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Magnetic reconnection underlies many explosive phenomena in the heliosphere and in laboratory plasmas. The new research capabilities in theory/simulations, observations, and laboratory experiments provide the opportunity to solve the grand scientific challenges summarized in this whitepaper. Success will require enhanced and sustained investments from relevant funding agencies, increased interagency/international partnerships, and close collaborations of the solar, heliospheric, and laboratory plasma communities. These investments will deliver transformative progress in understanding magnetic reconnection and related explosive phenomena including space weather events., Comment: 4 pages (including a title page), white paper submitted to Helio2050 workshop at https://www.hou.usra.edu/meetings/helio2050/. arXiv admin note: substantial text overlap with arXiv:2004.00079

This white paper summarizes major scientific challenges and opportunities in understanding magnetic reconnection and related explosive phenomena as a fundamental plasma process., Comment: 6 pages, 1 figure, white paper submitted to both Plasma 2020 and Astro 2020 Decadal Surveys

We study the linear stability of the flow of a viscous electrically conducting capillary fluid on a planar fixed plate in the presence of gravity and a uniform magnetic field. We first confirm that the Squire transformation for MHD is compatible with the stress and insulating boundary conditions at the free surface, but argue that unless the flow is driven at fixed Galilei and capillary numbers, the critical mode is not necessarily two-dimensional. We then investigate numerically how a flow-normal magnetic field, and the associated Hartmann steady state, affect the soft and hard instability modes of free surface flow, working in the low magnetic Prandtl number regime of laboratory fluids. Because it is a critical layer instability, the hard mode is found to exhibit similar behaviour to the even unstable mode in channel Hartmann flow, in terms of both the weak influence of Pm on its neutral stability curve, and the dependence of its critical Reynolds number Re_c on the Hartmann number Ha. In contrast, the structure of the soft mode's growth rate contours in the (Re, alpha) plane, where alpha is the wavenumber, differs markedly between problems with small, but nonzero, Pm, and their counterparts in the inductionless limit. As derived from large wavelength approximations, and confirmed numerically, the soft mode's critical Reynolds number grows exponentially with Ha in inductionless problems. However, when Pm is nonzero the Lorentz force originating from the steady state current leads to a modification of Re_c(Ha) to either a sublinearly increasing, or decreasing function of Ha, respectively for problems with insulating and conducting walls. In the former, we also observe pairs of Alfven waves, the upstream propagating wave undergoing an instability at large Alfven numbers., Comment: 58 pages, 16 figures

Many stormy events in astrophysics occur due to the sudden magnetic energy release. This is possible if a magnetic configuration abruptly changes its topology, an event usually referred to as magnetic reconnection. It is known that pure Ohmic decay is inefficient, occurring during cosmological times (due to the huge characteristic scales $L$). It is recognized that the presence of current sheets speeds up the process, but still insufficiently$^{1,2,3,4,5}$. We show that, in highly compressible and substantially gravitational media, the reconnection is fast enough to account for stormy events. Thus, highly compressible situations offer exiting opportunities in explanations of violent events, although full-scale compressible and gravitational simulations proved to be quite challenging., Comment: 10 pages, 4 figures

Accretion disks and astrophysical jets are used to model many active astrophysical objects, viz., young stars, relativistic stars, and active galactic nuclei. In this paper we present self-consistent time-dependent simulations of supersonic jets launched from magnetized accretion disks, using high resolution numerical techniques. In particular we study the effects of the disk magnetic resistivity, parametrized through an alpha-prescription, in determining the properties of the inflow-outflow system. Moreover we analyze under which conditions steady state solutions of the type proposed in the self similar models of Blandford and Payne can be reached and maintained in a self consistent nonlinear stage. We use the resistive MHD FLASH code with adaptive mesh refinement, allowing us to follow the evolution of the structure for a time scale long enough to reach steady state. A detailed analysis of the initial configuration state is given. We obtain the expected solutions in the axisymmetric (2.5D) limit. Assuming a magnetic field around equipartition with the thermal pressure of the disk, we show how the characteristics of the disk jet system, as the ejection efficiency and the energetics, are affected by the anomalous resistivity acting inside the disk., Comment: 20 pages, 18 figures, accepted for publication in Astronomy and Astrophysics

