arXiv subjects

David I. Pontin

Publications and source records attributed to David I. Pontin.

At least 19 recordsLinked to original sources

arXiv2026

Calculation of Field Line Slippage Rates in Coronal Simulations

We present the Universal Slippage calculator (\texttt{USlip}), an extension of the Universal Fieldline Tracer (\texttt{UFiT}), that computes magnetic field line slippage rates from an input magnetic field defined on a structured grid. The slippage rate provides a local, physically grounded diagnostic of non-ideal evolution, directly quantifying departures from ideal frozen-in motion. We outline the theoretical formulation and describe an efficient numerical implementation. The method is validated against analytical solutions and applied to simulations using three coronal codes treating varying spatial scales, all of which are standard inputs for \texttt{USlip}. In all the examples considered, the slippage rate not only identifies where magnetic connectivity changes occur, but also accurately reproduces the observed direction of field-line slippage. The consistency of the results across analytical tests and multiple simulation platforms demonstrates that \texttt{USlip} is a robust and versatile tool for diagnosing three-dimensional reconnection in dynamically evolving systems.

astro-ph.SR↗

arXiv2026

Distribution of energy release events due to magnetic braiding

Energy conversion by reconnection-powered nanoflare heating is one of the leading explanations for the heating of the solar chromosphere and corona. The aim of this paper is to shed light on this mechanism by exploring the magnetic Reynolds number dependence of the energy conversion process. To do this we employ boundary-driven, magnetohydrodynamic, flux-braiding simulations at different magnetic Reynolds numbers ($R_m$), and explore in detail the properties of the individual magnetic energy release events. The properties of the reconnecting current sheets that mediate the energy release are shown to depend on $R_m$. For increasing $R_m$, the current sheets become thinner, more intense, and more numerous. For sufficiently large $R_m$, the current sheets fragment along their length, leading to a sharp cutoff in the current sheet length distribution. The cutoff is consistent with the threshold for non-linear tearing/plasmoid instability. For increasing $R_m$ the magnetic field lines become increasingly tangled, the mean and peak values of the magnetic field strength increase, and the Poynting flux into the domain increases, implying that the heating rate also increases. The global reconnection rate is essentially independent of $R_m$. These results support the braiding mechanism as a viable way to effectively heat the internal portions of coherent flux tubes in the corona.

astro-ph.SR↗

arXiv2026

Solar Wind Dependence on Source Distance from the Open-Closed Boundary

The origin and variability of the slow solar wind remains an open question in solar physics, but is thought to be closely linked to dynamics at the Sun's open-closed magnetic flux boundary (OCB). Interchange magnetic reconnection at the OCB has been proposed as a mechanism for releasing closed-field plasma into the heliosphere, but observational evidence linking solar wind composition to OCB topology remains limited. We relate in situ solar wind measurements by Ulysses over a 10-year period to the magnetic topology of their source regions using two coronal magnetic field models: a potential field source surface model and a magnetofrictional model. We find a strong dependence of solar wind composition on the distance of the source magnetic flux from the OCB. Enhanced ion charge-state ratios, elemental abundances, and compositional variability are found to be concentrated within a supergranular-scale region (around 25 Mm) surrounding the OCB, consistent with the spatial scales of interchange magnetic reconnection. This variability decreases systematically with increasing distance from the boundary, with coronal hole wind exhibiting more uniform fast-wind signatures. We also find that the composition of solar wind emerging from regions close to the OCB is influenced by the strength of neighbouring closed magnetic fields, with stronger fields preferentially associated with slow-wind properties. These results indicate that the composition of the slow wind is strongly governed by the magnetic topology of the OCB, providing compelling evidence that interchange reconnection plays a crucial role in slow solar wind release and structure.

astro-ph.SR↗

arXiv2026

Dynamics of Streamers and Pseudostreamers and Implications for the Solar Wind

The origin of the Sun's slow wind and its inherent variability remain unknown, but there is increasing evidence that interactions between closed and open magnetic flux in the corona play a major role. This paper studies the dynamic evolution of streamers and pseudostreamers with a particular focus on the release of plasma from the closed to the open field region. We employ a global 2.5D magnetohydrodynamic model that extends from the solar chromosphere to 30 solar radii, and that extends previous interchange magnetic reconnection modelling by including self-consistent thermodynamics. We find that at both the helmet streamer and pseudostreamer there is a continual dynamic interaction between closed and open flux even in the absence of explicit driving. At the helmet streamer, the dynamics take the form of a ''breathing'' cycle in which the closed flux contracts and expands, and plasmoids are released along the heliospheric current sheet. The pseudostreamer exhibits a back-and-forth motion, driving interchange reconnection alternately on its opposite flanks. The resulting release of hot, dense plasma leads to density fluctuations in the open field that are significantly larger above the helmet streamer due to the persistence of the plasmoids there. Our model demonstrates that plasma is continually being released into the heliosphere from both streamers and pseudostreamers. We discuss the implications of our simulation results for observations of the corona and inner heliosphere.

