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Matthew Browning

Publications and source records attributed to Matthew Browning.

10 recordsLinked to original sources
arXiv2026

Superrotation and Jet Migration in Simulations of Jupiter's Convective Zone and Weather Layer

The mean zonal flow observed on Jupiter consists of an intricate pattern of jets, or bands of zonal flow moving prograde or retrograde compared to the bulk planetary rotation. The strongest flow is a superrotating (prograde) jet near the equator, which is flanked by 6-7 retrograde/prograde pairs of weaker jets per hemisphere. The two primary drivers of Jupiter's zonal flows are thought to be "shallow" baroclinically driven quasi-two-dimensional turbulence in an outer, stably stratified weather layer (WL) and "deep" rotationally constrained buoyantly driven three-dimensional Busse columns in the convective zone (CZ) just underneath the WL. To study both driving mechanisms simultaneously, we implement two rotating, three-dimensional, spherical-shell, anelastic convection simulations of a Jovian-like planet. In one case, the CZ is isolated, whereas in the other case, the upflows are allowed to overshoot into a stably stratified near-surface region, representing an idealized weather layer. We find that in both cases, homogenization of potential vorticity (whose forms in the CZ and WL are distinct) initially creates multiple jets at high latitudes, whereas angular momentum transport by Busse columns drives equatorial superrotation at low latitudes. The presence of an idealized WL significantly alters the thermal wind balance, resulting in large deviations of the meridional contours of the zonal flow from alignment with the rotation axis. Although the superrotation remains stable, the weaker high-latitude jets slowly migrate poleward and/or merge on a very long time scale (O(10) diffusion time scales or thousands of eddy turnover times).

astro-ph.EP↗
arXiv2010

Differential Rotation and Magnetism in Simulations of Fully Convective Stars

Stars of sufficiently low mass are convective throughout their interiors, and so do not possess an internal boundary layer akin to the solar tachocline. Because that interface figures so prominently in many theories of the solar magnetic dynamo, a widespread expectation had been that fully convective stars would exhibit surface magnetic behavior very different from that realized in more massive stars. Here I describe how recent observations and theoretical models of dynamo action in low-mass stars are partly confirming, and partly confounding, this basic expectation. In particular, I present the results of 3--D MHD simulations of dynamo action by convection in rotating spherical shells that approximate the interiors of 0.3 solar-mass stars at a range of rotation rates. The simulated stars can establish latitudinal differential rotation at their surfaces which is solar-like at ``rapid'' rotation rates (defined within) and anti-solar at slower rotation rates; the differential rotation is greatly reduced by feedback from strong dynamo-generated magnetic fields in some parameter regimes. I argue that this ``flip'' in the sense of differential rotation may be observable in the near future. I also briefly describe how the strength and morphology of the magnetic fields varies with the rotation rate of the simulated star, and show that the maximum magnetic energies attained are compatible with simple scaling arguments.

astro-ph.SR↗
arXiv2009

Research Science and Education: The NSF's Astronomy and Astrophysics Postdoctoral Fellowship

The NSF's Astronomy and Astrophysics Postdoctoral Fellowship (AAPF) is exceptional among the available postdoctoral awards in Astronomy and Astrophysics. The fellowship is one of the few that allows postdoctoral researchers to pursue an original research program, of their own design, at the U.S. institution of their choice. However, what makes this fellowship truly unique is the ability of Fellows to lead an equally challenging, original educational program simultaneously. The legacy of this singular fellowship has been to encourage and advance leaders in the field who are equally as passionate about their own research as they are about sharing that research and their passion for astronomy with students and the public. In this positional paper we address the importance of fellowships like the AAPF to the astronomical profession by identifying the science and educational contributions that Fellows have made to the community. Further, we recommend that fellowships that encourage leading postdoctoral researchers to also become leaders in Astronomy education be continued and expanded.

astro-ph.IM↗
arXiv2007

Simulations of dynamo action in fully convective stars

We present three-dimensional nonlinear magnetohydrodynamic simulations of the interiors of fully convective M-dwarfs. Our models consider 0.3 solar-mass stars using the Anelastic Spherical Harmonic code, with the spherical computational domain extending from 0.08-0.96 times the overall stellar radius. Like previous authors, we find that fully convective stars can generate kG-strength magnetic fields (in rough equipartition with the convective flows) without the aid of a tachocline of shear. Although our model stars are everywhere unstably stratified, the amplitudes and typical pattern sizes of the convective flows vary strongly with radius, with the outer regions of the stars hosting vigorous convection and field amplification while the deep interiors are more quiescent. Modest differential rotation is established in hydrodynamic calculations, but -- unlike in some prior work --strongly quenched in MHD simulations because of the Maxwell stresses exerted by the dynamo-generated magnetic fields. Despite the lack of strong differential rotation, the magnetic fields realized in the simulations possess significant mean (axisymmetric) components, which we attribute partly to the strong influence of rotation upon the slowly overturning flows.

astro-ph↗
arXiv2007

Dynamo Action in Fully Convective Low-Mass Stars

Recent observations indicate that fully convective stars can effectively build magnetic fields without the aid of a tachocline of shear, that those fields can possess large-scale components, and that they may sense the effects of rotation. Motivated by these puzzles, we present global three-dimensional simulations of convection and dynamo action in the interiors of fully convective M-dwarfs of 0.3 solar masses. We use the Anelastic Spherical Harmonic (ASH) code, adopting a spherical computational domain that extends from 0.08-0.96 times the overall stellar radius. We find that such fully convective stars can generate magnetic fields of several kG strength, roughly in equipartition with the convective flows. Differential rotation is established in hydrodynamic progenitor calculations, but strongly quenched in MHD simulations because of strong Maxwell stresses exerted by the magnetic fields. Despite the absence of interior angular velocity contrasts, the magnetic fields possess strong mean (axisymmetric) components, which we attribute partly to the very strong influence of rotation upon the slowly overturning flows.

