priest

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Author name code: priest
ADS astronomy entries on 2022-09-14
author:"Priest, Eric"

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Title: Magnetic reconnection: MHD theory and modelling
Authors: Pontin, David I.; Priest, Eric R.
2022LRSP...19....1P Altcode:
In this review we focus on the fundamental theory of magnetohydrodynamic
reconnection, together with applications to understanding a wide range
of dynamic processes in the solar corona, such as flares, jets, coronal
mass ejections, the solar wind and coronal heating. We summarise
only briefly the related topics of collisionless reconnection,
non-thermal particle acceleration, and reconnection in systems
other than the corona. We introduce several preliminary topics
that are necessary before the subtleties of reconnection can be
fully described: these include null points (Sects. 2.1-2.2), other
topological and geometrical features such as separatrices, separators
and quasi-separatrix layers (Sects. 2.3, 2.6), the conservation
of magnetic flux and field lines (Sect. 3), and magnetic helicity
(Sect. 4.6). Formation of current sheets in two- and three-dimensional
fields is reviewed in Sect. 5. These set the scene for a discussion of
the definition and properties of reconnection in three dimensions that
covers the conditions for reconnection, the failure of the concept of
a flux velocity, the nature of diffusion, and the differences between
two-dimensional and three-dimensional reconnection (Sect. 4). Classical
2D models are briefly presented, including magnetic annihilation
(Sect. 6), slow and fast regimes of steady reconnection (Sect. 7),
and non-steady reconnection such as the tearing mode (Sect. 8). Then
three routes to fast reconnection in a collisional or collisionless
medium are described (Sect. 9). The remainder of the review is
dedicated to our current understanding of how magnetic reconnection
operates in three dimensions and in complex magnetic fields such as
that of the Sun's corona. In Sects. 10-12, 14.1 the different regimes
of reconnection that are possible in three dimensions are summarised,
including at a null point, separator, quasi-separator or a braid. The
role of 3D reconnection in solar flares (Sect. 13) is reviewed, as
well as in coronal heating (Sect. 14), and the release of the solar
wind (Sect. 15.2). Extensions including the role of reconnection in the
magnetosphere (Sect. 15.3), the link between reconnection and turbulence
(Sect. 16), and the role of reconnection in particle acceleration
(Sect. 17) are briefly mentioned.

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Title: Ambipolar diffusion: Self-similar solutions and MHD code
testing. Cylindrical symmetry
Authors: Moreno-Insertis, F.; Nóbrega-Siverio, D.; Priest, E. R.;
Hood, A. W.
2022A&A...662A..42M Altcode: 2022arXiv220306272M
Context. Ambipolar diffusion is a process occurring in partially
ionised astrophysical systems that imparts a complicated mathematical
and physical nature to Ohm's law. The numerical codes that solve the
magnetohydrodynamic (MHD) equations have to be able to deal with the
singularities that are naturally created in the system by the ambipolar
diffusion term. Aims: The global aim is to calculate a set of
theoretical self-similar solutions to the nonlinear diffusion equation
with cylindrical symmetry that can be used as tests for MHD codes which
include the ambipolar diffusion term. Methods: First, following
the general methods developed in the applied mathematics literature,
we obtained the theoretical solutions as eigenfunctions of a nonlinear
ordinary differential equation. Phase-plane techniques were used to
integrate through the singularities at the locations of the nulls,
which correspond to infinitely sharp current sheets. In the second
half of the paper, we consider the use of these solutions as tests
for MHD codes. To that end, we used the Bifrost code, thereby testing
the capabilities of these solutions as tests as well as (inversely) the
accuracy of Bifrost's recently developed ambipolar diffusion module. Results: The obtained solutions are shown to constitute a demanding,
but nonetheless viable, test for MHD codes that incorporate ambipolar
diffusion. Detailed tabulated runs of the solutions have been made
available at a public repository. The Bifrost code is able to reproduce
the theoretical solutions with sufficient accuracy up to very advanced
diffusive times. Using the code, we also explored the asymptotic
properties of our theoretical solutions in time when initially perturbed
with either small or finite perturbations. Conclusions: The
functions obtained in this paper are relevant as physical solutions
and also as tests for general MHD codes. They provide a more stringent
and general test than the simple Zeldovich-Kompaneets-Barenblatt-Pattle
solution. Movies associated to Figs. 4 and 7 are available at <A
href="https://www.aanda.org/10.1051/0004-6361/202141449/olm">https://www.aanda.org</A>

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Title: Fast plasmoid-mediated reconnection in a solar flare
Authors: Yan, Xiaoli; Xue, Zhike; Jiang, Chaowei; Priest, E. R.;
Kliem, Bernhard; Yang, Liheng; Wang, Jincheng; Kong, Defang; Song,
Yongliang; Feng, Xueshang; Liu, Zhong
2022NatCo..13..640Y Altcode:
Magnetic reconnection is a multi-faceted process of energy conversion
in astrophysical, space and laboratory plasmas that operates at
microscopic scales but has macroscopic drivers and consequences. Solar
flares present a key laboratory for its study, leaving imprints of the
microscopic physics in radiation spectra and allowing the macroscopic
evolution to be imaged, yet a full observational characterization
remains elusive. Here we combine high resolution imaging and spectral
observations of a confined solar flare at multiple wavelengths with
data-constrained magnetohydrodynamic modeling to study the dynamics
of the flare plasma from the current sheet to the plasmoid scale. The
analysis suggests that the flare resulted from the interaction of a
twisted magnetic flux rope surrounding a filament with nearby magnetic
loops whose feet are anchored in chromospheric fibrils. Bright
cusp-shaped structures represent the region around a reconnecting
separator or quasi-separator (hyperbolic flux tube). The fast
reconnection, which is relevant for other astrophysical environments,
revealed plasmoids in the current sheet and separatrices and associated
unresolved turbulent motions.

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Title: Scaling theory of three-dimensional magnetic reconnection
spreading and applications
Authors: Arencibia, Milton; Cassak, Paul; Shay, Michael; Priest, Eric
2021AGUFMSM35E2015A Altcode:
We develop a first-principles scaling theory of the spreading of
three-dimensional (3D) magnetic reconnection of finite extent in the
out of plane direction. The results provide a theoretical framework for
understanding spreading and are important for applications including
two-ribbon solar flares and reconnection in Earth's magnetosphere. This
theory addresses systems with or without an out of plane (guide)
magnetic field, and with or without Hall physics. The theory reproduces
known spreading speeds and directions with and without guide fields,
unifying previous knowledge in a single theory. New results include:
(1) Reconnection spreads in a particular direction if an x-line is
induced at the interface between reconnecting and non-reconnecting
regions, which is controlled by the out of plane gradient of the
electric field in the outflow direction. (2) The spreading mechanism for
anti-parallel collisionless reconnection is convection, as is known,
but for guide field reconnection it is magnetic field bending. We
confirm the theory using 3D two-fluid and resistive-magnetohydrodynamics
simulations. (3) The theory explains why anti-parallel reconnection in
resistive-magnetohydrodynamics does not spread. (4) The simulation
domain aspect ratio, associated with the free magnetic energy,
influences whether reconnection spreads or convects with a fixed
x-line length. (5) We perform a simulation initiating anti-parallel
collisionless reconnection with a pressure pulse instead of a magnetic
perturbation, finding spreading is unchanged rather than spreading at
the magnetosonic speed as previously suggested.

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Title: Scaling theory of three-dimensional magnetic reconnection
spreading
Authors: Arencibia, Milton; Cassak, P. A.; Shay, M. A.; Priest, E. R.
2021PhPl...28h2104A Altcode: 2021arXiv210800294A
We develop a first-principles scaling theory of the spreading of
three-dimensional (3D) magnetic reconnection of finite extent in the out
of plane direction. This theory addresses systems with or without an out
of plane (guide) magnetic field, and with or without Hall physics. The
theory reproduces known spreading speeds and directions with and without
guide fields, unifying previous knowledge in a single theory. New
results include the following: (1) reconnection spreads in a particular
direction if an x-line is induced at the interface between reconnecting
and non-reconnecting regions, which is controlled by the out of
plane gradient of the electric field in the outflow direction. (2)
The spreading mechanism for anti-parallel collisionless reconnection
is convection, as is known, but for guide field reconnection it is
magnetic field bending. We confirm the theory using 3D two-fluid and
resistive-magnetohydrodynamics simulations. (3) The theory explains why
anti-parallel reconnection in resistive-magnetohydrodynamics does not
spread. (4) The simulation domain aspect ratio, associated with the free
magnetic energy, influences whether reconnection spreads or convects
with a fixed x-line length. (5) We perform a simulation initiating
anti-parallel collisionless reconnection with a pressure pulse instead
of a magnetic perturbation, finding spreading is unchanged rather
than spreading at the magnetosonic speed as previously suggested. The
results provide a theoretical framework for understanding spreading
beyond systems studied here and are important for applications including
two-ribbon solar flares and reconnection in Earth's magnetosphere.

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Title: Dynamic Evolution Of A Solar Flare Current Sheet
Authors: Chitta, L.; Priest, E. R.; Cheng, X.
2021AAS...23830301C Altcode:
Current sheets play a key role in solar flares as they are the locations
where magnetic energy is liberated through reconnection and is converted
to other forms. Yet, their formation and evolution during the impulsive
phase of a flare remain elusive. In this talk, we will report new
observations of a current-sheet formation and subsequent evolution
in the early stages of a solar flare. In particular, we will present
multi-phase evolution of a dynamic current sheet from its formation
to quasi-stable evolution and disruption. Implications for the onset
and evolution of reconnection will be discussed.

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Title: Chromospheric and coronal heating and jet acceleration due
to reconnection driven by flux cancellation. II. Cancellation of
two magnetic polarities of unequal flux
Authors: Syntelis, P.; Priest, E. R.
2021A&A...649A.101S Altcode: 2021arXiv210316184S
Context. Recent observations have shown that magnetic flux cancellation
occurs at the photosphere more frequently than previously thought. Aims: In order to understand the energy release by reconnection
driven by flux cancellation, we previously studied a simple model
of two cancelling polarities of equal flux. Here, we further develop
our analysis to achieve a more general setup where the two cancelling
polarities have unequal magnetic fluxes and where many new features are
revealed. Methods: We carried out an analytical study of the
cancellation of two magnetic fragments of unequal and opposite flux
that approach one another and are located in an overlying horizontal
magnetic field. Results: The energy release as microflares and
nanoflares occurs in two main phases. During phase 1a, a separator
is formed and reconnection is driven at it as it rises to a maximum
height and then moves back down to the photosphere, heating the
plasma and accelerating plasma jets in the process. During phase 1b,
once the separator moves back to the photosphere, it bifurcates into
two null points. Reconnection is no longer driven at the separator
and an isolated magnetic domain connecting the two polarities is
formed. During phase 2, the polarities cancel out at the photosphere
as magnetic flux submerges below the photosphere and as reconnection
occurs at and above the photosphere and plasma jets and a mini-filament
eruption can be produced.

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Title: Three-dimensional magnetic reconnection in astrophysical
plasmas
Authors: Li, Ting; Priest, Eric; Guo, Ruilong
2021RSPSA.47700949L Altcode: 2021arXiv210405174L
Magnetic reconnection is a fundamental process in laboratory,
magnetospheric, solar and astrophysical plasmas, whereby magnetic
energy is converted into heat, bulk kinetic energy and fast particle
energy. Its nature in two dimensions is much better understood than
that in three dimensions, where its character is completely different
and has many diverse aspects that are currently being explored. Here,
we focus on the magnetohydrodynamics of three-dimensional reconnection
in the plasma environment of the Solar System, especially solar
flares. The theory of reconnection at null points, separators and
quasi-separators is described, together with accounts of numerical
simulations and observations of these three types of reconnection. The
distinction between separator and quasi-separator reconnection is a
theoretical one that is unimportant for the observations of energy
release. A new paradigm for solar flares, in which three-dimensional
reconnection plays a central role, is proposed.

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Title: From Formation to Disruption: Observing the Multiphase
Evolution of a Solar Flare Current Sheet
Authors: Chitta, L. P.; Priest, E. R.; Cheng, X.
2021ApJ...911..133C Altcode: 2021arXiv210302352C
A current sheet, where magnetic energy is liberated through
reconnection and converted to other forms, is thought to play
the central role in solar flares, the most intense explosions in
the heliosphere. However, the evolution of a current sheet and its
subsequent role in flare-related phenomena such as particle acceleration
is poorly understood. Here we report observations obtained with
NASA's Solar Dynamics Observatory that reveal a multiphase evolution
of a current sheet in the early stages of a solar flare, from its
formation to quasi-stable evolution and disruption. Our observations
have implications for the understanding of the onset and evolution of
reconnection in the early stages of eruptive solar flares.

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Title: Chromospheric and coronal heating and jet acceleration due to
reconnection driven by flux cancellation. I. At a three-dimensional
current sheet
Authors: Priest, E. R.; Syntelis, P.
2021A&A...647A..31P Altcode: 2021arXiv210104600P
Context. The recent discovery of much greater magnetic flux cancellation
taking place at the photosphere than previously realised has led
us in our previous works to suggest magnetic reconnection driven by
flux cancellation as the cause of a wide range of dynamic phenomena,
including jets of various kinds and solar atmospheric heating. Aims: Previously, the theory considered energy release at a
two-dimensional current sheet. Here we develop the theory further by
extending it to an axisymmetric current sheet in three dimensions
without resorting to complex variable theory. Methods: We
analytically study reconnection and treat the current sheet as a
three-dimensional structure. We apply the theory to the cancellation
of two fragments of equal but opposite flux that approach each another
and are located in an overlying horizontal magnetic field. 
Results: The energy release occurs in two phases. During Phase 1,
a separator is formed and reconnection is driven at it as it rises to
a maximum height and then moves back down to the photosphere, heating
the plasma and accelerating a plasma jet as it does so. During Phase 2
the fluxes cancel in the photosphere and accelerate a mixture of cool
and hot plasma upwards.

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Title: Scaling theory of magnetic reconnection spreading
Authors: Arencibia, M.; Cassak, P.; Liang, H.; Shay, M.; Jiong, Q.;
Priest, E. R.
2020AGUFMSM0200006A Altcode:
Observations of magnetic reconnection at the dayside magnetopause,
magnetotail and in two-ribbon solar flares suggest that the magnetic
reconnection process often begins in a localized region and spreads
in the out-of-plane direction as it proceeds. Much has been learned
from 3D numerical simulations of quasi-2D current sheets about how
reconnection spreads, including an empirical understanding of the
direction and the out-of-plane spreading speed as a function of system
parameters for idealized systems. For anti-parallel reconnection the
spreading occurs at the speed and direction of the current carriers,
while for reconnection with a large out-of-plane (guide) magnetic
field it spreads bi-directionally at the Alfven speed. However,
the understanding of the physics of reconnection spreading from first
principles remains primitive. We develop a scaling theory of 3D magnetic
reconnection spreading from first principles. We identify the key
micro- and meso-scale physics causing the spreading of reconnection
with and without a guide field, and predict the spreading speed in
current sheets with uniform and non-uniform thicknesses. For current
sheets with uniform equilibrium thickness, the predictions reproduce
previous empirical results. For spreading with no guide field in current
sheets with a non-uniform equilibrium thickness, a key prediction is
that in the thicker regions, the spreading is slower than the local
current carriers. We confirm these predictions via a parametric study
using 3D two-fluid numerical simulations. The results are potentially
important for understanding systems in which reconnection spreads,
including why the observed spreading speed is often at sub-Alfvenic
speeds in two-ribbon solar flares and the dayside magnetopause. The
results also have applications to laboratory reconnection experiments
and in the solar wind.

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Title: Impulsive coronal heating during the interaction of surface
magnetic fields in the lower solar atmosphere
Authors: Chitta, L. P.; Peter, H.; Priest, E. R.; Solanki, S. K.
2020A&A...644A.130C Altcode: 2020arXiv201012560C
Coronal plasma in the cores of solar active regions is impulsively
heated to more than 5 MK. The nature and location of the magnetic
energy source responsible for such impulsive heating is poorly
understood. Using observations of seven active regions from the Solar
Dynamics Observatory, we found that a majority of coronal loops hosting
hot plasma have at least one footpoint rooted in regions of interacting
mixed magnetic polarity at the solar surface. In cases when co-temporal
observations from the Interface Region Imaging Spectrograph space
mission are available, we found spectroscopic evidence for magnetic
reconnection at the base of the hot coronal loops. Our analysis suggests
that interactions of magnetic patches of opposite polarity at the
solar surface and the associated energy release during reconnection
are key to impulsive coronal heating. Movies are available at <A
href="https://www.aanda.org/10.1051/0004-6361/202039099/olm">https://www.aanda.org</A>

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Title: Reconnection Spreading Theory and Application to Two-Ribbon
Flares
Authors: Arencibia, M.; Cassak, P.; Liang, H.; Priest, E.; Shay, M.;
Qiu, J.
2020SPD....5121109A Altcode:
Observations of solar flare ribbons and prominence eruptions suggest
that the magnetic reconnection process underlying them begins locally
and spreads in the out-of-plane direction as it proceeds. Much has been
learned from 3D numerical simulations of quasi-2D current sheets about
how reconnection spreads, including an empirical understanding of the
direction and the out-of-plane spreading speed as a function of system
parameters for idealized systems. It has been shown that in the absence
of an out-of-plane (guide) magnetic field, the spreading occurs at the
speed and direction of the current carriers; with a guide field, the
spreading is bi-directional at the Alfven speed. Here, we advance upon
previous knowledge in two key ways. First, we develop a first principles
theory of 3D magnetic reconnection spreading. We identify the key micro-
and meso-scale physics causing the spreading of reconnection with and
without a guide field, and predict the scaling for the spreading speed
in these configurations. The predictions reproduce the previously
determined empirical results for both configurations. Second, we
use the theory to predict the spreading speed for current sheets of
non-uniform thickness. We confirm these predictions via a parametric
study using 3D two-fluid numerical simulations. A key result is that the
spreading with no guide field in non-uniform current sheets is slower
than the speed of the current carriers in the thicker regions of the
current sheet. The results are potentially important for understanding
observations in which reconnection spreads, including why the observed
spreading speed is often at sub-Alfvénic speeds. The result may also
have applications to Earth's magnetosphere, laboratory reconnection
experiments, and reconnection in the solar wind.

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Title: Coronal Mini-jets in an Activated Solar Tornado-like Prominence
Authors: Chen, Huadong; Zhang, Jun; De Pontieu, Bart; Ma, Suli; Kliem,
Bernhard; Priest, Eric
2020ApJ...899...19C Altcode: 2020arXiv200608252C
High-resolution observations from the Interface Region Imaging
Spectrometer reveal the existence of a particular type of small
solar jet, which arose singly or in clusters from a tornado-like
prominence suspended in the corona. In this study, we perform a detailed
statistical analysis of 43 selected mini-jets in the tornado event. Our
results show that the mini-jets typically have (1) a projected length
of 1.0-6.0 Mm, (2) a width of 0.2-1.0 Mm, (3) a lifetime of 10-50 s,
(4) a velocity of 100-350 km s-1, and (5) an acceleration
of 3-20 km s-2. Based on spectral diagnostics and EM-Loci
analysis, these jets seem to be multithermal small-scale plasma
ejections with an estimated average electron density of ∼2.4 ×
1010 cm-3 and an approximate mean temperature of
∼2.6 × 105 K. Their mean kinetic energy density, thermal
energy density, and dissipated magnetic field strength are roughly
estimated to be ∼9 erg cm-3, 3 erg cm-3, and
16 G, respectively. The accelerations of the mini-jets, the UV and EUV
brightenings at the footpoints of some mini-jets, and the activation
of the host prominence suggest that the tornado mini-jets are probably
created by fine-scale external or internal magnetic reconnections (a)
between the prominence field and the enveloping or background field or
(b) between twisted or braided flux tubes within the prominence. The
observations provide insight into the geometry of such reconnection
events in the corona and have implications for the structure of the
prominence magnetic field and the instability that is responsible for
the eruption of prominences and coronal mass ejections.

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Title: The Creation of Twist by Reconnection of Flux Tubes
Authors: Priest, E. R.; Longcope, D. W.
2020SoPh..295...48P Altcode:
A fundamental process in a plasma is the magnetic reconnection of
one pair of flux tubes (such as solar coronal loops) to produce a new
pair. During this process magnetic helicity is conserved, but mutual
helicity can be transformed to self-helicity, so that the new tubes
acquire twist. However, until recently, when Wright (Astrophys. J.878,
102, 2019) supplied a solution, the partition of self-helicity between
the two tubes was an outstanding puzzle. Here we examine Wright's
result in detail and apply it to a variety of cases. The simplest case,
which Wright himself used to illustrate the result, is that of thin
ribbons or flux sheets. We first explicitly apply his method to the
usually expected standard case (when the tubes approach one another
without twisting before reconnection) and confirm his result is valid
for flux sheaths and tubes as well as sheets.

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Title: A Cancellation Nanoflare Model for Solar Chromospheric and
Coronal Heating. III. 3D Simulations and Atmospheric Response
Authors: Syntelis, P.; Priest, E. R.
2020ApJ...891...52S Altcode: 2020arXiv200110456S
Inspired by recent observations suggesting that photospheric magnetic
flux cancellation occurs much more frequently than previously thought,
we analytically estimated the energy released from reconnection
driven by photospheric flux cancellation, and propose that it can
act as a mechanism for chromospheric and coronal heating. Using
two-dimensional simulations we validated the analytical estimates
and studied the resulting atmospheric response. In the present work,
we set up 3D resistive MHD simulations of two canceling polarities in
a stratified atmosphere with a horizontal external field to further
validate and improve upon the analytical estimates. The computational
evaluation of the parameters associated with the energy release are
in good qualitative agreement with the analytical estimates. The
computational Poynting energy flux into the current sheet is in good
qualitative agreement with the analytical estimates, after correcting
the analytical expression to better account for the horizontal extent
of the current sheet. The atmospheric response to the cancellation is
the formation of hot ejections, cool ejections, or a combination of
both hot and cool ejections, which can appear with a time difference
and/or be spatially offset, depending on the properties of the canceling
region and the resulting height of the reconnection. Therefore, during
the cancellation, a wide spectrum of ejections can be formed, which
can account for the variety of multi-thermal ejections associated with
Ellerman bombs, UV bursts, and IRIS bombs, and also other ejections
associated with small-scale canceling regions and spicules.

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Title: Quantifying the Toroidal Flux of Preexisting Flux Ropes of
Coronal Mass Ejections
Authors: Xing, C.; Cheng, X.; Qiu, Jiong; Hu, Qiang; Priest, E. R.;
Ding, M. D.
2020ApJ...889..125X Altcode: 2019arXiv191210623X
In past decades, much progress has been achieved in understanding
the origin and evolution of coronal mass ejections (CMEs). In situ
observations of the counterparts of CMEs, especially magnetic clouds
(MCs) near the Earth, have provided measurements of the structure
and total flux of CME flux ropes. However, it has been difficult to
measure these properties in an erupting CME flux rope, in particular in
a preexisting flux rope. In this work, we propose a model to estimate
the toroidal flux of a preexisting flux rope by subtracting the flux
contributed by magnetic reconnection during the eruption from the
flux measured in the MC. The flux contributed by the reconnection
is derived from geometric properties of two-ribbon flares based on a
quasi-2D reconnection model. We then apply the model to four CME/flare
events and find that the ratio of toroidal flux in the preexisting flux
rope to that in the associated MC lies in the range 0.40-0.88. This
indicates that the toroidal flux of the preexisting flux rope makes an
important contribution to that of the CME flux rope and is usually at
least as large as the flux arising from the eruption process for the
selected events.

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Title: Sub-Alfvènic Spreading of 3D Collisionless Magnetic
Reconnection and Application to Two-Ribbon Solar Flares
Authors: Arencibia, M.; Cassak, P.; Liang, H.; Priest, E. R.; Qiu,
J.; Longcope, D. W.
2019AGUFMSH13D3421A Altcode:
Ribbons in two-ribbon solar flares are observed to elongate in time
along the polarity inversion line in addition to their well-known
apparent motion away from it. This has been attributed to the spreading
of initially localized magnetic reconnection in time. Similar spreading
of reconnection has been observed in other settings, including
laboratory experiments, Earth's magnetotail, and Earth's dayside
magnetopause. Understanding how reconnection spreads is an important
aspect of understanding flare evolution including plasma energization
and particle acceleration, since these phenomena are associated with
the properties of the magnetic reconnection including its physical
size. An interesting observational result, both in solar flares and
Earth's magnetosphere, is that the reconnection tends to spread slower
than the Alfven speed. We investigate the effect of the thickness of
the current sheet being non-uniform on the speed of 3D spreading of
magnetic reconnection. We perform a parametric study using 3D two-fluid
numerical simulations of initially localized anti-parallel magnetic
reconnection and present a theory for the out-of-plane spreading
velocity as a function of the initial current sheet thicknesses of the
fluted current sheet. We find the spreading in fluted current sheets
is slower than spreading in uniform sheets with the same thickness in
as its broader region. This result provides a potential explanation
of why 3D reconnection can spread at sub-Alfvenic speeds.

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Title: Evidence for downflows in the narrow plasma sheet of 10 Sep
2017, and their significance for flare reconnection
Authors: Longcope, Dana; Unverferth, John E.; Klein, Courtney;
McCarthy, Marika; Priest, Eric R.
2019AAS...23421604L Altcode:
Current sheets are believed to form in the wakes of erupting flux ropes
and to enable the magnetic reconnection responsible for an associated
flare. Multi-wavelength observations of an eruption on 10 Sep 2017
show a long, linear feature widely taken as evidence of a current
sheet viewed edge-on. The relation between the high-temperature,
high-density plasma thus observed and any current sheet is not yet
entirely clear. We estimate the magnetic field strength surrounding
the sheet, and from that conclude that approximately one-third of all
flux in the active region was opened by the eruption. Subsequently
decreasing field strength suggests that the open flux closed down
over the next several hours through reconnection at a rate dΦ/dt =
5 × 1017 Mx/s. We find in AIA observations evidence of
downward moving, dark structures analogous to either supra-arcade
downflows more typically observed above flare arcades viewed face-on,
or to supra-arcade downflowing loops, previously reported in flares
viewed in this perspective. This suggests that the plasma sheet is
composed of the magnetic flux retracting after being reconnected high
above the arcade. We use a model of flux tube retraction following
reconnection to show that this process can generate high densities
and temperatures as observed in the plasma sheet. The retracting flux
tubes reach their highest temperatures at the end of their retraction,
well below the site of reconnection. Previous analysis of AIA and EIS
data had revealed a peak in the plasma temperature very near the base
of this particular sheet. This is consistent with our hypothesis that
downflows descend from a higher reconnection point. This work
supported by grants from NASA/HSR and NSF/REU

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Title: A Cancellation Nanoflare Model for Solar Chromospheric and
Coronal Heating. II. 2D Theory and Simulations
Authors: Syntelis, P.; Priest, E. R.; Chitta, L. P.
2019ApJ...872...32S Altcode: 2019arXiv190102798S
Recent observations at high spatial resolution have shown that magnetic
flux cancellation occurs on the solar surface much more frequently than
previously thought, and so this led Priest et al. (2018) to propose
magnetic reconnection driven by photospheric flux cancellation as
a mechanism for chromospheric and coronal heating. In particular,
they estimated analytically the amount of energy released as heat
and the height of the energy release during flux cancellation. In
the present work, we take the next step in the theory by setting up a
two-dimensional resistive MHD simulation of two canceling polarities
in the presence of a horizontal external field and a stratified
atmosphere in order to check and improve upon the analytical
estimates. Computational evaluation of the energy release during
reconnection is found to be in good qualitative agreement with the
analytical estimates. In addition, we go further and undertake an
initial study of the atmospheric response to reconnection. We find
that, during the cancellation, either hot ejections or cool ones or a
combination of both hot and cool ejections can be formed, depending
on the height of the reconnection location. The hot structures can
have the density and temperature of coronal loops, while the cooler
structures are suggestive of surges and large spicules.

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Title: Chapter 7 - Magnetohydrodynamics and Solar Dynamo Action
Authors: Priest, E. R.
2019sgsp.book..239P Altcode:
Magnetohydrodynamics (MHD) describes the complex interaction between
magnetic fields and plasmas that are responsible for much dynamic
behavior in the Sun and many other cosmic objects. This chapter
introduces the fundamental equations and their physical effects,
including the basic physics inherent in the equations of induction and
motion and the key process of magnetic reconnection for converting
magnetic energy into other forms. MHD is important in astrophysical
processes such as magnetoconvection, magnetic flux emergence, flux
ropes, spots, atmospheric heating, wind acceleration, flares, and
eruptions. In particular, the focus is on the generation of magnetic
fields by dynamo action.

---------------------------------------------------------
Title: Evidence for Downflows in the Narrow Plasma Sheet of 2017
September 10 and Their Significance for Flare Reconnection
Authors: Longcope, Dana; Unverferth, John; Klein, Courtney; McCarthy,
Marika; Priest, Eric
2018ApJ...868..148L Altcode:
Current sheets are believed to form in the wakes of erupting flux ropes
and to enable the magnetic reconnection responsible for an associated
flare. Multiwavelength observations of an eruption on 2017 September
10 show a long, linear feature widely taken as evidence of a current
sheet viewed edge-on. The relation between the high-temperature,
high-density plasma thus observed and any current sheet is not yet
entirely clear. We estimate the magnetic field strength surrounding
the sheet and conclude that approximately one-third of all flux in
the active region was opened by the eruption. Subsequently decreasing
field strength suggests that the open flux closed down over the next
several hours through reconnection at a rate \dot{{{Φ }}}≃ 5×
{10}17 Mx s-1. We find in AIA observations
evidence of downward-moving, dark structures analogous to either
supra-arcade downflows, more typically observed above flare arcades
viewed face-on, or supra-arcade downflowing loops, previously reported
in flares viewed in this perspective. These features suggest that the
plasma sheet is composed of the magnetic flux retracting after being
reconnected high above the arcade. We model flux tube retraction
following reconnection to show that this process can generate high
densities and temperatures as observed in the plasma sheet. The
retracting flux tubes reach their highest temperatures at the end of
their retraction, well below the site of reconnection, consistent with
previous analysis of AIA and EIS data showing a peak in the plasma
temperature near the base of this particular sheet.

---------------------------------------------------------
Title: Self-similar Approach for Rotating Magnetohydrodynamic Solar
and Astrophysical Structures
Authors: Luna, M.; Priest, E.; Moreno-Insertis, F.
2018ApJ...863..147L Altcode: 2018arXiv180702473L
Rotating magnetic structures are common in astrophysics,
from vortex tubes and tornadoes in the Sun all the way to
jets in different astrophysical systems. The physics of these
objects often combine inertial, magnetic, gas pressure, and
gravitational terms. Also, they often show approximate symmetries
that help simplify the otherwise rather intractable equations
governing their morphology and evolution. Here we propose a
general formulation of the equations assuming axisymmetry and a
self-similar form for all variables: in spherical coordinates (r,
θ, ϕ), the magnetic field and plasma velocity are taken to be of
the form {\boldsymbol{B}}={\boldsymbol{f}}(θ )/{r}n
and {\boldsymbol{v}}={\boldsymbol{g}}(θ )/{r}m, with
corresponding expressions for the scalar variables like pressure
and density. Solutions are obtained for potential, force-free, and
non-force-free magnetic configurations. Potential field solutions
can be found for all values of n. Nonpotential force-free solutions
possess an azimuthal component B ϕ and exist only for n
≥ 2 the resulting structures are twisted and have closed field lines
but are not collimated around the system axis. In the non-force-free
case, including gas pressure, the magnetic field lines acquire an
additional curvature to compensate for an outward pointing pressure
gradient force. We have also considered a pure rotation situation
with no gravity, in the zero-β limit: the solution has cylindrical
geometry and twisted magnetic field lines. The latter solutions can be
helpful in producing a collimated magnetic field structure; but they
exist only when n &lt; 0 and m &lt; 0: for applications they must be
matched to an external system at a finite distance from the origin.

---------------------------------------------------------
Title: A Cancellation Nanoflare Model for Solar Chromospheric and
Coronal Heating
Authors: Priest, E. R.; Chitta, L. P.; Syntelis, P.
2018ApJ...862L..24P Altcode: 2018arXiv180708161P
Nanoflare models for heating the solar corona usually assume magnetic
braiding and reconnection as the source of the energy. However,
recent observations at record spatial resolution from the SUNRISE
balloon mission suggest that photospheric magnetic flux cancellation
is much more common than previously realized. We therefore examine
the possibility of three-dimensional reconnection driven by flux
cancellation as a cause of chromospheric and coronal heating. In
particular, we estimate how the heights and amount of energy release
produced by flux cancellation depend on flux size, flux cancellation
speed, and overlying field strength.

---------------------------------------------------------
Title: Flux Rope Formation Due to Shearing and Zipper Reconnection
Authors: Threlfall, J.; Hood, A. W.; Priest, E. R.
2018SoPh..293...98T Altcode: 2018arXiv180606760T
Zipper reconnection has been proposed as a mechanism for creating
most of the twist in the flux tubes that are present prior to eruptive
flares and coronal mass ejections. We have conducted a first numerical
experiment on this new regime of reconnection, where two initially
untwisted parallel flux tubes are sheared and reconnected to form
a large flux rope. We describe the properties of this experiment,
including the linkage of magnetic flux between concentrated flux
sources at the base of the simulation, the twist of the newly formed
flux rope, and the conversion of mutual magnetic helicity in the
sheared pre-reconnection state into the self-helicity of the newly
formed flux rope.

---------------------------------------------------------
Title: The Eruption of a Small-scale Emerging Flux Rope as the Driver
of an M-class Flare and of a Coronal Mass Ejection
Authors: Yan, X. L.; Jiang, C. W.; Xue, Z. K.; Wang, J. C.; Priest,
E. R.; Yang, L. H.; Kong, D. F.; Cao, W. D.; Ji, H. S.
2017ApJ...845...18Y Altcode: 2017arXiv170700073Y
Solar flares and coronal mass ejections are the most powerful explosions
in the Sun. They are major sources of potentially destructive space
weather conditions. However, the possible causes of their initiation
remain controversial. Using high-resolution data observed by the New
Solar Telescope of Big Bear Solar Observaotry, supplemented by Solar
Dynamics Observatory observations, we present unusual observations of
a small-scale emerging flux rope near a large sunspot, whose eruption
produced an M-class flare and a coronal mass ejection. The presence of
the small-scale flux rope was indicated by static nonlinear force-free
field extrapolation as well as data-driven magnetohydrodynamics modeling
of the dynamic evolution of the coronal three-dimensional magnetic
field. During the emergence of the flux rope, rotation of satellite
sunspots at the footpoints of the flux rope was observed. Meanwhile,
the Lorentz force, magnetic energy, vertical current, and transverse
fields were increasing during this phase. The free energy from the
magnetic flux emergence and twisting magnetic fields is sufficient to
power the M-class flare. These observations present, for the first time,
the complete process, from the emergence of the small-scale flux rope,
to the production of solar eruptions.

---------------------------------------------------------
Title: 3D Simulation Study of the Spreading/Elongation of Ribbons
in Two-Ribbon Flares
Authors: Arencibia, Milton; Cassak, Paul; Qiu, Jiong; Longscope,
Dana; Priest, Eric R.
2017SPD....4810812A Altcode:
Two-ribbon solar flares are characterized by the appearance in pairs of
bright ribbons on the surface of the Sun. The ribbons separate from each
other in time, which has been cited as one of many pieces of evidence
that magnetic reconnection participates in the release of magnetic
energy in solar flares. In addition to moving apart from each other,
observations have revealed that ribbons also elongate (or spread) in
time along the polarity inversion line. This is likely related to the
spreading of the magnetic reconnection process in the corona. Recent
observations have shown ribbons can elongate either unidirectionally or
bidirectionally. We investigate the physics of reconnection spreading
and its potential relation to two-ribbon flares via a parametric study
using 3D numerical simulations with the two-fluid (MHD + Hall effect +
electron inertia) model. We study how anti-parallel reconnection spreads
in current sheets with a non-uniform thickness in the out-of-plane
direction. Previous numerical work on spreading in current sheets of
uniform thickness revealed that anti-parallel reconnection spreads at
a speed given by the current carriers, but it is not obvious how the
spreading occurs in a current sheet with non-uniform thickness. We
compare spreading in this system with spreading in current sheets of
uniform thickness that are thicker than the dissipation scale. The
results may be useful not just for solar flares, but also for Earth’s
magnetotail, laboratory reconnection experiments, and reconnection in
the solar wind.

---------------------------------------------------------
Title: A Complex Solar Coronal Jet with Two Phases
Authors: Chen, Jie; Su, Jiangtao; Deng, Yuanyong; Priest, E. R.
2017ApJ...840...54C Altcode: 2017arXiv170402072C
Jets often occur repeatedly from almost the same location. In this
paper, a complex solar jet was observed with two phases to the west of
NOAA AR 11513 on 2012 July 2. If it had been observed at only moderate
resolution, the two phases and their points of origin would have
been regarded as identical. However, at high resolution we find that
the two phases merge into one another and the accompanying footpoint
brightenings occur at different locations. The phases originate from
different magnetic patches rather than being one phase originating
from the same patch. Photospheric line of sight (LOS) magnetograms
show that the bases of the two phases lie in two different patches
of magnetic flux that decrease in size during the occurrence of the
two phases. Based on these observations, we suggest that the driving
mechanism of the two successive phases is magnetic cancellation of
two separate magnetic fragments with an opposite-polarity fragment
between them.

---------------------------------------------------------
Title: Our Dynamic Sun (Hannes Alfvén Medal Lecture)
Authors: Priest, Eric
2017EGUGA..19.2273P Altcode:
The Sun, an object of worship for early civilisations, is the main
source of light and life on Earth and of our space weather, with many
subtle effects on our environment. The lecture will introduce you
to the Sun and its dynamic phenomena, and will aim to show how our
understanding of many aspects of the Sun has been revolutionized over
the past few years by current spacecraft observations and models. Much
of the dynamic behaviour is driven by the magnetic field since, in the
outer atmosphere (or corona), it represents by far the largest source
of energy. The interior of the Sun, revealed by solar seismology,
possesses a strong shear layer at the base of the convection zone,
where sunspot magnetic fields are generated. But a small-scale dynamo is
also operating near the surface of the Sun, generating magnetic fields
that thread the lowest layer of the solar atmosphere, the photosphere,
in a turbulent convective state. Above the photosphere lies the highly
dynamic fine-scale chromosphere and beyond that the rare corona at
high temperatures exceeding one million degrees K. Magnetic mechanisms
for heating the corona (an intriguing puzzle) will be described. Other
puzzles include the structure of giant flux ropes, known as prominences,
which have complex fine structure. Occasionally, they erupt and produce
huge ejections of mass and magnetic field (coronal mass ejections),
which can disrupt the space environment of the Earth. When such
eruptions originate in active regions around sunspots, they are also
associated with solar flares, where magnetic energy is converted to
kinetic, heat and fast particle energy. A new theory will be presented
for the origin of the twist that is observed in erupting prominences.

---------------------------------------------------------
Title: Elongation of Flare Ribbons
Authors: Qiu, Jiong; Longcope, Dana W.; Cassak, Paul A.; Priest,
Eric R.
2017ApJ...838...17Q Altcode: 2017arXiv170702478Q
We present an analysis of the apparent elongation motion of flare
ribbons along the polarity inversion line (PIL), as well as the shear
of flare loops in several two-ribbon flares. Flare ribbons and loops
spread along the PIL at a speed ranging from a few to a hundred
km s-1. The shear measured from conjugate footpoints
is consistent with the measurement from flare loops, and both show
the decrease of shear toward a potential field as a flare evolves
and ribbons and loops spread along the PIL. Flares exhibiting fast
bidirectional elongation appear to have a strong shear, which may
indicate a large magnetic guide field relative to the reconnection field
in the coronal current sheet. We discuss how the analysis of ribbon
motion could help infer properties in the corona where reconnection
takes place.

---------------------------------------------------------
Title: Imaging Observations of Magnetic Reconnection in a Solar
Eruptive Flare
Authors: Li, Y.; Sun, X.; Ding, M. D.; Qiu, J.; Priest, E. R.
2017ApJ...835..190L Altcode: 2016arXiv161209417L
Solar flares are among the most energetic events in the solar
atmosphere. It is widely accepted that flares are powered by magnetic
reconnection in the corona. An eruptive flare is usually accompanied
by a coronal mass ejection, both of which are probably driven by the
eruption of a magnetic flux rope (MFR). Here we report an eruptive
flare on 2016 March 23 observed by the Atmospheric Imaging Assembly
on board the Solar Dynamics Observatory. The extreme-ultraviolet
imaging observations exhibit the clear rise and eruption of an
MFR. In particular, the observations reveal solid evidence of
magnetic reconnection from both the corona and chromosphere during the
flare. Moreover, weak reconnection is observed before the start of the
flare. We find that the preflare weak reconnection is of tether-cutting
type and helps the MFR to rise slowly. Induced by a further rise of
the MFR, strong reconnection occurs in the rise phases of the flare,
which is temporally related to the MFR eruption. We also find that
the magnetic reconnection is more of 3D-type in the early phase,
as manifested in a strong-to-weak shear transition in flare loops,
and becomes more 2D-like in the later phase, as shown by the apparent
rising motion of an arcade of flare loops.

---------------------------------------------------------
Title: Flux-Rope Twist in Eruptive Flares and CMEs: Due to Zipper
and Main-Phase Reconnection
Authors: Priest, E. R.; Longcope, D. W.
2017SoPh..292...25P Altcode: 2017arXiv170100147P
The nature of three-dimensional reconnection when a twisted flux
tube erupts during an eruptive flare or coronal mass ejection is
considered. The reconnection has two phases: first of all, 3D "zipper
reconnection" propagates along the initial coronal arcade, parallel to
the polarity inversion line (PIL); then subsequent quasi-2D "main-phase
reconnection" in the low corona around a flux rope during its eruption
produces coronal loops and chromospheric ribbons that propagate away
from the PIL in a direction normal to it. One scenario starts with a
sheared arcade: the zipper reconnection creates a twisted flux rope
of roughly one turn (2 π radians of twist), and then main-phase
reconnection builds up the bulk of the erupting flux rope with a
relatively uniform twist of a few turns. A second scenario starts
with a pre-existing flux rope under the arcade. Here the zipper phase
can create a core with many turns that depend on the ratio of the
magnetic fluxes in the newly formed flare ribbons and the new flux
rope. Main phase reconnection then adds a layer of roughly uniform
twist to the twisted central core. Both phases and scenarios are
modeled in a simple way that assumes the initial magnetic flux is
fragmented along the PIL. The model uses conservation of magnetic
helicity and flux, together with equipartition of magnetic helicity,
to deduce the twist of the erupting flux rope in terms the geometry of
the initial configuration. Interplanetary observations show some flux
ropes have a fairly uniform twist, which could be produced when the
zipper phase and any pre-existing flux rope possess small or moderate
twist (up to one or two turns). Other interplanetary flux ropes have
highly twisted cores (up to five turns), which could be produced when
there is a pre-existing flux rope and an active zipper phase that
creates substantial extra twist.

---------------------------------------------------------
Title: The Formation of an Inverse S-shaped Active-region Filament
Driven by Sunspot Motion and Magnetic Reconnection
Authors: Yan, X. L.; Priest, E. R.; Guo, Q. L.; Xue, Z. K.; Wang,
J. C.; Yang, L. H.
2016ApJ...832...23Y Altcode: 2016arXiv160904871Y
We present a detailed study of the formation of an inverse S-shaped
filament prior to its eruption in active region NOAA 11884 from 2013
October 31 to November 2. In the initial stage, clockwise rotation of
a small positive sunspot around the main negative trailing sunspot
formed a curved filament. Then the small sunspot cancelled with the
negative magnetic flux to create a longer active-region filament
with an inverse S-shape. At the cancellation site a brightening was
observed in UV and EUV images and bright material was transferred to
the filament. Later the filament erupted after cancellation of two
opposite polarities below the upper part of the filament. Nonlinear
force-free field extrapolation of vector photospheric fields suggests
that the filament may have a twisted structure, but this cannot be
confirmed from the current observations.

---------------------------------------------------------
Title: 3D MHD modeling of twisted coronal loops
Authors: Reale, F.; Orlando, S.; Guarrasi, M.; Mignone, A.; Peres,
G.; Hood, A. W.; Priest, E. R.
2016ApJ...830...21R Altcode: 2016arXiv160705500R
We perform MHD modeling of a single bright coronal loop to include the
interaction with a non-uniform magnetic field. The field is stressed
by random footpoint rotation in the central region and its energy is
dissipated into heating by growing currents through anomalous magnetic
diffusivity that switches on in the corona above a current density
threshold. We model an entire single magnetic flux tube in the solar
atmosphere extending from the high-β chromosphere to the low-β corona
through the steep transition region. The magnetic field expands from
the chromosphere to the corona. The maximum resolution is ∼30 km. We
obtain an overall evolution typical of loop models and realistic loop
emission in the EUV and X-ray bands. The plasma confined in the flux
tube is heated to active region temperatures (∼3 MK) after ∼2/3
hr. Upflows from the chromosphere up to ∼100 km s-1
fill the core of the flux tube to densities above 109
cm-3. More heating is released in the low corona than the
high corona and is finely structured both in space and time.

---------------------------------------------------------
Title: Direct imaging of a classical solar eruptive flare
Authors: Li, Y.; Sun, X. D.; Ding, M. D.; Qiu, J.; Priest, E. R.;
Longcope, D. W.
2016usc..confE..21L Altcode:
Solar flares are the most energetic events in the solar system that
have a potential hazard on Earth. Although a standard model for
the eruptive flare accompanied by a coronal mass ejection has been
outlined and elaborated for decades, some key aspects are still under
debate, such as what drives the eruption, what is the role of magnetic
reconnection, and how the flare loops evolve. Here we present an
excellent event exhibiting nearly all the key elements involved in the
standard flare model. Using extreme-ultraviolet imaging observations,
we detect the unambiguous rise and eruption of a magnetic flux rope,
solid evidence for magnetic reconnection, and evident slipping and
rising motions in flare loops. Modeled coronal magnetic field supports
the interpretation of a pre-existing flux rope that persists after
the eruption with reduced twist. This flare, from the observational
view, shows a clear and comprehensive picture of how a classical solar
eruptive flare occurs and evolves, and helps to clarify some of the
controversial topics in the standard flare model.

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

---------------------------------------------------------
Title: Evolution of Magnetic Helicity During Eruptive Flares and
Coronal Mass Ejections
Authors: Priest, E. R.; Longcope, D. W.; Janvier, M.
2016SoPh..291.2017P Altcode: 2016arXiv160703874P; 2016SoPh..tmp..130P
During eruptive solar flares and coronal mass ejections, a non-potential
magnetic arcade with much excess magnetic energy goes unstable and
reconnects. It produces a twisted erupting flux rope and leaves behind
a sheared arcade of hot coronal loops. We suggest that the twist of the
erupting flux rope can be determined from conservation of magnetic flux
and magnetic helicity and equipartition of magnetic helicity. It depends
on the geometry of the initial pre-eruptive structure. Two cases are
considered, in the first of which a flux rope is not present initially
but is created during the eruption by the reconnection. In the second
case, a flux rope is present under the arcade in the pre-eruptive
state, and the effect of the eruption and reconnection is to add an
amount of magnetic helicity that depends on the fluxes of the rope
and arcade and the geometry.

---------------------------------------------------------
Title: MHD Structures in Three-Dimensional Reconnection
Authors: Priest, E.
2016ASSL..427..101P Altcode:
This review of three-dimensional reconnection focusses on the MHD
aspects of the process. It describes the different structures where
current tends to concentrate and so lead to reconnection, namely,
null points (where the magnetic field vanishes), separators (which are
magnetic field lines joining null points) and quasi-separators. Then
the role of topological invariants such as magnetic helicity (which
describes the twist and linkage of magnetic structures) is described,
together with the conditions for flux and field-line conservation
and for reconnection itself. The surprising and crucial differences
between 2D and 3D reconnection are highlighted and finally the different
regimes for 3D reconnection are summarised.

---------------------------------------------------------
Title: Are Tornado-Like Magnetic Structures Able to Support Solar
Prominence Plasma?
Authors: Ogunjo, S. T.; Luna Bennasar, M.; Moreno-Insertis, F.;
Priest, E. R.
2015AGUFMSH53B2483O Altcode:
Recent high-resolution and high-cadence observations have surprisingly
suggested that prominence barbs exhibit apparent rotating motions
suggestive of a tornado-like structure. Additional evidence has
been provided by Doppler measurements. The observations reveal
opposite velocities for both hot and cool plasma on the two sides of
a prominence barb. This motion is persistent for several hours and
has been interpreted in terms of rotational motion of prominence
feet. Several authors suggest that such barb motions are rotating
helical structures around a vertical axis similar to tornadoes
on Earth. One of the difficulties of such a proposal is how to
support cool prominence plasma in almost-vertical structures against
gravity. In this work we model analytically a tornado-like structure
and try to determine possible mechanisms to support the prominence
plasma. We have found that the Lorentz force can indeed support the
barb plasma provided the magnetic structure is sufficiently twisted
and/or significant poloidal flows are present.

