Author name code: parnell ADS astronomy entries on 2022-09-14 author:"Parnell, Clare E." ------------------------------------------------------------------------ Title: Critical Science Plan for the Daniel K. Inouye Solar Telescope (DKIST) Authors: Rast, Mark P.; Bello González, Nazaret; Bellot Rubio, Luis; Cao, Wenda; Cauzzi, Gianna; Deluca, Edward; de Pontieu, Bart; Fletcher, Lyndsay; Gibson, Sarah E.; Judge, Philip G.; Katsukawa, Yukio; Kazachenko, Maria D.; Khomenko, Elena; Landi, Enrico; Martínez Pillet, Valentín; Petrie, Gordon J. D.; Qiu, Jiong; Rachmeler, Laurel A.; Rempel, Matthias; Schmidt, Wolfgang; Scullion, Eamon; Sun, Xudong; Welsch, Brian T.; Andretta, Vincenzo; Antolin, Patrick; Ayres, Thomas R.; Balasubramaniam, K. S.; Ballai, Istvan; Berger, Thomas E.; Bradshaw, Stephen J.; Campbell, Ryan J.; Carlsson, Mats; Casini, Roberto; Centeno, Rebecca; Cranmer, Steven R.; Criscuoli, Serena; Deforest, Craig; Deng, Yuanyong; Erdélyi, Robertus; Fedun, Viktor; Fischer, Catherine E.; González Manrique, Sergio J.; Hahn, Michael; Harra, Louise; Henriques, Vasco M. J.; Hurlburt, Neal E.; Jaeggli, Sarah; Jafarzadeh, Shahin; Jain, Rekha; Jefferies, Stuart M.; Keys, Peter H.; Kowalski, Adam F.; Kuckein, Christoph; Kuhn, Jeffrey R.; Kuridze, David; Liu, Jiajia; Liu, Wei; Longcope, Dana; Mathioudakis, Mihalis; McAteer, R. T. James; McIntosh, Scott W.; McKenzie, David E.; Miralles, Mari Paz; Morton, Richard J.; Muglach, Karin; Nelson, Chris J.; Panesar, Navdeep K.; Parenti, Susanna; Parnell, Clare E.; Poduval, Bala; Reardon, Kevin P.; Reep, Jeffrey W.; Schad, Thomas A.; Schmit, Donald; Sharma, Rahul; Socas-Navarro, Hector; Srivastava, Abhishek K.; Sterling, Alphonse C.; Suematsu, Yoshinori; Tarr, Lucas A.; Tiwari, Sanjiv; Tritschler, Alexandra; Verth, Gary; Vourlidas, Angelos; Wang, Haimin; Wang, Yi-Ming; NSO and DKIST Project; DKIST Instrument Scientists; DKIST Science Working Group; DKIST Critical Science Plan Community Bibcode: 2021SoPh..296...70R Altcode: 2020arXiv200808203R The National Science Foundation's Daniel K. Inouye Solar Telescope (DKIST) will revolutionize our ability to measure, understand, and model the basic physical processes that control the structure and dynamics of the Sun and its atmosphere. The first-light DKIST images, released publicly on 29 January 2020, only hint at the extraordinary capabilities that will accompany full commissioning of the five facility instruments. With this Critical Science Plan (CSP) we attempt to anticipate some of what those capabilities will enable, providing a snapshot of some of the scientific pursuits that the DKIST hopes to engage as start-of-operations nears. The work builds on the combined contributions of the DKIST Science Working Group (SWG) and CSP Community members, who generously shared their experiences, plans, knowledge, and dreams. Discussion is primarily focused on those issues to which DKIST will uniquely contribute. Title: A comparison of methods for finding magnetic nulls in simulations and in situ observations of space plasmas Authors: Olshevsky, V.; Pontin, D. I.; Williams, B.; Parnell, C. E.; Fu, H. S.; Liu, Y.; Yao, S.; Khotyaintsev, Y. V. Bibcode: 2020A&A...644A.150O Altcode: 2021arXiv210102014O Context. Magnetic nulls are ubiquitous in space plasmas, and are of interest as sites of localised energy dissipation or magnetic reconnection. As such, a number of methods have been proposed for detecting nulls in both simulation data and in situ spacecraft data from Earth's magnetosphere. The same methods can be applied to detect stagnation points in flow fields.
Aims: In this paper we describe a systematic comparison of different methods for finding magnetic nulls. The Poincaré index method, the first-order Taylor expansion (FOTE) method, and the trilinear method are considered.
Methods: We define a magnetic field containing fourteen magnetic nulls whose positions and types are known to arbitrary precision. Furthermore, we applied the selected techniques in order to find and classify those nulls. Two situations are considered: one in which the magnetic field is discretised on a rectangular grid, and the second in which the magnetic field is discretised along synthetic "spacecraft trajectories" within the domain.
Results: At present, FOTE and trilinear are the most reliable methods for finding nulls in the spacecraft data and in numerical simulations on Cartesian grids, respectively. The Poincaré index method is suitable for simulations on both tetrahedral and hexahedral meshes.
Conclusions: The proposed magnetic field configuration can be used for grading and benchmarking the new and existing tools for finding magnetic nulls and flow stagnation points. Title: Analysing AIA Flare Observations using Convolutional Neural Networks Authors: Love, Teri; Neukirch, Thomas; Parnell, Clare E. Bibcode: 2020FrASS...7...34L Altcode: 2020arXiv200513287L In order to efficiently analyse the vast amount of data generated by solar space missions and ground-based instruments, modern machine learning techniques such as decision trees, support vector machines (SVMs) and neural networks can be very useful. In this paper we present initial results from using a convolutional neural network (CNN) to analyse observations from the Atmospheric Imaging Assembly (AIA) in the 1600 ̊A wavelength. The data is pre-processed to locate flaring regions where Hα flare ribbons are visible in the observations. The CNN is created and trained to automatically analyse the shape and position of the flare ribbons, by identifying whether a data set belongs into one of four classes: two-ribbon flare, compact/circular ribbon flare, limb flare or quiet Sun, with the final class acting as a control for any data included in the training or test sets where flaring regions are not present. The network created can classify flare ribbon observations into any of the four classes with a final accuracy of 94%. Initial results show that most of the flares are correctly classified with the limb flare class being the only class where accuracy drops and some observations are wrongly classified Title: Particle acceleration with anomalous pitch angle scattering in 3D separator reconnection Authors: Borissov, A.; Neukirch, T.; Kontar, E. P.; Threlfall, J.; Parnell, C. E. Bibcode: 2020A&A...635A..63B Altcode: 2020arXiv200107548B Context. Understanding how the release of stored magnetic energy contributes to the generation of non-thermal high energy particles during solar flares is an important open problem in solar physics. There is a general consensus that magnetic reconnection plays a fundamental role in the energy release and conversion processes taking place during flares. A common approach for investigating how reconnection contributes to particle acceleration is to use test particle calculations in electromagnetic fields derived from numerical magnetohydrodynamic (MHD) simulations of reconnecting magnetic fields. These MHD simulations use anomalous resistivities that are orders of magnitude larger than the Spitzer resistivity that is based on Coulomb collisions. The processes leading to such an enhanced resistivity should also affect the test particles, for example, through pitch angle scattering. This study explores the effect of such a link between the level of resistivity and its impact on particle orbits and builds on a previous study using a 2D MHD simulation of magnetic reconnection.
Aims: This paper aims to extend the previous investigation to a 3D magnetic reconnection configuration and to study the effect on test particle orbits.
Methods: We carried out orbit calculations using a 3D MHD simulation of reconnection in a magnetic field with a magnetic separator. The orbit calculations use the relativistic guiding centre approximation but, crucially, they also include pitch angle scattering using stochastic differential equations. The effects of varying the resistivity and the models for pitch angle scattering on particle orbit trajectories, final positions, energy spectra, final pitch angle distribution, and orbit duration are all studied in detail.
Results: Pitch angle scattering widens highly collimated beams of unscattered orbit trajectories, allowing orbits to access previously unaccessible field lines; this causes final positions to spread along other topological structures which could not be accessed without scattering. Scattered orbit energy spectra are found to be predominantly affected by the level of anomalous resistivity, with the pitch angle scattering model only playing a role in specific, isolated cases. This is in contrast to the study involving a 2D MHD simulation of magnetic reconnection, where pitch angle scattering had a more noticeable effect on the energy spectra. Pitch scattering effects are found to play a crucial role in determining the pitch angle and orbit duration distributions. Title: Determining whether the squashing factor, Q, would be a good indicator of reconnection in a resistive MHD experiment devoid of null points Authors: Reid, J.; Parnell, C. E.; Hood, A. W.; Browning, P. K. Bibcode: 2020A&A...633A..92R Altcode: The squashing factor of a magnetic field, Q, is commonly used as an indicator of magnetic reconnection, but few studies seek to evaluate how reliable it is in comparison with other possible reconnection indicators. By using a full, self-consistent, three-dimensional, resistive magnetohydrodynamic experiment of interacting magnetic strands constituting a coronal loop, Q and several different quantities are determined. Each is then compared with the necessary and sufficient condition for reconnection, namely the integral along a field line of the component of the electric field parallel to the magnetic field. Among the reconnection indicators explored, we find the squashing factor less successful when compared with alternatives, such as Ohmic heating. In a reconnecting magnetic field devoid of null points, our work suggests that Q, being a geometric measure of the magnetic field, is not a reliable indicator of the onset or a diagnostic of the location of magnetic reconnection in some configurations. Title: Coronal energy release by MHD avalanches: Heating mechanisms Authors: Reid, J.; Cargill, P. J.; Hood, A. W.; Parnell, C. E.; Arber, T. D. Bibcode: 2020A&A...633A.158R Altcode: The plasma heating associated with an avalanche involving three twisted magnetic threads within a coronal loop is investigated using three-dimensional magnetohydrodynamic simulations. The avalanche is triggered by the kink instability of one thread, with the others being engulfed as a consequence. The heating as a function of both time and location along the strands is evaluated. It is shown to be bursty at all times but to have no preferred spatial location. While there appears to be a level of "background" heating, this is shown to be comprised of individual, small heating events. A comparison between viscous and resistive (Ohmic) heating demonstrates that the strongest heating events are largely associated with the Ohmic heating that arises when the current exceeds a critical value. Viscous heating is largely (but not entirely) associated with smaller events. Ohmic heating dominates viscous heating only at the time of the initial kink instability. It is also demonstrated that a variety of viscous models lead to similar heating rates, suggesting that the system adjusts to dissipate the same amount of energy. Title: Coronal energy release by MHD avalanches: continuous driving Authors: Reid, J.; Hood, A. W.; Parnell, C. E.; Browning, P. K.; Cargill, P. J. Bibcode: 2018A&A...615A..84R Altcode: Previous work has confirmed the concept of a magnetohydrodynamic (MHD) avalanche in pre-stressed threads within a coronal loop. We undertook a series of full, three-dimensional MHD simulations in order to create three threads by twisting the magnetic field through boundary motions until an instability ensues. We find that, following the original instability, one unstable thread can disrupt its neighbours with continued driving. A "bursty" heating profile results, with a series of ongoing energy releases, but no evident steady state. For the first time using full MHD, we show that avalanches are a viable mechanism for the storing and release of magnetic energy in the solar corona, as a result of photospheric motions. Title: MHS Models of Current Layers in the Solar Atmosphere Authors: Parnell, Clare E. Bibcode: 2018GMS...235..219P Altcode: No abstract at ADS Title: Effects of Anomalous Resistivity on Particle Acceleration due to Pitch Angle Scattering. Authors: Borissov, A.; Kontar, E.; Neukirch, T.; Threlfall, J. W.; Stevenson, J.; Parnell, C. E. Bibcode: 2017AGUFMSH41B2765B Altcode: The mechanisms for generation of non-thermal accelerated particles in flares is one of the outstanding problems in solar physics. The energy for powering solar flares fundamentally comes from the coronal magnetic field and its release involves magnetic reconnection. One direct way of accelerating charged particles is due to the parallel electric field generated during magnetic reconnection. To achieve a sufficiently rapid release of energy an anomalous resistivity, several orders of magnitude larger than the Spitzer resistivity in the corona, is often invoked, particularly when performing mganetohydrodynamic (MHD) simulations of solar flares. Since resistivity is fundamentally connected to particle scattering, an enhanced anomalous resistivity relative to the Spitzer resistivity should result in an enhanced scattering frequency relative to the Coulomb scattering rate. We present results of test particle simulations that attempt to account for these phenomena by introducing pitch angle scattering at a rate dependent on the ratio of the anomalous to Spitzer resistivity in the context of MHD simulations of magnetic reconnection. We find that test particle trajectories and durations are significantly modified by the presence of resistivity dependent pitch angle scattering, with particle energy spectra also being affected in some cases Title: Particle Acceleration Due to Coronal Non-null Magnetic Reconnection Authors: Threlfall, James; Neukirch, Thomas; Parnell, Clare Elizabeth Bibcode: 2017SoPh..292...45T Altcode: Various topological features, for example magnetic null points and separators, have been inferred as likely sites of magnetic reconnection and particle acceleration in the solar atmosphere. In fact, magnetic reconnection is not constrained to solely take place at or near such topological features and may also take place in the absence of such features. Studies of particle acceleration using non-topological reconnection experiments embedded in the solar atmosphere are uncommon. We aim to investigate and characterise particle behaviour in a model of magnetic reconnection which causes an arcade of solar coronal magnetic field to twist and form an erupting flux rope, crucially in the absence of any common topological features where reconnection is often thought to occur. We use a numerical scheme that evolves the gyro-averaged orbit equations of single electrons and protons in time and space, and simulate the gyromotion of particles in a fully analytical global field model. We observe and discuss how the magnetic and electric fields of the model and the initial conditions of each orbit may lead to acceleration of protons and electrons up to 2 MeV in energy (depending on model parameters). We describe the morphology of time-dependent acceleration and impact sites for each particle species and compare our findings to those recovered by topologically based studies of three-dimensional (3D) reconnection and particle acceleration. We also broadly compare aspects of our findings to general observational features typically seen during two-ribbon flare events. Title: Particle dynamics in a non-flaring solar active region model Authors: Threlfall, J.; Bourdin, Ph. -A.; Neukirch, T.; Parnell, C. E. Bibcode: 2016A&A...587A...4T Altcode: 2015arXiv151004211T
Aims: The aim of this work is to investigate and characterise particle behaviour in an (observationally-driven) 3D magnetohydrodynamic (MHD) model of the solar atmosphere above a slowly evolving, non-flaring active region.
Methods: We use a relativistic guiding-centre particle code to investigate the behaviour of selected particle orbits, distributed throughout a single snapshot of the 3D MHD simulation.
Results: Two distinct particle acceleration behaviours are recovered, which affect both electrons and protons: (I) direct acceleration along field lines and (II) tangential drifting of guiding centres with respect to local magnetic field. However, up to 40% of all particles actually experience a form of (high energy) particle trap, because of changes in the direction of the electric field and unrelated to the strength of the magnetic field; such particles are included in the first category. Additionally, category (I) electron and proton orbits undergo surprisingly strong acceleration to non-thermal energies (≲42 MeV), because of the strength and extent of super-Dreicer electric fields created by the MHD simulation. Reducing the electric field strength of the MHD model does not significantly affect the efficiency of the (electric field-based) trapping mechanism, but does reduce the peak energies gained by orbits. We discuss the implications for future experiments, which aim to simulate non-flaring active region heating and reconnection. Title: Particle acceleration at reconnecting separator current layers Authors: Threlfall, J.; Stevenson, J. E. H.; Parnell, C. E.; Neukirch, T. Bibcode: 2016A&A...585A..95T Altcode: 2015arXiv151004215T
Aims: The aim of this work is to investigate and characterise particle behaviour in a 3D magnetohydrodynamic (MHD) model of a reconnecting magnetic separator.
Methods: We use a relativistic guiding-centre test-particle code to investigate electron and proton acceleration in snapshots from 3D MHD separator reconnection experiments, and compare the results with findings from an analytical separator reconnection model studied in a previous investigation.
