A Global Galactic Dynamo With a Corona Constrained by Relative Helicity

Prasad, A.; Mangalam, A.

doi:10.3847/0004-637x/817/1/12

Cited by 9 publications

(11 citation statements)

References 73 publications

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“…We separately studied the influence of differential rotation on the mean field dynamo solutions as a function of the SN input rate and the rotation period when these quantities are taken to be independent and also when they are proportional to each other. The latter would be expected from correlations of the SN rate with the star formation rate and in turn, the galactic surface density and rotation rate (Prasad & Mangalam 2016).…”

Section: Discussionmentioning

confidence: 93%

“…In the cases considered above, we have assumed that the eddy time and the rotation periods are independent but in practice, models of star formation rates (SFR) in galaxies both depend on radius. We now suppose that τ ed varies with the radial coordinate r according the the prescription adopted by Prasad & Mangalam (2016). Specifically, we adopt the relation…”

Section: Case When Correlation Time and ω Both Depend On Rmentioning

confidence: 99%

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Some consequences of shear on galactic dynamos with helicity fluxes

Zhou¹,

Blackman²

2017

Monthly Notices of the Royal Astronomical Society

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Galactic dynamo models sustained by supernova (SN) driven turbulence and differential rotation have revealed that the sustenance of large scale fields requires a flux of small scale magnetic helicity to be viable. Here we generalize a minimalist analytic version of such galactic dynamos to explore some heretofore unincluded contributions from shear on the total turbulent energy and turbulent correlation time, with the helicity fluxes maintained by either winds, diffusion, or magnetic buoyancy. We construct an analytic framework for modeling the turbulent energy and correlation time as function of SN rate and shear. We compare our prescription with previous approaches that only include rotation. The solutions depend separately on the rotation period and the eddy turnover time and not just on their ratio (the Rossby number). We consider models in which these two time scales are allowed to be independent and also a case in which they are mutually dependent on radius when a radial dependent SN rate model is invoked. For the case of a fixed rotation period (or fixed radius) we show that the influence of shear is dramatic for low Rossby numbers, reducing the correlation time of the turbulence, which in turn, strongly reduces the saturation value of the dynamo compared to the case when the shear is ignored. We also show that even in the absence of winds or diffusive fluxes, magnetic buoyancy may be able to sustain sufficient helicity fluxes to avoid quenching.

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Section: Discussionmentioning

confidence: 93%

Section: Case When Correlation Time and ω Both Depend On Rmentioning

confidence: 99%

Some consequences of shear on galactic dynamos with helicity fluxes

Zhou¹,

Blackman²

2017

Monthly Notices of the Royal Astronomical Society

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“…Sellwood & Balbus (1999) have shown that the existence of the high H i velocity dispersion in the extra stellar disk of the spiral galaxies can be produced by extraction of the energy from galactic differential rotation through magneto-hydro dynamic turbulence. If MRI causes turbulence at large galactic radii, an associated α effect may develop, and regular fields will also be seen out to such large radii (Prasad & Mangalam, 2016). A much larger sample size than discussed above will help to confirm such a conclusion.…”

Section: Comparison Of Disk Magnetic Fields Observations With Theoretmentioning

confidence: 87%

“…Mean field models with helicity fluxes can avoid catastrophic quenching of the dynamo, and the simplest example of such a flux arises from advection of the field through galactic outflows (Shukurov et al, 2006b). In the case of quasi-linear dynamo model (Prasad & Mangalam, 2016), a global axisymmetric turbulent dynamo operates in a galaxy with a corona. It treats the supernovae (SNe) and magneto-rotational instability (MRI) driven turbulence parameters under a common formalism where the diffusivity and alpha effect have radial variation as the shear.…”

Section: Probing Magnetic Fields At High Redshiftsmentioning

confidence: 99%

Probing Magnetic Fields with Square Kilometre Array and its Precursors

et al. 2016

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Origin of magnetic fields, its structure and effects on dynamical processes in stars to galaxies are not well understood. Lack of a direct probe has remained a problem for its study. The first phase of Square Kilometre Array (SKA-I), will have almost an order of magnitude higher sensitivity than the best existing radio telescope at GHz frequencies. In this contribution, we discuss specific science cases that are of interest to the Indian community concerned with astrophysical turbulence and magnetic fields. The SKA-I will allow observations of a large number of background sources with detectable polarization and measure their Faraday depths (FDs) through the Milky Way, other galaxies and their circum-galactic mediums. This will probe line-of-sight magnetic fields in these objects well and provide field configurations. Detailed comparison of observational data (e.g., pitch angles in spirals) with models which consider various processes giving rise to field amplification and maintenance (e.g., various types of dynamo models) will then be possible. Such observations will also provide the coherence scale of the fields and its random component through RM structure function. Measuring the random component is important to characterise turbulence in the medium. Observations of FDs with redshift will provide important information on magnetic field evolution as a function of redshift. The background sources could also be used to probe magnetic fields and its coherent scale in galaxy clusters and in bridges formed between interacting galaxies. Other than FDs, sensitive observations of synchrotron emission from galaxies will provide complimentary information on their magnetic field strengths in the sky plane. The core shift measurements of AGNs can provide more precise measurements of magnetic field in the sub parsec region near the black hole and its evolution. The low band of SKA-I will also be useful to study circularly polarized emission from Sun and comparing various models of field configurations with observations.

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“…S1 corresponding to Table 4. 5. Preliminary calculations using the constraint of relative helicity based on the formulations given in (Prasad et al 2014;Prasad & Mangalam 2016) and applying the constrained energy minimization principle (Mangalam & Krishan 2000;Finn & Antonsen 1983;Taylor 1974) indicate that stable configurations are possible for some regions in the parameter space of {B 0 , R}. We plan a complete solution of this allowed region and test it with numerical simulations in a paper in preparation.…”

Section: Discussionmentioning

confidence: 99%

Open and Closed Magnetic Configurations of Twisted Flux Tubes

Sen

Mangalam

2019

ApJ

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We construct two classes of magnetohydrostatic (MHS) equilibria for an axisymmetric vertical flux tube spanning from the photosphere to the lower part of the transition region within a realistic stratified solar atmosphere subject to solar gravity. We assume a general quadratic expression of the magnetic flux function for the gas pressure and poloidal current and solve the Grad-Shafranov equation analytically. The solution is a combination of a homogeneous and a particular part where the former is separable by a Coulomb function in r and exponential in z, while the particular part is an open configuration that has no z dependence. We also present another open field solution by using a self-similar formulation with two different profile functions and incorporating stratified solar gravity to maintain the magnetohydrostatic equilibria, which is a modification of earlier self-similar models with a twist. We study the admitted parameter space that is consistent with the conditions in the solar atmosphere and derive magnetic and the thermodynamic structures inside the flux tube that are reasonably consistent with the photospheric magnetic bright points (MBPs) for both open and closed field Coulomb function and self-similar models as estimated from observations and simulations. The obtained open and closed field flux tube solutions can be used as the background conditions for the numerical simulations for the study of the wave propagation through the flux tubes. The solutions can also be used to construct realistic magnetic canopies.

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A Global Galactic Dynamo With a Corona Constrained by Relative Helicity

Cited by 9 publications

References 73 publications

Some consequences of shear on galactic dynamos with helicity fluxes

Some consequences of shear on galactic dynamos with helicity fluxes

Probing Magnetic Fields with Square Kilometre Array and its Precursors

Open and Closed Magnetic Configurations of Twisted Flux Tubes

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