell,
C. E.; Welsch, B. T.
2008ApJ...674..520L Altcode:
We investigate the origin of small-scale flux concentrations in the
quiet Sun. In apparent violation of the physical requirement for
flux balance, 94% of the features containing newly detected flux
are unipolar at a resolution of 1.2”. We analyze 2619 of these
apparent unipolar emergences in an image sequence from the SOHO MDI
magnetograph and compare the ensemble average to a model of asymmetric
bipolar emergence that could in principle hide opposing flux under
the noise floor of MDI. We examine the statistical consequences of
this mechanism and find that it cannot be responsible for more than
a small fraction of the unipolar emergences. We conclude that the
majority of the newly detected flux in the quiet Sun is instead due
to the coalescence of previously existing but unresolved flux into
concentrations that are large and strong enough to be detected. We
estimate the rate of coalescence into arcsecond-scale magnetic
features averaged over the solar surface to be 7 × 1021
Mx hr-1, comparable to the reported flux injection rate
due to ephemeral regions. This implies that most flux in the solar
network has been processed by very small scale shredding, emergence,
cancellation, and/or coalescence that is not resolved at 1.2”, and
it suggests that currently unresolved emergences may be at least as
important as ephemeral region emergences to the overall flux budget.

---------------------------------------------------------
Title: Quiet Sun topology from Hinode/SOT
Authors: Regnier, Stephane; Parnell, Clare; Haynes, Andrew
2008cosp...37.2586R Altcode: 2008cosp.meet.2586R
The Hinode satellite was launched in 2006 with unprecedented high
spatial and temporal resolution revealing the detailed nature of the
quiet Sun. Based on the new data recorded by Hinode/SOT, we revisit
the magnetic topology of the quiet Sun. It has been found, using point
source models, that approximately one null point exists for each
source with only 9% of these above the photosphere. In this study,
we use a potential field extrapolation from a continuous photospheric
magnetic field and analyse the properties of the magnetic nulls. We
find that there are few photospheric nulls and most of the null points
are located in the chromosphere.

---------------------------------------------------------
Title: Feature Tracking of Hinode Magnetograms
Authors: Lamb, D.; Deforest, C. E.; Hagenaar, H. J.; Parnell, C. E.;
Welsch, B. T.
2007AGUFMSH53A1066L Altcode:
We present results of applying feature tracking to a sequence of Hinode
magnetograms. The single line wing Na D 5896 magnetograms have a high
signal-to-noise ratio, allowing the detection of flux approximately
30 times weaker than in MDI magnetograms. We find evidence that, even
with Hinode's improved resolution and sensitivity, we do not always
detect the bipolar emergence of new magnetic flux. This suggests that
we have not reached the ultimate resolution to observe the fundamental
flux generation processes in the photosphere.

---------------------------------------------------------
Title: Three-dimensional Resistive-MHD Model for X-ray Bright Points
and Coronal Jets
Authors: Parnell, C. E.
2007AGUFMSH21B..03P Altcode:
Thirteen years ago a simple two-dimensional model for an X-ray bright
point and associated cancelling magnetic feature was first proposed by
Priest et. al (1994). Over the next decade this model was extended into
three- dimensions and applied by many authors to many observed X-ray
bright points. A two-dimensional model was proposed at about the same
time for coronal jets associated with the emergence of magnetic flux
(e.g. Yokoyama &amp; Shibata 1995). Recently, however, new results from
Hinode hint to the fact that X-ray bright points may not actually be
created cancellation. Furthermore, images from XRT aboard Hinode reveal
that coronal jets occur far more frequency than previously reported. We
present results from resistive MHD experiments that show how it is
possible to create an X-ray bright point and coronal jet without the
need for either the emergence or cancellation of magnetic flux. Thus,
the numbers of each event are not limited to the numbers of flux
emergences or cancellations. Moreover, by following the evolution of the
magnetic topology during the interaction we can show that reconnection
at multiple (as opposed to single) separators is the energy release
mechanism. Also, we reveal how it is possible to get dramatic changes
in the coronal loop structures observed with hardly any changes in the
magnetic footpoints below, in line with recent observations from Hinode.

---------------------------------------------------------
Title: Solar Magnetic Tracking. I. Software Comparison and Recommended
Practices
Authors: DeForest, C. E.; Hagenaar, H. J.; Lamb, D. A.; Parnell,
C. E.; Welsch, B. T.
2007ApJ...666..576D Altcode: 2007arXiv0704.2921D
Feature tracking and recognition are increasingly common tools for
data analysis, but are typically implemented on an ad hoc basis
by individual research groups, limiting the usefulness of derived
results when selection effects and algorithmic differences are not
controlled. Specific results that are affected include the solar
magnetic turnover time, the distributions of sizes, strengths, and
lifetimes of magnetic features, and the physics of both small scale flux
emergence and the small-scale dynamo. In this paper, we present the
results of a detailed comparison between four tracking codes applied
to a single set of data from SOHO/MDI, describe the interplay between
desired tracking behavior and parameterization tracking algorithms,
and make recommendations for feature selection and tracking practice
in future work.

---------------------------------------------------------
Title: A trilinear method for finding null points in a
three-dimensional vector space
Authors: Haynes, A. L.; Parnell, C. E.
2007PhPl...14h2107H Altcode: 2007arXiv0706.0521H
Null points are important locations in vector fields, such as a magnetic
field. A new technique (a trilinear method for finding null points)
is presented for finding null points over a large grid of points,
such as those derived from a numerical experiment. The method was
designed so that the null points found would agree with any field
lines traced using the commonly used trilinear interpolation. It is
split into three parts: reduction, analysis, and positioning, which,
when combined, provide an efficient means of locating null points
to a user-defined subgrid accuracy. We compare the results of the
trilinear method with that of a method based on the Poincaré index,
and discuss the accuracy and limitations of both methods.

---------------------------------------------------------
Title: Multiply Connected Source and Null Pairs
Authors: Parnell, C. E.
2007SoPh..242...21P Altcode:
The magnetic fields within the solar atmosphere have a complex topology
owing to the fragmentary nature with which they thread the solar
surface. The topologies of the potential magnetic fields containing only
a few (up to four) point photospheric sources have been classified. For
small numbers of sources determining the connectivity of source pairs
is equivalent to counting the number of flux domains. As the numbers
of sources increase this, however, is no longer the case. Instead, a
pair of connected sources can have more than one distinct flux domain
linking them. We call these multiply connected source pairs. Pairs
of nulls connected by more than one separator are called multiply
connected null pairs. Multiply connected source and null pairs go
hand-in-hand such that two separators connecting the same pair of
nulls immediately implies multiple flux domains linking the same source
pair and vice versa. It is found that multiply connected source pairs
are common not only in fairly complex potential magnetic fields but
more interestingly in the resistive-MHD evolution of both simple and
complex magnetic fields. Magnetic energy release is often significant
around separators. Thus fields with multiply connected source pairs,
which naturally have more separators, (i) have more sites for intense
energy release and (ii) are likely to release energy more quickly
than other magnetic fields. Moreover, the combination of multiply
connected source and null pairs can give rise to a situation where
flux is reconnected repeatedly between two flux domains.

---------------------------------------------------------
Title: Magnetic Network Formation Due to Sub-arcsecond Flux Processing
Authors: Lamb, Derek; DeForest, C. E.; Parnell, C. E.; Hagenaar,
H. J.; Welsch, B. T.
2007AAS...210.9213L Altcode: 2007BAAS...39Q.210L
Kinematic models of solar magnetic network formation typically
employ the breakup of ephemeral regions by granular and supergranular
flow. We show that the coalescence of sub-arcsecond-scale magnetic
flux concentrations into features detectable with MDI is responsible
for injecting as much flux into the magnetic network as the published
emergence rate of ephemeral regions. We also show that the few fresh
bipoles we do detect have no preferential alignment, and thus violate
Joy's law at the arcsecond scale. These two items suggest that at least
half of the flux that makes its way into the network has been processed
at spatial scales below 1 arcsecond, indicative of a local dynamo.

---------------------------------------------------------
Title: Magnetohydrodynamic evolution of magnetic skeletons
Authors: Haynes, A. L.; Parnell, C. E.; Galsgaard, K.; Priest, E. R.
2007RSPSA.463.1097H Altcode: 2007astro.ph..2604H
The heating of the solar corona is probably due to reconnection of the
highly complex magnetic field that threads throughout its volume. We
have run a numerical experiment of an elementary interaction between
the magnetic field of two photospheric sources in an overlying field
that represents a fundamental building block of the coronal heating
process. The key to explaining where, how and how much energy is
released during such an interaction is to calculate the resulting
evolution of the magnetic skeleton. A skeleton is essentially the web of
magnetic flux surfaces (called separatrix surfaces) that separate the
coronal volume into topologically distinct parts. For the first time,
the skeleton of the magnetic field in a three-dimensional numerical
magnetohydrodynamic experiment is calculated and carefully analysed,
as are the ways in which it bifurcates into different topologies. A
change in topology normally changes the number of magnetic reconnection
sites. In our experiment, the magnetic field evolves through
a total of six distinct topologies. Initially, no magnetic flux
joins the two sources. Then, a new type of bifurcation, called a
global double-separator bifurcation, takes place. This bifurcation
is probably one of the main ways in which new separators are created
in the corona (separators are field lines at which three-dimensional
reconnection takes place). This is the first of five bifurcations
in which the skeleton becomes progressively more complex before
simplifying. Surprisingly, for such a simple initial state, at the peak
of complexity there are five separators and eight flux domains present.

