arXiv subjects

K. Subramanian

Publications and source records attributed to K. Subramanian.

13 recordsLinked to original sources

arXiv2018

Varying the forcing scale in low Prandtl number dynamos

Small-scale dynamos are expected to operate in all astrophysical fluids that are turbulent and electrically conducting, for example the interstellar medium, stellar interiors, and accretion disks, where they may also be affected by or competing with large-scale dynamos. However, the possibility of small-scale dynamos being excited at small and intermediate ratios of viscosity to magnetic diffusivity (the magnetic Prandtl number) has been debated, and the possibility of them depending on the large-scale forcing wavenumber has been raised. Here we show, using four values of the forcing wavenumber, that the small-scale dynamo does not depend on the scale-separation between the size of the simulation domain and the integral scale of the turbulence, i.e., the forcing scale. Moreover, the spectral bottleneck in turbulence, which has been implied as being responsible for raising the excitation conditions of small-scale dynamos, is found to be invariant under changing the forcing wavenumber. However, when forcing at the lowest few wavenumbers, the effective forcing wavenumber that enters in the definition of the magnetic Reynolds number is found to be about twice the minimum wavenumber of the domain. Our work is relevant to future studies of small-scale dynamos, of which several applications are being discussed.

physics.flu-dyn↗

arXiv2008

Nonlinear magnetic diffusivity and alpha tensors in helical turbulence

The effect of a dynamo-generated mean magnetic field of Beltrami type on the mean electromotive force is studied. In the absence of the mean magnetic field the turbulence is assumed to be homogeneous and isotropic, but it becomes inhomogeneous and anisotropic with this field. Using the testfield method the dependence of the alpha and turbulent diffusivity tensors on the magnetic Reynolds number Rm is determined for magnetic fields that have reached approximate equipartition with the velocity field. The tensor components are characterized by a pseudoscalar alpha and a scalar turbulent magnetic diffusivity etat. Increasing Rm from 2 to 600 reduces etat by a factor ~5, suggesting that the quenching of etat is, in contrast to the 2-dimensional case, only weakly dependent on Rm. Over the same range of Rm, however, alpha is reduced by a factor ~14, which can qualitatively be explained by a corresponding increase of a magnetic contribution to the alpha effect with opposite sign. The level of fluctuations of alpha and etat is only 10% and 20% of the respective kinematic reference values.

astro-ph↗

arXiv2007

Simulations of the anisotropic kinetic and magnetic alpha effects

The validity of a closure called the minimal tau approximation (MTA), is tested in the context of dynamo theory, wherein triple correlations are assumed to provide relaxation of the turbulent electromotive force. Under MTA, the alpha effect in mean field dynamo theory becomes proportional to a relaxation time scale multiplied by the difference between kinetic and current helicities. It is shown that the value of the relaxation time is positive and, in units of the turnover time at the forcing wavenumber, it is of the order of unity. It is quenched by the magnetic field -- roughly independently of the magnetic Reynolds number. However, this independence becomes uncertain at large magnetic Reynolds number. Kinetic and current helicities are shown to be dominated by large scale properties of the flow.

astro-ph↗

arXiv2006

The origin and evolution of cluster magnetism

Random motions can occur in the intergalactic gas of galaxy clusters at all stages of their evolution. Depending on the poorly known value of the Reynolds number, these motions can or cannot become turbulent, but in any case they can generate random magnetic fields via dynamo action. We argue that magnetic fields inferred observationally for the intracluster medium require dynamo action, and then estimate parameters of random flows and magnetic fields at various stages of the cluster evolution. Polarization in cluster radio halos predicted by the model would be detectable with the SKA.

