Astronomy and Astrophysics – Astronomy
Scientific paper
Dec 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993apj...418..709l&link_type=abstract
Astrophysical Journal v.418, p.709
Astronomy and Astrophysics
Astronomy
12
Galaxies: Jets, Ism: Individual Name: Crab Nebula, Magnetohydrodynamics: Mhd, Relativity, Stars: Mass Loss
Scientific paper
Two major classes of astrophysical systems are believed to involve magnetized relativistic outflows, viz., pulsar winds from pulsars and extragalactic radio jets from active galactic nuclei (AGNs). We study here some basic properties of a three-dimensional propagation of transverse incompressible perturbations and the associated second-order nonlinear effect in a relativistic magnetohydrodynamic (RMHD) radial outflow with spherical symmetry. Analytical solutions are derived in the high- and low-frequency limits, and in the asymptotic regime of large radial distance r, respectively. Numerical solutions are obtained to complement the analytical analysis. In essence, these perturbations are Alfvénic fluctuations. Several major results are as follows. The first one is that the transverse magnetic field perturbation b associated with Alfvénic fluctuations has a slower radial falloff at large r as compared with the background radial magnetic field B0 ∝ r-2. Since RMHD wind sources are expected to be noisy in general, this result implies that random transverse magnetic fields should have prominent observable effects in magnetized outflows far way from the central source. The second effect is due to the significant electric charge separation and thus the associated transverse electric force in the relativistic regime. This electric force always counteracts the magnetic pinching and in the extreme relativistic limit, these two forces nearly cancel each other. The third effect is associated with the significant displacement current in the relativistic regime so that a characteristic frequency ƒc determined by the outflow parameters can be extremely high. For a perturbation frequency ƒ higher than ƒc, the magnitude of b scales as ˜r-3/2; and for ƒ lower than ƒc, the magnitude of b scales as ˜r-3/2±1/2(1-ƒ2/ƒc2)1/2 at large r. In the contexts of the RMHD pulsar wind scenario for energizing the Crab Nebula and of the RMHD outflow models for AGN jets, we propose that the Poynting flux associated with Alfvénic fluctuations in a RMHD outflow is an integral part for effective transport of energy and momentum.
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