Helical Velocity Field Research Articles

Vortex simulations on a 3-sphere

We generate vortex tangles using a Hopf flow on a 3-sphere, in place of the standard torus defined by periodic boundary conditions. These tangles are highly anisotropic, with vortices tending to align along the flow direction. Standard power law dependences change accordingly from their values in more isotropic tangles. The line length density $\langle L\rangle$ is proportional to $v_{ns}^{1.28}$, where $v_{ns}$ is the drive velocity, and the reconnection rate depends roughly on $\langle L\rangle^2$. We also discuss the effect of the full Biot-Savart law versus the local induction approximation (LIA). Under LIA the tangle collapses so that all vortices are nearly aligned with a single flow line, in sharp contrast to the torus where they become perpendicular to the driving velocity. Finally we present a few torus simulations with a helical velocity field, which in some ways resembles the 3-sphere flow.

The Alpha-Effect by Supernova Explosions

The turbulence in galaxies is assumed to be driven by a sample of stochastically occurring supernova explosions. For simplicity, we replace the complicated real flow by the velocity field of the explosions themselves. The resulting turbulence model can be used to determine the turbulent electromotive force, < u′ × B′ >, a basic quantity in the dynamo theory.If the distribution function of the SN explosion rate is known, it is sufficient to compute the α-effect for a single explosion in a rotating, density stratified galaxy. We use an axisymmetric simulation code to determine the helical velocity field of the explosion, and consider only explosions located on the rotation axis. The (homogeneous) magnetic field is assumed to be purely vertical. Therefore, only the αzz-component of the α-tensor can be obtained with this method.The induced magnetic field fluctuations must be correlated with the velocity field in order to get the turbulent electromagnetic force. The resulting α-effect for a sample of explosions starting in the galactic midplane possesses a z-profile with an amplitude of about 60m/s. It is always negative in the northern hemisphere and positive in the southern hermisphere. The scale of the α-profile is about 60pc. The influence of the density distribution proved to be small. We consider the extreme case of uniform explosion rate and find α-values of only few meters per second for a density scale height of 100pc.

Hollow Cylindrical Lobes with a Helical Velocity Field of the L1551 Bipolar Flow

Observations of the structure and the velocity field in the L1551 bipolar flow were made with the 45m telescope at Nobeyama in the 115GHz 12CO J = 1 – 0 line with high spatial resolution. It was found that the bipolar flow lobes have a clear hollow cylindrical structure and show evidence of a helical velocity field. They appear to rotate in the same direction as the CS disk found by Kaifu et al. (1984). The velocity of the flow in the bipolar directions increases with distance up to ∼ 3′ from the central object, IRS 5. These characteristics coincide with those predicted by the magnetodynamic theory proposed by Uchida and Shibata and indicate the essential importance of the magnetic field in producing such flows and also in the star-formation process itself through the enhancement of angular-momentum loss.

Evidence for helical velocity field in molecular bipolar flows - Support for magnetodynamic model

A search for the helical velocity field that had been predicted in a magnetodynamic theory of Uchida and Shibata was made in the bipolar flows L1551 by using 12CO 115 GHz line, and evidence was obtained for it in the low velocity maps as the skew inter-invasion of the root part of the blue- and red shifted lobes into different sides of the opposite lobes. Theoretical implications of this and other findings are discussed, and the advantage of models with magnetic field is stressed.

Magnetodynamical acceleration of cosmic jets: sweeping-magnetic-twist mechanism

Characteristic behavior of cosmic jets predicted by a magnetodynamic mechanism proposed by Uchida and Shibata is discussed in terms of recent observational results of bipolar flows from star-forming regions as examples of low-energy cases. The theoretical model considers the twisting-up of part of the large-scale magnetic field with the driving mechanism being the contracting rotation of the accretion disk around the gravitating center. The twisted field screws out the mass from the surface layers of the disk along the large-scale external field, explaining the observed tuning-fork type of distribution of the cold CO bipolar flows, gradual acceleration of the flows from the vicinity of the disk, and the helical velocity field in the outflows, all of which are not easy to explain by previous hypotheses assuming the wind and blast from the central object. Prospects of application of this mechanism to high-energy jets from active galactic nuclei or such peculiar objects in the galaxy like SS433 or Sco X-1 are discussed from the point of view of the similarity inherent in the mechanism.

Sweeping-magnetic-twist mechanism for the acceleration of jets in the solar atmosphere

A magnetodynamic mechanism for the acceleration of jets in the solar atmosphere (surges, Brueckner's EUV jets, and so on) is proposed, and a 2.5-dimensional MHD simulation is performed to show how this mechanism operates in the situation of the chromosphere-corona region of the solar atmosphere. It is seen from the result of simulation that together with the release of the magnetic twist, e.g., into a reconnected open flux tube, the mass in the high density twisted loop is driven out into the open flux tube due both to the pinch effect progressing with the packet of the magnetic twist into the open flux tube, and to the j × B force at the front of the packet of the unwinding twist in the off-axis part of the tube. The former, the progressing pinch, is accompanied by an accelerated hot blob, while the latter, the unwinding front of the magnetic twist, drives a cool cylindrical flow, both with velocities of the order of the local Alfven velocity. One of the characteristic properties of the jet in our model is that the jet, consisting of hot core and cool sheath, has a helical velocity field in it, explaining the thus-far unexplained observed feature. The sudden release of the magnetic twist into an open flux tube is most likely to be due to the reconnection between a twisted loop and the open flux tube. The mass is driven out in the relaxation process of the magnetic twist from the twisted loop to the open flux tube.

Origin of the galactic Centre lobes

Recent observations of the 10-GHz continuum1 have indicated that there is a pair of lobes having ridges of intensity extending from the galactic centre region in a direction perpendicular to the galactic plane. This observation has attracted considerable attention as it is clearly an indication of dynamic processes occurring at the centre of our Galaxy that are similar to those occurring in the nuclei of radio galaxies, though different in scale and in strength2. Here we interpret these lobes as being due to a mag-netodynamic acceleration mechanism in which the production and relaxation of the magnetic twist induced by the rotation of the contracting gas disk play a part. The plasma is accelerated in a conical cylinder with a helical velocity field, reproducing the observed feature of radio lobes.