---------------------------------------------------------
Title: Book Review: FUNDAMENTALS OF PLASMA PHYSICS / Cambridge
University Press, 2006
Authors: Parnell, Clare
2007Obs...127...70P Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Book Review: LARGE-SCALE STRUCTURES AND THEIR ROLE IN SOLAR
ACTIVITY (ASP CONFERENCE SERIES, VOL. 346) / Astronomical Society
of the Pacific, 2005
Authors: Parnell, Clare
2007Obs...127...67P Altcode:
No abstract at ADS

---------------------------------------------------------
Title: 3D magnetic reconnection, flares and coronal heating
Authors: Parnell, C. E.
2007MmSAI..78..229P Altcode:
Magnetic reconnection is known to be an important energy release
mechanism in many solar, stellar, magnetospheric and astrophysical
phenomena. Also it is the only way in which magnetic fields can change
their topological structure. Reconnection in three dimensions is not
well understood. In particular, knowing where and how reconnection is
going to occur and at what rate it occurs at is not generally obvious in
time-dependent 3D resistive MHD systems. In order to find answers to the
above questions the simple interaction of two opposite-polarity magnetic
sources in an overlying field is considered. This simple interaction
represents a typical building block of the Sun's magnetic atmosphere. By
following the evolution of the skeleton of the magnetic field we are
able to explain where, how and at what rate the reconnection occurs
in this building block of the Sun's magnetic field. Remarkably there
were found to be up to five energy release sites and the reconnection
rate is significantly higher than one would expect.

---------------------------------------------------------
Title: Understanding Magnetic Structures in the Solar Corona Through
Topological Analysis
Authors: Maclean, R. C.; Parnell, C. E.; De Moortel, I.; Büchner,
J.; Priest, E. R.
2006ESASP.617E.156M Altcode: 2006soho...17E.156M
No abstract at ADS

---------------------------------------------------------
Title: Solar coronal heating by magnetic cancellation -
II. Disconnected and unequal bipoles
Authors: von Rekowski, B.; Parnell, C. E.; Priest, E. R.
2006MNRAS.369...43V Altcode: 2006MNRAS.tmp..503V
Two-dimensional numerical magnetohydrodynamic simulations of a
cancelling magnetic feature (CMF) and the associated coronal X-ray
bright point (XBP) are presented. Coronal magnetic reconnection is
found to produce the Ohmic heating required for a coronal XBP. During
the BP phase where reconnection occurs above the base, about 90-95 per
cent of the magnetic flux of the converging magnetic bipole cancels
at the base. The last ~5 to 10 per cent of the base magnetic flux is
cancelled when reconnection occurs at the base. Reconnection happens in
a time-dependent way in response to the imposed converging footpoint
motions. A potential field model gives a good first approximation to
the qualitative behaviour of the system, but the magnetohydrodynamics
(MHD) experiments reveal several quantitative differences: for example,
the effects of plasma inertia and a pressure build-up in-between the
converging bipole are to delay the onset of coronal reconnection above
the base and to lower the maximum X-point height.

---------------------------------------------------------
Title: Solar coronal heating by magnetic cancellation - I. Connected
equal bipoles
Authors: von Rekowski, B.; Parnell, C. E.; Priest, E. R.
2006MNRAS.366..125V Altcode: 2006MNRAS.tmp....9V
We present two-dimensional numerical magnetohydrodynamic simulations
of a cancelling magnetic feature and the associated coronal X-ray
bright point. Coronal reconnection is found to produce significant
Ohmic heating, and at the same time about 90 per cent of the magnetic
flux is cancelled. The presence of downflows accelerates the process
of flux cancellation in the early phase. The last 10 per cent of the
cancellation takes place by reconnection at the base. Reconnection
occurs in a time-dependent way in response to the footpoint motions, and
the resulting sequence of magnetic configurations is close to potential.

---------------------------------------------------------
Title: MHD simulations of photospheric cancelling magnetic features
causing coronal X-ray bright points
Authors: von Rekowski, B.; Parnell, C. E.; Priest, E. R.
2006cosp...36.2936V Altcode: 2006cosp.meet.2936V
begin document Discovering the mechanisms for heating the solar corona
represents one of the major challenges in astronomy at the present time
Long-period MHD waves have now been ruled out as a mechanism and so the
main focus is on various ways in which magnetic reconnection can heat
the three main elements of the Sun s corona namely X-ray bright points
coronal loops and coronal holes Coronal X-ray bright points XBPs have
been observed to account for about 20 to 30 percent of the heating of
the quiet-Sun corona releasing energies ranging from 10 27 to 10 29
erg About two thirds of XBPs are located above sites of cancelling
magnetic bipoles so-called cancelling magnetic features CMFs The
analytical converging flux model of Priest et al 1994 ApJ 427 459 is now
recognised as a likely explanation of the heating of these XBPs where
the heating takes place in response to the approach and cancellation
of underlying photospheric magnetic fragments of opposite polarity to
which the coronal magnetic loops are linked The CMFs trigger coronal
magnetic reconnection and the associated coronal heating in form of
XBPs Magnetic cancellation itself is driven by converging photospheric
footpoint motions of the bipolar sources and involves flux submergence
Building upon this model von Rekowski et al 2006 MNRAS 366 125 and
2006 MNRAS in press have recently begun to develop a greatly improved
numerical MHD model that investigates the dynamical behaviour of CMFs
and the associated reconnection and coronal heating

---------------------------------------------------------
Title: Solar Coronal Heating by Magnetic Cancellation
Authors: von Rekowski, B.; Parnell, C. E.; Priest, E. R.
2005ESASP.600E..95V Altcode: 2005dysu.confE..95V; 2005ESPM...11...95V
No abstract at ADS

---------------------------------------------------------
Title: Coronal Flux Recycling Times
Authors: Close, R. M.; Parnell, C. E.; Longcope, D. W.; Priest, E. R.
2005SoPh..231...45C Altcode:
High-cadence, high-resolution magnetograms have shown that the quiet-Sun
photosphere is very dynamic in nature. It is comprised of discrete
magnetic fragments which are characterized by four key processes -
emergence, coalescence, fragmentation and cancellation. All of this
will have consequences for the magnetic field in the corona above.

---------------------------------------------------------
Title: Elementary heating events - magnetic interactions between
two flux sources. III. Energy considerations
Authors: Galsgaard, K.; Parnell, C. E.
2005A&A...439..335G Altcode: 2005astro.ph..1602G
The magnetic field plays a crucial role in heating the solar corona
- this has been known for many years - but the exact energy release
mechanism(s) is(are) still unknown. Here, we investigate in detail,
using resistive, non-ideal, MHD models, the process of magnetic energy
release in a situation where two initially independent flux systems
are forced into each other. Work done by the foot point motions goes
into building a current sheet in which magnetic reconnection releases
some of the free magnetic energy leading to magnetic connectivity
changes. The scaling relations of the energy input and output are
determined as functions of the driving velocity and the strength of
fluxes in the independent flux systems. In particular, it is found that
the energy injected into the system is proportional to the distance
travelled. Similarly, the rate of Joule dissipation is related to the
distance travelled. Hence, rapidly driven foot points lead to bright,
intense, but short-lived events, whilst slowly driven foot points
produce weaker, but longer-lived brightenings. Integrated over the
lifetime of the events both would produce the same heating if all
other factors were the same. A strong overlying field has the effect
of creating compact flux lobes from the sources. These appear to lead
to a more rapid injection of energy, as well as a more rapid release
of energy. Thus, the stronger the overlying field the more compact
and more intense the heating. This means observers need to know not
only the flux of the magnetic fragments involved in an event, but
also their rate and direction of movement, as well as the strength
and orientation of the surrounding field to be able to predict the
energy dissipated. Furthermore, it is found that rough estimates of
the available energy can be obtained from simple models, starting
from initial potential situations, but that the time scale for the
energy release and, therefore its impact on the coronal plasma, can
only be determined from more detailed investigations of the non-ideal
behaviour of the plasma.

---------------------------------------------------------
Title: Destruction Mechanisms of Quiet-Sun Magnetic Flux
Authors: Lamb, D. A.; Deforest, C. E.; Hagenaar, H. J.; Parnell,
C. E.; Welsch, B. T.
2005AGUSMSP41B..02L Altcode:
We use SWAMIS, a freely available magnetic feature tracking suite,
to analyze the destruction of solar small-scale magnetic flux. We
track a sequence of high resolution MDI magnetograms to find the
destruction rates in a patch of quiet sun. We state criteria for
defining the individual magnetochemical destruction mechanisms
of merging, cancellation, and disappearance, and determine the
contribution of each process to the removal of detected flux from the
photosphere. Destruction mechanisms are important to determine because,
together with formation mechanisms, they provide information as to
the nature of the small-scale dynamo. We present preliminary results
and discuss the implications of these rates on models of quiet-sun
magnetic flux generation.