astro-ph.SR↗

arXiv2025

Generation of Surface Sausage Oscillations of a Current Sheet and Propagating Magnetoacoustic Waves by Impulsive Reconnection

Magnetic reconnection and Magnetohydrodynamic (MHD) waves may well be both playing a role in coronal heating. In this paper, we simulate reconnection in the corona as a response to the convergence of opposite-polarity magnetic sources at the base of the corona. A current sheet forms at a magnetic null and undergoes impulsive bursty reconnection which drives natural modes of oscillation of the current sheet by a process of symbiosis. These are leaky surface sausage modes which cause the length of the current sheet to oscillate. Interaction of the oscillations and reconnection outflows with the magnetic Y-points at the ends of the sheet acts as sources for magnetoacoustic waves. Fast-mode waves propagate outwards into the coronal environment, while slow-mode waves propagate along the separatrices extending from the ends of the current sheet. The periodicities for sausage oscillations of the current sheet, for the current sheet length, and for the propagating large-scale magnetoacoustic waves are all estimated to be approximately 91 s for the parameters of our experiment.

astro-ph.SR↗

arXiv2025

Coriolis force acting on near-surface horizontal flows during simulations of flux emergence produces a tilt angle consistent with Joy's law on the Sun

Joy's law describes the tilt of bipolar active regions on the Sun away from an east-west orientation, where the flux of the polarity concentrated at the prograde side tends to be closer to the equator than the polarity on the retrograde side. Joy's law is attributed to the Coriolis force because of the observed increase in tilt angle at higher latitudes. This tilt plays a crucial role in some solar dynamo models. Our goal is to model the effects of the Coriolis force on a flux tube as it rises through the near-surface convection zone. We use a three-dimensional Cartesian magnetohydrodynamic simulation of an untwisted flux tube ascending from a depth of 11 Mm. We model the Coriolis effect using the f-plane approximation, that only considers and acts on horizontal flows. On the Sun, Joy's law is weak and is only evident as an average over many active regions. To achieve a measurable effect in a single simulation, we consider a rotation rate 110 times faster than the Sun. The simulation shows that the flux tube emerges at the surface with a tilt angle consistent with Joy's law when scaled to the Sun's slower rotation, and the tilt angle does not substantially change after emergence. This shows that the Coriolis force acting on flows horizontal to the surface within the near-surface convection zone is consistent with Joy's law.

astro-ph.SR↗

arXiv2025

The Sun's open-closed flux boundary and the origin of the slow solar wind

The Sun's open-closed flux boundary (OCB) separates closed and open magnetic field lines, and is the site for interchange magnetic reconnection processes thought to be linked to the origin of the slow solar wind (SSW). We analyze the global magnetic field structure and OCB from 2010 December to 2019 December using three coronal magnetic field models: a potential-field source-surface (PFSS) model, a static equilibrium magnetofrictional model, and a time-dependent magnetofrictional model. We analyze the model and cycle dependence of the OCB length on the photosphere, as well as the magnetic flux in the vicinity of the OCB. Near solar maximum, the coronal magnetic field for each model consists predominantly of long, narrow coronal holes, and nearly all the open flux lies within 1 supergranule diameter (25 Mm) of the OCB. By comparing to interplanetary scintillation measurements of SSW speeds, we argue that the fraction of open flux within this 25 Mm band is a good predictor of the amount of SSW in the heliosphere. Importantly, despite its simplicity, we show that the PFSS model estimates this fraction as well as the time-dependent model. We discuss the implications of our results for understanding SSW origins and interchange reconnection at the OCB.

astro-ph.SR↗

arXiv2025

Localized Heating and Dynamics of the Solar Corona due to a Symbiosis of Waves and Reconnection