---------------------------------------------------------
Title: Are Tornado-like Magnetic Structures Able to Support Solar
Prominence Plasma?
Authors: Luna, M.; Moreno-Insertis, F.; Priest, E.
2015ApJ...808L..23L Altcode: 2015arXiv150701455L
Recent high-resolution and high-cadence observations have surprisingly
suggested that prominence barbs exhibit apparent rotating motions
suggestive of a tornado-like structure. Additional evidence has
been provided by Doppler measurements. The observations reveal
opposite velocities for both hot and cool plasma on the two sides of
a prominence barb. This motion is persistent for several hours and
has been interpreted in terms of rotational motion of prominence
feet. Several authors suggest that such barb motions are rotating
helical structures around a vertical axis similar to tornadoes
on Earth. One of the difficulties of such a proposal is how to
support cool prominence plasma in almost-vertical structures against
gravity. In this work we model analytically a tornado-like structure
and try to determine possible mechanisms to support the prominence
plasma. We have found that the Lorentz force can indeed support the
barb plasma provided the magnetic structure is sufficiently twisted
and/or significant poloidal flows are present.

---------------------------------------------------------
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.

---------------------------------------------------------
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: Hinode 7: Conference summary and future suggestions
Authors: Priest, Eric
2014PASJ...66S..18P Altcode: 2014arXiv1405.3523P; 2014PASJ..tmp..104P
This conclusion to the seventh Hinode science meeting (2013 November
in Takayama, Japan) attempts to summarise what we have learnt during
the conference (mainly from the review talks) about new observations
from Hinode and about theories stimulated by them. Suggestions for
future study are also offered.

---------------------------------------------------------
Title: The formation and stability of Petschek reconnection
Authors: Baty, H.; Forbes, T. G.; Priest, E. R.
2014PhPl...21k2111B Altcode:
No abstract at ADS

---------------------------------------------------------
Title: A Life of Fun Playing with Solar Magnetic Fields (Special
Historical Review)
Authors: Priest, E. R.
2014SoPh..289.3579P Altcode: 2014SoPh..tmp...93P; 2014arXiv1405.3481P
This invited memoire describes my fortunate life, which has been
enriched by meeting many wonderful people. The story starts at home
and university, and continues with accounts of St Andrews and trips
to the USA, together with musings on the book Solar MHD. The nature
and results of collaborations with key people from abroad and with
students is mentioned at length. Finally, other important aspects of
my life are mentioned briefly before wrapping up.

---------------------------------------------------------
Title: Catastrophe versus Instability for the Eruption of a Toroidal
Solar Magnetic Flux Rope
Authors: Kliem, B.; Lin, J.; Forbes, T. G.; Priest, E. R.; Török, T.
2014ApJ...789...46K Altcode: 2014arXiv1404.5922K
The onset of a solar eruption is formulated here as either a magnetic
catastrophe or as an instability. Both start with the same equation of
force balance governing the underlying equilibria. Using a toroidal
flux rope in an external bipolar or quadrupolar field as a model
for the current-carrying flux, we demonstrate the occurrence of a
fold catastrophe by loss of equilibrium for several representative
evolutionary sequences in the stable domain of parameter space. We
verify that this catastrophe and the torus instability occur at the same
point; they are thus equivalent descriptions for the onset condition
of solar eruptions.

---------------------------------------------------------
Title: Magnetohydrodynamics of the Sun
Authors: Priest, Eric
2014masu.book.....P Altcode:
Preface; 1. A description of the Sun; 2. Basic equations of MHD;
3. Magnetohydrostatics; 4. Waves; 5. Shock waves; 6. Magnetic
reconnection; 7. Instability; 8. Dynamo theory; 9. Magnetoconvection
and sunspots; 10. Heating of the upper atmosphere; 11. Prominences;
12. Solar flares and coronal mass ejections; 13. The solar wind;
Appendices; References; Index.

---------------------------------------------------------
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: Prominences: Conference Summary and Suggestions for the Future
Authors: Priest, Eric R.
2014IAUS..300..379P Altcode:
In this conclusion to the conference, I shall attempt to summarise
what we knew before about solar prominences and what we have learnt
during the conference (mainly from the review talks), as well as to
make suggestions for their future study.

---------------------------------------------------------
Title: On the Nature of Reconnection at a Solar Coronal Null Point
above a Separatrix Dome
Authors: Pontin, D. I.; Priest, E. R.; Galsgaard, K.
2013ApJ...774..154P Altcode: 2013arXiv1307.6874P
Three-dimensional magnetic null points are ubiquitous in the solar
corona and in any generic mixed-polarity magnetic field. We consider
magnetic reconnection at an isolated coronal null point whose fan
field lines form a dome structure. Using analytical and computational
models, we demonstrate several features of spine-fan reconnection
at such a null, including the fact that substantial magnetic flux
transfer from one region of field line connectivity to another can
occur. The flux transfer occurs across the current sheet that forms
around the null point during spine-fan reconnection, and there is no
separator present. Also, flipping of magnetic field lines takes place
in a manner similar to that observed in the quasi-separatrix layer or
slip-running reconnection.

---------------------------------------------------------
Title: The nature and significance of solar minima
Authors: Priest, Eric
2012IAUS..286....3P Altcode:
As an introduction to the theme of this symposium, I give a simple
review of the photospheric magnetic field, the properties of the solar
cycle, the way in which the magnetic field is thought to be generated by
dynamo action, and finally the unusual properties of the recent solar
minimum. This has awakened an interest in improving predictions of the
solar cycle and in the nature of solar minima not just as gaps between
maxima but as phenomena of intrinsic interest in their own right.

---------------------------------------------------------
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: 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: Cosmic magnetic fields in the Sun: Current Outstanding Problems
(Invited Review)
Authors: Priest, Eric
2011IAUS..273....1P Altcode:
In the Sun there has been much progress towards answering fundamental
problems with profound implications for the behaviour of cosmic
magnetic fields in other stars. A review is given here of such
problems, including identifying some of the outstanding questions that
remain. In the solar interior, the main dynamo operates at the base
of the convection zone, but its details have not been identified. In
the solar surface, recent observations have revealed many new and
surprising properties of magnetic fields, but understanding the key
processes of flux emergence, fragmentation, merging and cancellation
is rudimentary. Sunspots have until very recently been an enigma. In
the atmosphere, there are many new ideas for coronal heating and
solar wind acceleration, but the mechanisms have not yet been pinned
down. Also, the detailed mechanisms for solar flares and coronal mass
ejections remain controversial. In future, new generations of space
and ground-based measurements and computational modelling should enable
a definitive physical understanding of these puzzles.

---------------------------------------------------------
Title: The Flux Tube Tectonics model for coronal heating
Authors: Priest, Eric R.
2011JASTP..73..271P Altcode:
An account is presented of the Flux Tube Tectonics model for heating the
solar corona, in which a multitude of current sheets are continually
forming and dissipating. In addition, a model is summarised for the
time-dependent response of the corona to the sudden dissipation of
one such current sheet.

---------------------------------------------------------
Title: Nonlinear Plasma Physics of the Solar Corona
Authors: Priest, Eric R.
2011AIPC.1320....3P Altcode:
As a tribute to Dennis Papadopoulos, we present a review of some recent
ideas in solar coronal plasma physics. In particular we discuss some
models of coronal heating, notably the coronal tectonics model, as well
as some ideas on the nature of reconnection in three dimensions. In
addition, we summarise a model for the time-dependent response of the
corona to the sudden dissipation of a current sheet.

---------------------------------------------------------
Title: MHD reconnection
Authors: Priest, Eric
2011SchpJ...6.2371P Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Opening Remarks
Authors: Priest, E.
2010ASSP...19...10P Altcode: 2010mcia.conf...10P
I would like to add my warm welcome to this meeting on behalf of the
Scientific Organizing Committee. I am delighted that so many of the
world's leading solar physicists have agreed to attend this meeting.

---------------------------------------------------------
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: Three-dimensional null point reconnection regimes
Authors: Priest, E. R.; Pontin, D. I.
2009PhPl...16l2101P Altcode: 2009arXiv0910.3043P
Recent advances in theory and computational experiments have shown the
need to refine the previous categorization of magnetic reconnection
at three-dimensional null points-points at which the magnetic field
vanishes. We propose here a division into three different types,
depending on the nature of the flow near the spine and fan of the
null. The spine is an isolated field line which approaches the null
(or recedes from it), while the fan is a surface of field lines which
recede from it (or approach it). So-called torsional spine reconnection
occurs when field lines in the vicinity of the fan rotate, with current
becoming concentrated along the spine so that nearby field lines undergo
rotational slippage. In torsional fan reconnection field lines near
the spine rotate and create a current that is concentrated in the fan
with a rotational flux mismatch and rotational slippage. In both of
these regimes, the spine and fan are perpendicular and there is no
flux transfer across spine or fan. The third regime, called spine-fan
reconnection, is the most common in practice and combines elements
of the previous spine and fan models. In this case, in response to a
generic shearing motion, the null point collapses to form a current
sheet that is focused at the null itself, in a sheet that locally spans
both the spine and fan. In this regime the spine and fan are no longer
perpendicular and there is flux transfer across both of them.

---------------------------------------------------------
Title: Dynamic non-null magnetic reconnection in three dimensions-II:
composite solutions
Authors: Wilmot-Smith, A. L.; Hornig, G.; Priest, E. R.
2009GApFD.103..515W Altcode: 2008arXiv0811.4621W
In this series of papers we examine magnetic reconnection in a domain
where the magnetic field does not vanish and the non-ideal region
is localised in space. In a previous paper we presented a technique
for obtaining analytical solutions to the stationary resistive MHD
equations in such a situation and examined specific examples of
non-ideal reconnective solutions. Here we further develop the model,
noting that certain ideal solutions may be superimposed onto the
fundamental non-ideal solutions and examining the effect of imposing
various such flows. Significant implications are found for the evolution
of magnetic flux in the reconnection process. It is shown that, in
contrast to the two-dimensional case, in three-dimensions there is a
very wide variety of physically different steady reconnection solutions.

---------------------------------------------------------
Title: Relationship between the topological skeleton, current
concentrations, and 3D magnetic reconnection sites in the solar
atmosphere
Authors: Maclean, R. C.; Büchner, J.; Priest, E. R.
2009A&A...501..321M Altcode:
Aims: The aim of this work is to determine the relationship between
the 3D structure of the coronal magnetic field, diagnosed by the
topological skeleton, and current concentrations as potential sites of
3D reconnection. Methods: We utilised the results of 3D numerical
MHD simulations of an observed EUV bright point (BP) in the solar
atmosphere. The simulations are based on MDI line-of-sight magnetogram
data from 13 June 1998. We analysed the results of the simulations
using the method of magnetic charge topology. Three different methods
of reducing the magnetogram to a set of point magnetic sources are
tested. Results: Observations of the BP show a rotation of one
of its main magnetic source regions. Numerical simulations of this
rotational motion result in a localised build-up of parallel electric
current, which is dissipated by anomalous resistivity, causing
3D magnetic reconnection and BP heating. The magnetic topological
structure of the simulated BP was also calculated, and a portion
of the topological separatrix surface bounding the magnetic flux of
the rotating source region is found to correspond to the locations
of current build-up and heating. All three magnetogram reduction
methods produce similar results for the large-scale magnetic field
structure. Conclusions: Magnetic topology is a useful method
for predicting the locations of coronal current concentrations,
insofar as the results of our simulations show that strong integrated
parallel electric fields are found only along topological separatrix
surfaces. However, further investigation is necessary to determine
exactly which parts of the reconstructed separatrices will host the
electric currents. Topological magnetic field reconstructions also
cast light on the location of coronal BP heating, which occurs as
a result of the dissipation of the currents by 3D reconnection. The
choice of the magnetogram reduction algorithm does not greatly affect
the large-scale topological features of the resulting reconstructed
magnetic field. Further work is required to compare these results with
data for other observed BPs.

---------------------------------------------------------
Title: Slip-Squashing Factors as a Measure of Three-Dimensional
Magnetic Reconnection
Authors: Titov, V. S.; Forbes, T. G.; Priest, E. R.; Mikić, Z.;
Linker, J. A.
2009ApJ...693.1029T Altcode: 2008arXiv0807.2892T
A general method for describing magnetic reconnection in arbitrary
three-dimensional magnetic configurations is proposed. The method
is based on the field-line mapping technique previously used only
for the analysis of a magnetic structure at a given time. This
technique is extended here so as to analyze the evolution of a
magnetic structure. Such a generalization is made with the help of
new dimensionless quantities called "slip-squashing factors." Their
large values define the surfaces that border the reconnected or
to-be-reconnected magnetic flux tubes for a given period of time
during the magnetic evolution. The proposed method is universal,
since it assumes only that the time sequence of evolving magnetic
field and the tangential boundary flows are known. The application
of the method is illustrated for simple examples, one of which was
considered previously by Hesse and coworkers in the framework of the
general magnetic reconnection theory. The examples help us to compare
these two approaches; it reveals also that, just as for magnetic null
points, hyperbolic and cusp minimum points of a magnetic field serve
as favorable sites for magnetic reconnection. The new method admits a
straightforward numerical implementation and provides a powerful tool
for the diagnostics of magnetic reconnection in numerical models of
solar-flare-like phenomena in space and laboratory plasmas.

---------------------------------------------------------
Title: The Solar-Stellar Connection: Our New Sun
Authors: Priest, Eric R.
2009AIPC.1094....3P Altcode: 2009csss...15....3P
Our view of the Sun has changed dramatically over the past 10
years due mainly to a series of space satellites such as Yohkoh,
SoHO and TRACE. This state of ferment will continue with the coming
onto line last year of two other satellites, Hinode and STEREO, and
next year SDO. Here we give a brief overview of the progress made in
answering fundamental questions about the nature of the Sun which may
have profound implications for other stars. In the interior,
helioseismology has revealed the internal rotation structure and
suggested that the main solar dynamo responsible for active regions is
located at the tachocline, although the details are highly uncertain
and there may be a second dynamo responsible for generating small-scale
ephemeral regions. In the photosphere, flux is mainly concentrated at
the edges of supergranule cells, but recent high-resolution observations
have suggested that extra flux is also located at granulation boundaries
and Hinode has discovered much horizontal flux. The solar corona is
likely to be heated in myriads of tiny current sheets by reconnection,
according to the Coronal Tectonics Model. Observations suggest that
all the coronal field lines reconnect every 1.5 hours. Theory has shown
that reconnection in 3D has many features that are completely different
from the standard 2D picture. The solar wind is highly dynamic and
complex and its acceleration mechanism may possibly be high-frequency
ion-cyclotron waves. Many new features of solar flares and coronal
mass ejections have been discovered, but it is not known whether the
cause of the eruption is an instability or a lack of equilibrium.

---------------------------------------------------------
Title: Slip-Squashing Factors as a Measure of Three-Dimensional
Magnetic Reconnection
Authors: Titov, V. S.; Forbes, T. G.; Priest, E. R.; Mikic, Z.;
Linker, J. A.
2008AGUFMSM31A1713T Altcode:
A general method for describing magnetic reconnection in arbitrary
three-dimensional magnetic configurations is proposed. The method is
based on the field-line mapping technique previously used only for
the analysis of magnetic structure at a given time. This technique is
extended here so as to analyze the evolution of magnetic structure. Such
a generalization is made with the help of new dimensionless quantities
called "slip-squashing factors". Their large values define the surfaces
that border the reconnected or to-be-reconnected magnetic flux tubes
for a given period of time during the magnetic evolution. The proposed
method is universal, since it assumes only that the time sequence
of the evolving magnetic field and the tangential boundary flows are
known. The application of the method is illustrated for simple examples,
one of which was considered previously by Hesse and coworkers in the
framework of the general magnetic reconnection theory. The examples
help to compare these two approaches; they reveal also that, just as for
magnetic null points, hyperbolic and cusp minimum points of a magnetic
field may serve as favorable sites for magnetic reconnection. The new
method admits a straightforward numerical implementation and provides
a powerful tool for the diagnostics of magnetic reconnection in
numerical models of solar-flare-like phenomena in space and laboratory
plasmas. Research partially supported by NASA and NSF.

---------------------------------------------------------
Title: Coronal Alfvén speeds in an isothermal atmosphere. I. Global
properties
Authors: Régnier, S.; Priest, E. R.; Hood, A. W.
2008A&A...491..297R Altcode: 2008arXiv0809.1155R
Aims: Estimating Alfvén speeds is of interest in modelling the solar
corona, studying the coronal heating problem and understanding the
initiation and propagation of coronal mass ejections (CMEs). Methods: We assume here that the corona is in a magnetohydrostatic
equilibrium and that, because of the low plasma β, one may decouple
the magnetic forces from pressure and gravity. The magnetic field is
then described by a force-free field for which we perform a statistical
study of the magnetic field strength with height for four different
active regions. The plasma along each field line is assumed to be in a
hydrostatic equilibrium. As a first approximation, the coronal plasma
is assumed to be isothermal with a constant or varying gravity with
height. We study a bipolar magnetic field with a ring distribution
of currents, and apply this method to four active regions associated
with different eruptive events. Results: By studying the global
properties of the magnetic field strength above active regions, we
conclude that (i) most of the magnetic flux is localized within 50 Mm
of the photosphere; (ii) most of the energy is stored in the corona
below 150 Mm; (iii) most of the magnetic field strength decays with
height for a nonlinear force-free field slower than for a potential
field. The Alfvén speed values in an isothermal atmosphere can vary
by two orders of magnitude (up to 100 000 km s-1). The
global properties of the Alfvén speed are sensitive to the nature
of the magnetic configuration. For an active region with highly
twisted flux tubes, the Alfvén speed is significantly increased at
the typical height of the twisted flux bundles; in flaring regions,
the average Alfvén speeds are above 5000 km s-1 and depart
highly from potential field values. Conclusions: We discuss the
implications of this model for the reconnection rate and inflow speed,
the coronal plasma β and the Alfvén transit time.

---------------------------------------------------------
Title: Highlights from Hinode
Authors: Priest, E. R.
2008ASPC..397..147P Altcode:
A personal summary is presented of the main results that have been
presented at the conference. Particular emphasis is given to the new
discoveries that have made use of the SOT, XRT and EIS instruments on
board Hinode.

---------------------------------------------------------
Title: Global properties of Alfven speeds in the corona
Authors: Regnier, Stephane; Priest, Eric; Hood, Alan
2008cosp...37.2585R Altcode: 2008cosp.meet.2585R
We investigate the values and distribution of the Alfvén speed in the
solar corona. We assume e that the coronal magnetic field is force-free
and the plasma is isothermal and in hydrostatic equilibrium. Firstly
we consider a bipolar configuration in order to study the effect of
parameters such as the pressure scale-height and the density at the
base of the corona. Secondly, we apply the model to four active regions
at different stages of their evolution (before and after a flare or a
CME). At a given height in the low corona, the Alfvén speed values
can vary by two e orders of magnitude (up to 100000 km·s-1 ). For
an active region with highly twisted flux tubes, the Alfvén speed
is significantly increased at the typical height of the twisted flux
bundles; in e flaring regions, the average Alfvén speeds are above
5000 km·s-1 and depart strongly from e potential field values. We
discuss implications for coronal heating models and CME models in
terms of the plasma β, the inflow speed and the reconnection rate.

---------------------------------------------------------
Title: Free Magnetic Energy in Solar Active Regions above the
Minimum-Energy Relaxed State
Authors: Régnier, S.; Priest, E. R.
2007ApJ...669L..53R Altcode: 2008arXiv0805.1619R
To understand the physics of solar flares, including the local
reorganization of the magnetic field and the acceleration of energetic
particles, we have first to estimate the free magnetic energy available
for such phenomena, which can be converted into kinetic and thermal
energy. The free magnetic energy is the excess energy of a magnetic
configuration compared to the minimum-energy state, which is a linear
force-free field if the magnetic helicity of the configuration is
conserved. We investigate the values of the free magnetic energy
estimated from either the excess energy in extrapolated fields or
the magnetic virial theorem. For four different active regions,
we have reconstructed the nonlinear force-free field and the linear
force-free field corresponding to the minimum-energy state. The free
magnetic energies are then computed. From the energy budget and the
observed magnetic activity in the active region, we conclude that the
free energy above the minimum-energy state gives a better estimate
and more insights into the flare process than the free energy above
the potential field state.

---------------------------------------------------------
Title: Topological Aspects of Global Magnetic Field Reversal in the
Solar Corona
Authors: Maclean, R. C.; Priest, E. R.
2007SoPh..243..171M Altcode:
Every eleven years on average, the dipolar component of the Sun's global
coronal magnetic field reverses in sign - a consequence of the sunspot
cycle. In this paper we begin to investigate the complex changes in
coronal structure during the reversal. We present a simplified model
of the solar cycle containing six time-varying photospheric sources
of magnetic field and analyse the evolution of the global coronal
field using the technique of magnetic charge topology. Surprisingly,
a sequence of seventeen topological changes takes place in the model
between one solar minimum state and the next; many of the resultant
topological configurations correspond to observable magnetic field
structures in the real corona. We also show how descriptions of all the
six-source topological states from the model can be built up in terms
of combinations of simpler four-source states, providing a framework
for future descriptions of even more complicated topological states.

---------------------------------------------------------
Title: Nonlinear force-free models for the solar corona. I. Two
active regions with very different structure
Authors: Régnier, S.; Priest, E. R.
2007A&A...468..701R Altcode: 2007astro.ph..3756R
Context: With the development of new instrumentation providing
measurements of solar photospheric vector magnetic fields, we need
to develop our understanding of the effects of current density on
coronal magnetic field configurations. Aims: The object is to
understand the diverse and complex nature of coronal magnetic fields
in active regions using a nonlinear force-free model. Methods:
From the observed photospheric magnetic field we derive the photospheric
current density for two active regions: one is a decaying active region
with strong currents (AR8151), and the other is a newly emerged active
region with weak currents (AR8210). We compare the three-dimensional
structure of the magnetic fields for both active region when they are
assumed to be either potential or nonlinear force-free. The latter is
computed using a Grad-Rubin vector-potential-like numerical scheme. A
quantitative comparison is performed in terms of the geometry, the
connectivity of field lines, the magnetic energy and the magnetic
helicity content. Results: For the old decaying active region
the connectivity and geometry of the nonlinear force-free model include
strong twist and strong shear and are very different from the potential
model. The twisted flux bundles store magnetic energy and magnetic
helicity high in the corona (about 50 Mm). The newly emerged active
region has a complex topology and the departure from a potential field
is small, but the excess magnetic energy is stored in the low corona
and is enough to trigger powerful flares.

---------------------------------------------------------
Title: News and Views: Does the Sun affect the Earth's climate?
Authors: Priest, Eric; Lockwood, Mike; Solanki, Sami; Wolfendale,
Arnold; Coustenis, A.
2007A&G....48c...7P Altcode: 2007A&G....48c...7C
Svensmark's article in the February issue (A&amp;G 2007 48 1.18)
presented a possible mechanism for the way the Sun could influence
the Earth's climate. He suggested that water droplets condense in the
ionization trail left by cosmic rays, whose flux varies inversely with
solar activity: when the magnetic field of the solar wind is stronger,
it shields the Earth from galactic cosmic rays and so decreases their
flux on the Earth; according to Svensmark's ideas, this produces fewer
clouds and thereby heats the Earth.

---------------------------------------------------------
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: Magnetic Reconnection
Authors: Priest, Eric; Forbes, Terry
2007mare.book.....P Altcode:
Preface; 1. Introduction; 2. Current-sheet formation; 3. Magnetic
annihilation; 4. Steady reconnection: the classical solutions;
5. Steady reconnection: new generation of fast regimes; 6. Unsteady
reconnection: the tearing mode; 7. Unsteady reconnection: other
approaches; 8. Reconnection in three dimensions; 9. Laboratory
applications; 10. Magnetospheric applications; 11. Solar applications;
12. Astrophysical applications; 13. Particle acceleration; References;
Appendices; Index.

---------------------------------------------------------
Title: Reconnection of Magnetic Fields
Authors: Birn, J.; Priest, E. R.
2007rmf..book.....B Altcode:
Preface; Part I. Introduction: 1.1 The Sun E. R. Priest; 1.2 Earth's
magnetosphere J. Birn; Part II. Basic Theory of MHD Reconnection:
2.1 Classical theory of two-dimensional reconnection T. G. Forbes;
2.2 Fundamental concepts G. Hornig; 2.3 Three-dimensional
reconnection in the absence of magnetic null points G. Hornig; 2.4
Three-dimensional reconnection at magnetic null points D. Pontin; 2.5
Three-dimensional flux tube reconnection M. Linton; Part III. Basic
Theory of Collisionless Reconnection: 3.1 Fundamentals of collisionless
reconnection J. Drake; 3.2 Diffusion region physics M. Hesse; 3.3 Onset
of magnetic reconnection P. Pritchett; 3.4 Hall-MHD reconnection
A. Bhattacharjee and J. Dorelli; 3.5 Role of current-aligned
instabilities J. Büchner and W. Daughton; 3.6 Nonthermal particle
acceleration M. Hoshino; Part IV. Reconnection in the Magnetosphere:
4.1 Reconnection at the magnetopause: concepts and models J. G. Dorelli
and A. Bhattacharjee; 4.2 Observations of magnetopause reconnection
K.-H. Trattner; 4.3 On the stability of the magnetotail K. Schindler;
4.4 Simulations of reconnection in the magnetotail J. Birn; 4.5
Observations of tail reconnection W. Baumjohann and R. Nakamura; 4.6
Remote sensing of reconnection M. Freeman; Part V. Reconnection in
the Sun's Atmosphere: 5.1 Coronal heating E. R. Priest; 5.2 Separator
reconnection D. Longcope; 5.3 Pinching of coronal fields V. Titov;
5.4 Numerical experiments on coronal heating K. Galsgaard; 5.5
Solar flares K. Kusano; 5.6 Particle acceleration in flares: theory
T. Neukirch; 5.7 Fast particles in flares: observations L. Fletcher;
6. Open problems J. Birn and E. R. Priest; Bibliography; Index.

---------------------------------------------------------
Title: Reconnection of magnetic fields : magnetohydrodynamics and
collisionless theory and observations
Authors: Birn, J.; Priest, E. R.
2007rmfm.book.....B Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Solar Atmosphere
Authors: Priest, E. R.
2007hste.book...56P Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Transition-Region Explosive Events: Reconnection Modulated
by p-Mode Waves
Authors: Chen, P. F.; Priest, E. R.
2006SoPh..238..313C Altcode: 2006SoPh..tmp...61C
Transition-region explosive events (TREEs) have long been proposed
as a consequence of magnetic reconnection. However, several critical
issues have not been well addressed, such as the location of the
reconnection site, their unusually short lifetime (about one minute),
and the recently discovered repetitive behaviour with a period of three
to five minutes. In this paper, we perform MHD numerical simulations
of magnetic reconnection, where the effect of five-minute solar p-mode
oscillations is examined. UV emission lines are synthesised on the
basis of numerical results in order to compare with observations
directly. It is found that several typical and puzzling features of
the TREEs with impulsive bursty behaviour can only be explained if
there exist p-mode oscillations and the reconnection site is located
in the upper chromosphere at a height range of around 1900 km &lt; h
&lt; 2150 km above the solar surface. Furthermore, the lack of proper
motions of the high-velocity ejection may be due to a rapid change of
temperature along the reconnection ejecta.

---------------------------------------------------------
Title: Coronal Magnetic Topologies in a Spherical Geometry II. Four
Balanced Flux Sources
Authors: Maclean, R. C.; Beveridge, C.; Priest, E. R.
2006SoPh..238...13M Altcode: 2006SoPh..tmp...32M
The Sun's magnetic field is the primary factor determining the
structure and evolution of the solar corona. Here, magnetic topology
is used in combination with a Green's function method to model the
global coronal magnetic field with a spherical photosphere. We focus
on the case of three negative flux sources and one positive source,
completing our previous categorisation of the topological states
and bifurcations that are present in quadrupolar configurations in a
spherical geometry. Three fundamental varieties of topological state
are found, with three types of bifurcation taking one to the other. A
comparison to the equivalent results for a planar photosphere is then
carried out, and the differences between the two cases are explained.

---------------------------------------------------------
Title: Our Enigmatic Sun
Authors: Priest, E. R.
2006AIPC..848....3P Altcode:
Many major puzzles in Solar Physics have not yet been answered and
are of wide importance for astrophysics in general, such as: How is
the magnetic field generated? How is the corona heated? How do solar
flares occur? What is the effect of solar variations on the Earth's
climate? How is the solar wind accelerated? In each case, the questions
have multiple parts, some of which have been recently answered with
the help of spectacular space observations, while others are being
refined in ways that will be described.

---------------------------------------------------------
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: Coronal Magnetic Topologies in a Spherical Geometry I. Two
Bipolar Flux Sources
Authors: Maclean, R. C.; Hornig, G.; Priest, E. R.; Beveridge, C.
2006SoPh..235..259M Altcode:
The evolution of the solar corona is dominated to a large extent
by the hugely complicated magnetic field which threads it. Magnetic
topology provides a tool to decipher the structure of this field and
thus help to understand its behaviour. Usually, the magnetic topology
of a potential field is calculated due to flux sources on a locally
planar photospheric surface. We use a Green's function method to extend
this theory to sources on a global spherical surface. The case of two
bipolar flux-balanced source regions is studied in detail, with an
emphasis on how the distribution and relative strengths of the source
regions affect the resulting topological states. A new state with two
spatially distinct separators connecting the same two magnetic null
points, called the "dual intersecting" state, is discovered.

---------------------------------------------------------
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: Book Review: THE SOLAR-B MISSION AND THE FOREFRONT OF SOLAR
PHYSICS / Astronomical Society of the Pacific, 2005
Authors: Priest, Eric
2006Obs...126..215P Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Our Magnetic Sun
Authors: Priest, E. R.
2006msu..conf..197P Altcode:
No abstract at ADS

---------------------------------------------------------
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: Chromospheric and Coronal Magnetic Fields
Authors: Priest, E.
2005ESASP.596E..47P Altcode: 2005ccmf.confE..47P
No abstract at ADS

---------------------------------------------------------
Title: Low-order stellar dynamo models
Authors: Wilmot-Smith, A. L.; Martens, P. C. H.; Nandy, D.; Priest,
E. R.; Tobias, S. M.
2005MNRAS.363.1167W Altcode: 2005MNRAS.tmp..855W
Stellar magnetic activity - which has been observed in a diverse set
of stars including the Sun - originates via a magnetohydrodynamic
dynamo mechanism working in stellar interiors. The full set of
magnetohydrodynamic equations governing stellar dynamos is highly
complex, and so direct numerical simulation is currently out of
reach computationally. An understanding of the bifurcation structure,
likely to be found in the partial differential equations governing such
dynamos, is vital if we are to understand the activity of solar-like
stars and its evolution with varying stellar parameters such as rotation
rate. Low-order models are an important aid to this understanding,
and can be derived either as approximations of the governing equations
themselves or by using bifurcation theory to obtain systems with the
desired structure. We use normal-form theory to derive a third-order
model with robust behaviour. The model is able to reproduce many of the
basic types of behaviour found in observations of solar-type stars. In
the appropriate parameter regime, a chaotic modulation of the basic
cycle is present, together with varying periods of low activity such
as that observed during the solar Maunder minima.

---------------------------------------------------------
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: A topological analysis of the magnetic breakout model for an
eruptive solar flare
Authors: Maclean, Rhona; Beveridge, Colin; Longcope, Dana; Brown,
D. S.; Priest, E. R.
2005RSPSA.461.2099M Altcode:
The magnetic breakout model gives an elegant explanation for the onset
of an eruptive solar flare, involving magnetic reconnection at a coronal
null point which leads to the initially enclosed flux ‘breaking out’
to large distances. In this paper we take a topological approach to
the study of the conditions required for this breakout phenomenon to
occur. The evolution of a simple delta sunspot model, up to the point of
breakout, is analysed through several sequences of potential and linear
force-free quasi-static equilibria. We show that any new class of field
lines, such as those connecting to large distances, must be created
through a global topological bifurcation and derive rules to predict
the topological reconfiguration due to various types of bifurcation.

---------------------------------------------------------
Title: Coronal Heating at Separators and Separatrices
Authors: Priest, E. R.; Longcope, D. W.; Heyvaerts, J.
2005ApJ...624.1057P Altcode:
Several ways have been proposed for heating the solar corona by magnetic
reconnection in current sheets, depending on the nature of both the
coronal magnetic field and the photospheric driving. Two ways that
have recently been considered involve the formation of such current
sheets either along separatrices (surfaces that separate topologically
distinct regions) or along separators (intersections of separatrices
linking one null point to another). The effect of slow photospheric
motions on complex coronal magnetic configurations will in general
be to generate three forms of electric current, namely, nonsingular
distributed currents, singular currents on separatrices and singular
currents on separators. These currents are not mutually exclusive
but will in general coexist in the same configuration. The aim of
this paper is to compare energy storage and heating that occurs at
separatrices and separators. We use reduced MHD to model coronal loops
that are much longer than they are wide, and we construct a series of
examples for the formation of current sheets along separatrices and
separators. We deduce that coronal heating is of comparable importance
at separatrices and separators. Separatrices are twice as effective
for observed small footpoint motions, while separators are twice as
effective in the initial build-up of a new flux domain.

---------------------------------------------------------
Title: Forced magnetic reconnection
Authors: Birn, J.; Galsgaard, K.; Hesse, M.; Hoshino, M.; Huba, J.;
Lapenta, G.; Pritchett, P. L.; Schindler, K.; Yin, L.; Büchner, J.;
Neukirch, T.; Priest, E. R.
2005GeoRL..32.6105B Altcode: 2005GeoRL..3206105B
Using a multi-code approach, we investigate current sheet thinning and
the onset and progress of fast magnetic reconnection, initiated by
temporally limited, spatially varying, inflow of magnetic flux. The
present study extends an earlier collaborative effort into the
transition regime from thick to thin current sheets. Again we find
that full particle, hybrid, and Hall-MHD simulations lead to the same
fast reconnection rates, apparently independent of the dissipation
mechanism. The reconnection rate in MHD simulations is considerably
larger than in the earlier study, although still somewhat smaller than
in the particle simulations. All simulations lead to surprisingly
similar final states, despite differences in energy transfer and
dissipation. These states are contrasted with equilibrium models derived
for the same boundary perturbations. The similarity of the final states
indicates that entropy conservation is satisfied similarly in fluid
and kinetic approaches and that Joule dissipation plays only a minor
role in the energy transfer.

---------------------------------------------------------
Title: Numerical Simulations of the Flux Tube Tectonics Model for
Coronal Heating
Authors: Mellor, C.; Gerrard, C. L.; Galsgaard, K.; Hood, A. W.;
Priest, E. R.
2005SoPh..227...39M Altcode:
In this paper we present results from 3D MHD numerical simulations
based on the flux tube tectonics method of coronal heating proposed by
Priest, Heyvaerts, and Title (2002). They suggested that individual
coronal loops connect to the photosphere in many different magnetic
flux fragments and that separatrix surfaces exist between the
fingers connecting a loop to the photosphere and between individual
loops. Simple lateral motions of the flux fragments could then cause
currents to concentrate along the separatrices which may then drive
reconnection contributing to coronal heating. Here we have taken a
simple configuration with four flux patches on the top and bottom
of the numerical domain and a small background axial field. Then we
move two of the flux patches on the base between the other two using
periodic boundary conditions such that when they leave the box they
re-enter it at the other end. This simple motion soon causes current
sheets to build up along the quasi-separatrix layers and subsequently
magnetic diffusion/reconnection occurs.

---------------------------------------------------------
Title: Magnetic diffusion and the motion of field lines
Authors: Wilmot-Smith, A. L.; Priest, E. R.; Hornig, G.
2005GApFD..99..177W Altcode:
Diffusion of a magnetic field through a plasma is discussed in
one-, two- and three-dimensional configurations, together with the
possibility of describing such diffusion in terms of a magnetic flux
velocity, which, when it exists, is in general non-unique. Physically
useful definitions of such a velocity include doing so in terms
of the energy flow or in such a way that it vanishes in a steady
state. Straight field lines (or plane flux surfaces) diffuse as if
flux is disappearing at a neutral sheet, whereas circular field lines
(or cylindrical flux surfaces) do so as if flux is disappearing at an
0-type neutral line. In three dimensions it is not always possible to
define a flux velocity, for example when the magnetic flux through a
closed field line is changing in time. However, in at least some such
cases it is possible to describe the behaviour of the magnetic field
in terms of a pair of quasi-flux-velocities.

---------------------------------------------------------
Title: Kinematic reconnection at a magnetic null point: fan-aligned
current
Authors: Pontin, D. I.; Hornig, G.; Priest, E. R.
2005GApFD..99...77P Altcode:
Magnetic reconnection at a three-dimensional null point is a natural
extension of the familiar two-dimensional X-point reconnection. A
model is set up here for reconnection at a null point with current
directed parallel to the fan plane, by solving the kinematic, steady,
resistive magnetohydrodynamic equations in its vicinity. The magnetic
field is assumed to be steady, and a localised diffusion region
surrounding the null point is also assumed, outside which the plasma is
ideal. Particular attention is focussed on the way that the magnetic
flux changes its connections as a result of the reconnection. The
resultant plasma flow is found to cross the spine and fan of the
null, and thus transfer magnetic flux between topologically distinct
regions. Solutions are also found in which the flow crosses either
the spine or fan only.

---------------------------------------------------------
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: Effects of Complexity on the Flux-Tube Tectonics Model
Authors: Close, R. M.; Heyvaerts, J. F.; Priest, E. R.
2004SoPh..225..267C Altcode:
The quiet-Sun magnetic field emerges through the solar photosphere
in a multitude of mixed-polarity magnetic concentrations and is
subsequently tangled up into intricate regions of interconnecting
flux. Moreover, since these discrete concentrations are likely to be
extremely small in size, with fluxes of around only 1017
Mx, the number of such flux sources in, say, a supergranule, will be
extremely large. The flux-tube tectonics model of Priest, Heyvaerts,
and Title (2002) demonstrated how the formation and dissipation of
current sheets along the separatrices that separate the regions of
different connectivity are likely to make an important contribution
to coronal heating. Since the full complexity of the magnetic field
is below present observable scales, this study examines the effect
of having the magnetic flux emerge through configurations structured
on smaller and smaller scales. It is found that, by fixing the amount
of flux emerging into a given 2D region, the main factors influencing
the current build-up along the separatrices are the number of sources
through which the flux emerges and the spatial distribution of the
sources on the photosphere. The free energy (i.e., that above potential)
is stored lower and lower in the atmosphere as the complexity of the
system increases. A simple comparison is then made between coronal
heating by separator currents and by separatrix currents. It is
found that both result in comparable amounts of energy release, with
separatrix heating being the more dominant.

---------------------------------------------------------
Title: A Topological Analysis of the Magnetic Breakout Model for an
Eruptive Solar Flare
Authors: Maclean, R.; Beveridge, C.; Longcope, D.; Brown, D.;
Priest, E.
2004ESASP.575..485M Altcode: 2004soho...15..485M
No abstract at ADS

---------------------------------------------------------
Title: Kinematic Magnetic Reconnection at 3d Null Points
Authors: Pontin, D. I.; Hornig, G.; Priest, E. R.
2004ESASP.575..507P Altcode: 2004soho...15..507P
No abstract at ADS

---------------------------------------------------------
Title: Simple Numerical Simulations of the Flux Tube Tectonics Model
for Coronal Heating
Authors: Mellor, C.; Gerrard, C. L.; Galsgaard, K.; Hood, A. W.;
Priest, E. R.
2004ESASP.575...29M Altcode: 2004soho...15...29M
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: Magnetic Reconnection
Authors: Priest, E. R.; Pontin, D. I.
2004ASSL..317..397P Altcode: 2004shis.conf..397P
No abstract at ADS

---------------------------------------------------------
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: The Dynamics of Reconnection in a Three Dimensional Current
Sheet
Authors: Linton, M. G.; Priest, E. R.; Longcope, D. W.
2004AAS...204.9509L Altcode: 2004BAAS...36R.827L
Many models for solar flares and coronal heating rely on magnetic
reconnection in three dimensional current sheets. Yet the topology and
evolution of reconnection in such current sheets is not well known. We
will present a numerical MHD study of such reconnection. We will show
how the tearing mode evolves in a finite sized, 3D current sheet, and
how this affects the dynamics of the magnetic field reconnecting in the
sheet. We will show how the flux tubes formed in isolated reconnection
regions slingshot away from the the reconnection site, how they interact
with the unreconnected field surrounding them, and how this differs
from the 2D reconnection limit. Finally we will discuss the dynamics
of flux tubes reconnecting in a patchy reconnection scenario, where
many isolated reconnection regions occur simultaneously in a current
sheet. We will show how this causes reconnected flux tubes to become
topologically entangled with each other, and how this limits the level
of energy release which can be achieved in reconnection. This
work has been supported by NASA, ONR, and PPARC.

---------------------------------------------------------
Title: Kinematic reconnection at a magnetic null point: spine-aligned
current
Authors: Pontin, D. I.; Hornig, G.; Priest, E. R.
2004GApFD..98..407P Altcode:
Magnetic reconnection at a three-dimensional null point is the natural
extension of the familiar two-dimensional X-point reconnection. A model
is set up here for reconnection at a spiral null point, by solving
the kinematic, steady, resistive magnetohydrodynamic equations in its
vicinity. A steady magnetic field is assumed, as well as the existence
of a localised diffusion region surrounding the null point. Outside the
diffusion region the plasma and magnetic field move ideally. Particular
attention is focussed on the way that the magnetic flux changes its
connections as a result of the reconnection. The resultant plasma flows
are found to be rotational in nature, as is the change in connections
of the magnetic field lines.

---------------------------------------------------------
Title: Magnetic topologies in the solar corona due to four discrete
photospheric flux regions
Authors: Beveridge, C.; Priest, E. R.; Brown, D. S.
2004GApFD..98..429B Altcode:
Many dynamic phenomena in the solar corona are driven by the complex
and ever-changing magnetic field. It is helpful, in trying to model
these phenomena, to understand the structure of the magnetic field,
i.e. the magnetic topology. We study here the topological structure
of the coronal magnetic field arising from four discrete photospheric
flux patches, for which we find that seven distinct, topologically
stable states are possible; the changes between these are caused
by six types of bifurcation. Two bifurcation diagrams are produced,
showing how the changes occur as the relative positions and strengths
of the flux patches are varied. A method for extending the analysis
to higher numbers of sources is discussed.

---------------------------------------------------------
Title: Our Enigmatic Sun
Authors: Priest, Eric
2004IAUS..223..715P Altcode: 2005IAUS..223..715P
No abstract at ADS

---------------------------------------------------------
Title: Binary Reconnection and the Heating of the Solar Corona
Authors: Priest, E. R.; Longcope, D. W.; Titov, V. S.
2003ApJ...598..667P Altcode:
The relative motions of myriads of magnetic fragments in the solar
surface are likely to drive magnetic reconnection and therefore heating
among the magnetic field lines that spread from these fragments into
the solar corona. We suggest that the fundamental mechanism is one of
“binary reconnection” due to the motion of a given magnetic source
relative to its nearest neighbor. The heating is due to several effects:
(1) the three-dimensional reconnection of field lines that start out
joining the two sources and end up joining the largest source to other
more distant sources (or vice versa), so that the field line footpoints
are exchanged; (2) the viscous or resistive damping of the waves that
are emitted by the sources as their relative orientation rotates; and
(3) the relaxation of the nonlinear force-free fields that join the
two sources and that are built up by the relative motion of the sources.

---------------------------------------------------------
Title: Three-dimensional Reconnection of Untwisted Magnetic Flux Tubes
Authors: Linton, M. G.; Priest, E. R.
2003ApJ...595.1259L Altcode:
Understanding the reconnection of magnetic fields in flux tubes is
of key importance for modeling solar activity and space weather. We
are therefore studying this process via three-dimensional MHD
simulations. We report here on a simulation of the collision of
a pair of perpendicular, untwisted magnetic flux tubes. We find
that the collision proceeds in four stages. First, on contact, the
tubes flatten out into wide sheets. Second, they begin to reconnect
and the tearing mode instability is excited in the reconnection
region. Third, the nonlinear evolution of the tearing mode creates a
pair of reconnected flux tubes. Finally, these flux tubes reconnect
with each other to coalesce into a single flux tube. We then report
on a pair of simulations exploring how this behavior changes when
the speed of flux tube collision is increased and when the magnetic
resistivity is increased.

---------------------------------------------------------
Title: A model for elemental coronal flux loops
Authors: Beveridge, C.; Longcope, D. W.; Priest, E. R.
2003SoPh..216...27B Altcode:
The photosphere possesses many small, intense patches of magnetic
flux. Each of these patches (or sources) is connected magnetically
through the corona to several sources of opposite polarity. An
elemental flux loop consists of all of the flux joining one such
source to another. We find that each source is connected to twenty
other sources, on average, and that the typical flux and diameter
of elemental loops in the corona are 1016 Mx and 200 km;
there are approximately 17 separators for each source. We also model a
typical large-scale coronal loop consisting of many elemental loops and
determine its complex internal topology. Each upright null lies at the
end of about 22 separatrices, which tend to be clustered together in
trunk-like structures, analogous to river-valleys in a geographical
contour map. Prone nulls correspond to saddle points, while their
spines are analogous to watersheds.

---------------------------------------------------------
Title: Theory of 3D reconnection and coronal heating heating
Authors: Priest, E. R.
2003AdSpR..32.1021P Altcode:
Magnetic reconnection is a prime candidate for heating coronae. We here
summarise recent developments in the theory of 3D reconnection and the
different ways in which it is thought to heat the solar corona. At 3D
null points reconnection may occur by either spine, fan or separator
reconnection. In absence of null points, reconnection of two flux tubes
is completely different from 2D reconnection, since in general in 3D
a flux-conserving velocity w does not exist. Instead two flux tubes
split into four parts, which 'flip' via 'virtual flux tubes' to form
four flux tubes. Heating in the corona has been proposed to occur by:
driven reconnection at X-ray bright points; binary reconnection due to
the coronal interaction of a pair of opposite-polarity magnetic sources;
separator reconnection due to a higher-order interaction; braiding; and
coronal tectonics due to the formation and dissipation of current sheets
at myriads of separatix surfaces that thread the corona and separate
the flux coming from the many different sources in the photosphere.

---------------------------------------------------------
Title: On the distribution of magnetic null points above the solar
photosphere
Authors: Longcope, D. W.; Brown, D. S.; Priest, E. R.
2003PhPl...10.3321L Altcode:
Many theories predict magnetic energy dissipation at locations,
called null points, where the magnetic field vanishes. In several
astrophysical contexts, most notably the solar corona, energy is
released within a low-β magnetic field anchored to a lower boundary,
the photosphere. A general expression is derived for the distribution of
magnetic null points in potential magnetic fields anchored to a random,
homogeneous distribution of field on the lower boundary. For all such
fields the null point density decreases with height and scales with the
inverse cube of the field's characteristic length. For photospheric
fields which appear unipolar at the largest scales the nulls are
confined to a narrow layer. The results are applied to models of the
quiet Sun whose photospheric field consists of discrete sources of
mixed polarity. The number of coronal nulls depends on the degree of
imbalance between positive and negative sources. Numerical experiments
reveal that the greatest column density of null points occurs when
~20% of the sources are of the minority sign. Were the coronal energy
dissipation to occur at magnetic null points this result predicts an
observable relationship between flux imbalance and the amplitude and
distribution of dissipation.

---------------------------------------------------------
Title: On the nature of three-dimensional magnetic reconnection
Authors: Priest, E. R.; Hornig, G.; Pontin, D. I.
2003JGRA..108.1285P Altcode:
Three-dimensional magnetohydrodynamic reconnection in a finite diffusion
region is completely different in many respects from two-dimensional
reconnection at an X-point. In two dimensions a magnetic flux velocity
can always be defined: two flux tubes can break at a single point and
rejoin to form two new flux tubes. In three dimensions we demonstrate
that a flux tube velocity does not generally exist. The magnetic field
lines continually change their connections throughout the diffusion
region rather than just at one point. The effect of reconnection on
two flux tubes is generally to split them into four flux tubes rather
than to rejoin them perfectly. During the process of reconnection each
of the four parts flips rapidly in a virtual flow that differs from
the plasma velocity in the ideal region beyond the diffusion region.

---------------------------------------------------------
Title: Linear collapse of spatially linear, three-dimensional,
potential null points
Authors: Mellor, C.; Titov, V. S.; Priest, E. R.
2003GApFD..97..489M Altcode:
The collapse of the magnetic field in the vicinity of a null point
creates a large electrical current. Non-ideal effects in the surrounding
plasma can then allow the onset of magnetic reconnection, which is
important in many astrophysical phenomena. An elegant technique is used
to show that spatially linear, initially potential, three-dimensional
null points have a tendency to collapse with a growth of either the
spine current or the fan current or a combination of both. The rate
of collapse is determined for an incompressible plasma flow with open
boundary conditions. An initial plasma flow affects the rate of collapse
by speeding it up.

