arXiv subjects

M. Swisdak

Publications and source records attributed to M. Swisdak.

At least 19 recordsLinked to original sources

arXiv2026

Signatures of Reconnection and a Split Heliospheric Tail in High-Energy Energetic Neutral Atoms

The shape of the heliosphere, regarded as comet-like since the 1960s, has recently been the subject of intense debate in the last decade. There is disagreement whether the heliospheric tail extends to $\sim$10,000 au in a comet-like shape or if it is short ($\sim$400 au) with a split. Energetic neutral atom (ENA) maps from Cassini/INCA at energies from 5.2 to 13.5 keV revealed a global structure extending from the nose to the heliospheric tail known as the Belt whose origin has remained largely unexplored. Here, we use a state-of-the-art multi-ion magnetohydrodynamic (MHD) model and a novel reconnection simulation to establish that the Belt structure is consistent with a split tail heliosphere but not with a comet-like heliosphere. In a split-tail heliosphere there is a region of low-$\beta$ (ratio of thermal to magnetic pressure) in the downwind direction close to the heliopause. Direct simulations of this region reveal that magnetic reconnection is strong and drives the energetic particles that produce the >5.2keV ENAs measured by INCA in the low latitude portion of the Belt. Since the comet-like heliosphere does not produce this low-$\beta$ region and the resultant reconnection-drive mechanism for the >5.2keV ENAs, our results indicate that the INCA observations are inconsistent with a comet-like heliosphere. Further, these simulations and analysis establish for the first time that magnetic reconnection in the complex magnetic fields, expected in astrospheres across the universe, are likely to be a source of energetic particles and radiation.

astro-ph.SR↗

arXiv2026

The self-generation of core fields and electron scattering in flux ropes during magnetic reconnection

Two-dimensional particle-in-cell simulations with a realistic mass ratio reveal the generation mechanisms of the out-of-plane magnetic field in magnetic islands/flux ropes during magnetic reconnection. In the absence of an initial guide field, reconnection produces a large electron temperature anisotropy (around 4.5) inside magnetic islands that drives the Weibel instability. Strong out-of-plane magnetic fields (Bz/B0 around 0.4, greatly exceeding the Hall field) with a regular bipolar structure grow inside islands. A space-time analysis reveals a one-to-one correspondence between the temperature anisotropy and the development of the Weibel magnetic field. The instability relaxes the anisotropy, but island merging leads to anisotropy reemergence and re-excitation. In the presence of a strong ambient guide field (Bg/B0 = 0.5), the electron outflow from the X-point deflects along the separatrices and forms a circular current loop wrapping the flux ropes. This flux-rope separatrix current generates an out-ofplane magnetic field that reinforces the ambient guide field, reaching Bz/B0 around 1.4. The current can, in some cases, drive the electron Kelvin-Helmholtz instability, which produces electron vortices and strengthens the magnetic field. Mergers significantly broaden the islands and further strengthen the field. These self-generated out-of-plane magnetic fields scatter electrons and reduce their temperature anisotropy, which can potentially affect electron heating via Fermi reflection. The simulation results are supported by spacecraft observations suggesting that ambient guide fields can be enhanced within flux ropes in Earth's magnetotail.

physics.plasm-ph↗

arXiv2025

The impact of Alfvenic shear flow on magnetic reconnection and turbulence

Magnetic reconnection is a fundamental and omnipresent energy conversion process in plasma physics. Novel observations of fields and particles from Parker Solar Probe (PSP) have shown the absence of reconnection in a large number of current sheets in the near-Sun solar wind. Using near-Sun observations from PSP Encounters 4 to 11 (Jan 2020 to March 2022), we investigate whether reconnection onset might be suppressed by velocity shear. We compare estimates of the tearing mode growth rate in the presence of shear flow for time periods identified as containing reconnecting current sheets versus non-reconnecting times, finding systematically larger growth rates for reconnection periods. Upon examination of the parameters associated with reconnection onset, we find that 85% of the reconnection events are embedded in slow, non-Alfvenic wind streams. We compare with fast, slow non-Alfvenic, and slow Alfvenic streams, finding that the growth rate is suppressed in highly Alfvenic fast and slow wind and reconnection is not seen in these wind types, as would be expected from our theoretical expressions. These wind streams have strong Alfvenic flow shear, consistent with the idea of reconnection suppression by such flows. This could help explain the frequent absence of reconnection events in the highly Alfvenic, near-Sun solar wind observed by PSP. Finally, we find a steepening of both the trace and magnitude magnetic field spectra within reconnection periods in comparison to ambient wind. We tie this to the dynamics of relatively balanced turbulence within these reconnection periods and the potential generation of compressible fluctuations.