The Sun's outer atmosphere, the corona, is maintained at mega-Kelvin temperatures and fills the heliosphere with a supersonic outflowing wind. The dissipation of magnetic waves and direct electric currents are likely to be the most significant processes for heating the corona, but a lively debate exists on their relative roles. Here, we suggest that the two are often intrinsically linked, since magnetic waves may trigger current dissipation, and impulsive reconnection can launch magnetic waves. We present a study of the first of these processes by using a 2D physics-based numerical simulation using the Adaptive Mesh Refined (AMR) Versatile Advection Code (VAC). Magnetic waves such as fast magnetoacoustic waves are often observed to propagate in the large-scale corona and interact with local magnetic structures. The present numerical simulations show how the propagation of magnetic disturbances towards a null point or separator can lead to the accumulation of the electric currents. Lorentz forces can laterally push and vertically stretch the magnetic fields, forming a current sheet with a strong magnetic-field gradient. The magnetic field lines then break and reconnect, and so contribute towards coronal heating. Numerical results are presented that support these ideas and support the concept of a symbiosis between waves and reconnection in heating the solar corona.

astro-ph.SR↗

arXiv2025

Exact Nonlinear Decomposition of Ideal-MHD Waves Using Eigenenergies III: Gravity, Generalized Inhomogeneous Quasi-linear PDEs, Mode Conversion, and Numerical Implementation

Precise tracking and measurement of the energy carried by the individual magnetohydrodynamic (MHD) modes has important implications and utility in astrophysical and laboratory plasmas. Previously, this was only achievable in limited linear MHD cases in the $β\ll 1$ or $β\gg 1$ regimes. In a series of papers, of which this is the third, we introduced the Eigenenergy Decomposition Method (EEDM) and derived exact analytical expressions for the modal energy components--called eigenenergies--of nonlinear 3D disturbances governed by the homogeneous ideal-MHD equations. Here, we extend the method to inhomogeneous ideal-MHD by introducing a source term accounting for gravity, and provide detailed guidelines for applying the decomposition scheme to any general inhomogeneous quasi-linear PDEs that possess a globally conserved quantity, beyond the realm of MHD. Furthermore, we show that the eigenenergies can be used to locate and measure nonlinear mode conversions, which is an additional feature of the method. Finally, we provide well-categorized context for the application of the method to simulations and discuss the possible numerical inaccuracies that may inevitably arise due to discretization. This paper provides a more mature description of the method and its interpretation, and is recommended as the starting point for readers unfamiliar with the method.

astro-ph.SR↗

arXiv2024

Generation of fast magnetoacoustic waves in the corona by impulsive bursty reconnection

Fast-mode magnetohydrodynamic (MHD) waves in the solar corona are often known to be produced by solar flares and eruptive prominences. We here simulate the effect of the interaction of an external perturbation on a magnetic null in the solar corona which results in the formation of a current sheet (CS). Once the CS undergoes a sufficient extension in its length and squeezing of its width, it may go unstable to the formation of multiple impulsive plasmoids. Eventually, the plasmoids merge with one another to form larger plasmoids and/or are expelled from the sheet. The formation, motion and coalescence of plasmoids with each other and with magnetic Y-points at the outer periphery of the extended CS are found to generate wave-like perturbations. An analysis of the resultant quasi-periodic variations of pressure, density, velocity and magnetic field at certain locations in the model corona indicate that these waves are predominantly fast-mode magnetoacoustic waves. For typical coronal parameters, the resultant propagating waves carry an energy flux of $\mathrm{10^{5}~\mathrm{erg~cm^{-2}~s^{-1}}}$ to a large distance of at least 60 Mm away from the current sheet. In general, we suggest that both waves and reconnection play a role in heating the solar atmosphere and driving the solar wind and may interact with one another in a manner that we refer to as a $"$Symbiosis of WAves and Reconnection (SWAR)$"$.

astro-ph.SR↗

arXiv2024

Heating in the Solar Atmosphere at a Fin Current Sheet Driven by Magnetic Flux Cancellation

Magnetic reconnection before flux cancellation in the solar photosphere when two opposite-polarity photospheric magnetic fragments are approaching one another is usually modelled by assuming that a small so-called "floating current sheet" forms about a null point or separator that is situated in the overlying atmosphere. Here instead we consider the reconnection that is initiated as soon as the fragments become close enough that their magnetic fields interact. The resulting current sheet, which we term a "fin sheet" extends up from the null point or separator that is initially located in the solar surface. We develop here nonlinear analyses for finite-length models of both fin and floating current sheets that extend the previous models that were limited to short floating current sheets. These enable the length of the current sheet to be calculated in both cases as functions of the separation distance of the sources and the reconnection rate, as well as the rate of heating. Usually, the fin current sheet liberates more energy than a floating current sheet.

astro-ph.SR↗

arXiv2024

Exploring the generation and annihilation of three dimensional nulls through MHD simulations in initially chaotic magnetic field devoid of nulls