---------------------------------------------------------
Title: Three Dimensional Reconnection of Untwisted Magnetic Flux Tubes
Authors: Linton, M.; Priest, E.
2003SPD....34.0512L Altcode: 2003BAAS...35R.816L
Understanding the reconnection of magnetic fields in coronal flux
tubes is of key importance for modeling solar coronal flares. We
are therefore studying this process via 3D MHD simulations of the
reconnection of magnetic flux tubes. Here we will report on a series
of such simulations in which pairs of untwisted flux tubes collide
and reconnect. We find that in general these collisions flatten
the flux tubes into flux sheets and then excite the tearing mode
instability. In addition to the tearing mode reconnection, at high
collision speed reconnection is simultaneously initiated at a number
of other locations, leading to patchy reconnection and a significant
tangling of the reconnected fields. In contrast, at low collision speed
we find the reconnection is much smoother, and that the coalescence
instability follows the tearing mode, causing the reconnected field to
merge into a single twisted flux tube. In addition to discussing the
global dynamics of these reconnection processes, we will also address
the behavior of individual fieldlines as they reconnect, in an effort
to more clearly understand how such fieldline reconnection occurs in
truly 3D configurations. This work was supported by NASA, ONR,
and PPARC grants.

---------------------------------------------------------
Title: Flare activity in solar active region 8421 observed by the
TRACE satellite
Authors: Zuccarello, F.; Contarino, L.; Romano, P.; Priest, E. R.
2003A&A...402.1085Z Altcode:
Due to the wide range of wavelengths examined and to the high angular
and temporal resolution, TRACE allows one to carry out a spatial and
temporal analysis of active regions during highly transient phenomena
such as flares. This provides new input to the study of the mechanisms
involved in these phenomena. We have studied 3 flares that occurred in
AR 8421 between 29 and 30 December 1998 by comparing white light, 1600
Å, and 171 Å images obtained by TRACE with BBSO Halpha 
images, Mitaka magnetograms and Yohkoh hard X-ray data. The flares,
characterized by sudden intensity enhancements in EUV loops and by
moss brightenings, have been interpreted in the framework of the two
canonical flare models: i.e. simple loop and two ribbon flares. Our
analysis has shown that flare No. 1 may be interpreted as a two-ribbon
flare triggered by reconnection between a sheared arcade and a new
emerging flux tube. The analysis of flare No. 2 strongly supports the
model of two-ribbon flares characterized by reconnection occurring
at higher and higher levels as time proceeds. Finally, the analysis
of flare No. 3 has given the opportunity to relate moss brightening
with a probable process of chromospheric evaporation.

---------------------------------------------------------
Title: A Framework for Understanding the Topology of Complex Coronal
Structures
Authors: Pontin, D. I.; Priest, E. R.; Longcope, D. W.
2003SoPh..212..319P Altcode:
The Sun's coronal magnetic field is highly complex and provides the
driving force for many dynamical processes. The topology of this
complex field is made up mainly of discrete topological building
blocks produced by small numbers of magnetic fragments. In this work
we develop a method for predicting the possible topologies due to a
potential field produced by three photospheric sources, and describe
how this model accurately predicts the results of Brown and Priest
(1999). We then sketch how this idea may be extended to more general
non-symmetric configurations. It is found that, for the case of positive
total flux, a local separator bifurcation may take place with three
positive sources or with one positive and two negative sources, but
not for two positive sources and one negative.

---------------------------------------------------------
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: Numerical experiments on wave propagation towards a 3D null
point due to rotational motions
Authors: Galsgaard, K.; Priest, E. R.; Titov, V. S.
2003JGRA..108.1042G Altcode:
We describe 3D resistive magnetohydrodynamic (MHD) numerical experiments
at a null point driven by the rotation of magnetic field lines near
the spine of the null. When field lines around the spine are rotated,
a twist wave propagates toward the null along field lines, satisfying
a Klein-Gordon equation. While the helical Alfvén wave spreads out as
the null is approached, a fast-mode wave focuses on the null and wraps
around it. Only a weak diffusion of the twisted field line structure
is found to take place.

---------------------------------------------------------
Title: Solar magnetohydrodynamics
Authors: Priest, E. R.
2003dysu.book..217P Altcode:
The magnetic field exerts a force, stores energy, acts as a thermal
blanket, channels plasma, drives instabilities, and supports waves. For
many purposes the behaviour of the magnetic field and its interaction
with plasma is governed by the equations of magnetohydrodynamics
(MHD). This chapter gives a brief account of some of the basics of MHD,
and summarises the simple properties of the different kinds of waves
that are present in ideal MHD.

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

---------------------------------------------------------
Title: Magnetic topologies due to two bipolar regions
Authors: Beveridge, C.; Priest, E. R.; Brown, D. S.
2002SoPh..209..333B Altcode:
The Sun's atmosphere contains many diverse phenomena that are dominated
by the coronal magnetic field. To understand these phenomena it is
helpful to determine first the structure of the magnetic field, i.e.,
the magnetic topology. We study here the topological structure of the
coronal magnetic field arising from the interaction of two bipolar
regions, for which we find that four distinct, topologically stable
states are possible. A bifurcation diagram is produced, showing how the
magnetic configuration can change from one topology to another as the
relative orientation and sizes of the bipolar regions are varied. The
changes are produced either by a global separator bifurcation,
a local double-separator bifurcation, a new, global separatrix
quasi-bifurcation, or a new, global spine quasi-bifurcation.

---------------------------------------------------------
Title: A Flux-Tube Tectonics Model for Solar Coronal Heating Driven
by the Magnetic Carpet
Authors: Priest, Eric R.; Heyvaerts, Jean F.; Title, Alan M.
2002ApJ...576..533P Altcode:
We explore some of the consequences of the magnetic carpet for coronal
heating. Observations show that most of the magnetic flux in the
quiet Sun emerges as ephemeral regions and then quickly migrates
to supergranule boundaries. The original ephemeral concentrations
fragment, merge, and cancel over a time period of 10-40 hr. Since
the network photospheric flux is likely to be concentrated in units
of 1017 Mx or smaller, there will be myriads of coronal
separatrix surfaces caused by the highly fragmented photospheric
magnetic configuration in the quiet network. We suggest that the
formation and dissipation of current sheets along these separatrices
are an important contribution to coronal heating. The dissipation of
energy along sharp boundaries we call, by analogy with geophysical
plate tectonics, the tectonics model of coronal heating. Similar to
the case on Earth, the relative motions of the photospheric sources
will drive the formation and dissipation of current sheets along a
hierarchy of such separatrix surfaces at internal dislocations in the
corona. In our preliminary assessment of such dissipation we find
that the heating is fairly uniform along the separatrices, so that
each elementary coronal flux tube is heated uniformly. However, 95%
of the photospheric flux closes low down in the magnetic carpet and
the remaining 5% forms large-scale connections, so the magnetic carpet
will be heated more effectively than the large-scale corona. This
suggests that unresolved observations of coronal loops should exhibit
enhanced heating near their feet in the carpet, while the upper parts of
large-scale loops should be heated rather uniformly but less strongly.

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

---------------------------------------------------------
Title: Our Enigmatic Sun
Authors: Priest, E.
2002AAS...200.6701P Altcode: 2002BAAS...34..755P
The Sun remains of central importance for astronomy as a whole,
especially since it provides the closest cosmic view of many
fundamental plasma processes at work. In this review we summarise the
basic properties of the Sun and describe the major progress that has
recently been made on a set of key questions that have far-reaching
implications for many other parts of the Universe, namely: 1. How is
the magnetic field generated ? 2. How are winds accelerated ? 3. How
do eruptions occur ? 4. How is magnetic energy converted to other
forms ? 5. How are coronae heated ?

---------------------------------------------------------
Title: Exact Solutions for Spine Reconnective Magnetic Annihilation
Authors: Mellor, C.; Priest, E. R.; Titov, V. S.
2002GApFD..96..153M Altcode:
Solutions for spine reconnective annihilation are presented which
satisfy exactly the three-dimensional equations of steady-state
resistive incompressible magnetohydrodynamics (MHD). The magnetic flux
function (A) and stream function have the form $A = A_{0}(R) sin phi +
A_{1}(R)z, qquad Psi = Psi _{0}(R) sin phi + Psi _{1}(R)z,$ in terms
of cylindrical polar coordinates (R,,z). First of all, two non-linear
fourth-order equations for A1 and are solved by the method of matched
asymptotic expansions when the magnetic Reynolds number is much larger
than unity. The solution, for which a composite asymptotic expansion
is given in closed form, possesses a weak boundary layer near the spine
(R = 0). These solutions are used to solve the remaining two equations
for A0 and . Physically, the magnetic field is advected across the fan
separatrix surface and diffuses across the spine curve. Different
members of a family of solutions are determined by values of a
free parameter and the components (BRe, Bze) and (vRe, vze) of the
magnetic field and plasma velocity at a fixed external point (R,,z) =
(1,/2,0), say.

---------------------------------------------------------
Title: The Nature of Blinkers and the Solar Transition Region
Authors: Priest, E. R.; Hood, A. W.; Bewsher, D.
2002SoPh..205..249P Altcode:
Solar plasma that exists at around 105 K, which has
traditionally been referred to as the solar transition region, is
probably in a dynamic and fibril state with a small filling factor. Its
origin is as yet unknown, but we suggest that it may be produced
primarily by one of five different physical mechanisms, namely:
the heating of cool spicular material; the containment of plasma in
low-lying loops in the network; the thermal linking of cool and hot
plasma at the feet of coronal loops; the heating and evaporating of
chromospheric plasma in response to a coronal heating event; and the
cooling and draining of hot coronal plasma when coronal heating is
switched off. We suggest that, in each case, a blinker could be produced
by the granular compression of a network junction, causing subtelescopic
fibril flux tubes to spend more of their time at transition-region
temperatures and so to increase the filling factor temporarily.

---------------------------------------------------------
Title: Magnetic Reconnection and Coronal Heating
Authors: Priest, Eric
2002smra.progE..21P Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Theory of 3D Reconnection and Coronal Heating
Authors: Priest, E.
2002cosp...34E.450P Altcode: 2002cosp.meetE.450P
Magnetic reconnection is a prime candidate for heating coronae. We
here summarise recent developments in the theory of 3D reconnection
and the different ways in which it is thought to heat the solar
corona. Reconnection may occur at null points by either spine, fan or
separator reconnection and in the absence of null points by magnetic
flipping. Heating in the corona has been proposed to occur by: driven
reconnection at X-ray bright points; binary reconnection due to the
coronal interaction of a pair of opposite-polarity magnetic sources;
separator reconnection due to a higher-order interaction; braiding; and
coronal tectonics due to the formation and dissipation of current sheets
at myriads of separatrix surfaces that thread the corona and separate
the flux coming from the many different sources in the photosphere.

---------------------------------------------------------
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 nature of solar flares
Authors: Priest, E. R.; Forbes, T. G.
2002A&ARv..10..313P Altcode:
The main challenge for the theory of solar eruptions has been to
understand two basic aspects of large flares. These are the cause of
the flare itself and the nature of the morphological features which
form during its evolution. Such features include separating ribbons of
Hα emission joined by a rising arcade of soft x-ray loops, with hard
x-ray emission at their summits and at their feet. Two major advances
in our understanding of the theory of solar flares have recently
occurred. The first is the realisation that a magnetohydrodynamic (MHD)
catastrophe is probably responsible for the basic eruption and the
second is that the eruption is likely to drive a reconnection process
in the field lines stretched out by the eruption. The reconnection is
responsible for the ribbons and the set of rising soft x-ray loops,
and such a process is well supported by numerical experiments and
detailed observations from the Japanese satellite Yohkoh. Magnetic
energy conversion by reconnection in two dimensions is relatively well
understood, but in three dimensions we are only starting to understand
the complexity of the magnetic topology and the MHD dynamics which are
involved. How the dynamics lead to particle acceleration is even less
well understood. Particle acceleration in flares may in principle occur
in a variety of ways, such as stochastic acceleration by MHD turbulence,
acceleration by direct electric fields at the reconnection site, or
diffusive shock acceleration at the different kinds of MHD shock waves
that are produced during the flare. However, which of these processes
is most important for producing the energetic particles that strike
the solar surface remains a mystery.

---------------------------------------------------------
Title: Solar Orbiter science
Authors: Priest, E. R.
2001ESASP.493..127P Altcode: 2001sefs.work..127P
A summary is given of the scientific advances that are expected from
the Solar Orbiter. There will be unexpected discoveries from exploring
new regions of the Sun for the first time - namely, the polar regions,
the inner solar wind and increasing the current special resolution by
an order of magnitude. However, it is also likely to make substantial
advances or to provide definitive answers to several basic physics
questions that are of substantial importance for our understanding of
the wider universe. These include: the effect of the Sun on the Earth's
climate; the structure of the solar interior and the nature of the solar
dynamos; the nature and behaviour of the fundamental magnetic structures
of the solar surface; the nature of the transition region; the drivers
for the solar wind; and the mechanisms for heating the corona.

---------------------------------------------------------
Title: Application of an analytical MHD wind model with latitudinal
dependences to the solar wind
Authors: Lima, J. J. G.; Sauty, C.; Iro, N.; Tsinganos, K.; Priest,
E. R.
2001ESASP.493..269L Altcode: 2001sefs.work..269L
No abstract at ADS

---------------------------------------------------------
Title: Solar and stellar magnetic activity
Authors: Schrijver, Carollus J.; Zwaan, Cornelis; Priest, Eric R.
2001PhT....54i..54S Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Meeting report : News from PPARC's Astronomy Committee
Authors: Priest, Eric
2001A&G....42c..23P Altcode:
Eric Priest, Chair of the Astronomy Committee, reports on the March
meeting, which considered targets for future science and spending,
particularly in view of the decision to join the European Southern
Observatory.

---------------------------------------------------------
Title: Three-dimensional reconnection on the Sun
Authors: Priest, Eric R.
2001EP&S...53..483P Altcode:
A brief review is given of the theory of magnetic reconnection in three
dimensions. The key elements of a three-dimensional null point are its
spine and its fan, which consist, respectively, of a field line and
a surface of field lines that pass through the null. The fans of two
nulls intersect in general in a field line called a separator that joins
the nulls. Several different types of reconnection have been proposed,
namely: spine reconnection, fan reconnection, separator reconnection and
quasi-separatrix layer reconnection. In addition, a new exact solution
for reconnective annihilation has been recently discovered. A summary
is also given of the impressive evidence of reconnection at work on
the Sun provided by a range of observations from the Yohkoh and SOHO
satellites, both of solar flares and of coronal heating events.

---------------------------------------------------------
Title: An analytical MHD wind model with latitudinal dependences
obtained using separation of the variables
Authors: Lima, J. J. G.; Priest, E. R.; Tsinganos, K.
2001A&A...371..240L Altcode: 2001astro.ph..3289L
A new class of analytical 2-D solutions of the full set of the steady
magnetohydrodynamic (MHD) equations, describing an axisymmetric
helicoidal magnetized outflow originating from a rotating central
object, is presented. The solutions are systematically obtained via
a nonlinear separation of the variables in the momentum equation. The
analysis yields three parameters which measure the anisotropy in the
latitudinal distribution of various flow quantities. Topologically,
the wind speed is controlled by an X-type critical point that acts to
filter out a single wind-type branch and the Alfvén singularity. The
solutions can be regarded as an extension outside the equatorial plane
of the Weber &amp; Davis (\cite{Weber67}) model of magnetized winds
but with a variable polytropic index.

---------------------------------------------------------
Title: The topological behaviour of 3D null points in the Sun's corona
Authors: Brown, D. S.; Priest, E. R.
2001A&A...367..339B Altcode:
Many traditional two-dimensional models of flares and other reconnective
events rely on an X-point in the corona which collapses to form
a current sheet where reconnection takes place. However, the Sun's
corona is three-dimensional and the X-point is only a two-dimensional
artifact. Three-dimensional models can instead make use of 3D null
points in the corona, and \cite{priest96} have studied how reconnection
may take place at such locations. This paper aims to study possible
conditions for coronal nulls to occur, and how they can be born. It
starts with three negative sources and a positive source that is weaker
than the sum of the negative sources. As the positive source moves
inwards towards the negative sources, there is a local double-separator
bifurcation of the topology to a new state, referred to as a Coronal
Null state, which contains a null point above the photosphere.

---------------------------------------------------------
Title: Surprises from Our Sun
Authors: Priest, Eric R.
2001RvMA...14..133P Altcode:
No abstract at ADS

---------------------------------------------------------
Title: The Role of the Sun's Magnetic Field
Authors: Priest, E. R.
2001ASPC..248...25P Altcode: 2001mfah.conf...25P
No abstract at ADS

---------------------------------------------------------
Title: A review of: "Transport and energy conversion in the
heliosphere"
Authors: Priest, Eric
2001GApFD..95..333P Altcode:
Lectures given at the CNRS Summer School on Solar Astrophysics,
Oleron, France, 25-29 May 1998, edited by P.-P. Rozelot, L. Klein
and J.-C. Vial, Lecture Notes in Physics: Vol. 553, Springer-Verlag
Berlin Heidelberg New York, 2000. VIII+214 pp., DM 104.00, hardbound
(ISBN 3-540-67595-7)

---------------------------------------------------------
Title: Magnetohydrodynamics
Authors: Priest, E.; Forbes, T.
2000eaa..bookE1983P Altcode:
Magnetohydrodynamics (or MHD for short) is the study of the interaction
between a magnetic field and a plasma treated as a continuous medium
(e.g. Cowling 1957, Roberts 1967, Priest 1982, 1994). Most of the
universe is not a normal gas but is instead a plasma. We are all
familiar on Earth with the three states of matter (solid, liquid and
gas). You change from one state to another (such as ice to...

---------------------------------------------------------
Title: Sun and Solar-terrestrial Physics
Authors: Priest, E.
2000eaa..bookE5397P Altcode:
The Sun is of central importance in astronomy. On the one hand, many
basic processes that occur throughout the cosmos may be well observed
in the Sun and, on the other hand, the Sun acts as a source of the
radiation and plasma emissions that affect the Earth in many ways. The
study of the interaction of the Sun and its solar wind with the Earth
is the burgeoning field of solar-terrestrial physics...

---------------------------------------------------------
Title: Simulations of Three-Dimensional Reconnection in the Solar
Corona
Authors: Birn, Joachim; Gosling, John T.; Hesse, Michael; Forbes,
Terry G.; Priest, Eric R.
2000ApJ...541.1078B Altcode:
Using ideal and resistive MHD, we investigate the stability
and dynamic evolution of three-dimensional magnetic field
configurations, representing stretched arcade structures above a dipolar
“photospheric” magnetic field. Two types of configurations are studied
that differ by the amount of divergence (“fanning”) of the initial
field lines as viewed in the horizontal direction perpendicular to the
photospheric magnetic neutral line and, correspondingly, by the radial
decrease of magnetic field strength and current density. The two sets
of configurations are found to differ in their stability behavior. The
strongly fanning fields, associated with a rapid radial decrease of
the field strength, current density, and plasma pressure, are more
stable. A stability difference is found also when the configurations
are first subjected to a converging motion of photospheric footpoints
toward the neutral line, which leads to the buildup of thin current
sheets in the region above. This current sheet formation is more
pronounced for the weakly fanning fields. For similar current density
enhancements, the occurrence of anomalous dissipation (resistivity)
initiates magnetic reconnection in either configuration. However,
the effects are much more drastic in magnitude and spread in the
weakly diverging field structure. In the unstable cases, a strongly
localized electric field parallel to the magnetic field develops,
which results in integrated voltages with maximum values of the
order of a few hundred MeV, both on open and closed field lines. For
comparison, we studied both low-beta, force-free, and high-beta initial
states. The weakly fanning high-beta configurations tend to show more
drastic instability effects than the corresponding low-beta fields,
but the stabilization of the strongly fanning fields pertains to both
low-beta and high-beta fields. The three-dimensional reconnection in
the unstable cases generates a region of intertwined magnetic flux
tubes with different topologies that lie below a region of closed flux
ropes not affected by reconnection. The topological changes could be
the source of open flux tubes that are occasionally observed within
coronal mass ejections, as recently discussed by Gosling et al. The
fast outward flow generated in these simulations affects only the
regions of changing topology but does not cause the above-lying closed
flux ropes to move (within the times considered). This may be seen
as an indication that reconnection may be associated with the onset
of a flare, initiated after the eruption of a coronal mass ejection,
but is not the driver of the coronal mass ejection itself.

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

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

---------------------------------------------------------
Title: Magnetic Reconnection
Authors: Priest, Eric; Forbes, Terry
2000mare.book.....P Altcode:
Magnetic reconnection is at the core of many dynamic phenomena in
the universe, such as solar flares, geomagnetic substorms and tokamak
disruptions. Written by two world leaders on the subject, this volume
provides a comprehensive overview of this fundamental process. Coverage
gives both a pedagogical account of the basic theory and a wide-ranging
review of the physical phenomena created by reconnection--from
laboratory machines, the Earth's magnetosphere, and the Sun's atmosphere
to flare stars and astrophysical accretion disks. It also includes a
succinct account of particle acceleration by electric fields, stochastic
fields and shock waves, and how reconnection can be important in these
mechanisms. Clearly written and highly accessible, this volume serves
as an essential introduction for graduate students in solar physics,
astrophysics, plasma physics and space science. Researchers in these
fields also will find Magnetic Reconnection an authoritative reference.

---------------------------------------------------------
Title: Topological differences and similarities between force-free
and potential models of coronal magnetic fields
Authors: Brown, D. S.; Priest, E. R.
2000SoPh..194..197B Altcode:
Potential fields and linear force-free fields are often used as
models for the magnetic field of the Sun's corona. They can be
written as analytical expressions in terms of boundary values
at the photosphere. Because of their relative simplicity compared
with nonlinear force-free fields, these two models are of particular
importance in topological analysis of solar phenomena. However, it has
been suggested by Hudson and Wheatland (1999) that the topologies of
potential and force-free models are in general not even qualitatively
equivalent. In this paper, their example is re-examined and it is
found that the opposite conclusions hold. In general, potential and
force-free fields are topologically similar sufficiently close to
localized sources. The exception to this are structurally unstable
states, such as bifurcation states, where a small change of current
can produce a significant change of topology.

---------------------------------------------------------
Title: Three-dimensional Separator Reconnection - How Does It Occur?
Authors: Galsgaard, K.; Priest, E. R.; Nordlund, Å.
2000SoPh..193....1G Altcode:
In two dimensions magnetic energy release takes place at locations
where the magnetic field strength becomes zero and has an x-point
topology. The x-point topology can collapse into two y-points connected
by a current sheet when the advection of magnetic flux into the x-point
is larger than the dissipation of magnetic flux at the x-point. In
three dimensions magnetic fields may also contain singularities in the
form of three-dimensional null points. Three-dimensional nulls are
created in pairs and are therefore, at least in the initial stages,
always connected by at least one field line - the separator. The
separator line is defined by the intersection of the fan planes of the
two nulls. In the plane perpendicular to a single separator the field
line topology locally has a two dimensional x-point structure. Using
a numerical approach we find that the collapse of the separator can be
initiated at the two nulls by a velocity shear across the fan plane. It
is found that for a current concentration to connect the two nulls
along the separator, the current sheet can only obtain two different
orientations relative to the field line structure of the nulls. The
sheet has to have an orientation midway between the fan plane and the
spine axis of each null. As part of this process the spine axes are
found to lose their identity by transforming into an integrated part
of the separator surfaces that divide space into four magnetically
independent regions around the current sheet.

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

---------------------------------------------------------
Title: Reconnection Theory and the MHD of Solar Flares
Authors: Priest, Eric
2000IAUJD...7E...5P Altcode:
A review is given of the recent developments in the theory of magnetic
reconnection, including new exact solutions and the different regimes of
three-dimensional reconnection that have been proposed. In addition,
two key issues about the magnetohydrodynamics of solar flares are
assessed, namely: why do eruptions of magnetic arcades occur that can
produce coronal mass ejections, and what is the nature of the energy
release process in a solar flare ?

---------------------------------------------------------
Title: Magnetic reconnection : MHD theory and applications
Authors: Priest, Eric; Forbes, Terry
2000mrmt.conf.....P Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Solar Flare Theory and the Status of Flare Understanding
Authors: Priest, E. R.
2000ASPC..206...13P Altcode: 2000hesp.conf...13P
No abstract at ADS

---------------------------------------------------------
Title: Aspects of Three-Dimensional Magnetic Reconnection -
(Invited Review)
Authors: Priest, E. R.; Schrijver, C. J.
1999SoPh..190....1P Altcode: 1999SoPh..190....1S
In this review paper we discuss several aspects of magnetic reconnection
theory, focusing on the field-line motions that are associated with
reconnection. A new exact solution of the nonlinear MHD equations for
reconnective annihilation is presented which represents a two-fold
generalization of the previous solutions. Magnetic reconnection at
null points by several mechanisms is summarized, including spine
reconnection, fan reconnection and separator reconnection, where it
is pointed out that two common features of separator reconnection are
the rapid flipping of magnetic field lines and the collapse of the
separator to a current sheet. In addition, a formula for the rate of
reconnection between two flux tubes is derived. The magnetic field of
the corona is highly complex, since the magnetic carpet consists of
a multitude of sources in the photosphere. Progress in understanding
this complexity may, however, be made by constructing the skeleton of
the field and developing a theory for the local and global bifurcations
between the different topologies. The eruption of flux from the Sun may
even sometimes be due to a change of topology caused by emerging flux
break-out. A CD-ROM attached to this paper presents the results of a
toy model of vacuum reconnection, which suggests that rapid flipping
of field lines in fan and separator reconnection is an essential
ingredient also in real non-vacuum conditions. In addition, it gives
an example of binary reconnection between a pair of unbalanced sources
as they move around, which may contribute significantly to coronal
heating. Finally, we present examples in TRACE movies of geometrical
changes of the coronal magnetic field that are a likely result of
large-scale magnetic reconnection.

---------------------------------------------------------
Title: The Topological Behaviour of Stable Magnetic Separators
Authors: Brown, D. S.; Priest, E. R.
1999SoPh..190...25B Altcode:
It is important to understand the complex topology of the magnetic field
in the solar corona in order to be able to comprehend the mechanisms
which give rise to phenomena such as coronal loop structures and
x-ray bright points. A key feature of the magnetic topology is a
separator. A magnetic separator is a field line which connects two
magnetic null points, places where the magnetic field becomes zero. A
stable magnetic separator is important as it is the intersection of
two separatrix surfaces. These surfaces divide the magnetic field
lines into regions of different connectivity, so a separator usually
borders four regions of field-line connectivity. This work examines
the topological behaviour of separators that appear in a magnetic
field produced by a system of magnetic sources lying in a plane (the
photosphere). The questions of how separators arise and are destroyed,
the topological conditions for which they exist, how they interact
and their relevance to the coronal magnetic field are addressed.

---------------------------------------------------------
Title: Topological bifurcations in three-dimensional magnetic fields.
Authors: Brown, D. S.; Priest, E. R.
1999RSPSA.455.3931B Altcode:
Most of the dynamical processes that take place in the Sun's corona
(its outer atmosphere) are dominated by the magnetic field. The sources
of the coronal field are magnetic fragments scattered over the solar
surface and mostly clustered around the edges of large convection cells
called supergranules. These sources are not static but continually move
about over the surface, coalescing, fragmenting and cancelling with
one another. The resulting coronal magnetic field has an incredibly
complex topology. In order to begin to understand this complexity it
is important to consider, as building blocks, the field generated by
a small number of discrete sources.

---------------------------------------------------------
Title: Wavelet Analysis Of Active Region Oscillations
Authors: Ireland, J.; Walsh, R. W.; Priest, E. R.; Harrison, R. A.
1999ESASP.446..363I Altcode: 1999soho....8..363I
The wavelet transform is applied to the analysis of active region
oscillations. The localised (in time) nature of the wavelet transform
allows us to study both the duration of any statistically significant
oscillations as well as their period. Time series arising from SOHO
(Solar and Heliospheric Observatory) CDS-NIS (Coronal Diagnostic
Spectrometer Normal Incidence Spectrometer) active region observations
on 14th-15th November 1996 are used to demonstrate the applicability of
wavelet methods. High cadence (approximately 14 seconds) observations
were made in He I 584.33 Angstroms (log T = 4.3, T being the electron
temperature), O V 629.73 Angstroms(log T = 5.3), Mg IX 368.06 Angstroms
(log T = 6.0), Fe XVI 360.76 Angstroms (log T = 6.4) provide detailed
intensity information on the active region over a wide range of
temperatures. The distribution of statistically significant periods
found varies from line to line, as does their duration.

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

---------------------------------------------------------
Title: A wavelet analysis of active region oscillations
Authors: Ireland, J.; Walsh, R. W.; Harrison, R. A.; Priest, E. R.
1999A&A...347..355I Altcode:
The wavelet transform is applied to the analysis of active region
oscillations. The localised (in time) nature of the wavelet transform
allows us to study both the duration of any statistically significant
oscillations as well as their period. Time series arising from SOHO
(Solar and Heliospheric Observatory) CDS-NIS (Coronal Diagnostic
Spectrometer Normal Incidence Spectrometer) active region observations
on 14th-15th November 1996 are used to demonstrate the applicability of
wavelet methods. High cadence (approximately 14 seconds) observations
were made in {He I} 584.33 Ä (log Te=4.3), {O V} 629.73 Ä
(log Te=5.3), {Mg IX} 368.06 Ä (log Te=6.0),
{Fe XVI} 360.76 Ä (log Te=6.4) provide detailed intensity
information on the active region over a wide range of temperatures. The
distribution of statistically significant periods found varies from
line to line, as does their duration.

---------------------------------------------------------
Title: Magnetic Null Points due to Multiple Sources of Solar
Photospheric Flux
Authors: Inverarity, G. W.; Priest, E. R.
1999SoPh..186...99I Altcode:
How common are magnetic null points in the highly complex magnetic
field of the solar atmosphere? In this work we seek to model the
magnetic structure of quiet regions by placing magnetic sources
and sinks on a hexagonal network of supergranule cells to represent
the intense magnetic fields that occur at the boundaries of these
cells. The resulting potential coronal magnetic field is then computed
analytically and searched numerically for magnetic null points, which
are classified according to their types and spine directions. Two
relations from the theory of vector fields relate the numbers of null
points to the numbers of sources and sinks and these are used to check
the numerical results. Previous results relating these quantities
for monopolar and dipolar magnetic fields are described and a new one
for a particular class of quadrupolar fields arising in this study is
derived. We model a three-cell configuration and study the effects of
increasing the strength of a central sink and of moving the central
sink. A twelve-cell configuration is studied in lesser detail.

---------------------------------------------------------
Title: A spectacular new corona
Authors: Walsh, Robert; Priest, Eric
1999A&G....40b..29W Altcode:
No abstract at ADS

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

---------------------------------------------------------
Title: Heating the Solar Corona by Magnetic Reconnection
Authors: Priest, E. R.
1999Ap&SS.264...77P Altcode: 1998Ap&SS.264...77P
Here I review briefly the theory of magnetohydrodynamic reconnection
and ask what observational evidence is there that it is heating the
corona. In particular, the new directions in which three-dimensional
theory for reconnection is heading are outlined. Part of the coronal
heating problem has been solved with the identification of reconnection
driven by converging flux motions as the key for x-ray bright
points. Furthermore, it has been shown that the large-scale diffuse
corona is heated rather uniformly, so that turbulent reconnection by
braiding or ion-cyclotron waves driven by network micro-flares are prime
candidates. Finally, reconnection is the natural explanation for a wide
variety of phenomena discovered by SOHO including explosive events,
blinkers, the magnetic carpet and even possibly tornadoes.

---------------------------------------------------------
Title: How is the Solar Corona Heated?
Authors: Priest, E. R.
1999ASPC..158..321P Altcode: 1999ssa..conf..321P
No abstract at ADS

---------------------------------------------------------
Title: Our New View of the Sun According to SOHO
Authors: Priest, E. R.
1998ESASP.431..133P Altcode: 1998sslt.conf..133P
No abstract at ADS

---------------------------------------------------------
Title: Magnetic Flux Transport and the Formation of Filament Channels
on the Sun
Authors: van Ballegooijen, A. A.; Cartledge, N. P.; Priest, E. R.
1998ApJ...501..866V Altcode:
Observations of filaments and filament channels on the Sun indicate
that the magnetic fields in these structures exhibit a large-scale
organization: filament channels in the northern hemisphere predominantly
have axial fields directed to the right when viewed from the positive
polarity side of the channel (dextral orientation), while those in the
south have axial fields directed to the left (sinistral orientation). In
this paper we attempt to explain this pattern in terms of the most
natural mechanism, namely, solar differential rotation acting on
already emerged magnetic fields. We develop a model of global magnetic
flux transport that includes the effects of differential rotation,
meridional flow, and magnetic diffusion on photospheric and coronal
fields. The model is applied to National Solar Observatory/Kitt
Peak data1 on the photospheric magnetic flux distribution. We also
present results from a simulation of solar activity over a period
of two solar cycles, which gives a buildup of flux at the poles of a
magnitude, in agreement with observations. We find that differential
rotation acting on initially north-south oriented polarity inversion
lines (PILs) does produce axial fields consistent with the observed
hemispheric pattern. The fields associated with switchbacks in the
PILs are predicted to have a definite orientation: the high-latitude
“lead” arms of the switchbacks are preferentially sinistral (dextral)
in the north (south), while the lower latitude “return” arms are, in
agreement with observations, preferentially dextral (sinistral). The
predicted orientation of fields at the polar crown, however, appear
to be in conflict with observations. Further observational studies
are needed to determine whether the model can explain the observed
hemispheric pattern.

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

---------------------------------------------------------
Title: Nature of the heating mechanism for the diffuse solar corona
Authors: Priest, E. R.; Foley, C. R.; Heyvaerts, J.; Arber, T. D.;
Culhane, J. L.; Acton, L. W.
1998Natur.393..545P Altcode:
The temperature of the Sun's outer atmosphere (the corona) exceeds that
of the solar surface by about two orders of magnitude, but the nature
of the coronal heating mechanisms has long been a mystery. The corona
is a magnetically dominated environment, consisting of a variety of
plasma structures including X-ray bright points, coronal holes and
coronal loops. The latter are closed magnetic structures that occur
over a range of scales and are anchored at each end in the solar
surface. Large-scale regions of diffuse emission are made up of many
long coronal loops. Here we present X-ray observations of the diffuse
corona from which we deduce its likely heating mechanism. We find that
the observed variation in temperature along a loop is highly sensitive
to the spatial distribution of the heating. From a comparison of
the observations and models we conclude that uniform heating gives
the best fit to the loop temperature distribution, enabling us to
eliminate previously suggested mechanisms of low-lying heating near
the footpoints of a loop. Our findings favour turbulent breaking and
reconnection of magnetic field lines as the heating mechanism of the
diffuse solar corona.

---------------------------------------------------------
Title: A startling new Sun from SOHO
Authors: Priest, Eric
1998A&G....39c..10P Altcode:
No abstract at ADS

---------------------------------------------------------
Title: A crossroads for European solar and heliospheric
physics. Recent achievements and future mission
possibilities. Proceedings.
Authors: Priest, E. R.; Moreno-Insertis, F.; Harris, R. A.
1998cesh.conf.....P Altcode:
The following topics were dealt with: the solar interior, photosphere,
solar corona, the solar wind, SOHO spacecraft mission results, the
Ulysses mission, the Yohkoh mission, scientific objectives and future
missions, the RAMSES proposal, INTERHELIOS, SOLARNET, the solar STEREO
mission, HIREX, Solar-B mission, SOLAR LITE, UV coronagraphs.

---------------------------------------------------------
Title: Our new Sun
Authors: Priest, Eric
1998A&G....39b..25P Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Solar flare MHD processes
Authors: Priest, E. R.
1998PAICz..88...95P Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Crossroads for European Solar and Heliospheric Physics
Authors: Priest, E. R.
1998ESASP.417.....P Altcode:
No abstract at ADS

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

---------------------------------------------------------
Title: A Dynamic Dextral-Sinistral Model for the Structure and
Evolution of Prominence Magnetic Fields
Authors: Priest, E. R.
1998ASPC..150..453P Altcode: 1998npsp.conf..453P; 1998IAUCo.167..453P
No abstract at ADS

---------------------------------------------------------
Title: Magnetic Flux Transport and Formation of Filament Channels
Authors: van Ballegooijen, A. A.; Cartledge, N. P.; Priest, E. R.
1998ASPC..150..265V Altcode: 1998npsp.conf..265V; 1998IAUCo.167..265V
No abstract at ADS

---------------------------------------------------------
Title: The Topology of Coronal Magnetic Fields in Active Regions
Authors: Brown, D. S.; Priest, E. R.
1998ASPC..155...90B Altcode: 1998sasp.conf...90B
No abstract at ADS

---------------------------------------------------------
Title: The roles of advection and diffusion in planar magnetic
merging solutions
Authors: Watson, P. G.; Priest, E. R.; Craig, I. J. D.
1998GApFD..88..165W Altcode:
Since cosmic plasmas are highly conducting, large-scale magnetic fields
are tied almost completely to the velocity field of the fluid. Only
in localized regions of strong current density can the magnetic field
slip through the plasma, allowing magnetic energy to be converted into
ohmic heating or the kinetic energy of mass motion. Here we contrast
the roles of advection and resistive diffusion in three different
steady-state two-dimensional models for magnetic energy conversion:
magnetic annihilation, reconnective diffusion and a kinematic model
based on the classical magnetic reconnection picture. First we examine
the diagnostic of 'field-line slippage' and show that it provides a
useful indicator of the relative importance of advection and diffusion
in each solution. We then quantify the energy release characteristics
of the different models by examining the ratio of ohmic heat to kinetic
energy generation.

---------------------------------------------------------
Title: Magnetic mechanisms for heating the corona
Authors: Priest, E. R.
1998PAICz..88..142P Altcode:
No abstract at ADS

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

---------------------------------------------------------
Title: Alfven wave support of a dwarf molecular cloud. I. an
isothermal model.
Authors: Martin, C. E.; Heyvaerts, J.; Priest, E. R.
1997A&A...326.1176M Altcode:
Dwarf (or dark) molecular clouds and molecular clumps have a lifetime
which is greater than their dynamical time and must therefore be, in an
average sense, in mechanical equilibrium. Equilibrium perpendicular to
a global magnetic field is by magnetic forces and it is proposed that
along the field the gas is supported by an Alfven wave pressure force. A
self-consistent analytical model, utilising a WKB approximation, is
developed for such support. It is found that Alfven waves are indeed
a good candidate for this support, generating model cloud thicknesses
consistent with observations. The effect of damping by the linear
process of ion-neutral friction is considered. It is found that the
damping of the waves is not a necessary condition for the support of
the cloud although weak damping is an advantage. The possible sources
of these waves are discussed.

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

---------------------------------------------------------
Title: The Importance of Photospheric Intense Flux Tubes for Coronal
Heating
Authors: Démoulin, P.; Priest, E. R.
1997SoPh..175..123D Altcode:
Dissipation of magnetic energy in the corona requires the creation of
very fine scale-lengths because of the high magnetic Reynolds number
of the plasma. The formation of current sheets is a natural possible
solution to this problem and it is now known that a magnetic field
that is stressed by continous photospheric motions through a series
of equilibria can easily form such sheets. Furthermore, in a large
class of 3D magnetic fields without null points there are locations,
called `quasi-separatrix layers' (QSLs), where the field-line linkage
changes drastically. They are the relevant generalisation of normal
separatrices to configurations without nulls: along them concentrated
electric currents are formed by smooth boundary motions and 3D magnetic
reconnection takes place when the layers are thin enough. With a
homogenous normal magnetic field component at the boundaries, the
existence of thin enough QSL to dissipate magnetic energy rapidly
requires that the field is formed by flux tubes that are twisted by a
few turns. However, the photospheric field is not homogeneous but is
fragmented into a large number of thin flux tubes. We show that such
thin tubes imply the presence of a large number of very thin QSLs in
the corona. The main parameter on which their presence depends is the
ratio between the magnetic flux located outside the flux tubes to the
flux inside. The thickness of the QSLs is approximately given by the
distance between neighbouring flux tubes multiplied by the ratio of
fluxes to a power between two and three (depending on the density
of flux tubes). Because most of the photospheric magnetic flux is
confined in thin flux tubes, very thin QSLs are present in the corona
with a thickness much smaller than the flux tube size. We suggest that a
turbulent resistivity is triggered in a QSL, which then rapidly evolves
into a dynamic current sheet that releases energy by fast reconnection
at a rate that we estimate to be sufficient to heat the corona. We
conclude that the fragmentation of the photospheric magnetic field
stimulates the dissipation of magnetic energy in the corona.

---------------------------------------------------------
Title: Can we Extrapolate a Magnetic Field when its Topology is
Complex?
Authors: DÉmoulin, P.; HÉnoux, J. C.; Mandrini, C. H.; Priest, E. R.
1997SoPh..174...73D Altcode:
In order to understand various solar phenomena controlled by the
magnetic field, such as X-ray bright points, flares and prominence
eruptions, the structure of the coronal magnetic field must be
known. This requires a precise extrapolation of the photospheric
magnetic field. Presently, only potential or linear force-free
field approximations can be used easily. A more realistic modelling
of the field is still an active research area because of well-known
difficulties related to the nonlinear mixed elliptic-hyperbolic nature
of the equations. An additional difficulty arises due to the complexity
of the magnetic field structure which is caused by a discrete partition
of the photospheric magnetic field. This complexity is not limited
to magnetic regions having magnetic nulls (and so separatrices)
but also occurs in those containing thin elongated volumes (called
Quasi-Separatrix Layers) where the photospheric field-line linkage
changes rapidly. There is a wide range for the thickness of such layers,
which is determined by the character (bipolar or quadrupolar) of the
magnetic region, by the sizes of the photospheric field concentrations
and by the intensity of the electric currents. The aim of this paper
is to analyse the recent nonlinear force-free field extrapolation
techniques for complex coronal magnetic fields.

---------------------------------------------------------
Title: Reconnection in the solar corona and magnetospheric
substorms. Proceedings. D0.4 Symposium of COSPAR Scientific Commission
D which was held during the Thirty-first COSPAR Scientific Assembly,
Birmingham (UK), 14 - 21 Jul 1996.
Authors: Priest, E. R.; Baker, D. N.
1997AdSpR..19.1749P Altcode: 1997AdSpR..19R....P
The following topics were dealt with: basic MHD theory; collisionless
reconnection; solar reconnection; magnetospheric reconnection.

---------------------------------------------------------
Title: Phase-mixing in Dissipative Alfvén Waves
Authors: Ireland, Jack; Priest, Eric R.
1997SoPh..173...31I Altcode:
The phase-mixing mechanism first proposed as a coronal heating mechanism
by Heyvaerts and Priest (1983) is examined using a length-scale analysis
adapted from Cally (1991). This allows parameter ranges other than
those studied by Heyvaerts and Priest (1983) to be described, together
with a detailed examination of the transfer of energy to both longer
and shorter length-scales as the Alfvén wave front evolves in the
solar corona. The results of Heyvaerts and Priest (1983) are largely
confirmed, but with some notable differences. Energy initially at
smaller length-scales decays faster than their rate, because the
plasma is more strongly dissipative at smaller length-scales. The
full inclusion of diffusion across field lines also leads to smoother
Alfvén wavefronts.

---------------------------------------------------------
Title: Three-dimensional magnetic reconnection in the solar corona
Authors: Priest, E. R.
1997PhPl....4.1945P Altcode:
In two dimensions the notions of magnetic topology and null-point
bifurcations are straightforward. In addition, the nature
of magnetic reconnection is fairly well understood and can be
described by a new generation of fast reconnection mechanisms known
as almost-uniform reconnection and nonuniform reconnection. However,
in complex three-dimensional (3-D) magnetic fields, such as exist
in the solar corona, these phenomena are only just beginning to be
explored and are considerably more complex. The structural properties
of the magnetic field created in turn by two, three and more magnetic
sources at the photosphere have been recently studied. Passing through
each 3-D magnetic null there is an isolated spine field line and a
flux surface known as a fan. The fans form separatrix surfaces that
separate the volume into topologically distinct regions, and the fan of
one null can terminate at the spine of another null, while the spine
terminates either at a source or at infinity. The skeleton of complex
3-D fields in the corona, therefore, comprises the magnetic-null points
and a network of spine curves and separatrix fan surfaces. Magnetic
reconnection can occur in these fields by a variety of mechanisms,
including spine reconnection, fan reconnection, separator reconnection,
and quasiseparatrix layer reconnection. These types of reconnection
and the bifurcations of null points are described and found to be much
richer than in the relatively simple two-dimensional fields.

---------------------------------------------------------
Title: How is the Solar Corona Heated ?
Authors: Priest, Eric R.
1997SPD....28.0701P Altcode: 1997BAAS...29..911P
Trying to determine which mechanisms are heating the corona is one of
the main aims of the SOHO mission. A summary is given of the mechanisms
that have been proposed together with the observational properties
that have so far been determined. A new technique for determining
the heating mechanisms is presented together with the results from
applying it to Yohkoh data and SOHO data - it gives powerful evidence
in favour of one particular mechanism for most of the corona. The
technique provides a strong implication as to what observations could
be most usefully made from SOHO and what theoretical developments of
the different mechanisms are required in future.

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

---------------------------------------------------------
Title: Heating of coronal holes by phase mixing.
Authors: Hood, A. W.; Ireland, J.; Priest, E. R.
1997A&A...318..957H Altcode:
A two-dimensional, analytical, self-similar solution to the Alfven
wave phase mixing equations is presented for a coronal hole model. The
solution shows clearly that the damping of the waves with height follows
the scaling predicted by Heyvaerts &amp; Priest at low heights, before
switching to an algebraic decay at large heights. The ohmic dissipation
is calculated and it is shown that the maximum dissipation occurs at a
height that scales with η^1/3^. However, the total Ohmic dissipation
is, of course, independent of the resistivity. Using realistic solar
parameters it appears that phase mixing is a viable mechanism for
heating the lower corona provided either the frequency of photospheric
motions is sufficiently large or the background Alfven velocity is
sufficiently small.

---------------------------------------------------------
Title: Active Region Blinkers: Transient Events in the Solar
Atmosphere
Authors: Walsh, R. W.; Ireland, J.; Harrison, R. A.; Priest, E. R.
1997ESASP.404..717W Altcode: 1997cswn.conf..717W
No abstract at ADS

---------------------------------------------------------
Title: An Analytical MHD Model for Solar/Stellar Winds
Authors: Lima, J. J. G.; Priest, E. R.; Tsinganos, K.
1997ESASP.404..521L Altcode: 1997cswn.conf..521L
No abstract at ADS

---------------------------------------------------------
Title: Periodicities in Active Regions
Authors: Ireland, J.; Walsh, R. W.; Harrison, R. A.; Priest, E. R.
1997ESASP.404..433I Altcode: 1997cswn.conf..433I
No abstract at ADS

---------------------------------------------------------
Title: Magnetic Fields and Coronal Structures
Authors: Priest, E. R.
1997ESASP.404...93P Altcode: 1997cswn.conf...93P; 1997soho....5...93P
No abstract at ADS

---------------------------------------------------------
Title: Linear theory of steady X-point magnetic reconnection
Authors: Titov, V. S.; Priest, E. R.
1997AdSpR..19.1777T Altcode:
Slow magnetic reconnection at a neutral X-point of a two-dimensional
magnetic field is studied in an incompressible viscous resistive
fluid. It is shown analytically that the combined effect of viscosity
and resistivity resolves the current singularity appearing in both the
ideal and resistive magnetohydrodynamic approximations at the X-point
and along the separatrices when the flow is allowed to cross them. A
previous attempt had retained a weak singularity at third order. A
two-parameter family of exact solutions describing the structure of the
flow and current density distribution is found for the corresponding
basic equations.

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

---------------------------------------------------------
Title: The 3D topology and interaction of complex magnetic flux
systems
Authors: Priest, E. R.; Bungey, T. N.; Titov, V. S.
1997GApFD..84..127P Altcode:
In order to understand the topology of complex flux systems we analyse
in detail the rich structural properties of the magnetic field created
in turn by two or three balanced or unbalanced flux sources. Passing
through each three-dimensional magnetic null point (where the field
vanishes) is an isolated spine field line and a flux surface known
as a fan. The fans form separatrix surfaces which separate the volume
into topologically distinct regions and generally intersect each other
in separator field lines linking pairs of null points. Furthermore,
the fan of one null can terminate at the spine of another null, whilst
the spine terminates either at a source or at infinity. The skeleton
of complex three-dimensional magnetic fields therefore comprises the
magnetic null points and a network of spine curves and separatrix fan
surfaces. The skeletons due to three sources are classified and found
to be surprisingly complex. For example, a new discovery is that a
configuration with a separator may be produced by only three sources.