physics.space-ph↗

arXiv2025

Suppression of the collisionless tearing mode by flow shear: implications for reconnection onset in the Alfvénic solar wind

We analyse the collisionless tearing mode instability of a current sheet with a strong shear flow across the layer. The growth rate decreases with increasing shear flow, and is completely stabilized as the shear flow becomes Alfvénic. We also show that in the presence of strong flow shear, the tearing mode growth rate decreases with increasing background ion-to-electron temperature ratio, the opposite behaviour to the tearing mode without flow shear. We find that even a relatively small flow shear is enough to dramatically alter the scaling behaviour of the mode, because the growth rate is small compared to the shear flow across the ion scales (but large compared to shear flow across the electron scales). Our results may explain the relative absence of reconnection events in the near-Sun Alfvénic solar wind observed recently by NASA's Parker Solar Probe.

physics.space-ph↗

arXiv2025

Ion Beam Instabilities during Solar Flare Energy Release

The linear stability of waves driven by ion beams produced during solar flare energy release are explored to assess their role in driving abundance enhancements in minority species such as $^3$He and in controlling, through pitch-angle scattering, proton/alpha confinement during energy release. The Arbitrary Linear Plasma Solver (ALPS) is used to solve the linear dispersion relation for a population of energetic, reconnection-accelerated protons streaming into a cold background plasma. We assume equal densities of the two populations, using an anisotropic ($T_\parallel/T_\perp = 10$), one-sided kappa distribution for the energetic streaming population and a cold Maxwellian for the background. We find two unstable modes with parallel propagation: a right-handed wave with a frequency of the order of the proton cyclotron frequency ($Ω_{cp}$) and a left-handed, lower frequency mode. We also find highly oblique modes with frequencies below $Ω_{cp}$ that are unstable for higher beam energies. Through resonant interactions, all three modes will contribute to the scattering of the high-energy protons, thereby limiting their transport out of the flare-acceleration region. The higher-frequency oblique mode, which can be characterized as a kinetic Alfvén wave, will preferentially heat $^3$He, making it a good candidate for the driver of the abundance enhancements commonly observed for this species in impulsive events.

astro-ph.SR↗

arXiv2025

Magnetic reconnection-driven energization of protons up to 400 keV at the near-Sun heliospheric current sheet

We report observations of direct evidence of energetic protons being accelerated above ~400 keV within the reconnection exhaust of a heliospheric current sheet (HCS) crossing by NASA's Parker Solar Probe (PSP) at a distance of ~16.25 solar radii (Rs) from the Sun. Inside the extended exhaust, both the reconnection-generated plasma jets and the accelerated protons propagated toward the Sun, unambiguously establishing their origin from HCS reconnection sites located beyond PSP. Within the core of the exhaust, PSP detected stably trapped energetic protons up to ~400 keV, which is approximately 1000 times greater than the available magnetic energy per particle. The differential energy spectrum of the accelerated protons behaved as a pure power-law with spectral index of about -5. Supporting simulations using the kglobal model suggest that the trapping and acceleration of protons up to ~400 keV in the reconnection exhaust is likely facilitated by merging magnetic islands with a guide field between ~0.2-0.3 of the reconnecting magnetic field, consistent with the observations. These new results, enabled by PSP's proximity to the Sun, demonstrate that magnetic reconnection in the HCS is a significant new source of energetic particles in the near-Sun solar wind. The discovery of in-situ particle acceleration via magnetic reconnection at the HCS provides valuable insights into this fundamental process which frequently converts the large magnetic field energy density in the near-Sun plasma environment and may be responsible for heating the sun's atmosphere, accelerating the solar wind, and energizing charged particles to extremely high energies in solar flares.