astro-ph↗

arXiv2005

Minimal tau approximation and simulations of the alpha effect

astro-ph↗

arXiv2002

Magnetic helicity in stellar dynamos: new numerical experiments

The theory of large scale dynamos is reviewed with particular emphasis on the magnetic helicity constraint in the presence of closed and open boundaries. In the presence of closed or periodic boundaries, helical dynamos respond to the helicity constraint by developing small scale separation in the kinematic regime, and by showing long time scales in the nonlinear regime where the scale separation has grown to the maximum possible value. A resistively limited evolution towards saturation is also found at intermediate scales before the largest scale of the system is reached. Larger aspect ratios can give rise to different structures of the mean field which are obtained at early times, but the final saturation field strength is still decreasing with decreasing resistivity. In the presence of shear, cyclic magnetic fields are found whose period is increasing with decreasing resistivity, but the saturation energy of the mean field is in strong super-equipartition with the turbulent energy. It is shown that artificially induced losses of small scale field of opposite sign of magnetic helicity as the large scale field can, at least in principle, accelerate the production of large scale (poloidal) field. Based on mean field models with an outer potential field boundary condition in spherical geometry, we verify that the sign of the magnetic helicity flux from the large scale field agrees with the sign of alpha. For solar parameters, typical magnetic helicity fluxes lie around 10^{47} Mx^2 per cycle.

astro-ph↗

arXiv2000

Sheared helical turbulence and the helicity constraint in large-scale dynamos

The effect of shear on the growth of large scale magnetic fields in helical turbulence is investigated. The resulting large-scale magnetic field is also helical and continues to evolve, after saturation of the small scale field, on a slow resistive time scale. This is a consequence of magnetic helicity conservation. Because of shear, the time scale needed to reach an equipartition-strength large scale field is shortened proportionally to the ratio of the resulting toroidal to poloidal large scale fields.

astro-ph↗

arXiv1998

A Vishniac type contribution to the polarisation of the CMBR?

Radiation which has a quadrupole component of anisotropy, can get polarized by Thomson scattering from charged particles. In the cosmological context, the microwave background photons develop significant quadrupole anisotropy as they free stream away from the the epoch of standard recombination. Reionization in the post recombination era can provide free electrons to Thomson scatter the incident anisotropic $CMBR$ photons. We compute the resulting polarisation anisotropy on small (arc-minute) angular scales. We look for significant non-linear contributions, as in the case of Vishniac effect in temperature anisotropy, due to the coupling of small-scale electron density fluctuations, at the new last scattering surface, and the temperature quadrupole. We show that, while, in cold dark matter type models, this does not lead to very significant signals ($\sim 0.02 - 0.04 μK$), a larger small angular scale polarization anisotropy, ($\sim 0.1 - 0.5μK$), can result in isocurvature type models.

astro-ph↗

arXiv1997

Dynamics of fluctuating magnetic fields in turbulent dynamos incorporating ambipolar drifts

Turbulence with a large magnetic Reyonolds number, generically leads to rapidly growing magnetic noise over and above any mean field. We revisit the dynamics of this fluctuating field, in homogeneous, isotropic, helical turbulence. Assuming the turbulence to be Markovian, we first rederive, in a fairly transparent manner, the equation for the mean field, and corrected Fokker-Plank type equations for the magnetic correlations. In these equations, we also incorporate the effects of ambipolar drift which would obtain if the turbulent medium has a significant neutral component. We apply these equations to discuss a number of astrophysically interesting problems: (a) the small scale dynamo in galactic turbulence with a model Kolmogorov spectrum, incorporating the effect of ambipolar drift; (b) current helicity dynamics and the quasilinear corrections to the alpha effect; (c) growth of the current helicity and large-scale magnetic fields due to nonlinear effects.

astro-ph↗

arXiv1997

Can the turbulent galactic dynamo generate large-scale magnetic fields?

Large-scale magnetic fields in galaxies are thought to be generated by a turbulent dynamo. However the same turbulence also leads to a small-scale dynamo which generates magnetic noise at a more rapid rate. The efficiency of the large-scale dynamo depends on how this noise saturates. We examine this issue taking into account ambipolar drift, which obtains in a galaxy with significant neutral gas. We argue that, (1) the small-scale dynamo generated field does not fill the volume, but is concentrated into intermittent rope like structures. The flux ropes are curved on the turbulent eddy scales. Their thickness is set by the diffusive scale determined by the effective ambipolar diffusion; (2) For a largely neutral galactic gas, the small-scale dynamo saturates, due to inefficient random stretching, when the peak field in a flux rope has grown to a few times the equipartition value; (3) The average energy density in the saturated small-scale field is sub equipartition, since it does not fill the volume; (4) Such fields neither drain significant energy from the turbulence nor convert eddy motion of the turbulence on the outer scale into wavelike motion. The diffusive effects needed for the large-scale dynamo operation are then preserved until the large-scale field itself grows to near equipartition levels.

