arXiv subjects

Frederick A. Gent

Publications and source records attributed to Frederick A. Gent.

At least 19 recordsLinked to original sources

arXiv2026

Asymptotic behaviour of galactic small-scale dynamos at modest magnetic Prandtl number

Magnetic fields are critical at many scales to galactic dynamics and structure, including multiphase pressure balance, dust processing, and star formation. Dynamo action determines their dynamical structure and strength. Simulations of combined large- and small-scale dynamos have successfully developed mean fields with strength and topology consistent with observations but with turbulent fields much weaker than observed, while simulations of small-scale dynamos with parameters relevant to the interstellar medium yield turbulent fields an order of magnitude below the values observed or expected theoretically. We use the Pencil Code accelerated on GPUs with Astaroth to perform high-resolution simulations of a supernova-driven galactic dynamo including heating and cooling in a periodic domain. Our models show that the strength of the turbulent field produced by the small-scale dynamo approaches an asymptote at only modest magnetic Prandtl numbers. This allows us to use these models to suggest the essential characteristics of this constituent of the magnetic field for inclusion in global galactic models. The asymptotic limit occurs already at magnetic Prandtl number of only a few hundred, many orders of magnitude below physical values in the the interstellar medium and consistent with previous findings for isothermal compressible flows.

astro-ph.GA↗

arXiv2025

Mathematical Aspects of Geophysical and Astrophysical Fluid Dynamics: Magnetic buoyancy instability in galaxies

We study the nonlinear evolution of the magnetic buoyancy instability in rotating and non-rotating gas layers using numerical solutions of non-ideal, isothermal MHD equations. The unstable magnetic field is either imposed through the boundary conditions or generated by an imposed $α$-effect. In the case of an imposed field, we solve for the deviations from the background state with periodic boundary conditions. We also include cosmic rays as a weightless fluid which exerts a dynamically significant pressure and amplifies magnetic buoyancy, known as the Parker instability. Without rotation, systems with an imposed magnetic field evolve to a state with a very weak magnetic field, very different from the marginally stable eigenfunction, where the gas layer eventually becomes very thin as it is supported by thermal and turbulent pressures alone. However, this does not happen when the $α$-effect maintains the magnetic field. Rotation fundamentally changes the development of the instability. A rotating system develops nonlinear oscillations, and the magnetic field direction changes even with an imposed magnetic field. We demonstrate that the cause is a secondary $α$-effect at large altitudes as the gas flow produced by the instability becomes helical. The secondary $α$-effect has an anomalous sign with the $α$-coefficient being negative in the northern hemisphere, whereas the Coriolis force produces a positive $α$. The mean-field dynamo action outside the original gas layer can also lead to a switch in the magnetic field parity from quadrupolar to dipolar. Altogether, the magnetic buoyancy instability and the mean-field dynamo action become separated as distinct physical effects in a nonlinear rotating system. We show that none of the assumptions used in analytic studies of the Parker instability is corroborated by numerical results.

astro-ph.GA↗

arXiv2024

Supernova dust destruction in the magnetized turbulent ISM

Dust in the interstellar medium (ISM) is critical to the absorption and intensity of emission profiles used widely in astronomical observations, and necessary for star and planet formation. Supernovae (SNe) both produce and destroy ISM dust. In particular the destruction rate is difficult to assess. Theory and prior simulations of dust processing by SNe in a uniform ISM predict quite high rates of dust destruction, potentially higher than the supernova dust production rate in some cases. Here we show simulations of supernova-induced dust processing with realistic ISM dynamics including magnetic field effects and demonstrate how ISM inhomogeneity and magnetic fields inhibit dust destruction. Compared to the non-magnetic homogeneous case, the dust mass destroyed within 1 Myr per SNe is reduced by more than a factor of two, which can have a great impact on the ISM dust budget.