astro-ph.SR↗

arXiv2024

The Interplay Between Collisionless Magnetic Reconnection and Turbulence

Alongside magnetic reconnection, turbulence is another fundamental nonlinear plasma phenomenon that plays a key role in energy transport and conversion in space and astrophysical plasmas. From a numerical, theoretical, and observational point of view there is a long history of exploring the interplay between these two phenomena in space plasma environments; however, recent high-resolution, multi-spacecraft observations have ushered in a new era of understanding this complex topic. The interplay between reconnection and turbulence is both complex and multifaceted, and can be viewed through a number of different interrelated lenses - including turbulence acting to generate current sheets that undergo magnetic reconnection (turbulence-driven reconnection), magnetic reconnection driving turbulent dynamics in an environment (reconnection-driven turbulence) or acting as an intermediate step in the excitation of turbulence, and the random diffusive/dispersive nature of magnetic field lines embedded in turbulent fluctuations enabling so-called stochastic reconnection. In this paper, we review the current state of knowledge on these different facets of the interplay between turbulence and reconnection in the context of collisionless plasmas, such as those found in many near-Earth astrophysical environments, from a theoretical, numerical, and observational perspective. Particular focus is given to several key regions in Earth's magnetosphere - Earth's magnetosheath, magnetotail, and Kelvin-Helmholtz vortices on the magnetopause flanks - where NASA's Magnetospheric Multiscale mission has been providing new insights on the topic.

physics.space-ph↗

arXiv2024

Magnetic Reconnection on a Klein Bottle

We present a new boundary condition for simulations of magnetic reconnection based on the topology of a Klein bottle. When applicable, the new condition is computationally cheaper than fully periodic boundary conditions, reconnects more flux than systems with conducting boundaries, and does not require assumptions about regions external to the simulation as is necessary for open boundaries. The new condition reproduces the expected features of reconnection, but cannot be straightforwardly applied in systems with asymmetric upstream plasmas.

physics.plasm-ph↗

arXiv2024

Outstanding questions and future research of magnetic reconnection

This short article highlights the unsolved problems of magnetic reconnection in collisionless plasma. The advanced in-situ plasma measurements and simulations enabled scientists to gain a novel understanding of magnetic reconnection. Still, outstanding questions remain on the complex dynamics and structures in the diffusion region, on the cross-scale and regional couplings, on the onset of magnetic reconnection, and on the details of energetics. Future directions of the magnetic reconnection research in terms of new observations, new simulations and interdisciplinary approaches are discussed.

physics.plasm-ph↗

arXiv2024

Advanced methods for analyzing in-situ observations of magnetic reconnection

There is ample evidence for magnetic reconnection in the solar system, but it is a nontrivial task to visualize, to determine the proper approaches and frames to study, and in turn to elucidate the physical processes at work in reconnection regions from in-situ measurements of plasma particles and electromagnetic fields. Here an overview is given of a variety of single- and multi-spacecraft data analysis techniques that are key to revealing the context of in-situ observations of magnetic reconnection in space and for detecting and analyzing the diffusion regions where ions and/or electrons are demagnetized. We focus on recent advances in the era of the Magnetospheric Multiscale mission, which has made electron-scale, multi-point measurements of magnetic reconnection in and around Earth's magnetosphere.

physics.space-ph↗

arXiv2024

Wave Generation by Flare-Accelerated Ions and Implications for 3He Acceleration

The waves generated by high-energy proton and alpha particles streaming from solar flares into regions of colder plasma are explored using particle-in-cell simulations. Initial distribution functions for the protons and alphas consist of two populations: an energetic, streaming population represented by an anisotropic ($T_{\parallel} > T_{\perp}$), one-sided kappa function and a cold, Maxwellian background population. The anisotropies and non-zero heat fluxes of these distributions destabilize oblique waves with a range of frequencies below the proton cyclotron frequency. These waves scatter particles out of the tails of the initial distributions along constant energy surfaces in the wave frame. Overlap of the nonlinear resonance widths allows particles to scatter into near isotropic distributions by the end of the simulations. The dynamics of $^3$He are explored using test particles. Their temperatures can increase by a factor of nearly 20. Propagation of such waves into regions above and below the flare site can lead to heating and transport of $^3$He into the flare acceleration region. The amount of heated $^3$He that will be driven into the flare site is proportional to the wave energy. Using values from our simulations, we show that the abundance of $^3$He driven into the acceleration region should approach that of $^4$He in the corona. Therefore, waves driven by energetic ions produced in flares are a strong candidate to drive the enhancements of $^3$He observed in impulsive flares.