---------------------------------------------------------
Title: Comparison of blinkers and explosive events: A case study
Authors: Bewsher, D.; Innes, D. E.; Parnell, C. E.; Brown, D. S.
2005A&A...432..307B Altcode:
Blinkers are brightenings at network cell junctions that are
traditionally identified with SOHO/CDS and explosive events or high
velocity events are identified in high resolution UV spectra obtained
from HRTS and SOHO/SUMER. Criteria are determined to facilitate
objective automatic identification of both blinkers and explosive events
in both SOHO/CDS and SOHO/SUMER data. Blinkers are identified in SUMER
data, if the temporal resolution of the data is reduced to that of
CDS. Otherwise short lived, localised intensity enhancements that make
up the blinker are identified. Explosive events are identified in CDS
data when the line width is significantly increased, and occasionally if
there is an enhancement in the wing of the line profile. A theoretical
statistical model is presented which hypothesises that blinkers and
explosive events are random and not connected in any way. The results
given in this paper suggest that this hypothesis can not be rejected and
our probability interpretation of the recent results of Brković &amp;
Peter (2004, A&amp;A, 422, 709) are inconclusive. Appendices A and
B are only available in electronic form at http://www.edpscienc es.org

---------------------------------------------------------
Title: Domain structures in complex 3D magnetic fields
Authors: Close, R. M.; Parnell, C. E.; Priest, E. R.
2005GApFD..99..513C Altcode:
The numerous magnetic fragments that populate the mixed-polarity,
quiet-Sun photosphere give rise to many interesting topological
features in the corona. In light of this, much recent work has
gone into classifying the configurations that arise from simple,
point-source potential-field models in efforts to determine the
nature of the quiet-Sun magnetic field. These studies have ranged from
systematic and detailed examinations of magnetic fields arising from
only a handful of sources, involving classifying the configurations
that arise (and how some states may bifurcate into other states), to
statistical studies of the overall properties of fields arising from
hundreds of magnetic sources. Such studies have greatly increased our
understanding of what we might expect the magnetic field over the
quiet Sun to behave like; the purpose of the study presented here
is to extend this understanding further by examining the structure
of the individual domains (the regions in space through which pairs
of opposite-polarity sources are connected). In particular, the
features of lesser-known domain structures that are absent from fields
arising from only a few sources and overlooked by sweeping statistical
studies are documented. In spite of the incredible complexity of the
coronal field, previous studies have shown that there are only two
types of building block in a potential field arising from coplanar
point sources: namely, an isolated dome, bounded by a single unbroken
separatrix surface, and a separator-ring domain, engirdled by a ring
of separators. However, it is demonstrated here how both isolated
domains and separator-ring domains may be categorised further depending
upon their particular geometrical and topological traits. As many
models predict coronal heating at topologically distinct features
in magnetic fields such as null points, separators and separatrices,
for any such models to be applied to general fields would require a
scheme for identifying which topological features are related to a
given domain. The study here explores some of the issues that would
need to be taken into account by such a scheme, and in particular the
problems associated with trying to deduce the properties of a general
magnetic field from knowledge of domain footprints alone. Animated
3D-rotational views of some of the figures in this manuscript
may be viewed in AVI, MPEG and animated-GIF formats by visiting
http://www-solar.mcs.st-and.ac.uk/robertc/animations/blocks.html and
following the desired link.

---------------------------------------------------------
Title: Fragment Driven Magnetic Reconnection
Authors: Galsgaard, K.; Parnell, C.
2004ESASP.575..351G Altcode: 2004soho...15..351G; 2004astro.ph..9562G
In this paper, we investigate a simple model where two, initially
unconnected, flux systems are forced to interact in response to the
imposed boundary driving by solving the non-ideal 3D MHD equations
numerically. The reconnection rate of the dynamical process
is determined and compared with the corresponding rate for the
potential evolution of the magnetic field. This shows that the dynamic
reconnection rate is about a factor of two smaller than the potential
(perfect, instantaneous) rate for realistic solar driving velocities
demonstrating that this three-dimensional magnetic reconnection process
is fast. The energy input for a fixed advection distance is found to be
independent of the driving velocity. The Joule dissipation associated
with the reconnection process is also found to be basically dependent
on the advection distance rather than driving velocity. This implies
that the timescale for the event determines the effect the heating
has on the temperature increase. Finally, the numerical experiments
indicate that the observational structure of the reconnection site
changes dramatically depending on the phase of the evolution of
the passage of the two flux sources. In the initial phase, where
the sources become connected, the heating is confined to a compact
region. For the disconnecting phase the energy gets distributed over
a larger area due to the reconnected field line connectivity.

---------------------------------------------------------
Title: Probability Analysis of Coincident Blinkers and Explosive
Events
Authors: Bewsher, D.; Brown, D.; Innes, D.; Parnell, C.
2004ESASP.575..465B Altcode: 2004soho...15..465B
No abstract at ADS

---------------------------------------------------------
Title: Elementary heating events - magnetic interactions between
two flux sources. II. Rates of flux reconnection
Authors: Parnell, C. E.; Galsgaard, K.
2004A&A...428..595P Altcode:
Magnetic fragments in the photosphere are in continuous motion and, due
to the complex nature of the magnetic field in the solar atmosphere,
these motions are likely to drive a lucrative coronal energy source:
the passing of initially-unconnected opposite-polarity fragments
that release energy through both closing and then re-opening the
same fieldlines. Three-dimensional, time-dependent MHD and potential
models are used to investigate the passing of fragments in an overlying
field. The processes of closing and opening the field generally occur
through separator and separatrix reconnection, respectively. The
rates of flux reconnection in these processes are determined. They
are found to be dependent on the direction of the surrounding magnetic
field relative to the motion of the fragments and the velocity of the
sources. In particular, separator reconnection rates (closing) and
separatrix-surface reconnection rates (opening) are directly related
to the rate of flux transport perpendicular to the current sheet
(overlying field). The results suggest that both types of reconnection
are fast with the peak rates of separator and separatrix reconnection
occurring at 58% and 29% of the peak potential reconnection rate,
respectively, when the sources are driven at a hundredth of the
peak Alfvén velocity in the box. Moreover, the slower the system is
driven the closer the flux reconnection rates are to the instantaneous
potential rates. Furthermore, there is a maximum reconnection rate
for both types of reconnection as the driving speed tends to the
Alfvén speed with the separatrix reconnection rate typically half
that of separator reconnection. These results suggest that, on the
Sun, reconnection driven by the passing of small-scale network and
intranetwork fragments is a highly efficient process that is very likely
to contribute significantly to the heating of the background solar
corona. The three-dimensional reconnection processes are efficient
because, unlike in two-dimensions, there are many places within the
current sheets where reconnection can take place simultaneously giving
rise to fine-scale structure along the boundaries between the open,
closed and re-opened flux. Furthermore, due to the complexity of
the magnetic field above the photosphere the reconnection all takes
place low down at less than a quarter of the separation of the initial
fragments above the photosphere.

---------------------------------------------------------
Title: The Role of Dynamic Brightenings in Coronal Heating
Authors: Parnell, C. E.
2004ESASP.575..227P Altcode: 2004soho...15..227P
No abstract at ADS

---------------------------------------------------------
Title: Separators in 3D Quiet-Sun Magnetic Fields
Authors: Close, R. M.; Parnell, C. E.; Priest, E. R.
2004SoPh..225...21C Altcode:
At the confluence of four regions of different magnetic connectivity
lies a distinct topological candidate for coronal heating, namely
the magnetic separator. In this study, a method for tracing separator
curves is developed and the statistical properties of separators in
coronal fields are subsequently explored by analysing a model field
with an exponential source distribution, similar to that studied by
Schrijver and Title (2002). Magnetic fields based on data from an
observed sequence of MDI magnetograms are also considered as a case
study. The picture that emerges is one in which there are many more
magnetic separators than previously thought, since many separators
arise from each null point. For an exponential source distribution,
an average of 10.1±0.13 separators per null are found, of which
1.04±0.04 "multiply link" pairs of nulls (i.e., there is more than
one separator linking such pairs of nulls). For the observed sequence
of magnetograms, these figures are 7.63±0.2 and 0.99± 0.059,
respectively. The results obtained here show that separators have
a tendency to group together into trunks about a null. In the case
of prone nulls, these trunks lie either normal to the photospheric
surface or on it. It is also established that pairs of coronal nulls
are frequently interconnected, suggesting that they may have been
created by purely coronal bifurcations.

