The Dynamical State of the Interstellar Gas and Field

Parker, E. N.

doi:10.1086/148828

Cited by 693 publications

(557 citation statements)

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“…The top portion of the Ω-loop becomes denser than its surroundings as it approaches the top of the convective zone, its rise stops, and magnetic flux piles up under the photosphere. Here, the flux takes on an undulatory (serpentine) shape due to the magnetic Rayleigh-Taylor instability (Parker, 1966) and the effects of convective flows (Cheung et al, 2008(Cheung et al, , 2010.…”

Section: Peculiality Of Flux Emergence Phase: Serpentine Flux Tubesmentioning

confidence: 99%

Evolution of Active Regions

Driel-Gesztelyi

Green

2015

Living Rev. Sol. Phys.

250

131

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The evolution of active regions (AR) from their emergence through their long decay process is of fundamental importance in solar physics. Since large-scale flux is generated by the deepseated dynamo, the observed characteristics of flux emergence and that of the subsequent decay provide vital clues as well as boundary conditions for dynamo models. Throughout their evolution, ARs are centres of magnetic activity, with the level and type of activity phenomena being dependent on the evolutionary stage of the AR. As new flux emerges into a pre-existing magnetic environment, its evolution leads to re-configuration of small-and large-scale magnetic connectivities. The decay process of ARs spreads the once-concentrated magnetic flux over an ever-increasing area. Though most of the flux disappears through small-scale cancellation processes, it is the remnant of large-scale AR fields that is able to reverse the polarity of the poles and build up new polar fields. In this Living Review the emphasis is put on what we have learned from observations, which is put in the context of modelling and simulation efforts when interpreting them. For another, modelling-focused Living Review on the sub-surface evolution and emergence of magnetic flux see Fan (2009). In this first version we focus on the evolution of dominantly bipolar ARs. Article RevisionsLiving Reviews supports two ways of keeping its articles up-to-date:Fast-track revision. A fast-track revision provides the author with the opportunity to add short notices of current research results, trends and developments, or important publications to the article. A fast-track revision is refereed by the responsible subject editor. If an article has undergone a fast-track revision, a summary of changes will be listed here.Major update. A major update will include substantial changes and additions and is subject to full external refereeing. It is published with a new publication number.For detailed documentation of an article's evolution, please refer to the history document of the article's online version at http://dx

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Section: Peculiality Of Flux Emergence Phase: Serpentine Flux Tubesmentioning

confidence: 99%

Evolution of Active Regions

Driel-Gesztelyi

Green

2015

Living Rev. Sol. Phys.

250

131

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“…However, the wavelength of the most unstable mode, which is proportional to the field strength, becomes longer with the amplification of magnetic field, and it can be well resolved in the saturated state.The toroidal component is further amplified by the winding due to the radial differential rotation. Furthermore, magnetic buoyancy (Parker 1966) also plays a role in amplifying the vertical component (Nishikori, Machida & Matsumoto 2006;Machida et al 2013). Figure 2 presents the time evolution of the magnetic field strength averaged over azimuthal (φ) and vertical (z) directions,…”

Section: Overviewmentioning

confidence: 99%

Stochastic non-circular motion and outflows driven by magnetic activity in the Galactic bulge region

Suzuki

Fukui

Torii

et al. 2015

Mon. Not. R. Astron. Soc.

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By performing a global magneto-hydrodynamical simulation for the Milky Way with an axisymmetric gravitational potential, we propose that spatially dependent amplification of magnetic fields possibly explains the observed noncircular motion of the gas in the Galactic center region. The radial distribution of the rotation frequency in the bulge region is not monotonic in general. The amplification of the magnetic field is enhanced in regions with stronger differential rotation, because magnetorotational instability and field-line stretching are more effective. The strength of the amplified magnetic field reaches > ∼ 0.5 mG, and radial flows of the gas are excited by the inhomogeneous transport of angular momentum through turbulent magnetic field that is amplified in a spatially dependent manner. In addition, the magnetic pressuregradient force also drives radial flows in a similar manner. As a result, the simulated position-velocity diagram exhibits a time-dependent asymmetric parallelogram-shape owing to the intermittency of the magnetic turbulence; the present model provides a viable alternative to the bar-potential-driven model for the parallelogram-shape of the central molecular zone. This is a natural extension into the central few 100 pc of the magnetic activity, which is observed as molecular loops at radii from a few 100 pc to 1 kpc. Furthermore, the time-averaged net gas flow is directed outward, whereas the flows are highly time-dependent, which we discuss from a viewpoint of the outflow from the bulge.

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“…NEMPI is a convective type instability related to the interchange instability in plasmas (Tserkovnikov 1960;Newcomb 1961;Priest 1982) and the magnetic buoyancy instability in the astrophysical context (Parker 1966). The free energy in interchange and magnetic buoyancy instabilities is drawn from the gravitational field, while in NEMPI it is provided by the small-scale turbulence.…”

Section: Introductionmentioning

confidence: 99%

Properties of the negative effective magnetic pressure instability

et al. 2012

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As was demonstrated in earlier studies, turbulence can result in a negative contribution to the effective mean magnetic pressure, which, in turn, can cause a large-scale instability. In this study, hydromagnetic mean-field modelling is performed for an isothermally stratified layer in the presence of a horizontal magnetic field. The negative effective magnetic pressure instability (NEMPI) is comprehensively investigated. It is shown that, if the effect of turbulence on the mean magnetic tension force vanishes, which is consistent with results from direct numerical simulations of forced turbulence, the fastest growing eigenmodes of NEMPI are two-dimensional. The growth rate is found to depend on a parameter β characterizing the turbulent contribution of the effective mean magnetic pressure for moderately strong mean magnetic fields. A fit formula is proposed that gives the growth rate as a function of turbulent kinematic viscosity, turbulent magnetic diffusivity, the density scale height, and the parameter β . The strength of the imposed magnetic field does not explicitly enter provided the location of the vertical boundaries are chosen such that the maximum of the eigenmode of NEMPI fits into the domain. The formation of sunspots and solar active regions is discussed as possible applications of NEMPI.

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The Dynamical State of the Interstellar Gas and Field

Cited by 693 publications

References 0 publications

Evolution of Active Regions

Evolution of Active Regions

Stochastic non-circular motion and outflows driven by magnetic activity in the Galactic bulge region

Properties of the negative effective magnetic pressure instability

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