---------------------------------------------------------
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: Bifurcations of magnetic topology by the creation or
annihilation of null points
Authors: Priest, E. R.; Lonie, D. P.; Titov, V. S.
1996JPlPh..56..507P Altcode:
Linear null points of a magnetic field may come together and coalesce
at a secondorder null, or vice versa a second-order null may form and
split, giving birth to a pair of linear nulls. Such local bifurcations
lead to global changes of magnetic topology and in some cases release
of magnetic energy. In two dimensions the null points are of X or O
type and the flux function is a Hamiltonian; the magnetic field may
undergo addle-centre, pitchfork or degenerate resonant bifurcations. In
three dimensions the null points and their creation or annihilation
by bifurcations are considerably more complex. The nulls possess a
skeleton consisting of a spine curve and a fan surface and are of
radial-type (proper or improper) or spiral-type; the type of null
and the inclination of spine and fan depend on the magnitudes of the
current components along and normal to the spine. In cylindrically
symmetric fields a comprehensive treatment is given of the various
types of saddle-node, Hopf and saddle-node—Hopfbifurcations. In fully
three-dimensional situations examples are given of saddle-node and
degenerate bifurcations, in which generically two nulls are created
or destroyed and are joined by a separator field line, which is the
intersection of the two fans. Furthermore, global bifurcations can
create chaotic field lines that could perhaps trigger energy release
in, for example, solar flares.

---------------------------------------------------------
Title: Magnetic Reconnection at Three-Dimensional Null Points
Authors: Priest, E. R.; Titov, V. S.
1996RSPSA.354.2951P Altcode:
The skeleton of an isolated null point in three dimensions consists of a
`spine curve' and a `fan surface'. Two isolated magnetic field lines
approach (or recede from) the null point from both directions along
the spine, and a continuum of field lines recedes from (or approaches)
the null in the plane of the fan surface. Two bundles of field lines
approach the null point around the spine (one from each direction)
and spread out near the fan. The kinematics of steady reconnection
at such a null point is considered, depending on the nature of
the imposed boundary conditions on the surface that encloses the
null, in particular on a cylindrical surface with its axis along
the spine. Three kinds of reconnection are discovered. In `spine
reconnection' continuous footpoint motions are imposed on the curved
cylindrical surface, crossing the fan and driving singular jetting flow
along the spine. In `fan reconnection' continuous footpoint motions
are prescribed on the ends of the cylinder, crossing the spine and
driving a singular swirling motion at the fan. An antireconnection
theorem is proved, which states that steady MHD reconnection in three
dimensions with plasma flow across the spine or fan is impossible in
an inviscid plasma with a highly subAlfvenic flow and uniform magnetic
diffusivity. One implication of this is that reconnection tends to be
an inherently nonlinear phenomenon. A linear theory for slow steady
reconnection is developed which demonstrates explicitly the nature of
the spine singularity in spine reconnection. Finally, the properties of
`separator reconnection' in complex configurations containing two null
points are discussed by means of analytical examples.

---------------------------------------------------------
Title: Magnetic Reconnection at Three-Dimensional Null Points
Authors: Priest, E. R.; Titov, V. S.
1996RSPTA.354.2951P Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Magnetic Field Diffusion in Self-consistently Turbulent
Accretion Disks
Authors: Heyvaerts, J.; Priest, E. R.; Bardou, A.
1996ApJ...473..403H Altcode:
We show how the level of turbulence in accretion disks can be derived
from a self-consistency requirement that the associated effective
viscosity should match the instantaneous accretion rate. This method
is applicable when turbulence has a direct energy cascade. Only
limited information on the origin and properties of the turbulence,
such as its injection scale and anisotropy, is needed. The method
is illustrated by considering the case of turbulence originating
from the magnetic shearing instability. The corresponding effective
kinematic viscosity coefficient is shown to scale as the 1/3 power
of surface mass density at a given radius in optically thick disks,
and to be describable by a Shakura-Sunyaev law with α≍0.04. Mass
flow in disks fed at a localized hot spot is calculated for accretion
regimes driven by such turbulence, as well as passive magnetic field
diffusion and dragging. An important result of this analysis is that
thin disks supported by turbulence driven by the magnetic shearing
instability, and more generally any turbulence with injection scale
of order of the disk thickness, are very low magnetic Reynolds number
systems. Turbulent viscosity-driven solutions with negligible field
dragging and no emission of cold winds or jets are natural consequences
of such regimes. Disks of accreting objects that are magnetized enough
to be shielded by a magnetopause, however, may not operate in their
innermost regions in the magnetic shearing instability regime. The
possibility therefore remains to be explored of centrifugally driven
winds emanating from such regions.

---------------------------------------------------------
Title: Quasi-Separatrix Layers in Simple Magnetic Configurations
Authors: Mandrini, C. H.; Demoulin, P.; Henoux, J. C.; Priest, E.
1996RMxAC...4Q.104M Altcode:
No abstract at ADS

---------------------------------------------------------
Title: A Strong Limitation on the Rapidity of Flux-Pile-Up
Reconnection
Authors: Litvinenko, Y. E.; Forbes, T. G.; Priest, E. R.
1996SoPh..167..445L Altcode:
The reconnection rate which can be achieved in the steady-state
flux-pile-up regime is severely limited by the gas pressure of the
plasma. Using the family of solutions obtained previously by Priest and
Forbes, we show that the Alfvén Mach number of the plasma flowing
towards the reconnection site cannot exceed πβe/(8
ln Rme), where βe and Rme are
the plasma β and magnetic Reynolds numbers at large distance. This
limit corresponds to a very weak flux-pile-up, and it is a factor of
βe slower than the maximum Petschek rate. Thus the maximum
flux-pile-up reconnection rate in the corona is at least two orders
of magnitude smaller than the rate implied by flare observations.

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

---------------------------------------------------------
Title: A 2-D model for the support of a polar-crown solar prominence
Authors: Cartledge, N. P.; Titov, V. S.; Priest, E. R.
1996SoPh..166..287C Altcode: 1996SoPh..166..287G
We present a 2-D potential-field model for the magnetic structure in
the environment of a typical quiescent polar-crown prominence. The
field is computed using the general method of Titov (1992) in which
a curved current sheet, representing the prominence, is supported
in equilibrium by upwardly directed Lorentz forces to balance the
prominence weight. The mass density of the prominence sheet is computed
in this solution using a simple force balance and observed values of
the photospheric and prominence magnetic field. This calculation gives
a mass density of the correct order of magnitude. The prominence sheet
is surrounded by an inverse-polarity field configuration adjacent to
a region of vertical, open polar field in agreement with observations.

---------------------------------------------------------
Title: Magnetohydrodynamic Waves in Solar Coronal Arcades
Authors: Oliver, R.; Hood, A. W.; Priest, E. R.
1996ApJ...461..424O Altcode:
The propagation of magnetohydrodynamic (MRD) disturbances in a solar
coronal arcade is investigated. The equations of magnetoacoustic
fast and slow waves are presented in a very general form: a pair
of second-order, two-dimensional partial differential equations in
which the two dependent variables are the components of the velocity
perturbation parallel and normal to the magnetic field. In deriving
these equations, a general two-dimensional equilibrium structure with
no longitudinal magnetic field component has been assumed. Thus, the
equations are valid for rather general configurations. Alfvén waves are
decoupled from the magnetoacoustic modes and give rise to an Alfvén
continuous spectrum. The solutions to the wave equations have
been obtained numerically, and the perturbed restoring forces (plasma
pressure gradient, magnetic pressure gradient, and magnetic tension),
responsible for the oscillatory modes, have also been computed. These
forces give rise to the propagation of MRD waves, and their interaction
determines the physical properties of the various modes. Therefore,
the spatial structure of the forces and their interplay are basic
in characterizing fast and slow modes. Pure fast and pure slow
waves do not exist in the present configuration, although for the
considered parameter values, all modes possess either fast-mode or
slow-mode properties. "Slow" modes in these two-dimensional equilibria
can propagate across the magnetic field only with difficulty and so
display a structure of bands, centred about certain field lines, of
alternate positive and negative parallel velocity component. On the
other hand, "fast" modes are isotropic in nature, and their spatial
structure is not so intimately linked to the shape of field lines. In
addition, as a consequence of the distinct characteristic propagation
speeds of fast and slow modes, their frequencies typically differ by
an order of magnitude.

---------------------------------------------------------
Title: Basic topological elements of coronal magnetic fields.
Authors: Bungey, T. N.; Titov, V. S.; Priest, E. R.
1996A&A...308..233B Altcode:
We model the magnetic field of a coronal active region using discrete
magnetic sources and look for regions of the photospheric inversion
line which are tangentially touched by coronal field lines. These
locations are called `Bald Patches'. Field lines threading these
bald patch regions, together with those threading any coronal null
points, determine the `Topological Skeleton' of the field. We examine
the properties of these null points and bald patch regions for two,
three and four magnetic sources, and study the evolution of the coronal
topological skeleton as a result of moving and varying the sources. In
particular, for a four-source magnetic configuration, we establish the
existence of a well-defined `separator' field line in the absence of any
coronal null points. We conclude that bald patches as well as coronal
nulls constitute an elemental topological feature of the coronal field:
they may be significant as possible regions for coronal current sheet
formation and for prominence formation.

---------------------------------------------------------
Title: Three-dimensional magnetic reconnection without null points
2. Application to twisted flux tubes
Authors: Démoulin, P.; Priest, E. R.; Lonie, D. P.
1996JGR...101.7631D Altcode:
Magnetic reconnection has traditionally been associated
exclusively with the presence of magnetic null points or field
lines tangential to a boundary. However, in many cases introducing a
three-dimensional perturbation in a two-and-half-dimensional magnetic
configuration implies the disappearance of separatrices. Faced
with this structural instability of separatrices when going from
two-and-half to three-dimensional configurations, several approaches
have been investigated to replace the topological ideas familiar
in two-dimensional, but no unanimity has yet emerged on the way
reconnection should be defined. While it is true that the field line
linkage is continuous in three-dimensional, we show here that extremely
thin layers (called quasi-separatrix layers (QSLs)) are present. In
these layers the gradient of the mapping of field lines from one part
of a boundary to another is very much larger than normal (by many
orders of magnitude). Even for highly conductive media these extremely
thin layers behave physically like separatrices. Thus reconnection
without null points can occur in QSLs with a breakdown of ideal MHD
and a change in connectivity of plasma elements. We have analyzed
several twisted flux tube configurations, going progressively from
two-and-half to three-dimensional, showing that QSLs are structurally
stable features (in contrast to separatrices). The relative thickness w
of QSLs depends mainly on the maximum twist; typically, with two turns,
w~10-6, while with four turns, w~10-12. In these
twisted configurations the shape of the QSLs, at the intersection with
the lower planar boundary, is typical of the two ribbons observed in
two-ribbon solar flares, confirming that the accompanying prominence
eruption involves the reconnection of twisted magnetic structures. We
conclude that reconnection occurs in three-dimensional in thin layers
or QSLs, which generalise the traditional separatrices (related only
to magnetic null points or field lines tangential to the boundary).

---------------------------------------------------------
Title: Quasi-Separatrix layers in solar flares. I. Method.
Authors: Demoulin, P.; Henoux, J. C.; Priest, E. R.; Mandrini, C. H.
1996A&A...308..643D Altcode:
Magnetic reconnection is usually thought to be linked to the presence
of magnetic null points and to be accompanied by the transport of
magnetic field lines across separatrices, the set of field lines where
the mapping of field lines is discontinuous. In view of the variety
of observed flaring configurations, we show that this view is too
restrictive. Instead, Priest and Demoulin (1995) have explored a way
of generalising the concept of separatrices to magnetic configurations
without field-line linkage discontinuities. They propose that magnetic
reconnection may also occur in 3D in the absence of null points at
"quasi-separatrix layers" (QSLs), which are regions where there is
drastic change in field-line linkage. In previous studies we have shown
that solar flare kernels are linked to the topology of the active-region
magnetic field. The observed photospheric field was extrapolated to
the corona using subphotospheric magnetic sources and the topology was
defined by the magnetic linkage between these sources, the method being
called SM (for Source Method). In this paper we define a new method,
called QSLM (for Quasi-Separatrix Layers Method), which finds the
location of QSLs above the photosphere. It is designed to be applied to
any kind of magnetic field representation, while, in the present paper,
we apply it only to simple theoretical magnetic configurations in order
to compare it with the SM. It generalises the concept of separatrices to
magnetic configurations without field-line linkage discontinuities. The
QSLM determines elongated regions that are in general located along
small portions of the separatrices defined by the SM, and in the limit
of very concentrated photospheric fields both methods give the same
result. In bipolar magnetic configurations two QSLs are found at both
sides of the inversion line, while in quadrupolar configurations four
appear. We find that there is a wide range for the thickness of the
QSLs, which is determined by the character (bipolar or quadrupolar)
of the magnetic region and by the sizes of the photospheric field
concentrations. We then show that smooth photospheric motions induce
concentrated currents at the locations defined by the QSLM. We prove
this only for initially potential configurations but, due to the form
of the equations, we conjecture that it is also valid for any kind
of initial magnetic equilibrium. We conclude that, even in bipolar
configurations, there are localized places where current build-up can
be induced by photospheric motions, leading to ideal MHD breakdown
with strong flows and magnetic energy release.

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

---------------------------------------------------------
Title: Coronal Heating by Magnetic Reconnection
Authors: Priest, Eric R.
1996Ap&SS.237...49P Altcode:
The theory of magnetic reconnection has advanced substantially
over the past few years. There now exists a new generation of fast
two-dimensional models known as almost-uniform reconnection and
nonuniform reconnection, depending on the boundary conditions. Also,
we are beginning to explore the uncharted region of three-dimensional
reconnection, where regimes of “spine reconnection” and “fan
reconnection” have been discovered. Furthermore, part of the coronal
heating problem appears to have been solved with recent observational
support for the Converging Flux Model in which heating is produced by
coronal reconnection driven by footpoint motions.

---------------------------------------------------------
Title: A Model for Polar-Crown Prominences
Authors: Cartledge, N. P.; Titov, V. S.; Priest, E. R.
1996ApL&C..34...89C Altcode:
No abstract at ADS

---------------------------------------------------------
Title: 3D Reconnection in Complex Topologies (Invited)
Authors: Priest, E. R.
1996ASPC..111..331P Altcode: 1997ASPC..111..331P
In order to understand the topology of complex flux systems such as
occur in the solar corona, the author studies the structural properties
of the magnetic field created in turn by 2, 3 and 4 magnetic sources
at the photosphere. Passing through each 3D magnetic null there is an
isolated spine field line and a flux surface known as a fan. The fans
form separatrix surfaces which separate the volume into topologically
distinct regions, and the fan of one null can terminate at the spine
of another null, while the spine terminates either at a source or at
infinity. Magnetic reconnection can occur in these fields by a variety
of mechanisms, including spine reconnection, fan reconnection, separator
reconnection and quasi-separatrix layer reconnection. The skeleton of
complex 3D fields in the corona therefore comprises the magnetic null
points and a network of spine curves and separatrix fan surfaces. The
types of reconnection in these fields and the bifurcations of null
points are described in some detail and found to be much richer than
in the relatively simple 2D fields that have been the main object of
study up to now.

---------------------------------------------------------
Title: Separatrix Characteristics in Steady MHD Flows
Authors: Surlantzis, G.; Tsinganos, K.; Priest, E.
1996ApL&C..34..251S Altcode:
No abstract at ADS

---------------------------------------------------------
Title: New developments in magnetic reconnection theory.
Authors: Priest, E. R.
1996ASIC..481..171P Altcode:
Although two-dimensional reconnection is now fairly well understood,
there have recently been some interesting new developments. Four
distinct types of reconnection are possible, namely: viscous (or
kinematic) reconnection; extra-slow (or linear) reconnection; slow (or
Sweet-Parker) reconnection; and fast (Almost-uniform or Nonuniform)
reconnection. An antireconnection theorem has been discovered which
states that steady two-dimensional reconnection (with flow across the
separatrices) is impossible for slow inviscid flow (and it has also been
generalised to three dimensions). Solutions for linear reconnection have
been presented and a new theory for the self-consistent time-dependent
collapse of an X-point to form a reconnecting current sheet has been
developed. The new field of three-dimensional reconnection is only
just beginning. Reconnection at null points is discussed.

---------------------------------------------------------
Title: Two-Dimensional MHD Models for Stellar Winds
Authors: Lima, J. J. G.; Tsinganos, K.; Priest, E. R.
1996ApL&C..34..281L Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Plasma Beta Limitations for Magnetic Annihilation Models
Authors: Inverarity, G. W.; Priest, E. R.
1996ASPC..111..296I Altcode: 1997ASPC..111..296I
The need to keep the plasma pressure positive in fast reconnection
solutions introduces a minimum plasma beta (ratio of plasma pressure
to magnetic pressure) at the edge of the region in which annihilation
or reconnection is occurring. Such constraints are presented for the
models of Sonnerup and Priest (1975) and Craig and Henton (1995). It
is found that in astrophysical contexts the models are only applicable
at very slow reconnection rates.

---------------------------------------------------------
Title: New Paradigms for Solar Prominences
Authors: Priest, E. R.
1996ASPC...95..229P Altcode: 1996sdit.conf..229P
No abstract at ADS

---------------------------------------------------------
Title: Interaction of turbulent accretion disks with embedded
magnetic fields.
Authors: Heyvaerts, J. F.; Bardou, A.; Priest, E. R.
1996ASIC..481..659H Altcode:
The authors show that the level of turbulence in accretion disks can
be derived from a self-consistency requirement that the associated
effective viscosity should match the instantaneous accretion rate. When
turbulence originates in the magnetic shearing instability, the
effective kinematic viscosity coefficient is shown to be describable
by a Shakura-Sunyaev law with α ≍ 0.04. It is shown that thin disks
suported by any turbulence with injection scale of order of the disk
thickness, are very low magnetic Reynolds number systems. Turbulent
viscosity-driven solutions with negligible field dragging and no
emission of cold winds or jets are natural consequences of such
regimes. Such disks are shown to expell the magnetic field of the
accreting object from their Keplerian regions radially outwards,
resulting in a flux distribution in the disk which differs very much
from a dipolar one.

---------------------------------------------------------
Title: Reconnection of magnetic lines of force.
Authors: Priest, E. R.
1996ASIC..481..151P Altcode:
Parker laid the foundation for the subject of magnetic reconnection in
his fundamental early papers. The author summarises his contributions
and gives a new generalisation of the Sweet-Parker relations for a
current sheet in which the outflow pressure is an extra parameter. Then
he reviews the models for fast reconnection that have since been
proposed, beginning with the Petschek mechanism and continuing to the
more general Almost-Uniform and Nonuniform families. A comparison with
numerical experiments is also made and the conditions under which fast
reconnection exists are elucidated.

---------------------------------------------------------
Title: 3D Magnetic Reconnection: Example of an X-Ray Bright Point
Authors: Demoulin, P.; Mandrini, C. H.; van Driel-Gesztelyi, L.;
Priest, E. R.; Henoux, J. C.; Schmieder, B.
1996ASPC..111...49D Altcode: 1997ASPC..111...49D
In the classical view magnetic reconnection occurs at neutral points
and implies transport of magnetic field-lines across separatrices. The
authors show that reconnection may also occur in the absence of neutral
points at so-called "quasi-separatrix layers" (QSLs), where there is a
steep gradient in field-line linkage at the boundaries. Reconnection
occurs in QSLs where the field-line velocity becomes larger than the
allowed maximal plasma velocity or where the electric-current density
becomes too great. The authors describe both a theoretical and an
observed configuration. In the case of a simple sheared X-field they
show that even a smooth continuous shear flow, imposed at the boundary,
gives strong plasma jetting inside and parallel to the QSLs. Applying
the QSL method to an X-ray bright point observed by Yohkoh, they find
field lines in the extrapolated field which are on both sides of QSLs
and which are in good agreement with loops observed in Hα and X-rays
related to emerging flux. The evolution of the QSL width may explain
the brightness evolution of the XBP.

---------------------------------------------------------
Title: Book-Review - Lectures on Solar and Planetary Dynamos
Authors: Proctor, M. R. E.; Gilbert, A. D.; Priest, E. R.
1995Obs...115..338P Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Three-dimensional magnetic reconnection without null
points. 1. Basic theory of magnetic flipping
Authors: Priest, E. R.; Démoulin, P.
1995JGR...10023443P Altcode:
In two or three dimensions, magnetic reconnection may occur at
neutral points and is accompanied by the transport of magnetic field
lines across separatrices, the field lines (or flux surfaces in three
dimensions) at which the mapping of field lines is discontinuous. Here
we show that reconnection may also occur in three dimensions in the
absence of neutral points at so-called “quasi-separatrix layers,”
where there is a steep gradient in field line linkage. Reconnection is
a global property, and so, in order to determine where it can occur,
the first step is to enclose the volume being considered by a boundary
(such as a spherical surface). Then the mapping of field lines from
one part of the boundary to another is determined, and quasi-separatrix
layers may be identified as regions where the gradient of the mapping
or its inverse is very much larger than normal. The most effective
measure of the presence of such layers is the norm of the displacement
gradient tensor; their qualitative location is robust and insensitive
to the particular surface that is chosen. Reconnection itself occurs
when there is a breakdown of ideal MHD and a change of connectivity
of plasma elements, where the field line velocity becomes larger
than the plasma velocity, so that the field lines slip through the
plasma. This breakdown can occur in the quasi-separatrix layers
with an electric field component parallel to the magnetic field. In
three dimensions the electric field E (and therefore the field line
velocity v⊥) depends partly on the imposed values of E
(or v⊥) at the boundary and partly on the gradients of the
inverse mapping function. We show that the inverse mapping determines
the location of the narrow layers where the breakdown of ideal MHD
can occur, while the imposed boundary values of v⊥
determine mainly the detailed flow pattern inside the layers. Thus,
in general, E (and therefore v⊥) becomes much larger than
its boundary values at locations where the gradients of the inverse
mapping function are large. An example is given of a sheared X field,
where a slow smooth continuous shear flow imposed on the boundary
across one quasi-separatrix produces a flipping of magnetic field lines
as they slip rapidly through the plasma in the other quasi-separatrix
layer. It results in a strong plasma jetting localized in, and parallel
to, the separatrix layers.

---------------------------------------------------------
Title: Disques d'accrétion turbulents auto-cohérents.
Authors: Bardou, A.; Heyvaerts, J.; Priest, E.
1995JAF....49...51B Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Turbulent coronal heating. III. Wave heating in coronal loops.
Authors: Inverarity, G. W.; Priest, E. R.
1995A&A...302..567I Altcode:
The heating which results from damped oscillations of a uniform
solar coronal loop driven by photospheric motions is considered for
an incompressible, visco-resistive, turbulent medium. Photospheric
disturbances on a time-scale faster than the time for an Alfven wave
to cross the loop drive damped standing waves in the loop. Including
turbulent effects permits the generation of gradients sufficiently
intense to allow dissipation of the kinetic and magnetic energy of the
waves to balance the rate of injection of energy from the photosphere,
thus maintaining a steady state. Assuming a single global driving
frequency, levels of heating approaching 10^4^Wm^-2^ (10^7^erg cm^-2^/s)
are attainable for the fundamental resonance and first few harmonics,
which could account for the heating of short (10^4^km) loops with
fields of order 100G if sufficiently rapid photospheric oscillations
occur. Also, for fields of about 10G, which occur in the longer loops
(10^5^km) found outside active regions, levels approaching 100Wm^-2^
(10^5^ergcm^-2^/s) are obtained -- sufficient to heat the quiet corona.

---------------------------------------------------------
Title: Models for the Motions of Flare Loops and Ribbons
Authors: Lin, J.; Forbes, T. G.; Priest, E. R.; Bungey, T. N.
1995SoPh..159..275L Altcode:
We have found a conformal mapping which is valid for any magnetic
boundary condition at the photosphere and which can be used to determine
the evolution of an open, two-dimensional magnetic field configuration
as it relaxes to a closed one. Solutions obtained with this mapping
are in quasi-static equilibrium, and they contain a vertical current
sheet and have line-tied boundary conditions. As a specific example,
we determine the solution for a boundary condition corresponding to a
submerged, two-dimensional dipole below the photosphere. We assume that
the outer edges of the hottest X-ray loops correspond to field lines
mapping from the outer edges of the Hα ribbon to the lower tip of
the current sheet where field lines reconnect at aY-type neutral line
which rises with time. The cooler Hα loops are assumed to lie along
the field lines mapping to the inner edges of the flare ribbons. With
this correspondence between the plasma structures and the magnetic
field we determine the shrinkage that field lines are observed to
undergo as they are disconnected from the neutral line. During the
early phase of the flare, we predict that shrinkage inferred from
the height of the Hα and X-ray loops is close to 100% of the loop
height. However, the shrinkage should rapidly decrease with time to
values on the order of 20% by the late phase. We also predict that
the shrinkage in very large loops obeys a universal scaling law which
is independent of the boundary condition, provided that the field
becomes self-similar (i.e., all field lines have the same shape) at
large distances. Specifically, for any self-similar field containing
aY-type neutral line, the observed shrinkage at large distances should
decrease as (ΔX/XR)−2/3, where ΔX is the
ribbon width andXRis the ribbon separation. Finally, we
discuss the relation between the electric field at the neutral line
and the motions of the flare loops and ribbons.

---------------------------------------------------------
Title: Erratum - Turbulent Coronal Heating
Authors: Inverarity, G. W.; Priest, E. R.; Heyvaerts, J.
1995A&A...299..640I Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Book Review: Fundamentals of cosmic electrodynamics /
Kluwer, 1994
Authors: Priest, E. R.
1995Obs...115..142P Altcode: 1995Obs...115..142S
No abstract at ADS

---------------------------------------------------------
Title: Photospheric Magnetic Field Evolution and Eruptive Flares
Authors: Forbes, T. G.; Priest, E. R.
1995ApJ...446..377F Altcode:
Using an eruptive flare model based on a loss of equilibrium in a
coronal flux rope, we show that the average horizontal component
of the photospheric field does not necessarily become more like
a potential (current-free) field immediately after the eruption
begins. Therefore, recent observations showing that the average
horizontal field becomes less potential during the impulsive phase of
a flare do not necessarily imply that the magnetic energy in the corona
has increased as has sometimes been assumed. The flux-rope model which
we use also has relevance to coronal mass ejections. It differs from
previous models because eruption is triggered solely by the converging
motion of two photospheric field sources that lie below the coronal
flux rope. Because this boundary condition is much simpler than that
used in previous formulations, this particular version of the model
is especially well-suited for two-dimensional numerical simulations
with translational symmetry.

---------------------------------------------------------
Title: Book Review: Solar surface magnetism / Kluwer, 1994
Authors: Priest, E.
1995Obs...115..103P Altcode: 1995Obs...115..103R
No abstract at ADS

---------------------------------------------------------
Title: Turbulent coronal heating. II. Twisted flux tube.
Authors: Inverarity, G. W.; Priest, E. R.
1995A&A...296..395I Altcode:
Inverarity et al. extended the model of Hevyaerts &amp; Priest for
turbulent coronal heating in a sheared arcade, again achieving
the observed levels of heating but with improved mathematical
consistency. A similar model for heating in a flux tube twisted
by vortical photospheric motions is here developed. Two models
for the turbulence are investigated: three-dimensional, helically
isotropic turbulence and two-dimensional turbulence, isotropic in
the plane perpendicular to the background field. It is found that the
three-dimensional turbulence model can reproduce the observed levels
of coronal heating. However, the purely two-dimensional model cannot
be used when modelling steady-state heating.

---------------------------------------------------------
Title: On the Nature of 3D Reconnection
Authors: Priest, E. R.
1995LNP...462..303P Altcode: 1995ssst.conf..303P
In two dimensions the theory of magnetic reconnection is now highly
developed and fairly well understood. A general configuration will
contain X-type neutral points and separatrix field lines which link to
it. Reconnection occurs by the breaking and rejoining of field lines
at the X-point and the transfer of flux across the separatrices from
one topological region to another.

---------------------------------------------------------
Title: Current sheet configurations in potential and force-free
fields.
Authors: Bungey, T. N.; Priest, E. R.
1995A&A...293..215B Altcode:
Using complex variables and the method of conformal mappings, an
analytical expression is produced for the possible configurations of
constant-current, force-free or magnetostatic magnetic fields in the
neighbourhood of a linear current sheet. The configurations obtained
include the well-known potential solutions of Green and Syrovatskii as
special cases. New potential and force-free solutions possess asymmetry
along the current sheet and a closing over of the field lines for the
force-free case.

---------------------------------------------------------
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: Turbulent coronal heating. I. Sheared arcade.
Authors: Inverarity, G. W.; Priest, E. R.; Heyvaerts, J.
1995A&A...293..913I Altcode:
The model of Heyvaerts &amp; Priest for heating the solar corona by
turbulent processes in a sheared arcade is examined in more detail by
numerically solving their basic equations for the turbulent viscosity,
magnetic diffusivity, velocity and energy flux density. A weighted
spectrum for the mean square velocity in the photosphere and a
scale-dependent turbulent viscosity and magnetic diffusivity for local
eddy relaxation are introduced which improve the self-consistency of
the model. Estimates are made of the turbulent variables listed above
which are found to accord well with the requirements for heating a
solar active region (=~10^7^erg/cm^2^/s).

---------------------------------------------------------
Title: Magnetic Reconnection in the Sun's Atmosphere
Authors: Priest, E. R.
1995fras.conf..191P Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Solar Flares
Authors: Priest, Eric R.
1995LNP...454....3P Altcode: 1995flfl.conf....3P; 1995IAUCo.151....3P; 1995LNP...454....1P
There are many types of solar flare, but the classic type is a
two-ribbon flare with three phases — a preflare phase, a rise
phase and a main phase. The properties of these phases are described,
together with some recent observational advances in understanding the
conditions for solar flares. Such flares are thought to be caused by
an eruptive MHD instability which drives reconnection and therefore
energy conversion. A review is given of our current understanding of
the nature of this instability and the resulting reconnection process,
including a recent attempt to describe its three-dimensional nature.

---------------------------------------------------------
Title: Thermal equilibria in solar coronal magnetic loops:
gravitational considerations.
Authors: Steele, C. D. C.; Priest, E. R.
1994A&A...292..291S Altcode:
Equations for thermal equilibria along coronal magnetic loops (with
constant cross-sectional area) are solved in the absence and the
presence of gravity. When the effect of gravity is negligible, loops
may exist with cool summits; these loops consist of two portions, an
isothermal portion at the summit and a non-isothermal portion near the
footpoint. In the presence of gravity, the two-component structure no
longer exists and there is a restriction (dependent on the heating)
on the lengths of loops with cool summits. Loops with hot summits
are cooled slightly by the incorporation of gravity. A model of a
cylindrical arcade is set up where the heating and the magnetic field
vary with position. Hot arcades exist as do arcades with a vertical
cool sheet surrounded by hotter plasma - a situation that occurs for a
prominence. It is anticipated that loops with cool summits may exist
for higher values of gravity when the loop area is allowed to vary
with position.

---------------------------------------------------------
Title: Book-Review - Solar Magnetic Fields
Authors: Schussler, M.; Schmidt, W.; Priest, E. R.
1994Ap&SS.222..266S Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Book reviews
Authors: Burgess, D.; Kahn, F. D.; Dyson, John; Roche, Patrick;
Priest, E. R.; Budding, Edwin; Keenan, F. P.
1994Ap&SS.222..263B Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Steady linear X-point magnetic reconnection
Authors: Priest, E. R.; Titov, V. S.; Vekestein, G. E.; Rikard, G. J.
1994JGR....9921467P Altcode:
In the main part of this paper a model for linear reconnection
is developed with a current spike around the X-point and vortex
current sheets along the separatrices, which are resolved by the
effects of viscosity and magnetic diffusivity. The model contains
three regions. In the external ideal region, diffusion effects are
negligible, and the flow is purely radial but becomes singular both
along the separatrices and at the X-point. Near the separatrix there
is a self-similar boundary layer with strong electric current and
vorticity, where resistivity and viscosity resolve the singularity
and allow the flow to cross the separatrix. A composit solution
is set up that matches the external and separatrix solutions. Near
the origin diffusion also resolves the singularity and is described
approximately by a biharmonic solution. A classification of steady
two-dimensional reconnection regimes is proposed into viscous
reconnection (Me greater than Re,) extra slow (linear)
reconnection (Me less than Rme-1),
slow reconnection (Rme-1/2 Me less
than or = Rme-1/2), and fast reconnection
(Rme-1/2 less than Me is the
dimensionless reconnection rate, Rme the magnetic
Reynolds number, and Re the Reynolds number, all based on the Alfven
speed far from the reconnection point. Also, an anti-reconnection
theorem is proved, which has profound effects on the nature of linear
reconnection. It states that steady two-dimensional magnetohydrodynamcis
(MHD) reconnection with plasma flow across the separatrices is
impossible in a plasma which is inviscid, highly sub-Alfvenic, and
has uniform magnetic diffusivity.

---------------------------------------------------------
Title: Coronal magnetic field evolution under reconnective relaxation
Authors: Vekstein, G. E.; Priest, E. R.; Wolfson, R.
1994SSRv...70..303V Altcode:
The nonlinear evolution of a partially open coronal magnetic
configuration is considered, assuming that corona responds to
photospheric footpoint motions by small-scale reconnection events
that produce a relaxed lower-energy state while conserving the global
magnetic helicity of the system. The results of numerical calculations
for such a relaxed equilibrium show an essential role of the amount
of helicity injected to the closed-field region. If photospheric
perturbations are incoherent (small-scale shearing with inefficient
helicity injection), the relaxed state becomes close to an initial
potential field. In this case reconnective relaxation does not result
in a substantial global evolution, just providing heating of the
corona (Vekstein et al, 1993). On the contrary, sufficient injection
of the magnetic helicity can lead to a considerable restructuring
of the coronal magnetic configuration, with possible change of its
topology (formation of magnetic islands), and even catastrophic loss
of equilibrium (Wolfson et al, 1994)

---------------------------------------------------------
Title: Working group 2: Loops and prominences
Authors: Priest, E. R.
1994SSRv...70..221P Altcode:
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: Preflare State
Authors: Rust, David M.; Sakurai, Takashi; Gaizauskas, Victor; Hofmann,
Axel; Martin, Sara F.; Priest, Eric R.; Wang, Jing-Xiu
1994SoPh..153....1R Altcode:
Discussion on the preflare state held at the Ottawa Flares 22
Workshop focused on the interpretation of solar magnetograms and
of Hα filament activity. Magnetograms from several observatories
provided evidence of significant build up of electric currents in
flaring regions. Images of X-ray emitting structures provided a clear
example of magnetic relaxation in the course of a flare. Emerging
and cancelling magnetic fields appear to be important for triggering
flares and for the formation of filaments, which are associated with
eruptive flares. Filaments may become unstable by the build up of
electric current helicity. Examples of heliform eruptive filaments
were presented at the Workshop. Theoretical models linking filaments
and flares are briefly reviewed.

---------------------------------------------------------
Title: Energy Release in Solar Flares
Authors: Brown, John C.; Correia, Emilia; Farnik, Frantisek; Garcia,
Howard; Henoux, Jean-Claude; Larosa, Ted N.; Machado, Marcos E.;
Nakajima, Hiroshi; Priest, Eric R.
1994SoPh..153...19B Altcode:
Team 2 of the Ottawa FLARES 22 Workshop dealt with observational and
theoretical aspects of the characteristics and processes of energy
release in flares. Main results summarized in this article stress
the global character of the flaring phenomenon in active regions, the
importance of discontinuities in magnetic connectivity, the role of
field-aligned currents in free energy storage, and the fragmentation
of energy release in time and space.

---------------------------------------------------------
Title: Dynamic Coronal Heating by Magnetic Flux Interaction
Authors: Priest, E. R.
1994kofu.symp...93P Altcode:
The elements of a Converging Flux Model for heating X-ray bright points
are here put forward which also accounts for the cancelling magnetic
features that are usually observed to be present in the photosphere
below bright points and the jets that are observed with Yohkoh. The
model has three phases: a preinteraction phase in which two opposite
polarity photospheric fragments are unconnected magnetically; an
interaction phase when the fragments reconnect in the corona and
create the bright point and x-ray jet; a cancellation phase when
reconnection in the photosphere produces the cancelling magnetic
feature. An analytical model is presented together with preliminary
results of a numerical experiment and a three-dimensional modelling of
particular bright points observed by the NIXT telescope. Furthermore,
dynamic reconnection driven by footpoint motions in a bright point may
represent an elementary heating event that could be heating coronal
loops and coronal holes and not just bright points.

---------------------------------------------------------
Title: Nonlinear Evolution of the Coronal Magnetic Field under
Reconnective Relaxation
Authors: Wolfson, R.; Vekstein, G. E.; Priest, E. R.
1994ApJ...428..345W Altcode:
Recently, Vekstein et al. (Vekstein, Priest, &amp; Steele 1993) have
developed a model for coronal heating in which the corona responds
to photospheric footpoint motions by small-scale reconnection events
that bring about a relaxed state while conserving magnetic helicity but
not field-line connectivity. Vekstein et al. consider a partially open
field configuration in which magnetic helicity is ejected to infinity
on open field lines but retained in the closed-field region. Under this
scheme, they describe the evolution of an initially potential field,
in response to helicity injection, in the linear regime. The present
work uses numerical calculations to extend the model of Vekstein
et al. into the fully nonlinear regime. The results show a rise and
bulging of the field lines of the closed-field region with increasing
magnetic helicity, to a point where further solutions are impossible. We
interpret these solution-sequence endpoints as indicating a possible
loss of equilibrium, in the sense that a relaxed equilibrium state
may no longer be available to the corona when sufficient helicity has
been injected. The rise and bulging behavior is reminiscent of what
is observed in a helmet streamer just before the start of a coronal
mass ejection (CME), and so our model suggests that a catastrophic
loss of magnetic equilibrium might be the initiation mechanism for
CMEs. We also find that some choices of boundary conditions can result
in qualitative changes in the magnetic topology, with the appearance
of magnetic islands. Whether or not this behavior occurs depends on
the relative strengths of the fields in the closed- and open-field
regions; in particular, island formation is most likely when the open
field (which is potential) is strong and thus acts to confine the
force-free closed field. Finally, we show that the energy released
through reconnective relaxation can be a substantial fraction of the
magnetic energy injected into the corona through footpoint motions
and may be sufficient for heating the corona above active regions.

---------------------------------------------------------
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: Magnetic Energy Dissipation via Reconnective Relaxation in
Astrophysical Jets
Authors: Vekstein, G. E.; Priest, E. R.; Steele, D. C.
1994ApJS...92..111V Altcode:
The magnetic energy release through field-line reconnection is
considered for a force-free jet which expands (or contracts) radially
in response to variations in the confining external pressure. The
main new result obtained in comparison with previous publications
on the subject, where this mechanism has been proposed, is that a
self-consistent theory for the reconnective relaxation of the jet's
magnetic configuration has been constructed. It enables us to calculate
the magnetic energy dissipation rate inside a jet for an arbitrary
value of the ratio T_r_/Tnu_ where T_r_ is a typical
reconnection time and Tnu_, is the expansion time. Thus
these results provide a well-grounded support to the conclusion that
this magnetic energy dissipation mechanism could, in principle, power
the observed radio emission in extragalactic jets. Moreover, knowing
the observed radiation power (and, hence, the required energy supply)
it is possible to obtain information about the value of T_r_, which
enters in the theory as a phenomenological parameter.

---------------------------------------------------------
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: The Dynamics of Driven Magnetic Reconnection in Coronal Arcades
Authors: Rickard, G. J.; Priest, E. R.
1994SoPh..151..107R Altcode:
The dynamics of interacting coronal loops and arcades have recently been
highlighted by observations from theYohkoh satellite and may represent a
viable mechanism for heating the solar corona. Here such an interaction
is studied using two-dimensional resistive magnetohydrodynamic (MHD)
simulations. Initial potential field structures evolve in response to
imposed photospheric flows. In addition to the anticipated current sheet
about theX-point separating the colliding flux systems, significant
current layers are found to lie all the way along the separatrices that
intersect at theX-point and divide the coronal magnetic field into
topologically distinct regions. Shear flows across the separatrices
are also observed. Both of these features are shown to be compatible
with recent analytical studies of two-dimensional linear steady-state
magnetic reconnection, even though the driven system that has been
simulated is not strictly `open' in the sense implied by steady-state
calculations. The implications for future steady-state models are
also discussed. The presence of the neutral point also brings into
question any constant-density approximations that have previously been
used for such quasi-steady coronal evolution models. This results from
the intimate coupling between the neutral point and its separatrices
communicated via the gas pressure.

---------------------------------------------------------
Title: On the Maximum Energy Release in Flux Rope Models of Eruptive
Flares
Authors: Forbes, T. G.; Priest, E. R.; Isenberg, P. A.
1994SoPh..150..245F Altcode:
We determine the photospheric boundary conditions which maximize
the magnetic energy released by a loss of ideal-MHD equilibrium in
two-dimensional flux-rope models. In these models a loss of equilibrium
causes a transition of the flux rope to a lower magnetic energy state at
a higher altitude. During the transition a vertical current sheet forms
below the flux rope, and reconnection in this current sheet releases
additional energy. Here we compute how much energy is released by the
loss of equilibrium relative to the total energy release. When the
flux-rope radius is small compared to its height, it is possible to
obtain general solutions of the Grad-Shafranov equation for a wide
range of boundary conditions. Variational principles can then be
used to find the particular boundary condition which maximizes the
magnetic energy released for a given class of conditions. We apply
this procedure to a class of models known as cusp-type catastrophes,
and we find that the maximum energy released by the loss of equilibrium
is 20.8% of the total energy release for any model in this class. If
the additional restriction is imposed that the photospheric magnetic
field forms a simple arcade in the absence of coronal currents, then
the maximum energy release reduces to 8.6%.

---------------------------------------------------------
Title: General family of nonuniform reconnection models with
separatrix jets
Authors: Strachan, N. R.; Priest, E. R.
1994GApFD..74..245S Altcode:
The Nonuniform Magnetic Reconnection model presented by Priest and
Lee (1990) was the first to incorporate several features observed in
previous numerical models, in particular the highly curved field in the
inflow region, which relaxes one of the key assumptions of the classical
Almost-Uniform reconnection model of Petschek (1964). Also present is
a strong jet of plasma emerging from the reconnection region along the
separatrix and a negative current spike at the outflow from the central
diffusion region. In this paper we present a generalization of the
Priest-Lee model to include pressure gradients in the inflow region,
so relaxing the other main assumption of the Petschek and Priest-Lee
models, namely that the inflow is current-free or potential. This
produces several reconnection regimes similar to those described by
Priest and Forbes (1986) in their Unified Almost-Uniform solutions,
where significant pressure gradients are included, but their inflow,
like Petschek's, is almost-uniform with slightly curved field
lines. We investigate the behaviour of the inflow magnetic field
without linearizing about a uniform field. We study both a shockless,
incompressible outflow and one containing weak shocks, where the
type of reconnection is dictated by both the inflow and outflow
boundary conditions. In order to introduce a pressure gradient into
the Priest-Lee model an extra non-potential field in the x-direction
is added. This field has a uniform current and does not alter the
Y-point nature of the neutral points at the ends of the diffusion
region but does alter the position of the separatrix, where the flow
becomes singular, and hence the position of the Alfvenic discontinuity
and the strength of the plasma jet. The maximum reconnection rate
given by the model scales as (1 + c)(exp 3/2)R(sub me)(exp -1/2),
where the parameter c is a measure of the current and is such that
c greater than 0 gives fast- or slow-mode expansions and c less than
0 gives slow-mode compressions. These regimes are also found in the
Unified Almost-Uniform model. Fast reconnection with a reconnection
rate in excess of the pure Sweet-Parker scaling is therefore possible
in the flux pile-up regimes with c greater than 0. For a given large
magnetic Reynolds number, a fast reconnection rate of, say, 0.1 times
the Alfven speed is possible provided c is large enough (e.g. c greater
than 10 for R(sub me) = 10(exp 5)).

---------------------------------------------------------
Title: Magnetic Coronal Heating and Global Restructuring of the
Coronal Magnetic Field by Reconnective Relaxation
Authors: Vekstein, G.; Priest, E.; Steele, C.; Wolfson, R.
1994ASPC...68..194V Altcode: 1994sare.conf..194V
No abstract at ADS

---------------------------------------------------------
Title: Book-Review - Advances in Solar System Magnetohydrodynamics
Authors: Priest, E. R.; Hood, A. W.; Burlaga, L. F.
1994SSRv...67..225P Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Magnetohydrodynamics (With 87 figures)
Authors: Priest, Eric R.
1994plas.conf....1P Altcode: 1994SAAS...24....1P
Prelude The Sun Brief History Overall Properties The Atmosphere
Solar Activity The Solar Revolution Magnetohydrodynamic Equations
Flux Tubes Fundamental Equations Induction Equation The Lorentz Force
Concluding Comment Magnetohydrostatics Introduction Potential Fields
Force-Free Fields Magnetic Flux Tubes Magnetohydrodynamic Waves Sound
Waves Alfven Waves Compressional Alfven Waves Magnetoacoustic Waves
Effect of Magnetic Diffusion on Alfven Waves Shock Waves The Solar
Wind Introduction Parker Solution Effect of the Magnetic Field Magnetic
Reconnection Introduction Magnetic Annihilation (Sonnerup and Priest,
1975) Qualitative Effects of Reconnection Formation of a Current Sheet
Linear Reconnection Fast Steady-State Reconnection Coronal Heating
Introduction Magnetically Open Regions - Heating by Magnetic Waves
Magnetically Closed Regions - Heating by Current Sheets Self-Consistent
Model for Heating by MHD Turbulence Conclusion Prominences Observations
Formation of a Prominence Internal Structure External Structure A
Flux Tube Model Conclusion Solar Flares Introduction Energy Release
by Magnetic Reconnection Conditions for Flare Occurrence Conclusion
References

---------------------------------------------------------
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

---------------------------------------------------------
Title: Plasma Astrophysics
Authors: Kirk, J. G.; Melrose, D. B.; Priest, E. R.; Benz, A. O.;
Courvoisier, T. J. -L.
1994plas.conf.....K Altcode: 1994QB462.7.K57....; 1994plas.conf.....B
This volume presents the lecture notes of the 24th Advanced Course
of the Swiss Society for Astrophysics and Astronomy in March 1994 at
Les Diablerets. In three lectures on magnetohydrodynamics, on kinetic
plasma physics and on particle acceleration leading experts describe
the physical basis of their subjects and extend the discussion to
several applications in modern problems of astrophysics. In style and
presentation the texts are well-suited for graduate work in plasma
astrophysics, one of the very important tools of modern astronomy. The
themes developed in this book will be helpful in understanding many
processes in the universe from the solar corona to active galaxies.

---------------------------------------------------------
Title: Motions of Loops and Ribbons of Two-Ribbon Flares
Authors: Lin, J.; Forbes, T. G.; Priest, E. R.
1994ASPC...68..202L Altcode: 1994sare.conf..202L
No abstract at ADS

---------------------------------------------------------
Title: Heating of X-ray bright points and other coronal structures.
Authors: Priest, E. R.
1994GMS....84....1P Altcode:
A brief summary is given of the observations of X-ray bright points in
the solar corona and of their relation to cancelling magnetic features
in the underlying photosphere. A new Converging Flux Model for bright
points is proposed with three phases. In the Preinteraction Phase
photospheric magnetic fragments of opposite polarity are unconnected
and approach one another. In the Interaction Phase the X-ray bright
points is created by coronal reconnection. Finally, in the Cancellation
Phase the cancelling magnetic feature is produced by photospheric
reconnection. It is suggested that coronal reconnection driven by
footpoint motions may represent an elementary heating event that is
heating all coronal loops and not just bright points.

---------------------------------------------------------
Title: Aspects of Filament Evolution
Authors: Dumitrache, C.; Dinulescu, S.; Priest, E. R.
1994emsp.conf...65D Altcode:
No abstract at ADS

---------------------------------------------------------
Title: A Comparison of Theories and Numerical Experiments on Fast
Steady-State Reconnection
Authors: Priest, E. R.
1994swms.conf...93P Altcode:
No abstract at ADS

---------------------------------------------------------
Title: The Propagation of Sound Waves in a Randomly Magnetized
Medium. II. The Interaction of an Unsteady Wave Packet with an
Ensemble of Magnetic Flux Tubes
Authors: Ryutova, M. P.; Priest, E. R.
1993ApJ...419..371R Altcode:
The interaction of an unsteady wave packet of acoustic waves with plasma
containing ensembles of random magnetic flux tubes and the physical
mechanisms of energy transfer to the plasma are described. It is
shown that the propagation of an acoustic wave-packet in the randomly
magnetized solar atmosphere is accompanied by clear morphological
effects which include the spreading of the energy absorption region
over scales much larger than the size of the initial wave packet,
and which, in principle, can be observed. The regions of an efficient
energy input and their localization crucially depend on the distribution
of magnetic flux tubes in space and over their physical parameters
(including noncollinearity). The very existence of these effects is
determined by the fact that the solar atmosphere is randomly magnetized,
and magnetic flux tubes are essentially nonidentical. The results
may be useful for the diagnostics of the visible layers of the solar
atmosphere and magnetic helioseismology.