astro-ph.GA↗

arXiv2024

Computational approaches to modeling dynamos in galaxies

Galaxies are observed to host magnetic fields with a typical total strength of around 15microgauss. A coherent large-scale field constitutes up to a few microgauss of the total, while the rest is built from strong magnetic fluctuations over a wide range of spatial scales. This represents sufficient magnetic energy for it to be dynamically significant. Several questions immediately arise: What is the physical mechanism that gives rise to such magnetic fields? How do these magnetic fields affect the formation and evolution of galaxies? In which physical processes do magnetic fields play a role, and how can that role be characterized? Numerical modelling of magnetized flows in galaxies is playing an ever-increasing role in finding those answers. We review major techniques used for these models. Current results strongly support the conclusion that field growth occurs during the formation of the first galaxies on timescales shorter than their accretion timescales due to small-scale turbulent dynamos. The saturated small-scale dynamo maintains field strengths at a few percent of equipartition with turbulence. The subsequent action of large-scale dynamos in differentially rotating discs produces observed modern field strengths in equipartition with the turbulence and having power at large scales. The field structure resulting appears consistent with observations including Faraday rotation and polarisation from synchrotron and dust thermal emission. Major remaining challenges include scaling numerical models toward realistic scale separations and Prandtl and Reynolds numbers.

astro-ph.GA↗

arXiv2023

Nonlinear magnetic buoyancy instability and turbulent dynamo

Stratified disks with strong horizontal magnetic fields are susceptible to magnetic buoyancy instability (MBI). Modifying the magnetic field and gas distributions, this can play an important role in galactic evolution. The MBI and the Parker instability, in which cosmic rays exacerbate MBI, are often studied using an imposed magnetic field. However, in galaxies and accretion discs, the magnetic field is continuously replenished by a large-scale dynamo action. Using non-ideal MHD equations, we model a section of the galactic disc (we neglect rotation and cosmic rays considered elsewhere), in which the large-scale field is generated by an imposed $α$-effect of variable intensity to explore the interplay between dynamo instability and MBI. The system evolves through three distinct phases: the linear (kinematic) dynamo stage, the onset of linear MBI when the magnetic field becomes sufficiently strong and the nonlinear, statistically steady state. Nonlinear effects associated with the MBI introduce oscillations which do not occur when the field is produced by the dynamo alone. The MBI initially accelerates the magnetic field amplification but the growth is quenched by the vertical motions produced by MBI. We construct a 1D model, which replicates all significant features of 3D simulations to confirm that magnetic buoyancy alone can quench the dynamo and is responsible for the magnetic field oscillations. Unlike with an imposed magnetic field (arXiv:2305.03318,arXiv:2212.03215), the nonlinear interactions do not reduce the gas scale height, so the consequences of the magnetic buoyancy depend on how the magnetic field is maintained.

astro-ph.GA↗

arXiv2023

Transition from small-scale to large-scale dynamo in a supernova-driven, multiphase medium

Magnetic fields are widely recognised as critical at many scales to galactic dynamics and structure, including multiphase pressure balance, dust processing, and star formation. Using imposed magnetic fields cannot reliably model the interstellar medium's (ISM) dynamical structure nor phase interactions. Dynamos must be modelled. ISM models exist of turbulent magnetic fields using small-scale dynamo (SSD). Others model the large-scale dynamo (LSD) organising magnetic fields at scale of the disc or spiral arms. Separately, neither can fully describe the galactic magnetic field dynamics nor topology. We model the LSD and SSD together at sufficient resolution to use the low explicit Lagrangian resistivity required. The galactic SSD saturates within 20 Myr. We show that the SSD is quite insensitive to the presence of an LSD and is even stronger in the presence of a large-scale shear flow. The LSD grows more slowly in the presence of SSD, saturating after 5 Gyr vs. 1--2 Gyr in studies where the SSD is weak or absent. The LSD primarily grows in warm gas in the galactic midplane. Saturation of the LSD occurs due to $α$-quenching near the midplane as the growing mean field produces a magnetic $α$ that opposes the kinetic $α$. The magnetic energy in our models of the LSD shows slightly sublinear response to increasing resolution, indicating that we are converging towards the physical solution at 1 pc resolution. Clustering supernovae in OB associations increases the growth rates for both the SSD and the LSD, compared to a horizontally uniform supernova distribution.

astro-ph.GA↗

arXiv2023

Numerical evidence for a small-scale dynamo approaching solar magnetic Prandtl numbers