Results: The behaviour and acceleration of large distributions of particles are examined in detail for both analytical and numerical separator reconnection models. Accelerated particle orbit trajectories are shown to follow the separator before leaving the system along the separatrix surface of one of the nulls (determined by particle species) in the system of both models. A sufficiently localised electric field about the separator causes the orbits to appear to follow the spine bounding the separatrix surface field lines instead. We analyse and discuss the locations and spread of accelerated particle orbit final positions, which are seen to change over time in the numerical separator reconnection model. We deduce a simple relationship between the final energy range of particle orbits and the model dimensions, and discuss its implications for observed magnetic separators in the solar corona. Title: A Comparison of Global Magnetic Field Skeletons and Active-Region Upflows Authors: Edwards, S. J.; Parnell, C. E.; Harra, L. K.; Culhane, J. L.; Brooks, D. H. Bibcode: 2016SoPh..291..117E Altcode: 2015SoPh..tmp..161E Plasma upflows have been detected in active regions using Doppler velocity maps. The origin and nature of these upflows is not well known with many of their characteristics determined from the examination of single events. In particular, some studies suggest these upflows occur along open field lines and, hence, are linked to sources of the solar wind. To investigate the relationship these upflows may have with the solar wind, and to probe what may be driving them, this paper considers seven active regions observed on the solar disc using the Extreme ultraviolet Imaging Spectrometer aboard Hinode between August 2011 and September 2012. Plasma upflows are observed in all these active regions. The locations of these upflows are compared to the global potential magnetic field extrapolated from the Solar Dynamics Observatory, Helioseismic and Magnetic Imager daily synoptic magnetogram taken on the day the upflows were observed. The structure of the magnetic field is determined by constructing its magnetic skeleton in order to help identify open-field regions and also sites where magnetic reconnection at global features is likely to occur. As a further comparison, measurements of the temperature, density and composition of the plasma are taken from regions with active-region upflows. In most cases the locations of the upflows in the active regions do not correspond to areas of open field, as predicted by a global coronal potential-field model, and therefore these upflows are not always sources of the slow solar wind. The locations of the upflows are, in general, intersected by separatrix surfaces associated with null points located high in the corona; these could be important sites of reconnection with global consequences. Title: Spontaneous reconnection at a separator current layer: 2. Nature of the waves and flows Authors: Stevenson, J. E. H.; Parnell, C. E. Bibcode: 2015JGRA..12010353S Altcode: 2015arXiv150907743S Sudden destabilizations of the magnetic field, such as those caused by spontaneous reconnection, will produce waves and/or flows. Here we investigate the nature of the plasma motions resulting from spontaneous reconnection at a 3-D separator. In order to clearly see these perturbations, we start from a magnetohydrostatic equilibrium containing two oppositely signed null points joined by a generic separator along which lies a twisted current layer. The nature of the magnetic reconnection initiated in this equilibrium as a result of an anomalous diffusivity is discussed in detail in Stevenson and Parnell (2015). The resulting sudden loss of force balance inevitably generates waves that propagate away from the diffusion region carrying the dissipated current. In their wake a twisting stagnation flow, in planes perpendicular to the separator, feeds flux back into the original diffusion site (the separator) in order to try to regain equilibrium. This flow drives a phase of slow weak impulsive bursty reconnection that follows on after the initial fast-reconnection phase. Title: Spontaneous reconnection at a separator current layer: 1. Nature of the reconnection Authors: Stevenson, J. E. H.; Parnell, C. E. Bibcode: 2015JGRA..12010334S Altcode: 2015arXiv150907729S Magnetic separators, which lie on the boundary between four topologically distinct flux domains, are prime locations in three-dimensional magnetic fields for reconnection, especially in the magnetosphere between the planetary and interplanetary magnetic fields and also in the solar atmosphere. Little is known about the details of separator reconnection, and so the aim of this paper, which is the first of two, is to study the properties of magnetic reconnection at a single separator. Three-dimensional, resistive magnetohydrodynamic numerical experiments are run to study separator reconnection starting from a magnetohydrostatic equilibrium which contains a twisted current layer along a single separator linking a pair of opposite-polarity null points. The resulting reconnection occurs in two phases. The first is short involving rapid reconnection in which the current at the separator is reduced by a factor of around 2.3. Most (75%) of the magnetic energy is converted during this phase, via Ohmic dissipation, directly into internal energy, with just 0.1% going into kinetic energy. During this phase the reconnection occurs along most of the separator away from its ends (the nulls) but in an asymmetric manner which changes both spatially and temporally over time. The second phase is much longer and involves slow impulsive bursty reconnection. Again, Ohmic heating dominates over viscous damping. Here the reconnection occurs in small localized bursts at random anywhere along the separator. Title: The appearance, motion, and disappearance of three-dimensional magnetic null points Authors: Murphy, Nicholas A.; Parnell, Clare E.; Haynes, Andrew L. Bibcode: 2015PhPl...22j2117M Altcode: 2015arXiv150905915M While theoretical models and simulations of magnetic reconnection often assume symmetry such that the magnetic null point when present is co-located with a flow stagnation point, the introduction of asymmetry typically leads to non-ideal flows across the null point. To understand this behavior, we present exact expressions for the motion of three-dimensional linear null points. The most general expression shows that linear null points move in the direction along which the magnetic field and its time derivative are antiparallel. Null point motion in resistive magnetohydrodynamics results from advection by the bulk plasma flow and resistive diffusion of the magnetic field, which allows non-ideal flows across topological boundaries. Null point motion is described intrinsically by parameters evaluated locally; however, global dynamics help set the local conditions at the null point. During a bifurcation of a degenerate null point into a null-null pair or the reverse, the instantaneous velocity of separation or convergence of the null-null pair will typically be infinite along the null space of the Jacobian matrix of the magnetic field, but with finite components in the directions orthogonal to the null space. Not all bifurcating null-null pairs are connected by a separator. Furthermore, except under special circumstances, there will not exist a straight line separator connecting a bifurcating null-null pair. The motion of separators cannot be described using solely local parameters because the identification of a particular field line as a separator may change as a result of non-ideal behavior elsewhere along the field line. Title: The appearance, motion, and disappearance of 3D magnetic null points Authors: Murphy, Nicholas A.; Parnell, Clare; Haynes, Andrew Bibcode: 2015shin.confE...7M Altcode: While theoretical models and simulations of magnetic reconnection often assume symmetry such that the magnetic null point when present is co-located with a flow stagnation point, the introduction of asymmetry typically leads to non-ideal flows across the null point. To understand this behavior, we present exact expressions for the motion of three-dimensional linear null points. The most general expression shows that linear null points move in the direction along which the vector field and its time derivative are antiparallel. Null point motion in resistive magnetohydrodynamics results from advection by the bulk plasma flow and resistive diffusion of the magnetic field, which allows non-ideal flows across topological boundaries. Null point motion is described intrinsically by parameters evaluated locally; however, global dynamics help set the local conditions at the null point. Linear null points typically appear and disappear in pairs during the bifurcation of a degenerate null point that has a linear structure with a one-dimensional null space. During a bifurcation, the instanteous velocity of separation or convergence of the null-null pair will typically be infinite along the null space but with finite components in the directions orthogonal to the null space. Except under special circumstances, there will not exist a straight line separator connecting a bifurcating null-null pair. The motion of separators cannot be described using solely local parameters because the identification of a particular field line as a separator may change as a result of non-ideal behavior elsewhere along the field line. Title: Null Point Distribution in Global Coronal Potential Field Extrapolations Authors: Edwards, S. J.; Parnell, C. E. Bibcode: 2015SoPh..290.2055E Altcode: 2015SoPh..tmp...93E Magnetic null points are points in space where the magnetic field is zero. Thus, they can be important sites for magnetic reconnection by virtue of the fact that they are weak points in the magnetic field and also because they are associated with topological structures, such as separators, which lie on the boundary between four topologically distinct flux domains and therefore are also locations where reconnection occurs. The number and distribution of nulls in a magnetic field acts as a measure of the complexity of the field. 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. Bibcode: 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 >=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: Is magnetic topology important for heating the solar atmosphere? Authors: Parnell, Clare E.; Stevenson, Julie E. H.; Threlfall, James; Edwards, Sarah J. Bibcode: 2015RSPTA.37340264P Altcode: 2015arXiv150505701P Magnetic fields permeate the entire solar atmosphere weaving an extremely complex pattern on both local and global scales. In order to understand the nature of this tangled web of magnetic fields, its magnetic skeleton, which forms the boundaries between topologically distinct flux domains, may be determined. The magnetic skeleton consists of null points, separatrix surfaces, spines and separators. The skeleton is often used to clearly visualize key elements of the magnetic configuration, but parts of the skeleton are also locations where currents and waves may collect and dissipate. In this review, the nature of the magnetic skeleton on both global and local scales, over solar cycle time scales, is explained. The behaviour of wave pulses in the vicinity of both nulls and separators is discussed and so too is the formation of current layers and reconnection at the same features. Each of these processes leads to heating of the solar atmosphere, but collectively do they provide enough heat, spread over a wide enough area, to explain the energy losses throughout the solar atmosphere? Here, we consider this question for the three different solar regions: active regions, open-field regions and the quiet Sun. We find that the heating of active regions and open-field regions is highly unlikely to be due to reconnection or wave dissipation at topological features, but it is possible that these may play a role in the heating of the quiet Sun. In active regions, the absence of a complex topology may play an important role in allowing large energies to build up and then, subsequently, be explosively released in the form of a solar flare. Additionally, knowledge of the intricate boundaries of open-field regions (which the magnetic skeleton provides) could be very important in determining the main acceleration mechanism(s) of the solar wind. Title: Particle acceleration at a reconnecting magnetic separator Authors: Threlfall, J.; Neukirch, T.; Parnell, C. E.; Eradat Oskoui, S. Bibcode: 2015A&A...574A...7T Altcode: 2014arXiv1410.6465T Context. While the exact acceleration mechanism of energetic particles during solar flares is (as yet) unknown, magnetic reconnection plays a key role both in the release of stored magnetic energy of the solar corona and the magnetic restructuring during a flare. Recent work has shown that special field lines, called separators, are common sites of reconnection in 3D numerical experiments. To date, 3D separator reconnection sites have received little attention as particle accelerators.
Aims: We investigate the effectiveness of separator reconnection as a particle acceleration mechanism for electrons and protons.
Methods: We study the particle acceleration using a relativistic guiding-centre particle code in a time-dependent kinematic model of magnetic reconnection at a separator.
Results: The effect upon particle behaviour of initial position, pitch angle, and initial kinetic energy are examined in detail, both for specific (single) particle examples and for large distributions of initial conditions. The separator reconnection model contains several free parameters, and we study the effect of changing these parameters upon particle acceleration, in particular in view of the final particle energy ranges that agree with observed energy spectra. Title: The nature of separator current layers in MHS equilibria. I. Current parallel to the separator Authors: Stevenson, J. E. H.; Parnell, C. E.; Priest, E. R.; Haynes, A. L. Bibcode: 2015A&A...573A..44S Altcode: 2014arXiv1410.8691S Context. Separators, which are in many ways the three-dimensional equivalent to two-dimensional nulls, are important sites for magnetic reconnection. Magnetic reconnection occurs in strong current layers which have very short length scales.
Aims: The aim of this work is to explore the nature of current layers around separators. A separator is a special field line which lies along the intersection of two separatrix surfaces and forms the boundary between four topologically distinct flux domains. In particular, here the current layer about a separator that joins two 3D nulls and lies along the intersection of their separatrix surfaces is investigated.
Methods: A magnetic configuration containing a single separator embedded in a uniform plasma with a uniform electric current parallel to the separator is considered. This initial magnetic setup, which is not in equilibrium, relaxes in a non-resistive manner to form an equilibrium. The relaxation is achieved using the 3D MHD code, Lare3d, with resistivity set to zero. A series of experiments with varying initial current are run to investigate the characteristics of the resulting current layers present in the final (quasi-)equilibrium states.
Results: In each experiment, the separator collapses and a current layer forms along it. The dimensions and strength of the current layer increase with initial current. It is found that separator current layers formed from current parallel to the separator are twisted. Also the collapse of the separator is a process that evolves like an infinite-time singularity where the length, width and peak current in the layer grow slowly whilst the depth of the current layer decreases. Title: The Distribution and Behaviour of Photospheric Magnetic Features Authors: Parnell, C. E.; Lamb, D. A.; DeForest, C. E. Bibcode: 2014AGUFMSH34A..05P Altcode: Over the past two decades enormous amounts of data on the magnetic fields of the solar photosphere have been produced by both ground-based (Kitt Peak & SOLIS), as well as space-based instruments (MDI, Hinode & HMI). In order to study the behaviour and distribution of photospheric magnetic features, efficient automated detection routines need to be utilised to identify and track magnetic features. In this talk, I will discuss the pros and cons of different automated magnetic feature identification and tracking routines with a special focus on the requirements of these codes to deal with the large data sets produced by HMI. By patching together results from Hinode and MDI (high-res & full-disk), the fluxes of magnetic features were found to follow a power-law over 5 orders of magnitude. At the strong flux tail of this distribution, the power law was found to fall off at solar minimum, but was maintained over all fluxes during solar maximum. However, the point of deflection in the power-law distribution occurs at a patching point between instruments and so questions remain over the reasons for the deflection. The feature fluxes determined from the superb high-resolution HMI data covers almost all of the 5 orders of magnitude. Considering both solar mimimum and solar maximum HMI data sets, we investigate whether the power-law over 5 orders of magnitude in flux still holds. Furthermore, we investigate the behaviour of magnetic features in order to probe the nature of their origin. In particular, we analyse small-scale flux emergence events using HMI data to investigate the existence of a small-scale dynamo just below the solar photosphere. Title: Particle Acceleration at a Reconnecting Magnetic Separator Authors: Threlfall, J. W.; Neukirch, T.; Parnell, C. E.; Stevenson, J. Bibcode: 2014AGUFMSH23A4153T Altcode: We present first results of test particle orbit calculations in two different environments which model separator reconnection in three dimensions. The test particle (electron and proton) orbits are calculated using the relativistic guiding-centre approximation. We compare and contrast test particle orbits in a time-dependent (analytical) electro-magnetic field configuration with those based upon the results of large-scale (numerical) 3D MHD simulations of a reconnecting magnetic separator. We will discuss how the test-particle orbits and the energy gain depend on the initial conditions, and how observations (for example, of solar flares) may be used to constrain model parameters. Title: The solar cycle variation of topological structures in the global solar corona Authors: Platten, S. J.; Parnell, C. E.; Haynes, A. L.; Priest, E. R.; Mackay, D. H. Bibcode: 2014A&A...565A..44P Altcode: 2014arXiv1406.5333P Context. The complicated distribution of magnetic flux across the solar photosphere results in a complex web of coronal magnetic field structures. To understand this complexity, the magnetic skeleton of the coronal field can be calculated. The skeleton highlights the (separatrix) surfaces that divide the field into topologically distinct regions, allowing open-field regions on the solar surface to be located. Furthermore, separatrix surfaces and their intersections with other separatrix surfaces (i.e., separators) are important likely energy release sites.
Aims: The aim of this paper is to investigate, throughout the solar cycle, the nature of coronal magnetic-field topologies that arise under the potential-field source-surface approximation. In particular, we characterise the typical global fields at solar maximum and minimum.
Methods: Global magnetic fields are extrapolated from observed Kitt Peak and SOLIS synoptic magnetograms, from Carrington rotations 1645 to 2144, using the potential-field source-surface model. This allows the variations in the coronal skeleton to be studied over three solar cycles.
Results: The main building blocks which make up magnetic fields are identified and classified according to the nature of their separatrix surfaces. The magnetic skeleton reveals that, at solar maximum, the global coronal field involves a multitude of topological structures at all latitudes criss-crossing throughout the atmosphere. Many open-field regions exist originating anywhere on the photosphere. At solar minimum, the coronal topology is heavily influenced by the solar magnetic dipole. A strong dipole results in a simple large-scale structure involving just two large polar open-field regions, but, at short radial distances between ± 60° latitude, the small-scale topology is complex. If the solar magnetic dipole if weak, as in the recent minimum, then the low-latitude quiet-sun magnetic fields may be globally significant enough to create many disconnected open-field regions between ± 60° latitude, in addition to the two polar open-field regions. Title: Determining the location of open field regions in active regions and their potential as source regions of the slow solar wind. Authors: Harra, Louise K.; Culhane, J. Leonard; Parnell, Clare; Brooks, David; Platten, Sarah Bibcode: 2014cosp...40E1158H Altcode: One of the significant discoveries from the Hinode EUV Imaging Spectrometer (EIS) instrument is the observation of persistent upflows at edges of active regions. These had been observed in the pre-Hinode era with TRACE imaging, and with SOHO. However, with Hinode these upflows are now observed regularly spectroscopically, and are seen in some form in every active region observed. These tend to occur in regions of low intensity. Although the upflows are always seen, it is far from clear whether they form part of the plasma that flows out into the heliosphere. In this work, we study six active regions in various time periods, with different characteristics - not all are in the same hemisphere, some are located next to coronal holes, others are not. All of them show upflowing plasma. Our aim is to study each active region and compare their physical characteristics. We will then carry out modelling to determine where the truly open magnetic field is, and correlate this with the observations. We want to attempt to distinguish between open and closed field regions. If this can be done in a consistent and fast way, this would prove to be extremely valuable in understanding the source of the slow solar wind. Indeed in terms of a practical use, it could be used as a tool for choosing regions to observe for the future Solar Orbiter mission - the purpose of which is to understand the source of the solar wind from its creation on the Sun through its propagation into the heliosphere Title: Coronal heating and nanoflares: current sheet formation and heating Authors: Bowness, R.; Hood, A. W.; Parnell, C. E. Bibcode: 2013A&A...560A..89B Altcode:
Aims: Solar photospheric footpoint motions can produce strong, localised currents in the corona. A detailed understanding of the formation process and the resulting heating is important in modelling nanoflares, as a mechanism for heating the solar corona.
Methods: A 3D MHD simulation is described in which an initially straight magnetic field is sheared in two directions. Grid resolutions up to 5123 were used and two boundary drivers were considered; one where the boundaries are continuously driven and one where the driving is switched off once a current layer is formed.
Results: For both drivers a twisted current layer is formed. After a long time we see that, when the boundary driving has been switched off, the system relaxes towards a lower energy equilibrium. For the driver which continuously shears the magnetic field we see a repeating cycle of strong current structures forming, fragmenting and decreasing in magnitude and then building up again. Realistic coronal temperatures are obtained. Title: Solar Magnetic Tracking. IV. The Death of Magnetic Features Authors: Lamb, D. A.; Howard, T. A.; DeForest, C. E.; Parnell, C. E.; Welsch, B. T. Bibcode: 2013ApJ...774..127L Altcode: 2013arXiv1307.4019L The removal of magnetic flux from the quiet-Sun photosphere is important for maintaining the statistical steady state of the magnetic field there, for determining the magnetic flux budget of the Sun, and for estimating the rate of energy injected into the upper solar atmosphere. Magnetic feature death is a measurable proxy for the removal of detectable flux, either by cancellation (submerging or rising loops, or reconnection in the photosphere) or by dispersal of flux. We used the SWAMIS feature tracking code to understand how nearly 2 × 104 magnetic features die in an hour-long sequence of Hinode/SOT/NFI magnetograms of a region of the quiet Sun. Of the feature deaths that remove visible magnetic flux from the photosphere, the vast majority do so by a process that merely disperses the previously detected flux so that it is too small and too weak to be detected, rather than completely eliminating it. The behavior of the ensemble average of these dispersals is not consistent with a model of simple planar diffusion, suggesting that the dispersal is constrained by the evolving photospheric velocity field. We introduce the concept of the partial lifetime of magnetic features, and show that the partial lifetime due to Cancellation of magnetic flux, 22 hr, is three times slower than previous measurements of the flux turnover time. This indicates that prior feature-based estimates of the flux replacement time may be too short, in contrast with the tendency for this quantity to decrease as resolution and instrumentation have improved. This suggests that dispersal of flux to smaller scales is more important for the replacement of magnetic fields in the quiet Sun than observed bipolar cancellation. We conclude that processes on spatial scales smaller than those visible to Hinode dominate the processes of flux emergence and cancellation, and therefore also the quantity of magnetic flux that threads the photosphere. Title: Solar Magnetic Carpet III: Coronal Modelling of Synthetic Magnetograms Authors: Meyer, K. A.; Mackay, D. H.; van Ballegooijen, A. A.; Parnell, C. E. Bibcode: 2013SoPh..286..357M Altcode: 2013arXiv1303.1342M This article is the third in a series working towards the construction of a realistic, evolving, non-linear force-free coronal-field model for the solar magnetic carpet. Here, we present preliminary results of 3D time-dependent simulations of the small-scale coronal field of the magnetic carpet. Four simulations are considered, each with the same evolving photospheric boundary condition: a 48-hour time series of synthetic magnetograms produced from the model of Meyer et al. (Solar Phys.272, 29, 2011). Three simulations include a uniform, overlying coronal magnetic field of differing strength, the fourth simulation includes no overlying field. The build-up, storage, and dissipation of magnetic energy within the simulations is studied. In particular, we study their dependence upon the evolution of the photospheric magnetic field and the strength of the overlying coronal field. We also consider where energy is stored and dissipated within the coronal field. The free magnetic energy built up is found to be more than sufficient to power small-scale, transient phenomena such as nanoflares and X-ray bright points, with the bulk of the free energy found to be stored low down, between 0.5 - 0.8 Mm. The energy dissipated is currently found to be too small to account for the heating of the entire quiet-Sun corona. However, the form and location of energy-dissipation regions qualitatively agree with what is observed on small scales on the Sun. Future MHD modelling using the same synthetic magnetograms may lead to a higher energy release. Title: A Non-Linear Force-Free Field Model for the Solar Magnetic Carpet Authors: Meyer, Karen; Mackay, D.; van Ballegooijen, A.; Parnell, C. Bibcode: 2013SPD....4430201M Altcode: The magnetic carpet is defined to be the small-scale photospheric magnetic field of the quiet-Sun. Recent high resolution, high cadence observations have shown that although small-scale, the magnetic carpet is far from 'quiet', it is continually evolving in a complex and dynamic manner. I will present a two-component model for the dynamic evolution of the Sun's magnetic carpet. The first component is a 2D model for the photospheric evolution of the small-scale solar magnetic field, that reproduces many observed parameters. The basic evolution of magnetic elements within the model is governed by a supergranular flow profile. In addition, magnetic elements may evolve through the processes of emergence, cancellation, coalescence and fragmentation. The synthetic magnetograms produced by the 2D model are then applied as photospheric boundary data to drive the continuous evolution of a 3D non-linear force-free coronal field. We studied the resultant complex, small-scale coronal magnetic field, in particular the energetics of the field. Title: The Emergence, Motion, and Disappearance of Magnetic Null Points Authors: Murphy, Nicholas A.; Parnell, C.; Haynes, A. L.; Pontin, D. Bibcode: 2013SPD....44..103M Altcode: Magnetic reconnection frequently occurs at and around magnetic nulls: locations where the magnetic field strength equals zero. While theoretical models and simulations of magnetic reconnection often assume that the magnetic field null is co-located with a flow stagnation point, the introduction of asymmetry typically leads to flow across the magnetic null. We derive an exact expression for the three dimensional motion of a magnetic null point in a smoothly varying magnetic field. We define xn as the position of a null, U≡dxn/dt as the null's velocity, and M as the Jacobian matrix of the magnetic field at the null. By using Faraday's law and evaluating the convective derivative of the magnetic field at xn with velocity U, the velocity of the null is given by U=M-1▽×E. This expression is independent of Ohm's law. For resistive magnetohydrodynamics with uniform resistivity η, this reduces to U=V(xn)-ηM-1▽2B. This indicates that any difference between the plasma flow velocity at the null and the velocity of the null itself is due to resistive diffusion of the magnetic field. Null points must diffuse in and out of existence. Null-null pairs first appear (or disappear) as a single degenerate null with singular M, and then instantaneously move apart (or together) infinitely fast. However, the motion of separators cannot be described using solely local parameters because the identification of a particular magnetic field line as a separator may change due to non-ideal behavior at another location. Title: Magnetohydrodynamics dynamical relaxation of coronal magnetic fields. IV. 3D tilted nulls Authors: Fuentes-Fernández, J.; Parnell, C. E. Bibcode: 2013A&A...554A.145F Altcode: 2013arXiv1309.3019F Context. There are various types of reconnection that may take place at 3D magnetic null points. Each different reconnection scenario must be associated with a particular type of current layer.