---------------------------------------------------------
Title: The Propagation of Sound Waves in a Randomly Magnetized
Medium. I. General Considerations
Authors: Ryutova, M. P.; Priest, E. R.
1993ApJ...419..349R Altcode:
We study the time-dependent response of a randomly magnetized medium
(such as the solar atmosphere) to the propagation of acoustic waves,
including energy transfer to the medium due to different physical
processes. It is shown that the details of the interaction of a sound
wave with an ensemble of magnetic flux tubes, and, in particular,
the maximum energy input, crucially depends on the magnetic filling
factor of the medium as well as on the distribution of the random
tubes in space. The interaction of acoustic waves and unsteady wave
packets with an ensemble of magnetic flux tubes reveals some simple
and important features, which, in principle, are observable. A most
important role in these effects is played by resonant interaction both
absorption and scattering of the sound wave by flux tubes. We focus
on the case when the incident wavelength (λ) is much larger than the
separation (d) between tubes, which is in turn much larger than the
tube radii (R). In the case of resonant absorption (an effect
similar to Landau damping in the collisionless plasma) the energy of
the incident acoustic wave is accumulated in the system of magnetic
flux tubes and causes the acoustic wave (of frequency ω) to damp at
a rate νL ∼ (R2/d2)ω. The energy
remains for a long time in the form of flux-tube oscillations. Then,
in a time ν-1rad which is much longer than the
damping time of the sound wave, the resonant flux tubes radiate their
energy as secondary acoustic (or MHD) waves, where νrad
∼ ωk2R2. The incident acoustic wave can also
be resonantly scattered with the main contribution coming from the
kink mode; it leads to a linear frequency shift and to the appearance
of incoherent noise without a preliminary build up of wave energy in
flux-tube oscillations. When the distribution of flux-tube natural
frequencies is broader than νL the Landau-like resonant
absorption process is more important than resonant scattering, but when
the distribution is narrow the tubes are essentially identical and
resonant absorption is generally absent so that resonant scattering
dominates. A nonlinear analysis allows us to find the maximum energy
input and the frequency shift and their dependence on the parameters
of medium. Also, a detailed treatment is given of Landau-like damping
due to excitation of sausage modes.

---------------------------------------------------------
Title: Time-dependent magnetic annihilation at a stagnation point
Authors: Anderson, C.; Priest, E. R.
1993JGR....9819395A Altcode:
Magnetic reconnection is a funcamental process that can take place in
astrophysical or laboratory plasmas. It occurs within regions of large
magnetic gradients where the magnetic field is no longer frozen to the
plasma but instead diffuses through it, releasing magnetic energy and
causing a change in the connectivity of the field lines. In particular,
on the Sun, magnetic reconnection is believed to play an important role
in coronal heating, X ray bright points, solar flares and cancelling
magnetic features. Here, an exact time-dependent solution of the MHD
equations for magnetic annihilation in response to a time-varying
stagnation point flow is presented. The main assumptions in this model
are that the magnetic field lines are straight (so that there is no
magnetic tension acting on the plasma) and the flow that carries the
field lines together is of stagnation point type. This a reasonable
model for the resistive MHD behavior near the X point of a reconnecting
field, especially when the central diffusion region is long, as in the
flux pile up regime (Priest and Forbes 1986). The general solution is
used to conduct a series of numerical experiments, namely, the evolution
of different initial magnetic profiles in a steady flow; the effect of a
sudden change in magnetic diffusivity on an initially steady state; the
effects of a velocity that either increases linearly in time or ramps
up from one steady value to another. The results exhibit the effects
of subtle imbalances in diffusion and advection but have the following
general features: (1) a diffusion layer is created, the thickness of
which is determined by the nature of the plasma flow; (2) the magnetic
field outside the diffusion region is determined by advection and will
either exponentially increase, exponentially decay or remain in a steady
state, depending on whether the initial magnetic profile By
varies as y-n has n less than 1, n greater than 1, or n =
1, respectively; and (3) the magnetic field within the diffusion layer
tends to respond to the advected magnetic field at its edge.

---------------------------------------------------------
Title: On the Problem of Magnetic Coronal Heating by Turbulent
Relaxation
Authors: Vekstein, G. E.; Priest, E. R.; Steele, C. D. C.
1993ApJ...417..781V Altcode:
Three developments in the basic theory for heating the solar
corona by Taylor-Heyvaerts relaxation are presented. First of all,
a general expression is derived for coronal heating in response
to small footpoint motions by intermediate relaxation to a state
that lies between the ideal MHD state and the lowest energy linear
force-free field. It depends on the ratio (ωτr) of the
relaxation time (τr) to the timescale (ω-1)
for footpoint motions: when this is much smaller than unity, there
is complete relaxation to the linear force-free state, and when it is
much larger the deviation from the ideal state is small but it is in
practice still sufficient to provide the observed heating. Second,
a well-known difficulty with linear force-free fields, namely, that in
a semi-infinite region such fields have infinite energy, is resolved:
it is shown that the only possible relaxed state is a potential field,
so that any excess magnetic helicity is ejected to infinity. Third,
it is suggested that in practice the solar corona may be in a state
not of complete relaxation but of partial relaxation, in which open
fields continually relax to a potential state and eject magnetic
helicity into the solar wind, while closed fields relax to a linear
force-free state by means of small-scale reconnections.

---------------------------------------------------------
Title: Conditions for the appearance of "bald patches" at the
solar surface
Authors: Titov, V. S.; Priest, E. R.; Demoulin, P.
1993A&A...276..564T Altcode:
A general criterion for the existence of sections of the photospheric
polarity inversion line where the overlying magnetic field lines
are parallel to the photosphere is presented. It is shown that
such sections, called "bald patches", exist for a wide range of
parameters in potential and constant-α force-free fields created by
four concentrated sources of magnetic flux. Bald patches appear when
the polarity inversion line is bent too much in an S-like manner. The
appearance of bald patches may be important for the formation of
prominences and possibly for solar flares.

---------------------------------------------------------
Title: MHD equilibria with flows in unifrom gravity. II. A class of
exact 2-D loop-like solutions.
Authors: Tsinganos, K.; Surlantzis, G.; Priest, E. R.
1993A&A...275..613T Altcode:
We present a novel class of two-dimensional MHD equilibria which emerge
as exact solutions of the coupled transfield and Bernoulli nonlinear
equations governing the dynamics of steady flows along magnetic lines
in an atmosphere that is horizontally compressible and vertically
stratified in the presence of a uniform gravitational field. The
topology of the solutions is analysed in detail and is found to be
controlled by a classical sonic and a novel X-type critical point
corresponding to a new characteristic speed for MHD wave propagation
in an inhomogeneous medium. A subclass of low Alfvén numbers loop-like
solutions is found for a mildly stratified atmosphere; for very strong
stratification no solutions exist while for moderate stratification only
periodic solutions are allowed. The results of the study are compared
with those of models without flows and models with low Alfvén number
flows while the approach followed here extends that of treating the flux
tube as rigid, or slender. Among our conclusions is that an increase
of the magnitude of the flow speed increases the height of the loops,
while for stronger flows there are no equilibrium solutions and it
is conjectured that the loop is disrupted. Thus, the general trends
emerging from this analysis may be contrasted with solar observations
wherein, for example, some solar loops extend over heights much higher
than those predicted by models without flows. Then, in the quiet Sun
and in chromospheric or coronal loops with plasma-β and Alfvén number
not much smaller than unity, the above nonlinear effects of the flows
should be taken into account.

---------------------------------------------------------
Title: Prominence Support in Helical Coronal Fields Formed by
Photospheric Motions
Authors: Ridgway, C.; Priest, E. R.
1993SoPh..146..277R Altcode:
We consider the effect of converging photospheric motions acting upon
a sheared cylindrically-symmetric line-tied arcade. The motions drive
a reconnection process whereby a helical structure is formed, in which
cool, dense plasma may accumulate at the lowest points of the helical
windings thus creating a prominence.

---------------------------------------------------------
Title: Magnetostatic Equilibria and Current Sheets in an Sheared
Magnetic Field with an X-Point
Authors: Vekstein, G.; Priest, E. R.
1993SoPh..146..119V Altcode:
When a two-dimensional magnetic field containing X-points is sheared,
in general current sheets appear all along the separatrices and the
X-points are structurally unstable. Their splitting into pairs of
cusp points near which the magnetic field is of self-similar form is
suggested. Solutions both inside and outside the cusps are obtained
and are matched together by the conditions of continuity of the flux
function and continuity of magnetic pressure.

---------------------------------------------------------
Title: Oscillations of a Quiescent Solar Prominence Embedded in a
Hot Corona
Authors: Oliver, R.; Ballester, J. L.; Hood, A. W.; Priest, E. R.
1993ApJ...409..809O Altcode:
The magneto-acoustic-gravity modes of vibration of a
Kippenhahn-Schlueter prominence model, surrounded by an external medium
representing the solar corona, are investigated. Modes which do not
disappear when the coronal region is removed are called internal modes,
while modes which do not disappear when the prominence is removed are
called external modes. Three modes were found which could be labeled
as either internal or external modes, since they do not disappear
when either region is removed. The results of this study suggest that
short-period oscillations in limb prominences may be caused by the
fundamental and first harmonics of the internal magneto-acoustic-gravity
slow and Alfven modes, while the reported long-period oscillations in
limb prominences may be the result of the hybrid slow mode. Short-period
oscillations lasting a few minutes and detected in filaments may be
related to internal fast modes, while the hybrid fast mode causes
vertical oscillations with periods of approximately 20 min.

---------------------------------------------------------
Title: Upper Limits for Impulsive Energy Release in Flux-Rope Models
of Eruptive Flares
Authors: Forbes, T. G.; Isenberg, P. A.; Priest, E. R.
1993BAAS...25.1198F Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Magnetohydrodynamic waves in a potential coronal arcade
Authors: Oliver, R.; Ballester, J. L.; Hood, A. W.; Priest, E. R.
1993A&A...273..647O Altcode:
An important result obtained from solar observations made in coronal
emission lines has been to establish the existence of periodic
oscillations or waves in the corona. These are detected, although
not always simultaneously, in line intensity, line-width or Doppler
velocity with oscillation periods of several minutes. For this reason,
we have studied the magnetohydrodynamic modes of oscillation of a
coronal potential arcade under different boundary conditions, solving
the wave equations for different density profiles. Our results show
that only the Alfvén and the fast mode exist in such a structure
and that the slow mode is absent, because of the assumed low-β
limit. The Alfvén mode produces a continuous spectrum of frequencies
with periods which can vary from zero to infinity, depending on the
assumed density profile. Due to the polarization direction of the plasma
motions, these waves could in principle be detected in front-view
observations of coronal arcades at the limb. Concerning the fast
mode, we have obtained several diagnostic diagrams using different
density profiles and boundary conditions. These diagrams would be
useful for comparing the predicted and observed periods. However,
the observational identification of fast modes could be harder since
we would need lateral observations of the coronal arcades.

---------------------------------------------------------
Title: Magnetic reconnection with large separatrix angles
Authors: Yan, M.; Lee, L. C.; Priest, E. R.
1993JGR....98.7593Y Altcode:
A two-dimensional incompressible MHD simulation model is developed to
study magnetic reconnection with large magnetic separatrix angles. It
is found that the normal magnetic field on the inflow boundary plays
an important role in determining the reconnection configuration. When
the normal magnetic field on the inflow boundary is large, steady state
reconnection with large magnetic separatrix angles is obtained. When
the normal magnetic field is small, a localized resistivity is required
to maintain the steady state Petschek-like reconnection configuration,
which has small magnetic separatrix angles. Field-aligned plasma jets
are observed when the normal magnetic field and magnetic Reynolds
number Rm are large. The plasma jets are located slightly
downstream of the magnetic separatrices and each plasma jet consists
of two parts: (1) a slow-mode shock and (2) a fast-mode compressional
structure. The length and width of the diffusion region depend on the
values of Me, Rm, and the normal magnetic field
on the inflow boundary. For a fixed normal inflow magnetic field the
diffusion region becomes longer and thinner as Me becomes
larger; the width of the diffusion region decreases with an increasing
Rm, while the length of the diffusion region has a minimum
value at about Rm=500. The length of the diffusion region
increases with a decreasing normal magnetic field, while the width
does not change remarkably. It is also found that the multiple X line
bursty reconnection tends to occur with the development of tearing-mode
instability in cases with a large Me, a large Rm,
or a small normal magnetic field on the inflow boundary.

---------------------------------------------------------
Title: A Model for an Inverse Polarity Prominence Supported in a
Dip of a Quadrupolar Region
Authors: Demoulin, P.; Priest, E. R.
1993SoPh..144..283D Altcode:
We investigate the formation and support of solar prominences in a
quadrupolar magnetic configuration. The prominence is modeled as a
current sheet with mass in equilibrium in a two-dimensional field. The
model possesses an important property which is now thought to be
necessary, namely that the prominence forms within the dip, rather
than the dip being created by the prominence.

---------------------------------------------------------
Title: Book Review: Physical processes in solar flares / Kluwer, 1991
Authors: Priest, E. R.
1993SoPh..144..203P Altcode: 1993SoPh..144..203S
No abstract at ADS

---------------------------------------------------------
Title: Book Review: Solar interior and atmosphere / U Arizona
Press, 1991
Authors: Heinzel, P.; Priest, E. R.; Heinzel, P.
1993SoPh..144..203H Altcode: 1993SoPh..144..203C
No abstract at ADS

---------------------------------------------------------
Title: Two-dimensional models for solar and stellar winds -
Hydrodynamic effects
Authors: Lima, J. J. G.; Priest, E. R.
1993A&A...268..641L Altcode:
. A new family of solutions of the system of steady hydrodynamic
equations describing the axisymmetric outflow originating from a
rotating central object, surrounded by a compressible, non-polytropic
and inviscid fluid, is deduced. The system is solved self-consistently
without any explicit assumption on the form of the velocity field
and density; the solutions result purely from the assumed set of
equations, the boundary conditions and the assumption of separation
of variables. The analysis gives three dimensionless parameters
that control the degree of collimation of the outflow as well as the
anisotropy of the density distribution. The solution reproduces in a
natural way the presence of jets around the polar axis and of disks
around the equator that are observed in many astrophysical objects.

---------------------------------------------------------
Title: On the nonlinear theory of the radiation-driven thermal
instability of a magnetized plasma
Authors: Meerson, B.; Priest, E. R.; Steele, C. D. C.
1993GApFD..71..243M Altcode:
The nonlinear evolution of perturbations in a magnetized plasma
subject to the radiation-driven thermal instability (RDTI) is
investigated analytically in a simplified model. The perturbed plasma
motions are assumed to be one-dimensional and perpendicular to the
magnetic field. The intermediate- and long-wavelength limits of the
RDTI are considered. In the former limit, the force balance sets in
rapidly, on the magneto-acoustic time scale and we assume the total
(thermal-magnetic) pressure remains constant. By transforming to
Lagrangian variables, the problem is reduced to a single generalized
reaction-diffusion equation, which is employed to analyze the two
following stages of the RDTI. The first develops on the radiative time
scale, when the heat conduction is insignificant, while the second
usually occurs on a much longer, heat conduction-related time scale. For
the first stage, a simple analytical solution is found, which describes
the development of a strong plasma stratification (coexisting hot
rarefied phase and cool dense phase) across the magnetic field. Slow
erosion of the stratification in the form of almost uniform motion,
collision and "annihilation" of the inter-phase boundaries generally
occurs at the second stage.

---------------------------------------------------------
Title: The collapse of an X-type neutral point to form a reconnecting
time-dependent current sheet
Authors: Titov, Vyacheslav S.; Priest, Eric R.
1993GApFD..72..249T Altcode:
A general method for solving the two-dimensional time-dependent ideal
MHD equations is developed when the Alfven-Mach number is small and the
Mach number large in comparison with unity. The method is applied to the
problem of the time-dependent nonlinear collapse of an X-type neutral
point. A current sheet is formed which grows in a self-similar manner.

---------------------------------------------------------
Title: MHD Waves in a Solar Prominence
Authors: Oliver, R.; Ballester, J. L.; Hood, A. W.; Priest, E. R.
1993ASSL..183..191O Altcode: 1993pssc.symp..191O
No abstract at ADS

---------------------------------------------------------
Title: Implications of Rapid Footpoint Motions of Photospheric Flux
Tubes for Coronal Heating
Authors: Choudhuri, Arnab R.; Auffret, Herve; Priest, Eric R.
1993SoPh..143...49C Altcode:
Some recent observations at Pic-du-Midi (Mulleret al., 1992a) suggest
that the photospheric footpoints of coronal magnetic field lines
occasionally move rapidly with typical velocities of the order 3 km
s−1 for about 3 or 4 min. We argue that such occasional
rapid footpoint motions could have a profound impact on the heating of
the quiet corona. Qualitative estimates indicate that these occasional
rapid motions can account for the entire energy flux needed to heat the
quiet corona. We therefore carry out a mathematical analysis to study
in detail the response of a vertical thin flux tube to photospheric
footpoint motions in terms of a superposition of linear kink modes
for an isothermal atmosphere. We find the resulting total energy that
is asymptotically injected into an isothermal atmosphere (i.e., an
atmosphere without any back reflection). By using typical parameter
values for fast and slow footpoint motions, we show that, even if the
footpoints spend only 2.5% of the time undergoing rapid motions, still
these rapid motions could be more efficient in transporting energy to
the corona than the slow motions that take place most of the time.

---------------------------------------------------------
Title: Coronal Heating Mechanisms
Authors: Priest, E. R.
1993ASSL..183..515P Altcode: 1993pssc.symp..515P
No abstract at ADS

---------------------------------------------------------
Title: Unsteady Wave-Packet in the Random Ensemble of Magnetic Flux
Tubes: Acoustic Halos
Authors: Ryutova, M.; Priest, E.
1993ASPC...46..554R Altcode: 1993mvfs.conf..554R; 1993IAUCo.141..554R
No abstract at ADS

---------------------------------------------------------
Title: Steady MHD Flows in Uniform Gravity
Authors: Tsinganos, K.; Surlantzis, G.; Priest, E. P.
1993ASSL..183..623T Altcode: 1993pssc.symp..623T
No abstract at ADS

---------------------------------------------------------
Title: Quiescent Solar Prominences: A Two-Dimensional Model
Authors: Oliver, R.; Ballester, J. L.; Priest, E. R.
1993ASSL..183..187O Altcode: 1993pssc.symp..187O
No abstract at ADS

---------------------------------------------------------
Title: Does fast magnetic reconnection exist?
Authors: Priest, E. R.; Forbes, T. G.
1992JGR....9716757P Altcode:
The classical Petschek model of fast, steady state reconnection
has been generalized in two families of reconnection regimes. The
first family, which we refer to as “almost uniform,” models the
reconnection of nearly uniform, antiparallel magnetic fields, and
it includes Petschek's model as a special case. The second family,
which we refer to as nonuniform, models the reconnection of strongly
curved magnetic fields, and it includes separatrix jets and reversed
current spikes at the ends of the diffusion region. In general,
both families contain regimes having fast reconnection rates,
but we show here that these fast reconnection regimes do not occur
when the boundary conditions often used in numerical experiments
are adopted. In 1986, D. Biskamp carried out a series of numerical
experiments to check Petschek's prediction that the maximum reconnection
rate should scale with the magnetic Reynolds number, Rme,
as [ln(Rme)]-1. Biskamp found that the maximum
reconnection rate in his experiments did not scale in this way but
instead as Rme-1/2. Because this corresponds to
the scaling predicted by the slow reconnection theory of Sweet (1958)
and Parker (1957), Biskamp has argued that his numerical experiments
show that fast reconnection does not exist at high magnetic Reynolds
numbers. However, by applying boundary conditions similar to Biskamp's
to the “nonuniform” family of reconnection regimes, we are able
to explain Biskamp's scaling results and to explain why he did not
achieve fast reconnection in his numerical experiments. Therefore,
we conclude that numerical experiments with suitably designed boundary
conditions are highly likely to exhibit fast reconnection and that such
reconnection is a common process in astrophysical and space plasmas.

---------------------------------------------------------
Title: Magnetohydrodynamic Waves in a Solar Prominence
Authors: Oliver, R.; Ballester, J. L.; Hood, A. W.; Priest, E. R.
1992ApJ...400..369O Altcode:
The presence of oscillations in solar prominences has been known for
a long time, and at first they were thought to be initiated by shock
waves emitted by a flare. However, short- and long-period oscillations,
not related to flares, have been abundantly reported during recent
years. In limb prominences they have been detected mainly in the
velocity field, whereas in filaments they are hardly detected at
all. In this paper we try to provide a theoretical explanation for
these oscillations by investigating the magneto-acoustic-gravity modes
of vibration of a Kippenhahn-Schlueter prominence model. First of
all, we obtain the magneto-acoustic modes for a very thin prominence
with horizontal magnetic field, and later we solve the full problem
numerically. Our results suggest that short-period oscillations detected
in limb prominences could be due to the fundamental and first harmonic
of the magneto-acoustic-gravity slow modes of the prominence itself,
while the long-period oscillations could be due to global oscillations
of the whole surrounding arcade.

---------------------------------------------------------
Title: Thermal profiles of coronal loops - gravitational
considerations.
Authors: Steele, Colin D. C.; Priest, E. R.
1992ESASP.348..193S Altcode: 1992cscl.work..193S
Equations of thermal equilibrium along coronal magnetic loops are
solved in the absence and presence of gravity. Without gravity, four
categories of loop are found - hot, cool, hot-cool and warm. Which
categories exist depends on two parameters involving the heating,
the loop length and the footpoint pressure. Including gravity, the
longer hot loops suffer a decrease in their summit temperatures. The
hot-cool loops are restricted to a very small range of lengths. A
hot-cool arcade can still exist if either the coronal heating or the
loop footpoint pressure varies. When the cross-sectional area does
not vary from footpoint to summit, hot-cool loops may exist only for
unrealistically low gravity. However, causing the cross-sectional area
to vary may allow hot-cool loops to exist for realistic gravity.

---------------------------------------------------------
Title: Book-Review - Dynamics and Structure of Quiescent Solar
Prominences
Authors: Priest, E. R.; Sakurai, T.
1992SSRv...61..430P Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Solar coronal magnetic structure.
Authors: Priest, E. R.
1992ESASP.346...13P Altcode: 1992ssts.rept...13P
The solar corona has a four-fold structure of coronal loops, coronal
holes, X-ray bright points and prominences, which evolve usually
slowly but often dramatically in response to photospheric footpoint
motions and which are dominated by the magnetic field. A brief review
is given of recent developments in the theory for heating the corona
including a self-consistent MHD turbulence mechanism and new ways of
creating current sheets by shearing force-free fields or collapsing
X-points. Prominences may be formed in huge flux tubes in response to
flux cancellations at the photosphere, and flares may result from a
magnetic catastrophe.

---------------------------------------------------------
Title: A Model for the Fibril Structure of Normal Polarity Solar
Prominences
Authors: Steele, C. D. C.; Priest, E. R.
1992SoPh..140..289S Altcode:
A normal-polarity prominence is modelled as a series of cool fibrils
set in the hotter corona. Equations of magnetostatic equilibrium
are solved and each fibril corresponds to a dip in the mgnetic
field. The ratio of fibril width to interfibril spacing is dependent
on the prominence-coronal temperature ratio and the ratio of plasma
to magnetic pressure. The prominence mass is found to depend on the
square of the magnetic field strength. When variations along the
prominence are allowed in addition to those across the prominence,
an apparently random pattern of fibrils results.

---------------------------------------------------------
Title: Fast magnetic reconnection with small shock angles
Authors: Yan, M.; Lee, L. C.; Priest, E. R.
1992JGR....97.8277Y Altcode:
The steady state fast magnetic reconnection process with small
separatrix angles proposed by Pestchek (1964) and generalized
by Priest and Forbes (1986) is studied by a two-dimensional
incompressible magnetohydrodynamic (MHD) code. In the code a uniform
tangential magnetic field and normal plasma speed are specified on
the inflow boundary. On the outflow boundary the tangential flow
speed and tangential magnetic field are specified to be those in
the Priest-Forbes' model in order to obtain different reconnection
regimes. In our simulations both a uniform and a nonuniform resistivity
are used. For a nonuniform resistivity model in which the resistivity
in the outflow region is highly reduced, our simulations are in many
aspects consistent with those in the analytical results. A steady
state magnetic reconnection configuration with small separatrix angle
is obtained. The plasma is heated and accelerated by the current
sheets associated with slow shocks. We also obtain various regimes
predicted by Priest and Forbes for different boundary conditions
on the outflow boundary which are characterized by a parameter
b0. They are a weak fast-mode expansion (b0=0),
slow-mode compression (b0&lt;0), slow-mode expansion
(b0&gt;=0), and a hybrid regime of fast-mode and slow-mode
expansion (0&lt;b0&lt;1). The width and length of the
current sheet for different parameters obtained in the simulations
are found to be consistent with theoretical values. When the magnetic
Reynolds number or the reconnection rate given on the inflow boundary
increases, the diffusion region becomes smaller. However, for cases
with a uniform resistivity imposed in the simulation domain it is found
that the diffusion region tends to lengthen indefinitely and no steady
state configuration is obtained. Magnetic islands are usually formed
at the later stage of the simulation.

---------------------------------------------------------
Title: Book-Review - Advances in Solar System Magnetohydrodynamics
Authors: Priest, E. R.; Hood, A. W.; Hartquist, T. W.
1992Ap&SS.192..159P Altcode:
No abstract at ADS

---------------------------------------------------------
Title: A Self-consistent Turbulent Model for Solar Coronal Heating
Authors: Heyvaerts, J.; Priest, E. R.
1992ApJ...390..297H Altcode:
The rate of solar coronal heating induced by the slow random
motions of the dense photosphere is calculated in the framework of an
essentially parameter-free model. This model assumes that these motions
maintain the corona in a state of small-scale MHD turbulence. The
associated dissipative effects then allow a large-scale stationary
state to be established. The solution for the macroscopic coronal
flow and the heating flux is first obtained assuming the effective
(turbulent) dissipation coefficients to be known. In a second step
these coefficients are calculated by the self-consistency argument
that they should result from the level of turbulence associated with
this very heating flux. For the sake of tractability the derivation
is restricted to a two-dimensional situation where boundary flows are
translationally symmetric. The resulting value of the heating rate and
the predicted level of microturbulent velocity compare satisfactorily
with the observational data.

---------------------------------------------------------
Title: The properties of sources and sinks of a linear force-free
field
Authors: Demoulin, P.; Priest, E. R.
1992A&A...258..535D Altcode:
In a highly conducting plasma, the magnetic field topology
determines where, for example, current sheets can form, which is
of great importance as a potential coronal heating source. With the
classical extrapolation of a continuous weak photospheric field, the
determination of topology is in general a difficult challenge. Because
of the concentration of the photospheric field at intense flux tubes in
supergranulation boundaries a more realistic field representation may
be a description in terms of magnetic singularities located just below
the photosphere. In this paper we analyze in detail the generalization
to linear force-free fields of the standard multipole expansion
for singular potential fields. Solutions are presented in spherical
coordinates with the constraint that all singularities are located
in the half-space z is less than 0 below the solar photospheric plane
(z = 0). A great variety of solutions is shown to exist depending on
two continuous and one discrete parameter. The properties of monopole
and dipole solutions in particular are discussed and it is shown that
isolated magnetic charges exist only in the potential limit and not
in a linear force-free field.

---------------------------------------------------------
Title: The Fibril Structure of Prominences
Authors: Hood, A. W.; Priest, E. R.; Anzer, U.
1992SoPh..138..331H Altcode:
In this paper we present several magneto-hydrostatic equilibrium
models for prominences with fibril-like fine structure. For all the
models ad hoc temperature profiles are used without discussing the
energetics. For our models we assume fine structure to occur either
across the prominence axis or along it. This approach is intended
as a first step towards more realistic models based upon a series of
vertical fibril structures.

---------------------------------------------------------
Title: A Family of Two-Dimensional Nonlinear Solutions for Magnetic
Field Annihilation
Authors: Jardine, M.; Allen, H. R.; Grundy, R. E.; Priest, E. R.
1992JGR....97.4199J Altcode:
We present a family of nonlinear solutions for magnetic field
annihilation in two dimensions. These solutions include fully the
effects of viscosity and resistivity and are a generalization of
the Sonnerup and Priest model, where an irrotational stagnation
point flow carriers straight field lines toward a long, thin current
sheet. Here, we allow for vorticity in the inflow. When this is low,
there is a unique solution for the flow and magnetic field. The current
sheet adjusts its dimensions to accommodate different inflows. It is
widest for a negative imposed vorticity and increases in width as the
resistivity or viscosity is increased. When the imposed vorticity
is large and negative, however, the solutions become nonunique,
the flow pattern becomes cellular, and current sheets develop at the
cell boundaries. These results, then, show that it is possible to have
many more different types of inflow matched to full solutions for the
current sheet than have been considered hitherto.

---------------------------------------------------------
Title: Prominence Sheets Supported by Constant-Current Force-free
Fields. II. Imposition of Normal Photospheric Field Component and
Prominence Surface Current
Authors: Ridgway, C.; Amari, T.; Priest, E. R.
1992ApJ...385..718R Altcode:
Attention is given to a method of constructing longitudinally invariant
magnetic field configurations in which a symmetric finite vertical
current sheet is in magnetohydrostatic equilibrium between the
combined forces exerted by a background constant-current force-free
field and a uniform gravitational field. Both the normal magnetic
field component along the photosphere and the current density along
the prominence sheet are imposed as functions of position. The method
is used to generate both N- and I-type configurations by selecting a
convenient form for the imposed functions. Consideration is given to
the evolution of these configurations as the strength of the current
(and hence the mass) is increased while all other parameters are held
fixed. It is shown that in general the sheet loses equilibrium near
its upper extremity as I is increased beyond a certain value.

---------------------------------------------------------
Title: Magnetic Flipping: Reconnection in Three Dimensions Without
Null Points
Authors: Priest, E. R.; Forbes, T. G.
1992JGR....97.1521P Altcode:
In three dimensions, magnetic reconnection may take place in a sheared
magnetic field at any singular field line, where the nearby field
has X-type topology in planes perpendicular to the field line and
where an electric field is present parallel to the field line. In
the ideal region around the singular line there will, in general,
be singularities in the plasma flow and electric field, both at the
singular line and at “magnetic flipping layers,” which are remnants
of local magnetic separatrices. In the absence of a three-dimensional
magnetic neutral point or null point, reconnection of field lines can
still occur by a process of magnetic flipping, in which the plasma
crosses the flipping layers but the field lines rapidly flip along
them by magnetic diffusion. Depending on the boundary conditions,
there may be two or four flipping layers which converge on the singular
line. A boundary layer analysis of a flipping layer is given in which
the magnetic field parallel to the layer decreases as one crosses it
while the plasma pressure (or magnetic pressure associated with the
field along the singular line) increases. The width of the flipping
layer decreases with distance from the singular line.

---------------------------------------------------------
Title: Structural instability of nonlinear Alfven waves
Authors: Nocera, L.; Priest, E. R.
1992MmSAI..63..773N Altcode:
Results are presented of a study of the nonlinear dynamics of O(sq
rt epsilon) amplitude Alfven waves forced into a solar coronal loop
where they drive and interact with slow magnetosonic waves. The
evolution of the loop is described as the solution of a simple forced
dissipative dynamical system led to be a regular Galerkin analysis of
the one-dimensional MHD equations.

---------------------------------------------------------
Title: Basic Magnetic Configuration and Energy Supply Processes for
an Interacting Flux Model of Eruptive Solar Flares
Authors: Priest, E. R.
1992LNP...399...15P Altcode: 1992esf..coll...15P; 1992IAUCo.133...15P
No abstract at ADS

---------------------------------------------------------
Title: Book-Review - Advances in Solar System Magnetohydrodynamics
Authors: Priest, E. R.; Hood, A. W.; Ghizaru, M.
1992RoAJ....2...95P Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Magnetohydrodynamic Equilibria and Cusp Formation at an X-Type
Neutral Line by Footpoint Shearing
Authors: Vekstein, G. E.; Priest, E. R.
1992ApJ...384..333V Altcode:
Two-dimensional magnetohydrodynamic equilibria in the presence of
an X-type natural line are investigated in the context of ideal
magnetohydrodynamics. The poloidal field geometry with a neutral
X-line is shown to be inconsistent with general sheared magnetostatic
equilibria. Shearing of an initial potential poloidal magnetic field
results in the splitting of an X-point into a current sheet with
cusp-points (or Y-points in some particular cases) at its ends. The
global geometry of the poloidal magnetic field produced by different
types of footpoint shearing motions is discussed.

---------------------------------------------------------
Title: Steady flows in solar magnetic solutions structures-a class
of exact MHD solutions
Authors: Ville, De. A.; Priest, E. R.
1991GApFD..59..253V Altcode:
A class of exact solutions to the steady, two-dimensional
magnetohydrodynamic equations ina cylindrical geometry is
presented. These may model both closed and open magnetic structures
found in the solar atmosphere. For closed structures, it is found that
increasing the flow speed causes the summit of the arcade of closed
magnetic fieldlines to rise. Parameter ranges also exist where the
solution has regions of open and closed field, and so the solutions may
be relevant for modelling flows in solar magnetic structures such as
coronal streamers, X-ray bright points coronal plumes and coronal holes.

---------------------------------------------------------
Title: Steady flows in solar magnetic structures - a class of exact
MHD solutions.
Authors: de Ville, A.; Priest, E. R.
1991GApFD..59..253D Altcode:
A class of exact solutions to the steady, two-dimensional
magnetohydrodynamic equations in a cylindrical geometry is
presented. These may model both closed and open magnetic structures
found in the solar atmosphere. For closed structures, it is found that
increasing the flow speed causes the summit of the arcade of closed
magnetic fieldlines to rise. Parameter ranges also exist where the
solution has regions of open and closed field, and so the solutions
may be relevant for modelling flows in solar magnetic structures
such as coronal streamers, X-ray bright points, coronal plumes and
coronal holes.

---------------------------------------------------------
Title: Book-Review - Advances in Solar System Magnetohydrodynamics
Authors: Priest, E. R.; Hood, A. W.
1991JBAA..101..300P Altcode:
No abstract at ADS

---------------------------------------------------------
Title: The Magnetohydrodynamics of Energy Release in Solar Flares:
Discussion
Authors: Phillips, K. J. H.; Priest, E. R.; Haines, M. G.
1991RSPTA.336..380P Altcode:
No abstract at ADS

---------------------------------------------------------
Title: The Magnetohydrodynamics of Energy Release in Solar Flares
Authors: Priest, E. R.
1991RSPTA.336..363P Altcode:
A large solar flare is thought to occur when a sheared magnetic arcade
loses equilibrium or goes unstable and erupts, and drives magnetic
reconnection in the stretched-out magnetic field lines. These two
key processes of magnetic eruption and magnetic energy conversion by
reconnection are reviewed briefly, with an account of recent analytical
and numerical models. When the height or length of a prominence in a
sheared coronal arcade is too great it may erupt and drive the formation
and reconnection of a current sheet below it. Recent progress in fast
steady-state reconnection theory has explained many puzzling features
of numerical experiments, and has shown how a new process of magnetic
flipping can reconnect fields in three dimensions. Also numerical
modelling of the formation of flare loops and ribbons by reconnection
has accounted for many observational properties.

---------------------------------------------------------
Title: Prominence Sheets Supported by Constant-Current Force-free
Fields. I. Imposition of Normal Magnetic Field Components at the
Current Sheet and the Photosphere
Authors: Ridgway, C.; Amari, T.; Priest, E. R.
1991ApJ...378..773R Altcode:
We present an analytical model for the support of a prominence
(represented by a sheet of mass and current) in a constant-current
force-free field. The model allows us to produce both normal-type and
inverse-type configurations which are in equilibrium everywhere along
the sheet and for which the field is locally bounded.

---------------------------------------------------------
Title: Bistability of a forced hydromagnetic cavity
Authors: Nocera, Luigi; Priest, Eric R.
1991JPlPh..46..153N Altcode:
We study the nonlinear stability of a one-dimensional hydromagnetic
cavity into which Alfvén waves are fed by harmonic shear motions
of its boundaries and where they interact with slow magnetosonic
waves. We use characteristic conditions for the outgoing and ingoing
Alfven waves at the boundaries where the magnetosonic oscillations are
required to vanish. Forcing of Alfven waves takes place at a frequency
close to the eigenfrequency of the lowest-order mode of the cavity. We
let the frequency detuning δω vary as a free parameter together
with the amplitude of the forcing, the plasma β and the compressive
Reynolds number Re0. Given these last three parameters and
varying δω, we calculate the amplitude of the nonlinear equilibrium
state of the cavity as the stationary solution of a simple forced,
dissipative dynamical system that governs the evolution of the cavity
over a slow time scale and to which we are led by multiple-scale and
Galerkin analyses of the one-dimensional MHD equations. This amplitude
is a multi-valued function of δω (bistability), and we discuss the
possibility of nonlinear stabilization of the Alfven wave by locking
it in one of the bistable states. This amplitude undergoes saddle-node
bifurcations: we calculate the two values of δω at which this occurs
and the lowest value of the Reynolds number (27/2) for this
to happen. We show that the magnetic energy density released during a
bistable transition scales as (Re0)2; it has a
maximum at β = 1 - (⅔)½ and it may amount to a substantial part of
the energy originally stored in the unperturbed cavity. The magnetic
power density released scales as (Re0)3 and has
a maximum at β = 1 ± (⅓)½5. We conclude that the cavity is a good
site for plasma heating such as that of the solar corona.

---------------------------------------------------------
Title: Advances in solar system magnetohydrodynamics
Authors: Priest, Eric R.; Hood, Alan W.
1991gamp.conf.....P Altcode:
Most of the solar system is in the plasma state and its subtle
non-linear interaction with the magnetic field is described for many
purposes by the equations of magnetohydrodynamics (MHD). Over the
past few years this important and complex field has become one of the
most actively pursued areas of research, with increasingly diverse
applications in geophysics, space physics and astrophysics. This
book examines the basic MHD topics, such as equilibria, waves,
instabilities and reconnection and examines each in the context of
different areas that utilize MHD. Many of the world's leading experts
have contributed to this volume, which has been edited by two of the
key enthusiasts. It is hoped that it can help the reader to appreciate
and understand the common threads between the different branches of
magnetohydrodynamics. This book will be a timely exposition of recent
advances made in the field.

---------------------------------------------------------
Title: A Two-Dimensional Model for a Solar Prominence - Effect of
an External Magnetic Field
Authors: Oliver, R.; Ballester, J. L.; Priest, E. R.
1991SoPh..134..123O Altcode:
Using analytical approximations we study the effects of different
external magnetic configurations on the half-width, mass, and internal
magnetic structure of a quiescent solar prominence, modelled as a thin
vertical sheet of cool plasma. Firstly, we build up a zeroth-order
model and analyse the effects produced by a potential coronal field
or a constant-α force-free field. This model allows us to obtain
the half-width and mass of the prominence for different values of
the external field, pressure and shear angle. Secondly, the effects
of these external magnetic configurations on a two-dimensional model
proposed by Ballester and Priest (1987) are studied. The main effects
are a change of the half-width with height, an increase of the mass,
a decrease of the magnetic field strength with height and a change in
the shape of the magnetic field lines.

---------------------------------------------------------
Title: Thermal Equilibria of Isobaric Coronal Magnetic Arcades
Authors: Steele, C. D. C.; Priest, E. R.
1991SoPh..134...73S Altcode:
A coronal magnetic arcade can be thought of as consisting of an
assembly of coronal loops. By solving equations of isobaric thermal
equilibrium along each loop and assuming a base temperature of
2 × 104 K, the thermal structure of the arcade can be
found. The possible thermal equilibria can be shown to depend on two
parameters L*p* and h*/p*
representing the ratios of cooling (radiation) to condu and heating
to cooling, respectively. Arcades can contain four types of loops:
hot loops with summits hotter than 400000 K; cool loops at temperatures
less than 80000 K along their lengths; hot-cool loops with cool summits
and cool footpoints but hotter intermediate portions; and warm loops,
cooler than 80000 K along most of their lengths but with summits as
hot as 400000 K. Two possibilities for coronal heating are considered,
namely a heating that is independent of magnetic field and a heating
that is proportional to the square of the local magnetic field. When
the arcade is sheared the thermal structure of the arcade may change,
leading in some cases to non-equilibrium or in other cases to the
formation of a cool core.

---------------------------------------------------------
Title: Advances in Solar System Magnetohydrodynamics, 1991
Authors: Priest, Eric R.; Hood, Alan W.
1991assm.conf.....P Altcode: 1991QB460.A38......
Most of the solar system exists in the plasma state (the fourth state
of matter). Its subtle nonlinear interaction with the magnetic field
can be described by the equations of magnetohydrodynamics (MHD). Over
the past few years this important and complex field of research has
been actively pursued and increasingly diversely applied to the fields
of geophysics, space physics, and astrophysics. It is, for instance,
relevant to the study of many dynamic phenomena such as solar flares,
and the origins of magnetic fields in the Sun and the Earth. This book
examines basic MHD topics, such as equilibria, waves, instabilities,
and reconnection, and examines each in the context of different areas
that utilize MHD. Many of the world's leading experts have contributed
to this volume, which has been edited by two of the key enthusiasts. It
is hoped that it will help researchers to appreciate and understand
the common threads among the different branches of magnetohydrodynamics.

---------------------------------------------------------
Title: Instability of a prominence supported in a linear force-free
field. II - Effect of twist or flux conservation
Authors: Demoulin, P.; Ferreira, J.; Priest, E. R.
1991A&A...245..289D Altcode:
Ideal MHD equations are used to study the vertical stability of
a prominence in a linear force-free field in two dimensions. The
prominence is modeled by a line current in equilibrium at a height
between the background magnetic force, the repulsion of image currents
and gravity. Attention is given to boundary conditions, current
evolution, parameters, stability results with boundary conditions,
and the effects of lateral boundaries.

---------------------------------------------------------
Title: Thermal Equilibria of Coronal Magnetic Loops with Non Constant
Cross-Secitonal Area
Authors: Steele, C. D. C.; Priest, E. R.
1991SoPh..132..293S Altcode:
Equations of thermal equilibrium along coronal loops are solved in the
absence of gravity but where the cross-sectional area changes along the
loop. The footpoint temperature is assumed to be 2 × 104
K. Several fundamental types of solution are found, namely hot loops,
cool loops, hot-cool loops (where the footpoints and summits are cool
but the intermediate parts are hotter) and warm loops (cool along most
of their lengths except the summits). On increasing the cross-sectional
area the summit temperature generally increases slightly except for
warm loops where no increase in temperature is recorded and hot-cool
loops where a dramatic increase in summit temperature may occur. The
cool and hot-cool loops may model elementary fibril structures within
prominences.

---------------------------------------------------------
Title: Formation of current sheets in force-free magnetic fields.
Authors: Vekstein, G.; Priest, E. R.; Amari, T.
1991A&A...243..492V Altcode:
It is shown that the evolution of coronal magnetic fields in response
to slow photospheric footpoint motions in general produces states
with current sheets rather than smooth force-free equilibria. When a
separatrix surface is present, shearing motions of a two-dimensional
double arcade in general produce a current sheet all along the
separatrix, whether the motions are infinitesimal or of finite
amplitude; local singularities in the horizontal field component
occur at the footpoints of the separatrix where it becomes tangential
to the solar surface although the flux function is regular there. The
particular case of equal lobes and an antisymmetric shear gives a smooth
field without current sheets. When there is an X-type neutral line (or
separator) present, converging or divering motions create a current
sheet with Y-points at its ends; shearing motions create a current
sheet all along the separattrices, which coalesce at two cusp-points
to give a single current sheet near the original separator. In general
these current sheets could heat the corona as they dissipate, but when
new flux emerges into a highly sheared field they could also trigger
a solar flare.

---------------------------------------------------------
Title: The Fibril Structure of Prominences
Authors: Priest, E. R.; Hood, A. W.; Anzer, U.
1991SoPh..132..199P Altcode:
We suggest that the fibril structure of prominences may be caused by
filamentation during its formation by radiative instability. We also
discuss the effects of other types of instability and give a mechanism
for the formation of vertical threads. The models indicate that highly
inhomogeneous density structures can exist in the presence of smooth
profiles for the plasma pressure and magnetic field. In our particular
models the plasma pressure of a fibril prominence is higher and the
vertical magnetical field is weaker than in a uniform prominence model,
while the mass is substantially smaller.

---------------------------------------------------------
Title: Magnetic Reconnection and Energy Release in the Solar Corona
by Taylor Relaxation
Authors: Vekstein, G. E.; Priest, E. R.; Steele, C. D. C.
1991SoPh..131..297V Altcode:
The heating of the solar corona by resistive turbulence of coronal
magnetic fields is considered. The theory of this process, based on the
Taylor-Heyvaerts-Priest hypothesis and a magnetic relaxation equation,
is developed. Such an approach allows one to obtain the successive
magnetic reconnection configurations and energy balance of the coronal
magnetic field in response to prescribed motions of the photospheric
footpoints. Two specific models of the coronal magnetic configuration
are investigated, namely an array of closely packed flux tubes and a
two-dimensional magnetic arcade.

---------------------------------------------------------
Title: Introduction to Solar System Magnetohydrodynamics
Authors: Priest, E. R.
1991assm.conf....1P Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Current Sheet Formation in Force-Free Magnetic Fields (With
2 Figures)
Authors: Vekstein, G.; Priest, E. R.
1991mcch.conf..536V Altcode:
No abstract at ADS

---------------------------------------------------------
Title: The structure of magnetic neutral points in two dimensions
Authors: Strachan, N. R.; Priest, E. R.
1991GApFD..61..199S Altcode:
Magnetic neutral points are of central importance in many solar
phenomena, such as flares, reconnection and coronal structures. Despite
their importance, the mathematical nature of magnetic neutral points has
not previously come under detailed mathematical scrutiny. We present
here a series of models for different types of neutral points in two
dimensions. X-points have four separatrix field lines meeting at the
neutral point and are a particular class of star-point which have an
even number of separatrices. Asymmetric star-points (with an odd number
of separatrices, at least one of which is a current sheet or a neutral
sheet) include: Y-points with three separatrices intersecting at angles
of 2/3; cusp-points with two of the three separatrices touching at
the neutral point; and T-points with two of the separatrices meeting
the third at right angles.

---------------------------------------------------------
Title: Effect of Coronal Heating on Coronal Arcades (With 3 Figures)
Authors: Steele, C. D. C.; Priest, E. R.
1991mcch.conf..544S Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Mechanisms of Chromospheric and Coronal Heating
Authors: Ulmschneider, Peter; Priest, Eric R.; Rosner, Robert
1991mcch.conf.....U Altcode: 1991QB809.M43......
One of the great problems of astrophysics is the unanswered
question about the origin and mechanism of chromospheric and coronal
heating. Just how these outer stellar envelopes are heated is of
fundamental importance, since all stars have hot chromospheric and
coronal shells where the temperature rises to millions of degrees,
comparable to the temperatures in the stars' cores. Here for the first
time is a comprehensive inventory of the proposed chromospheric and
coronal heating theories. The proposed heating processes are critically
compared, and the observational evidence for the various mechanisms
is reviewed. This is essential reading for all those working in such
fields as stellar activity, radio and XUV emission, rotation, and mass
loss, for whom a detailed and consistent presentation of our knowledge
of chromospheric and coronal heating mechanisms is urgently needed.

---------------------------------------------------------
Title: Report of IAU Commission 10: Solar activity (Activité
solaire).
Authors: Priest, E. R.
1991IAUTA..21...53P Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Steady flows in magnetic arcades - a class of exact mhd
solutions
Authors: de Ville, A.; Priest, E. R.
1991GApFD..61..225D Altcode:
A class of exact solutions to the steady, two-dimensional
magnetohydrodynamic equations in a cartesian geometry is presented. The
method developed by Tsinganos (1981, 1982) is employed. These solutions
may mode! siphon flow in magnetic arcades, the flow being driven by
pressure differences between the footpoints. The basic result is that
the presence of inertial forces causes the arcade to rise in comparison
to the static case. Eventually, the arcade may be caused to erupt by
continuing to increase the magnitude of inertial forces.

---------------------------------------------------------
Title: A Twisted Flux-Tube Model for Solar Prominences. III. Magnetic
Support
Authors: Ridgway, C.; Priest, E. R.; Amari, T.
1991ApJ...367..321R Altcode:
The model for the support of a solar prominence in a twisted flux tube
by Priest et al. (1989) is here extended to consider large deviations
of the tube from cylindrical symmetry. The prominence is modeled as a
finite current sheet with mass which is locally free from singularities
in the magnetic field. Analytical solutions are found to the nonlinear
force-free equations for the structure around the prominence for two
functional forms of the field component parallel to the prominence
axis. An associated mixed boundary-value problem is also solved in
which the prominence is connected at its base to the photosphere,
and the normal magnetic field components along the prominence and the
photosphere are imposed.