---------------------------------------------------------
Title: Recycling of the Solar Corona's Magnetic Field
Authors: Close, R. M.; Parnell, C. E.; Longcope, D. W.; Priest, E. R.
2004ApJ...612L..81C Altcode:
Magnetic fields play a dominant role in the atmospheres of the Sun
and other Sun-like stars. Outside sunspot regions, the photosphere
of the so-called quiet Sun contains myriads of small-scale magnetic
concentrations, with strengths ranging from the detection limit of
~1016 Mx up to ~3×1020 Mx. The tireless motion
of these magnetic flux concentrations, along with the continual
appearance and disappearance of opposite-polarity pairs of fluxes,
releases a substantial amount of energy that may be associated with
a whole host of physical processes in the solar corona, not least
the enigma of coronal heating. We find here that the timescale for
magnetic flux to be remapped in the quiet-Sun corona is, surprisingly,
only 1.4 hr (around 1/10 of the photospheric flux recycling time),
implying that the quiet-Sun corona is far more dynamic than previously
thought. Besides leading to a fuller understanding of the origins of
magnetically driven phenomena in our Sun's corona, such a process may
also be crucial for the understanding of stellar atmospheres in general.

---------------------------------------------------------
Title: Dynamics of Transition Region Blinkers
Authors: Bewsher, D.; Parnell, C.; Pike, D.; Harrison, R.
2004ESASP.547..267B Altcode: 2004soho...13..267B
Analysis of CDS data has shown that transition region blinkers and the
chromosphere directly below, are preferentially more redshifted and
have larger non-thermal velocities than the normal transition region
and chromospheric plasma. The ranges of these enhanced velocities,
however, are no larger than the typical spread of Doppler and
non-thermal velocities in these regions. An overview of the Doppler
and non-thermal velocities of blinkers found with CDS in the quiet
Sun and active region are presented. The anticipated range of Doppler
velocities of blinkers are 10 - 15 km s-1 in the quiet-Sun (10 - 20 km
s-1 in active-regions) in the chromosphere (He I) and 25 - 30 km s-1
in the quiet-Sun (20 - 40 km s-1 in activeregions) in the transition
region (O V). The range of non-thermal velocities of blinkers in both
the quiet- Sun and active-regions are estimated to be 15 - 25 km s-1
in He I and 30 - 45 km s-1 in O V. There are more blinkers with larger
Doppler and non-thermal velocities than would be expected in the whole
of the chromosphere and transition region.

---------------------------------------------------------
Title: A study of the unification of quiet-Sun transient-event
phenomena
Authors: Harrison, R. A.; Harra, L. K.; Brković, A.; Parnell, C. E.
2003A&A...409..755H Altcode:
A number of small-scale, globally distributed solar transient
event-types have been reported in the literature. Their potential
role in fundamental processes in the solar atmosphere, such as coronal
heating and wind acceleration, is under active investigation. However,
the event-types, such as those known as blinkers, explosive events,
EUV (extreme-UV) network and cell brightenings, network flares, heating
events, nanoflares and EUV brightenings are basically classifications
which are driven to a large extent by different observational techniques
and different instruments rather than the identification of a clear
differing physical phenomenon. We investigate the different instrumental
and technique limitations and attempt to identify any unification
of the reported quiet-Sun transient, small-scale phenomena. We find
that once observational techniques have been considered, a number of
the different classifications appear to be the same. This suggests
that events known as blinkers, network and cell brightenings and EUV
brightenings are the same event-type. We suggest that the term blinker
be used as a generic term to describe these events. However, there
appears to be little evidence that blinkers and explosive events are
directly related. Furthermore, although a small percentage of blinkers
and nanoflares/heating events appear to be related to one another,
these events pose a number of important questions suggesting that either
(i) blinkers and nanoflare/heating events are all created by the same
mechanism, i.e. for some blinker events, the conditions are such that
higher temperatures are found, or (ii) there are two types of event,
including the “traditional” blinker which is effectively a transition
region brightening driven by a density or filling factor enhancement,
and a mini-flare-like event which reaches higher temperatures,
presumably driven by reconnection.

---------------------------------------------------------
Title: Dynamics of Blinkers
Authors: Bewsher, D.; Parnell, C. E.; Pike, C. D.; Harrison, R. A.
2003SoPh..215..217B Altcode:
The relative Doppler and non-thermal velocities of quiet-Sun
and active-region blinkers identified in O v with CDS are
calculated. Relative velocities for the corresponding chromospheric
plasma below are also determined using the He i line. O v blinkers
and the chromosphere directly below, have a preference to be more
red-shifted than the normal transition region and chromospheric
plasma. The ranges of these enhanced velocities, however, are no larger
than the typical spread of Doppler velocities in these regions. The
anticipated ranges of Doppler velocities of blinkers are 10-15 km
s−1 in the quiet Sun (10-20 km s−1 in active
regions) for He i and 25-30 km s−1 in the quiet Sun
(20-40 km s−1 in active regions) for O v. Blinkers and
the chromosphere below also have preferentially larger non-thermal
velocities than the typical background chromosphere and transition
region. Again the increase in magnitude of these non-thermal velocities
is no greater than the typical ranges of non-thermal velocities. The
ranges of non-thermal velocities of blinkers in both the quiet Sun
and active regions are estimated to be 15-25 km s−1 in
He i and 30-45 km s−1 in O v. There are more blinkers
with larger Doppler and non-thermal velocities than would be expected
in the whole of the chromosphere and transition region. The recently
suggested mechanisms for blinkers are revisited and discussed further
in light of the new results.

---------------------------------------------------------
Title: Determination of coronal loop properties from trace
observations
Authors: De Moortel, I.; Parnell, C. E.; Hood, A. W.
2003SoPh..215...69D Altcode:
In this paper, we determine the temperature profile along the footpoints
of large coronal loops observed by TRACE in both the 171 Å and 195
Å passbands. The temperature along the lower part of these coronal
loops only shows small variations and can probably be considered to
be isothermal. Using the obtained temperature profile T(s) and an
estimate of the column depth along the loop, we then determine the
pressure along the lower part of the observed coronal loops and hence
the value of the pressure scale length. The obtained scale lengths
correspond in order-of-magnitude with the theoretically predicted
gravitational scale height. We show that the differences between
the observed and predicted scale heights are unlikely to be caused by
(significant) flows along the loops but could possibly be a consequence
of the inclination of the loops. This implies that the quasi-periodic
intensity oscillations observed in the loops are most probably caused
by compressive waves propagating upward at the coronal sound speed.

---------------------------------------------------------
Title: Comparison of Blinkers and Explosive Events
Authors: Bewsher, D.; Innes, D.; Parnell, C. E.
2003SPD....34.1617B Altcode: 2003BAAS...35Q.836B
There has been much speculation over recent years as to whether blinkers
and explosive events are the same phenomena observed with different
instruments. Blinkers were first observed by Harrison (1997) by eye
using SOHO/CDS, but more recently Brković et al. (2001) and Bewsher
et al. (2002) have developed automated methods of identifying these
events. Blinkers are small scale intensity enhancements seen in the
transition region, with a mean area of 3 x 107 km2
and a mean lifetime of 16 minutes. The Doppler velocities of blinkers
are preferentially more red-shifted than the typical transition region
plasma. The range of these enhanced velocities, however, are no larger
than the typical spread of Doppler velocities in these regions. Explosive events were first observed using HRTS (Bruckener and Bartoe,
1983) as high energy turbulent events and jets. More recently, they
have been observed using SOHO/SUMER. They have a typical size of 2"
- 4" and have an average lifetime of 60 seconds. Line profiles of
explosive events show strong non-Gaussian enhancements of both wings of
the profile, and velocities associated with them range from +/- 50 -
+/- 250 km s-1. Examples will be presented showing
(i) a co-spatial and co-incident blinker and explosive event; (ii)
a blinker with no associated explosive event; and (iii) an explosive
event with no associated blinker. We investigate the lightcurves and
line profiles of the regions to explain the relationship, if any,
between blinkers and explosive events.

---------------------------------------------------------
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: Magnetic structure of transition region blinkers
Authors: Bewsher, D.; Parnell, C. E.; Brown, D. S.; Hood, A. W.
2002ESASP.505..239B Altcode: 2002IAUCo.188..239B; 2002solm.conf..239B
Analysis of the photospheric magnetic field has shown that the
majority of blinkers, small-scale intensity enhancements seen in the
transition region, occur above single fragments. We investigate the
relationship between the strength of these single magnetic fragments
or the ratio of any mixed magnetic fields beneath blinkers and blinker
characteristics. In all cases, no correlation is found between the
strength of the magnetic field and the blinker properties. We suggest,
therefore, that blinkers are not caused by reconnection and that other
mechanisms should be explored further.

---------------------------------------------------------
Title: Small-scale transient events in the quiet and active solar
atmosphere
Authors: Parnell, Clare E.
2002ESASP.505..231P Altcode: 2002IAUCo.188..231P; 2002solm.conf..231P
Over the last decade the unprecedented uninterrupted, high resolution,
coverage of the Sun has led to the discovery of many new types
of small-scale phenomena, as well as a better understanding of
phenomena such as X-ray bright points. Due to the excellent range of
telescopes aboard these many spacecraft the small-scale phenomena
have been observed in many different wavelengths. Much has been
discovered over the past 10 years about them, but there are still many
questions to be answered. What are the connections between all these
different small-scale transient phenomena. How are they created? Do
they contribute to the heating of the solar corona and if so, how
significant is their contribution? Are any of them the source of the
fast solar wind? Six key types of phenomena from the quiet and active
Sun are reviewed. Their connections to the magnetic field below and
to other coronal phenomena are discussed.