Magnetic fields on small scales are ubiquitous in the universe. Though they can often be observed in detail, their generation mechanisms are not fully understood. One possibility is the so-called small-scale dynamo (SSD). Prevailing numerical evidence, however, appears to indicate that an SSD is unlikely to exist at very low magnetic Prandtl numbers ($Pr_M$) such as are present in the Sun and other cool stars. We have performed high-resolution simulations of isothermal forced turbulence employing the lowest $Pr_M$ values so far achieved. Contrary to earlier findings, the SSD turns out to be not only possible for $Pr_M$ down to 0.0031, but even becomes increasingly easier to excite for $Pr_M$ below $\simeq\,$0.05. We relate this behaviour to the known hydrodynamic phenomenon referred to as the bottleneck effect. Extrapolating our results to solar values of $Pr_M$ indicates that an SSD would be possible under such conditions.

astro-ph.SR↗

arXiv2023

Steady states of the Parker instability: the effects of rotation

We model the Parker instability in vertically stratified isothermal gas using non-ideal MHD three-dimensional simulations. Rotation, especially differential, more strongly and diversely affects the nonlinear state than the linear stage (where we confirm the most important conclusions of analytical models), and stronger than any linear analyses predict. Steady state magnetic fields are stronger and cosmic ray energy density higher than in comparable nonrotating systems. Transient gas outflows induced by the nonlinear instability persist longer, of order 2 Gyr, with rotation. Stratification combined with (differential) rotation drives helical flows, leading to mean-field dynamo. Consequently, the nonlinear state becomes oscillatory (while both the linear instability and the dynamo are non-oscillatory). The horizontal magnetic field near the midplane reverses its direction propagating to higher altitudes as the reversed field spreads buoyantly. The spatial pattern of the large-scale magnetic field may explain the alternating magnetic field directions in the halo of the edge-on galaxy NGC 4631. Our model is unique in producing a large-scale magnetic structure similar to such observation. Furthermore, our simulations show that the mean kinetic helicity of the magnetically driven flows has the sign opposite to that in the conventional non-magnetic flows. This has profound consequences for the nature of the dynamo action and large-scale magnetic field structure in the coronae of spiral galaxies which remain to be systematically explored and understood. We show that the energy density of cosmic rays and magnetic field strength are not correlated at scales of order a kiloparsec.

astro-ph.GA↗

arXiv2022

Steady states of the Parker instability

We study the linear properties, nonlinear saturation and a steady, strongly nonlinear state of the Parker instability in galaxies. We consider magnetic buoyancy and its consequences with and without cosmic rays. Cosmic rays are described using the fluid approximation with anisotropic, non-Fickian diffusion. To avoid unphysical constraints on the instability (such as boundary conditions often used to specify an unstable background state), nonideal MHD equations are solved for deviations from a background state representing an unstable magnetohydrostatic equilibrium. We consider isothermal gas and neglect rotation. The linear evolution of the instability is in broad agreement with earlier analytical and numerical models; but we show that most of the simplifying assumptions of the earlier work do not hold, such that they provide only a qualitative rather than quantitative picture. In its nonlinear stage the instability has significantly altered the background state from its initial state. Vertical distributions of both magnetic field and cosmic rays are much wider, the gas layer is thinner, and the energy densities of both magnetic field and cosmic rays are much reduced. The spatial structure of the nonlinear state differs from that of any linear modes. A transient gas outflow is driven by the weakly nonlinear instability as it approaches saturation.