Recently X-ray emission from protostellar jets has been detected with both XMM-Newton and Chandra satellites, but the physical mechanism which can give rise to this emission is still unclear. We performed an extensive exploration of a wide space of the main parameters influencing the jet/ambient interaction. Aims include: 1) to constrain the jet/ambient interaction regimes leading to the X-ray emission observed in Herbig-Haro objects in terms of the emission by a shock forming at the interaction front between a continuous supersonic jet and the surrounding medium; 2) to derive detailed predictions to be compared with optical and X-ray observations of protostellar jets; 3) to get insight into the protostellar jet's physical conditions. We performed a set of bidimensional hydrodynamic numerical simulations, in cylindrical coordinates, modeling supersonic jets ramming a uniform ambient medium. The model takes into account the most relevant physical effects, namely the thermal conduction and the radiative losses. Our model explains the observed X-ray emission from protostellar jets in a natural way. In particular we find that the case of a protostellar jet less dense than the ambient medium reproduces well the observations of the nearest Herbig-Haro object, HH154, and allows us to make detailed predictions of a possible X-ray source proper motion (vsh = 500 km/s), detectable with Chandra. Furthermore our results suggest that the simulated protostellar jets which best reproduce the X-rays observations cannot drive molecular outflows., Comment: 13 pages, 10 figures, accepted for publication in Astronomy and Astrophysics

AIMS. We study and discuss the time-dependent X-ray emission predicted by hydrodynamic modeling of the interaction of a SNR shock wave with an interstellar gas cloud. The scope includes: 1) to study the correspondence between modeled and X-ray emitting structures, 2) to explore two different physical regimes in which either thermal conduction or radiative cooling plays a dominant role, and 3) to investigate the effects of the physical processes at work on the emission of the shocked cloud in the two different regimes. METHODS. We use a detailed hydrodynamic model, including thermal conduction and radiation, and explore two cases characterized by different Mach numbers of the primary shock: M = 30 in which the cloud dynamics is dominated by radiative cooling and M = 50 dominated by thermal conduction. From the simulations, we synthesize the expected X-ray emission, using available spectral codes. RESULTS. The morphology of the X-ray emitting structures is significantly different from that of the flow structures originating from the shock-cloud interaction. The hydrodynamic instabilities are never clearly visible in the X-ray band. Shocked clouds are preferentially visible during the early phases of their evolution. Thermal conduction and radiative cooling lead to two different phases of the shocked cloud: a cold cooling dominated core emitting at low energies and a hot thermally conducting corona emitting in the X-ray band. The thermal conduction makes the X-ray image of the cloud smaller, more diffuse, and shorter-lived than that observed when thermal conduction is neglected., Comment: 9 pages, 8 Figures; paper accepted for publication in A&A; the paper with high-resolution figures can be downloaded at http://www.astropa.unipa.it/Library/OAPA_preprints/shock_cloud2.pdf

We model the hydrodynamic interaction of a shock wave of an evolved supernova remnant with a small interstellar gas cloud like the ones observed in the Cygnus loop and in the Vela SNR. We investigate the interplay between radiative cooling and thermal conduction during cloud evolution and their effect on the mass and energy exchange between the cloud and the surrounding medium. Through the study of two cases characterized by different Mach numbers of the primary shock (M = 30 and 50, corresponding to a post-shock temperature $T\approx 1.7\times 10^6$ K and $\approx 4.7\times 10^6$ K, respectively), we explore two very different physical regimes: for M = 30, the radiative losses dominate the evolution of the shocked cloud which fragments into cold, dense, and compact filaments surrounded by a hot corona which is ablated by the thermal conduction; instead, for M = 50, the thermal conduction dominates the evolution of the shocked cloud, which evaporates in a few dynamical time-scales. In both cases we find that the thermal conduction is very effective in suppressing the hydrodynamic instabilities that would develop at the cloud boundaries., Comment: 18 pages, 11 figures, A&A in press, full res. paper at http://www.astropa.unipa.it/Library/OAPA_preprints/orl_AA_2896.ps.gz

Recent Chandra and XMM-Newton observations of galaxy cluster cooling flows have revealed X-ray emission voids of up to 30 kpc in size that have been identified with buoyant, magnetized bubbles. Motivated by these observations, we have investigated the behavior of rising bubbles in stratified atmospheres using the FLASH adaptive-mesh simulation code. We present results from 2-D simulations with and without the effects of magnetic fields, and with varying bubble sizes and background stratifications. We find purely hydrodynamic bubbles to be unstable; a dynamically important magnetic field is required to maintain a bubble's integrity. This suggests that, even absent thermal conduction, for bubbles to be persistent enough to be regularly observed, they must be supported in large part by magnetic fields. Thermal conduction unmitigated by magnetic fields can dissipate the bubbles even faster. We also observe that the bubbles leave a tail as they rise; the structure of these tails can indicate the history of the dynamics of the rising bubble., Comment: 44 pgs, accepted to ApJ. 25 figs; replaced with single-spaced version