astro-ph↗
arXiv2006

Dynamo Action in the Solar Convection Zone and Tachocline: Pumping and Organization of Toroidal Fields

We present the first results from three-dimensional spherical shell simulations of magnetic dynamo action realized by turbulent convection penetrating downward into a tachocline of rotational shear. This permits us to assess several dynamical elements believed to be crucial to the operation of the solar global dynamo, variously involving differential rotation resulting from convection, magnetic pumping, and amplification of fields by stretching within the tachocline. The simulations reveal that strong axisymmetric toroidal magnetic fields (about 3000 G in strength) are realized within the lower stable layer, unlike in the convection zone where fluctuating fields are predominant. The toroidal fields in the stable layer possess a striking persistent antisymmetric parity, with fields in the northern hemisphere largely of opposite polarity to those in the southern hemisphere. The associated mean poloidal magnetic fields there have a clear dipolar geometry, but we have not yet observed any distinctive reversals or latitudinal propagation. The presence of these deep magnetic fields appears to stabilize the sense of mean fields produced by vigorous dynamo action in the bulk of the convection zone.

astro-ph↗
arXiv2004

Simulations of core convection and resulting dynamo action in rotating A-type stars

We present the results of 3--D nonlinear simulations of magnetic dynamo action by core convection within A-type stars of 2 solar masses, at a range of rotation rates. We consider the inner 30% by radius of such stars, with the spherical domain thereby encompassing the convective core and a portion of the surrounding radiative envelope. The compressible Navier-Stokes equations, subject to the anelastic approximation, are solved to examine highly nonlinear flows that span multiple scale heights, exhibit intricate time dependence, and admit magnetic dynamo action. Small initial seed magnetic fields are found to be amplified greatly by the convective and zonal flows. The central columns of strikingly slow rotation found in some of our progenitor hydrodynamic simulations continue to be realized in some simulations to a lesser degree, with such differential rotation arising from the redistribution of angular momentum by the nonlinear convection and magnetic fields. We assess the properties of the magnetic fields thus generated, the extent of convective penetration, the magnitude of the differential rotation, and the excitation of gravity waves within the radiative envelope.

astro-ph↗
arXiv2003

Simulations of core convection in rotating A-type stars: Differential rotation and overshooting

We present the results of 3--D simulations of core convection within A-type stars of 2 solar masses, at a range of rotation rates. We consider the inner 30% by radius of such stars, thereby encompassing the convective core and some of the surrounding radiative envelope. We utilize our anelastic spherical harmonic (ASH) code, which solves the compressible Navier-Stokes equations in the anelastic approximation, to examine highly nonlinear flows that can span multiple scale heights. The cores of these stars are found to rotate differentially, with central cylindrical regions of strikingly slow rotation achieved in our simulations of stars whose convective Rossby number (R_{oc}) is less than unity. Such differential rotation results from the redistribution of angular momentum by the nonlinear convection that strongly senses the overall rotation of the star. Penetrative convective motions extend into the overlying radiative zone, yielding a prolate shape (aligned with the rotation axis) to the central region in which nearly adiabatic stratification is achieved. This is further surrounded by a region of overshooting motions, the extent of which is greater at the equator than at the poles, yielding an overall spherical shape to the domain experiencing at least some convective mixing. We assess the overshooting achieved as the stability of the radiative exterior is varied, and the weak circulations that result in that exterior. The convective plumes serve to excite gravity waves in the radiative envelope, ranging from localized ripples of many scales to some remarkable global resonances.

astro-ph↗
arXiv2003

Looking Deep Within an A-type Star: Core Convection Under the Influence of Rotation

The advent of massively parallel supercomputing has begun to permit explicit 3--D simulations of turbulent convection occurring within the cores of early-type main sequence stars. Such studies should complement the stellar structure and evolution efforts that have so far largely employed 1--D nonlocal mixing length descriptions for the transport, mixing and overshooting achieved by core convection. We have turned to A-type stars as representative of many of the dynamical challenges raised by core convection within rotating stars. The differential rotation and meridional circulations achieved deep within the star by the convection, the likelihood of sustained magnetic dynamo action there, and the bringing of fresh fuel into the core by overshooting motions, thereby influencing main sequence lifetimes, all constitute interesting dynamical questions that require detailed modelling of global-scale convection. Using our anelastic spherical harmonic (ASH) code tested on the solar differential rotation problem, we have conducted a series of 3--D spherical domain simulations that deal with a simplified description of the central regions of rotating A-type stars, i.e a convectively unstable core is surrounded by a stable radiative envelope. A sequence of 3--D simulations are used to assess the properties of the convection (its global patterns, differential rotation, meridional circulations, extent and latitudinal variation of the overshooting) as transitions are made between laminar and turbulent states by changing the effective diffusivities, rotation rates, and subadiabaticity of the radiative exterior. We report on the properties deduced from these models for both the extent of penetration and the profile of rotation sustained by the convection.

astro-ph↗