astro-ph.GA↗

arXiv2022

The small-scale dynamo in a multiphase supernova-driven medium

Magnetic fields grow quickly, even at early cosmological times, suggesting the action of a small-scale dynamo (SSD) in the interstellar medium (ISM) of galaxies. Many studies have focused on idealized, isotropic, homogeneous, turbulent driving of the SSD. Here we analyze more realistic simulations of supernova-driven turbulence to understand how it drives an SSD. We find that SSD growth rates are intermittently variable as a result of the evolving multiphase ISM structure. Rapid growth in the magnetic field typically occurs in hot gas, with the highest overall growth rates occurring when the fractional volume of hot gas is large. SSD growth rates correlate most strongly with vorticity and fluid Reynolds number, which also both correlate strongly with gas temperature. Rotational energy exceeds irrotational energy in all phases, but particularly in the hot phase while SSD growth is most rapid. Supernova (SN) rate does not significantly affect the ISM average kinetic energy density. Rather, higher temperatures associated with high SN rates tend to increase SSD growth rates. SSD saturates with total magnetic energy density around 5% of equipartition to kinetic energy density, increasing slightly with magnetic Prandtl number. While magnetic energy density in the hot gas can exceed that of the other phases when SSD grows most rapidly, it saturates below 5% of equipartition with kinetic energy in the hot gas, while in the cold gas it attains 100%. Fast, intermittent growth of the magnetic field appears to be a characteristic behavior of SN-driven, multiphase turbulence.

astro-ph.GA↗

arXiv2021

Supernova induced processing of interstellar dust: impact of ISM gas density and gas turbulence

Quantifying the efficiency of dust destruction in the interstellar medium (ISM) due to supernovae (SNe) is crucial for the understanding of galactic dust evolution. We present 3D hydrodynamic simulations of an SN blast wave propagating through the ISM. The interaction between the forward shock of the remnant and the surrounding ISM leads to destruction of ISM dust by the shock heated gas. We consider the dust processing due to ion sputtering, accretion of atoms/molecules and grain-grain collisions. Using 2D slices from the simulation timeseries, we apply post-processing calculations using the Paperboats code. We find that efficiency of dust destruction depends strongly on the rate of grain shattering due to grain-grain collisions. The effective dust destruction is similar to previous theoretical estimates when grain-grain collisions are omitted, but with grain shattering included, the net destruction efficiency is roughly one order of magnitude higher. This result indicates that the dust destruction rate in the ISM may have been severely underestimated in previous work, which only exacerbates the dust-budget crises seen in galaxies at high redshifts.

astro-ph.GA↗

arXiv2021

Small-Scale Dynamo in Supernova-Driven Interstellar Turbulence

Magnetic fields grow quickly even at early cosmological times, suggesting the action of a small-scale dynamo (SSD) in the interstellar medium of galaxies. Many studies have focused on idealized turbulent driving of the SSD. Here we simulate more realistic supernova-driven turbulence to determine whether it can drive an SSD. Magnetic field growth occurring in our models appears inconsistent with simple tangling of magnetic fields, but consistent with SSD action, reproducing and confirming models by Balsara et al. (2004) that did not include physical resistivity $η$. We vary $η$, as well as the numerical resolution and supernova rate, $\dotσ$, to delineate the regime in which an SSD occurs. For a given $\dotσ$ we find convergence for SSD growth rate with resolution of a parsec. For $\dotσ\simeq\dotσ_{\rm sn}$, with $\dotσ_{\rm sn}$ the solar neighbourhood rate, the critical resistivity below which an SSD occurs is $0.005>η_{\rm crit}>0.001\,\rm kpc^{-1}\,\rm km s^{-1}$, and this increases with the supernova rate. Across the modelled range of 0.5--4 pc resolution we find that for $η<η_{\rm crit}$, the SSD saturates at about 5% of kinetic energy equipartition, independent of growth rate. In the range $0.2\dotσ_{\rm sn}\leq \dotσ\leq8\dotσ_{\rm sn}$ growth rate increases with $\dotσ$. SSDs in the supernova-driven interstellar medium commonly exhibit erratic growth.

astro-ph.GA↗

arXiv2019

Modelling 3D magnetic networks in a realistic solar atmosphere

The magnetic network extending from the photosphere (solar radius $\simeq R_\odot$) to lower corona ($R_\odot + 10$ Mm) plays an important role in the heating mechanisms of the solar atmosphere. Here we further develop the models with realistic open magnetic flux tubes of the authors in order to model more complicated configurations. Closed magnetic loops, and combinations of closed and open magnetic flux tubes are modelled. These are embedded within a stratified atmosphere, derived from observationally motivated semi-empirical and data-driven models subject to solar gravity and capable of spanning from the photosphere up into the chromosphere and lower corona. Constructing a magnetic field comprising self-similar magnetic flux tubes, an analytic solution for the kinetic pressure and plasma density is derived. Combining flux tubes of opposite polarity it is possible to create a steady background magnetic field configuration modelling a solar atmosphere exhibiting realistic stratification. The result can be applied to SOHO/MDI and SDO/HMI and other magnetograms from the solar surface, upon which photospheric motions can be simulated to explore the mechanism of energy transport. We demonstrate this powerful and versatile method with an application to Helioseismic and Magnetic Imager data.