Aims: A range of current layers may form because the topology of 3D nulls permits currents to form by either twisting the field about the spine of the null or by folding the fan and spine into each other. Additionally, the initial geometry of the field can lead to variations in the currents that are accumulated. Here, we study current accumulations in so-called 3D "tilted" nulls formed by a folding of the spine and fan. A non-zero component of current parallel to the fan is required such that the null's fan plane and spine are not perpendicular. Our aims are to provide valid magnetohydrostatic equilibria and to describe the current accumulations in various cases involving finite plasma pressure.
Methods: To create our equilibrium current structures we use a full, non-resistive, magnetohydrodynamic (MHD) code so that no reconnection is allowed. A series of experiments are performed in which a perturbed 3D tilted null relaxes towards an equilibrium via real, viscous damping forces. Changes to the initial plasma pressure and to magnetic parameters are investigated systematically.
Results: An initially tilted fan is associated with a non-zero Lorentz force that drives the fan and spine to collapse towards each other, in a similar manner to the collapse of a 2D X-point. In the final equilibrium state for an initially radial null with only the current perpendicular to the spine, the current concentrates along the tilt axis of the fan and in a layer about the null point with a sharp peak at the null itself. The continued growth of this peak indicates that the system is in an asymptotic regime involving an infinite time singularity at the null. When the initial tilt disturbance (current perpendicular to the spine) is combined with a spiral-type disturbance (current parallel to the spine), the final current density concentrates in three regions: one on the fan along its tilt axis and two around the spine, above and below the fan. The increased area of current accumulation leads to a weakening of the singularity formed at the null. The 3D spine-fan collapse with generic current studied here provides the ideal setup for non-steady reconnection studies. Title: The Emergence, Motion, and Disappearance of Magnetic Null Points Authors: Murphy, Nicholas A.; Parnell, Clare; Haynes, Andrew L.; Pontin, David Bibcode: 2013shin.confE.118M Altcode: Magnetic reconnection frequently occurs at and around magnetic nulls: locations where the magnetic field strength equals zero. While theoretical models and simulations of laminar, non-turbulent magnetic reconnection often assume that the magnetic field null is co-located with a flow stagnation point, the introduction of asymmetry typically leads to flows across the magnetic null. We derive an exact expression for the three dimensional motion of a magnetic null point in a smoothly varying magnetic field by using Faraday's law and evaluating the convective derivative of the magnetic field at the null using the null's velocity. In resistive magnetohydrodynamics, any difference between the plasma flow velocity at the null and the velocity of the null itself must be due to resistive diffusion of the magnetic field. Null points must diffuse in and out of existence. Null-null pairs first appear (disappear) as a single degenerate null with a singular Jacobian matrix, and then instantaneously move apart (together) infinitely fast. However, the motion of separators cannot be described using solely local parameters because the identification of a particular magnetic field line as a separator may change due to non-ideal behavior at another location. Title: SWIFF: Space weather integrated forecasting framework Authors: Lapenta, Giovanni; Pierrard, Viviane; Keppens, Rony; Markidis, Stefano; Poedts, Stefaan; Šebek, Ondřej; Trávníček, Pavel M.; Henri, Pierre; Califano, Francesco; Pegoraro, Francesco; Faganello, Matteo; Olshevsky, Vyacheslav; Restante, Anna Lisa; Nordlund, Åke; Trier Frederiksen, Jacob; Mackay, Duncan H.; Parnell, Clare E.; Bemporad, Alessandro; Susino, Roberto; Borremans, Kris Bibcode: 2013JSWSC...3A..05L Altcode: SWIFF is a project funded by the Seventh Framework Programme of the European Commission to study the mathematical-physics models that form the basis for space weather forecasting. The phenomena of space weather span a tremendous scale of densities and temperature with scales ranging 10 orders of magnitude in space and time. Additionally even in local regions there are concurrent processes developing at the electron, ion and global scales strongly interacting with each other. The fundamental challenge in modelling space weather is the need to address multiple physics and multiple scales. Here we present our approach to take existing expertise in fluid and kinetic models to produce an integrated mathematical approach and software infrastructure that allows fluid and kinetic processes to be modelled together. SWIFF aims also at using this new infrastructure to model specific coupled processes at the Solar Corona, in the interplanetary space and in the interaction at the Earth magnetosphere. Title: Nonlinear wave propagation and reconnection at magnetic X-points in the Hall MHD regime Authors: Threlfall, J.; Parnell, C. E.; De Moortel, I.; McClements, K. G.; Arber, T. D. Bibcode: 2012A&A...544A..24T Altcode: 2012arXiv1202.3648T Context. The highly dynamical, complex nature of the solar atmosphere naturally implies the presence of waves in a topologically varied magnetic environment. Here, the interaction of waves with topological features such as null points is inevitable and potentially important for energetics. The low resistivity of the solar coronal plasma implies that non-magnetohydrodynamic (MHD) effects should be considered in studies of magnetic energy release in this environment.
Aims: This paper investigates the role of the Hall term in the propagation and dissipation of waves, their interaction with 2D magnetic X-points and the nature of the resulting reconnection.
Methods: A Lagrangian remap shock-capturing code (Lare2d) was used to study the evolution of an initial fast magnetoacoustic wave annulus for a range of values of the ion skin depth (δi) in resistive Hall MHD. A magnetic null-point finding algorithm was also used to locate and track the evolution of the multiple null-points that are formed in the system.
Results: Depending on the ratio of ion skin depth to system size, our model demonstrates that Hall effects can play a key role in the wave-null interaction. In particular, the initial fast-wave pulse now consists of whistler and ion-cyclotron components; the dispersive nature of the whistler wave leads to (i) earlier interaction with the null; (ii) the creation of multiple additional, transient nulls and, hence, an increased number of energy release sites. In the Hall regime, the relevant timescales (such as the onset of reconnection and the period of the oscillatory relaxation) of the system are reduced significantly, and the reconnection rate is enhanced. Title: Magnetohydrodynamics dynamical relaxation of coronal magnetic fields. III. 3D spiral nulls Authors: Fuentes-Fernández, J.; Parnell, C. E. Bibcode: 2012A&A...544A..77F Altcode: 2012arXiv1206.5527F Context. The majority of studies on stressed 3D magnetic null points consider magnetic reconnection driven by an external perturbation, but the formation of a genuine current sheet equilibrium remains poorly understood. This problem has been considered more extensively in two dimensions, but lacks a generalization into 3D fields.
Aims: 3D magnetic nulls are more complex than 2D nulls and the field can take a greater range of magnetic geometries local to the null. Here, we focus on one type and consider the dynamical non-resistive relaxation of 3D spiral nulls with initial spine-aligned current. We aim to provide a valid magnetohydrostatic equilibrium, and describe the electric current accumulations in various cases, involving a finite plasma pressure.
Methods: A full MHD code was used, with the resistivity set to zero so that reconnection is not allowed, to run a series of experiments in which a perturbed spiral 3D null point was allowed to relax towards an equilibrium via real, viscous damping forces. Changes to the initial plasma pressure and other magnetic parameters were systematically investigated.
Results: For the axisymmetric case, the evolution of the field and the plasma is such that it concentrates the current density into two cone-shaped regions along the spine, thus concentrating the twist of the magnetic field around the spine, leaving a radial configuration in the fan plane. The plasma pressure redistributes to maintain the current density accumulations. However, it is found that changes in the initial plasma pressure do not significantly modify the final state. In the cases where the initial magnetic field is not axisymmetric, an infinite-time singularity of current perpendicular to the fan is found at the location of the null. Title: The onset of impulsive bursty reconnection at a two-dimensional current layer Authors: Fuentes-Fernández, J.; Parnell, C. E.; Priest, E. R. Bibcode: 2012PhPl...19g2901F Altcode: 2012arXiv1205.2120F The sudden reconnection of a non-force free 2D current layer, embedded in a low-beta plasma, triggered by the onset of an anomalous resistivity, is studied in detail. The resulting behaviour consists of two main phases. First, a transient reconnection phase, in which the current in the layer is rapidly dispersed and some flux is reconnected. This dispersal of current launches a family of small amplitude magnetic and plasma perturbations, which propagate away from the null at the local fast and slow magnetosonic speeds. The vast majority of the magnetic energy released in this phase goes into internal energy of the plasma, and only a tiny amount is converted into kinetic energy. In the wake of the outwards propagating pulses, an imbalance of Lorentz and pressure forces creates a stagnation flow which drives a regime of impulsive bursty reconnection, in which fast reconnection is turned on and off in a turbulent manner as the current density exceeds and falls below a critical value. During this phase, the null current density is continuously built up above a certain critical level, then dissipated very rapidly, and built up again, in a stochastic manner. Interestingly, the magnetic energy converted during this quasi-steady phase is greater than that converted during the initial transient reconnection phase. Again essentially all the energy converted during this phase goes directly to internal energy. These results are of potential importance for solar flares and coronal heating, and set a conceptually important reference for future 3D studies. Title: A contemporary view of coronal heating Authors: Parnell, C. E.; De Moortel, I. Bibcode: 2012RSPTA.370.3217P Altcode: 2012arXiv1206.6097P Determining the heating mechanism (or mechanisms) that causes the outer atmosphere of the Sun, and many other stars, to reach temperatures orders of magnitude higher than their surface temperatures has long been a key problem. For decades the problem has been known as the coronal heating problem, but it is now clear that `coronal heating' cannot be treated or explained in isolation and that the heating of the whole solar atmosphere must be studied as a highly coupled system. The magnetic field of the star is known to play a key role, but, despite significant advancements in solar telescopes, computing power and much greater understanding of theoretical mechanisms, the question of which mechanism or mechanisms are the dominant supplier of energy to the chromosphere and corona is still open. Following substantial recent progress, we consider the most likely contenders and discuss the key factors that have made, and still make, determining the actual (coronal) heating mechanism (or mechanisms) so difficult. Title: Astrophysical processes on the Sun Authors: Parnell, C. E. Bibcode: 2012RSPTA.370.3043P Altcode: No abstract at ADS Title: Consequences of spontaneous reconnection at a two-dimensional non-force-free current layer Authors: Fuentes-Fernández, J.; Parnell, C. E.; Hood, A. W.; Priest, E. R.; Longcope, D. W. Bibcode: 2012PhPl...19b2901F Altcode: 2012arXiv1202.0161F Magnetic neutral points, where the magnitude of the magnetic field vanishes locally, are potential locations for energy conversion in the solar corona. The fact that the magnetic field is identically zero at these points suggests that for the study of current sheet formation and of any subsequent resistive dissipation phase, a finite beta plasma should be considered, rather than neglecting the plasma pressure as has often been the case in the past. The rapid dissipation of a finite current layer in non-force-free equilibrium is investigated numerically, after the sudden onset of an anomalous resistivity. The aim of this study is to determine how the energy is redistributed during the initial diffusion phase, and what is the nature of the outward transmission of information and energy. The resistivity rapidly diffuses the current at the null point. The presence of a plasma pressure allows the vast majority of the free energy to be transferred into internal energy. Most of the converted energy is used in direct heating of the surrounding plasma, and only about 3% is converted into kinetic energy, causing a perturbation in the magnetic field and the plasma which propagates away from the null at the local fast magnetoacoustic speed. The propagating pulses show a complex structure due to the highly non-uniform initial state. It is shown that this perturbation carries no net current as it propagates away from the null. The fact that, under the assumptions taken in this paper, most of the magnetic energy released in the reconnection converts internal energy of the plasma, may be highly important for the chromospheric and coronal heating problem. Title: Magnetohydrodynamics dynamical relaxation of coronal magnetic fields. II. 2D magnetic X-points Authors: Fuentes-Fernández, J.; Parnell, C. E.; Hood, A. W. Bibcode: 2011A&A...536A..32F Altcode: 2011arXiv1110.5253F Context. Magnetic neutral points are potential locations for energy conversion in the solar corona. 2D X-points have been widely studied in the past, but only a few of those studies have taken finite plasma beta effects into consideration, and none of them look at the dynamical evolution of the system. At the moment there exists no description of the formation of a non-force-free equilibrium around a two-dimensional X-point.
Aims: Our aim is to provide a valid magnetohydrostatic equilibrium from the collapse of a 2D X-point in the presence of a finite plasma pressure, in which the current density is not simply concentrated in an infinitesimally thin, one-dimensional current sheet, as found in force-free solutions. In particular, we wish to determine if a finite pressure current sheet will still involve a singular current, and if so, what is the nature of the singularity.
Methods: We use a full MHD code, with the resistivity set to zero, so that reconnection is not allowed, to run a series of experiments in which an X-point is perturbed and then is allowed to relax towards an equilibrium, via real, viscous damping forces. Changes to the magnitude of the perturbation and the initial plasma pressure are investigated systematically.
Results: The final state found in our experiments is a "quasi-static" equilibrium where the viscous relaxation has completely ended, but the peak current density at the null increases very slowly following an asymptotic regime towards an infinite time singularity. Using a high grid resolution allows us to resolve the current structures in this state both in width and length. In comparison with the well known pressureless studies, the system does not evolve towards a thin current sheet, but concentrates the current at the null and the separatrices. The growth rate of the singularity is found to be tD, with 0 < D < 1. This rate depends directly on the initial plasma pressure, and decreases as the pressure is increased. At the end of our study, we present an analytical description of the system in a quasi-static non-singular equilibrium at a given time, in which a finite thick current layer has formed at the null. The dynamical evolution of the system and the dependence of the final state on the initial plasma and magnetic quantities is discussed, as are the energetic consequences. Title: 3D Magnetic Reconnection Authors: Parnell, Clare E.; Maclean, Rhona C.; Haynes, Andrew L.; Galsgaard, Klaus Bibcode: 2011IAUS..271..227P Altcode: Magnetic reconnection is an important process that is prevalent in a wide range of astrophysical bodies. It is the mechanism that permits magnetic fields to relax to a lower energy state through the global restructuring of the magnetic field and is thus associated with a range of dynamic phenomena such as solar flares and CMEs. The characteristics of three-dimensional reconnection are reviewed revealing how much more diverse it is than reconnection in two dimensions. For instance, three-dimensional reconnection can occur both in the vicinity of null points, as well as in the absence of them. It occurs continuously and continually throughout a diffusion volume, as opposed to at a single point, as it does in two dimensions. This means that in three-dimensions field lines do not reconnect in pairs of lines making the visualisation and interpretation of three-dimensional reconnection difficult.