---------------------------------------------------------
Title: The magnetohydrodynamics of energy release in solar flares.
Authors: Priest, E. R.
1991psf..conf..363P Altcode:
A large solar flare is thought to occur when a sheared magnetic arcade
loses equilibrium or goes unstable and erupts, and drives magnetic
reconnection in the stretched-out magnetic field lines. These two
key processes of magnetic eruption and magnetic energy conversion by
reconnection are reviewed briefly, with an account of recent analytical
and numerical models. When the height or length of a prominence in
a sheared coronal arcade is too great it may erupt and drive the
formation and reconnection of a current sheet below it.

---------------------------------------------------------
Title: Magnetic Field Annihilation Within a Stagnation Point Flow
(With 2 Figures)
Authors: Jardine, M.; Priest, E. R.; Allen, H. R.
1991mcch.conf..601J Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Introduction to solar system MHD.
Authors: Priest, E. R.
1991gamp.conf....1P Altcode:
Most of the solar systems are in the plasma state and its subtle
non-linear interaction with the magnetic field is described for
many purposes adequately by the equations of magnetohydrodynamics
(MHD). These equations are also applicable to the electrically
conducting liquid cores of planets, such as our Earth, where magnetic
fields are being generated by dynamo action. Indeed, this realisation of
the existence of a strong coupling between a magnetic field and a plasma
has revolutionised our understanding of the solar system. Furthermore,
the complementary remote sensing observations at high resolution of
the Sun and in-situ spacecraft measurements of solar system plasmas
have revealed a spectacular world of intriguing dynamic activity over
a wide range of length- and time-scales.

---------------------------------------------------------
Title: Two-Dimensional Magnetic Neutral Points (With 1 Figure)
Authors: Strachan, N. R.; Priest, E. R.
1991mcch.conf..539S Altcode:
No abstract at ADS

---------------------------------------------------------
Title: The Formation of Current Sheets and Coronal Heating (With
12 Figures)
Authors: Priest, E. R.
1991mcch.conf..520P Altcode:
No abstract at ADS

---------------------------------------------------------
Title: The creation of the magnetic environment for prominence
formation in a coronal arcade
Authors: Amari, T.; Démoulin, P.; Browning, P.; Hood, A.; Priest, E.
1991A&A...241..604A Altcode:
The possibility of prominence formation in sheared coronal arcades is
investigated. The creation of a dip at the summit of field lines is
a likely requirement before a prominence can form; then dense plasma
can be supported against gravity by the Lorentz force. It is proved
that, in fact, no shear profile can create a dip in a two-dimensional
force-free arcade if the photospheric field is bipolar. However,
numerical investigations show that shearing an arcade can induce very
flat field lines. It is investigated, in order of magnitude, how this
flattening of the field can increase the free fall time of a dense
plasma. Also, the interaction between shear and twist is analyzed; the
critical twist needed to have a dip is a decreasing function of shear.

---------------------------------------------------------
Title: The Evolution of Coronal Magnetic Fields
Authors: Priest, E. R.; Forbes, T. G.
1990SoPh..130..399P Altcode:
Slow photospheric motions can produce flow speeds in the corona
which are fast enough to violate quasi-static evolution. Therefore,
high-speed flows observed in the corona are not necessarily due to a
loss of equilibrium or stability. In this letter we present an example
where the flow speed increases indefinitely with, height, while the
coronal magnetic energy increases quadratically with time.

---------------------------------------------------------
Title: Nonlinear magnetic reconnection models with separatrix jets
Authors: Priest, E. R.; Lee, L. C.
1990JPlPh..44..337P Altcode:
A new theory for fast steady-state magnetic reconnection is proposed
that includes many features of recent numerical experiments. The
inflow region differs from that in the classical model of Petschek
(1964) and the unified linear solutions of Priest &amp; Forbes (1986)
in possessing highly curved magnetic field lines rather than ones that
are almost straight. A separatrix jet of plasma is ejected from the
central diffusion region along the magnetic separatrix. Two types of
outflow are studied, the simplest possessing an outflow magnetic field
that is potential. The other contains weak standing shock waves attached
to the ends of the diffusion region and either slowing down the flow
(fast-mode shock) after it crosses the separatrix jet or speeding it up
(slow-mode), depending on the downstream boundary conditions. A spike
of reversed current slows down the plasma that emerges rapidly from
the diffusion region into the more slowly moving downstream region,
and diverts most of it along the separatrix jets. In the simplest
case the outflow possesses no vorticity over most of the downstream
region. The models demonstrate that both upstream and downstream
boundary conditions are important in determining which regime of
reconnection is produced from a wide variety of possibilities.

---------------------------------------------------------
Title: Thermal Equilibria of Coronal Magnetic Loops
Authors: Steele, C. D. C.; Priest, E. R.
1990SoPh..125..295S Altcode:
Equations of thermal equilibrium along coronal loops with footpoint
temperatures of 2 × 104 K are solved. Three fundamentally
different categories of solution are found, namely hot loops
with summit temperatures above about 4 × 105 K, cool
loops which are cooler than 8 × 104 K along their whole
length and hot-cool loops which have summit temperatures around 2 ×
104 K but much hotter parts at intermediate points between
the summit and the footpoints. Hot loops correspond to the hot corona
of the Sun. The cool loops are of relevance for fibrils, for the cool
cores observed by Foukal and also for active-region prominences where
the magnetic field is directed mainly along the prominence. Quiescent
prominences consist of many cool threads inclined to the prominence
axis, and each thread may be modelled as a hot-cool loop. In addition,
it is possible for warm loops at intermediate summit temperatures
(8 × 104K to 4 × 105 K) to exist, but the
observed differential emission measure suggests that most of the
plasma in the solar atmosphere is in either the hot phase or the cool
phase. Thermal catastrophe may occur when the length or pressure of
a loop is so small that the hot solution ceases to exist and there
are only cool loop solutions. Many loops can be superimposed to form
a coronal arcade which contains loops of several different types.

---------------------------------------------------------
Title: The Nonuniform Magnetohydrodynamic Nature of the Solar
Corona. IV. Effect of Magnetogravity Interactions
Authors: de Ville, A.; Priest, E. R.
1990ApJ...359..560D Altcode:
Magnetogravity interactions in the solar corona are considered
by extending the method proposed by Priest for modeling standing
magnetohydrodynamic disturbances. The effects of gravity on the basic
solutions are to increase the maxima of the plasma and magnetic
pressures and to decrease the curvature of the field lines. Also,
the classification of the solutions is altered. When the transverse
component of the magnetic field has a zero, more complex magnetic
field structures are produced, reminiscent of coronal streamers,
in which the type of interaction occurring in the plasma alters with
height in the corona.

---------------------------------------------------------
Title: Relaxed states in a spheromak with inhomogeneous boundary
fields
Authors: Dixon, A. M.; Browning, P. K.; Bevir, M. K.; Gimblett, C. G.;
Priest, E. R.
1990JPlPh..43..357D Altcode:
In this paper we consider force-free equilibrium solutions of the MHD
equations in a spherical geometry for the case in which magnetic flux
crosses the boundary of the containing vessel. The main motivation is
to model more faithfully actual spheromak experiments in the laboratory,
for which boundaries are unlikely to be magnetic surfaces. We show how a
general inhomogeneous boundary field may be constructed from individual
components. In particular, we consider the cases of a boundary field
of dipolar form and one of quadrupolar form. We then go on to discuss
solutions for fields embedded in point or ring electrodes using the
‘general solution’, some of which can be used to model experiments
such as the PS-1- or CTX-type spheromaks.

---------------------------------------------------------
Title: Thermal Equilibria of Coronal Magnetic Arcades
Authors: Steele, C. D. C.; Priest, E. R.
1990SoPh..127...65S Altcode:
A coronal magnetic arcade can be thought of as consisting of an assembly
of coronal loops. By solving equations of thermal equilibrium along
each loop and assuming a base temperature of 2 × 104 K,
the thermal structure of the arcade can be found. By assuming a form for
the plasma pressure in the arcade, the possible thermal structures can
be shown to depend on three parameters. Arcades can contain hot loops
with summits hotter than 400 000 K, cool loops at temperatures less
than 80 000 K along their lengths, hot-cool loops with cool summits
and cool footpoints but hotter intermediate portions, and warm loops,
cooler than 80 000 K along most of their lengths but with summits as
hot as 400 000 K. For certain arcades, there exist regions where more
than one kind of loop is possible. If the parameters describing the
arcade are varied, it is possible for non-equilibrium to occur when a
type of solution ceases to exist. For example, hot or warm loops can
cease to exist so that only cool solutions are possible when the arcade
size or pressure is decreased, while warm or cool loops may give way to
hot-cool loops when the heating is reduced or the pressure is increased.

---------------------------------------------------------
Title: Magnetic Field Evolution during Prominence Eruptions and
Two-Ribbon Flares
Authors: Priest, E. R.; Forbes, T. G.
1990SoPh..126..319P Altcode:
Simple models for the MHD eruption of a solar prominence are presented,
in which the prominence is treated as a twisted magnetic flux tube
that is being repelled from the solar surface by magnetic pressure
forces. The effects of different physical assumptions to deal with
this magneto-hydrodynamically complex phenomenon are evaluated,
such as holding constant the prominence current, radius, flux or
twist or modelling the prominence as a current sheet. Including a
background magnetic field allows the prominence to be in equilibrium
initially with an Inverse Polarity and then to erupt due to magnetic
non-equilibrium when the background magnetic field is too small or the
prominence twist is too great. The electric field at the neutral point
below the prominence rapidly increases to a maximum value and then
declines. Including the effect of gravity also allows an equilibrium
with Normal Polarity to exist. Finally, an ideal MHD solution is
found which incorporates self-consistently a current sheet below the
prominence and which implies that a prominence will still erupt and
form a current sheet even if no reconnection occurs. When reconnection
is allowed it is, therefore, driven by the eruption.

---------------------------------------------------------
Title: Energetics of compressible models of fast steady-state
magnetic reconnection
Authors: Jardine, M.; Priest, E. R.
1990JPlPh..43..141J Altcode:
An understanding of the energy transfer that takes place during
magnetic reconnection is crucial to the study of this fundamental
process. It depends on two factors: the type of reconnection regime
(which is determined by the boundary conditions) and the degree of
compressibility. Here we examine the role of compressibility in the
energetics of a family of reconnection models. When the inflow Mach
number (or reconnection rate) Me is small the effects of
compressibility may be more important than the differences between
regimes. We find that for a slow-compression regime with Me
= 0·005 compressibility decreases by 39% the efficiency of the shocks
in converting magnetic energy and increases by 14% the ratio of thermal
to kinetic energy in the outflow jet. This compares with a 13% decrease
in the shock efficiency and a 7% decrease in the jet ratio obtained by
choosing instead a flux-pile-up regime. As Me is increased,
however, the differences between regimes become larger and may be
comparable to or greater than the effects of compressibility. Thus when
the above Mach number is doubled we find that a change of regime now
has 1-6 times the effect on the jet energy ratio as the introduction
of compressibility. For those regimes, therefore, which only exist at
low inflow Mach numbers, compressibility will always be important. At
higher values of Me the type of regime may be the dominant
factor governing the energetics.

---------------------------------------------------------
Title: Physics of magnetic flux ropes
Authors: Russell, C. T.; Priest, E. R.; Lee, L. C.
1990GMS....58.....R Altcode: 1990QB528.P48......
The present work encompasses papers on the structure, waves, and
instabilities of magnetic flux ropes (MFRs), photospheric flux tubes
(PFTs), the structure and heating of coronal loops, solar prominences,
coronal mass ejections and magnetic clouds, flux ropes in planetary
ionospheres, the magnetopause, magnetospheric field-aligned currents and
flux tubes, and the magnetotail. Attention is given to the equilibrium
of MFRs, resistive instability, magnetic reconnection and turbulence
in current sheets, dynamical effects and energy transport in intense
flux tubes, waves in solar PFTs, twisted flux ropes in the solar
corona, an electrodynamical model of solar flares, filament cooling
and condensation in a sheared magnetic field, the magnetopause, the
generation of twisted MFRs during magnetic reconnection, ionospheric
flux ropes above the South Pole, substorms and MFR structures, evidence
for flux ropes in the earth magnetotail, and MFRs in 3D MHD simulations.

---------------------------------------------------------
Title: How to Form a Dip in a Magnetic Field Before the Formation
of a Solar Prominence
Authors: Démoulin, P.; Priest, E. R.
1990LNP...363..269D Altcode: 1990doqp.coll..269D; 1990IAUCo.117..269D
Magnetic fields with downward curvature are not favourable for
prominence formation since the presence of a small quantity of dense
material at the summit of a low-beta arcade cannot deform sufficiently
the magnetic field lines to remain there in a stable manner. Thus a
dip at the field line summit is needed before a prominence can form. We
investigate different ways of forming such an upward curvature. Results
with a twisted flux tube or a sheared arcade are reviewed, and a third
possibility, namely a quadrupolar region is proposed.

---------------------------------------------------------
Title: Magnetic Reconnection on the Sun
Authors: Priest, E. R.
1990IAUS..142..271P Altcode:
Several solar phenomena where reconnection is believed to be operating
are described. The type of reconnection regime and the rate of
reconnection are found to depend sensitively on the inflow boundary
conditions, with the b = 0 and b = 1 solutions being particular members
of a much wider class. Explanations are provided for four puzzling
features that are not present in the classical models of reconnection:
different types of inflow, separatrix jets, reversed current spikes, and
highly curved field lines in the inflow region. The mechanisms at work
in heating different parts of the corona are discussed. It is argued
that as oppositely directed magnetic fragments come together, their
magnetic field lines reconnect in the atmosphere above, creating one
loop which moves out while another submerges. The question of how the
global magnetic flux balance in the corona is maintained is addressed.

---------------------------------------------------------
Title: Hvar Reference Atmosphere of Quiescent Prominences
Authors: Engvold, Oddbjørn; Hirayama, Tadashi; Leroy, Jean Louis;
Priest, Eric R.; Tandberg-Hanssen, Einar
1990LNP...363..294E Altcode: 1990IAUCo.117..294E; 1990doqp.coll..294E
No abstract at ADS

---------------------------------------------------------
Title: Two-Dimensional Magnetic Neutral Points
Authors: Strachan, N. R.; Priest, E. R.
1990PDHO....7..168S Altcode: 1990ESPM....6..168S; 1990dysu.conf..168S
No abstract at ADS

---------------------------------------------------------
Title: Magnetic reconnection theory
Authors: Priest, E. R.
1990MmSAI..61..383P Altcode:
There are many phenomena on the sun where magnetic reconnection is
thought to be playing a central role, such as cancelling magnetic
features, prominences, coronal heating and solar flares. A few recent
advances in the basic theory of steady-state magnetic reconnection
are reviewed. The classical theory due to Sweet, Parker, Petschek and
Sonnerup has been unified in a theory which possesses many new regimes
that depend on the boundary conditions at large distances. For example,
the flux pile-up regime possesses diverging flows, a long central
diffusion region and a reconnection rate that is much larger than the
Petschek value. Recent numerical experiments however, often possess
three features that are not present in the earlier theoretical models,
namely highly curved inflow field lines, separatrix jets and reversed
current spikes, and so an attempt is described to include these features
in a new theoretical model. Also some new ideas on three-dinmensional
aspects of reconnection are discussed.

---------------------------------------------------------
Title: Support of a Prominence by a Constant-Current Force-Free Field
Authors: Ridgway, C.; Amari, T.; Priest, E. R.
1990PDHO....7..166R Altcode: 1990dysu.conf..166R; 1990ESPM....6..166R
No abstract at ADS

---------------------------------------------------------
Title: Basic plasma processes on the sun
Authors: Priest, E. R.; Krishan, V.
1990IAUS..142.....P Altcode:
The present symposium on basic plasma processes on the sun discusses
the solar interior, stellar plasmas, photospheric flows and magnetic
fields, photospheric fluxtubes, chromospheric and coronal heating,
magnetic reconnection and coronal evolution, solar flares, and
solar radio emission. Attention is given to the interior structure
of the sun, the electrodynamics of neutrinos in dispersive media,
problems of solar convection, mechanisms for dynamo mode excitation,
and plasma damping of gravitational waves. Topics addressed include
magnetic braking, energy release in stellar flares, large-scale flow
patterns in the solar atmosphere, the magnetohydrodynamics of sunspots,
waves in magnetic flux tubes, and wave propagation in sunspots. Also
discussed are relaxed states of MHD turbulence, the interaction of flux
tubes with sound waves, the magnetic helicity of oscillating coronal
loops, the existence of hydromagnetic interface waves in a structured
atmosphere, and magnetic reconnection on the sun.

---------------------------------------------------------
Title: Fibril Structure of Solar Prominences
Authors: Ballester, J. L.; Priest, E. R.
1990LNP...363..241B Altcode: 1990doqp.coll..241B; 1990IAUCo.117..241B
No abstract at ADS

---------------------------------------------------------
Title: Effect of an External Magnetic Field on Prominence Properties
Authors: Oliver, R.; Ballester, J. L.; Priest, E. R.
1990PDHO....7..170O Altcode: 1990dysu.conf..170O; 1990ESPM....6..170O
No abstract at ADS

---------------------------------------------------------
Title: Thermal Equilibrium of Coronal Loops and Prominence Formation
Authors: Steele, C. D. C.; Priest, E. R.
1990LNP...363..275S Altcode: 1990IAUCo.117..275S; 1990doqp.coll..275S
No abstract at ADS

---------------------------------------------------------
Title: Magnetic Helicity of Oscillating Coronal Loops
Authors: Krishan, V.; Priest, E. R.
1990IAUS..142..256K Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Fibril structure of solar prominences
Authors: Ballester, J. L.; Priest, E. R.
1990GMS....58..321B Altcode:
While previous modelings of solar active-region prominences have taken
the form of single, cool loops, observations suggest a more accurate
interpretation in terms of many loops of plasma that are inclined to
the filament. An effort is presently made to follow those suggestions
in the construction of a model for the fibrillar structure of solar
prominences in terms of slender flux tubes reproducing the observed
parameters of both quiescent and active region prominences. The model
structure encompasses a hot component and a depressed cool component;
the most realistic results include the effect of an external magnetic
field in the corona.

---------------------------------------------------------
Title: Comparison of Energetics of Magnetic Reconnection for
Compressible and Incompressible Models.
Authors: Jardine, M.; Priest, E. R.
1990ppsa.conf..219J Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Heating of Coronal Arcades
Authors: Steele, C. D. C.; Priest, E. R.
1990PDHO....7..164S Altcode: 1990dysu.conf..164S; 1990ESPM....6..164S
The thermal structure of a coronal arcade can be found by considering
it as an assembly of individual magnetic loops. This procedure is
valid as the thermal conductivity parallel to the magnetic field is
orders of magnitude greater than that perpendicular to the field. For
the present work, the arcade considered is a cylindrical one with its
axis on or below the photosphere.

---------------------------------------------------------
Title: Magnetic structure of prominences: (Invited Review)
Authors: Priest, E. R.
1990LNP...363..150P Altcode: 1990IAUCo.117..150P; 1990doqp.coll..150P
Models for magnetic structures of prominences are reviewed. Starting
with the internal structure of the prominence sheet, the external
prominence magnetic fields of normal polarity and of inverse polarity
are considered, followed by brief comments on fibril structure and
feet. A new flux tube model is summarized, and some comments about
the longitudinal magnetic component and the long-term evolution of
prominences are presented.

---------------------------------------------------------
Title: A three-dimensional model for solar prominences
Authors: Démoulin, P.; Priest, E. R.; Anzer, U.
1990LNP...363..268D Altcode: 1990doqp.coll..268P; 1990LNP...363..268P; 1990IAUCo.117..268P
We suggest here a model for the 3D structure of quiescent prominences by
a superposition of two fields. A 3D force-free field with constant is
assumed to exist in the corona prior to the prominence formation. The
prominence itself is represented by a line current which interacts
with the coronal field. The three-dimensional field is represented by
analytical functions and concentration of the magnetic field at the
photospheric level by convection cells is taken into account. When the
field created by the photospheric pattern supports the prominence,
the prominence feet are found to be located at supergranule centres
otherwise; they are located at cell boundaries.

---------------------------------------------------------
Title: The Nonuniform Magnetohydrodynamic Nature of the Solar
Corona. III. Cylindrical Geometry
Authors: de Ville, A.; Priest, E. R.
1989ApJ...347.1167D Altcode:
The method developed by Priest in 1988 for modeling steady MHD
disturbances in the solar corona is extended to a cylindrical geometry,
which is more realistic for three-dimensional structures, such as
plumes and coronal holes, which are observed in the corona. Both
axial symmetric and nonaxial magnetic fields are treated. The
basic characteristics of the axisymmetric solutions are found
to be similar to the previous Cartesian case. Quantitatively, the
interactions are stronger in the central region and weaker at the outer
boundary. Pressure gradients are also found to be smaller. Solutions
dependent on all three spatial variables exhibit an asymmetry because
of the angular dependence. They depend upon the azimuthal magnetic
field imposed at the coronal base. The solutions found in this paper
may be useful in interpreting the physics of MHD interactions observed
in numerical experiments and also in the solar atmosphere.

---------------------------------------------------------
Title: A generalization of the Woltjer minimum-energy principle
Authors: Dixon, A. M.; Berger, M. A.; Priest, E. R.; Browning, P. K.
1989A&A...225..156D Altcode:
The theorem of Woltjer (1958) for the minimization of the magnetic
energy of a magnetic structure is extended to include the case of
a free boundary subjected to external magnetic or plasma pressure
forces. The case where the boundary is not a magnetic surface is also
treated. Applications to a finite cylindrical flux tube and a spheromak
are given to illustrate the theory. It is also shown how the theory
may be applied to the construction of stationary Euler flows.

---------------------------------------------------------
Title: Model for the fibril structure of solar prominences
Authors: Ballester, J. L.; Priest, E. R.
1989A&A...225..213B Altcode:
Limb observations of quiescent solar prominences have revealed them to
be composed of many fine structures. Also observations in H-alpha and
UV lines suggest that quiescent filaments are made up of many clusters
of small-scale loops at different temperatures inclined to the filament
axis, with the C IV structures more extended than the H-alpha ones. In
H-alpha, the dimensions of these structures are about 7000 km long
and 1000 km thick, and they evolve over a typical time scale of about
8 min. The aim has been to take into account such observations and to
construct a model for the fibril structure of solar prominences in terms
of slender magnetic flux tubes, in which the main observed parameters
of both quiescent and active-region prominences are reproduced.

---------------------------------------------------------
Title: A three-dimensional model for solar prominences
Authors: Demoulin, P.; Priest, E. R.; Anzer, U.
1989A&A...221..326D Altcode:
In an attempt to model the external force field of a prominence,
a three-dimensional linear force-free field configuration was
studied. The model consists of a fundamental together with a harmonic
that is periodic along the prominence. The variation of the prominence
height along the prominence is calculated and it is suggested that
feet occur where the prominence sags down to low heights.

---------------------------------------------------------
Title: A Twisted Flux-Tube Model for Solar Prominences. I. General
Properties
Authors: Priest, E. R.; Hood, A. W.; Anzer, U.
1989ApJ...344.1010P Altcode:
It is proposed that a solar prominence consists of cool plasma supported
in a large-scale curved and twisted magnetic flux tube. As long as
the flux tube is untwisted, its curvature is concave toward the solar
surface, and so it cannot support dense plasma against gravity. However,
when it is twisted sufficiently, individual field lines may acquire a
convex curvature near their summits and so provide support. Cool plasma
then naturally tends to accumulate in such field line dips either
by injection from below or by thermal condensation. As the tube is
twisted up further or reconnection takes place below the prominence, one
finds a transition from normal to inverse polarity. When the flux tube
becomes too long or is twisted too much, it loses stability and its true
magnetic geometry as an erupting prominence is revealed more clearly.

---------------------------------------------------------
Title: The Formation of Flare Loops by Magnetic Reconnection and
Chromospheric Ablation
Authors: Forbes, T. G.; Malherbe, J. M.; Priest, E. R.
1989SoPh..120..285F Altcode:
Slow-mode shocks produced by reconnection in the corona can provide the
thermal energy necessary to sustain flare loops for many hours. These
slow shocks have a complex structure because strong thermal conduction
along field lines dissociates the shocks into conduction fronts and
isothermal subshocks. Heat conducted along field lines mapping from
the subshocks to the chromosphere ablates chromospheric plasma and
thereby creates the hot flare loops and associated flare ribbons. Here
we combine a non-coplanar compressible reconnection theory with simple
scaling arguments for ablation and radiative cooling, and predict
average properties of hot and cool flare loops as a function of the
coronal vector magnetic field. For a coronal field strength of 100 G the
temperature of the hot flare loops decreases from 1.2 × 107
K to 4.0 × 106 K as the component of the coronal magnetic
field perpendicular to the plane of the loops increases from 0% to 86%
of the total field. When the perpendicular component exceeds 86% of
the total field or when the altitude of the reconnection site exceeds
106km, flare loops no longer occur. Shock enhanced radiative
cooling triggers the formation of cool Hα flare loops with predicted
densities of ≈ 1013 cm−3, and a small gap
of ≈ 103 km is predicted to exist between the footpoints
of the cool flare loops and the inner edges of the flare ribbons.

---------------------------------------------------------
Title: Compressible models of fast steady-state magnetic reconnection
Authors: Jardine, M.; Priest, E. R.
1989JPlPh..42..111J Altcode:
We investigate the effects of compressibility on magnetic reconnection,
using as a basis the incompressible models of Priest &amp; Forbes and
Jardine &amp; Priest. Our results show that compressibility modifies
the reconnection process, without changing its essential character. In
the region of inflowing plasma, compressibility tends to increase
the convergence or divergence of the flow. Also, for regimes with a
compression in the inflow the maximum rate of reconnection is increased,
while for regimes with an expansion in the inflow the magnetic Mach
number at the entrance to the diffusion region is increased. In the
region of outflowing plasma the main effects of compressibility are
to produce faster and narrower outflow jets, with a lower magnetic
field strength.

---------------------------------------------------------
Title: Solar Physics. (Book Reviews: The Physics of Solar Flares)
Authors: Priest, Eric
1989Sci...245..770T Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Book Review: The physics of solar flares. / CUP, 1988
Authors: Priest, E.
1989Sci...245..770P Altcode:
No abstract at ADS

---------------------------------------------------------
Title: A model for a non-Keplerian magnetic accretion disk with a
magnetically heated corona
Authors: Heyvaerts, J. F.; Priest, E. R.
1989A&A...216..230H Altcode:
MHD stresses are used to model a thin disk dynamically interacting with
a self-created magnetic corona. A simple model of the coronal magnetic
structure produced by the turbulent MHD relaxation of the stresses
exerted by the disk's differential rotation can explain the effect
of coronal heating and the back reaction of the corona on the energy
balance and angular momentum flow in the disk. The results show the
disk to acquire a non-Keplerian rotation profile, with the trailing
foot point of the magnetic loops being accelerated, and the leading
one being decelerated, in comparison to a Keplerian distribution. It
is found that the coronal heating and non-Keplerian effects are weak
when the coronal scale becomes much larger than the disk size.

---------------------------------------------------------
Title: Book-Review - Dynamics and Structure of Quiescent Solar
Prominences
Authors: Priest, E. R.
1989JBAA...99..152P Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Book-Review - Dynamics and Structure of Quiescent Solar
Prominences
Authors: Priest, E. R.
1989S&T....77..619P Altcode:
No abstract at ADS

---------------------------------------------------------
Title: The Nonuniform Magnetohydrodynamic Nature of the Solar
Corona. II. Generalization of Basic Solutions
Authors: de Ville, A.; Priest, E. R.
1989ApJ...340..579D Altcode:
The method developed recently by Priest (1988) for modeling standing MHD
disturbances in the solar corona is extended in several ways. Including
inertial effects of the uniform plasma flow distorts the isobars and
makes the flow and magnetic field vary with altitude in a manner that
depends on the value of the flow speed relative to the cusp speed, the
sound speed, and the Alfven speed. More general classes of solutions
to the governing equation may be determined by seeking nonseparable
solutions with a Green's function technique. Allowing magnetic flux to
escape across the side boundaries leads to models which are relevant
for coronal holes. The relation of this governing equation to the
nonlinear MHD equations is indicated.

---------------------------------------------------------
Title: A twisted flux model for solar prominences. II - Formation of a
dip in a magnetic structure before the formation of a solar prominence
Authors: Demoulin, P.; Priest, E. R.
1989A&A...214..360D Altcode:
The possibility of dip formation in a linear force-free field before the
formation of a filament is investigated. The creation of a dip prior
to prominence formation requires a greatly sheared magnetic field and
a particular magnetic base flux. In the present work, attention is
given to a bipolar region devoid of parasite polarity.

---------------------------------------------------------
Title: Steady Magnetic Reconnection in Three Dimensions
Authors: Priest, E. R.; Forbes, T. G.
1989SoPh..119..211P Altcode:
The concepts of magnetic reconnection that have been developed
in two dimensions need to be generalised to three-dimensional
configurations. Reconnection may be defined to occur when there is
an electric field (E∥) parallel to field lines (known as
potential singular lines) which are potential reconnection locations and
near which the field has an X-type topology in a plane normal to that
field line. In general there is a continuum of neighbouring potential
singular lines, and which one supports reconnection depends on the
imposed flow or electric field. For steady reconnection the nearby
flow and electric field are severely constrained in the ideal region
by the condition that E∥ = 0 there. Potential singular
lines may occur in twisted prominence fields or in the complex magnetic
configuration above sources of mixed polarity of an active region or a
supergranulation cell. When reconnection occurs there is dynamic MHD
behaviour with current concentration and strong plasma jetting along
the singular line and the singular surfaces which map onto them.

---------------------------------------------------------
Title: Slow Shock Heating in POST Flare Arches
Authors: Hick, P.; Priest, E. R.
1989SoPh..122..111H Altcode:
The heating of a coronal arch, following the occurrence of a dynamic
(two-ribbon) flare, is discussed. We investigate whether slow-shock
heating, occurring during the reconnection process in the dynamic
flare and responsible for the heating of the post-flare loops, is also
a workable proposition for the heating of a coronal arch. Contrary
to the flare loops, the shock structure in the arch is generally
not modified greatly by thermal conduction effects. As a result
slow-shock heating may be investigated in terms of the familiar MHD
shock jump relations. The observed enhanced arch density with respect
to the surrounding corona is explained as a direct consequence of the
reconnection process. For a combination of high arch temperatures and
low values of coronal magnetic field and density thermal conduction may
become important and will lead to an extra density enhancement in the
arch. Our interpretation of the arch of 21-22 May, 1980 suggests that
the formation of the arch took approximately one hour, and that observed
temperature, density and maximum energy content can be consistently
explained by the slow-shock heating mechanism.

---------------------------------------------------------
Title: Non-equilibrium of a cylindrical magnetic arcade
Authors: Steele, C. D. C.; Hood, A. W.; Priest, E. R.; Amari, T.
1989SoPh..123..127S Altcode:
A cylindrically-symmetric magnetic arcade with its axis on the
photosphere is perturbed by means of an alteration in the pressure
along the base. The perturbation is examined with a view to finding
equilibrium configurations close to the original equilibrium. It
is found that equilibria can only be found when the integral of the
excess pressure along the base is zero. In other cases no equilibria
can be found and the arcade is likely either to collapse or, in the
case of a coronal mass ejection, to erupt. For an initial arcade
whose field increases linearly with radial distance from the axis,
the neighbouring equilibria have been found.

---------------------------------------------------------
Title: The Eruption of a Prominence and Coronal Mass Ejection which
Drive Reconnection
Authors: Steele, C. D. C.; Priest, E. R.
1989SoPh..119..157S Altcode:
Two possible limiting scenarios are proposed for the production of
a coronal mass ejection. In the first the magnetic field around a
prominence evolves until it loses equilibrium and erupts, which drives
reconnection below the prominence and an eruption of the overlying
magnetic arcade. In the second a large-scale magnetic arcade evolves
until it loses equilibrium and erupts, thereby causing a prominence
to erupt. In general it is likely to be the non-equilibrium of the
coupled system which creates the eruption. Furthermore, large quiescent
prominences are expected to be centred within the magnetic bubble of
a coronal mass ejection whereas when active-region prominences erupt
they are likely to be located initially to one side of the bubble.

---------------------------------------------------------
Title: The magnetic field around quiescent solar prominences computed
from observational boundary conditions
Authors: Demoulin, P.; Malherbe, J. M.; Priest, E. R.
1989A&A...211..428D Altcode:
A generalization of Anzer's model (1972) for quiescent prominence
support is presented. The coronal field is assumed to be current-free
except inside the prominence where currents support dense material
against gravity. The prominence is taken to be an infinitely thin
current sheet of finite extent along the vertical axis. The hypothesis
of two-dimensional fields allows the use of complex functions to
solve the mixed boundary problem which is defined by the observed
vertical field in the photosphere and the horizontal magnetic field
in the prominence. These boundary conditions are not sufficient to
determine a unique solution for the magnetic field. The indeterminacy
is decreased by physical considerations, and some models for Normal
(N) and Inverse (I) configurations are presented. In both cases it
is possible to find field configurations which can support the fintie
prominence against gravity.

---------------------------------------------------------
Title: Book-Review - Dynamics and Structure of Quiescent Solar
Prominences
Authors: Priest, E. R.; Tandberg-Hanssen, E. A.
1989SoPh..124..193P Altcode:
No abstract at ADS

---------------------------------------------------------
Title: How to Form a Dip in a Magnetic Field Before the Formation
of a Solar Prominence
Authors: Demoulin, P.; Priest, E. R.
1989HvaOB..13..261D Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Dynamics and structure of quiescent solar prominences
Authors: Priest, Eric Ronald
1989ASSL..150.....P Altcode: 1989dsqs.work.....P
Recent observational and theoretical investigations of quiescent
solar prominences (QSPs) are reviewed, in greatly expanded versions
of lectures presented at the workshop. Chapters are devoted to an
introduction to QSPs, the overall properties of QSPs and the role
of steady flows, the QSP environment, observations of QSP magnetic
fields, the formation of QSPs, the structure and equilibrium of QSPs,
and stability and eruption phenomena. Extensive graphs, diagrams,
and sample images are provided.

---------------------------------------------------------
Title: A Three-Dimensional Model for Solar Prominences
Authors: Demoulin, P.; Priest, E. R.; Anzer, U.
1989HvaOB..13..253D Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Magnetic Energy Conversion on the Sun
Authors: Priest, E. R.
1989ESASP.285...73P Altcode: 1989rsp..conf...73P
For a 3D magnetic field such as exists above sunspots or magnetic
fragments of complex polarity or in prominence fields, it is suggested
that reconnection may be defined to occur when there is a singular line
(a field line with neighboring X-type topology and an electric field
along it). In general there is a continuum of potential singular lines
and which one supports reconnection depends on the flow or electric
field. An example is given of how to drive reconnection all along
any curved field line in a configuration with shear and it is found
that strong plasma jetting occurs along the singular line. Thus in
a sheared coronal configuration of potential singular lines, coronal
heating may occur at small current sheets created at singular lines
by the appropriate flows.

---------------------------------------------------------
Title: Introduction to quiescent solar prominences
Authors: Priest, E. R.
1989ASSL..150....1P Altcode: 1988dsqs.work....1P; 1989dsqs.work....1P
The basic properties of quiescent solar prominences (QSPs) and
the theoretical models used to characterize them are reviewed and
illustrated with sample images. QSPs are located in the solar corona
but have temperatures 100 times lower and densities 100 times greater
than the corona. Particular attention is given to QSP development,
structures, eruptions, and instabilities and to the governing
magnetohydrodynamic, magnetohydrostatic, and wave equations for QSPs.

---------------------------------------------------------
Title: Structuring of plasmas by a magnetic field.
Authors: Priest, E. R.
1989plap.work..189P Altcode:
The highly structured nature of the solar corona is created by the
magnetic field. It is suggested that such structure can be understood
in terms of standing MHD disturbances of several types, spread over
extended regions. They can be compressions, in which the plasma pressure
increases, or expansions in which it decreases. Such interactions
are also present in the inflow region of 2D steady-state magnetic
reconnection models. Reconnection of 3D magnetic fields occurs when
an electric field exists along a singular line. In general a sheared
field possesses a continuum of potential singular lines and which one
supports reconnection depends on the flow or electric field. Constraints
on the electric field and flow are presented for reconnection at a
given singular line. It is proposed that prominences located in large
curved flux tubes are twisted up by Coriolis forces. When the twist
is large enough the magnetic field near the summit has the required
curvature to support plasma against gravity. As the twist increases
the prominence grows in length and eventually loses equilibrium and
erupts outwards into interplanetary space with a coronal mass ejection.

---------------------------------------------------------
Title: Thermal instability in a stratified plasma.
Authors: Hermanns, D. F. M.; Priest, E. R.
1989plap.work...99H Altcode:
The thermal instability mechanism has been studied in connection
to observed coronal features, like, e.g. prominences or cool cores
in loops. Although these features show a lot of structure, most
studies concern the thermal instability in a uniform medium. The
authors investigate the thermal instability and the interaction
between thermal modes and the slow magneto-acoustic subspectrum for a
stratified plasma slab. They formulate the relevant system of equations
and give some straightforward properties of the linear spectrum of a
non-uniform plasma slab, i.e. the existence of continuous parts in the
spectrum. They present a numerical scheme with which one can investigate
the linear spectrum for equilibrium states with stratification. The
slow and thermal subspectra of a crude coronal model are given as a
preliminary result.

---------------------------------------------------------
Title: Comparison of compressible and incompressible models of
reconnection.
Authors: Jardine, M.; Priest, E. R.
1989ESASP.285...45J Altcode: 1989rsp..conf...45J
The authors consider two recent families of reconnection models, one
for a compressible and one for an incompressible plasma and examine
the differences between them.

---------------------------------------------------------
Title: Global energetics of fast magnetic reconnection
Authors: Jardine, M.; Priest, E. R.
1988JPlPh..40..505J Altcode:
We examine the global energetics of a recent weakly nonlinear theory
of fast steady-state reconnection in an incompressible plasma (Jardine
&amp; Priest 1988). This is itself an extension to second order of the
Priest &amp; Forbes (1986) family of models, of which Petschek-like and
Sonnerup-like solutions are special cases. While to first order we find
that the energy conversion is insensitive to the type of solution (such
as slow compression or flux pile-up), to second order not only does the
total energy converted vary but so also does the ratio of the thermal
to kinetic energies produced. For a slow compression with a strongly
converging flow, the amount of energy converted is greatest and is
dominated by the thermal contribution, while for a flux pile-up with
a strongly diverging flow, the amount of energy converted is smallest
and is dominated by the kinetic contribution. We also find that the
total energy flowing out of the downstream region can be increased
either by increasing the external magnetic Mach number Me
or the external plasma beta βe Increasing Me
also enhances the variations between different types of solutions.

---------------------------------------------------------
Title: Book Review: Cool stars, stellar systems and the sun. /
Springer-Verlag, 1987
Authors: Priest, E. R.
1988Obs...108..234P Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Instability of a prominence supported in a linear force-free
field
Authors: Demoulin, P.; Priest, E. R.
1988A&A...206..336D Altcode:
The authors analyse the equilibrium of a prominence by modelling it as
a line of current under the action of gravity and of a two-dimensional
constant force-free field. They show that a region of non-equilibrium
may occur, if the field strength is great enough, for some distribution
of the base flux. This occurs for both Kippenhahn-Schlüter and
Kuperus-Raadu topologies and the region of non-equilibrium increases
with the shear.

---------------------------------------------------------
Title: Weakly nonlinear theory of fast steady-state magnetic
reconnection
Authors: Jardine, M.; Priest, E. R.
1988JPlPh..40..143J Altcode:
A family of models for fast steady-state reconnection has recently
been presented by Priest and Forbes, of which the Petschek-like
and Sonnerup-like solutions are special cases. This essentially
linear treatment involves expanding about a uniform flow and field
in powers of the external Alfvén Mach number Me, and
hence is valid for small values of that parameter. To lowest order,
the discrete slow-mode compressions attached to the diffusion region
are straight, while downstream of them the plasma flows at simply
the external Alfvén speed vAe and the field lines are
straight. Here we present an extension of these solutions to the next
order, which not only reveals that the wave itself is curved (as are the
downstream magnetic field lines), but also that the downstream solution
is sensitive to changes in the upstream boundary conditions. In the
downstream solution there is a free parameter, which may be specified
as a downstream boundary condition. Thus the boundary conditions at
both the inflow and the outflow boundaries are crucial in determining
the nature of the reconnection.

---------------------------------------------------------
Title: Nonuniform Magnetohydrodynamic Nature of the Solar
Corona. I. Basic Solutions
Authors: Priest, E. R.
1988ApJ...329.1009P Altcode:
The highly structured nature of the solar corona is created largely
by the magnetic field, and the author shows how such structure can
be understood in terms of standing MHD disturbances of several types,
spread over extended regions rather than being concentrated in narrow
regions like shock waves. They can be compressions, in which the
plasma pressure increases, or expansions in which it decreases. The
compressions can be of fast-mode type if the magnetic field increases
or of slow-mode type when it decreases, and vice-versa for the
expansions. A general method is developed for describing these
structures, and it is applied in simple examples to coronal plumes
(fast-mode compressions), coronal holes or depletions (slow-mode
compressions), the deflection of coronal rays (fast-mode compressions),
and the legs of coronal mass ejections (fast-mode compressions).

---------------------------------------------------------
Title: The Initiation of Solar Coronal Mass Ejections by Magnetic
Nonequilibrium
Authors: Priest, E. R.
1988ApJ...328..848P Altcode:
It is suggested that coronal mass ejections (CMEs) may be initiated
when the plasma pressure of magnetic shear in a magnetic arcade (or
helmet streamer) builds up so much that magnetic equilibrium is no
longer possible. The resulting withdrawal of stabilizing magnetic field
lines from the underlying prominence, as the CME moves out, then causes
the prominence to rise slowly as a secondary effect, stretching out the
neighboring field lines until reconnection allows a rapid eruption of
the prominence - in the case of an active region prominence, this in
turn may initiate a solar flare. Two simple examples of model coronal
arcades are studied which suggest that eruption of arcades and therefore
initiation of CMEs may be produced if the magnetic flux or axial plasma
(or magnetic) pressure become too great or if the external plasma
(or magnetic) pressure or temperature become too small.

---------------------------------------------------------
Title: Reverse currents in fast magnetic reconnection
Authors: Jardine, Mira; Priest, Eric R.
1988GApFD..42..163J Altcode:
It is suggested that reverse currents seen in recent numerical
reconnection experiments (Biskamp, 1986; Forbes and Priest, 1983) are
caused by the choice of outflow boundary conditions. The specification
of the normal velocity at the outflow boundary may result in a mismatch
in velocity at the diffusion region which is manifested as a spike of
reverse current.

---------------------------------------------------------
Title: Coronal heating by relaxation in a sunspot magnetic field
Authors: Dixon, A. M.; Browning, P. K.; Priest, E. R.
1988GApFD..40..293D Altcode:
The heating by resistive turbulence of solar coronal magnetic fields is
evaluated by means of the Taylor-Heyvaerts hypothesis, which enables one
to calculate the evolution of the magnetic helicity and magnetic field
in the corona in response to prescribed motions of the photospheric
footpoints. The relationship between the photospheric velocity field and
the helicity generation rate is considered for a coronal arcade and the
energy release is proved to be positive definite in general. Also, the
evolution and dissipation is determined for the axisymmetric magnetic
field above a single sunspot which is being twisted up from below.

---------------------------------------------------------
Title: A 2D prominence model.
Authors: Ballester, J. L.; Priest, E. R.
1988dssp.conf..137B Altcode:
A two-dimensional magnetohydrostatic model of a vertical prominence
sheet is set up by allowing slow variations of the magnetic field and
plasma properties with height. The width of the prominence is found
to decrase with height and in many cases the field lines become less
curved, while the strength of the horizontal magnetic field increases
with height, in agreement with some observations.

---------------------------------------------------------
Title: Dynamics and structure of solar prominences. Proceedings of
the workshop held November 18 - 20, 1987, at the Universitat de les
Illes Balears, Palma de Mallorca, Spain
Authors: Ballester, J. L.; Priest, E. R.
1988dssp.conf.....B Altcode:
No abstract at ADS

---------------------------------------------------------
Title: The eruption of a prominence and coronal transient.
Authors: Steele, C. D. C.; Priest, E. R.
1988dssp.conf..157S Altcode:
A model is set up for the eruption of a magnetically coupled prominence
and coronal mass ejection. The prominence is modelled as a twisted flux
tube and the transient as an overlying bubble. Stable equilibrium is
found when the magnetic flux below the prominence is small. However,
when a threshold flux is reached, the equilibrium ceases to exist
so that the prominence and the transient accelerate upwards before
reaching constant velocities.

---------------------------------------------------------
Title: Solar plasma physics.
Authors: Priest, E. R.
1988PhB....39...66P Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Dynamics of magnetic and velocity fields in coronal loops.
Authors: Krishan, V.; Berger, M.; Priest, E. R.
1988sscd.conf..236K Altcode:
The coronal loop plasma is represented by a superposition of the three
lowest order Chandrasekhar-Kendall modes. The temporal evolution of
the velocity and magnetic field in each of these mode is determined
using ideal MHD equations under two simplified cases viz (1) allowing
small departures from the equilibrium and (2) the pump approximation.

---------------------------------------------------------
Title: Non equilibrium of a prominence current in a linear force
free field.
Authors: Demoulin, P.; Priest, E.
1988dssp.conf...45D Altcode:
The authors analyse the equilibrium of a prominence by modelling
it as a line of current under the action of gravity and of a two
dimensional constant force free field. They show that a region of non
equilibrium may occur, if the field strength is great enough, for some
distribution of the base flux. This occurs for both Kippenhahn-Schluter
and Kuperus-Raadu topologies and the region of non equilibrium increases
with the shear.

---------------------------------------------------------
Title: Book Review: Instabilities in space and laboratory plasmas. /
CUP, 1986.
Authors: Priest, E. R.
1987Obs...107..228P Altcode: 1987Obs...107..228M
No abstract at ADS

---------------------------------------------------------
Title: Book-Review - Solar System Magnetic Fields
Authors: Priest, E. R.; Schrijver, J.
1987SoPh..108..417P Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Line-Tied Magnetic Reconnection
Authors: Robertson, J. A.; Priest, E. R.
1987SoPh..114..311R Altcode:
We present two-dimensional numerical simulations of magnetic
reconnection in a configuration relevant to two-ribbon solar flares. The
calculations extend those of Forbes and Priest (1982a, b, 1983) and
some puzzling aspects of their results are clarified. In particular, the
roles of magnetic diffusion, of the tearing mode and of turbulence are
individually examined. We stress the important part played by boundary
conditions in determining the evolution of the initial current sheet and
suggest that in future the evolution of the entire overlying magnetic
arcade be modelled as well as the current sheet that is created below
the rising arcade. Tearing at very high magnetic Reynolds numbers
is likely to develop into an impulsive bursty regime of reconnection
after a time which depends on the initial level of turbulence.

---------------------------------------------------------
Title: A Two-Dimensional Model for a Solar Prominence
Authors: Ballester, J. L.; Priest, E. R.
1987SoPh..109..335B Altcode:
A two-dimensional magnetohydrostatic model of a vertical prominence
sheet is set up by allowing slow variations of the magnetic field and
plasma properties with height. The width of the prominence is found
to decrease with height and in many cases the field lines become
less curved, while the strength of the horizontal magnetic field
increases with height, in agreement with observations. Since we are
only considering a local analysis, the model applies to a general
prominence sheet, whether of Kippenhahn-Schlüter or Kuperus-Raadu
type. The challenge in the future is to understand the detailed
fine-scale microstructure which takes place in the mould formed by
the present global macro-models.

---------------------------------------------------------
Title: Dynamical Model of Prominence Formation and Oscillation -
Part One
Authors: Sakai, J.; Colin, A.; Priest, E.
1987SoPh..114..253S Altcode:
We investigate a dynamical model of prominence formation in
a current sheet at the boundary between two regions of opposite
magnetic polarity. Coupled nonlinear equations describing the temporal
compression and condensation of plasma in the current sheet are set up
as a natural extension of the usual equations for current sheet collapse
(Imshennik and Syrovatskii, 1967). It is shown that under certain
conditions the current sheet undergoes a nonlinear oscillation during
the compression. The thermal instability with cooling is driven by a
density enhancement produced during the current sheet formation stage.