astro-ph.SR↗

arXiv2019

Modelling supernova driven turbulence

High Mach number shocks are ubiquitous in interstellar turbulence. The Pencil Code is particularly well suited to the study of magnetohydrodynamics in weakly compressible turbulence and the numerical investigation of dynamos because of its high-order advection and time evolution algorithms. However, the high-order algorithms and lack of Riemann solver to follow shocks make it less well suited to handling high Mach number shocks, such as those produced by supernovae (SNe). Here, we outline methods required to enable the code to efficiently and accurately model SNe, using parameters that allow stable simulation of SN-driven turbulence, in order to construct a physically realistic galactic dynamo model. These include the resolution of shocks with artificial viscosity, thermal conductivity, and mass diffusion; the correction of the mass diffusion terms; and a novel generalization of the Courant condition to include all source terms in the momentum and energy equations. We test our methods with the numerical solution of the one-dimensional (1D) Riemann shock tube (Sod, J. Comput. Phys. 1978, 27), also extended to a 1D adiabatic shock with parameters and Mach number relevant to SN shock evolution, including shocks with radiative losses. We extend our test with the three-dimensional (3D) numerical simulation of individual SN remnant evolution for a range of ambient gas densities typical of the interstellar medium and compare these to the analytical solutions of Sedov-Taylor (adiabatic) and the snowplough and Cioffi, McKee and Bertschinger (Astrophys. J. 1988, 334) results incorporating cooling and heating processes. We show that our new timestep algorithm leads to linear rather than quadratic resolution dependence as the strength of the artificial viscosity varies, because of the corresponding change in the strength of interzone gradients.

astro-ph.GA↗

arXiv2019

Sensitivity to luminosity, centrifugal force, and boundary conditions in spherical shell convection

We test the sensitivity of hydrodynamic and magnetohydrodynamic turbulent convection simulations with respect to Mach number, thermal and magnetic boundary conditions, and the centrifugal force. We find that varying the luminosity, which also controls the Mach number, has only a minor effect on the large-scale dynamics. A similar conclusion can also be drawn from the comparison of two formulations of the lower magnetic boundary condition with either vanishing electric field or current density. The centrifugal force has an effect on the solutions, but only if its magnitude with respect to acceleration due to gravity is by two orders of magnitude greater than in the Sun. Finally, we find that the parameterisation of the photospheric physics, either by an explicit cooling term or enhanced radiative diffusion, is more important than the thermal boundary condition. In particular, runs with cooling tend to lead to more anisotropic convection and stronger deviations from the Taylor-Proudman state. In summary, the fully compressible approach taken here with the Pencil Code is found to be valid, while still allowing the disparate timescales to be taken into account.

astro-ph.SR↗

arXiv2018

The supernova-regulated ISM. IV. A comparison of simulated polarization with Planck observations