astro-ph.SR↗

arXiv2023

Constraints on the Alfvénicity of Switchbacks

Switchbacks (SBs) are localized structures in the solar wind containing deflections of the magnetic field direction relative to the background solar wind magnetic field. The amplitudes of the magnetic field deflection angles for different SBs vary from ~40 to ~160-170 degrees. Alignment of the perturbations of the magnetic field and the bulk solar wind velocity is observed inside SBs and causes spiky enhancements of the radial bulk velocity inside SBs. We have investigated the deviations of SB perturbations from Alfvénicity by evaluating the distribution of the parameter defined as the ratio of the parallel to $Δ\vec{B}$ component of $Δ\vec{V}$ to $Δ\vec{V}_A = Δ\vec{B}/4πn_i m_i$ inside SBs, i.e. $α=V_{||}/|Δ\vec{V}_A|$ ( $α=Δ\vec{V}/|Δ\vec{V}_A|$ when $Δ\vec{V}~Δ\vec{B}$), which quantifies the deviation of the perturbation from an Alfvénic one. Based on Parker Solar Probe (PSP) observations, we show that $α$ inside SBs has systematically lower values than it has in the pristine solar wind: $α$ inside SBs observed during PSP Encounter 1 were distributed in a range of 0.2-0.9. The upper limit on $α$ is constrained by the requirement that the jump in velocity across the switchback boundary be less than the local Alfvén speed. This prevents the onset of shear flow instabilities. The consequence is that the perturbation of the proton bulk velocity in SBs with deflection greater than 60 degrees cannot reach $α=1$ (the Alfvénicity condition) and the highest possible $α$ for a SB with the full reversal of B is 0.5. These results have consequences for the interpretation of switchbacks as large amplitude Alfvén waves.

astro-ph.SR↗

arXiv2023

Development of a structured, turbulent solar wind as a result of interchange reconnection

The role of interchange reconnection as a drive mechanism for the solar wind is explored by solving the global magnetic-field-aligned equations describing wind acceleration. Boundary conditions in the low corona, including a reconnection-driven Alfvénic outflow and associated heating differ from previous models. Additional heating of the corona associated with Alfvén waves or other MHD turbulence, which has been the foundation of many earlier models, is neglected. For this simplified model a sufficient condition for interchange reconnection to overcome gravity to drive the wind is derived. The combination of Alfvénic ejection and reconnection-driven heating yields a minimum value of the Alfvén speed of the order of 350-400$km/s$ that is required to drive the wind. Recent evidence based on Parker Solar Probe (PSP) observations suggests that this threshold is typically exceeded in the coronal holes that are the source regions of the fast wind. On the other hand, since reconnection in the coronal environment is predicted to have a bursty character, the magnitude of reconnection outflows can be highly variable. The consequence is a highly non-uniform wind in which in some regions the velocity increases sharply to super-Alfvénic values while in adjacent regions the formation of an asymptotic wind fails. A simple model is constructed to describe the turbulent mixing of these highly-sheared super-Alfvénic flows that suggests these flows are the free-energy source of the Alfvénic turbulence and associated switchbacks that have been documented in the PSP data in the near coronal environment. The global wind profiles are presented and benchmarked with Parker Solar Probe (PSP) observations at 12 solar radii.

astro-ph.SR↗

arXiv2023

The Role of Magnetic Shear in Reconnection-Driven Flare Energy Release

Using observations from the Solar Dynamics Observatory's Atmosphere Imaging Assembly and the Ramaty High Energy Solar Spectroscopic Imager, we present novel measurements of the shear of post-reconnection flare loops (PRFLs) in SOL20141218T21:40 and study its evolution with respect to magnetic reconnection and flare emission. Two quasi-parallel ribbons form adjacent to the magnetic polarity inversion line (PIL), spreading in time first parallel to the PIL and then mostly in a perpendicular direction. We measure magnetic reconnection rate from the ribbon evolution, and also the shear angle of a large number of PRFLs observed in extreme ultraviolet passbands ($\lesssim$1 MK). For the first time, the shear angle measurements are conducted using several complementary techniques allowing for a cross-validation of the results. In this flare, the total reconnection rate is much enhanced before a sharp increase of the hard X-ray emission, and the median shear decreases from 60$^\circ$-70$^\circ$ to 20$^\circ$, on a time scale of ten minutes. We find a correlation between the shear-modulated total reconnection rate and the non-thermal electron flux. These results confirm the strong-to-weak shear evolution suggested in previous observational studies and reproduced in numerical models, and also confirm that, in this flare, reconnection is not an efficient producer of energetic non-thermal electrons during the first ten minutes when the strongly sheared PRFLs are formed. We conclude that an intermediate shear angle, $\le 40^\circ$, is needed for efficient particle acceleration via reconnection, and we propose a theoretical interpretation.

astro-ph.SR↗

arXiv2023

Interchange reconnection as the source of the fast solar wind within coronal holes