---------------------------------------------------------
Title: Magnetic reconnection throughout the solar atmosphere
Authors: Hood, A. W.; Galsgaard, K.; Parnell, C. E.
2002ESASP.505..285H Altcode: 2002solm.conf..285H; 2002IAUCo.188..285H
Magnetic reconnection is responsible for many different solar phenomena
and it is the release of magnetic energy through reconnection that
is believed to (i) drive flares, (ii) generate CMEs, (III) heat the
corona and (iv) generate MHD waves. In basic models of 2D magnetic
reconnection, the particular choise of boundary conditions influences
the form of reconnection obtained. Reconnection in 3D can occur
with and without null points. Numerical experiments have attempted to
investigate different types of reconnection but a basic understanding of
reconnection at 3D magnetic null points is essential in understanding
these fumdamental processes. The structure of magnetic regions depends
on features such as the magnetic skeleton, the mull points, the spine
and fan plane. Numerical simulations are important but, at present,
are unable to fully resolve the reconnection region. Recent analytical
and numerical results of 3D reconnection will be presented. Applications
of reconnection in the solar atmosphere will be discussed also.

---------------------------------------------------------
Title: Nature of the magnetic carpet - I. Distribution of magnetic
fluxes
Authors: Parnell, C. E.
2002MNRAS.335..389P Altcode:
The distribution of magnetic fluxes found in the solar magnetic
carpet, the photospheric magnetic field of the quiet-Sun, is
investigated. A total of 27 500 concentrations are studied with
fluxes ranging between a few times 1017 and a few times
1020 Mx. A histogram of the fluxes shows that there
are more small fluxes and more large fluxes than anticipated by an
exponential distribution. However, there are significantly fewer
large and small fluxes than a power-law distribution fitted to the
middle of the range. Thus, the fluxes are not distributed in the form
of either an exponential or a power-law distribution, as previously
suggested. Instead, the Weibull distribution, which involves both a
power law and an exponential, is found to provide both a statistically
good fit to the data and a physically reasonable prediction for the
total absolute flux density. The best-fitting Weibull distribution
has a Kolmogorov-Smirnov D statistic of approximately 0.07 - well
below the 5 per cent significance level and the Weibull distribution
predicts the observed total absolute flux densities to within better
than 92 per cent. Physically, the observed distribution of fluxes
can be explained as being made up of the following three elements:
(i) emergence of new flux over all scales gives rise to an initial
exponential distribution (observed); (ii) fragmentation and partial
cancellation produce smaller and smaller fluxes thus creating excess
small fluxes; (iii) excess large fluxes are created by coalescence and
an additional (possibly significant) injection of flux from remnants
of active regions. Finally, the equations of `magneto-chemistry' are
used to derive suitable forms for the rate of emergence, cancellation,
coalescence and fragmentation consistent with a Weibull distribution
of fluxes.

---------------------------------------------------------
Title: Solar Corona: Magnetic carpets and hot coronae
Authors: Parnell, Clare
2002A&G....43d..16P Altcode:
Solar physicists have known for many decades now that the outer
atmosphere of the Sun, the solar corona, is some 200 times hotter than
the surface of the Sun. Understanding just how the corona is heated
is one of the Sun's greatest mysteries and although many mechanisms
have been proposed it is not clear which is the most important. What
is clear, however, is that small-scale phenomena are a very important
piece in this puzzle.

---------------------------------------------------------
Title: Nanoflare Statistics from First Principles: Fractal Geometry
and Temperature Synthesis
Authors: Aschwanden, Markus J.; Parnell, Clare E.
2002ApJ...572.1048A Altcode:
We derive universal scaling laws for the physical parameters
of flarelike processes in a low-β plasma, quantified in terms
of spatial length scales l, area A, volume V, electron density
ne, electron temperature Te, total emission
measure M, and thermal energy E. The relations are specified as
functions of two independent input parameters, the power index a
of the length distribution, N(l)~l-a, and the fractal
Haussdorff dimension D between length scales l and flare areas,
A(l)~lD. For values that are consistent with the data,
i.e., a=2.5+/-0.2 and D=1.5+/-0.2, and assuming the RTV scaling
law, we predict an energy distribution N(E)~E-α with a
power-law coefficient of α=1.54+/-0.11. As an observational test,
we perform statistics of nanoflares in a quiet-Sun region covering a
comprehensive temperature range of Te~1-4 MK. We detected
nanoflare events in extreme-ultraviolet (EUV) with the 171 and 195
Å filters from the Transition Region and Coronal Explorer (TRACE),
as well as in soft X-rays with the AlMg filter from the Yohkoh soft
X-ray telescope (SXT), in a cospatial field of view and cotemporal time
interval. The obtained frequency distributions of thermal energies of
nanoflares detected in each wave band separately were found to have
power-law slopes of α~1.86+/-0.07 at 171 Å (Te~0.7-1.1 MK),
α~1.81+/-0.10 at 195 Å (Te~1.0-1.5 MK), and α~1.57+/-0.15
in the AlMg filter (Te~1.8-4.0 MK), consistent with earlier
studies in each wavelength. We synthesize the temperature-biased
frequency distributions from each wavelength and find a corrected
power-law slope of α~1.54+/-0.03, consistent with our theoretical
prediction derived from first principles. This analysis, supported by
numerical simulations, clearly demonstrates that previously determined
distributions of nanoflares detected in EUV bands produced a too
steep power-law distribution of energies with slopes of α~2.0-2.3
mainly because of this temperature bias. The temperature-synthesized
distributions of thermal nanoflare energies are also found to be more
consistent with distributions of nonthermal flare energies determined
in hard X-rays (α~1.4-1.6) and with theoretical avalanche models
(α~1.4-1.5).

---------------------------------------------------------
Title: Transition-Region Blinkers - II. Active-Region Properties
Authors: Parnell, C. E.; Bewsher, D.; Harrison, R. A.
2002SoPh..206..249P Altcode:
The distribution and general properties of events identified in an
active region that have the same characteristics as quiet-Sun blinkers
are discussed and named `active-region blinkers'. The events are
identified using an automated scheme `BLinker Identification Program'
(BLIP) which was designed for and tested on quiet-Sun blinkers. Like
quiet-Sun blinkers, the active-region events are most easily identified
in the 629 Å emission line from O v although evidence for them is
also found in other extreme UV lines emitted from He i, O iii and O
iv. Unlike quiet-Sun blinkers, however, they may also have coronal
signatures in the lines Mg ix and Mg x. Their properties are very
similar to those of quiet-Sun blinkers with mean lifetimes of 16-19
min, mean areas of 2.4-4.3×107 km2 and mean
intensity enhancements factors of 1.8-3.3. Their global frequency of
7-28 s−1 is about 42%-700% higher than that for quiet-Sun
blinkers. The blinkers discussed here are found above both active-region
(plage) magnetic fields, as well as above the umbra and penumbra of
a sunspot.

---------------------------------------------------------
Title: Transition Region Blinkers I. Quiet-Sun Properties
Authors: Bewsher, D.; Parnell, C. E.; Harrison, R. A.
2002SoPh..206...21B Altcode:
An automated method of identifying transition region blinkers is
presented. The distribution and general properties of blinkers
identified in the quiet Sun are discussed. The blinkers are seen
most clearly in the O v (629 Å) transition region emission line,
but they also have strong signatures in O iv (554 Å), and the
chromospheric line, He i (584 Å). The strongest O v blinkers can
also be identified in O iii (599 Å). No significant signatures are
found for blinkers in the Mg ix (368 Å) and Mg x (624 Å) coronal
lines. A few hundred O v blinkers are analyzed. Their global frequency
is between 1 and 20 s−1 depending on how significant the
peaks of the blinkers are. They have a typical area of 3×107
km2, a typical lifetime of 16 min and a typical intensity
enhancement factor of 1.8. We find the ratios of the oxygen lines
to be flat confirming the result that blinkers are not temperature
events, but are density enhancements or due to increases in filling
factor. Blinkers are found to occur preferentially over regions of
enhanced chromospheric or transition region emission such as network
boundaries, however, it is not so clear that they appear below the
brightest coronal regions. A rough analysis of the magnetic fragments
show that blinkers preferentially occur above regions where there are
large or strong magnetic fragments with 75% occurring in regions where
one polarity dominates.

---------------------------------------------------------
Title: X-Ray Bright Points and other Quiet Sun Transient Phenomena
[Invited]
Authors: Parnell, C. E.
2002mwoc.conf...47P Altcode:
Over the last decade the unprecedented uninterrupted, high resolution,
coverage of the Sun has led to the discovery of many new types of
small-scale phenomena, as well as a better understanding of phenomena
such as X-ray bright points and coronal plumes. Due to the excellent
range of telescopes aboard the many spacecraft currently viewing the
Sun these small-scale phenomena have been observed in many different
wavelengths. Much has been discovered over the past 10 years, but there
are still many questions to be answered. What are the connections
between all these different small-scale transient phenomenaNULL How
are they createdNULL Do they contribute to the heating of the solar
corona and if so how significant is their contributionNULL Are any of
them the source of the fast solar windNULL I will be reviewing what
has been discovered about both the new and well known dynamic quiet Sun
phenomena from not only Yohkoh, but also SOHO and TRACE. In particular,
I shall discuss the connections they appear to have to X-ray bright
points and coronal plumes and will consider if any of them make a
major contribution to the heating of the corona.