---------------------------------------------------------
Title: Book Review: The sun and the heliosphere in three dimensions. /
Reidel, 1986.
Authors: Priest, E. R.
1987Obs...107...35P Altcode: 1987Obs...107...35M
No abstract at ADS

---------------------------------------------------------
Title: The effect of gravity on the stability of a line-tied coronal
magnetohydrostatic equilibrium
Authors: Melville, J. P.; Hood, A. W.; Priest, E. R.
1987GApFD..39...83M Altcode:
The magnetohydrodynamic stability of a class of magnetohydrostatic
equilibria is investigated. The effect of gravity is included as well as
the stabilising influence of the dense photospheric line-tying. Although
the two-dimensional equilibria exhibit a catastrophe point, when the
ratio of plasma pressure to magnetic pressure exceeds a critical value,
arcade structures, with both footpoints connected to the photosphere,
become unstable to three-dimensional disturbances before the catastrophe
point is reached. Numerical results for field lines that are open into
the solar corona suggest that they are completely stable. Although
there is no definite proof of stability, this would allow the point
of non-equilibrium to be reached.

---------------------------------------------------------
Title: Appearance and Disappearance of Magnetic Flux at the Solar
surface
Authors: Priest, E. R.
1987rfsm.conf..297P Altcode:
Theory and observation of the emergence and submergence of magnetic
flux and the role of magnetic reconnection are briefly reviewed,
together with a new unified theory for fast steady-state reconnection
that has recently been proposed. New flux appears on a wide variety of
scales due to magnetic buoyancy or convective motions. However, only a
small fraction of the flux which emerges is likely to escape. Active
regions seem to disappear by the fragmentation and cancellation of
flux. Cancelling magnetic features (discovered by Sara Martin and
coworkers) probably represent sites where magnetic flux is disappearing
on small scales by reconnection submergence rather than by simple
submergence. On large scales flux is also escaping in coronal transients
and is probably submerging below quiescent prominences.

---------------------------------------------------------
Title: Magnetic Reconnection in Flares
Authors: Priest, E. R.
1987sman.work..157P Altcode:
A review of the basic theory of reconnection in current sheets is
given, including current sheet formation, tearing modes, fast nonlinear
reconnection regimes (such as the Petschek-Sonnerup, flux pile-up and
impulsive bursty regimes) and recent numerical experiments.

---------------------------------------------------------
Title: The Eruption of a Prominence and Coronal Transient
Authors: Steele, C.; Priest, E. R.
1987dssp.work..157S Altcode: 1987ASSL..150..157S
No abstract at ADS

---------------------------------------------------------
Title: Solar magneto-hydrodynamics.
Authors: Priest, E. R.
1987smh..book.....P Altcode:
No abstract at ADS

---------------------------------------------------------
Title: A Two-Dimensional Prominence Model
Authors: Ballester, J. L.; Priest, E. R.
1987dssp.work..137B Altcode: 1987ASSL..150..137B
No abstract at ADS

---------------------------------------------------------
Title: Theoretical Description of Magnetic Fields
Authors: Priest, E.
1987mfeo.conf...17P Altcode: 1991mfeo.conf...17P
No abstract at ADS

---------------------------------------------------------
Title: Nonequilibrium of a Prominence Current in a Linear Force-Free
Field
Authors: Demoulin, P.; Priest, E. R.
1987dssp.work...45D Altcode: 1987ASSL..150...45D
No abstract at ADS

---------------------------------------------------------
Title: Magnetohydrodynamic instability
Authors: Priest, E. R.; Cargill, P.; Forbes, T. G.; Hood, A. W.;
Steinolfson, R. S.
1986epos.conf..1.3P Altcode: 1986epos.confA...3P
There have been major advances in the theory of magnetic reconnection
and of magnetic instability, with important implications for the
observations, as follows: (1) Fast and slow magnetic shock waves are
produced by the magnetohydrodynamics of reconnection and are potential
particle accelerators. (2) The impulsive bursty regime of reconnection
gives a rapid release of magnetic energy in a series of bursts. (3)
The radiative tearing mode creates cool filamentary structures in the
reconnection process. (4) The stability analyses imply that an arcade
can become unstable when either its height or twist of plasma pressure
become too great.

---------------------------------------------------------
Title: The Preflare State
Authors: Priest, E. R.; Gaizauskas, V.; Hagyard, M. H.; Schmahl,
E. J.; Webb, D. F.
1986epos.conf..1.1P Altcode: 1986epos.confA...1P
No abstract at ADS

---------------------------------------------------------
Title: Book-Review - Solar System Magnetic Fields
Authors: Priest, E. R.; Russell, C. T.
1986SSRv...44..394P Altcode:
No abstract at ADS

---------------------------------------------------------
Title: The Shape of Buoyant Coronal Loops in a Magnetic Field and
the Eruption of Coronal Transients and Prominences
Authors: Browning, P. K.; Priest, E. R.
1986SoPh..106..335B Altcode:
The equilibrium and non-equilibrium properties of a coronal loop
embedded in a stratified isothermal atmosphere are investigated. The
shape of the loop is determined by a balance between magnetic tension,
buoyancy, and external pressure gradients. The footpoints of the loop
are anchored in the photosphere; if they are moved too far apart, no
equilibrium is possible and the loop erupts upwards. This critical
separation is independent of the pressure differential between the
loop and the external medium if the loop has enhanced magnetic field,
but varies if instead the loop pressure is increased. The maximum width
is proportional to the larger of the gravitational scale-height and the
length-scale of the ambient field. In some circumstances, it is shown
that multiple solutions exist for the tube path. These results may be
relevant to the eruption of prominences during the preflare phase of
two-ribbon flares and to the onset of coronal loop transients. Such
eruptions may occur if the footpoint separation, internal pressure or
internal magnetic field are too great.

---------------------------------------------------------
Title: Book-Review - Solar System Magnetic Fields
Authors: Priest, E. R.
1986S&T....72...41P Altcode:
No abstract at ADS

---------------------------------------------------------
Title: The Ideal Magnetohydrodynamic Stability of a Line-Tied Coronal
Magnetohydrostatic Equilibrium
Authors: Melville, J. P.; Hood, A. W.; Priest, E. R.
1986SoPh..105..291M Altcode:
An energy method is used to determine a condition for local instability
of field lines in magnetohydrostatic equilibrium which are rooted in
the photosphere. The particular equilibrium studied is isothermal
and two-dimensional and may model a coronal arcade of loops where
variations along the axis of the arcade are weak enough to be
ignorable. If line tying conditions are modelled by perturbations
that vanish on the photosphere, then, when the field is unsheared,
the condition for stability is necessary and sufficient. However, when
the axial field component is non-zero, so that the field is sheared,
the stability condition is only sufficient.

---------------------------------------------------------
Title: Book-Review - Solar System Magnetic Fields
Authors: Priest, E. R.
1986Sci...232.1656P Altcode:
No abstract at ADS

---------------------------------------------------------
Title: New models for fast steady state magnetic reconnection
Authors: Priest, E. R.; Forbes, T. G.
1986JGR....91.5579P Altcode:
A new unified family of models for incompressible, steady state
magnetic reconnection in a finite region is presented. They are
obtained by expanding in powers of the Alfvén Mach number and may
be used to elucidate some of the puzzling properties of numerical
experiments on reconnection which are not present in the classical
models. The conditions imposed on the inflow boundary of the finite
region determine which member of the family occurs. Petscheklike and
Sonneruplike solutions are particular members. The Sonneruplike regime
is a special case of a weak slow mode expansion in the inflow region,
and it separates two classes of members with reversed currents. These
are the hybrid regime with a mixture of strong fast mode and slow mode
expansions and the flux pileup regime with a stong slow mode expansion,
in which the magnetic field strength increases as it approaches the
diffusion region and the flow diverges. The Petscheklike regime is a
singular case of a weak fast mode expansion, and it separates the hybrid
regime from a regime of slow mode compressions. The hybrid expansions
are fast mode in character in the center of the inflow and slow mode
near the edges of the region, while the flux pileup expansions possess
long thin diffusion regions and no maximum reconnection rate. The
maximum rate is calculated for the other solutions as a function of the
magnetic Reynolds number and compared with the classical Sweet-Parker
and Petschek rates. For the flux pileup and hybrid regimes, reconnection
can be ;much faster than the maximum Petschek rate. Care should be
taken in deciding which type of reconnection is operating in a numerical
experiment. Indeed, no experiment to date has used boundary conditions
appropriate for demonstrating steady state Petschek reconnection.

---------------------------------------------------------
Title: Viscous Normal Modes on Coronal Inhomogeneities and Their
Role as a Heating Mechanism
Authors: Steinolfson, R. S.; Priest, E. R.; Poedts, S.; Nocera, L.;
Goossens, M.
1986ApJ...304..526S Altcode:
Viscous damping of Alfven surface waves is examined both analytically
and numerically using incompressible MHD. Normal modes are shown to
exist on discontinuous as well as continuously varying interfaces in
Alfven speed. The waves experience negligible decay below the transition
zone. High-frequency waves damp just above the transition region,
while those of lower frequency lose energy further out. A comparison of
dissipative decay rates shows that wave damping by viscosity proceeds
approximately two orders of magnitude faster than by resistivity.

---------------------------------------------------------
Title: Coronal heating in closely-packed flux tubes: a
Taylor-Heyvaerts relaxation theory.
Authors: Browning, P. K.; Sakurai, T.; Priest, E. R.
1986A&A...158..217B Altcode:
The aim of this paper is to take a more quantitative and detailed look
at dissipation in an array of closely-packed flux tubes. An initially
potential coronal loop is investigated, whose footpoints are twisted
up by cellular photospheric motions, forming a network of twisted flux
tubes. The motions are assumed to be slow compared with the reconnection
time-scale, so that the stressed field reconnects and dissipates some
of its energy as heat. The generalised Taylor's hypothesis is used
to investigate the effects of reconnection on the flux tubes and to
determine the efficiency of the dissipation. A basic mathematical
model is set up and the procedure for calculating the evolution is
outlined. The authors investigate the response of the field to the
footpoint motions and evaluate the heating produced. The results are
discussed, applications to the coronal heating problem are considered,
and the predictions are compared with the known heating requirements
of the corona.

---------------------------------------------------------
Title: Magnetic field-line reconnection with jets
Authors: Soward, A. M.; Priest, E. R.
1986JPlPh..35..333S Altcode:
Some recent numerical simulations of driven magnetic field-line
reconnection by Biskamp show no evidence of the Petschek mechanism when
the reconnection rate or magnetic Reynolds number are large. Instead,
an electric current sheet forms on the symmetry axis, across which a
magnetic field is annihilated. The sheet terminates at a Y-point. Fluid
driven into the current sheet escapes as jets along the separatrices
emanating from the Y-point. This paper shows how many of the features
such as the jets can be explained by a simple analytical model. Since
the numerical simulations are necessarily on a bounded domain,
the importance of the external boundary conditions in setting up a
steady-state solution is stressed by illustrative examples.

---------------------------------------------------------
Title: Heating of coronal arcades by magnetic tearing turbulence,
using the Taylor-Heyvaerts hypothesis
Authors: Browning, P. K.; Priest, E. R.
1986A&A...159..129B Altcode:
The heating of the solar corona by direct currents, which are dissipated
by magnetic reconnection, is studied. The coronal field responds to slow
photospheric motions by evolving through a series of equilibria, which
may be unstable to resistive modes. According to a generalization of
Taylor's hypothesis (Heyvaerts and Priest, 1984), the field reconnects
and relaxes to a linear force-free state (satisfying Delta X B = alpha
B), where the parameter alpha is uniform. During the relaxation process,
the field reconnects and dissipates some magnetic energy as heat. The
value of alpha at each time and the energy released during relaxation
may be determined from the evolution of magnetic helicity. Two theorems
concerning this method are proved: First, the invariance of the method
with respect to gauge transformations of the vector potential is
discussed, and it is shown that the helicity evolution equation in any
gauge predicts the same evolution of the field. Second, it is shown
that the energy release always vanishes in the limit of infinitely
fast reconnection. It is found that similar footpoint motions heat an
arcade more efficiently if it is already strongly sheared, such as
in a rapidly evolving active region. The general conclusion is that
tearing turbulence is a viable heating mechanism for the solar corona.

---------------------------------------------------------
Title: Magnetohydrodynamic Theories of Solar Flares
Authors: Priest, E. R.
1986SoPh..104....1P Altcode:
Our current understanding of the MHD of the flare process is summarised,
with some emphasis on processes which produce strong impulsive electric
fields and current filamentation. As an introduction, a description of
the two main types of flare (i.e., simple-loop and two-ribbon) is given,
together with an account of the two branches of reconnection theory
(tearing modes and the Petschek-Sonnerup mechanism). Modern numerical
experiments of reconnection suggest impulsive bursty acceleration
of particles in many small regions of width a hundred kilometres or
less. This is followed by a discussion of the eruptive instability
thought to initiate a large flare and of the reconnection process of
energy release. Finally, the role of emerging flux and horizontally
moving satellite sunspots is discussed briefly.

---------------------------------------------------------
Title: Criteria for the stability of a line-tied magnetohydrostatic
equilibrium in the solar corona
Authors: Melville, J.; Hood, A.; Priest, E. R.
1986AdSpR...6f..49M Altcode: 1986AdSpR...6...49M
Arcades of loop structures in the solar corona have been associated
with the onset of solar flares. Changes in the plasma and/or magnetic
pressure could initiate a flare if the equilibrium structure becomes
unstable. It is shown that for a model magnetohydrostatic equilibrium,
if the plasma β &gt; β* where β* is the β-value for which a magnetic
island just appears on the photosphere, then the closed field lines
and some field lines tied to the photosphere are unstable to localised
linear perturbations. If the field remains unsheared by photospheric
motions, then the condition β &lt; β* for stability is necessary
and sufficient.

---------------------------------------------------------
Title: Preflare activity.
Authors: Priest, E. R.; Gaizauskas, V.; Hagyard, M. J.; Schmahl, E. J.;
Webb, D. F.; Cargill, P.; Forbes, T. G.; Hood, A. W.; Steinolfson,
R. S.; Chapman, G. A.; Deloach, A. C.; Gary, G. A.; Jones, H. P.;
Karpen, J. T.; Martres, M. -J.; Porter, J. G.; Schmieder, B.; Smith,
J. B., Jr.; Toomre, J.; Woodgate, B.; Waggett, P.; Bentley, R.;
Hurford, G.; Schadee, A.; Schrijver, J.; Harrison, R.; Martens, P.
1986NASCP2439....1P Altcode:
Contents: 1. Introduction: the preflare state - a review of previous
results. 2. Magnetohydrodynamic instability: magnetic reconnection,
nonlinear tearing, nonlinear reconnection experiments, emerging flux and
moving satellite sunspots, main phase reconnection in two-ribbon flares,
magnetic instability responsible for filament eruption in two-ribbon
flares. 3. Preflare magnetic and velocity fields: general morphology of
the preflare magnetic field, magnetic field shear, electric currents in
the preflare active region, characterization of the preflare velocity
field, emerging flux. 4. Coronal manifestations of preflare activity:
defining the preflare regime, specific illustrative events, comparison
of preflare X-rays and ultraviolet, preflare microwave intensity and
polarization changes, non-thermal precursors, precursors of coronal
mass ejections, short-lived and long-lived HXIS sources as possible
precursors.

---------------------------------------------------------
Title: Book-Review - Solar Flare Magnetohydrodynamics - the Fluid
Mechanics of Astrophysics and Geophysics - Volume I
Authors: Priest, E. R.; Staude, H.
1986AN....307..288P Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Magnetic Reconnection on the Sun
Authors: Priest, E. R.
1986MitAG..65...41P Altcode:
A brief review is given of the process of magnetic reconnection on the
Sun, where it can convert magnetic energy into heat and other forms
and where it is probably responsible for solar flares and coronal
heating. Recent numerical experiments are linking the classical
theory of resistive instability and fast steady-state reconnection and
are revealing a regime of impulsive bursty reconnection. The current
discoveries about plasma behaviour on the Sun are potentially of great
value for the rest of astrophysics.

---------------------------------------------------------
Title: Nonlinear development of phase-mixed alfvén waves
Authors: Nocera, L.; Priest, E. R.; Hollweg, J. V.
1986GApFD..35..111N Altcode:
We derive an equation governing the nonlinear propagation of a
linearly polarized Alfvén wave in a two-dimensional, anisotropic,
slightly compressible, highly magnetized, viscous plasma, where
nonlinearities arise from the interaction of the Alfvén wave with fast
and slow magnetoacoustic waves. The phase mixing of such a wave has
been suggested as a mechanism for heating the outer solar atmosphere
(Heyvaerts and Priest, 1983). We find that cubic wave damping dominates
shear linear dissipation whenever the Alfvén wave velocity amplitude
vy exceeds a few times ten metres per second. In the nonlinear regime,
phase-mixed waves are marginally stable, while non-phase-mixed waves
of wavenumber ka are damped over a timescale kuRe0|δ vy/vA|-2, Re0
being the Reynolds number corresponding to the Braginskij viscosity
coefficient η0 and vA the Alfvén speed. Dissipation is most effective
where β = (vs/vA)2 ≈ 1, vs being the speed of sound.

---------------------------------------------------------
Title: Onset of an energy cascade and nonperiodic behaviour in the
nonlinear propagation of MHD waves in the solar atmosphere
Authors: Nocera, Luigi; Priest, Eric R.
1986GApFD..37..193N Altcode:
We study the nonlinear stability of MHD waves propagating in
a two-dimensional, compressible, highly magnetized, viscous
plasma. These waves are driven by a weak, shear body force which
could be imposed by large scale internal fluctuations present in the
solar atmosphere. The effects of anisotropic viscosity (leading to
a cubic damping) and of the nonlinear coupling of the Alfven and the
magnetoacoustic waves are analysed using Galerkin and multiple-scale
analysis: the MHD equations are reduced to a set of nonlinear ordinary
differential equations which is then suitably truncated to give a
model dynamical system, representing the interaction of two complex
Galerkin modes. For propagation oblique to the background magnetic
field, analytical integration shows that the low-wavenumber mode is
physically unstable. For propagation parallel to the background magnetic
field the high-wavenumber wave can undergo saddlenode bifurcations, in
way that is similar to the van der Pol oscillator; these bifurcations
lead to the appearance of a hysteresis cycle. A numerical integration
of the dynamical system shows that a sequence of Hopf bifurcations
takes place as the Reynolds number is increased, up to the onset of
nonperiodic behaviour. It also shows that energy can be transferred
from the low- wavenumber to the high-wavenumber mode.

---------------------------------------------------------
Title: The MHD of solar flares.
Authors: Priest, E. R.
1986RALR...85..140P Altcode:
The object of this review is to summarize the kind of problems that are
met when studying the magnetohydrodynamics of solar flares. Comments
are given on flare observations of relevance and recent developments
in basic magnetic reconnection theory are reviewed.

---------------------------------------------------------
Title: Magnetic Reconnection
Authors: Priest, E.
1986ppm..conf....1P Altcode:
A brief review is given of reconnection on the sun, where it is probably
responsible for a wide variety of phenomena including coronal heating,
solar flares and the newly discovered cancelling magnetic features.

---------------------------------------------------------
Title: Role of magnetic reconnection in solar flares
Authors: Priest, E. R.
1986AdSpR...6f..73P Altcode: 1986AdSpR...6...73P
The behaviour of the magnetic field in the solar atmosphere is governed
by the equations of MHD rather than the electromagnetism of wires. In
particular, the roles of magnetic reconnection are: to create small
flares; to trigger large flares; and to release magnetic energy in large
flares. Observational evidence for reconnection is reviewed together
with recent theory and observation of (post-) flare loops, which
enable one to deduce the reconnection electric field from the motion
of either the flare loops or Hα ribbons. Also, an account is given of
a new unified theory for fast steady-state reconnection which includes
the Sweet-Parker, Petschek and Sonnerup-like models as special cases.

---------------------------------------------------------
Title: Corrigendum
Authors: Forbes, T. G.; Priest, E. R.; Hood, A. W.
1985JPlPh..34..481F Altcode:
Numerical solutions were obtained by Forbes, Priest &amp; Hood (1982)
for the resistive decay of a current sheet in an MHD fluid. To check
the accuracy of the numerical solutions, a linear, analytical solution
was also deived for the regime where diffusion is dominant. In a
subsequent reinvestigation of this problem an error in the linear,
analytical solution has been discovered. For the parameter values used
in the numerical solution this error is too small ( 2%) to produce
any significant change in the previous test comparison between the
numerical and analytical solutions. However, for parameter values much
different from those used in the numerical solution, the error in the
linear solution can be significant.

---------------------------------------------------------
Title: REVIEW ARTICLE: The magnetohydrodynamics of current sheets
Authors: Priest, E. R.
1985RPPh...48..955P Altcode:
Examples of current sheets are summarized and their formation is
described. A universal phenomenon in cosmic plasmas is the creation
of sheets off intense current near X-type neutral points (where the
magnetic field vanishes). These sheets are important as sites where
the magnetic-field energy is converted efficiently into heat and
bulk kinetic energy and where particles can be accelerated to high
energies. Examples include disruptions in laboratory tokamaks, substorms
in the earth's magnetosphere, and flares on the sun. The basic behavior
of a one-dimensional sheet is presented, together with an account of the
linear tearing-mode instability that can cause the field lines in such
a sheet to reconnect. Such reconnection may develop in different ways:
it may arise from a spontaneous instability or it may be driven, either
from outside by motions or locally by a resistivity enhancement. Various
processes are described that may occur during the nonlinear development
of tearing, along with the many numerical and laboratory experiments
that are aiding our understanding of this intriguing cosmical process.

---------------------------------------------------------
Title: Small-Scale Reconnection
Authors: Priest, E. R.
1985ESASP.235..137P Altcode: 1985fmsh.work..137P; 1985shpp.rept..137P
The basic theory of reconnection consists of two branches, namely
linear tearing modes and the fast nonlinear state of Petschek-Sonnerup
reconnection. Recent developments have linked them and have revealed
two fast unsteady regimes, namely the flux pile-up regime and the
impulsive bursty regime. Applications include: mini flux-transfer events
discovered by the AMPTE satellite at the magnetopause; cancelling
magnetic features in photospheric magnetograms from Big Bear Solar
Observatory; and the heating of the solar corona by tearing turbulence
at many small current sheets.

---------------------------------------------------------
Title: Remarks on the Magnetic Support of Quiescent Prominences
Authors: Anzer, U.; Priest, E.
1985SoPh...95..263A Altcode:
The development of magnetic field structures which can
lead to prominence configurations of the Kuperus-Raadu type is
discussed. Starting from streamer type configurations and preserving the
total current in the system we find that simple two-dimensional static
configurations lead to prominences which in general lie systematically
much lower than the heights found from observations. We therefore
conclude that either more complex field configurations are needed to
explain the recent observations by Leroy et al. (1983) or the initial
configurations must be very special.

---------------------------------------------------------
Title: Introduction to solar activity.
Authors: Priest, E. R.
1985ssmf.conf....1P Altcode:
The article summarises some of the basic properties of the sun, reminds
the reader of the magnetohydrodynamic equations and outlines some of
the major problems in solar activity.

---------------------------------------------------------
Title: The Magnetohydrodynamics of Solar Flares
Authors: Priest, E. R.
1985spit.conf...73P Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Current sheets in solar flares
Authors: Priest, E. R.
1985IAUS..107..233P Altcode:
Until recently magnetic reconnection in solar flares was discussed
simplistically in terms of either a spontaneous tearing mode instability
or a driven Petschek mode. Now the subtle relationship between these
two extremes is much better understood. Current sheets may form and
reconnection may be initiated in many different ways. There are also
a variety of nonlinear pathways from a reconnection instability and
several types of driven reconnection. In solar flares current sheets
may be important as new flux emerges from below the photosphere and also
as a magnetic arcade closes down after being blown open by an eruptive
instability. Numerical simulations of these sheets are described.

---------------------------------------------------------
Title: Phase Mixing of Propagating Alfven Waves
Authors: Nocera, L.; Leroy, B.; Priest, E. R.
1985IAUS..107..365N Altcode:
Among MHD waves, Alfvén waves have been proved to be the best
candidates to reach the solar corona and, eventually, to be responsible
for the heating of this outer part of the solar atmosphere. The problem
concerning the mechanism able to transform the energy stored in the
waves into heat is considered.

---------------------------------------------------------
Title: Coronal heating in closely packed flux tubes: a
Taylor-Heyvaerts relaxation theory.
Authors: Browning, P. K.; Sakurai, T.; Priest, E. R.
1985MPARp.181.....B Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Solar system magnetic fields. Based on lectures presented at
the Summer School on Solar System Plasmas, held at Imperial College,
London, September 1984.
Authors: Priest, E. R.
1985ssmf.conf.....P Altcode:
The physics of solar-system plasmas is explored in introductory
chapters reviewing the results of recent theoretical investigations
and observational studies. Topics examined include solar activity,
magnetospheric MHD, MHD waves, MHD instabilities, magnetic reconnection,
magnetoconvection, dynamo theory, the solar wind and the earth bow
shock, planetary magnetospheres, and comets. Graphs, photographs,
diagrams, and drawings are provided.

---------------------------------------------------------
Title: Coronal heating by turbulent phase mixed MHD waves.
Authors: Nocera, L.; Priest, E. R.
1984ESASP.220..249N Altcode: 1984ESPM....4..249N
The authors study the possibility of transferring mechanical and
magnetic energy from MHD waves propagating in the solar corona to
smaller and smaller scales via nonlinear effects, starting from a
phase-mixed configuration. The efficiency of this turbulent cascade
and the turbulent spectra (both in space and time) are worked
out numerically. The time spectrum suggests the onset of chaotic
behaviour. The resulting heating rate meets the energetic requirements
to heat the corona.

---------------------------------------------------------
Title: A numerical simulation of the formation of solar prominences.
Authors: Malherbe, J. M.; Forbes, T. G.; Priest, E. R.
1984ESASP.220..119M Altcode: 1984ESPM....4..119M
The radiative-resistive MHD equations are numerically solved in
two-dimensions for a magnetic field configuration that starts with
a vertical current sheet which is line-tied at its base and is in
mechanical, but not radiative, equilibrium. The aim of the present
study is to determine whether this initial configuration can achieve
a prominence-like equilibrium in the presence of magnetic reconnection
and tearing in the current-sheet.

---------------------------------------------------------
Title: The development and cooling of a solar limb-flare
Authors: Veck, N. J.; Strong, K. T.; Jordan, C.; Simnett, G. M.;
Cargill, P. J.; Priest, E. R.
1984MNRAS.210..443V Altcode:
Observations of a flare that began in soft X-rays at 20:37 UT on 1980
April 12, at the west limb of the Sun are discussed. The Solar Maximum
Mission (SMM) satellite was used to obtain X-ray images before the flare
and for a period during the decay phase. Hα photographs and the soft
X-ray flux measured by the GOES-3 satellite are available throughout
the flare. Some alternative models are discussed but insufficient
data are available for a full treatment. Overall, the observations are
best fitted qualitatively by a model where pre-existing magnetic loop
structures are perturbed by the intrusion of new magnetic flux.

---------------------------------------------------------
Title: Numerical Simulation of Reconnection in an Emerging Magnetic
Flux Region
Authors: Forbes, T. G.; Priest, E. R.
1984SoPh...94..315F Altcode:
The resistive MHD equations are numerically solved in two dimensions for
an initial-boundary-value problem which simulates reconnection between
an emerging magnetic flux region and an overlying coronal magnetic
field. The emerging region is modelled by a cylindrical flux tube with
a poloidal magnetic field lying in the same plane as the external,
coronal field. The plasma betas of the emerging and coronal regions
are 1.0 and 0.1, respectively, and the magnetic Reynolds number for the
system is 2 × 103. At the beginning of the simulation the
tube starts to emerge through the base of the rectangular computational
domain, and, when the tube is halfway into the computational domain, its
position is held fixed so that no more flux of plasma enters through
the base. Because the time-scale of the emergence is slower than
the Alfvén time-scale, but faster than the reconnection time-scale,
a region of closed loops forms at the base. These loops are gradually
opened and reconnected with the overlying, external magnetic field as
time proceeds.

---------------------------------------------------------
Title: Coronal heating by reconnection in DC current systems -
A theory based on Taylor's hypothesis
Authors: Heyvaerts, J.; Priest, E. R.
1984A&A...137...63H Altcode:
The rate of coronal heating expected from complex reconnection
processes is analyzed by adapting Taylor's hypothesis to solar and
stellar physical conditions. The fact that the magnetic helicity of
a region which is not a closed flux tube is not a gauge-invariant
quantity is addressed, and the time evolution of a magnetic arcade
undergoing slow footpoint motions and infinitely rapid relaxation by
reconnection is calculated. It is shown that, when the stresses are
relaxed instantaneously the amount of heating vanishes exactly. A
second order theory is developed which permits the heating effect
due to a small but finite reconnection time to be calculated. It is
concluded that DC current coronal heating is mainly due to motions
comparable in size to or smaller than the characteristic size of
the magnetic structure, and that these motions must tend to produce a
nonconstant alpha force-free magnetic structure. The theory illuminates
the connection between general coronal heating and solar flares.

---------------------------------------------------------
Title: Book-Review - Solar Magnetohydrodynamics
Authors: Priest, E. R.; Livshits, M. A.
1984SvA....28..484P Altcode:
No abstract at ADS

---------------------------------------------------------
Title: The Magnetic Non-Equilibrium of Buoyant Flux Tubes in the
Solar Corona
Authors: Browning, P. K.; Priest, E. R.
1984SoPh...92..173B Altcode:
The equilibrium shape of a slender flux tube in the stratified solar
atmosphere is studied. The path is determined by a balance between the
downwards magnetic tension, which depends on the curvature of the loop,
and the upwards buoyancy force. Previous results for untwisted slender
tubes are extended to include twisted tubes embedded in an external
magnetic field.

---------------------------------------------------------
Title: Magnetohydrostatic Structures in the Solar Atmosphere
Authors: Melville, J. P.; Hood, A. W.; Priest, E. R.
1984SoPh...92...15M Altcode:
Separable two-dimensional solutions to the isothermal magnetohydrostatic
equations are presented which include the effect of gravity. Examples of
three types of linear solution are given in which photospheric magnetic
fields are prescribed and the field topologies are discussed. In
addition, a new nonlinear solution is discussed. The functional
form of the pressure distribution is restricted by the separable
assumption. An analysis suggests that these are the only separable
analytical solutions.

---------------------------------------------------------
Title: Phase mixing of propagating Alfven waves
Authors: Nocera, L.; Priest, E. R.; Leroy, B.
1984A&A...133..387N Altcode:
The fundamental wave solution found by Heyvaerts and Priest
(1983) for the propagation and damping of shear Alfven waves in an
inhomogeneous medium is checked against their assumptions, and a range
of self-consistency is provided. By analyzing wave behavior outside
this range, novel wave propagation properties are discovered. In the
limit of weak damping, a uniformly valid solution is obtained by the
method of multiple scales. For long wavelengths, the diffusion of
energy in the direction transverse to the wave propagation is found
to be important, and leads to weaker damping laws than those found by
Heyvaerts and Priest.

---------------------------------------------------------
Title: Book-Review - Solar and Stellar Magnetic Fields - Origins
and Effects - I.A.U. Symposium NO.102
Authors: Stenflo, J. O.; Priest, E. R.
1984Obs...104..102S Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Global magnetohydrostatic fields in stellar atmosphere
Authors: Browning, P. K.; Priest, E. R.
1984GApFD..28..141B Altcode:
The equilibrium properties of the magnetic field of an axisymmetric star
are studied. A family of analytical solutions to the magnetohydrostatic
equations is found, which are used to model the slow evolution of the
field through a series of equilibria. Firstly, a model is set up for
a force-free dipole-like field, which has a toroidal field component;
it is found that, as such a field is twisted up, a critical point is
reached, at which the field topology changes. If the twist is increased
beyond this point, there is no physically reasonable equilibrium. Next,
an untwisted magnetostatic dipole-like field is studied, with
an increasing pressure differential between pole and equator. A
critical point again occurs when the pressure differential becomes
too large. Finally a force-free quadrupole-like field is modelled,
which is being twisted up, for example by differential rotation;
this has similar properties to the dipole-like field. In each case,
it is suggested that, when the critical point is reached, the field
will no longer evolve smoothly, but will change catastrophically to a
new stable, releasing energy. Such an event could represent the onset
of a stellar flare or some other dynamic stellar process.

---------------------------------------------------------
Title: Book-Review - Solar Magnetohydrodynamics
Authors: Priest, E. R.; Jordan, C.
1984Obs...104...32P Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Kelvin-Helmholtz instability of a phased-mixed Alfven wave
Authors: Browning, P. K.; Priest, E. R.
1984A&A...131..283B Altcode:
The development of the Kelvin-Helmholtz instability at the velocity
antinodes of a standing Alfven wave is studied. The problem is
investigated at large times, when the velocity profile has a sinusoidal
form, and at the onset of instability. At large times it is found
that the growth rates of the sinusoidal profile are much smaller, and
that the most unstable wavelengths are about 12 times the phase-mixed
velocity inhomogeneity length scale, whereas for the square wave, short
waves are the most unstable. In a temporally local stability analysis,
a critical time is found after which the instability grows significantly
within one Alfven wave period. This critical time is related to the
dimensionless wave frequency Omega1 = (k-parallel)(a)/M,
where k-parallel is the Alfven wavenumber and M is the magnetic Mach
number. The growth rate of the instability is determined as a function
of time for several values of Omega1, and the critical time
is found at which the growth rate is equal to the wave frequency. It
is shown that the instability develops within very few wave periods,
thus it is expected that a shear Alfven wave would rapidly be disrupted
by Kelvin-Helmholtz instability.

---------------------------------------------------------
Title: Role of newly emerging flux in the flare process
Authors: Priest, E. R.
1984AdSpR...4g..37P Altcode: 1984AdSpR...4...37P
New flux emerging from below the photosphere is believed to give rise
to small flares and also to be capable of triggering large events when
extra energy is stored in the overlying field. A summary is given of
the observations of emerging flux, together with the current theoretical
ideas on its behaviour.

---------------------------------------------------------
Title: Magnetic reconnection at the sun
Authors: Priest, E. R.
1984GMS....30...63P Altcode:
Theoretical models of magnetic reconnection processes in the sun are
reviewed and illustrated with diagrams, drawings, graphs, and visual
and X-ray images. Topics examined include pathways to reconnection
(nonlinear development of tearing instabilities; ideal versus resistive
instability; and Petschek-Sonnerup, supercritical/flux-pile-up,
and impulsive-bursty regimes of fast reconnection), quiescent solar
prominences, coronal heating, (wave heating and tearing turbulence in
slowly evolving fields), and solar flares (observations, small flares
with emerging flux, and large two-ribbon flares). Some important
similarities between solar and magnetospheric phenomena are indicated.

---------------------------------------------------------
Title: Report of ESA's topical team on solar and heliospheric physics.
Authors: Christensen-Dalsgaard, J.; Delache, P.; Hoyng, P.; Priest,
E. R.; Schwenn, R.; Stenflo, J. O.
1984ESASP1070...26C Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Magnetic Reconnection at the Sun
Authors: Priest, E. R.
1984mrsl.conf...63P Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Book-Review: Solar Magnetohydrodynamics
Authors: Priest, E. R.; Martens, P. C. H.
1984Ruimt..33..119P Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Solar magneto-hydrodynamics
Authors: Priest, Eric Ronald
1984smh..book.....P Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Reconnection of magnetic fields.
Authors: Sonnerup, B. U. Ö.; Baum, P. J.; Birn, J.; Cowley, S. W. H.;
Forbes, T. G.; Hassam, A. B.; Kahler, S. W.; Matthaeus, W. H.; Park,
W.; Paschmann, G.; Priest, E. R.; Russell, C. T.; Spicer, D. S.;
Stenzel, R.
1984NASRP1120....1S Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Remarks on the magnetic support of quiescent prominences.
Authors: Anzer, U.; Priest, E.
1984MPARp.157.....A Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Book-Review - Cosmic Electrodynamics
Authors: Piddington, J. H.; Priest, E. R.
1983SSRv...36..422P Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Book-Review - Solar Magnetohydrodynamics
Authors: Priest, E. R.; Kuperus, M.
1983SSRv...36..424P Altcode:
No abstract at ADS

---------------------------------------------------------
Title: BOOK REVIEW: Solar Magnetohydrodynamics
Authors: Gokhale, M. H.; Priest, E. R.
1983BASI...11..347G Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Book-Review - Solar Magnetohydrodynamics
Authors: Priest, E. R.; Karlicky, M.
1983BAICz..34..378P Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Book reviews
Authors: Kleczek, J.; Nussbaumer, H.; van der Hucht, K. A.; De Greve,
J. P.; Ooms, G.; Rutten, R. J.; van der Laan, H.; Jäger, F. W.;
Reijnen, G. C. M.; Bijleveld, W.; Kistemaker, J.; de Jager, C.;
Mustel, E. R.; Ne'Eman, Y.; Priest, E. R.; Stiller, H.; Seifert, W.;
Namba, O.; Kuperus, M.; Hoekstra, Roel; Stumpers, F. L. H. M.; Frank,
S.; Zimmerman, J. T. F.; De Loore, C.; Gendrin, R.; Schrijver, J.;
Mulder, P. S.; Pounds, K. A.; Young, R. S.; Houziaux, L.; Engvold,
O.; Bok, B. J.; de Graaff, W.
1983SSRv...36..415K Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Current sheet models for solar prominences. II - Energetics
and condensation process
Authors: Malherbe, J. M.; Priest, E. R.; Forbes, T. G.; Heyvaerts, J.
1983A&A...127..153M Altcode:
A steady state dynamic model for solar prominences of the Kuperus and
Raadu type was previously proposed by Malherbe and Priest (1983), but
only the motion through series of quasi-static states was investigated
there. The mechanisms for formation, condensation and cooling of
plasma in this model are studied. As hot coronal material approaches
the filament sheet, it is expected to cool and condense. Cold material
is then carried up through the prominence by rising magnetic field
lines due to converging photospheric motions below the filament. Two
possible ways are suggested of triggering a thermal instability and so
producing such a stationary condensation process: a larger pressure in
the sheet, or a smaller wave heating in the reconnected field than in
the surrounding corona. This paper presents a simple model to simulate
the plasma condensation: the thermodynamics of the cooling process,
as well as the dynamics of new material entering the prominence sheet,
are described in detail.

---------------------------------------------------------
Title: Mass upflows in `post'-flare loops
Authors: Forbes, T. G.; Priest, E. R.
1983SoPh...88..211F Altcode:
A self-consistent numerical model of a reconnecting magnetic field
configuration similar to that occurring during the main-phase
of two-ribbon flares is used to estimate the upflow caused by the
fast-mode expansion of the magnetic field moving into the reconnection
region. Such an expansion creates a field-aligned pressure gradient
which accelerates plasma upward from the chromospheric base of magnetic
field lines in the region external to the loops. The numerical results
imply that the amount of mass sucked up in this way is even smaller
than was previously estimated by Kopp and Pneuman who used a kinematic
model. Therefore, some indirect mechanism (such as evaporation),
which would probably derive its motive power from the thermal energy
generated by the reconnection, is required to explain the large mass
upflows inferred from observations.

---------------------------------------------------------
Title: Solar and stellar magnetic fields
Authors: Priest, E. R.; Weiss, N. O.
1983Obs...103..239P Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Magnetic equilibrium in coronal arcades.
Authors: Melville, J. P.; Hood, A. W.; Priest, E. R.
1983SoPh...87..301M Altcode:
An analytical solution to the magnetohydrostatic equations is presented
that generalises a solution due to Birn et al. (1978) to include the
effect of gravity. There exist two solutions to the same boundary
conditions for small base pressures. If the base pressure exceeds
a critical value, then `non-equilibrium' may occur and initiate a
two-ribbon flare.

---------------------------------------------------------
Title: Magnetic Theories of Solar Flares
Authors: Priest, E. R.
1983SoPh...86...33P Altcode:
The basic processes of evaporation and magnetic reconnection have
recently been developed in much greater detail. They may be important
in the two main types of flaring event, namely simple-loop flares and
two-ribbon flares. The first type could be produced by kink instability,
thermal nonequilibrium, or emerging flux. The second type is thought to
be the result of an eruptive MHD instability that is either spontaneous
or triggered from outside. After the eruption the magnetic field lines
that have been blown open reconnect back down in a way that has now
been simulated numerically.

---------------------------------------------------------
Title: Current sheet models for solar prominences. I
Magnetohydrostatics of support and evolution through quasi-static
models
Authors: Malherbe, J. M.; Priest, E. R.
1983A&A...123...80M Altcode:
Several 2D current sheet models for solar prominences, supported against
gravity by potential magnetic configurations, are presented. Simple
magnetostatic solutions, both of Kuperus-Raadu type (KR) and of
Kippenhahn-Schlüter type (KS) are built with the help of complex
functions and discussed. The authors suggest that, in the case of KS
models, slow (&lt;100 ms-1) diverging photospheric flows
below prominences could explain the upward motions reported by Malherbe
et al. (1983) in disk filaments; while, in the case of KR models, slow
converging flows could explain them. To account for such motions and
magnetic field observations by Leroy et al. (1983) it is proposed in
Raadu's spirit (1979), a quasi-static reconnection model, in which new
material entering the prominence condenses and is brought up through
it by the moving magnetic field. The resulting dynamics is studied in
a simple magnetic geometry of KR type.

---------------------------------------------------------
Title: A Numerical Experiment Relevant to Line-Tied Reconnection in
Two-Ribbon Flares
Authors: Forbes, T. G.; Priest, E. R.
1983SoPh...84..169F Altcode:
The nonlinear evolution of a reconnecting magnetic field configuration
similar to that occurring just before the onset of `post'-flare loops
in two-ribbon flares is determined. The evolution, which is obtained
by numerically solving the resistive MHD equations, shows two new
features that have not yet been incorporated into contemporary models of
`post'-flare loops. The first of these new features is the formation
of a nearly stationary fast-mode shock above the region corresponding
to the top of the loops. This fast-mode shock occurs just below the
magnetic neutral line and between the slow-mode shocks associated with
fast magnetic reconnection at the neutral line. The second new feature
is the creation and annihilation of large-scale magnetic islands in the
current sheet above the loops. The annihilation of the islands occurs
very rapidly and appears to be a manifestation of the coalescence
instability. The creation and annihilation of magnetic islands could
be important in understanding the energetics of `post'-flare loops
since the coalescence instability can produce an intermittent energy
release more than an order of magnitude faster than that predicted by
steady-state reconnection theories.

---------------------------------------------------------
Title: Book-Review - Solar Magnetohydrodynamics
Authors: Priest, E. R.
1983Sci...220..338P Altcode:
No abstract at ADS

---------------------------------------------------------
Title: The Decay of a Large Limb Flare
Authors: Cargill, P. J.; Veck, N. J.; Jordan, C.; Strong, K. T.;
Simnett, G. M.; Priest, E. R.
1983BAAS...15..712C Altcode:
No abstract at ADS

---------------------------------------------------------
Title: The heating of postflare loops
Authors: Cargill, P. J.; Priest, E. R.
1983ApJ...266..383C Altcode:
The heating of postflare loops by slow MHD shocks is
considered. Although such loops were once thought to represent the
dying remnant of a two-ribbon flare, it is now realized that they
are the main part of such an event and begin very early on. It is
shown that, if the local temperature of a loop, its local angle of
inclination to the vertical, and ambient coronal conditions are known
from observations, then simple analytical formulae give the rise speed
of the shocks and the magnitude of the evaporative flow incident upon
them. The formulae are applied as an example to the 1973 July 29 and
1980 May 21 two-ribbon flares, and reasonable agreement with rise-speed
observations is found. It is therefore claimed that shock heating is
a viable method for heating postflare loops for the long times observed.

---------------------------------------------------------
Title: The structure of twisted magnetic flux tubes
Authors: Browning, P. K.; Priest, E. R.
1983ApJ...266..848B Altcode:
The basic physics of untwisted flux tubes confined by an external
plasma pressure has been developed by Parker (1979). However,
observations indicate that in many situations on the sun flux tubes
are twisted. There is, for instance, evidence for helical structures
in erupting prominences. It is, therefore, important to extend the
results already found for untwisted tubes to include the effects of
twist. The present investigation is concerned with the structure of
a thick twisted flux tube in an inhomogeneous atmosphere. Previous
work on untwisted flux tubes and straight cylindrical twisted tubes is
generalized, in order to describe the properties of twisted flux tubes
confined by a varying external pressure. The governing equations are
nonlinear, and, if the confining pressure is specified, they define a
free surface problem. This problem is solved analytically in certain
limiting cases, for which the complete structure of the field is
found. Two exact solutions for force-free fields are studied in detail.

---------------------------------------------------------
Title: Book-Review - Solar Magneto-Hydrodynamcs
Authors: Priest, E. R.
1983JBAA...93..100P Altcode:
No abstract at ADS

---------------------------------------------------------
Title: On reconnection and plasmoids in the geomagnetic tail
Authors: Forbes, T. G.; Priest, E. R.
1983JGR....88..863F Altcode:
The nonlinear evolution of the collisional tearing mode is numerically
determined for a two-dimensional current sheet configuration whose
magnetic field lines are tied at one end to a stationary surface. The
configuration is analogous to that occurring in the geomagnetic tail
at the start of a substorm. The numerical results suggest that the
formation of a near-earth neutral line at substorm onset is due to
the asymmetric tearing that occurs because the field lines in the
geomagnetic tail are partly line-tied, or anchored, by the earth's
ionosphere. The results also suggest that during substorm recovery
the neutral line in the tail moves away from the earth at a speed on
the order of the speed of the plasma flowing into the neutral line
region. Overall, the solution is consistent with proposed reconnection
models of the substorm but suggests that the recovery phase may be more
complex than previously expected, due to the growth and coalescence
of multiple magnetic islands (i.e., plasmoids).

---------------------------------------------------------
Title: The magnetohydrodynamics of solar activity
Authors: Priest, E. R.
1983PlPh...25..161P Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Coronal heating by phase-mixed shear Alfven waves.
Authors: Heyvaerts, J.; Priest, E. R.
1983A&A...117..220H Altcode:
The authors consider the physical processes which occur when a shear
Alfvén wave propagates in a structure with a large gradient of the
Alfvén velocity. Although these waves do not possess local resonances
(unlike magneto acoustic modes) they nevertheless suffer intense phase
mixing during which the oscillations of neighbouring field lines become
rapidly out of phase. The authors study this effect and show that the
resulting large growth of gradients dramatically enhances the viscous
and ohmic dissipation. The cases of propagating and standing waves
are considered, and a detailed calculation is given of the rate of
dissipation achieved in a finite length structure like a loop, in
the presence of a random excitation at its ends. The authors prove
that, after a long enough time, phase mixing can actually ensure the
dissipation of all the wave mechanical energy that a loop can pick up
from the excitation, in agreement with a previous claim by Ionson. The
general conclusion of the study is that phase mixing is the process
most able to ensure the dissipation of shear Alfvén waves in loops and
in open regions of strong reflectivity, and that loops, in particular,
must be in a permanent state of Kelvin-Helmholtz and tearing turbulence.

---------------------------------------------------------
Title: Solar Magneto-hydrodynamics
Authors: Priest, E. R.
1983C&T....99..159P Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Book-Review - Solar Magnetohydrodynamics
Authors: Priest, E. R.
1983S&T....66R..36P Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Magnetic instabilities in stellar atmospheres
Authors: Priest, E. R.
1983ASSL..102..545P Altcode: 1983IAUCo..71..545P; 1983ards.proc..545P
The extensive theory for magnetohydrodynamic instability of a flux
tube is briefly reviewed, together with its application to tokamaks
and solar flares. In a star a single coronal loop whose footprints
are anchored in the dense photosphere may become unstable to the kink
instability when it is twisted too much. Magnetic arcades may also be
subject to an eruptive instability when they are sheared too much. After
the eruption the magnetic field closes back down by reconnection and
continues to heat the plasma long after the impulsive phase. Global
instability of a large part of the coronal magnetic field is also
possible when the stored energy is too great.

---------------------------------------------------------
Title: Numerical Study of Line-Tied Magnetic Reconnection
Authors: Forbes, T. G.; Priest, E. R.
1982SoPh...81..303F Altcode:
A two-dimensional configuration, analogous to that at the start of
the main phase in two-ribbon flares, is modelled numerically by
self-consistently solving the time-dependent MHD equations. The
initial state consists of a vertical current sheet with an
external plasma beta value of 0.1 and a magnetic Reynolds number of
10−3. Although the model does not yet include gravity or
a full energy equation, many of the principal dynamical features of
the main phase in a flare are present. In particular, the numerical
results confirm the earlier prediction of the kinematic Kopp-Pneuman
(1976) model that a neutral line forms at the base of the corona and
rises upwards as open, extended field lines close back down to form
loops (i.e., `post'-flare loops). By the end of the computation a
state of nonlinear reconnection containing slow shocks has developed,
and the velocity of the plasma flowing into the neutral line region is
approximately 0.06 times the corresponding inflow Alfvén velocity -
a value consistent with the steady-state nonlinear reconnection theory
of Soward and Priest (1977). The speed at which the neutral line rises
in the numerical simulation varies from an initial value of ≲ 0.02
to a final value of ∼- 0.12 times the inflow Alfvén speed.

