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Astrophysical Jets

Astrophysical jets are beautifully puzzling structures that have resisted comprehensive theoretical interpretation despite 30 years of observation. The bottom panel of Figure 1 presents a Hubble Space Telescope (HST) image of an extragalactic jet while the top panel shows HST images of galactic jets from young stellar objects (YSO). Microquasars (not shown) are yet another example of a (recently discovered) galactic jet. These astrophysical jets share three characteristics aside from the observed jet: (i) a spinning magnetic field; (ii) a strong magnetic field, and (iii) an accretion disk [25]. The consequence of an accretion disk is that as the disk collapses toward the central object (stellar or black hole) the disk material is heated and ionized providing a convenient source of hot plasma which is injected into the inhomogeneous magnetic field of the central attractor. The similarity of boundary conditions displayed by these example astrophysical jets suggests that each of these systems might be influenced by a QNC mechanism allowing these disparate systems to produce similar jets.

All these jets are observed at scales much larger than the central magnetic field, so there is little data on the structure of the magnetic field. However, we can infer from variations in the jet brightness that the acceleration region is highly compact, which is ideal for producing large $\nabla
B$-drifts [26,27]. The plasma that is injected at the magnetic equator of the accretion disk would then separate via the QNC mechanism into a heavy core of ions grouped around the equator and a hot halo of electrons that bounce vigorously around the ions. This is analogous to an ambipolar electric field that forms when an isothermal plasma is placed in a gravitational potential, only in this case, it is the mirror force plus the centrifugal barrier that plays the part of the gravitational well. The symmetry of the magnetic dipole field results in an ambipolar electric quadrupole, where the perpendicular energy of the plasma determines the potential difference between the equator and the poles.

The result of this quadrupole is the acceleration of positive particles from the equator toward the poles. For example, should the potential exceed 1.022 MV, electron-positron pairs would be formed on collisions of the electrons with the accretion disk, resulting in positrons being accelerated toward the poles. As the positron gained energy, it would get less magnetized and eventually escape from the dipole field. (This does not occur for the electrons, because they decelerate as they move away from the equator, becoming more highly magnetized.) Such a demagnetization would be enhanced at a kink in the field, which itself may be the result of the extended electron cloud. The interaction of relativistic plasma with magnetic fields is an area of ongoing research, but the overall effect is predictable, a pair of beams of matter emanating perpendicular to the accretion plane, and accelerated to high velocity. Since nothing in our QNC model specifies the maximum perpendicular energy (as long as it is finite), one would imagine that an astrophysical instability would be driven until it saturates.

We note that the parallel electric field produced by separation of charge in a magnetic field also causes a perpendicular or polarization electric field. It is precisely because the first order drift of plasma in such a polarization electric field, $v_d = E \times B$ is perpendicular to the electric field that the plasma is not able to short out the polarization field, which is what sets up the equilibrium between the mirror force and the space charge in the first place. However, when the perpendicular field is strong enough, second order drifts, $v_{dd}=(E \times B) \times B$ are in the same direction as the applied electric field and the polarization field can short out the space charge. Thus the QNC mechanism saturates as some power of the space charge, which occurs before other saturation mechanisms such as demagnetization.

Scaling this second order drift as described by Rothwell [28] to astrophysical dimensions predicted correctly that AGN's, with a 1 G field extending 1 AU in diameter, should have a jet energy of $\sim$ 1 GeV, whereas YSO's should have keV jet speeds. This excellent agreement over many orders of magnitude encouraged us to build a table top laboratory experiment to produce parallel potentials using a spinning magnet and a DC plasma injector.


next up previous
Next: Experimental Setup Up: The spinning terrella plasma Previous: The Earth's Magnetosphere
Rob Sheldon 2001-03-09