The efforts for comparing dust polarization measurements with synthetic observations from MHD models have previously concentrated on the scale of molecular clouds. Here we extend the model comparisons to kiloparsec scales, taking into account hot shocked gas generated by supernovae, and a non-uniform dynamo-generated magnetic field at both large and small scales down to 4 pc spatial resolution. Radiative transfer calculations are used to model dust emission and polarization on the top of MHD simulations. We compute synthetic maps of column density $N_H$, polarization fraction $p$, and polarization angle dispersion $S$, and study their dependencies on the important properties of the MHD simulations. These include the large-scale magnetic field and its orientation, the small-scale magnetic field, and supernova-driven shocks. Similar filament-like structures of $S$ as seen in the Planck all-sky maps are visible in our synthetic results, although the smallest scale structures are absent from our maps. Supernova-driven shock fronts and $S$ do not show significant correlation. Instead, $S$ can clearly be attributed to the distribution of the small-scale magnetic field. We also find that the large-scale magnetic field influences the polarization properties, such that, for a given strength of magnetic fluctuation, a strong plane-of-the-sky mean field weakens the observed $S$, while strengthening $p$. The anticorrelation of $p$ and $S$, and decreasing $p$ as a function of $N_H$ are consistent across all synthetic observations. The magnetic fluctuations follow an exponential distribution, rather than Gaussian, characteristic of flows with intermittent repetitive shocks. The observed polarization properties and column densities are sensitive to the line of sight distance over which the emission is integrated.

astro-ph.GA↗

arXiv2017

Supernova-regulated ISM -- V. Space- and time-correlations

We apply correlation analysis to random fields in numerical simulations of the supernova-driven interstellar medium (ISM) with the magnetic field produced by dynamo action. We solve the thermo-magnetohydrodynamic (MHD) equations in a shearing, Cartesian box representing a local region of the ISM, subject to thermal and kinetic energy injection by supernova explosions, and parameterized optically-thin radiative cooling. We consider the cold, warm\, and hot phases of the ISM separately; the analysis mostly considers the warm gas, which occupies the bulk of the domain. Various physical variables have different correlation lengths in the warm phase: $40 pc$, $50 pc$, and $60 pc$ for random magnetic field, density, and velocity, respectively, in the midplane. The correlation time of the random velocity is comparable to the eddy turnover time, about $10^7 yr$, although it may be shorter in regions with higher star formation rate. The random magnetic field is anisotropic, with the standard deviations of its components $b_x/b_y/b_z$ having approximate ratios $0.5/0.6/0.6$ in the midplane. The anisotropy is attributed to the global velocity shear from galactic differential rotation, and locally inhomogeneous outflow to the galactic halo. The correlation length of Faraday depth along the $z$-axis, $120 pc$, is greater than for electron density, $60 - 90 pc$, and vertical magnetic field, $60 pc$. Such comparisons may be sensitive to the orientation of the line of sight. Uncertainties of the structure functions of synchrotron intensity rapidly increase with the scale. This feature is hidden in power spectrum analysis, which can undermine the usefulness of power spectra for detailed studies of interstellar turbulence.

astro-ph.GA↗

arXiv2017

The supernova-regulated ISM. III. Generation of vorticity, helicity and mean flows

The forcing of interstellar turbulence, driven mainly by supernova explosions, is irrotational in nature, but the development of significant amounts of vorticity and helicity, accompanied by large-scale dynamo action, has been reported. Several earlier investigations examined vorticity production in simpler systems; here all the relevant processes can be considered simultaneously. We also investigate the mechanisms for the generation of net helicity and large-scale flow in the system. We use a three-dimensional, stratified, rotating and shearing local simulation domain of the size 1x1x2 kpc$^3$, forced with SN explosions occurring at the rate typical of the solar neighbourhood in the Milky Way. In addition to the nominal simulation run with realistic Milky Way parameters, we vary the rotation and shear rates, but keep the absolute value of their ratio fixed. Reversing the sign of shear vs. rotation allows us to separate the rotation- and shear-generated contributions. As in earlier studies, we find the generation of significant amounts of vorticity, with on average 65% of the kinetic energy being in the rotational modes. The vorticity production can be related to the baroclinicity of the flow, especially in the regions of hot, dilute clustered supernova bubbles. In these regions, the vortex stretching acts as a sink of vorticity. The net helicities produced by rotation and shear are of opposite signs for physically motivated rotation laws, with the solar neighbourhood parameters resulting in the near cancellation of the total net helicity. We also find the excitation of oscillatory mean flows, the strength and oscillation period of which depend on the Coriolis and shear parameters; we interpret these as signatures of the anisotropic kinetic (AKA) effect. We use the method of moments to fit for the turbulent transport coeffcients, and find $α_{\rm AKA}$ values of the order 3-5 km/s.

astro-ph.GA↗