Many classical nova ejecta are enriched in CNO and Ne. Rosner et al. recently suggested that the enrichment might originate in the resonant interaction between large-scale shear flows in the accreted H/He envelope and gravity waves at the interface between the envelope and the underlying C/O white dwarf. The shear flow amplifies the waves, which eventually form cusps and break. This wave breaking injects a spray of C/O into the superincumbent H/He. Using two-dimensional simulations, we formulate a quantitative expression for the amount of C/O per unit area that can be entrained into the H/He at saturation. The fraction of the envelope that is enriched depends on the horizontal distribution of shear velocity and the density contrast between the C/O white dwarf and the H/He layer but is roughly independent of the vertical shape of the shear profile. Using this parameterization for the mixed mass, we then perform several one-dimensional Lagrangian calculations of an accreting white dwarf envelope and consider two scenarios: that the wave breaking and mixing is driven by the convective flows; and that the mixing occurs prior to the onset of convection. In the absence of enrichment prior to ignition, the base of the convective zone, as calculated from mixing-length theory with the Ledoux instability criterion, does not reach the C/O interface. As a result, there is no additional mixing, and the runaway is slow. In contrast, the formation of a mixed layer during the accretion of H/He, prior to ignition, causes a more violent runaway. The envelope can be enriched by <25% of C/O by mass (consistent with that observed in some ejecta) for shear velocities, over the surface, with Mach numbers <0.4., Comment: 6 pages, 6 figures, uses emulateapj.cls (included); minor changes to text from v1, to appear in ApJ

Critically understanding the `standard candle'-like behavior of Type Ia supernovae requires understanding their explosion mechanism. One family of models for Type Ia Supernovae begins with a deflagration in a Carbon-Oxygen white dwarf which greatly accelerates through wrinkling and flame instabilities. While the planar speed and behavior of astrophysically-relevant flames is increasingly well understood, more complex behavior, such as the flame's response to stretch and curvature, has not been extensively explored in the astrophysical literature; this behavior can greatly enhance or suppress instabilities and local flame-wrinkling, which in turn can increase or decrease the bulk burning rate. In this paper, we explore the effects of curvature on both nuclear flames and simpler model flames to understand the effect of curvature on the flame structure and speed., Comment: 25 pages; accepted to ApJ; fixed author fields

We have studied the fully nonlinear behavior of pre-mixed flames in a gravitationally stratified medium, subject to the Boussinesq approximation. Key results include the establishment of criterion for when such flames propagate as simple planar flames; elucidation of scaling laws for the effective flame speed; and a study of the stability properties of these flames. The simplicity of some of our scalings results suggests that analytical work may further advance our understandings of buoyant flames., Comment: 11 pages, 14 figures, RevTex, gzipped tar file

We look at the spreading of localized temperature perturbations in the accreted fuel layer of a non-rotating neutron star. The pressure at the base of the accreted fuel layer is large and the material is only partially degenerate. Any temperature perturbations and resulting pressure gradients will lead to enormous accelerations (both laterally and vertically) of the material in the fuel layer. If the burning of this fuel cannot proceed more rapidly than the spreading of this perturbation, then localized burning cannot take place, and it is likely that the ignition would have to proceed simultaneously throughout the envelope. We present some multidimensional simulations of the spreading of temperature perturbations in a helium atmosphere on a neutron star., Comment: 7 pages, to appear in the Proceedings of the 3D Stellar Evolution Workshop, Livermore, CA, July 2002

We look in detail at the process of mapping an astrophysical initial model from a stellar evolution code onto the computational grid of an explicit, Godunov type code while maintaining hydrostatic equilibrium. This mapping process is common in astrophysical simulations, when it is necessary to follow short-timescale dynamics after a period of long timescale buildup. We look at the effects of spatial resolution, boundary conditions, the treatment of the gravitational source terms in the hydrodynamics solver, and the initialization process itself. We conclude with a summary detailing the mapping process that yields the lowest ambient velocities in the mapped model., Comment: 59 pages, 21 figures, accepted to ApJS. Some figures are degraded for size constraints

Observed novae abundances and explosion energies estimated from observations indicate that there must be significant mixing of the heavier material of the white dwarf (C+O) into the lighter accreted material (H+He). Accordingly, nova models must incorporate a mechanism that will dredge up the heavier white dwarf material, and fluid motions from an early convection phase is one proposed mechanism. We present results from two-dimensional simulations of classical nova precursor models that demonstrate the beginning of a convective phase during the `simmering' of a Nova precursor. We use a new hydrostatic equilibrium hydrodynamics module recently developed for the adaptive-mesh code FLASH. The two-dimensional models are based on the one-dimensional models of Ami Glasner, and were evolved with FLASH from a pre-convective state to the onset of convection., Comment: 5 pages, 4 figures, from the 2002 International Conference on Classical Novae in Sitges, Spain