---------------------------------------------------------
Title: Magnetic structure and reconnection of x-ray bright points
in the solar corona
Authors: Brown, D. S.; Parnell, C. E.; DeLuca, E. E.; McMullen, R. A.;
Golub, L.; Priest, E. R.
2002AdSpR..29.1093B Altcode:
The three-dimensional magnetic topology of the solar corona is
incredibly complex and its effect on the nature of 3D reconnection is
profound. We study the supposedly simple topology of a small scale X-ray
bright point observed by TRACE and SOHO/MDI, and how it is driven by
reconnection when it forms and during the early stages of its lifetime.

---------------------------------------------------------
Title: The Magnetic Structure of a Coronal X-Ray Bright Point
Authors: Brown, D. S.; Parnell, C. E.; Deluca, E. E.; Golub, L.;
McMullen, R. A.
2001SoPh..201..305B Altcode:
X-ray bright points are small dynamic loop structures that are observed
all over the solar corona. The high spatial and temporal resolution of
the TRACE instrument allows bright points to be studied in much greater
detail than previously possible. This paper focuses on a specific
bright point which occurred for about 20 hours on 13-14 June 1998 and
examines its dynamic structure in detail. This example suggests that
the mechanisms that cause bright points to form and evolve are more
complex than previously thought. In this case, reconnection probably
plays a major part during the formation and brightening of the loop
structure. However, later on the foot points rotate injecting twist
into the bright point which may cause an instability to occur with
dynamic results.

---------------------------------------------------------
Title: A model of the Solar Magnetic Carpet
Authors: Parnell, C. E.
2001SoPh..200...23P Altcode:
There are four key processes that dictate the behaviorof the magnetic
flux concentrations that form the so-called `magnetic carpet' of
the quiet photosphere. These processes are emergence, cancellation,
coalescence, and fragmentation. Rates of emergence have been estimated
from observations, but the rates of cancellation, coalescence, and
fragmentation are much more difficult to determine observationally. A
model is set up to simulate an area of magnetic carpet in the quiet
Sun. In the model there are three imposed parameters: the rate of
emergence of new flux, the distribution of emerged flux and the rate
of fragmentation of flux concentrations. The rate of cancellation
and the rate of coalescence are deduced from the model. From the
simulations it is estimated that the average emergence rate of new flux
in the quiet Sun must be between 6×10−6 and 10−
5 Mx cm−2 s−1 to maintain an absolute
flux density of between 2.5 and 3 G. For this rate of emergence a
fragmentation rate of more than 12×10−5 s−1
is required to produce the observed exponential index for the number
density of flux concentrations. This is equivalent to each fragment
canceling more than once every 200 minutes. The rate of cancellation
is calculated from the model and is found naturally to be equivalent
to the rate of emergence. However, it is found that the frequency
of cancellation is much greater than the frequency of emergence. In
fact, it is likely that there are several orders of magnitude more
cancellation events than emergence events. This implies that flux
is injected in relatively large concentrations whereas cancellation
occurs though the disappearance of many small concentrations.

---------------------------------------------------------
Title: Transition Region Blinkers
Authors: Parnell, C. E.; Bewsher, D.; Harrison, R. A.; Hood, A. W.
2001IAUS..203..359P Altcode:
Blinkers are small bright emission events observed best in the O V
transition region line that occur above the supergranular network. They
were first observed using SoHO/CDS data and were identified manually
by Harrison (1997). They are believed to be density enhancements,
but how they are created and what their properties are is not well
known. We have developed the first program to automatically identify
blinkers and their characteristics. The evolution of the magnetic
field observed by SoHO/MDI below these blinkers has then be analysed to
determine what magnetic field configuration is required for a blinker
to occur. Also, the coronal emission above has been investigated using
SoHO/CDS and TRACE data to determine the relation between blinkers,
x-ray bright points and nanoflares. All three of these events are
known to occur at the network, but as yet the relation between them
is not understood. Putting together the results from these multi-wave
length studies we have been able to determine a model for how blinkers
occur and what their effect is on the transition region around and
the corona above.

---------------------------------------------------------
Title: Solar Corona: X-ray Bright Points
Authors: Parnell, C.
2000eaa..bookE2022P Altcode:
X-ray (or coronal) bright points are small knots of intense x-ray
emission scattered randomly throughout the solar CORONA. They are
associated with pairs of opposite-polarity magnetic flux seen below in
the SOLAR PHOTOSPHERE and are most likely to be heated by the motion
of the magnetic fluxes causing reconnection in the corona....

---------------------------------------------------------
Title: Elementary heating events - Magnetic interactions between
two flux sources
Authors: Galsgaard, K.; Parnell, C. E.; Blaizot, J.
2000A&A...362..395G Altcode:
Observations taken by the SoHO MDI instrument have revealed that the
quiet photospheric magnetic flux is, on average, recycled within a few
days. As new flux emerges from the convection zone into the photosphere
it is moved around by horizontal motions resulting from overshoots of
convection cells. These motions cause the magnetic fields extending
from flux fragments to tangle, forcing different magnetic flux systems
to interact. Only the process of magnetic reconnection limits the
complexity of magnetic field line connectivity. The energy liberated
by these detangling or destressing processes act as a natural energy
source which may heat the solar coronal plasma. In this paper, we use
a numerical approach to solve the MHD equations in a three-dimensional
domain to examine the dynamical behaviour of one simple magnetic flux
interaction. The model consists of a uniform magnetic field overlying
two flux sources of opposite polarity that are initially unconnected and
are forced to interact as they are driven passed each other. We find
that the development from initially unconnected sources to connected
sources proceeds quite quickly and simply. This change takes place
through driven separator reconnection in a systematically twisted
current sheet. The out flow velocity from the reconnection is highly
asymmetric with much higher velocities in the region defined by the
field lines connected to both sources. However, the change back to two
independent sources after the nearest approach has past takes place on
a much longer time scale even though the distance between the sources
increases significantly. This is because the opening of the field has
to take place through separatrix reconnection and at this phase of
the development there are no forcing of the fluxes to drive a fast
opening of the magnetic field.

---------------------------------------------------------
Title: Magnetic reconnection and some solar applications
Authors: Parnell, C. E.
2000RSPTA.358..669P Altcode:
Magnetic reconnection is a fundamental plasma-physics process that
is of great importance for the Sun, the Earth's magnetosphere and
all astrophysical objects which have magnetic fields. It is a process
central to the generation of magnetic fields in stars and also plays a
major role in the heating of solar and stellar coronae. The development
of both large (e.g. flares) and small (e.g. coronal bright points)
dynamic phenomena observed on the Sun depends on reconnection and it is
likely that reconnection may also be important for the acceleration of
solar and stellar winds. It is 50 years since the first seeds of ideas
for magnetic reconnection were sown and over 40 years since the classic
Sweet-Parker mechanism was suggested. Since then the majority of the
research has focused on reconnection in two dimensions. However, in the
last few years attentions have turned to understanding the intricacies
of reconnection in three dimensions. In this paper, the classical
aspects of two-dimensional reconnection are reviewed, together with
various mechanisms for reconnection in three dimensions, in particular,
spine, fan and quasi-separatrix layer reconnection. The paper is then
rounded off with examples of some solar phenomena where reconnection
is believed to be present. In particular, the heating of some observed
small-scale events in the corona is investigated and the question of
quiet coronal heating due to nanoflares and microflares is addressed.

---------------------------------------------------------
Title: Statistical Analysis of the Energy Distribution of Nanoflares
in the Quiet Sun
Authors: Parnell, C. E.; Jupp, P. E.
2000ApJ...529..554P Altcode:
For many years it has been debated whether the quiet solar
corona is heated by nanoflares and microflares or by magnetic
waves. In this paper TRACE data of events with energies in the
range 1023-1026 ergs are investigated. A new
stable and objective statistical technique is proposed to determine
the index, -γ, of a power-law relation between the frequency of
the events and their energy. We find that γ is highly dependent
on the form of the line-of-sight depth assumed to determine the
event energies. If a constant line-of-sight depth is assumed, then
γ lies between 2.4 and 2.6; however, if a line-of-sight depth of
the form (Ae/k2)1/2 is assumed,
where Ae is event area and k is a constant, then γ lies
between 2.0 and 2.1. In all cases the value of γ is greater than 2 and
therefore implies that the events with the lowest energies dominate
the heating of the quiet solar corona. Moreover, there are strong
indications that there is insufficient energy from events with nanoflare
energies (i.e., energies in the range 1024-1027
ergs) to explain the total energy losses in the quiet corona. However,
our results do not rule out the possibility that events with picoflare
energies (i.e., energies in the range 1021-1024
ergs) heat the quiet corona. From analysis of the spatial distribution
of the events, we find that events are mainly confined to regions with
the brightest EUV emission, which are presumably the regions connected
to the strongest magnetic fields. Indeed, just 16% of the quiet corona
possesses such events.