By considering particular numerical 3D magnetohydrodynamic models of reconnection, we consider how magnetic reconnection can lead to complex magnetic topologies and current sheet formation. Indeed, it has been found that even simple interactions, such as the emergence of a flux tube, can naturally give rise to `turbulent-like' reconnection regions. Title: Solar Magnetic Carpet I: Simulation of Synthetic Magnetograms Authors: Meyer, K. A.; Mackay, D. H.; van Ballegooijen, A. A.; Parnell, C. E. Bibcode: 2011SoPh..272...29M Altcode: 2011SoPh..tmp..294M; 2011SoPh..tmp..198M; 2011SoPh..tmp..319M; 2011SoPh..tmp..267M; 2011arXiv1108.1080M This paper describes a new 2D model for the photospheric evolution of the magnetic carpet. It is the first in a series of papers working towards constructing a realistic 3D non-potential model for the interaction of small-scale solar magnetic fields. In the model, the basic evolution of the magnetic elements is governed by a supergranular flow profile. In addition, magnetic elements may evolve through the processes of emergence, cancellation, coalescence and fragmentation. Model parameters for the emergence of bipoles are based upon the results of observational studies. Using this model, several simulations are considered, where the range of flux with which bipoles may emerge is varied. In all cases the model quickly reaches a steady state where the rates of emergence and cancellation balance. Analysis of the resulting magnetic field shows that we reproduce observed quantities such as the flux distribution, mean field, cancellation rates, photospheric recycle time and a magnetic network. As expected, the simulation matches observations more closely when a larger, and consequently more realistic, range of emerging flux values is allowed (4×1016 - 1019 Mx). The model best reproduces the current observed properties of the magnetic carpet when we take the minimum absolute flux for emerging bipoles to be 4×1016 Mx. In future, this 2D model will be used as an evolving photospheric boundary condition for 3D non-potential modeling. Title: Small-Scale Flux Emergence Observed Using Hinode/SOT Authors: Thornton, L. M.; Parnell, C. E. Bibcode: 2011SoPh..269...13T Altcode: 2010SoPh..tmp..220T The aim of this paper is to determine the flux emergence rate due to small-scale magnetic features in the quiet Sun using high-resolution Hinode SOT NFI data. Small-scale magnetic features are identified in the data using two different feature identification methods (clumping and downhill); then three methods are applied to detect flux emergence events. The distribution of the intranetwork peak emerged fluxes is determined. When combined with previous emergence results, from ephemeral regions to sunspots, the distribution of all fluxes are found to follow a power-law distribution which spans nearly seven orders of magnitude in flux (1016 - 1023 Mx) and 18 orders of magnitude in frequency. The power-law fit to all these data is of the form dN/dΨ = n_0/Ψ0Ψ/Ψ0-2.7 where Ψ0=1016 Mx and is used to predict a global flux emergence rate of ≈ 450 Mx cm−2 day−1 from all features with fluxes of 1016 Mx or more. Since the slope of all emerged fluxes is less than −2, this implies that most of the new flux that is fed into the solar atmosphere is from small-scale emerging events. This suggests that the rate of flux emergence is independent of the solar cycle and is equivalent to a global rate of flux emergence of more than a few times 1025 Mx day−1. The single power-law distribution over all emerged fluxes implies a scale-free dynamo, therefore indicating that a turbulent dynamo may act throughout the convection zone. Moreover, from the slope of the emerging flux distribution the (turbulent?) dynamo producing small-scale features produces considerably more flux than the active-region dynamo at the tachocline. Title: Three Dimensional Magnetic Reconnection at Null Points and Separators Authors: Parnell, Clare E.; Haynes, Andrew L.; Maclean, Rhona C. Bibcode: 2011sswh.book..147P Altcode: No abstract at ADS Title: The Detection of Numerous Magnetic Separators in a Three-Dimensional Magnetohydrodynamic Model of Solar Emerging Flux Authors: Parnell, C. E.; Maclean, R. C.; Haynes, A. L. Bibcode: 2010ApJ...725L.214P Altcode: Magnetic separators in three-dimensional (3D) magnetic fields are believed to be often associated with locations of magnetic reconnection. In this preliminary study, we investigate this relationship using data from a numerical resistive 3D MHD experiment of a solar flux emergence event. For the first time separators are detected in complex magnetic fields resulting from a 3D resistive MHD model of flux emergence. Two snapshots of the model, taken from different stages of its evolution, are analyzed. Numerous separators are found in both snapshots, and their properties, including their geometry, length, relationship to the magnetic null points, and integrated parallel electric field are studied. The separators reside at the junctions between the emerging flux, the overlying field, and two other flux domains that are newly formed by reconnection. The long separators, which connect clusters of nulls that lie either side of the emerging flux, pass through spatially localized regions of high parallel electric field and correspond to local maxima in integrated parallel electric field. These factors indicate that strong magnetic reconnection takes place along many of the separators, and that separators play a key role during the interaction of emerging and overlying flux. Title: Formation, Evolution, and Associated Flows for Flux Transfer Events: 3D Nature of Reconnection Authors: Loring, B.; Karimabadi, H.; Raeder, J.; Vu, H.; Omelchenko, Y. A.; Parnell, C.; Haynes, A.; Daughton, W. S.; Roytershteyn, V.; Dorelli, J. Bibcode: 2010AGUFMSM43B..05L Altcode: In this presentation, we examine the formation and evolution of flux transfer events (FTEs) in our 3D global hybrid simulation of the magnetosphere during southward IMF with no dipole tilt. Some of the questions that we address include: (1) Do FTEs form during periods of southward IMF under steady IMF conditions? (2) What is the trigger mechanism for FTEs and does vortex formation cause FTEs? (3) What is the structure and plasma composition of FTEs? (4) What are the flows associated with FTEs? (5) How does the structure of FTEs evolve in time? (6) How are FTEs in a 3D global hybrid simulation different from FTEs in an equivalent global MHD simulation? 3D reconnection at the dayside magnetopause is found to be quite complex and identification of separatrices in the turbulent environment of magnetosheath is challenging. Preliminary results show evidence of separator reconnection. We compare and contrast the results with FTE formation in 2D global hybrid and 2D global full PIC simulation. The 2D global full PIC simulations show formation of elongated electron layers which become unstable to formation of secondary islands (FTEs). This is consistent with our previous work on 2D Harris sheet. 2D global full PIC and 2D global hybrid simulations show qualitatively similar results. However, there are significant differences in structure, size, evolution, and plasma composition within the FTE in 2D and 3D global hybrid simulations. We will discuss the cause of these differences and their implications for spacecraft studies of FTEs. Title: Solar Magnetic Tracking. III. Apparent Unipolar Flux Emergence in High-resolution Observations Authors: Lamb, D. A.; DeForest, C. E.; Hagenaar, H. J.; Parnell, C. E.; Welsch, B. T. Bibcode: 2010ApJ...720.1405L Altcode: Understanding the behavior of weak magnetic fields near the detection limit of current instrumentation is important for determining the flux budget of the solar photosphere at small spatial scales. Using 0farcs3-resolution magnetograms from the Solar Optical Telescope's Narrowband Filter Imager (NFI) on the Hinode spacecraft, we confirm that the previously reported apparent unipolar magnetic flux emergence seen in intermediate-resolution magnetograms is indeed the coalescence of previously existing flux. We demonstrate that similar but smaller events seen in NFI magnetograms are also likely to correspond to the coalescence of previously existing weak fields. The uncoalesced flux, detectable only in the ensemble average of hundreds of these events, accounts for 50% of the total flux within 3 Mm of the detected features. The spatial scale at which apparent unipolar emergence can be directly observed as coalescence remains unknown. The polarity of the coalescing flux is more balanced than would be expected given the imbalance of the data set, however without further study we cannot speculate whether this implies that the flux in the apparent unipolar emergence events is produced by a granulation-scale dynamo or is recycled from existing field. Title: Sunspots and starspots, by John H. Thomas and Nigel O. Weiss Authors: Parnell, Clare Bibcode: 2010GApFD.104..453P Altcode: No abstract at ADS Title: Magnetic reconnection in the solar atmosphere: from proposal to paradigm Authors: Cargill, Peter; Parnell, Clare; Browning, Philippa; de Moortel, Ineke; Hood, Alan Bibcode: 2010A&G....51c..31C Altcode: MEETING REPORT On 13 November 2009, the RAS hosted a discussion meeting to commemorate the formal retirement of Prof. Eric Priest. Here Peter Cargill, Clare Parnell, Philippa Browning, Ineke de Moortel and Alan Hood examine how magnetic reconnection has evolved over the past 50 years from an important but controversial proposal, to a general paradigm. Title: Magnetohydrodynamics dynamical relaxation of coronal magnetic fields . I. Parallel untwisted magnetic fields in 2D Authors: Fuentes-Fernández, J.; Parnell, C. E.; Hood, A. W. Bibcode: 2010A&A...514A..90F Altcode: 2011arXiv1110.5258F Context. For the last thirty years, most of the studies on the relaxation of stressed magnetic fields in the solar environment have only considered the Lorentz force, neglecting plasma contributions, and therefore, limiting every equilibrium to that of a force-free field.
Aims: Here we begin a study of the non-resistive evolution of finite beta plasmas and their relaxation to magnetohydrostatic states, where magnetic forces are balanced by plasma-pressure gradients, by using a simple 2D scenario involving a hydromagnetic disturbance to a uniform magnetic field. The final equilibrium state is predicted as a function of the initial disturbances, with aims to demonstrate what happens to the plasma during the relaxation process and to see what effects it has on the final equilibrium state.
Methods: A set of numerical experiments are run using a full MHD code, with the relaxation driven by magnetoacoustic waves damped by viscous effects. The numerical results are compared with analytical calculations made within the linear regime, in which the whole process must remain adiabatic. Particular attention is paid to the thermodynamic behaviour of the plasma during the relaxation.
Results: The analytical predictions for the final non force-free equilibrium depend only on the initial perturbations and the total pressure of the system. It is found that these predictions hold surprisingly well even for amplitudes of the perturbation far outside the linear regime.
Conclusions: Including the effects of a finite plasma beta in relaxation experiments leads to significant differences from the force-free case. Title: Structure of magnetic separators and separator reconnection Authors: Parnell, C. E.; Haynes, A. L.; Galsgaard, K. Bibcode: 2010JGRA..115.2102P Altcode: 2010JGRA..11502102P Magnetic separators are important locations of three-dimensional magnetic reconnection. They are field lines that lie along the edges of four flux domains and represent the intersection of two separatrix surfaces. Since the intersection of two surfaces produces an X-type structure, when viewed along the line of intersection, the global three-dimensional topology of the magnetic field around a separator is hyperbolic. It is therefore usually assumed that the projection of the magnetic field lines themselves onto a two-dimensional plane perpendicular to a separator is also hyperbolic in nature. In this paper, we use the results of a three-dimensional MHD experiment of separator reconnection to show that, in fact, the projection of the magnetic field lines in a cut perpendicular to a separator may be either hyperbolic or elliptic and that the structure of the magnetic field projection may change in space, along the separator, as well as in time, during the life of the separator. Furthermore, in our experiment, we find that there are both spatial and temporal variations in the parallel component of current (and electric field) along the separator, with all high parallel current regions (which are associated with reconnection) occurring between counterrotating flow regions. Importantly, reconnection occurs not only at locations where the structure of the projected perpendicular magnetic field is hyperbolic but also where it is elliptic. Title: Three-Dimensional Magnetic Reconnection Authors: Parnell, C. E.; Haynes, A. L. Bibcode: 2010ASSP...19..261P Altcode: 2010mcia.conf..261P; 2009arXiv0903.0274P The importance of magnetic reconnection as an energy release mechanism in many solar, stellar, magnetospheric and astrophysical phenomena has long been recognised. Reconnection is the only mechanism by which magnetic fields can globally restructure, enabling them to access a lower energy state. Over the past decade, there have been some major advances in our understanding of three-dimensional reconnection. In particular, the key characteristics of 3D magnetohydrodynamic (MHD) reconnection have been determined. For instance, 3D reconnection (1) occurs with or without nulls, (2) occurs continuously and continually throughout a diffusion region and (3) is driven by counter rotating flows. Furthermore, analysis of resistive 3D MHD magnetic experiments have revealed some intriguing effects relating to where and how reconnection occurs. To illustrate these new features, a series of constant-resistivity experiments, involving the interaction of two opposite-polarity magnetic sources in an overlying field, are considered. Such a simple interaction represents a typical building block of the Sun's magnetic atmosphere. By following the evolution of the magnetic topology, we are able to explain where, how and at what rate the reconnection occurs. Remarkably, there can be up to five energy release sites at any one time (compared to one in the potential case) and the duration of the interaction increases (more than doubles) as the resistivity decreases (by a factor of 16). The decreased resistivity also leads to a higher peak ohmic dissipation and more energy being released in total, as a result of a greater injection of Poynting flux. Title: Interaction of twisted curved flux tubes Authors: Selwa, Malgorzata; Parnell, Clare; Priest, Eric Bibcode: 2010cosp...38.1947S Altcode: 2010cosp.meet.1947S Most solar eruptions are initiated from sigmoidal structures. We perform 3D MHD numerical experiments of the interaction of force-free dipolar flux tubes. The magnetic configuration is initialized as either a potential or a force-free dipole with a constant density. Next we perturb the dipoles by twisting or rotating them leading to reconnection in a resistive MHD regime. We compare the connectivity, energetics and topological features in both models, vary the contact angle of the dipoles and check if the initial configuration (sigmoidal or not) affects flares and eruption initiation leading to faster and stronger reconnection. Title: Is Null-Point Reconnection Important for Solar Flux Emergence? Authors: Maclean, R. C.; Parnell, C. E.; Galsgaard, K. Bibcode: 2009SoPh..260..299M Altcode: 2009arXiv0910.0368M The role of null-point reconnection in a three-dimensional numerical magnetohydrodynamic (MHD) model of solar emerging flux is investigated. The model consists of a twisted magnetic flux tube rising through a stratified convection zone and atmosphere to interact and reconnect with a horizontal overlying magnetic field in the atmosphere. Null points appear as the reconnection begins and persist throughout the rest of the emergence, where they can be found mostly in the model photosphere and transition region, forming two loose clusters on either side of the emerging flux tube. Up to 26 nulls are present at any one time, and tracking in time shows that there is a total of 305 overall, despite the initial simplicity of the magnetic field configuration. We find evidence for the reality of the nulls in terms of their methods of creation and destruction, their balance of signs, their long lifetimes, and their geometrical stability. We then show that due to the low parallel electric fields associated with the nulls, null-point reconnection is not the main type of magnetic reconnection involved in the interaction of the newly emerged flux with the overlying field. However, the large number of nulls implies that the topological structure of the magnetic field must be very complex and the importance of reconnection along separators or separatrix surfaces for flux emergence cannot be ruled out. Title: How skeletons turn into quasi-separatrix layers in source models Authors: Restante, A. L.; Aulanier, G.; Parnell, C. E. Bibcode: 2009A&A...508..433R Altcode: Context: In situations where there are no magnetic null points located above a reference photospheric plane, and when the photospheric magnetic field is modeled by discrete flux concentrations, the magnetic connectivity is defined by the magnetic skeleton of the configuration. For a continuous distribution of non-zero photospheric flux, the connectivity is defined by quasi-separatrix layers (QSLs). Both the magnetic skeleton and QSLs can account for current sheet formation and dissipation. Observationally, though, only some portions of the skeleton are found to be related to flare ribbons, which are generally associated with QSL footpoints.
Aims: In potential magnetic source models, a transition from the skeleton to QSLs has been shown to occur when the sources are displaced below the photospheric plane. The objective of this paper is to understand the topological and geometrical nature of this transition, and to derive rules to predict which parts of a given skeleton will give rise to QSLs.
Methods: We consider magnetic configurations, derived from potential magnetic sources, which possess no coronal null points. We have calculated their skeletons, composed of null points, spine field lines and separatrix (fan) surfaces. Choosing a reference photospheric plane above the sources, we have calculated their QSL footprints.
Results: As already known, the latter mostly match with subphotospheric spine field lines since, above these lines, field lines tend to diverge as a result of approaching a null and lying either side of the separatrix surface extending out of from this null. However, many non-spine related QSL footprints are also found, which we call branches. They correspond to the intersection with the photosphere of portions of fan field lines which “branch” away from the sources and result in QSLs due to the inclination of the coronal field lines.
Conclusions: Our findings allow a better geometrical understanding of the relations between QSLs and skeletons. We show that in the absence of coronal null points, spines, as well as specific portions of fans as calculated in standard potential source models, are good predictors for the location of QSL footprints and of flare ribbons. Title: The Density of Coronal Null Points from Hinode and MDI Authors: Longcope, D.; Parnell, C.; DeForest, C. Bibcode: 2009ASPC..415..178L Altcode: 2009arXiv0901.0865L Magnetic null points can be located numerically in a potential field extrapolation or their average density can be estimated from the Fourier spectrum of a magnetogram. We use both methods to compute the null point density from a quiet Sun magnetogram made with Hinode's NFI and from magnetograms from SOHO's MDI in both its high-resolution and low-resolution modes. All estimates of the super-chromospheric column density (z>1.5 Mm) agree with one another and with the previous measurements: 3×10-3 null points per square Mm of solar surface. Title: A Power-Law Distribution of Solar Magnetic Fields Over More Than Five Decades in Flux Authors: Parnell, C. E.; DeForest, C. E.; Hagenaar, H. J.; Johnston, B. A.; Lamb, D. A.; Welsch, B. T. Bibcode: 2009ApJ...698...75P Altcode: Solar flares, coronal mass ejections, and indeed phenomena on all scales observed on the Sun, are inextricably linked with the Sun's magnetic field. The solar surface is covered with magnetic features observed on many spatial scales, which evolve on differing timescales: the largest features, sunspots, follow an 11-year cycle; the smallest seem to follow no cycle. Here, we analyze magnetograms from Solar and Heliospheric Observatory (SOHO)/Michelson Doppler Imager (full disk and high resolution) and Hinode/Solar Optical Telescope to determine the fluxes of all currently observable surface magnetic features. We show that by using a "clumping" algorithm, which counts a single "flux massif" as one feature, all feature fluxes, regardless of flux strength, follow the same distribution—a power law with slope -1.85 ± 0.14—between 2 × 1017 and 1023 Mx. A power law suggests that the mechanisms creating surface magnetic features are scale-free. This implies that either all surface magnetic features are generated by the same mechanism, or that they are dominated by surface processes (such as fragmentation, coalescence, and cancellation) in a way which leads to a scale-free distribution. Title: A Power-law Distribution of Solar Magnetic Fields Over More Than Five Decades in Flux Authors: Parnell, Clare; DeForest, C. E.; Hagenaar, H. J.; Johnston, B. A.; Lamb, D. A.; Welsch, B. T. Bibcode: 2009SPD....40.0603P Altcode: The surface of the Sun is covered with magnetic features observed on many spatial scales, which evolve on differing time scales: the largest features, sunspots, follow an 11 year cycle; the smallest apparently follow no cycle. Magnetograms from SoHO/MDI (full disk and high-resolution) and Hinode/SOT are analysed to determine the fluxes of all currently observable surface magnetic features. To identify features we use a 'clumping' algorithm, which defines a single feature as a group of contiguous, same-sign pixels, each of which exceeds an absolute flux cutoff. We show that, using this feature identification method, all feature fluxes, regardless of flux strength, follow the same distribution - a power-law with slope -1.85±0.14 - between 2x 1017 and 1023 Mx. This result implies that the processes that determine the spatial structure of surface magnetic features are scale-free. Hence, suggesting that either all surface magnetic features are generated by the same mechanism, or that their spatial structure is dominated by processes in the interior or at the surface (e.g., fragmentation, coalescence and cancellation) that produce a scale-free distribution. We will discuss the likelihood of these two mechanisms for generating the powerlaw distribution of feature fluxes. Title: The Number of Magnetic Null Points in the Quiet Sun Corona Authors: Longcope, D. W.; Parnell, C. E. Bibcode: 2009SoPh..254...51L Altcode: 2008SoPh..tmp..185L; 2008arXiv0811.0097L The coronal magnetic field above a particular photospheric region will vanish at a certain number of points, called null points. These points can be found directly in a potential field extrapolation or their density can be estimated from the Fourier spectrum of the magnetogram. The spectral estimate, in which the extrapolated field is assumed to be random and homogeneous with Gaussian statistics, is found here to be relatively accurate for quiet Sun magnetograms from SOHO's MDI. The majority of null points occur at low altitudes, and their distribution is dictated by high wavenumbers in the Fourier spectrum. This portion of the spectrum is affected by Poisson noise, and as many as five-sixths of null points identified from a direct extrapolation can be attributed to noise. The null distribution above 1500 km is found to depend on wavelengths that are reliably measured by MDI in either its low-resolution or high-resolution mode. After correcting the spectrum to remove white noise and compensate for the modulation transfer function we find that a potential field extrapolation contains, on average, one magnetic null point, with altitude greater than 1.5 Mm, above every 322 Mm2 patch of quiet Sun. Analysis of 562 quiet Sun magnetograms spanning the two latest solar minima shows that the null point density is relatively constant with roughly 10% day-to-day variation. At heights above 1.5 Mm, the null point density decreases approximately as the inverse cube of height. The photospheric field in the quiet Sun is well approximated as that from discrete elements with mean flux «|φ|»=1.0×1019 Mx distributed randomly with density n=0.007 Mm−2. Title: Quiet-Sun: A Comparison of MDI and SOT Fluxes Authors: Parnell, C. E.; Deforest, C. E.; Hagenaar, H. J.; Lamb, D. A.; Welsch, B. T. Bibcode: 2008ASPC..397...31P Altcode: The SOT-NFI on Hinode has both higher resolution and better sensitivity than MDI on SOHO. Line-of-sight magnetograms of the quiet Sun taken simultaneously by both MDI and SOT are investigated to show how the observed flux differs between the two instruments. We find that: (i) the total unsigned flux observed by SOT is approximately 50% greater than that observed by MDI and (ii) the total signed flux remains approximately constant. Thus, the extra flux observed by SOT is made up of equal amounts of positive and negative flux. By comparing the observed flux distributions from MDI and SOT we find that the extra flux is contained in features with fluxes less than the smallest observed by MDI. Indeed, the smallest features in SOT have just ≥ 10^{16} Mx, a factor of thirty less than the smallest observed by MDI.