The fast solar wind that fills the heliosphere originates from deep within regions of open magnetic field on the Sun called coronal holes. The energy source responsible for accelerating the plasma to high speeds is widely debated, however there is evidence that it is ultimately magnetic in nature with candidate mechanisms including wave heating^(1,2) and interchange reconnection^(3,4,5). The coronal magnetic field near the solar surface is structured on scales associated with supergranulation convection cells, where descending flows create intense fields. The energy density in these network magnetic field bundles is a likely candidate as an energy source of the wind. Here we report measurements of fast solar wind streams from the Parker Solar Probe (PSP) spacecraft^6 which provides strong evidence for the interchange reconnection mechanism. We show that supergranulation structure at the coronal hole base remains imprinted in the near-Sun solar wind resulting in asymmetric patches of magnetic 'switchbacks'^(7,8) and bursty wind streams with power law-like energetic ion spectra to beyond 100 keV. Computer simulations of interchange reconnection support key features of the observations, including the ion spectra. Important characteristics of interchange reconnection in the low corona are inferred from the data including that the reconnection is collisionless and that the energy release rate is sufficient to power the fast wind. In this scenario, open magnetic flux undergoes continuous reconnection and the wind is driven both by the resulting plasma pressure and the radial Alfvenic flow bursts.

astro-ph.SR↗

arXiv2021

Flux rope merging and the structure of switchbacks in the solar wind

A major discovery of Parker Solar Probe (PSP) was the presence of large numbers of localized increases in the radial solar wind speed and associated sharp deflections of the magnetic field - switchbacks (SB). A possible generation mechanism of SBs is through magnetic reconnection between open and closed magnetic flux near the solar surface, termed interchange reconnection that leads to the ejection of flux ropes (FR) into the solar wind. Observations also suggest that SBs undergo merging, consistent with a FR picture of these structures. The role of FRs merging in controlling the structure of SB in the solar wind is explored through direct observations, through analytic analysis, and numerical simulations. Analytic analysis reveals key features of the structure of FR and their scaling with the heliocentric distance R that are consistent with observations and that reveal the critical role of merging in controlling the SB structure. FR merging is shown to be energetically favorable to reduce the strength of the wrapping magnetic field and drive the observed elongation of SBs. A further consequence is the resulting dominance of the axial magnetic field within SBs that leads to the characteristic sharp rotation of the magnetic field into the axial direction at the SB boundary that is revealed in observations. Finally, the radial scaling of the SB area in the FR model of SBs suggests that the observational probability of SB identification should be insensitive to R, which is consistent with the most recent statistical analysis of SB observations from PSP.

astro-ph.SR↗

arXiv2021

Whistler-regulated MHD: Transport equations for electron thermal conduction in the high $β$ intracluster medium of galaxy clusters

Transport equations for electron thermal energy in the high $β_e$ intracluster medium (ICM) are developed that include scattering from both classical collisions and self-generated whistler waves. The calculation employs an expansion of the kinetic electron equation along the ambient magnetic field in the limit of strong scattering and assumes whistler waves with low phase speeds $V_w\sim{v}_{te}/β_e\ll{v}_{te}$ dominate the turbulent spectrum, with $v_{te}$ the electron thermal speed and $β_e\gg1$ the ratio of electron thermal to magnetic pressure. We find: (1) temperature-gradient-driven whistlers dominate classical scattering when $L_c>L/β_e$, with $L_c$ the classical electron mean-free-path and $L$ the electron temperature scale length, and (2) in the whistler dominated regime the electron thermal flux is controlled by both advection at $V_w$ and a comparable diffusive term. The findings suggest whistlers limit electron heat flux over large regions of the ICM, including locations unstable to isobaric condensation. Consequences include: (1) the Field length decreases, extending the domain of thermal instability to smaller length-scales, (2) the heat flux temperature dependence changes from $T_e^{7/2}/L$ to $V_wnT_e\sim{T}_e^{1/2}$, (3) the magneto-thermal and heat-flux driven buoyancy instabilities are impaired or completely inhibited, and (4) sound waves in the ICM propagate greater distances, as inferred from observations. This description of thermal transport can be used in macroscale ICM models.

astro-ph.GA↗

arXiv2021

The Reversibility Of Magnetic Reconnection

The reversibility of the transfer of energy from the magnetic field to the surrounding plasma during magnetic reconnection is examined. Trajectories of test particles in an analytic model of the fields demonstrate that irreversibility is associated with separatrix crossings and regions of weaker magnetic field. Inclusion of a guide field increases the degree of reversibility. Full kinetic simulations with a particle-in-cell code support these results and demonstrate that while time-reversed simulations at first "un-reconnect", they eventually evolve into a reconnecting state.

physics.plasm-ph↗