We present the results of a simulation of a wind-driven non-linear gravity wave breaking on the surface of a white dwarf. The ``wind'' consists of H/He from an accreted envelope, and the simulation demonstrates that this breaking wave mechanism can produce a well-mixed layer of H/He with C/O from the white dwarf above the surface. Material from this mixed layer may then be transported throughout the accreted envelope by convection, which would enrich the C/O abundance of the envelope as is expected from observations of novae., Comment: 5 pages, 3 figures, to appear in the proceedings of the International Conference on Classical Nova Explosions, Sitges, Spain, 20-24 May 2002

We present a case study of validating an astrophysical simulation code. Our study focuses on validating FLASH, a parallel, adaptive-mesh hydrodynamics code for studying the compressible, reactive flows found in many astrophysical environments. We describe the astrophysics problems of interest and the challenges associated with simulating these problems. We describe methodology and discuss solutions to difficulties encountered in verification and validation. We describe verification tests regularly administered to the code, present the results of new verification tests, and outline a method for testing general equations of state. We present the results of two validation tests in which we compared simulations to experimental data. The first is of a laser-driven shock propagating through a multi-layer target, a configuration subject to both Rayleigh-Taylor and Richtmyer-Meshkov instabilities. The second test is a classic Rayleigh-Taylor instability, where a heavy fluid is supported against the force of gravity by a light fluid. Our simulations of the multi-layer target experiments showed good agreement with the experimental results, but our simulations of the Rayleigh-Taylor instability did not agree well with the experimental results. We discuss our findings and present results of additional simulations undertaken to further investigate the Rayleigh-Taylor instability., Comment: 76 pages, 26 figures (3 color), Accepted for publication in the ApJS

The aim of this paper is to validate a methodology for connecting the emission measure of individual solar coronal loops to the integrated emission measure of the entire solar corona, and using this connection to deduce the energetic properties of the corona, and then to show how this methodology can be applied to observations of solar-like stellar coronae. The solar validation is carried out by using spatially resolved X-ray observations of the Sun obtained from the Yohkoh satellite. This work is a further step in our effort to place the "solar-stellar connection" on a quantitative footing. In particular, we show how this analysis procedure can be used in the context of archival Einstein, ROSAT and EUVE data, as well as Chandra and XMM Newton data, as a complementary analysis tool to existing multi-thermal component models., Comment: 37 pages, 6 figures, ApJ latex format. In press, Ap.J., Dec. 20, 2001 issue

We examine the linear stability of fluid interfaces subjected to a shear flow. Our main object is to generalize previous work to arbitrary Atwood number, and to allow for surface tension and weak compressibility. The motivation derives from instances in astrophysical systems where mixing across material interfaces driven by shear flows may significantly affect the dynamical evolution of these systems., Comment: 45 pages (preprint format), 13 pages, 1 table accepted for publication in Phys. Rev. E

Using the results of recent work in shear instabilities in stratified fluids, we show that the resonant interaction between large-scale flows in the accreted H/He envelope of white dwarf stars and interfacial gravity waves can mix with the star's envelope with the white dwarf's surface material, leading to the enhancement of the envelope's C/O abundance to levels required by extant models for nova outbursts., Comment: 8 pages, 0 figures, accepted for publication in ApJ Lett

We investigate the miscible Rayleigh-Taylor (RT) instability in both 2 and 3 dimensions using direct numerical simulations, where the working fluid is assumed incompressible under the Boussinesq approximation. We first consider the case of randomly perturbed interfaces. With a variety of diagnostics, we develop a physical picture for the detailed temporal development of the mixed layer: We identify three distinct evolutionary phases in the development of the mixed layer, which can be related to detailed variations in the growth of the mixing zone. Our analysis provides an explanation for the observed differences between two and three-dimensional RT instability; the analysis also leads us to concentrate on the RT models which (1) work equally well for both laminar and turbulent flows, and (2) do not depend on turbulent scaling within the mixing layer between fluids. These candidate RT models are based on point sources within bubbles (or plumes) and interaction with each other (or the background flow). With this motivation, we examine the evolution of single plumes, and relate our numerical results (of single plumes) to a simple analytical model for plume evolution., Comment: 31 pages, 27 figures, to appear in November issue of JFM, 2001. For better figures: http://astro.uchicago.edu/~young/ps/jfmtry08.ps.Z