The distributions of feature fluxes observed by the two instruments are also compared. We find that by using a `clumping' algorithm, which counts a single `flux massif' as one feature, the fluxes in MDI and SOT follow the same distribution - a power-law - between 2× 10^{17} and 10^{20} Mx. Thus, the mechanism producing network and intranetwork features appears to be the same. Furthermore, the power-law index of this distribution is found to be -1.85. This value is neither the Kolomogrov -5/3 slope of hydrodynamic turbulence nor the Krichenen -2 slope of magneto-hydrodynamic turbulence, although both of these numbers may be within the error bars of our analysis. Title: A new view of quiet-Sun topology from Hinode/SOT Authors: Régnier, S.; Parnell, C. E.; Haynes, A. L. Bibcode: 2008A&A...484L..47R Altcode: 2008arXiv0805.1602R Context: With the recent launch of the Hinode satellite our view of the nature and evolution of quiet-Sun regions has been improved. In light of the new high resolution observations, we revisit the study of the quiet Sun's topological nature.
Aims: Topology is a tool to explain the complexity of the magnetic field, the occurrence of reconnection processes, and the heating of the corona. This Letter aims to give new insights to these different topics.
Methods: Using a high-resolution Hinode/SOT observation of the line-of-sight magnetic field on the photosphere, we calculate the three dimensional magnetic field in the region above assuming a potential field. From the 3D field, we determine the existence of null points in the magnetic configuration.
Results: From this model of a continuous field, we find that the distribution of null points with height is significantly different from that reported in previous studies. In particular, the null points are mainly located above the bottom boundary layer in the photosphere (54%) and in the chromosphere (44%) with only a few null points in the corona (2%). The density of null points (expressed as the ratio of the number of null points to the number of photospheric magnetic fragments) in the solar atmosphere is estimated to be between 3% and 8% depending on the method used to identify the number of magnetic fragments in the observed photosphere.
Conclusions: This study reveals that the heating of the corona by magnetic reconnection at coronal null points is unlikely. Our findings do not rule out the heating of the corona at other topological features. We also report the topological complexity of the chromosphere as strongly suggested by recent observations from Hinode/SOT. Title: The Small-Scale Field Measured With Hinode/SOT and Feature Tracking: Where is the mixed- polarity flux? Authors: Deforest, C. E.; Lamb, D. A.; Berger, T.; Hagenaar, H.; Parnell, C.; Welsch, B. Bibcode: 2008AGUSMSP51D..01D Altcode: We report on the results of the first feature tracking study of the solar magnetic field with Hinode/SOT. We processed a SOT Na-D line-of-sight magnetogram sequence with five different magnetic tracking codes. The SOT data allow us to probe the evolving magnetic field on the granular scale for hours at a time, something that was not possible with either ground-based observations (which are limited to short periods of good seeing) or prior space-based observations (which are limited to arcsecond spatial scales). We find that the field is much less mixed than previously supposed: while Hinode resolves small-scale structure within features that, to SOHO/MDI, would appear as monolithic flux concentrations, this substructure has but a single sign. Furthermore, the average distance between identifiable flux concentrations of opposite sign remains nearly unchanged at the higher resolution, a result that is quite surprising in light of the common picture of a sea of strong mixed-polarity flux concentrations dotting the inter-granular lanes. We discuss possible mechanisms for this surprising result, and implications for the small-scale dynamo. Title: Recursive Reconnection and Magnetic Skeletons Authors: Parnell, C. E.; Haynes, A. L.; Galsgaard, K. Bibcode: 2008ApJ...675.1656P Altcode: By considering a simple driven model involving the resistive 3D MHD interaction of magnetic sources, it is shown that it is essential to know the magnetic skeleton to determine (1) the locations of reconnection, (2) type of reconnection, (3) the rate of reconnection, and (4) how much reconnection is occurring. In the model, two opposite-polarity magnetic fragments interact in an overlying magnetic field with reconnection, first closing and then opening the magnetic field from the sources. There are two main reconnection phases: the first has one reconnection site at which the flux is closed, and the second has three sites. The latter is a hybrid case involving both closing and reopening reconnection processes. Each reconnection site coincides with its own separator, and hence all reconnection is via separator reconnection. All the separators connect the same two nulls and thus mark the intersection between the same four types of flux domain. In the hybrid state, the two competing reconnection processes (which open and close flux connecting the same two source pairs) run simultaneously, leading to recursive reconnection. That is, the same flux may be closed and then reopened not just once, but many times. This leads to two interesting consequences: (1) the global reconnection rate is enhanced and (2) heating occurs for a longer period and over a wider area than in the single-separator case. Title: Solar Magnetic Tracking. II. The Apparent Unipolar Origin of Quiet-Sun Flux Authors: Lamb, D. A.; DeForest, C. E.; Hagenaar, H. J.; Parnell, C. E.; Welsch, B. T. Bibcode: 2008ApJ...674..520L Altcode: We investigate the origin of small-scale flux concentrations in the quiet Sun. In apparent violation of the physical requirement for flux balance, 94% of the features containing newly detected flux are unipolar at a resolution of 1.2''. We analyze 2619 of these apparent unipolar emergences in an image sequence from the SOHO MDI magnetograph and compare the ensemble average to a model of asymmetric bipolar emergence that could in principle hide opposing flux under the noise floor of MDI. We examine the statistical consequences of this mechanism and find that it cannot be responsible for more than a small fraction of the unipolar emergences. We conclude that the majority of the newly detected flux in the quiet Sun is instead due to the coalescence of previously existing but unresolved flux into concentrations that are large and strong enough to be detected. We estimate the rate of coalescence into arcsecond-scale magnetic features averaged over the solar surface to be 7 × 1021 Mx hr-1, comparable to the reported flux injection rate due to ephemeral regions. This implies that most flux in the solar network has been processed by very small scale shredding, emergence, cancellation, and/or coalescence that is not resolved at 1.2'', and it suggests that currently unresolved emergences may be at least as important as ephemeral region emergences to the overall flux budget. Title: Quiet Sun topology from Hinode/SOT Authors: Regnier, Stephane; Parnell, Clare; Haynes, Andrew Bibcode: 2008cosp...37.2586R Altcode: 2008cosp.meet.2586R The Hinode satellite was launched in 2006 with unprecedented high spatial and temporal resolution revealing the detailed nature of the quiet Sun. Based on the new data recorded by Hinode/SOT, we revisit the magnetic topology of the quiet Sun. It has been found, using point source models, that approximately one null point exists for each source with only 9% of these above the photosphere. In this study, we use a potential field extrapolation from a continuous photospheric magnetic field and analyse the properties of the magnetic nulls. We find that there are few photospheric nulls and most of the null points are located in the chromosphere. Title: Feature Tracking of Hinode Magnetograms Authors: Lamb, D.; Deforest, C. E.; Hagenaar, H. J.; Parnell, C. E.; Welsch, B. T. Bibcode: 2007AGUFMSH53A1066L Altcode: We present results of applying feature tracking to a sequence of Hinode magnetograms. The single line wing Na D 5896 magnetograms have a high signal-to-noise ratio, allowing the detection of flux approximately 30 times weaker than in MDI magnetograms. We find evidence that, even with Hinode's improved resolution and sensitivity, we do not always detect the bipolar emergence of new magnetic flux. This suggests that we have not reached the ultimate resolution to observe the fundamental flux generation processes in the photosphere. Title: Three-dimensional Resistive-MHD Model for X-ray Bright Points and Coronal Jets Authors: Parnell, C. E. Bibcode: 2007AGUFMSH21B..03P Altcode: Thirteen years ago a simple two-dimensional model for an X-ray bright point and associated cancelling magnetic feature was first proposed by Priest et. al (1994). Over the next decade this model was extended into three- dimensions and applied by many authors to many observed X-ray bright points. A two-dimensional model was proposed at about the same time for coronal jets associated with the emergence of magnetic flux (e.g. Yokoyama & Shibata 1995). Recently, however, new results from Hinode hint to the fact that X-ray bright points may not actually be created cancellation. Furthermore, images from XRT aboard Hinode reveal that coronal jets occur far more frequency than previously reported. We present results from resistive MHD experiments that show how it is possible to create an X-ray bright point and coronal jet without the need for either the emergence or cancellation of magnetic flux. Thus, the numbers of each event are not limited to the numbers of flux emergences or cancellations. Moreover, by following the evolution of the magnetic topology during the interaction we can show that reconnection at multiple (as opposed to single) separators is the energy release mechanism. Also, we reveal how it is possible to get dramatic changes in the coronal loop structures observed with hardly any changes in the magnetic footpoints below, in line with recent observations from Hinode. Title: Solar Magnetic Tracking. I. Software Comparison and Recommended Practices Authors: DeForest, C. E.; Hagenaar, H. J.; Lamb, D. A.; Parnell, C. E.; Welsch, B. T. Bibcode: 2007ApJ...666..576D Altcode: 2007arXiv0704.2921D Feature tracking and recognition are increasingly common tools for data analysis, but are typically implemented on an ad hoc basis by individual research groups, limiting the usefulness of derived results when selection effects and algorithmic differences are not controlled. Specific results that are affected include the solar magnetic turnover time, the distributions of sizes, strengths, and lifetimes of magnetic features, and the physics of both small scale flux emergence and the small-scale dynamo. In this paper, we present the results of a detailed comparison between four tracking codes applied to a single set of data from SOHO/MDI, describe the interplay between desired tracking behavior and parameterization tracking algorithms, and make recommendations for feature selection and tracking practice in future work. Title: A trilinear method for finding null points in a three-dimensional vector space Authors: Haynes, A. L.; Parnell, C. E. Bibcode: 2007PhPl...14h2107H Altcode: 2007arXiv0706.0521H Null points are important locations in vector fields, such as a magnetic field. A new technique (a trilinear method for finding null points) is presented for finding null points over a large grid of points, such as those derived from a numerical experiment. The method was designed so that the null points found would agree with any field lines traced using the commonly used trilinear interpolation. It is split into three parts: reduction, analysis, and positioning, which, when combined, provide an efficient means of locating null points to a user-defined subgrid accuracy. We compare the results of the trilinear method with that of a method based on the Poincaré index, and discuss the accuracy and limitations of both methods. Title: Multiply Connected Source and Null Pairs Authors: Parnell, C. E. Bibcode: 2007SoPh..242...21P Altcode: The magnetic fields within the solar atmosphere have a complex topology owing to the fragmentary nature with which they thread the solar surface. The topologies of the potential magnetic fields containing only a few (up to four) point photospheric sources have been classified. For small numbers of sources determining the connectivity of source pairs is equivalent to counting the number of flux domains. As the numbers of sources increase this, however, is no longer the case. Instead, a pair of connected sources can have more than one distinct flux domain linking them. We call these multiply connected source pairs. Pairs of nulls connected by more than one separator are called multiply connected null pairs. Multiply connected source and null pairs go hand-in-hand such that two separators connecting the same pair of nulls immediately implies multiple flux domains linking the same source pair and vice versa. It is found that multiply connected source pairs are common not only in fairly complex potential magnetic fields but more interestingly in the resistive-MHD evolution of both simple and complex magnetic fields. Magnetic energy release is often significant around separators. Thus fields with multiply connected source pairs, which naturally have more separators, (i) have more sites for intense energy release and (ii) are likely to release energy more quickly than other magnetic fields. Moreover, the combination of multiply connected source and null pairs can give rise to a situation where flux is reconnected repeatedly between two flux domains. Title: Magnetic Network Formation Due to Sub-arcsecond Flux Processing Authors: Lamb, Derek; DeForest, C. E.; Parnell, C. E.; Hagenaar, H. J.; Welsch, B. T. Bibcode: 2007AAS...210.9213L Altcode: 2007BAAS...39Q.210L Kinematic models of solar magnetic network formation typically employ the breakup of ephemeral regions by granular and supergranular flow. We show that the coalescence of sub-arcsecond-scale magnetic flux concentrations into features detectable with MDI is responsible for injecting as much flux into the magnetic network as the published emergence rate of ephemeral regions. We also show that the few fresh bipoles we do detect have no preferential alignment, and thus violate Joy's law at the arcsecond scale. These two items suggest that at least half of the flux that makes its way into the network has been processed at spatial scales below 1 arcsecond, indicative of a local dynamo. Title: Magnetohydrodynamic evolution of magnetic skeletons Authors: Haynes, A. L.; Parnell, C. E.; Galsgaard, K.; Priest, E. R. Bibcode: 2007RSPSA.463.1097H Altcode: 2007astro.ph..2604H The heating of the solar corona is probably due to reconnection of the highly complex magnetic field that threads throughout its volume. We have run a numerical experiment of an elementary interaction between the magnetic field of two photospheric sources in an overlying field that represents a fundamental building block of the coronal heating process. The key to explaining where, how and how much energy is released during such an interaction is to calculate the resulting evolution of the magnetic skeleton. A skeleton is essentially the web of magnetic flux surfaces (called separatrix surfaces) that separate the coronal volume into topologically distinct parts. For the first time, the skeleton of the magnetic field in a three-dimensional numerical magnetohydrodynamic experiment is calculated and carefully analysed, as are the ways in which it bifurcates into different topologies. A change in topology normally changes the number of magnetic reconnection sites.