---------------------------------------------------------
Title: Neutral line motion due to reconnection in two-ribbon solar
flares and magnetospheric substorms
Authors: Forbes, T. G.; Priest, E. R.
1982P&SS...30.1183F Altcode:
Two kinematic models of line-tied reconnection are considered which
describe the motion of a magnetic neutral line (NL) during the main
phase of a two-ribbon solar flare and during the recovery phase of
a magnetospheric substorm in the geomagnetic tail. The models are
kinematic in that they use only the magnetic induction equation, which
suffices to determine the position and velocity of the NL as functions
of time if the rate of reconnection is prescribed. The solar flare model
shows that the observed large decrease in the rate at which "post"-flare
loops rise upward from the photosphere during the main phase does not
require a corresponding decrease in the rate of reconnection. Instead
it is found that a constant rate of reconnection can account for the
motion of the loops for almost the entire period during which they
are observed. By contrast, application of the same procedures to the
recovery phase of the magnetospheric substorm in the tail predicts a
slightly increasing speed of NL motion if the rate of reconnection is
constant. Furthermore, it is found that the motion of the NL relative
to the ambient medium may account for much of the observed asymmetry
in the magnetic field in the plasma sheet during recovery. Due to this
motion, the plasma sheet thickness may be up to 4 times smaller and
the normal magnetic field component up to 2 times weaker in the region
tailward of the NL than in the corresponding region earthward of the NL.

---------------------------------------------------------
Title: Thermally Isolated Coronal Loops in Hydrostatic Equilibrium
Authors: Wragg, M. A.; Priest, E. R.
1982SoPh...80..309W Altcode:
Numerical solutions are presented for the summit temperature and
heating in a thermally isolated coronal loop that is in hydrostatic
equilibrium. The extent to which gravity modifies the usual
uniform-pressure scaling law is shown, and plots of the differential
emission measure are also given.

---------------------------------------------------------
Title: Fast magnetic field-line reconnexion in a compressible
fluid. Part 1. Coplanar field lines
Authors: Soward, A. M.; Priest, E. R.
1982JPlPh..28..335S Altcode:
The Petschek model for incompressible reconnexion has been put on a firm
mathematical foundation in an earlier paper by Soward &amp; Priest,
who discovered a ‘local’ similarity solution for the process. The
present paper extends that analysis to compressible reconnexion, in
which the previous Alfvén waves are replaced by slow magneto-acoustic
shocks of switch-off type. By contrast with a previous suggestion,
it is found unnecessary to include intermediate waves standing ahead
of the slow shocks. The maximum reconnexion rate is typically half
of Petschek's stated value, though faster rates are achieved when the
external gas pressure is sufficiently low.

---------------------------------------------------------
Title: The structure of untwisted magnetic flux tubes
Authors: Browning, P. K.; Priest, E. R.
1982GApFD..21..237B Altcode:
The equilibrium structure of an axisymmetric magnetic flux tube confined
by an external pressure pe(Z) that varies along the length of the tube
is studie. In the past, most work has concentrated on slender flux
tubes, where the effects of transverse structure and radial fields are
neglected. Here the aim is to explore the properties of thick tubes,
in order to see to what extent the slender tube theory is valid. The
main results are:

---------------------------------------------------------
Title: The thermal stability of solar coronal loops in hydrostatic
equilibrium
Authors: Wragg, M. A.; Priest, E. R.
1982A&A...113..269W Altcode:
Many static models of solar coronal loops have been set up, and more
recently there have been attempts to look at their stability. A loop
is globally stable or unstable to thermal effects depending on whether
the net conduction (stabilizing) or the net radiation (destabilizing)
dominates. It has previously been found that uniform-pressure loops are
unstable if the base heat flux is too small. In particular, thermally
isolated loops are always unstable. In the present paper the effects
of gravity are included by considering a basic state in hydrostatic
equilibrium. It is found that the inclusion of gravity is a stabilizing
influence because it lowers the density (and hence the radiation)
in the upper parts of a loop. Thermally isolated loops are shown to
be thermally unstable when they are short but stable when they are
long enough. Thus, small coronal loops would be expected to exhibit
dynamic fluctuations and longer ones to possess a steadier emission.

---------------------------------------------------------
Title: Book-Review - Solar Flare Magnetohydrodynamics
Authors: Priest, E. R.; Rosner, R.
1982Natur.298..691P Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Present Theories for Simple-Loop and Two-Ribbon Flares
Authors: Priest, E.
1982Obs...102..118P Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Slow-shock heating and the Kopp-Pneuman model for `post'-flare
loops
Authors: Cargill, P. J.; Priest, E. R.
1982SoPh...76..357C Altcode:
The heating of `post'-flare loops in the Kopp-Pneuman (1976) model
is here reconsidered. In that kinematic model the loops are heated by
gas-dynamic shocks to at most 3-4 × 106 K. However, in a
full dynamic model they would be replaced by slow magnetohydrodynamic
shocks, which may provide more heating due to the additional release
of magnetic energy. It is shown from a local compressible analysis
that such shock waves can account for the observed temperatures of 5
× 106-107 K and also for the observed upward
loop speeds of 1-50 km s-1. The above values are obtained
when the ambient plasma beta is 0.01 and the shocks propagate at highly
sub-Alfvénic velocities. However, if the velocity of shock propagation
approaches the Alfvén speed, then temperatures of 108
K are produced. This may explain the extremely high temperatures that
have been observed with the Solar Maximum Mission, when it is realised
that the `post'-flare loop phenomenon may well be occurring very early
on in the flare.

---------------------------------------------------------
Title: Evolution of current sheets following the onset of enhanced
resistivity
Authors: Forbes, T. G.; Priest, E. R.; Hood, A. W.
1982JPlPh..27..157F Altcode:
An important aspect of pre-flare current sheets in the solar atmosphere
is the sudden enhancement of the effective electrical resistivity in
the sheet due to the onset of a plasma micro-instability. Numerical
and analytical solutions to the isothermal MHD equations are here
presented that describe the evolution of a current sheet subsequent to
such an enhancement in the resistivity. The solutions show that, if the
initial width of the current sheet is less than the acoustic-diffusion
length obtained by dividing the resistivity by the sound speed,
then isomagnetic shocks are formed. These shocks propagate outward
from the the centre of the current sheet and are transformed into
fast-mode magneto-acoustic waves when they reach the edges of the
current sheet. The fast-mode waves thus formed continue to propagate
outward beyond the confines of the current sheet. In contrast to a
previous study by Cheng, the present solutions demonstrate that flow
speeds several times greater than the local fast-mode wave speed can
be produced if the plasma beta parameter and the initial sheet width
are sufficiently small. The results may be relevant to the triggering
of a solar flare, as in the emerging flux model of flares.

---------------------------------------------------------
Title: Solar magneto-hydrodynamics
Authors: Priest, Eric Ronald
1982soma.book.....P Altcode: 1982QB539.M23P74...
No abstract at ADS

---------------------------------------------------------
Title: Solar magneto-hydrodynamics.
Authors: Priest, E. R.
1982GAM....21.....P Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Theories for simple-loop and two-ribbon solar flares
Authors: Priest, E. R.
1982FCPh....7..363P Altcode: 1982FCoPh...7..363P
A review of several basic flare processes is presented that includes
hydrodynamic motions in a loop, magnetic reconnection, and particle
acceleration. This is followed by a description of several possible
mechanisms for the two types of flares. It is noted that there are
two possibilities for explaining the types of flare. One is that
simple-loop and two-ribbon events are effected by completely different
instabilities, one being a local loop instability and the other a
global arcade instability. The other possibility is that the basic
instability is in essence the same in both cases (a kink instability,
for instance) but that in the one case the overlying field is so strong
that the instability is contained, whereas in the other the instability
spreads to other loops and the ambient field is too weak to contain
the explosion. This would make possible the eruption and subsequent
close down to give 'post' -flare loops as a consequence of the rupture.

---------------------------------------------------------
Title: Book-Review - Solar Flare Magnetohydrodynamics
Authors: Priest, E. R.; Machado, M. E.
1982SSRv...32..468P Altcode:
No abstract at ADS

---------------------------------------------------------
Title: The energetics of steady - state flows in the solar corona
Authors: Cargill, P. J.; Priest, E. R.
1982GApFD..20..227C Altcode:
In this paper the energetics of steady-state flows in coronal loops
are examined as an example of flows in flux tubes. The equations of
continuity, momentum, energy and state are solved numerically (with
some analytical special cases) subject to prescribed temperatures and
pressures at the footpoints. A large range of subsonic and supersonic
solutions is found, and it is pointed out that the static solutions
discussed previously in the literature are just one of a large
family of more general dynamic solutions. A steady flow is found to
remove the symmetry present in static loops and to lower the maximum
loop temperature. Also, the possibility of a catastrophe, which can
exist in static loops as non-equilibrium, is found to be enhanced
by the presence of a flow; it implies that for certain pressure and
temperature differences between the footpoints, a hot (&gt;106K)
steady flow is impossible, and so either a cool (&lt;105 K) steady
flow or an unsteady flow ensues. If the footpoint density is lowered,
the threshold for catastrophe is raised.

---------------------------------------------------------
Title: Effect of pressure gradients and line-tying on the magnetic
stability of coronal loops
Authors: Hood, A. W.; Priest, E. R.; Einaudi, G.
1982GApFD..20..247H Altcode:
In this paper we study the stability of an idealised magnetostatic
coronal loop, incorporating both the effect of line-tying, due to the
dense photosphere, and of pressure gradients. The stability equations
may be solved analytically for our particular equilibrium. From the
marginally stable case, the critical conditions separating instability
from stability are derived. It is found that stretching or twisting
a loop eventually makes it kink unstable.

---------------------------------------------------------
Title: Thermal Nonequilibrium - a Trigger for Solar Flares
Authors: Hood, A. W.; Priest, E. R.
1981SoPh...73..289H Altcode:
In this paper, we suggest that a solar flare may be triggered by a lack
of thermal equilibrium rather than by a magnetic instability. The
possibility of such a thermal nonequilibrium (or catastrophe)
is demonstrated by solving approximately the energy equation for
a loop under a balance between thermal conduction, optically thin
radiation and a heating source. It is found that, if one starts with
a cool equilibrium at a few times 104 K and gradually
increases the heating or decreases the loop pressure (or decreases
the loop length), then, ultimately, critical metastable conditions
are reached beyond which no cool equilibrium exists. The plasma heats
up explosively to a new quasi-equilibrium at typically 107
K. During such a thermal flaring, any magnetic disruption or particle
acceleration are secondary in nature. For a simple-loop (or compact)
flare, the cool core of an active-region loop heats up and the magnetic
tube of plasma maintains its position. For a two-ribbon flare, the
material of an active-region (or plage) filament heats up and expands
along the filament; it slowly rises until, at a critical height, the
magnetic configuration becomes magnetohydrodynamically unstable and
erupts violently outwards. In this case thermal nonequilibrium acts
as a trigger for the magnetic eruption and subsequent magnetic energy
release as the field closes back down.

---------------------------------------------------------
Title: Book-Review - Solar Flare Magnetohydrodynamics
Authors: Priest, E. R.
1981Sci...214..356P Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Internal Structure of Reconnecting Current Sheets and the
Emerging Flux Model for Solar Flares
Authors: Milne, A. M.; Priest, E. R.
1981SoPh...73..157M Altcode:
We present a steady-state model for reconnecting current sheets, which
relates the central values of temperature, density and pressure within
the sheet to the prescribed external values of these parameters as well
as the magnetic field strength and inflow velocity (or reconnection
rate). The simplifying feature of our model is the assumption of
quasi-one-dimensionality so that only variations across the sheet at
the centre of symmetry are considered in detail. The dimensions of the
sheet, the spatial profiles and their variation with the prescribed
dimensionless parameters are obtained from the model. We also obtain
the conditions on the dimensionless parameters for the existence
of a steady state. A beta-limitation is discovered, such that steady
reconnection is impossible when the plasma beta is too small. Also, the
sheet dimensions may be an order of magnitude larger than previously
thought. Finally, these general results are applied to the emerging
flux model for solar flares. A state of thermal nonequilibrium ensues
when the current sheet between the emerging and ambient flux reaches
a critical height. The effect of the beta-limitation is to make this
critical height decrease with increasing magnetic field strength.

---------------------------------------------------------
Title: The Temperature-Density Structure of Coronal Loops in
Hydrostatic Equilibrium
Authors: Wragg, M. A.; Priest, E. R.
1981SoPh...70..293W Altcode:
The temperature and density structure are computed for a
comprehensive set of coronal loops that are in hydrostatic and thermal
equilibrium. The effect of gravity is to produce significant deviations
from the usual uniform-pressure scaling law (T∼(pL)1/3)
when the loops are taller than a scale height. For thermally isolated
loops it lowers the pressure throughout the loop, which in turn lowers
the density significantly and also the temperature slightly; this
modifies the above scaling law considerably. For more general loops,
where the base conductive flux does not vanish, gravity lowers the
summit pressure and so makes the radiation decrease by more than the
heating. This in turn raises the temperature above its uniform pressure
value for loops of moderate length but lowers it for longer loops. A
divergence in loop cross-section increases the summit temperature by
typically a factor of 2, and decreases the density, while an increase
in loop height (for constant loop length) changes the temperature very
little but can halve the density.

---------------------------------------------------------
Title: Models of the Open Solar Atmosphere
Authors: Wragg, M. A.; Priest, E. R.
1981SoPh...69..257W Altcode:
McWhirter et al. (1975) have presented a standard model for the
transition region and inner corona that matches with the Harvard
Smithsonian Reference Atmosphere. They assume an open field line
configuration and solve numerically the equations of energy and
hydrostatic equilibrum. The purpose of the present paper is to
generalise their model for the temperature and density as functions
of height in several ways and, in particular, to determine the
temperature maxium and its location. The effect of varying the following
characteristics of the model is determined:

---------------------------------------------------------
Title: Solar flares: magnetohydrodynamic instabilities.
Authors: Hood, A. W.; Priest, E. R.
1981ASIC...68..509H Altcode: 1981spss.conf..509H
It is pointed out that most solar flares are either simple-loop (or
compact) flares or two-ribbon flares. A single loop brightens and
decays without moving, whereas the appearance of the two-ribbon type
involves the eruption of a magnetic arcade. The present investigation
is concerned with the basic MHD instabilities which may be responsible
for the solar flares. A summary of some basic theoretical concepts is
provided, taking into account the tearing-mode instability, magnetic
reconnection, and the energy method. Three mechanisms for producing
a simple-loop flares are discussed, giving attention to the kink
instability, a thermal nonequilibrium, and the theory of the emerging
flux model. It is found that a two-ribbon flare is probably caused
by the instability of the sheared magnetic configuration in which a
filament is situated.

---------------------------------------------------------
Title: Critical conditions for magnetic instabilities in force-free
coronal loops
Authors: Hood, A. W.; Priest, E. R.
1981GApFD..17..297H Altcode:
The remarkable magnetohydrodynamic stability of solar coronal loops
has been attributed to the anchoring of the ends of loops in the dense
photosphere. However, all the previous analyses of such line-tying
have been approximate, in the sense that they give only upper or lower
bounds on the critical amount of twist (or the critical loop-length)
required for the breakdown of stability. The object of the present
paper is to remove these approximations and determine the exact value
for the critical twist. When it is exceeded the magnetic field becomes
kink unstable and a flare may be initiated. A simple analytic stability
calculation is described for an idealised loop. This is followed by the
development of a general numerical technique for any loop profile, which
involves solving the partial differential equations of motion. It is
found, for example, that a force-free field of uniform twist possesses
a critical twist of 2.49 , by comparison with the previous bounds of 2,
for stability, and 3.3, for instability.

---------------------------------------------------------
Title: Solar flare magnetohydrodynamics
Authors: Priest, E. R.
1981sfmh.book.....P Altcode:
This book is aimed at first-year postgraduate students, with each
chapter presenting a simple description of previous work and also
some more recent unpublished calculations. Flare observations are
considered along with current sheets. Simple-loop flares are discussed,
taking into account magnetic instabilities and aspects of thermal
evolution. Attention is also given to magnetostatic equilibria
in the case of two-ribbon flares, postflare loops, and particle
acceleration in solar flares. Information presented in connection with
an introduction is related to the basic description of a flare, recent
advances concerning the understanding of the flare phenomenon, earlier
theories for the solar flare, and the magnetohydrodynamic equations.

---------------------------------------------------------
Title: Current sheets
Authors: Priest, E. R.
1981sfmh.book..139P Altcode:
Current sheets are of central importance for solar flare models
since they represent regions of greatly enhanced magnetic dissipation
and are a possible site for particle acceleration, either by direct
electric fields or by time-varying fields as current sheets explode or
in regions between the shocks that emanate from current sheets. Means
of formation are considered, taking into account the X-type collapse,
nonequilibrium, forced magnetic evolution, and the development of
current sheets during the course of an instability. The behavior of
current sheets is discussed, giving attention to magnetic annihilation
in one-dimensional sheets, the interchange instability, the growth of
reconnection, the nonlinear phase of steady reconnection, laboratory
experiments, and numerical experiments. The role of current sheets
in flare models is investigated, and a description of the emerging
(or evolving) flux model is presented.

---------------------------------------------------------
Title: Theory of loop flows and instability.
Authors: Priest, E. R.
1981sars.work..213P Altcode:
A preliminary theory for the steady and transient coronal loop flows
in solar active regions and their magnetohydrodynamic instability
is presented. Siphon flow is shown to be possible in the loops if
a pressure difference is maintained between the footpoints, and to
account for the presence of cool cores and appearances of only half a
loop. The evolution of active region magnetic loops is found to lead
to the continual evaporation and draining of the plasma contained
within them, particularly as a result of an increase in heating
rate. Consideration of static models for thermally isolated loops
reveals them to be thermally unstable, implying that in the absence of
some atmospheric stabilizing mechanism, the loops must be in a dynamic
state of thermal activity. It is shown that kilogauss photospheric
fields may be formed by an intense magnetic field instability, with
an associated transient downflow which may induce coronal flows at
enhanced velocities. Magnetohydrodynamic stability analysis suggests
that the major cause of magnetic stability may be line-tying of loop
footpoints in the dense photosphere.

---------------------------------------------------------
Title: Globular cluster systems - Implications for galaxy formation
Authors: Priest, E. R.
1981seng.proc..251P Altcode: 1981seng.proc..251F
Some properties of globular cluster systems in galaxies relevant to
an understanding of galaxy formation are reviewed. Consideration
is given to globular cluster populations in elliptical and spiral
galaxies, the galactic distribution of globular clusters, abundance
gradients among globular clusters in the same galaxy, cluster ages
in relation to abundances, the abundance distribution of Galactic
globular clusters, globular cluster initial mass functions, and the
orbital properties of galactic globular clusters. It is noted that
these properties reveal significant differences between the cluster
systems and diffuse outer halo populations of galaxies, suggestive of
different chemical and dynamical histories, and the probable earlier
formation of the cluster systems.

---------------------------------------------------------
Title: Introduction
Authors: Priest, E. R.
1981sfmh.book....1P Altcode:
An outline is provided of the two main types of solar flare, taking
into account the simple-loop (or compact) flare, the two-ribbon flare,
and the flare theories which have been proposed. Attention is given to
ground-based observations, space observations from Skylab, the preflare
build-up, an analysis of the instabilities, the energy release, bulk
motions, aspects of particle acceleration, the produced radiation,
the storage of magnetic energy and its release, the flux-loop models,
the current-sheet theories, and the magnetohydrodynamic equations. It
is pointed out that the solar flare owes its existence to the
magnetic field. The behavior of the magnetic field during the flare
is considered, taking into account the control of the flare plasma by
the magnetic field.

---------------------------------------------------------
Title: Are solar coronal loops in thermal equilibrium
Authors: Hood, A. W.; Priest, E. R.
1980A&A....87..126H Altcode:
Many authors have recently set up static models for coronal loops. In
this paper the thermal stability of such loops is tested by the
development of two simple methods which apply to a wide class of
equilibria. Stability is found to depend on the boundary conditions
adopted but not critically on the form of the heating. A loop is shown
to be stable if its base conductive flux is large enough that it lies
on the upper of two equilibrium branches. One particular model that has
attracted much attention is the thermally isolated loop, which has a
vanishing conductive flux at its base; it is found to be unstable to
perturbations that maintain the value of either the base temperature
or the base flux. Individual coronal loops may therefore be in a
dynamic state of ceaseless thermal activity unless some stabilizing
mechanism exists.

---------------------------------------------------------
Title: Magnetic instability of coronal arcades as the origin of
two-ribbon flares
Authors: Hood, A. W.; Priest, E. R.
1980SoPh...66..113H Altcode:
The generally accepted scenario for the events leading up to a
two-ribbon flare is that a magnetic arcade (supporting a plage filament)
responds to the slow photospheric motions of its footpoints by evolving
passively through a series of (largely) force-free equilibria. At
some critical amount of shear the configuration becomes unstable and
erupts outwards. Subsequently, the field closes back down in the manner
modelled by Kopp and Pneuman (1976); but the main problem has been to
explain the eruptive instability.

---------------------------------------------------------
Title: Force-Free Magnetic Arcades Relevant to Two-Ribbon Solar Flares
Authors: Priest, E. R.; Milne, A. M.
1980SoPh...65..315P Altcode:
Simple analytic models for the passive evolution of arcade-like magnetic
fields through a series of force-free equilibria are presented. At
the photospheric boundary, the normal magnetic field component is
prescribed together with either the longitudinal field component
or the photospheric shear. Analytic progress is made by considering
either cylindrically symmetric solutions or using the separation of
variables technique.

---------------------------------------------------------
Title: Siphon Flows in Coronal Loops - Part One - Adiabatic Flow
Authors: Cargill, P. J.; Priest, E. R.
1980SoPh...65..251C Altcode:
It is now known that the corona is filled with a multitude of
loop-like structures. The likelihood of these loops being in static
equilibrium is small and so this paper explores the possibility of
steady isothermal or adiabatic flows, driven by a pressure difference
between the loop feet. For a symmetric loop the flow becomes supersonic
at the summit and is then retarded by a shock-wave at some point on
the downflowing leg. The effect of adiabatic flow is to lower both
pressure and temperature by at least a factor of two and so provide
a possible explanation for the cool cores that are sometimes observed
in coronal loops.

---------------------------------------------------------
Title: The Structure of Coronal Arcades and the Formation of Solar
Prominences
Authors: Priest, E. R.; Smith, E. A.
1979SoPh...64..267P Altcode:
The temperature and density are obtained for coronal plasma in thermal
and hydrostatic equilibrium and located in a force-free magnetic
arcade. The isotherms are found to be inclined to the magnetic field
lines and so care should be taken in inferring the magnetic structure
from observed emission.

---------------------------------------------------------
Title: Kink Instability of Solar Coronal Loops as the Cause of
Solar Flares
Authors: Hood, A. W.; Priest, E. R.
1979SoPh...64..303H Altcode:
Solar coronal loops are observed to be remarkably stable structures. A
magnetohydrodynamic stability analysis of a model loop by the energy
method suggests that the main reason for stability is the fact that the
ends of the loop are anchored in the dense photosphere. In addition
to such line-tying, the effect of a radial pressure gradient is
incorporated in the analysis.

---------------------------------------------------------
Title: A model for quiescent solar prominences.
Authors: Milne, A. M.; Priest, E. R.; Roberts, B.
1979ApJ...232..304M Altcode:
A one-dimensional model is computed for a quiescent prominence
in both magnetohydrostatic equilibrium and thermal equilibrium
(under a balance between the thermal condition, radiation, and
wave heating). The effects of changing the coronal plasma pressure,
the horizontal magnetic field strength, and the inclination of the
horizontal magnetic field to the prominence normal are investigated. It
is found that an equilibrium state is impossible when either the plasma
beta or the magnetic field shear is too high. One feature of this
model is that the magnetohydrostatics is coupled to the energetics,
giving a fourth-order two-point boundary value problem, with two
symmetric conditions applied at the center of the structure and the
coronal temperature and density specified at a fixed outer edge.

---------------------------------------------------------
Title: The equilibrium of solar coronal magnetic loops.
Authors: Hood, A. W.; Priest, E. R.
1979A&A....77..233H Altcode:
A method is described for obtaining the temperature and density along
a magnetic field line for plasma that is in thermal equilibrium at
uniform pressure. One of the main features of the results is that,
when the pressure becomes too great or the length of the field line too
large, a lack of thermal equilibrium ensues and the plasma cools down
to a new equilibrium below 100,000 K. The stretching of a coronal loop
may be caused either by the straightforward motion of the photospheric
footprints or by the twisting up of a field that remains force-free. It
is suggested, therefore, that such an increase in pressure or length
is what causes some coronal loops to develop very cool cores. This is
proposed as a mechanism for the creation of active region filaments
or prominences.

---------------------------------------------------------
Title: A Modified KIPPENHAHN-SCHLÜTER Model for Quiescent
Prominences.
Authors: Priest, E. R.; Milne, A. M.; Roberts, B.
1979phsp.coll..184P Altcode: 1979IAUCo..44..184P
The effect of varying magnetic field strengths and shears on a model
quiescent solar prominence is assessed. Following Kippenhahn and
Schlueter (1957) the prominence is modeled by a one-dimensional slab
configuration. Their work is extended to include thermal effects, using
an energy equation which expresses a balance between thermal conduction,
radiative loss and wave heating. At some specified distance from the
center of the prominence, the coronal temperature and pressure are
prescribed while at the center of the prominence there are two symmetry
conditions, which results in a two-point boundary value problem.

---------------------------------------------------------
Title: A trigger mechanism for the emerging flux model of solar
flares.
Authors: Tur, T. J.; Priest, E. R.
1978SoPh...58..181T Altcode:
The energetics of a current sheet that forms between newly emerging flux
and an ambient field are considered. As more and more flux emerges,
so the sheet rises in the solar atmosphere. The various contributions
to the thermal energy balance in the sheet are approximated and the
resulting equation solved for the internal temperature of the sheet. It
is found that, for certain choices of the ambient magnetic field
strength and velocity, the internal temperature increases until, when
the sheet reaches some critical height, no neighbouring equilibrium
state exists. The temperature then increases rapidly, seeking a
hotter branch of the solution curve. During this dynamic heating, the
threshold temperature for the onset of plasma microinstabilities may be
attained. It is suggested that this may be a suitable trigger mechanism
for the recently proposed `emerging flux' model of a solar flare.

---------------------------------------------------------
Title: The structure of coronal loops.
Authors: Priest, E. R.
1978SoPh...58...57P Altcode:
With the advent of space telescopes, coronal magnetic loops,
both within and outside active regions, are being observed with
renewed interest. This paper is an attempt to outline some general
physical considerations pertinent to such loops, as a prelude to
more sophisticated modelling. For example, a loop that is stretched
(or possibly twisted) too much may be subject to a thermal instability
that cools its core to a new equilibrium below 105 K. Also
a simple consequence of hydrostatic balance along an equilibrium loop
is that, under some circumstances, the density inside a cool loop
can be comparable with that outside, despite the much smaller scale
height. Finally, when the equilibrium loop density is less than the
ambient density, several small scale magnetohydrodynamic instabilities
are sometimes efficient enough to generate a circulation that tends
to equalize the densities.

---------------------------------------------------------
Title: An emerging flux model for the solar phenomenon.
Authors: Heyvaerts, J.; Priest, E. R.; Rust, D. M.
1977ApJ...216..123H Altcode:
An outline is presented of the physical processes involved in the
emerging flux model, which appears to explain naturally many solar
flare observations. The separate physical phases of the basic model
include a preflare heating phase as the new flux emerges, an impulsive
phase as high-energy particles are accelerated, a flash (or explosive)
phase when the H-alpha intensity increases, and a main phase while
it decreases. The extent and morphology of the main phase emission
depend on the structure of the magnetic field region in which the new
flux finds itself imbedded. It is suggested that a (small) simple loop
flare occurs if the new flux appears in a region where no great amount
of magnetic energy in excess of potential is stored. A two-ribbon
flare occurs if the flux emerges near the polarity inversion line of
an active region that has begun to develop filaments.

---------------------------------------------------------
Title: An emerging flux model for solar flares.
Authors: Heyvaerts, J.; Priest, E.; Rust, D. M.
1977SoPh...53..255H Altcode:
No abstract at ADS

---------------------------------------------------------
Title: The formation of solar prominences by thermal instability in
a current sheet.
Authors: Smith, E. A.; Priest, E. R.
1977SoPh...53...25S Altcode:
The energy balance equation for the upper chromosphere or lower corona
contains a radiative loss term which is destabilizing, because a slight
decrease in temperature from the equilibrium value causes more radiation
and hence a cooling of the plasma; also a slight increase in temperature
has the effect of heating the plasma. In spite of this tendency towards
thermal instability, most of the solar atmosphere is remarkably stable,
since thermal conduction is very efficient at equalizing any temperature
irregularity which may arise. However, the effectiveness of thermal
conduction in transporting heat is decreased considerably in a current
sheet or a magnetic flux tube, since heat can be conducted quickly only
along the magnetic field lines. This paper presents a simple model for
the thermal equilibrium and stability of a current sheet. It is found
that, when its length exceeds a certain maximum value, no equilibrium
is possible and the plasma in the sheet cools. The results may be
relevant for the formation of a quiescent prominence.

---------------------------------------------------------
Title: Fast magnetic field line reconnection
Authors: Soward, A. M.; Priest, E. R.
1977RSPTA.284..369S Altcode: 1977RSLPT.284..369S
Petschek's (1964) model of fast magnetic-field-line reconnection
is placed on a sound mathematical basis by obtaining asymptotic
solutions that contain only one discontinuity in each quadrant. A
detailed self-consistent analytical model is developed in which the
magnetic-field strength varies to the lowest order as the square root of
the logarithm of the distance from the origin, increases with distance
in the inflow region, and decreases in the outflow region; also, the
Alfven lines curve away from the incoming flows. The solutions are
found to be valid everywhere outside the central diffusion region when
the inflow Alfven Mach number is much less than unity and to be valid
at large distances from the diffusion region when that Mach number is
of the order of unity. The results are compared with those obtained by
Sonnerup (1970), Yeb and Axford (1970), and Roberts and Priest (1975).

---------------------------------------------------------
Title: The formation of current sheets during the emergence of new
magnetic flux from below the photosphere.
Authors: Tur, T. J.; Priest, E. R.
1976SoPh...48...89T Altcode:
Solar flares are frequently observed to occur where new magnetic flux
is emerging and pressing up against strong active region magnetic
fields. Since the solar plasma is highly conducting, current sheets
develop at the boundary between the emergent and ambient flux, provided
the two magnetic fields are inclined at a non-zero angle to one another.

---------------------------------------------------------
Title: Current Sheets
Authors: Priest, E. R.
1976RSPTA.281..497P Altcode: 1976RSLPT.281..497P
Current sheets are believed to be of prime importance in the solar
atmosphere. Low down they may form at supergranulation boundaries,
whereas up in the corona they have been suggested as a prominence
formation site. In addition, they may occur when rapidly emerging flux
presses up against pre-existing magnetic fields: if rapid magnetic field
annihilation and reconnection is then triggered, a surge or a flare
may be produced. Comments are given about three aspects of general
current sheet theory. The position and shape of the current sheet
which forms between two-dimensional dipole sources is calculated. The
thermal instability which occurs when the length of the sheet exceeds
a critical value is described. Finally, a simple model of magnetic
field annihilation is presented.

---------------------------------------------------------
Title: Thermal evolution of current sheets and flash phase of
solar flares.
Authors: Heyvaerts, J.; Priest, E. R.
1976SoPh...47..223H Altcode:
The physical conditions in a stationary flow of the Petchek type,
allowing reconnection between flux emerging from below the solar
photosphere and a preexisting magnetic field, are discussed. It is
shown that, when rising in the solar atmosphere, the reconnection
region has at first a rather low temperature as compared with its
environment. Above a certain critical height, however, this low
temperature thermal equilibrium often ceases to be possible, and the
sheet rapidly heats, seeking a new thermal equilibrium. During this
dynamical process, current-driven microinstabilities may be triggered in
the current sheet, giving rise to an enhanced resistivity. High energy
particles might be produced by the induced electric field developed
during the rapid readjustment of MHD flows that results from this
change in the transport properties of the plasma.

---------------------------------------------------------
Title: Current sheet models of solar flares.
Authors: Priest, E. R.
1976SoPh...47...41P Altcode:
Current sheets have been suggested as the site for flare energy release
because they can convert magnetic energy very rapidly into both heat
and directed plasma energy. Also they contain electric fields with
the potential of accelerating particles to high energies.

---------------------------------------------------------
Title: Report on the solar physics - plasma physics workshop. held
at Stanford University, 17 - 20 September 1974.
Authors: Sturrock, P. A.; Baum, P. J.; Beckers, J. M.; Newman, C. E.;
Priest, E. R.; Rosenberg, H.; Smith, D. F.; Wentzel, D. G.
1976SoPh...46..411S Altcode:
This report summarizes the proceedings of a meeting held on
17-20 September 1974, at Stanford University. The purpose was to
explore plasma physics problems which arise in the study of solar
physics. Sessions were concerned with specific questions including
the following: Is the solar plasma thermal or non-thermal? What
spectroscopic data are required? What types of magnetic field structures
exist? Do MHD instabilities occur? Do resistive or non-MHD instabilities
occur? What mechanisms of particle acceleration have been proposed? What
information do we have concerning shock waves? Very few questions were
answered categorically but, for each question, there was discussion
concerning the observational evidence, theoretical analyses, and
existing or potential laboratory and numerical experiments.

---------------------------------------------------------
Title: The solar flare phenomenon.
Authors: Priest, E. R.
1976pspe.proc..144P Altcode: 1976pspe.conf..144P
The basic features of a solar flare are first described in terms of a
preflare heating phase, an impulsive phase, a flash phase and a main
phase. Then some recent observations are surveyed, including the results
of recent high resolution H-alpha photography and photospheric magnetic
and velocity field measurements; they indicate the importance of new
flux which emerges from below the photosphere prior to the start of the
flare. Also, the structure and time evolution of the high energy flare
plasma which emits X-radiation is described. Finally, flare theories
are classified into flux tube and current sheet types. In the former
category, the Alfven-Carlqvist and 'Alfven-wave' theories are described,
while, in the latter, Syrovatsky's mechanism and Sturrock's model are
summarized, as well as a new 'emerging flux model'.

---------------------------------------------------------
Title: On Fast Magnetic Field Reconnection
Authors: Priest, E. R.; Soward, A. M.
1976IAUS...71..353P Altcode:
No abstract at ADS

---------------------------------------------------------
Title: On the maximum rate of magnetic-field reconnexion for
Petschek's mechanism
Authors: Roberts, B.; Priest, E. R.
1975JPlPh..14..417R Altcode:
The standard model for fast magnetic-field reconnexion (Petschek 1964)
is qualitatively valid, despite numerous criticisms of its quantitative
details. It contains four slow magnetohydrodynamic shock waves, which
radiate from a central diffusion region. On the basis of Petschek's
rough analysis, it is generally stated that, for large values of the
magnetic Reynolds number Rm, reconnexion can occur at a
rate no faster than a fraction π/(4 log Rm) of the Alfvén
speed. Alternative models of the region outside that of diffusion have
been put forward by Yeh &amp; Axford (1970), whose general solutions
Vasyliusnas (1975) proved invalid, and by Sonnerup (1970), whose model
is mathematically useful, but of limited practical applicability. But
their results suggest that reconnexion can occur at any rate whatsoever,
with the diffusion-region dimensions responding accordingly. The present
paper analyses the external region for Petschek's mechanism in greater
detail than hitherto, with the object of deciding whether or not there
is a maximum rate. The inclinations of the shock waves are calculated
as a function of the fluid speed ve at large distances,
which is taken as a measure of the reconnexion rate. It is found that,
in agreement with Petschek's rough analysis, there is indeed an upper
limit on the allowable rate of magnetic-field reconnexion. Its variation
with Rm is calculated, and it is shown, for log10
Rm 1, to be approximately 20% of Petschek's value. Typical
values are 0·10vAe for Rm = 10·2 and
0·02vAe for Rm = 106. (vAe
is the Alfvén speed at large distances from the diffusion region.)

---------------------------------------------------------
Title: Some comments on magnetic field reconnection
Authors: Priest, E. R.; Cowley, S. W. H.
1975JPlPh..14..271P Altcode:
Some comments are made about how to determine the speed with
which magnetic flux is carried towards an X-type neutral point and
reconnected. Conditions in the diffusion region near the neutral point
are also investigated, with the conclusion that the streamlines and
magnetic field lines cannot both be locally hyperbolic. Instead, two
distinct modes may be possible. In the first, the magnetic field lines
are straight, and the diffusion region does not differ greatly from
that described by Parker (1963). In the second, the fluid velocity
components increase cubically away from the neutral point with the
result that, for a given reconnection rate, the diffusion region is
typically five times greater than in the Parker model.

---------------------------------------------------------
Title: Resistive MHD stagnation-point flows at a current sheet
Authors: Sonnerup, B. U. O.; Priest, E. R.
1975JPlPh..14..283S Altcode:
A family of exact solutions to the MHD equations is presented for
steady incompressible two- and three-dimensional flow in the vicinity
of the stagnation point, which forms in a current sheet separating two
colliding plasma streams. The magnetic field in each plasma is strictly
parallel to the current sheet, but can have different magnitudes and
directions. Resistive and viscous effects are accounted for. These
flows are of considerable interest in connexion with the magnetic
field merging process. They represent the limit of resistive field
annihilation with zero reconnexion.

---------------------------------------------------------
Title: Preflare Current Sheets in the Solar Atmosphere
Authors: Priest, E. R.; Raadu, M. A.
1975SoPh...43..177P Altcode:
Neutral current sheets are expected to form in the solar atmosphere
when photospheric motions or the emergence of new magnetic flux causes
oppositely directed magnetic fields to be pressed together. Magnetic
energy may thus be stored slowly in excess of the minimum energy
associated with a purely potential field and released suddenly during
a solar flare. For simplicity, we investigate the neutral sheet which
forms between two parallel line dipoles when either the distance
between them decreases or their dipole moments increase. It is found
that, when the dipoles have approached by an amount equal to a tenth
of their original separation distance, the stored energy is comparable
with that released in a major flare. In addition, a similarity solution
for one-dimensional magnetohydro-dynamic flow within such a neutral
sheet is presented; it demonstrates that rapid conversion of magnetic
energy into heat is possible provided conditions at the edge of the
neutral sheet are changing sufficiently quickly.

---------------------------------------------------------
Title: A model for the solar flare
Authors: Canfield, R. C.; Priest, E. R.; Rust, D. M.
1975STIN...7615007C Altcode:
It is suggested that many solar flares occur due to an interaction
between newly emerging magnetic flux and an active region filament. A
current sheet forms between the new and old flux and, when its
electric current density exceeds a critical value, rapid magnetic
fields reconnection takes place. Electrons are then accelerated
to high energies and follow the magnetic field lines down to the
chromosphere where they produce several (typically three) bright H
alpha knots. Magnetic energy is continuously released as reconnected
prominence fields are allowed to untwist. Two ribbons of H alpha
emission are produced by one or more of three mechanisms: energetic
particles, thermal conduction or shocks due to infalling material.

---------------------------------------------------------
Title: Theories of magnetic field annihilation.
Authors: Priest, E. R.; Sonnerup, B. U. Oe.
1975GeoJ...41..405P Altcode: 1975GeoJI..41..405P
No abstract at ADS

---------------------------------------------------------
Title: A Clue to the Trigger for Both the Type III Solar Radioburst
and the Solar Flare
Authors: Priest, E. R.; Heyvaerts, J.
1974SoPh...36..433P Altcode:
Recent observations of `neutral line absorbing features' in the solar
atmosphere may give an important clue to the mechanism whereby both
type III solar radiobursts and solar flares are triggered. It is
suggested that as new satellite magnetic flux emerges at the edge
of an active region in an area of opposite polarity a neutral sheet
builds up between the new and old flux. When the sheet has a length of
about a megametre its thermal insulation from the surrounding plasma
is effective enough for a thermal instability to occur. The resulting
compression and inflow of plasma is observed in Hα on the disc as a
neutral line absorbing feature. Furthermore, the electric field of
the accompanying collisionless tearing mode instability in a thin
slab near the centre of the sheet exceeds the runaway field; it may
therefore accelerate electrons to high enough energies to produce the
type III burst which usually occurs at the same time as the absorbing
feature. Perhaps the flare which sometimes ensues is triggered when the
quasi-equilibrium state is destroyed by the development of turbulence
in the neutral sheet.

---------------------------------------------------------
Title: The Influence of Non-Uniform Solar Wind Expansion on the
Angular Momentum Loss from the Sun
Authors: Priest, E. R.; Pneuman, G. W.
1974SoPh...34..231P Altcode:
The influence on the rate of angular momentum loss from the
Sun of magnetic geometries which are not spherically symmetric is
estimated. Departures from spherical symmetry are expected to influence
significantly the loss rate by two effects - the presence of closed
magnetic field regions with no loss and also the variability in the
radial distance to the Alfvénic point, as stressed by Mestel (1968).

---------------------------------------------------------
Title: On Neutral Line Absorbing Features
Authors: Heyvaerts, J.; Priest, E.
1974cesra...4..147H Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Solar physics. Plasma physics workshop.
Authors: Baum, P. J.; Beckers, J. M.; Newman, C. E.; Priest, E. R.;
Rosenberg, H.; Smith, D. F.; Sturrock, P. A.; Wentzel, D. G.
1974sppp.book.....B Altcode:
No abstract at ADS

---------------------------------------------------------
Title: The Nonlinear Acceleration of a Magnetic Disturbance in the
Solar Corona
Authors: Altschuler, Martin D.; Smith, Dean F.; Swarztrauber, Paul N.;
Priest, Eric R.
1973SoPh...32..153A Altcode:
The simple form of Ohm's law (SI units)J = σ(E+ v × B)is valid for
high density magnetofluids (where the mean free path for collisions
is less than the Larmor radius) but is not strictly valid for the
tenuous solar corona. We examine the nonlinear evolution of a magnetic
disturbance using a more general form of Ohm's law which includes
the Hall term. The Hall term dominates MHD development in the corona
when the product of the magnetic scale length and the square root
of the density is small enough; in particular when (1) the electron
density is less than about 1013 m-3 and (2) the
scale length is less than a few hundred meters. For these parameters,
a magnetic disturbance may carry electrons at a drift speed in excess of
the Alfvén speed. We believe this nonlinear phenomenon may be important
for the impulsive acceleration of charged particles in the solar corona.

---------------------------------------------------------
Title: The Trapped Magnetoatmospheric Waves
Authors: Nakagawa, Y.; Priest, E. R.; Wellck, R. E.
1973ApJ...184..931N Altcode:
The trapped (vertically nonpropagating) magnetoatmospheric waves are
examined on the basis of local dispersion relations governing the
propagation of magnetoacoustic and magnetogravitational waves in an
inviscid atmosphere of infinite electrical conductivity permeated by
a uniform magnetic field. It is shown from the necessary condition
that trapped waves are possible, (1) when the magnetic field is purely
horizontal, (2) when the magnetic field is purely vertical, and (3)
when the magnetic field has both vertical and horizontal components
for waves propagating perpendicular to the magnetic field. Through
examinations of the sufficient conditions for each of these three
specific circumstances, the domain of trapped waves is determined in the
(angular frequency, horizontal wavenumber)-plane. It is shown that for
a horizontal magnetic field, most of the magnetoatmospheric waves become
trapped, except for the magnetoacoustic waves. It is shown also that for
a vertical or a general magnetic field, most of the magnetoatmospheric
waves with the frequency below the local Brunt- frequency could become
trapped. The physical significance and possible practical application
of the results to solar observations are discussed. Subject headings:
atmospheres, solar - hydromagnetics - solar atmospheric motions

---------------------------------------------------------
Title: On Steady Magnetic-Field Reconnection
Authors: Priest, E. R.
1973ApJ...181..227P Altcode:
Magnetic-field reconnection may be essential in a variety of solar
and astrophysical phenomena, and there has been much debate as to
the maximum allowable rate of reconnection. Petschek's mechanism,
for instance, permits a maximum rate of about one-tenth of the Alfven
speed, UA, but is probably unworkable. Yeh and Axford, on the other
hand, have recently claimed that any rate is possible up to UA. A
more careful matching of the internal and external regions in Yeh
and Axford's mechanism is here performed, with the condusion. that
the highest rate of reconnection is about UA/l8. Subject headings:
flares, solar - hydromagnetics - plasmas

---------------------------------------------------------
Title: Evolution of a Magnetic Disturbance in the Solar Corona With
a General Ohm's Law
Authors: Altschuler, M. D.; Smith, D. F.; Swarztrauber, P.; Priest,
E. R.
1973BAAS....5S.268A Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Erratum: Current Limitation in Solar Flares
Authors: Smith, Dean F.; Priest, E. R.
1973ApJ...180..667S Altcode:
No abstract at ADS

---------------------------------------------------------
Title: The Energy Spectrum of Small-Scale Solar Magnetic Fields
Authors: Nakagawa, Y.; Priest, E. R.
1973ApJ...179..949N Altcode:
On the basis of observations that magnetic flux is transported by
fluid motions at the photospheric level of the Sun, the possible
interpretation of the energy spectra of small-scale solar magnetic
fields in terms of a passive response of the (longitudinal) magnetic
field to turbulent fluid motions is examined. In consideration of the
prevailing physical conditions, a theory is developed which accounts
for the two-dimensional passive response of longitudinal magnetic
fields to a three- dimensionally isotropic turbulent convection at
the level of vanishing vertical velocity. The theoretical results
are compared with the energy spectra of small-scale magnetic fields
obtained from longitudinal magnetograms for typical active, quiet,
and mixed regions of the Sun. It is shown that the observed spectra
can be interpreted in terms of the theory developed with the choice of
eddy kinematic viscosity 1014 cm2 5 -` and eddy magnetic diffusivity
1012 cm2 -1, including differences in the spectral shape for different
regions of solar activity. The details of the theory are described
together with discussions on the limitations and other implications
of the results. Subject headings: hydromagnetics - magnetic fields,
solar - turbulence

---------------------------------------------------------
Title: A Possible New Interpretation of Power Spectra of
Solar-Granulation Brightness Fluctuations
Authors: Nakagawa, Y.; Priest, E. R.
1972ApJ...178..251N Altcode:
The brightness fluctuations of solar granulation are attributed
to local temperature fluctuations through the photosphere. It is
plausible to consider that the temperature fluctuations result from a
passive response of temperature to turbulent fluid convections. The
theoretical spectrum governing such a passive response given by
Batchelor is compared with the power spectra derived by Edmonds from
Stratoscope photographs, and it is shown that a satisfactory agreement
can be obtained. Discrepancies between this result and observations
from the ground are discussed.

---------------------------------------------------------
Title: Current Sheets in Coronal Streamers
Authors: Priest, E. R.; Smith, D. F.
1972ApL....12...25P Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Current Limitation in Solar Flares
Authors: Smith, Dean F.; Priest, E. R.
1972ApJ...176..487S Altcode:
It is shown that the ion-sound current instability will be the relevant
mechanism of current dissipation in most instances in the fully ionized
part of the solar atmosphere (upper chromosphere and corona). Since
this instability has the same initial threshold as the Alfven-Cariqvist
instability, but a considerably lower threshold once the instability
has started toward saturation, the Affven-Carlqvist instability will
not occur. The linear and nonlinear stages of the ion-sound current
instability are examined and a turbulent resistivity is given for the
saturated state which is in agreement with laboratory experiments. This
turbulent resistivity is applied to the Alfven-Carlqvist model of a
cylindrical current filament, and it is shown that enough energy could
not be dissipated sufficiently fast for a solar flare. This turbulent
resistivity is also applied to the "hard" phase of Syrovatsky's dynamic
dissipation model, and it is shown that Syrovatsky's electromagnetic
instability for instantaneous current-sheet interruption cannot
occur. The possibility that the ion-sound can systematically accelerate
a select group of particles is investigated. It is concluded that
heating rather than acceleration will result. It is suggested that
a small electric field acting over large distances is a potentially
much more efficient accelerating mechanism.

---------------------------------------------------------
Title: Current Limitation in Solar Flares
Authors: Smith, D. F.; Priest, E. R.
1972BAAS....4Q.392S Altcode:
No abstract at ADS

---------------------------------------------------------
Title: An Inconsistency in Petschek's Mechanism
Authors: Priest, E. R.
1972BAAS....4R.389P Altcode:
No abstract at ADS

---------------------------------------------------------
Title: A modification and criticism of Petschek's mechanism
Authors: Priest, E. R.
1972MNRAS.159..389P Altcode:
Petschek's mechanism is shown to be internally inconsistent when
variations in magnetic field strength along the current sheet are
included. The inconsistency can be removed for both incompressible and
compressible flow provided the magnetic field lines in the narrow region
between a pair of shock waves are pulled out into long loops rather than
being only slightly bowed. If the mechanism is modified in this way,
the rate of energy conversion is not significantly altered. However,
there are several difficulties which remain and, unless they can be
overcome, Petschek's mechanism must be considered unworkable.

---------------------------------------------------------
Title: Magnetohydrodynamic neutral point theory
Authors: Priest, Eric Ronald
1970PhDT........87P Altcode:
No abstract at ADS