Three-dimensional (3D) magnetic nulls are abundant in the solar atmosphere, as been firmly established through contemporary observations. They are established to be important magnetic structures in, for example, jets and circular ribbon flares. While simulations and extrapolations support this, the mechanisms behind 3D null generation remain an open question. Recent magnetohydrodynamics (MHD) simulations propose that magnetic reconnection is responsible for both generating and annihilating 3D nulls, a novel concept. However, these simulations began with initial magnetic fields already supporting pre-existing nulls, raising the question of whether magnetic reconnection can create nulls in fields initially devoid of them. Previously, this question was briefly explored in a simulation with an initial chaotic magnetic field. However, the study failed to precisely identify locations, topological degrees, and natures (spiral or radial) of nulls, and it approximated magnetic reconnection without fully tracking field line in time. In this paper these findings are revisited in light of recent advancements and tools used to locate and trace nulls, along with the tracing of field lines, through which the concept of generation/annihilation of 3D nulls from chaotic fields is established in a precise manner.

astro-ph.SR↗

arXiv2024

Generation and annihilation of three dimensional magnetic nulls in extrapolated solar coronal magnetic field: Data-based Implicit Large Eddy Simulation

Three-dimensional magnetic nulls are the points where magnetic field vanishes and are preferential sites for magnetic reconnection: a process which converts magnetic energy into heat and accelerates charged particles along with a rearrangement of magnetic field lines. In the solar corona, the reconnections manifest as coronal transients including solar flares, coronal mass ejections and coronal jets. The nulls are generally found to be collocated with complex active regions on the solar photosphere. Extrapolation of magnetic field from corresponding photospheric magnetogram indicate an abundance of these nulls in solar atmosphere. Nevertheless, their generation is still not well understood. Recently, Maurya et al. (2023) have demonstrated magnetic reconnection to be a cause for generation and annihilation of magnetic nulls through magnetohydrodynamics simulation, where the initial magnetic field is idealized to have a single radial null. This article further extends the study in a more realistic scenario where the initial magnetic field is constructed by extrapolating photospheric magnetogram data and hence, incorporates field line complexities inherent to a complex active region. For the purpose, the active region NOAA 11977 hosting a C6.6 class flare is selected. The simulation is initiated using non-force-free extrapolated magnetic field from the photospheric vector magnetogram at around 02:48:00 UT on 17 February 2014, 16 minutes before the flare peak. The generation, annihilation and dynamics of nulls are explored by a complimentary usage of trilinear null detection technique and tracing of magnetic field line dynamics. It is found that the nulls can spontaneously generate/annihilate in pairs while preserving the topological degree and can have observational implications like footpoint brightenings. Magnetic reconnection is found to be the cause of such generation and annihilation.

astro-ph.SR↗

arXiv2024

2.5-D MHD Simulation of the Formation and Evolution of Plasmoids in Coronal Current Sheets

In the present paper, using MPI-AMRVAC, we perform a 2.5-D numerical MHD simulation of the dynamics and associated thermodynamical evolution of an initially force-free Harris current sheet subjected to an external velocity perturbation under the condition of uniform resistivity. The amplitude of the magnetic field is taken to be 10 Gauss, typical of the solar corona. We impose a Gaussian velocity pulse across this current sheet mimicking the interaction of fast magnetoacoustic waves with a current sheet in corona. This leads to a variety of dynamics and plasma processes in the current sheet, which is initially quasi-static. The initial pulse interacts with the current sheet and splits into a pair of counter-propagating wavefronts, which forms a rarefied region and leads to inflow and a thinning of the current sheet. The thinning results in Petschek-type magnetic reconnection followed by tearing instability and plasmoid formation. The reconnection outflows containing outward-moving plasmoids have accelerated motions with velocities ranging from 105-303 km/s. The average temperature and density of the plasmoids are found to be 8 MK and twice the background density of the solar corona, respectively. These estimates of velocity, temperature and density of plasmoids are similar to values reported from various solar coronal observations. Therefore, we infer that the external triggering of a quasi-static current sheet by a single velocity pulse is capable of initiating magnetic reconnection and plasmoid formation in the absence of a localized enhancement of resistivity in the solar corona.

astro-ph.SR↗

arXiv2023

Reconnection generated plasma flows in the quasi-separatrix layer in localised solar corona