In our experiment, the magnetic field evolves through a total of six distinct topologies. Initially, no magnetic flux joins the two sources. Then, a new type of bifurcation, called a global double-separator bifurcation, takes place. This bifurcation is probably one of the main ways in which new separators are created in the corona (separators are field lines at which three-dimensional reconnection takes place). This is the first of five bifurcations in which the skeleton becomes progressively more complex before simplifying. Surprisingly, for such a simple initial state, at the peak of complexity there are five separators and eight flux domains present. Title: Book Review: FUNDAMENTALS OF PLASMA PHYSICS / Cambridge University Press, 2006 Authors: Parnell, Clare Bibcode: 2007Obs...127...70P Altcode: No abstract at ADS Title: Book Review: LARGE-SCALE STRUCTURES AND THEIR ROLE IN SOLAR ACTIVITY (ASP CONFERENCE SERIES, VOL. 346) / Astronomical Society of the Pacific, 2005 Authors: Parnell, Clare Bibcode: 2007Obs...127...67P Altcode: No abstract at ADS Title: 3D magnetic reconnection, flares and coronal heating Authors: Parnell, C. E. Bibcode: 2007MmSAI..78..229P Altcode: Magnetic reconnection is known to be an important energy release mechanism in many solar, stellar, magnetospheric and astrophysical phenomena. Also it is the only way in which magnetic fields can change their topological structure. Reconnection in three dimensions is not well understood. In particular, knowing where and how reconnection is going to occur and at what rate it occurs at is not generally obvious in time-dependent 3D resistive MHD systems. In order to find answers to the above questions the simple interaction of two opposite-polarity magnetic sources in an overlying field is considered. This simple interaction represents a typical building block of the Sun's magnetic atmosphere. By following the evolution of the skeleton of the magnetic field we are able to explain where, how and at what rate the reconnection occurs in this building block of the Sun's magnetic field. Remarkably there were found to be up to five energy release sites and the reconnection rate is significantly higher than one would expect. Title: Understanding Magnetic Structures in the Solar Corona Through Topological Analysis Authors: Maclean, R. C.; Parnell, C. E.; De Moortel, I.; Büchner, J.; Priest, E. R. Bibcode: 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. Bibcode: 2006MNRAS.369...43V Altcode: 2006MNRAS.tmp..503V Two-dimensional numerical magnetohydrodynamic simulations of a cancelling magnetic feature (CMF) and the associated coronal X-ray bright point (XBP) are presented. Coronal magnetic reconnection is found to produce the Ohmic heating required for a coronal XBP. During the BP phase where reconnection occurs above the base, about 90-95 per cent of the magnetic flux of the converging magnetic bipole cancels at the base. The last ~5 to 10 per cent of the base magnetic flux is cancelled when reconnection occurs at the base. Reconnection happens in a time-dependent way in response to the imposed converging footpoint motions. A potential field model gives a good first approximation to the qualitative behaviour of the system, but the magnetohydrodynamics (MHD) experiments reveal several quantitative differences: for example, the effects of plasma inertia and a pressure build-up in-between the converging bipole are to delay the onset of coronal reconnection above the base and to lower the maximum X-point height. Title: Solar coronal heating by magnetic cancellation - I. Connected equal bipoles Authors: von Rekowski, B.; Parnell, C. E.; Priest, E. R. Bibcode: 2006MNRAS.366..125V Altcode: 2006MNRAS.tmp....9V We present two-dimensional numerical magnetohydrodynamic simulations of a cancelling magnetic feature and the associated coronal X-ray bright point. Coronal reconnection is found to produce significant Ohmic heating, and at the same time about 90 per cent of the magnetic flux is cancelled. The presence of downflows accelerates the process of flux cancellation in the early phase. The last 10 per cent of the cancellation takes place by reconnection at the base. Reconnection occurs in a time-dependent way in response to the footpoint motions, and the resulting sequence of magnetic configurations is close to potential. Title: MHD simulations of photospheric cancelling magnetic features causing coronal X-ray bright points Authors: von Rekowski, B.; Parnell, C. E.; Priest, E. R. Bibcode: 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. Bibcode: 2005ESASP.600E..95V Altcode: 2005dysu.confE..95V; 2005ESPM...11...95V No abstract at ADS Title: Coronal Flux Recycling Times Authors: Close, R. M.; Parnell, C. E.; Longcope, D. W.; Priest, E. R. Bibcode: 2005SoPh..231...45C Altcode: High-cadence, high-resolution magnetograms have shown that the quiet-Sun photosphere is very dynamic in nature. It is comprised of discrete magnetic fragments which are characterized by four key processes - emergence, coalescence, fragmentation and cancellation. All of this will have consequences for the magnetic field in the corona above. Title: Elementary heating events - magnetic interactions between two flux sources. III. Energy considerations Authors: Galsgaard, K.; Parnell, C. E. Bibcode: 2005A&A...439..335G Altcode: 2005astro.ph..1602G The magnetic field plays a crucial role in heating the solar corona - this has been known for many years - but the exact energy release mechanism(s) is(are) still unknown. Here, we investigate in detail, using resistive, non-ideal, MHD models, the process of magnetic energy release in a situation where two initially independent flux systems are forced into each other. Work done by the foot point motions goes into building a current sheet in which magnetic reconnection releases some of the free magnetic energy leading to magnetic connectivity changes. The scaling relations of the energy input and output are determined as functions of the driving velocity and the strength of fluxes in the independent flux systems. In particular, it is found that the energy injected into the system is proportional to the distance travelled. Similarly, the rate of Joule dissipation is related to the distance travelled. Hence, rapidly driven foot points lead to bright, intense, but short-lived events, whilst slowly driven foot points produce weaker, but longer-lived brightenings. Integrated over the lifetime of the events both would produce the same heating if all other factors were the same. A strong overlying field has the effect of creating compact flux lobes from the sources. These appear to lead to a more rapid injection of energy, as well as a more rapid release of energy. Thus, the stronger the overlying field the more compact and more intense the heating. This means observers need to know not only the flux of the magnetic fragments involved in an event, but also their rate and direction of movement, as well as the strength and orientation of the surrounding field to be able to predict the energy dissipated. Furthermore, it is found that rough estimates of the available energy can be obtained from simple models, starting from initial potential situations, but that the time scale for the energy release and, therefore its impact on the coronal plasma, can only be determined from more detailed investigations of the non-ideal behaviour of the plasma. Title: Destruction Mechanisms of Quiet-Sun Magnetic Flux Authors: Lamb, D. A.; Deforest, C. E.; Hagenaar, H. J.; Parnell, C. E.; Welsch, B. T. Bibcode: 2005AGUSMSP41B..02L Altcode: We use SWAMIS, a freely available magnetic feature tracking suite, to analyze the destruction of solar small-scale magnetic flux. We track a sequence of high resolution MDI magnetograms to find the destruction rates in a patch of quiet sun. We state criteria for defining the individual magnetochemical destruction mechanisms of merging, cancellation, and disappearance, and determine the contribution of each process to the removal of detected flux from the photosphere. Destruction mechanisms are important to determine because, together with formation mechanisms, they provide information as to the nature of the small-scale dynamo. We present preliminary results and discuss the implications of these rates on models of quiet-sun magnetic flux generation. Title: Comparison of blinkers and explosive events: A case study Authors: Bewsher, D.; Innes, D. E.; Parnell, C. E.; Brown, D. S. Bibcode: 2005A&A...432..307B Altcode: Blinkers are brightenings at network cell junctions that are traditionally identified with SOHO/CDS and explosive events or high velocity events are identified in high resolution UV spectra obtained from HRTS and SOHO/SUMER. Criteria are determined to facilitate objective automatic identification of both blinkers and explosive events in both SOHO/CDS and SOHO/SUMER data. Blinkers are identified in SUMER data, if the temporal resolution of the data is reduced to that of CDS. Otherwise short lived, localised intensity enhancements that make up the blinker are identified. Explosive events are identified in CDS data when the line width is significantly increased, and occasionally if there is an enhancement in the wing of the line profile. A theoretical statistical model is presented which hypothesises that blinkers and explosive events are random and not connected in any way. The results given in this paper suggest that this hypothesis can not be rejected and our probability interpretation of the recent results of Brković & Peter (2004, A&A, 422, 709) are inconclusive.

Appendices A and B are only available in electronic form at http://www.edpscienc es.org Title: Domain structures in complex 3D magnetic fields Authors: Close, R. M.; Parnell, C. E.; Priest, E. R. Bibcode: 2005GApFD..99..513C Altcode: The numerous magnetic fragments that populate the mixed-polarity, quiet-Sun photosphere give rise to many interesting topological features in the corona. In light of this, much recent work has gone into classifying the configurations that arise from simple, point-source potential-field models in efforts to determine the nature of the quiet-Sun magnetic field. These studies have ranged from systematic and detailed examinations of magnetic fields arising from only a handful of sources, involving classifying the configurations that arise (and how some states may bifurcate into other states), to statistical studies of the overall properties of fields arising from hundreds of magnetic sources. Such studies have greatly increased our understanding of what we might expect the magnetic field over the quiet Sun to behave like; the purpose of the study presented here is to extend this understanding further by examining the structure of the individual domains (the regions in space through which pairs of opposite-polarity sources are connected). In particular, the features of lesser-known domain structures that are absent from fields arising from only a few sources and overlooked by sweeping statistical studies are documented. In spite of the incredible complexity of the coronal field, previous studies have shown that there are only two types of building block in a potential field arising from coplanar point sources: namely, an isolated dome, bounded by a single unbroken separatrix surface, and a separator-ring domain, engirdled by a ring of separators. However, it is demonstrated here how both isolated domains and separator-ring domains may be categorised further depending upon their particular geometrical and topological traits. As many models predict coronal heating at topologically distinct features in magnetic fields such as null points, separators and separatrices, for any such models to be applied to general fields would require a scheme for identifying which topological features are related to a given domain. The study here explores some of the issues that would need to be taken into account by such a scheme, and in particular the problems associated with trying to deduce the properties of a general magnetic field from knowledge of domain footprints alone. Animated 3D-rotational views of some of the figures in this manuscript may be viewed in AVI, MPEG and animated-GIF formats by visiting http://www-solar.mcs.st-and.ac.uk/robertc/animations/blocks.html and following the desired link. Title: Fragment Driven Magnetic Reconnection Authors: Galsgaard, K.; Parnell, C. Bibcode: 2004ESASP.575..351G Altcode: 2004soho...15..351G; 2004astro.ph..9562G In this paper, we investigate a simple model where two, initially unconnected, flux systems are forced to interact in response to the imposed boundary driving by solving the non-ideal 3D MHD equations numerically. The reconnection rate of the dynamical process is determined and compared with the corresponding rate for the potential evolution of the magnetic field. This shows that the dynamic reconnection rate is about a factor of two smaller than the potential (perfect, instantaneous) rate for realistic solar driving velocities demonstrating that this three-dimensional magnetic reconnection process is fast. The energy input for a fixed advection distance is found to be independent of the driving velocity. The Joule dissipation associated with the reconnection process is also found to be basically dependent on the advection distance rather than driving velocity. This implies that the timescale for the event determines the effect the heating has on the temperature increase. Finally, the numerical experiments indicate that the observational structure of the reconnection site changes dramatically depending on the phase of the evolution of the passage of the two flux sources. In the initial phase, where the sources become connected, the heating is confined to a compact region. For the disconnecting phase the energy gets distributed over a larger area due to the reconnected field line connectivity. Title: Probability Analysis of Coincident Blinkers and Explosive Events Authors: Bewsher, D.; Brown, D.; Innes, D.; Parnell, C. Bibcode: 2004ESASP.575..465B Altcode: 2004soho...15..465B No abstract at ADS Title: Elementary heating events - magnetic interactions between two flux sources. II. Rates of flux reconnection Authors: Parnell, C. E.; Galsgaard, K. Bibcode: 2004A&A...428..595P Altcode: Magnetic fragments in the photosphere are in continuous motion and, due to the complex nature of the magnetic field in the solar atmosphere, these motions are likely to drive a lucrative coronal energy source: the passing of initially-unconnected opposite-polarity fragments that release energy through both closing and then re-opening the same fieldlines. Three-dimensional, time-dependent MHD and potential models are used to investigate the passing of fragments in an overlying field. The processes of closing and opening the field generally occur through separator and separatrix reconnection, respectively. The rates of flux reconnection in these processes are determined. They are found to be dependent on the direction of the surrounding magnetic field relative to the motion of the fragments and the velocity of the sources. In particular, separator reconnection rates (closing) and separatrix-surface reconnection rates (opening) are directly related to the rate of flux transport perpendicular to the current sheet (overlying field). The results suggest that both types of reconnection are fast with the peak rates of separator and separatrix reconnection occurring at 58% and 29% of the peak potential reconnection rate, respectively, when the sources are driven at a hundredth of the peak Alfvén velocity in the box. Moreover, the slower the system is driven the closer the flux reconnection rates are to the instantaneous potential rates. Furthermore, there is a maximum reconnection rate for both types of reconnection as the driving speed tends to the Alfvén speed with the separatrix reconnection rate typically half that of separator reconnection. These results suggest that, on the Sun, reconnection driven by the passing of small-scale network and intranetwork fragments is a highly efficient process that is very likely to contribute significantly to the heating of the background solar corona. The three-dimensional reconnection processes are efficient because, unlike in two-dimensions, there are many places within the current sheets where reconnection can take place simultaneously giving rise to fine-scale structure along the boundaries between the open, closed and re-opened flux. Furthermore, due to the complexity of the magnetic field above the photosphere the reconnection all takes place low down at less than a quarter of the separation of the initial fragments above the photosphere. Title: The Role of Dynamic Brightenings in Coronal Heating Authors: Parnell, C. E. Bibcode: 2004ESASP.575..227P Altcode: 2004soho...15..227P No abstract at ADS Title: Separators in 3D Quiet-Sun Magnetic Fields Authors: Close, R. M.; Parnell, C. E.; Priest, E. R. Bibcode: 2004SoPh..225...21C Altcode: At the confluence of four regions of different magnetic connectivity lies a distinct topological candidate for coronal heating, namely the magnetic separator. In this study, a method for tracing separator curves is developed and the statistical properties of separators in coronal fields are subsequently explored by analysing a model field with an exponential source distribution, similar to that studied by Schrijver and Title (2002). Magnetic fields based on data from an observed sequence of MDI magnetograms are also considered as a case study. The picture that emerges is one in which there are many more magnetic separators than previously thought, since many separators arise from each null point. For an exponential source distribution, an average of 10.1±0.13 separators per null are found, of which 1.04±0.04 "multiply link" pairs of nulls (i.e., there is more than one separator linking such pairs of nulls). For the observed sequence of magnetograms, these figures are 7.63±0.2 and 0.99± 0.059, respectively. The results obtained here show that separators have a tendency to group together into trunks about a null. In the case of prone nulls, these trunks lie either normal to the photospheric surface or on it. It is also established that pairs of coronal nulls are frequently interconnected, suggesting that they may have been created by purely coronal bifurcations. Title: Recycling of the Solar Corona's Magnetic Field Authors: Close, R. M.; Parnell, C. E.; Longcope, D. W.; Priest, E. R. Bibcode: 2004ApJ...612L..81C Altcode: Magnetic fields play a dominant role in the atmospheres of the Sun and other Sun-like stars. Outside sunspot regions, the photosphere of the so-called quiet Sun contains myriads of small-scale magnetic concentrations, with strengths ranging from the detection limit of ~1016 Mx up to ~3×1020 Mx. The tireless motion of these magnetic flux concentrations, along with the continual appearance and disappearance of opposite-polarity pairs of fluxes, releases a substantial amount of energy that may be associated with a whole host of physical processes in the solar corona, not least the enigma of coronal heating. We find here that the timescale for magnetic flux to be remapped in the quiet-Sun corona is, surprisingly, only 1.4 hr (around 1/10 of the photospheric flux recycling time), implying that the quiet-Sun corona is far more dynamic than previously thought. Besides leading to a fuller understanding of the origins of magnetically driven phenomena in our Sun's corona, such a process may also be crucial for the understanding of stellar atmospheres in general. Title: Dynamics of Transition Region Blinkers Authors: Bewsher, D.; Parnell, C.; Pike, D.; Harrison, R. Bibcode: 2004ESASP.547..267B Altcode: 2004soho...13..267B Analysis of CDS data has shown that transition region blinkers and the chromosphere directly below, are preferentially more redshifted and have larger non-thermal velocities than the normal transition region and chromospheric plasma. The ranges of these enhanced velocities, however, are no larger than the typical spread of Doppler and non-thermal velocities in these regions. An overview of the Doppler and non-thermal velocities of blinkers found with CDS in the quiet Sun and active region are presented. The anticipated range of Doppler velocities of blinkers are 10 - 15 km s-1 in the quiet-Sun (10 - 20 km s-1 in active-regions) in the chromosphere (He I) and 25 - 30 km s-1 in the quiet-Sun (20 - 40 km s-1 in activeregions) in the transition region (O V). The range of non-thermal velocities of blinkers in both the quiet- Sun and active-regions are estimated to be 15 - 25 km s-1 in He I and 30 - 45 km s-1 in O V. There are more blinkers with larger Doppler and non-thermal velocities than would be expected in the whole of the chromosphere and transition region. Title: A study of the unification of quiet-Sun transient-event phenomena Authors: Harrison, R. A.; Harra, L. K.; Brković, A.; Parnell, C. E. Bibcode: 2003A&A...409..755H Altcode: A number of small-scale, globally distributed solar transient event-types have been reported in the literature. Their potential role in fundamental processes in the solar atmosphere, such as coronal heating and wind acceleration, is under active investigation. However, the event-types, such as those known as blinkers, explosive events, EUV (extreme-UV) network and cell brightenings, network flares, heating events, nanoflares and EUV brightenings are basically classifications which are driven to a large extent by different observational techniques and different instruments rather than the identification of a clear differing physical phenomenon. We investigate the different instrumental and technique limitations and attempt to identify any unification of the reported quiet-Sun transient, small-scale phenomena. We find that once observational techniques have been considered, a number of the different classifications appear to be the same. This suggests that events known as blinkers, network and cell brightenings and EUV brightenings are the same event-type. We suggest that the term blinker be used as a generic term to describe these events. However, there appears to be little evidence that blinkers and explosive events are directly related. Furthermore, although a small percentage of blinkers and nanoflares/heating events appear to be related to one another, these events pose a number of important questions suggesting that either (i) blinkers and nanoflare/heating events are all created by the same mechanism, i.e. for some blinker events, the conditions are such that higher temperatures are found, or (ii) there are two types of event, including the ``traditional'' blinker which is effectively a transition region brightening driven by a density or filling factor enhancement, and a mini-flare-like event which reaches higher temperatures, presumably driven by reconnection. Title: Dynamics of Blinkers Authors: Bewsher, D.; Parnell, C. E.; Pike, C. D.; Harrison, R. A. Bibcode: 2003SoPh..215..217B Altcode: The relative Doppler and non-thermal velocities of quiet-Sun and active-region blinkers identified in O v with CDS are calculated. Relative velocities for the corresponding chromospheric plasma below are also determined using the He i line. O v blinkers and the chromosphere directly below, have a preference to be more red-shifted than the normal transition region and chromospheric plasma. The ranges of these enhanced velocities, however, are no larger than the typical spread of Doppler velocities in these regions. The anticipated ranges of Doppler velocities of blinkers are 10-15 km s−1 in the quiet Sun (10-20 km s−1 in active regions) for He i and 25-30 km s−1 in the quiet Sun (20-40 km s−1 in active regions) for O v. Blinkers and the chromosphere below also have preferentially larger non-thermal velocities than the typical background chromosphere and transition region. Again the increase in magnitude of these non-thermal velocities is no greater than the typical ranges of non-thermal velocities. The ranges of non-thermal velocities of blinkers in both the quiet Sun and active regions are estimated to be 15-25 km s−1 in He i and 30-45 km s−1 in O v. There are more blinkers with larger Doppler and non-thermal velocities than would be expected in the whole of the chromosphere and transition region. The recently suggested mechanisms for blinkers are revisited and discussed further in light of the new results. Title: Determination of coronal loop properties from trace observations Authors: De Moortel, I.; Parnell, C. E.; Hood, A. W. Bibcode: 2003SoPh..215...69D Altcode: In this paper, we determine the temperature profile along the footpoints of large coronal loops observed by TRACE in both the 171 Å and 195 Å passbands. The temperature along the lower part of these coronal loops only shows small variations and can probably be considered to be isothermal. Using the obtained temperature profile T(s) and an estimate of the column depth along the loop, we then determine the pressure along the lower part of the observed coronal loops and hence the value of the pressure scale length. The obtained scale lengths correspond in order-of-magnitude with the theoretically predicted gravitational scale height. We show that the differences between the observed and predicted scale heights are unlikely to be caused by (significant) flows along the loops but could possibly be a consequence of the inclination of the loops. This implies that the quasi-periodic intensity oscillations observed in the loops are most probably caused by compressive waves propagating upward at the coronal sound speed. Title: Comparison of Blinkers and Explosive Events Authors: Bewsher, D.; Innes, D.; Parnell, C. E. Bibcode: 2003SPD....34.1617B Altcode: 2003BAAS...35Q.836B There has been much speculation over recent years as to whether blinkers and explosive events are the same phenomena observed with different instruments.

Blinkers were first observed by Harrison (1997) by eye using SOHO/CDS, but more recently Brković et al. (2001) and Bewsher et al. (2002) have developed automated methods of identifying these events. Blinkers are small scale intensity enhancements seen in the transition region, with a mean area of 3 x 107 km2 and a mean lifetime of 16 minutes. The Doppler velocities of blinkers are preferentially more red-shifted than the typical transition region plasma. The range of these enhanced velocities, however, are no larger than the typical spread of Doppler velocities in these regions.