astro-ph↗

arXiv1996

The origin of large scale magnetic fields

Magnetic fields correlated on several kiloparsec scales are seen in spiral galaxies. Their origin could be due to the winding up of a primordial cosmological field or due to amplification of a small seed field by a turbulent galactic dynamo. Both options have difficulties: There is no known battery mechanism for producing the required primordial field. Equally the turbulent dynamo may self destruct before being able to produce the large scale field, due to excess generation of small scale power. The current status of these difficulties is discussed. The resolution could depend on the nature of the saturated field produced by the small scale dynamo. We argue that the small scale fields do not fill most of the volume of the fluid and instead concentrate into intermittent ropes, with their peak value of order equipartition fields, and radii much smaller than their lengths. In this case these fields neither drain significant energy from the turbulence nor convert eddy motion of the turbulence on the outer scale to wave like motion. This preserves the diffusive effects needed for the large scale dynamo operation.

astro-ph↗

arXiv1994

Constraints on the Models for Structure Formation from the Abundance of Damped Lyman Alpha Systems

Models for structure formation attempt to predict the power spectrum of density perturbations in the present universe from the initial power spectrum and the nature of dark matter. Observational constraints on the power spectrum at different scales in the present epoch can, therefore, be used to eliminate (or choose between) different theoretical models. Such a comparison is fairly easy at large scales (at which linear theory is valid), and one can use observations like the MBR anisotropy, large scale steaming motions etc to constrain the models. But to discriminate between the models effectivley, it is necessay to constrain the power spectrum at small scales. The most reliable constraints on the power spectra at small scales come from the predicted abundance of bound systems which can be estimated reasonably accurately using Press-Schecter (or similar) methods$^1$. In the past, this method has been used in conjunction with the quasar abundance$^{2-4}$ and cluster abundance$^{5-7}$. We show here that the abundance of damped lyman alpha systems (DLAS, hereafter), provides a far stronger constraint on the models for structure formation. Models with a mixture of hot and cold dark matter $^{8-11}$ (which are consistent with large scale observations) are strongly ruled out by the DLAS constraints while models with cosmological constant $^{12}$ are marginally inconsistent. It is also possible to combine the constraints from the abundance of clusters, DLAS and QSO's to obtain model-independent bounds on the power spectrum at the nonlinear scales. These bounds are to be respected by any viable model for structure formation.

astro-ph↗

arXiv1993

Baryonic Content of Galactic Halos and Constraints on Models for Structure Formation

The recent detection of microlensing of stars of LMC by compact objects in the halo of our galaxy suggests that our galaxy is surrounded by a non-luminous halo made of compact objects with mass of about $(0.03-0.5) \msun$. The rate of detection could be consistent with the assumption that these halo objects are distributed with a softened isothermal profile with a core radius of $(2-8)$Kpc and asymptotic circular velocity of $220\kms$. Taken in isolation, this observation is consistent with a universe having only baryonic dark matter (BDM, hereafter) contributing $Ω_b=Ω_{total}\simeq0.06$ which will, however, violently contradict several other large scale observations, notably the COBE-DMR results. Alternatively one can assume that galaxies like ours are surrounded by both BDM and non-baryonic dark matter (NBDM, hereafter). A model with a single component for NBDM with, say, $Ω_b\simeq0.06,Ω_{cdm}\simeq0.94$, is also ruled out if we demand that: (i) at least thirty percent of the dark matter density within $100$ kpc is baryonic and (ii) galactic structures should have collapsed by redshift of $z=1$. Further if half or more of the dark matter within $100 kpc$ is baryonic then the maximum value of $Ω_{dm}$, contributed by NBDM clustered at galactic scales, can only be about $Ω_{max}\simeq0.29$. Models with C+HDM cannot satisfy this constraint but $Λ$+ CDM models are still

astro-ph↗