---------------------------------------------------------
Title: Magnetic Reconnection: Classical Aspects
Authors: Parnell, Clare E.
2000LNP...553...61P Altcode: 2000tech.conf...61P
Magnetic reconnection is an important mechanism in astrophysics
for converting magnetic energy to both thermal energy and bulk
acceleration of plasma and also for changing the global topology
of the magnetic field. For over 50 years now solar theorists have
investigated reconnection. This paper provides a basic review of the
classical aspects of reconnection in both one, two and three dimensions,
as well as, giving a potted history of reconnection theory in solar
physics. Magnetic annihilation, Sweet-Parker reconnection and Petschek
reconnection will all be discussed as will spine and fan reconnection
in three dimensions.

---------------------------------------------------------
Title: Observed Magnetic Structure of X-Ray Bright Points from TRACE
and MDI
Authors: Brown, D. S.; Parnell, C.; Deluca, E.; McMullen, R.; Golub, L.
1999ASPC..184...81B Altcode:
From 13th-17th June 1998, TRACE and MDI simultaneously observed the
same quiet region of the Sun. From these observations the fascinating
and complex structure of x-ray bright points, intense small scale
brightenings that occur throughout the solar corona, can be seen in
great detail. For the first time, it has been possible to study bright
points for their entire lifetime with a cadence of 2 minutes and a
temporal resolution of 0.5 arcsecs. One particular bright point which
lasted two days exhibited dynamic structural behaviour which became
increasingly complex and lead to its sudden eruptive demise. With the
use of MDI magnetograms, it is possible to extrapolate the magnetic
structure using an analytical constant α force-free approximation. This
has been used to help us to explain the topology and behaviour of the
bright point. By comparing two of TRACE's Fe lines (FeIX and FeXII) the
spatial and temporal temperature and density structure of the bright
point has been investigated. This analysis indicates that this bright
point is made up of a complex system of dense loops. By understanding
the magnetic, temperature and density structure of the bright point,
it is hoped that the mechanism by which it is heated can be gained.

---------------------------------------------------------
Title: Peeling back the Sun.
Authors: Parnell, C.
1999PhyEd..34..108P Altcode:
Observations show that the Sun is not simply a ball of hot plasma but
contains several discrete layers: the interior where the energy is
generated is covered by the photosphere, the chromosphere and finally
the corona. This article describes a photon's journey to the outside
and links features such as prominences and sunspots to the regions in
which they occur.

---------------------------------------------------------
Title: Our New View of the Solar Corona from YOHKOH and SOHO
Authors: Parnell, C. E.
1998Ap&SS.261...81P Altcode: 1999Ap&SS.261...81P
The Yohkoh satellite has now been orbiting the Earth for about 6
years and during this time it has revealed a number of new features
in the solar corona. These include the discovery of X-ray jets and
active region transient brightenings, as well as observations of hard
X-ray sources above and at the feet of soft X-ray flare loops. SOHO,
the newest solar space mission, is not orbiting the Earth, but is in
fact orbiting the Sun and has been at the Earth's L1 point for about 2
years. During this time it too has identified some new and interesting
characteristics of both the solar corona, the photosphere and the
solar interior. For example, studies of high resolution MDI/SOHO
magnetograms indicate that the magnetic carpet may play an important
role in the heating of the coronal. Also polar plume studies from
EIT/SOHO data suggest that they may be a possible source for the fast
solar wind. Together these two missions have dominated solar research
throughout the 90s and are expected to continue to do so during the
rise from solar minimum to the next solar maximum.

---------------------------------------------------------
Title: Coronal Heating and the Photospheric Magnetic Field
Authors: Parnell, C. E.; Sturrock, P. A.
1997SPD....28.0506P Altcode: 1997BAAS...29..909P
Since magnetic field typically plays a role (either active or passive)
in coronal heating theories, it may be possible to evaluate these
theories by investigating the relationship between the coronal energy
budget (the total power requirement of the corona) and measurable
properties of the photospheric magnetic field. The X-ray flux is a
useful proxy for the total power required to maintain the corona, so we
have examined the relationship between the total X-ray flux, as measured
by the GOES instruments, and the total magnetic flux, as estimated
from the NSO instrumentation at Kitt Peak. We use this relationship to
test the recent proposal that coronal heating is due to sudden magnetic
relaxation. According to this concept, reconnection in the chromosphere
of the footpoint regions of two oppositely directed flux tubes leads to
a new flux tube, with widely separated footpoints, which erupts rapidly
and generates sound waves that heat the corona. We adopt a simple
"kinetic theory" model for the photospheric and chromospheric processes,
and so obtain an estimate of the magnetic flux reconnection rate in
terms of the mean field strength and of the parameters (diameter, flux
and random speed) that characterize the elementary flux elements. The
sudden magnetic relaxation model gives a simple relation between the
magnetic flux budget and the coronal energy budget. In this way, we
obtain a theoretical relationship between the coronal energy budget and
the mean photospheric magnetic field strength that we compare with the
available observational data. This work was supported by NASA grants
NAS 8-37334 and NAGW-2265, and by Air Force grant F49620-95-1-0008.

---------------------------------------------------------
Title: Coronal heating by reconnection
Authors: Parnell, C. E.
1997AdSpR..19.1853P Altcode:
The outermost atmosphere of the Sun, called the corona, is some
200 times hotter than the surface of the Sun. The main source of
energy for heating the corona is believed to be the magnetic field
which dominates the corona. Magnetic reconnection is probably the most
important mechanism for releasing magnetic energy and may, therefore, be
important for coronal heating or micro-flaring. The best observational
examples of reconnection in the corona are thought to be X-ray bright
points, which are small-scale brightenings seen randomly throughout
the whole corona. Theoretical models can not only explain the key
observations relating to bright points, but they can also explain the
complex three-dimensional structures often seen in bright points. In
these models magnetic neutral points play a significant role as the
centres for reconnection both in two and three dimensions.

---------------------------------------------------------
Title: Structure and collapse of three-dimensional magnetic neutral
points
Authors: Parnell, C. E.; Neukirch, T.; Smith, J. M.; Priest, E. R.
1997GApFD..84..245P Altcode:
The structure and collapse of linear three-dimensional magnetic neutral
points is studied by varying the four parameters (p, q,j|,j) that
define, in general, the linear field of a neutral point. The effect of
these parameters on both the skeleton structure (i.e. the fan and spine)
and the actual field line structure of the null is considered. It is
found that one current component (j) causes the skeleton structure of
the null to fold up from its potential state, whereas the other current
component (j|;) causes the field lines to bend. The two other parameters
(p,q) determine the potential structure of the null and cause the null
to transform from a three-dimensional null to a two-dimensional null
and from a positive (type B) null to a negative (type A) null. To
investigate the collapse of three-dimensional nulls, solutions to the
linear, low-β ideal magnetohydrodynamic equations are found. It is
found that three-dimensional null points can collapse if the field
line foot-points are free and energy can propagate into the system.

---------------------------------------------------------
Title: The Role of Magnetic Reconnection in Small-Scale Coronal Events
(Invited)
Authors: Parnell, C. E.
1996ASPC..111...19P Altcode: 1997ASPC..111...19P
To illustrate the importance of magnetic reconnection in the heating of
small-scale coronal events examples of reconnection models for X-ray
bright points are given. These models include, in two dimensions, the
converging flux and the emerging flux models and, in three dimensions,
models of particular bright points are discussed. A brief general
introduction to reconnection is given which focuses on reconnection
at null points in three dimensions.

---------------------------------------------------------
Title: The Three-Dimensional Structures of Elementary Coronal
Heating Events
Authors: Parnell, C. E.
1996mpsa.conf..455P Altcode: 1996IAUCo.153..455P
No abstract at ADS

---------------------------------------------------------
Title: Discussion and application of X-ray bright point models.
Authors: Parnell, C. E.
1996joso.proc..121P Altcode:
A basic model for X-ray bright points which explains many of the key
observations is discussed. Then, using the philosophy of this model,
an explanation for the three-dimensional structure of bright points
is given. Further application of the basic model is considered, in
particular, a reason for the similarities between bright point flares
and active region transient brightenings is suggested.

---------------------------------------------------------
Title: A converging flux model for the formation of an X-ray bright
point above a supergranule cell
Authors: Parnell, C. E.; Priest, E. R.
1995GApFD..80..255P Altcode:
The many complex regions of positive and negative flux that thread the
surface of the Sun are mainly grouped around the edges of supergranule
cells. These cells have large concentrations of magnetic flux on their
boundaries and very little flux inside, with the magnetic fragments
that appear in the centre of the cells swept to the boundaries by
convective motions. Thus, a small bipolar pair of magnetic fragments
(such as an ephemeral region) emerges inside a cell and moves towards
the cell boundary as it grows. On reaching the boundary the fragments
encounter unipolar regions of network flux with which they may merge
or cancel. When cancellation takes place there is often an associated
X-ray bright point in the overlying corona. Here, the emergence and
interaction of an ephemeral region in a quiet-region or active-region
super-granule cell is considered. It is found that there are three
possible scenarios for the evolution of an ephemeral region in a
supergranule cell and these are all investigated. The magnetic fields
for the supergranule cell and ephemeral region are modelled by finite
sources of flux and are studied as the ephemeral region moves through
a series of quasi-static states. It is found that the ratio of the
cancelling fragment widths (strengths) is important in determining
the lifetime and path of the bright point, while the actual sizes
of the fragments is important for determining the intensity of the
bright point, the lifetime of the cancelling magnetic feature and the
relative times of the bright point completion and cancelling magnetic
feature onset. From this we suggest that transient brightenings in
active regions and bright points on the quiet Sun may both be created
by the converging flux mechanism detailed here.

