Significant Field Amplification Research Articles

Combined dynamo of gravitational and magneto-rotational instability in irradiated accretion discs

Aims. We aim to assess whether magneto-rotational instability (MRI) can exist in a turbulent state generated by gravitational instability (GI). We investigated the magnetic field saturation and elucidated the ability of GI turbulence to act as a dynamo. Methods. The results were obtained by numerical simulations using the magnetohydrodynamics code Athena. A sub-routine to solve the Poisson equation for self-gravity using three-dimensional Fourier transforms was implemented for that purpose. A GI-turbulent state was then restarted, with a zero-net-flux type magnetic seed field being introduced. The seed field was chosen with β ≈ 1010 to make sure that the magnetic field of the stationary state is exclusively generated by the dynamo. Results. Shortly after introducing the magnetic seed field, a significant field amplification is observed, despite MRI not being active. This shows that GI acts as a kinematic dynamo. The growing magnetic field allows MRI to become active, which leads to the emergence of a butterfly diagram. The turbulent stress of the saturated state is found to be consistent with the superposition of GI stresses and MRI stresses. Moreover, the ratio of magnetic stress to magnetic pressure is found to lie in the 0.3−0.4 range, which is typical for MRI turbulence. Furthermore, it is found that the magnetic energy significantly decreases if self-gravity is turned off. This indicates, in accordance with the initial field amplification, that GI provides the dominant dynamo contribution and that MRI is not simply added but rather grows on the magnetic field provided by GI turbulence. Finally, it is shown that the combined GI-MRI-dynamo is consistent with an α − Ω model and that the observed oscillation frequency of the butterfly diagram roughly agrees with the model prediction.

Effects of Large-Scale Simple Velocity Shear on a Fluctuating Interplanetary Magnetic Field

The effects of large-scale simple velocity shear on a magnetic field in a low-β MHD fluid are examined in the limit that the magnetic field does not significantly affect the fluid flow. The large-scale fluid shear stretches the magnetic field and can cause significant field amplification. Application to the solar wind suggests that the latitudinal shear observed may contribute significantly to the observed periods of underwound or radial magnetic field.

Magnetic field amplification and saturation in turbulence behind a relativistic shock

We have investigated via two-dimensional relativistic MHD simulations the long-term evolution of turbulence created by a relativistic shock propagating through an inhomogeneous medium. In the postshock region, magnetic field is strongly amplified by turbulent motions triggered by preshock density inhomogeneities. Using a long-simulation box we have followed the magnetic-field amplification until it is fully developed and saturated. The turbulent velocity is sub-relativistic even for a strong shock. Magnetic-field amplification is controled by the turbulent motion and saturation occurs when the magnetic energy is comparable to the turbulent kinetic energy. Magnetic-field amplification and saturation depend on the initial strength and direction of the magnetic field in the preshock medium, and on the shock strength. If the initial magnetic field is perpendicular to the shock normal, the magnetic field is first compressed at the shock and then can be amplified by turbulent motion in the postshock region. Saturation occurs when the magnetic energy becomes comparable to the turbulent kinetic energy in the postshock region. If the initial magnetic field in the preshock medium is strong, the postshock region becomes turbulent but significant field amplification does not occur. If the magnetic energy after shock compression is larger than the turbulent kinetic energy in the postshock region, significant field amplification does not occur. We discuss possible applications of our results to gamma-ray bursts and active galactic nuclei.

The influences of particle number on hot spots in strongly coupled metal nanoparticles chain

In understanding of the hot spot phenomenon in single-molecule surface enhanced Raman scattering (SM-SERS), the electromagnetic field within the gaps of dimers (i.e., two particle systems) has attracted much interest as it provides significant field amplification over single isolated nanoparticles. In addition to the existing understanding of the dimer systems, we show in this paper that field enhancement within the gaps of a particle chain could maximize at a particle number N>2, due to the near-field coupled plasmon resonance of the chain. This particle number effect was theoretically observed for the gold (Au) nanoparticles chain but not for the silver (Ag) chain. We attribute the reason to the different behaviors of the dissipative damping of gold and silver in the visible wavelength range. The reported effect can be utilized to design effective gold substrate for SM-SERS applications.