Multiwavelength observations of the propagating disturbances (PDs), discovered by Atmospheric Imaging Assembly (AIA) onboard Solar Dynamics Observatory (SDO), are analyzed to determine its driving mechanism and physical nature. Two magnetic strands in the localised corona are observed to approach and merge with each other followed by the generation of brightening, which further propagates in a cusp-shaped magnetic channel. Differential emission measure analysis shows an occurrence of heating in this region-of-interest (ROI). We extrapolate potential magnetic field lines at coronal heights from observed Helioseismic and Magnetic Imager (HMI) vector magnetogram via Green's function method using MPI-AMRVAC. We analyze the field to locate magnetic nulls and quasi-separatrix layers (QSLs) which are preferential locations for magnetic reconnection. Dominant QSLs including a magnetic null are found to exist and match the geometry followed by PDs, therefore, it provides conclusive evidence of magnetic reconnection. In addition, spectroscopic analysis of Interface Region Imaging Spectrograph (IRIS) Si IV 1393.77 Å line profiles show a rise of line-width in the same time range depicting presence of mass motion in the observed cusp-shaped region. PDs are observed to exhibit periodicities of around four minutes. The speeds of PDs measured by Surfing Transform Technique are almost close to each other in four different SDO/AIA bandpasses, i.e., 304, 171, 193 and 131 Å excluding the interpretation of PDs in terms of slow magnetoacoustic waves. We describe comprehensively the observed PDs as quasi-periodic plasma flows generated due to periodic reconnection in vicinity of a coronal magnetic null.

astro-ph.SR↗

arXiv2022

Parallel plasma loops and the energization of the solar corona

The outer atmosphere of the Sun is composed of plasma heated to temperatures well in excess of the visible surface. We investigate short cool and warm (<1 MK) loops seen in the core of an active region to address the role of field-line braiding in energising these structures. We report observations from the High-resolution Coronal imager (Hi-C) that have been acquired in a coordinated campaign with the Interface Region Imaging Spectrograph (IRIS). In the core of the active region, the 172 A band of Hi-C and the 1400 A channel of IRIS show plasma loops at different temperatures that run in parallel. There is a small but detectable spatial offset of less than 1 arcsec between the loops seen in the two bands. Most importantly, we do not see observational signatures that these loops might be twisted around each other. Considering the scenario of magnetic braiding, our observations of parallel loops imply that the stresses put into the magnetic field have to relax while the braiding is applied: the magnetic field never reaches a highly braided state on these length-scales comparable to the separation of the loops. This supports recent numerical 3D models of loop braiding in which the effective dissipation is sufficiently large that it keeps the magnetic field from getting highly twisted within a loop.

astro-ph.SR↗

arXiv2019

Reconnection of Vortex Tubes with Axial Flow

This paper addresses the interaction of initially anti-parallel vortex tubes containing an axial flow that induces a twisting of the vortex lines around the tube axes, using numerical simulations. Vortex tube configurations with both the same and opposite senses of twist -- corresponding to same and opposite signs of kinetic helicity density -- are considered. It is found that the topology of the reconnection process is very different between the two cases. For tubes with the same sense of twist, the reconnection is fully three-dimensional (3D): vortex lines reconnect at a finite angle, and 3D vortex null points may be created. Following reconnection the vortex line topology in both bridge and thread structures exhibits a high degree of complexity. For oppositely-twisted tubes the reconnection is locally two-dimensional, occurring along vorticity null lines, that in contrast to the untwisted case are not perpendicular to the tube axes. This leads to a break in the symmetry between the two vortex bridges generated during reconnection. For all cases studied, increasing the twist in the vortex tubes leads to a later, faster, and more complete reconnection process.

physics.flu-dyn↗

arXiv2018

Magnetic Structures at the Boundary of the Closed Corona: Interpretation of S-web Arcs

The topology of magnetic fields near the open-closed flux boundary in the Sun's corona is an important influencing factor in the process of interchange reconnection, whereby plasma is exchanged between open and closed flux domains. Maps of the magnetic squashing factor at the radial outer boundary in coronal field models reveal the presence of the so-called `S-web', and suggest that interchange reconnection could potentially deposit closed coronal material into high-latitude regions far from the heliospheric current sheet. Here we demonstrate that certain features of the S-web reveal the underlying topological structure of the magnetic field. Specifically, in order for the arcing bands of highly squashed magnetic flux of the S-web to terminate or intersect away from the helmet streamer apex, there must be a null spine line that maps a finite segment of the photospheric open-closed boundary up to a singular point in the open flux domain. We propose that this association between null spine lines and arc termination points may be used to identify locations in the heliosphere that are preferential for the appearance of solar energetic particles or slow solar wind plasma with certain characteristics.

astro-ph.SR↗