---------------------------------------------------------
Title: Models of X-ray bright points and cancelling magnetic features
Authors: Parnell, Clare Elizabeth
1995PhDT.......196P Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Models of x-ray bright points and cancelling magnetic features
Authors: Parnell, Clare E.
1995PhDT.......163P Altcode:
Small brightenings called x-ray bright points (Golub et al, 1974) occur
in the solar corona. They are observed with the soft x-ray telescope
on Skylab to be approximately 22 Mm in diameter with a brighter inner
core of width 4-7 Mm although with the Normal Incidence X-ray Telescope
their dimensions are observed to be typically 6 Mm x 9 Mm. By comparison
with magnetograms of the photosphere it has been noticed recently
that there is a high correlation between the occurrence of x-ray
bright points and the mutual reduction of flux between two opposite
polarity magnetic fragments. These fragments are originally unconnected
magnetically, but move towards each other and simultaneously lose equal
amounts of flux (cancel): they are called cancelling magnetic features
(Martin et al, 1984). The observations relating to these features were
reviewed by Priest et al. (1994) who suggested that they naturally
evolve through three phases: the pre-interaction, interaction and
cancellation phases. From this evidence qualitative pictures of the
magnetic field structure for an x-ray bright point and associated
cancelling magnetic feature were established. The aim of this thesis
has been to build on the ideas of Priest et al. (1994) to produce a
detailed theoretical model of an x-ray bright point and a cancelling
magnetic feature. The magnetic field structures are estimated, and
the position and lifetime of the bright point are calculated, as is
the total amount of energy released during the bright point. This
work is also extended to study more complex cancelling configurations
representing the main basic types of cancelling magnetic feature. The
results of these models determine the factors that affect the lifetime
and position of a bright point and indicate which types of cancelling
magnetic features are most likely to produce bright points that
are long-lived, lie directly above the cancellation site and occur
simultaneously with the cancellation phase. The complex structure
of a bright point cannot be explained from the above two-dimensional
models: thus two recently observed bright points were studied to see
if the above model could be extended into three dimensions to explain
the structure seen in soft x-ray images. The available observational
data was used and leads to reasonable explanations for the complex
shapes of both bright points. Finally, a more realistic model for
the overlying field was set up involving a model of the field above a
supergranule cell field with fragments of finite width. The interaction
of an ephemeral region within this field was then studied and led to
five different scenarios. The results obtained reaffirmed those found
in the previous simpler models and suggest where bright points may
appear in a cell relative to the cancelling magnetic feature and for
how long the bright points might last. Predictions for the lifetimes
of cancelling magnetic features are also made, indicating when the
cancelling magnetic feature occurs relative to the bright point.

---------------------------------------------------------
Title: Modelling of dynamic coronal heating
Authors: Parnell, C. E.
1994ESASP.373..149P Altcode: 1994soho....3..149P
No abstract at ADS

---------------------------------------------------------
Title: A Model for X-Ray Bright Points due to Unequal Cancelling
Flux Sources
Authors: Parnell, C. E.; Priest, Eric R.; Titov, V. S.
1994SoPh..153..217P Altcode:
A recent discovery from the Big Bear Solar Observatory has linked the
cancellation of opposite polarity magnetic fragments in the photosphere
(i.e., so-called cancelling magnetic features) to X-ray bright points
and has stimulated the setting up of a converging flux model for
the process. Cancelling magnetic features can occur between magnetic
fragments of differing strengths in many different situations. Here,
therefore, we model two opposite polarity fragments of different
strengths in the photosphere by two unequal sources in an overlying
uniform field. Initially in thepre-interaction phase these sources
are assumed to be unconnected, but as they move closer together
theinteraction phase starts with an X-type neutral point forming,
initially on the photosphere, then rising up into the chromosphere
and corona before lowering back down to the photosphere. Thecapture
phase then follows with the sources fully connected as they move
together. Finally, after they come in to contact, during thecancellation
phase the weaker source is cancelled by part of the stronger source. The
height of the X-type neutral point varies with the separation of the
sources and the ratio of the source strengths, as do the positions of
the neutral points before connection and after complete reconnection
of the two sources. The neutral point is the location of magnetic
reconnection and therefore energy release which is believed to
power the X-ray bright point in the corona. By using a current sheet
approximation, where it is assumed no reconnection takes place as
the two sources move together, the total amount of energy released
during reconnection may be estimated. The typical total free magnetic
energy is found to be of the order of 1020-1021
J, which is as required for an X-ray bright point. It is also found
that, as the ratio of the source strengths increases, the height of
the X-type neutral point decreases, as do the total energy released,
and the lifetime of the bright point.

---------------------------------------------------------
Title: A Converging Flux Model of an X-Ray Bright Point and an
Associated Canceling Magnetic Feature
Authors: Priest, E. R.; Parnell, C. E.; Martin, S. F.
1994ApJ...427..459P Altcode:
X-ray bright points are an important part of the solar corona and
therefore of the coronal heating problem. When it was first realized
that bright points are always situated above opposite polarity
magnetic fragments in the photosphere, it was natural to suggest
that such fragments represent emerging flux and that an X-ray
bright point is caused by reconnection of the emerging flux with
an overlying coronal magnetic field. However, a recent important
discovery at the Big Bear Solar Observatory is that the magnetic
fragments of opposite polarity are usually not emerging but are
instead coming together and disappearing and so are referred to as
canceling magnetic features. Sometimes a tiny filament is observed
to form and erupt at the same time. A unified model is here proposed
which explains these observational features and has several phases:
(1) a preinteraction phase, in which two photospheric fragments are
unconnected magnetically and begin to approach one another, until
eventually oppositely directed fields from the fragments come into
contact at a second-order null point; (2) an interaction phase, in which
the null point becomes an X-point and rises into the corona; an X-ray
bright point is created for typically 8 hr by coronal reconnection,
driven by the continued approach of the photospheric sources; long
hot loops and Yohkoh X-ray jets may be created by the reconnection,
and rapid variability in bright point emission may be produced by
an impulsive burst regime of reconnection; the explosive events
seen with High Resolution Telescope and Spectrograph (HRTS) may be
produced as the X-point passes through the upper chromosphere; (3) a
cancellation phase, in which a canceling magnetic feature is produced
by photospheric reconnection as the fragments come into contact and
decrease in strength; above the canceling fragments a small filament
may form and erupt over typically an hour. An important role is played
by the interaction distance (d), which is proportional to the magnetic
flux of the fragments and inversely proportional to the overlying
magnetic field strength. It determines the fragment separation at
which the interaction phase begins and the resulting maximum height
of the reconnection point. It is suggested that coronal reconnection
driven by footpoint motion represents an elementary heating event
that may be heating normal coronal loops and may be at the root of
the nanoflare/microflare process. Bright points may well be at the
large-scale end of a broad spectrum of events of the type modeled in
this paper, which are heating the solar corona. At very small scales,
such events in 'furnaces' in the coronal hole network may even produce
high-frequency waves that propagate out and drive the solar wind
(Axford 1993).

---------------------------------------------------------
Title: The Three-Dimensional Structures of X-Ray Bright Points
Authors: Parnell, C. E.; Priest, E. R.; Golub, L.
1994SoPh..151...57P Altcode:
Recently, the Converging Flux Model has been proposed for X-ray bright
points and cancelling magnetic features. The aim of this peice of
work is to try and model theoretically specific X-ray bright points
in the framework of the Converging Flux Model. The observational
data used includes a magnetogram showing the normal component of the
magnetic field at the photosphere and a high-resolution soft X-ray
image from NIXT showing the brightenings in the lower solar corona. By
approximating the flux concentrations in the magnetograms with poles
of the appropriate sign and sense, the overlying three-dimensional
potential field structure is calculated. Deduction of plausible motions
of the flux sources are made which produce brightenings of the observed
shape due to reconnection between neighbouring flux regions. Also the
three-dimensional separarix and separator structure and the way the
magnetic field lines reconnect in three dimensions is deduced.

---------------------------------------------------------
Title: A Model of an X-Ray Bright Point
Authors: Parnell, C. E.; Priest, E. R.
1994swms.conf....1P Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Solar Coronal Heating by Magnetic Flux Interaction
Authors: Priest, E. R.; Parnell, C. E.; Rickard, G. J.
1994ASIC..422...11P Altcode: 1994coma.conf...11P
No abstract at ADS