Particle acceleration by electrostatic polarization fields that arise in plasmas streaming across magnetic fields is discussed as a possible acceleration mechanism of highest-energy cosmic rays. Specifically, plasmoids arising in planetoid impacts onto neutron star magnetospheres are considered. We find that such impacts at plausible rates may account for the observed flux and energy spectrum of the highest energy cosmic rays., Comment: 4 pages, 4 figures, accepted for publication in Phys. Rev. Lett., uses REVTEX

A recently proposed mechanism of acceleration of highest energy cosmic rays by polarization electric fields arising in plasmoids injected into neutron star magnetospheres is discussed., Comment: 6 pages, to appear in Proc. of 20th Texas Symposium on Relativistic Astrophysics, uses aipproc (included)

We present direct numerical simulations of mixing at Rayleigh-Taylor unstable interfaces performed with the FLASH code, developed at the ASCI/Alliances Center for Astrophysical Thermonuclear Flashes at the University of Chicago. We present initial results of single-mode studies in two and three dimensions. Our results indicate that three-dimensional instabilities grow significantly faster than two-dimensional instabilities and that grid resolution can have a significant effect on instability growth rates. We also find that unphysical diffusive mixing occurs at the fluid interface, particularly in poorly resolved simulations., Comment: 3 pages, 1 figure. To appear in the proceedings of the 20th Texas Symposium on Relativistic Astrophysics

We assess the stability of Kruskal-Schwarzschild (magnetic Rayleigh-Taylor) type modes for accreted matter on the surface of a neutron star confined by a strong (>= 1.E12 G) magnetic field. Employing the energy principle to analyze the stability of short-wavelength ballooning modes, we find that line-tying to the neutron star crust stabilizes these modes until the overpressure at the top of the neutron star crust exceeds the magnetic pressure by a factor ~ 8(a/h), where a and h are respectively the lateral extent of the accretion region and the density scale height. The most unstable modes are localized within a density scale height above the crust. We calculate the amount of mass that can be accumulated at the polar cap before the onset of instability., Comment: 8 pages, 2 figures, accepted for publication by ApJ, uses AASTEX 5.0 and emulateapj5.sty (included)

We discuss some of the computational challenges encountered in simulating the evolution of clusters of galaxies. Eulerian adaptive mesh refinement (AMR) techniques can successfully address these challenges but are currently being used by only a few groups. We describe our publicly available AMR code, FLASH, which uses an object-oriented framework to manage its AMR library, physics modules, and automated verification. We outline the development of the FLASH framework to include collisionless particles, permitting it to be used for cluster simulation., Comment: 3 pages, 3 figures, to appear in Proceedings of the VII International Workshop on Advanced Computing and Analysis Techniques in Physics Research (ACAT 2000), Fermilab, Oct. 16-20, 2000

The analysis of the stability properties of astrophysical jets against Kelvin-Helmholtz (or shear-layer) instabilities plays a basic role in the understanding the origin and physical characteristics of these objects. Numerical simulations by Bodo et al. (1998) have shown that the three-dimensional non-linear evolution of KH instabilities in supersonic jets is substantially faster than in the two-dimensional case, leading to a cascade of modes towards smaller scales and a very effective mixing and momentum transfer to the ambient medium. On the other hand, Rossi et al. (1997) and Micono et al. (1998) found, in two dimensions, that radiative losses tend to reduce and delay mixing effects and momentum transfer to the ambient medium. In this paper, as a logical next step, we investigate the effects of radiative losses on the stability of 3D supersonic jets, assuming that the internal jet density is initially lower, equal and higher than the ambient medium, respectively. We find that light and equal-density radiative jets evolve in a qualitatively similar fashion with respect to the corresponding adiabatic ones. Conversely, we note substantial differences in the evolution of heavy jets: they remain more collimated and do not spread out, while the momentum gained by the ambient medium stays within ~ 5 jet radii., Comment: 15 pages containing 13 postscript figures; uses A&A macros. To appear in Astronomy and Astrophysics

We use direct numerical simulations to study the evolution, or relaxation, of magnetic configurations to an equilibrium state. We use the full single-fluid equations of motion for a magnetized, non-resistive, but viscous fluid; and a Lagrangian approach is used to obtain exact solutions for the magnetic field. As a result, the topology of the magnetic field remains unchanged, which makes it possible to study the case of topological nonequilibrium. We find two cases for which such nonequilibrium appears, indicating that these configurations may develop singular current sheets., Comment: 10 pages, 5 figures