Primordial Magnetic Field Amplification Research Articles

Non‐Linear Electrodynamics in Blandford–Znajek Energy Extraction

AbstractNon‐linear electrodynamics (NLED) is a generalization of Maxwell's electrodynamics for strong fields. It has significant implications for the study of black holes and cosmology and has been extensively studied in the literature, extending from quantum to cosmological contexts. In this work, two new ways to investigate these non‐linear theories are investigated. First, the Blandford‐Znajek mechanism is analyzed in light of this promising theoretical context, providing the general form of the extracted power up to second order in the black hole spin parameter a. It is found that, depending on the NLED model, the emitted power can be extremely increased or decreased, and that the magnetic field lines around the black hole seem to become vertical quickly. Considering only separated solutions, it is found that no monopole solutions exist and this could have interesting astrophysical consequences (not considered here). Last but not least, it is attempted to confine the NLED parameters by inducing the amplification of primordial magnetic fields (“seeds”), thus admitting non‐linear theories already during the early stages of the Universe. However, the latter approach proves to be useful for NLED research only in certain models. These (analytical) results emphasize that the behavior of non‐linear electromagnetic phenomena strongly depends on the physical context and that only a power‐law model seems to have any chance to compete with Maxwell.

On the amplification of magnetic fields in cosmic filaments and galaxy clusters★

The amplification of primordial magnetic fields via a small-scale turbulent dynamo during structure formation might be able to explain the observed magnetic fields in galaxy clusters. The magnetization of more tenuous large-scale structures such as cosmic filaments is more uncertain, as it is challenging for numerical simulations to achieve the required dynamical range. In this work, we present magnetohydrodynamical cosmological simulations on large uniform grids to study the amplification of primordial seed fields in the intracluster medium (ICM) and in the warm–hot-intergalactic medium (WHIM). In the ICM, we confirm that turbulence caused by structure formation can produce a significant dynamo amplification, even if the amplification is smaller than what is reported in other papers. In the WHIM inside filaments, we do not observe significant dynamo amplification, even though we achieve Reynolds numbers of Re ∼ 200–300. The maximal amplification for large filaments is of the order of ∼100 for the magnetic energy, corresponding to a typical field of a few ∼nG starting from a primordial weak field of 10−10 G (comoving). In order to start a small-scale dynamo, we found that a minimum of ∼102 resolution elements across the virial radius of galaxy clusters was necessary. In filaments we could not find a minimum resolution to set off a dynamo. This stems from the inefficiency of supersonic motions in the WHIM in triggering solenoidal modes and small-scale twisting of magnetic field structures. Magnetic fields this small will make it hard to detect filaments in radio observations.

Primordial magnetic field amplification from turbulent reheating

We analyze the possibility of primordial magnetic field amplification by astochastic large scale kinematic dynamo during reheating. We consider acharged scalar field minimally coupled to gravity. During inflation thisfield is assumed to be in its vacuum state. At the transition to reheatingthe state of the field changes to a many particle/anti-particle state. Wecharacterize that state as a fluid flow of zero mean velocity but with astochastic velocity field. We compute the scale-dependent Reynolds number Re(k), and the characteristic times for decay of turbulence, td andpair annihilation ta, finding ta << td. We calculate the rmsvalue of the kinetic helicity of the flow over a scale ℒ andshow that it does not vanish. We use this result to estimate theamplification factor of a seed field from the stochastic kinematic dynamoequations. Although this effect is weak, it shows that the evolution of thecosmic magnetic field from reheating to galaxy formation may well be morecomplex than as dictated by simple flux freezing.

Numerical study of primordial magnetic field amplification by inflation-produced gravitational waves

We numerically study the interaction of inflation-produced magnetic fields with gravitational waves, both of which originate from quantum fluctuations during inflation. The resonance between the magnetic field perturbations and the gravitational waves has been suggested as a possible mechanism for magnetic field amplification. However, some analytical studies suggest that the effect of the inflationary gravitational waves is too small to provide significant amplification. Our numerical study shows more clearly how the interaction affects the magnetic fields and confirms the weakness of the influence of the gravitational waves. We present an investigation based on the magnetohydrodynamic approximation and take into account the differences of the Alfven speed.

Amplification of primordial magnetic fields by anisotropic gravitational collapse

AbstractIf a magnetic field, B , is frozen into a plasma that undergoes spherical compression then B varies with the plasma density ρ according to B ∝ ρ 2/3. In the gravitational collapse of cosmological density perturbations, however, quasi‐spherical evolution is very unlikely. In anisotropic collapses the magnetic field can be a much steeper function of gas density than in the isotropic case. We investigate the distribution of amplifications in realistic gravitational collapses from Gaussian initial fluctuations using the Zel'dovich approximation. Representing our results using a relation of the form B ∝ ρ α , we show that the median value of α can be much larger than the α = 2/ 3 resulting from spherical collapse, even if there is no initial correlation between magnetic .eld and principal collapse directions. (© 2006 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Amplification of primordial magnetic fields by anisotropic gravitational collapse

If a magnetic field is frozen into a plasma that undergoes spherical compression then the magnetic field B varies with the plasma density \rho according to B \propto \rho^{2/3}. In the gravitational collapse of cosmological density perturbations, however, quasi-spherical evolution is very unlikely. In anisotropic collapses the magnetic field can be a much steeper function of gas density than in the isotropic case. We investigate the distribution of amplifications in realistic gravitational collapses from Gaussian initial fluctuations using the Zel'dovich approximation. Representing our results using a relation of the form B\propto \rho^{\alpha}, we show that the median value of \alpha can be much larger than the \alpha=2/3 resulting from spherical collapse, even if there is no initial correlation between magnetic field and principal collapse directions. These analytic arguments go some way towards understanding the results of numerical simulations.