Explosive events were first observed using HRTS (Bruckener and Bartoe, 1983) as high energy turbulent events and jets. More recently, they have been observed using SOHO/SUMER. They have a typical size of 2" - 4" and have an average lifetime of 60 seconds. Line profiles of explosive events show strong non-Gaussian enhancements of both wings of the profile, and velocities associated with them range from +/- 50 - +/- 250 km s-1.

Examples will be presented showing (i) a co-spatial and co-incident blinker and explosive event; (ii) a blinker with no associated explosive event; and (iii) an explosive event with no associated blinker. We investigate the lightcurves and line profiles of the regions to explain the relationship, if any, between blinkers and explosive events. Title: Statistical Flux Tube Properties of 3D Magnetic Carpet Fields Authors: Close, R. M.; Parnell, C. E.; Mackay, D. H.; Priest, E. R. Bibcode: 2003SoPh..212..251C Altcode: The quiet-Sun photosphere consists of numerous magnetic flux fragments of both polarities that evolve with granular and supergranular flow fields. These concentrations give rise to a web of intermingled magnetic flux tubes which characterise the coronal magnetic field. Here, the nature of these flux tubes is studied. The photosphere is taken to be the source plane and each photospheric fragment is represented by a series of point sources. By analysing the potential field produced by these sources, it is found that the distribution of flux tube lengths obtained by (i) integrating forward from positive sources and (ii) tracing back from negative sources is highly dependent on the total flux imbalance within the region of interest. It is established that the relation between the footpoint separation of a flux tube and its height cannot be assumed to be linear. Where there is a significant imbalance of flux within a region, it is found that fragments of the dominant polarity will have noticeably more connections, on average, than the minority polarity fragments. Despite this difference, the flux from a single fragment of either polarity is typically divided such that (i) 60-70% connects to one opposite-polarity fragment, (ii) 25-30% goes to a further 1 to 2 opposite-polarity fragments, and (iii) any remaining flux may connect to as many as another 50 or more other opposite-polarity fragments. This is true regardless of any flux imbalance within the region. It is found that fragments connect preferentially to their nearest neighbours, with, on average, around 60-70% of flux closing down within 10 Mm of a typical fragment. Only 50% of the flux in a quiet region extends higher than 2.5 Mm above the solar surface and 5-10% extends higher than 25 Mm. The fragments that contribute to the field above this height cover a range of sizes, with even the smallest of fragments contributing to the field at heights of over 50 Mm. Title: Magnetic structure of transition region blinkers Authors: Bewsher, D.; Parnell, C. E.; Brown, D. S.; Hood, A. W. Bibcode: 2002ESASP.505..239B Altcode: 2002IAUCo.188..239B; 2002solm.conf..239B Analysis of the photospheric magnetic field has shown that the majority of blinkers, small-scale intensity enhancements seen in the transition region, occur above single fragments. We investigate the relationship between the strength of these single magnetic fragments or the ratio of any mixed magnetic fields beneath blinkers and blinker characteristics. In all cases, no correlation is found between the strength of the magnetic field and the blinker properties. We suggest, therefore, that blinkers are not caused by reconnection and that other mechanisms should be explored further. Title: Small-scale transient events in the quiet and active solar atmosphere Authors: Parnell, Clare E. Bibcode: 2002ESASP.505..231P Altcode: 2002IAUCo.188..231P; 2002solm.conf..231P Over the last decade the unprecedented uninterrupted, high resolution, coverage of the Sun has led to the discovery of many new types of small-scale phenomena, as well as a better understanding of phenomena such as X-ray bright points. Due to the excellent range of telescopes aboard these many spacecraft the small-scale phenomena have been observed in many different wavelengths. Much has been discovered over the past 10 years about them, but there are still many questions to be answered. What are the connections between all these different small-scale transient phenomena. How are they created? Do they contribute to the heating of the solar corona and if so, how significant is their contribution? Are any of them the source of the fast solar wind? Six key types of phenomena from the quiet and active Sun are reviewed. Their connections to the magnetic field below and to other coronal phenomena are discussed. Title: Magnetic reconnection throughout the solar atmosphere Authors: Hood, A. W.; Galsgaard, K.; Parnell, C. E. Bibcode: 2002ESASP.505..285H Altcode: 2002solm.conf..285H; 2002IAUCo.188..285H Magnetic reconnection is responsible for many different solar phenomena and it is the release of magnetic energy through reconnection that is believed to (i) drive flares, (ii) generate CMEs, (III) heat the corona and (iv) generate MHD waves. In basic models of 2D magnetic reconnection, the particular choise of boundary conditions influences the form of reconnection obtained. Reconnection in 3D can occur with and without null points. Numerical experiments have attempted to investigate different types of reconnection but a basic understanding of reconnection at 3D magnetic null points is essential in understanding these fumdamental processes. The structure of magnetic regions depends on features such as the magnetic skeleton, the mull points, the spine and fan plane. Numerical simulations are important but, at present, are unable to fully resolve the reconnection region. Recent analytical and numerical results of 3D reconnection will be presented. Applications of reconnection in the solar atmosphere will be discussed also. Title: Nature of the magnetic carpet - I. Distribution of magnetic fluxes Authors: Parnell, C. E. Bibcode: 2002MNRAS.335..389P Altcode: The distribution of magnetic fluxes found in the solar magnetic carpet, the photospheric magnetic field of the quiet-Sun, is investigated. A total of 27 500 concentrations are studied with fluxes ranging between a few times 1017 and a few times 1020 Mx. A histogram of the fluxes shows that there are more small fluxes and more large fluxes than anticipated by an exponential distribution. However, there are significantly fewer large and small fluxes than a power-law distribution fitted to the middle of the range. Thus, the fluxes are not distributed in the form of either an exponential or a power-law distribution, as previously suggested. Instead, the Weibull distribution, which involves both a power law and an exponential, is found to provide both a statistically good fit to the data and a physically reasonable prediction for the total absolute flux density. The best-fitting Weibull distribution has a Kolmogorov-Smirnov D statistic of approximately 0.07 - well below the 5 per cent significance level and the Weibull distribution predicts the observed total absolute flux densities to within better than 92 per cent. Physically, the observed distribution of fluxes can be explained as being made up of the following three elements: (i) emergence of new flux over all scales gives rise to an initial exponential distribution (observed); (ii) fragmentation and partial cancellation produce smaller and smaller fluxes thus creating excess small fluxes; (iii) excess large fluxes are created by coalescence and an additional (possibly significant) injection of flux from remnants of active regions. Finally, the equations of `magneto-chemistry' are used to derive suitable forms for the rate of emergence, cancellation, coalescence and fragmentation consistent with a Weibull distribution of fluxes. Title: Solar Corona: Magnetic carpets and hot coronae Authors: Parnell, Clare Bibcode: 2002A&G....43d..16P Altcode: Solar physicists have known for many decades now that the outer atmosphere of the Sun, the solar corona, is some 200 times hotter than the surface of the Sun. Understanding just how the corona is heated is one of the Sun's greatest mysteries and although many mechanisms have been proposed it is not clear which is the most important. What is clear, however, is that small-scale phenomena are a very important piece in this puzzle. Title: Nanoflare Statistics from First Principles: Fractal Geometry and Temperature Synthesis Authors: Aschwanden, Markus J.; Parnell, Clare E. Bibcode: 2002ApJ...572.1048A Altcode: We derive universal scaling laws for the physical parameters of flarelike processes in a low-β plasma, quantified in terms of spatial length scales l, area A, volume V, electron density ne, electron temperature Te, total emission measure M, and thermal energy E. The relations are specified as functions of two independent input parameters, the power index a of the length distribution, N(l)~l-a, and the fractal Haussdorff dimension D between length scales l and flare areas, A(l)~lD. For values that are consistent with the data, i.e., a=2.5+/-0.2 and D=1.5+/-0.2, and assuming the RTV scaling law, we predict an energy distribution N(E)~E-α with a power-law coefficient of α=1.54+/-0.11. As an observational test, we perform statistics of nanoflares in a quiet-Sun region covering a comprehensive temperature range of Te~1-4 MK. We detected nanoflare events in extreme-ultraviolet (EUV) with the 171 and 195 Å filters from the Transition Region and Coronal Explorer (TRACE), as well as in soft X-rays with the AlMg filter from the Yohkoh soft X-ray telescope (SXT), in a cospatial field of view and cotemporal time interval. The obtained frequency distributions of thermal energies of nanoflares detected in each wave band separately were found to have power-law slopes of α~1.86+/-0.07 at 171 Å (Te~0.7-1.1 MK), α~1.81+/-0.10 at 195 Å (Te~1.0-1.5 MK), and α~1.57+/-0.15 in the AlMg filter (Te~1.8-4.0 MK), consistent with earlier studies in each wavelength. We synthesize the temperature-biased frequency distributions from each wavelength and find a corrected power-law slope of α~1.54+/-0.03, consistent with our theoretical prediction derived from first principles. This analysis, supported by numerical simulations, clearly demonstrates that previously determined distributions of nanoflares detected in EUV bands produced a too steep power-law distribution of energies with slopes of α~2.0-2.3 mainly because of this temperature bias. The temperature-synthesized distributions of thermal nanoflare energies are also found to be more consistent with distributions of nonthermal flare energies determined in hard X-rays (α~1.4-1.6) and with theoretical avalanche models (α~1.4-1.5). Title: Transition-Region Blinkers - II. Active-Region Properties Authors: Parnell, C. E.; Bewsher, D.; Harrison, R. A. Bibcode: 2002SoPh..206..249P Altcode: The distribution and general properties of events identified in an active region that have the same characteristics as quiet-Sun blinkers are discussed and named `active-region blinkers'. The events are identified using an automated scheme `BLinker Identification Program' (BLIP) which was designed for and tested on quiet-Sun blinkers. Like quiet-Sun blinkers, the active-region events are most easily identified in the 629 Å emission line from O v although evidence for them is also found in other extreme UV lines emitted from He i, O iii and O iv. Unlike quiet-Sun blinkers, however, they may also have coronal signatures in the lines Mg ix and Mg x. Their properties are very similar to those of quiet-Sun blinkers with mean lifetimes of 16-19 min, mean areas of 2.4-4.3×107 km2 and mean intensity enhancements factors of 1.8-3.3. Their global frequency of 7-28 s−1 is about 42%-700% higher than that for quiet-Sun blinkers. The blinkers discussed here are found above both active-region (plage) magnetic fields, as well as above the umbra and penumbra of a sunspot. Title: Transition Region Blinkers I. Quiet-Sun Properties Authors: Bewsher, D.; Parnell, C. E.; Harrison, R. A. Bibcode: 2002SoPh..206...21B Altcode: An automated method of identifying transition region blinkers is presented. The distribution and general properties of blinkers identified in the quiet Sun are discussed. The blinkers are seen most clearly in the O v (629 Å) transition region emission line, but they also have strong signatures in O iv (554 Å), and the chromospheric line, He i (584 Å). The strongest O v blinkers can also be identified in O iii (599 Å). No significant signatures are found for blinkers in the Mg ix (368 Å) and Mg x (624 Å) coronal lines. A few hundred O v blinkers are analyzed. Their global frequency is between 1 and 20 s−1 depending on how significant the peaks of the blinkers are. They have a typical area of 3×107 km2, a typical lifetime of 16 min and a typical intensity enhancement factor of 1.8. We find the ratios of the oxygen lines to be flat confirming the result that blinkers are not temperature events, but are density enhancements or due to increases in filling factor. Blinkers are found to occur preferentially over regions of enhanced chromospheric or transition region emission such as network boundaries, however, it is not so clear that they appear below the brightest coronal regions. A rough analysis of the magnetic fragments show that blinkers preferentially occur above regions where there are large or strong magnetic fragments with 75% occurring in regions where one polarity dominates. Title: X-Ray Bright Points and other Quiet Sun Transient Phenomena [Invited] Authors: Parnell, C. E. Bibcode: 2002mwoc.conf...47P Altcode: Over the last decade the unprecedented uninterrupted, high resolution, coverage of the Sun has led to the discovery of many new types of small-scale phenomena, as well as a better understanding of phenomena such as X-ray bright points and coronal plumes. Due to the excellent range of telescopes aboard the many spacecraft currently viewing the Sun these small-scale phenomena have been observed in many different wavelengths. Much has been discovered over the past 10 years, but there are still many questions to be answered. What are the connections between all these different small-scale transient phenomenaNULL How are they createdNULL Do they contribute to the heating of the solar corona and if so how significant is their contributionNULL Are any of them the source of the fast solar windNULL I will be reviewing what has been discovered about both the new and well known dynamic quiet Sun phenomena from not only Yohkoh, but also SOHO and TRACE. In particular, I shall discuss the connections they appear to have to X-ray bright points and coronal plumes and will consider if any of them make a major contribution to the heating of the corona. Title: Magnetic structure and reconnection of x-ray bright points in the solar corona Authors: Brown, D. S.; Parnell, C. E.; DeLuca, E. E.; McMullen, R. A.; Golub, L.; Priest, E. R. Bibcode: 2002AdSpR..29.1093B Altcode: The three-dimensional magnetic topology of the solar corona is incredibly complex and its effect on the nature of 3D reconnection is profound. We study the supposedly simple topology of a small scale X-ray bright point observed by TRACE and SOHO/MDI, and how it is driven by reconnection when it forms and during the early stages of its lifetime. Title: The Magnetic Structure of a Coronal X-Ray Bright Point Authors: Brown, D. S.; Parnell, C. E.; Deluca, E. E.; Golub, L.; McMullen, R. A. Bibcode: 2001SoPh..201..305B Altcode: X-ray bright points are small dynamic loop structures that are observed all over the solar corona. The high spatial and temporal resolution of the TRACE instrument allows bright points to be studied in much greater detail than previously possible. This paper focuses on a specific bright point which occurred for about 20 hours on 13-14 June 1998 and examines its dynamic structure in detail. This example suggests that the mechanisms that cause bright points to form and evolve are more complex than previously thought. In this case, reconnection probably plays a major part during the formation and brightening of the loop structure. However, later on the foot points rotate injecting twist into the bright point which may cause an instability to occur with dynamic results. Title: A model of the Solar Magnetic Carpet Authors: Parnell, C. E. Bibcode: 2001SoPh..200...23P Altcode: There are four key processes that dictate the behaviorof the magnetic flux concentrations that form the so-called `magnetic carpet' of the quiet photosphere. These processes are emergence, cancellation, coalescence, and fragmentation. Rates of emergence have been estimated from observations, but the rates of cancellation, coalescence, and fragmentation are much more difficult to determine observationally. A model is set up to simulate an area of magnetic carpet in the quiet Sun. In the model there are three imposed parameters: the rate of emergence of new flux, the distribution of emerged flux and the rate of fragmentation of flux concentrations. The rate of cancellation and the rate of coalescence are deduced from the model. From the simulations it is estimated that the average emergence rate of new flux in the quiet Sun must be between 6×10−6 and 10− 5 Mx cm−2 s−1 to maintain an absolute flux density of between 2.5 and 3 G. For this rate of emergence a fragmentation rate of more than 12×10−5 s−1 is required to produce the observed exponential index for the number density of flux concentrations. This is equivalent to each fragment canceling more than once every 200 minutes. The rate of cancellation is calculated from the model and is found naturally to be equivalent to the rate of emergence. However, it is found that the frequency of cancellation is much greater than the frequency of emergence. In fact, it is likely that there are several orders of magnitude more cancellation events than emergence events. This implies that flux is injected in relatively large concentrations whereas cancellation occurs though the disappearance of many small concentrations. Title: Transition Region Blinkers Authors: Parnell, C. E.; Bewsher, D.; Harrison, R. A.; Hood, A. W. Bibcode: 2001IAUS..203..359P Altcode: Blinkers are small bright emission events observed best in the O V transition region line that occur above the supergranular network. They were first observed using SoHO/CDS data and were identified manually by Harrison (1997). They are believed to be density enhancements, but how they are created and what their properties are is not well known. We have developed the first program to automatically identify blinkers and their characteristics. The evolution of the magnetic field observed by SoHO/MDI below these blinkers has then be analysed to determine what magnetic field configuration is required for a blinker to occur. Also, the coronal emission above has been investigated using SoHO/CDS and TRACE data to determine the relation between blinkers, x-ray bright points and nanoflares. All three of these events are known to occur at the network, but as yet the relation between them is not understood. Putting together the results from these multi-wave length studies we have been able to determine a model for how blinkers occur and what their effect is on the transition region around and the corona above. Title: Solar Corona: X-ray Bright Points Authors: Parnell, C. Bibcode: 2000eaa..bookE2022P Altcode: X-ray (or coronal) bright points are small knots of intense x-ray emission scattered randomly throughout the solar CORONA. They are associated with pairs of opposite-polarity magnetic flux seen below in the SOLAR PHOTOSPHERE and are most likely to be heated by the motion of the magnetic fluxes causing reconnection in the corona.... Title: Elementary heating events - Magnetic interactions between two flux sources Authors: Galsgaard, K.; Parnell, C. E.; Blaizot, J. Bibcode: 2000A&A...362..395G Altcode: Observations taken by the SoHO MDI instrument have revealed that the quiet photospheric magnetic flux is, on average, recycled within a few days. As new flux emerges from the convection zone into the photosphere it is moved around by horizontal motions resulting from overshoots of convection cells. These motions cause the magnetic fields extending from flux fragments to tangle, forcing different magnetic flux systems to interact. Only the process of magnetic reconnection limits the complexity of magnetic field line connectivity. The energy liberated by these detangling or destressing processes act as a natural energy source which may heat the solar coronal plasma. In this paper, we use a numerical approach to solve the MHD equations in a three-dimensional domain to examine the dynamical behaviour of one simple magnetic flux interaction. The model consists of a uniform magnetic field overlying two flux sources of opposite polarity that are initially unconnected and are forced to interact as they are driven passed each other. We find that the development from initially unconnected sources to connected sources proceeds quite quickly and simply. This change takes place through driven separator reconnection in a systematically twisted current sheet. The out flow velocity from the reconnection is highly asymmetric with much higher velocities in the region defined by the field lines connected to both sources. However, the change back to two independent sources after the nearest approach has past takes place on a much longer time scale even though the distance between the sources increases significantly. This is because the opening of the field has to take place through separatrix reconnection and at this phase of the development there are no forcing of the fluxes to drive a fast opening of the magnetic field. Title: Magnetic reconnection and some solar applications Authors: Parnell, C. E. Bibcode: 2000RSPTA.358..669P Altcode: Magnetic reconnection is a fundamental plasma-physics process that is of great importance for the Sun, the Earth's magnetosphere and all astrophysical objects which have magnetic fields. It is a process central to the generation of magnetic fields in stars and also plays a major role in the heating of solar and stellar coronae. The development of both large (e.g. flares) and small (e.g. coronal bright points) dynamic phenomena observed on the Sun depends on reconnection and it is likely that reconnection may also be important for the acceleration of solar and stellar winds. It is 50 years since the first seeds of ideas for magnetic reconnection were sown and over 40 years since the classic Sweet-Parker mechanism was suggested. Since then the majority of the research has focused on reconnection in two dimensions. However, in the last few years attentions have turned to understanding the intricacies of reconnection in three dimensions. In this paper, the classical aspects of two-dimensional reconnection are reviewed, together with various mechanisms for reconnection in three dimensions, in particular, spine, fan and quasi-separatrix layer reconnection. The paper is then rounded off with examples of some solar phenomena where reconnection is believed to be present. In particular, the heating of some observed small-scale events in the corona is investigated and the question of quiet coronal heating due to nanoflares and microflares is addressed. Title: Statistical Analysis of the Energy Distribution of Nanoflares in the Quiet Sun Authors: Parnell, C. E.; Jupp, P. E. Bibcode: 2000ApJ...529..554P Altcode: For many years it has been debated whether the quiet solar corona is heated by nanoflares and microflares or by magnetic waves. In this paper TRACE data of events with energies in the range 1023-1026 ergs are investigated. A new stable and objective statistical technique is proposed to determine the index, -γ, of a power-law relation between the frequency of the events and their energy. We find that γ is highly dependent on the form of the line-of-sight depth assumed to determine the event energies. If a constant line-of-sight depth is assumed, then γ lies between 2.4 and 2.6; however, if a line-of-sight depth of the form (Ae/k2)1/2 is assumed, where Ae is event area and k is a constant, then γ lies between 2.0 and 2.1. In all cases the value of γ is greater than 2 and therefore implies that the events with the lowest energies dominate the heating of the quiet solar corona. Moreover, there are strong indications that there is insufficient energy from events with nanoflare energies (i.e., energies in the range 1024-1027 ergs) to explain the total energy losses in the quiet corona. However, our results do not rule out the possibility that events with picoflare energies (i.e., energies in the range 1021-1024 ergs) heat the quiet corona. From analysis of the spatial distribution of the events, we find that events are mainly confined to regions with the brightest EUV emission, which are presumably the regions connected to the strongest magnetic fields. Indeed, just 16% of the quiet corona possesses such events. Title: Magnetic Reconnection: Classical Aspects Authors: Parnell, Clare E. Bibcode: 2000LNP...553...61P Altcode: 2000tech.conf...61P Magnetic reconnection is an important mechanism in astrophysics for converting magnetic energy to both thermal energy and bulk acceleration of plasma and also for changing the global topology of the magnetic field. For over 50 years now solar theorists have investigated reconnection. This paper provides a basic review of the classical aspects of reconnection in both one, two and three dimensions, as well as, giving a potted history of reconnection theory in solar physics. Magnetic annihilation, Sweet-Parker reconnection and Petschek reconnection will all be discussed as will spine and fan reconnection in three dimensions. Title: Observed Magnetic Structure of X-Ray Bright Points from TRACE and MDI Authors: Brown, D. S.; Parnell, C.; Deluca, E.; McMullen, R.; Golub, L. Bibcode: 1999ASPC..184...81B Altcode: From 13th-17th June 1998, TRACE and MDI simultaneously observed the same quiet region of the Sun. From these observations the fascinating and complex structure of x-ray bright points, intense small scale brightenings that occur throughout the solar corona, can be seen in great detail. For the first time, it has been possible to study bright points for their entire lifetime with a cadence of 2 minutes and a temporal resolution of 0.5 arcsecs. One particular bright point which lasted two days exhibited dynamic structural behaviour which became increasingly complex and lead to its sudden eruptive demise. With the use of MDI magnetograms, it is possible to extrapolate the magnetic structure using an analytical constant α force-free approximation. This has been used to help us to explain the topology and behaviour of the bright point. By comparing two of TRACE's Fe lines (FeIX and FeXII) the spatial and temporal temperature and density structure of the bright point has been investigated. This analysis indicates that this bright point is made up of a complex system of dense loops. By understanding the magnetic, temperature and density structure of the bright point, it is hoped that the mechanism by which it is heated can be gained. Title: Peeling back the Sun. Authors: Parnell, C. Bibcode: 1999PhyEd..34..108P Altcode: Observations show that the Sun is not simply a ball of hot plasma but contains several discrete layers: the interior where the energy is generated is covered by the photosphere, the chromosphere and finally the corona. This article describes a photon's journey to the outside and links features such as prominences and sunspots to the regions in which they occur. Title: Our New View of the Solar Corona from YOHKOH and SOHO Authors: Parnell, C. E. Bibcode: 1998Ap&SS.261...81P Altcode: 1999Ap&SS.261...81P The Yohkoh satellite has now been orbiting the Earth for about 6 years and during this time it has revealed a number of new features in the solar corona. These include the discovery of X-ray jets and active region transient brightenings, as well as observations of hard X-ray sources above and at the feet of soft X-ray flare loops. SOHO, the newest solar space mission, is not orbiting the Earth, but is in fact orbiting the Sun and has been at the Earth's L1 point for about 2 years. During this time it too has identified some new and interesting characteristics of both the solar corona, the photosphere and the solar interior. For example, studies of high resolution MDI/SOHO magnetograms indicate that the magnetic carpet may play an important role in the heating of the coronal. Also polar plume studies from EIT/SOHO data suggest that they may be a possible source for the fast solar wind. Together these two missions have dominated solar research throughout the 90s and are expected to continue to do so during the rise from solar minimum to the next solar maximum. Title: Coronal Heating and the Photospheric Magnetic Field Authors: Parnell, C. E.; Sturrock, P. A. Bibcode: 1997SPD....28.0506P Altcode: 1997BAAS...29..909P Since magnetic field typically plays a role (either active or passive) in coronal heating theories, it may be possible to evaluate these theories by investigating the relationship between the coronal energy budget (the total power requirement of the corona) and measurable properties of the photospheric magnetic field. The X-ray flux is a useful proxy for the total power required to maintain the corona, so we have examined the relationship between the total X-ray flux, as measured by the GOES instruments, and the total magnetic flux, as estimated from the NSO instrumentation at Kitt Peak. We use this relationship to test the recent proposal that coronal heating is due to sudden magnetic relaxation. According to this concept, reconnection in the chromosphere of the footpoint regions of two oppositely directed flux tubes leads to a new flux tube, with widely separated footpoints, which erupts rapidly and generates sound waves that heat the corona. We adopt a simple "kinetic theory" model for the photospheric and chromospheric processes, and so obtain an estimate of the magnetic flux reconnection rate in terms of the mean field strength and of the parameters (diameter, flux and random speed) that characterize the elementary flux elements. The sudden magnetic relaxation model gives a simple relation between the magnetic flux budget and the coronal energy budget. In this way, we obtain a theoretical relationship between the coronal energy budget and the mean photospheric magnetic field strength that we compare with the available observational data. This work was supported by NASA grants NAS 8-37334 and NAGW-2265, and by Air Force grant F49620-95-1-0008. Title: Coronal heating by reconnection Authors: Parnell, C. E. Bibcode: 1997AdSpR..19.1853P Altcode: The outermost atmosphere of the Sun, called the corona, is some 200 times hotter than the surface of the Sun. The main source of energy for heating the corona is believed to be the magnetic field which dominates the corona. Magnetic reconnection is probably the most important mechanism for releasing magnetic energy and may, therefore, be important for coronal heating or micro-flaring. The best observational examples of reconnection in the corona are thought to be X-ray bright points, which are small-scale brightenings seen randomly throughout the whole corona. Theoretical models can not only explain the key observations relating to bright points, but they can also explain the complex three-dimensional structures often seen in bright points. In these models magnetic neutral points play a significant role as the centres for reconnection both in two and three dimensions. Title: Structure and collapse of three-dimensional magnetic neutral points Authors: Parnell, C. E.; Neukirch, T.; Smith, J. M.; Priest, E. R. Bibcode: 1997GApFD..84..245P Altcode: The structure and collapse of linear three-dimensional magnetic neutral points is studied by varying the four parameters (p, q,j|,j) that define, in general, the linear field of a neutral point. The effect of these parameters on both the skeleton structure (i.e. the fan and spine) and the actual field line structure of the null is considered. It is found that one current component (j) causes the skeleton structure of the null to fold up from its potential state, whereas the other current component (j|;) causes the field lines to bend. The two other parameters (p,q) determine the potential structure of the null and cause the null to transform from a three-dimensional null to a two-dimensional null and from a positive (type B) null to a negative (type A) null.

To investigate the collapse of three-dimensional nulls, solutions to the linear, low-β ideal magnetohydrodynamic equations are found. It is found that three-dimensional null points can collapse if the field line foot-points are free and energy can propagate into the system. Title: The Role of Magnetic Reconnection in Small-Scale Coronal Events (Invited) Authors: Parnell, C. E. Bibcode: 1996ASPC..111...19P Altcode: 1997ASPC..111...19P To illustrate the importance of magnetic reconnection in the heating of small-scale coronal events examples of reconnection models for X-ray bright points are given. These models include, in two dimensions, the converging flux and the emerging flux models and, in three dimensions, models of particular bright points are discussed. A brief general introduction to reconnection is given which focuses on reconnection at null points in three dimensions. Title: The Three-Dimensional Structures of Elementary Coronal Heating Events Authors: Parnell, C. E. Bibcode: 1996mpsa.conf..455P Altcode: 1996IAUCo.153..455P No abstract at ADS Title: Discussion and application of X-ray bright point models. Authors: Parnell, C. E. Bibcode: 1996joso.proc..121P Altcode: A basic model for X-ray bright points which explains many of the key observations is discussed. Then, using the philosophy of this model, an explanation for the three-dimensional structure of bright points is given. Further application of the basic model is considered, in particular, a reason for the similarities between bright point flares and active region transient brightenings is suggested. Title: A converging flux model for the formation of an X-ray bright point above a supergranule cell Authors: Parnell, C. E.; Priest, E. R. Bibcode: 1995GApFD..80..255P Altcode: The many complex regions of positive and negative flux that thread the surface of the Sun are mainly grouped around the edges of supergranule cells. These cells have large concentrations of magnetic flux on their boundaries and very little flux inside, with the magnetic fragments that appear in the centre of the cells swept to the boundaries by convective motions. Thus, a small bipolar pair of magnetic fragments (such as an ephemeral region) emerges inside a cell and moves towards the cell boundary as it grows. On reaching the boundary the fragments encounter unipolar regions of network flux with which they may merge or cancel. When cancellation takes place there is often an associated X-ray bright point in the overlying corona. Here, the emergence and interaction of an ephemeral region in a quiet-region or active-region super-granule cell is considered. It is found that there are three possible scenarios for the evolution of an ephemeral region in a supergranule cell and these are all investigated. The magnetic fields for the supergranule cell and ephemeral region are modelled by finite sources of flux and are studied as the ephemeral region moves through a series of quasi-static states. It is found that the ratio of the cancelling fragment widths (strengths) is important in determining the lifetime and path of the bright point, while the actual sizes of the fragments is important for determining the intensity of the bright point, the lifetime of the cancelling magnetic feature and the relative times of the bright point completion and cancelling magnetic feature onset. From this we suggest that transient brightenings in active regions and bright points on the quiet Sun may both be created by the converging flux mechanism detailed here. Title: Models of X-ray bright points and cancelling magnetic features Authors: Parnell, Clare Elizabeth Bibcode: 1995PhDT.......196P Altcode: No abstract at ADS Title: Models of x-ray bright points and cancelling magnetic features Authors: Parnell, Clare E. Bibcode: 1995PhDT.......163P Altcode: Small brightenings called x-ray bright points (Golub et al, 1974) occur in the solar corona. They are observed with the soft x-ray telescope on Skylab to be approximately 22 Mm in diameter with a brighter inner core of width 4-7 Mm although with the Normal Incidence X-ray Telescope their dimensions are observed to be typically 6 Mm x 9 Mm. By comparison with magnetograms of the photosphere it has been noticed recently that there is a high correlation between the occurrence of x-ray bright points and the mutual reduction of flux between two opposite polarity magnetic fragments. These fragments are originally unconnected magnetically, but move towards each other and simultaneously lose equal amounts of flux (cancel): they are called cancelling magnetic features (Martin et al, 1984). The observations relating to these features were reviewed by Priest et al. (1994) who suggested that they naturally evolve through three phases: the pre-interaction, interaction and cancellation phases. From this evidence qualitative pictures of the magnetic field structure for an x-ray bright point and associated cancelling magnetic feature were established. The aim of this thesis has been to build on the ideas of Priest et al. (1994) to produce a detailed theoretical model of an x-ray bright point and a cancelling magnetic feature. The magnetic field structures are estimated, and the position and lifetime of the bright point are calculated, as is the total amount of energy released during the bright point. This work is also extended to study more complex cancelling configurations representing the main basic types of cancelling magnetic feature. The results of these models determine the factors that affect the lifetime and position of a bright point and indicate which types of cancelling magnetic features are most likely to produce bright points that are long-lived, lie directly above the cancellation site and occur simultaneously with the cancellation phase. The complex structure of a bright point cannot be explained from the above two-dimensional models: thus two recently observed bright points were studied to see if the above model could be extended into three dimensions to explain the structure seen in soft x-ray images. The available observational data was used and leads to reasonable explanations for the complex shapes of both bright points. Finally, a more realistic model for the overlying field was set up involving a model of the field above a supergranule cell field with fragments of finite width. The interaction of an ephemeral region within this field was then studied and led to five different scenarios. The results obtained reaffirmed those found in the previous simpler models and suggest where bright points may appear in a cell relative to the cancelling magnetic feature and for how long the bright points might last. Predictions for the lifetimes of cancelling magnetic features are also made, indicating when the cancelling magnetic feature occurs relative to the bright point. Title: Modelling of dynamic coronal heating Authors: Parnell, C. E. Bibcode: 1994ESASP.373..149P Altcode: 1994soho....3..149P No abstract at ADS Title: A Model for X-Ray Bright Points due to Unequal Cancelling Flux Sources Authors: Parnell, C. E.; Priest, Eric R.; Titov, V. S. Bibcode: 1994SoPh..153..217P Altcode: A recent discovery from the Big Bear Solar Observatory has linked the cancellation of opposite polarity magnetic fragments in the photosphere (i.e., so-called cancelling magnetic features) to X-ray bright points and has stimulated the setting up of a converging flux model for the process. Cancelling magnetic features can occur between magnetic fragments of differing strengths in many different situations. Here, therefore, we model two opposite polarity fragments of different strengths in the photosphere by two unequal sources in an overlying uniform field. Initially in thepre-interaction phase these sources are assumed to be unconnected, but as they move closer together theinteraction phase starts with an X-type neutral point forming, initially on the photosphere, then rising up into the chromosphere and corona before lowering back down to the photosphere. Thecapture phase then follows with the sources fully connected as they move together. Finally, after they come in to contact, during thecancellation phase the weaker source is cancelled by part of the stronger source. The height of the X-type neutral point varies with the separation of the sources and the ratio of the source strengths, as do the positions of the neutral points before connection and after complete reconnection of the two sources. The neutral point is the location of magnetic reconnection and therefore energy release which is believed to power the X-ray bright point in the corona. By using a current sheet approximation, where it is assumed no reconnection takes place as the two sources move together, the total amount of energy released during reconnection may be estimated. The typical total free magnetic energy is found to be of the order of 1020-1021 J, which is as required for an X-ray bright point. It is also found that, as the ratio of the source strengths increases, the height of the X-type neutral point decreases, as do the total energy released, and the lifetime of the bright point. Title: A Converging Flux Model of an X-Ray Bright Point and an Associated Canceling Magnetic Feature Authors: Priest, E. R.; Parnell, C. E.; Martin, S. F. Bibcode: 1994ApJ...427..459P Altcode: X-ray bright points are an important part of the solar corona and therefore of the coronal heating problem. When it was first realized that bright points are always situated above opposite polarity magnetic fragments in the photosphere, it was natural to suggest that such fragments represent emerging flux and that an X-ray bright point is caused by reconnection of the emerging flux with an overlying coronal magnetic field. However, a recent important discovery at the Big Bear Solar Observatory is that the magnetic fragments of opposite polarity are usually not emerging but are instead coming together and disappearing and so are referred to as canceling magnetic features. Sometimes a tiny filament is observed to form and erupt at the same time. A unified model is here proposed which explains these observational features and has several phases: (1) a preinteraction phase, in which two photospheric fragments are unconnected magnetically and begin to approach one another, until eventually oppositely directed fields from the fragments come into contact at a second-order null point; (2) an interaction phase, in which the null point becomes an X-point and rises into the corona; an X-ray bright point is created for typically 8 hr by coronal reconnection, driven by the continued approach of the photospheric sources; long hot loops and Yohkoh X-ray jets may be created by the reconnection, and rapid variability in bright point emission may be produced by an impulsive burst regime of reconnection; the explosive events seen with High Resolution Telescope and Spectrograph (HRTS) may be produced as the X-point passes through the upper chromosphere; (3) a cancellation phase, in which a canceling magnetic feature is produced by photospheric reconnection as the fragments come into contact and decrease in strength; above the canceling fragments a small filament may form and erupt over typically an hour. An important role is played by the interaction distance (d), which is proportional to the magnetic flux of the fragments and inversely proportional to the overlying magnetic field strength. It determines the fragment separation at which the interaction phase begins and the resulting maximum height of the reconnection point. It is suggested that coronal reconnection driven by footpoint motion represents an elementary heating event that may be heating normal coronal loops and may be at the root of the nanoflare/microflare process. Bright points may well be at the large-scale end of a broad spectrum of events of the type modeled in this paper, which are heating the solar corona. At very small scales, such events in 'furnaces' in the coronal hole network may even produce high-frequency waves that propagate out and drive the solar wind (Axford 1993). Title: The Three-Dimensional Structures of X-Ray Bright Points Authors: Parnell, C. E.; Priest, E. R.; Golub, L. Bibcode: 1994SoPh..151...57P Altcode: Recently, the Converging Flux Model has been proposed for X-ray bright points and cancelling magnetic features. The aim of this peice of work is to try and model theoretically specific X-ray bright points in the framework of the Converging Flux Model. The observational data used includes a magnetogram showing the normal component of the magnetic field at the photosphere and a high-resolution soft X-ray image from NIXT showing the brightenings in the lower solar corona. By approximating the flux concentrations in the magnetograms with poles of the appropriate sign and sense, the overlying three-dimensional potential field structure is calculated. Deduction of plausible motions of the flux sources are made which produce brightenings of the observed shape due to reconnection between neighbouring flux regions. Also the three-dimensional separarix and separator structure and the way the magnetic field lines reconnect in three dimensions is deduced. Title: A Model of an X-Ray Bright Point Authors: Parnell, C. E.; Priest, E. R. Bibcode: 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. Bibcode: 1994ASIC..422...11P Altcode: 1994coma.conf...11P No abstract at ADS