Martin Erick Sulkanen

Self-collimated electromagnetic jets from magnetized accretion disks

The global electrodynamics of a viscous resistive accretion disk around a Schwarzschild black hole with a force-free plasma outside of the disk is worked out. The magnetic field in the disk is assumed to include a well-ordered component. The magnetic field and fluid dynamics of the disk are tested, simplifying the induction equation and solving for the flux function and the toroidal magnetic field. A Greens function method is used to obtain far-field solutions of the basic electromagnetic field equation for the plasma outside of the disk. The solutions are found to include self-collimated electromagnetic jets. The overall energy conservation for the disk-jet system is considered, and a global condition is derived which imposes an upper bound on the reaction of the accretion luminosity which can be carried away by the jets. 36 references.

Where have all the cluster halos gone

A new LF (330 MHz) VLA image of the Perseus cluster confirms the presence of a miniradio halo with diameter of about 430 kpc (H0 = 75 km/s Mpc) surrounding 3C 84. A careful comparison with the Coma cluster shows that there is no evidence for a similar, very extended halo in Perseus despite the large number of cluster radio galaxies which could power such a halo. These two clusters represent two classes of radio halos which differ by the absence (Coma) or presence (Perseus) of cooling inflows. It is argued that smaller halos as in Perseus result form insufficient clusterwide magnetic fields. A simple model is presented which suggests that cooling flows can suppress the diffusion of turbulently amplified B-fields outward from the cluster core. Such a suppression leads to the development of minihalos which are confined to the cores of cooling flow clusters. 25 refs.

Intrinsically asymmetric astrophysical jets

Previous work on the origin of self-collimated electromagnetic jets is extended to the general case where there is no reflection symmetry of the magnetic field about the equatorial plane of the disk. The axisymmetric field structure inside the disk is obtained by solving for the magnetic flux function, and the toroidal magnetic field, from the generalized thin-disk induction equation, which is derived. The asymptotic (large-z) magnetic field structure outside the disk is obtained by solving the force-free Grad-Shafranov equation semianalytically. Jet solutions are found in which the power flow is carried mainly by the Poynting flux of the electromagnetic field and the angular momentum outflow from the disk is carried by the magnetic field. The ratio of jet luminosities (top/bottom) depends directly on the degree of asymmetry of the field and can easily be much greater than unity. It is argued that the degree of field asymmetry in the disk is determined by the asymmetry of the weak galactic field fed into the disk at large distances over long periods of time.

Self-collimated electromagnetic jets from magnetized accretion disks - The even-symmetry case

This paper extends the previous treatment (Lovelace et al., 1987) of the origin of self-collimated EM jets to the case of even field symmetry, where the magnetic flux function Psi(r, z) is an even function of z. A viscous resistive accretion disk is assumed to surround a black hole with a force-free plasma outside of the disk. Inside the disk, the induction equation is solved for Psi(r, z) and the toroidal magnetic field. Outside the disk, previous results are used to study the formation of self-collimated EM jets. In contrast with the odd-symmetry case, for even symmetry the toroidal magnetic field acts to vertically compress the disk; a comparatively large toroidal magnetic field can exist inside the disk; and an appreciable fraction (possibly all) of the available accretion power can go into the jets. 7 refs.

Pulsar magnetospheres with jets

The present study of Grad-Shafranov solutions for the ideal MHD flows around a magnetized rotating neutron star whose axes of rotation and magnetism are identical notes the force-free limit of this equation to be identical to the pulsar equation derived from a variational principle which minimizes the electromagnetic field energy within the constraints of fixed total angular momentum and total magnetic helicity. The pulsar equation is found to be a peacewise linear pdf with a class of solutions encompassing self-collimated electromagnetic jets along the stars rotation axis. 37 refs.

Instabilities in astrophysical jets. II - Numerical simulations of slab jets

Numerical simulations of an unstable supersonic slab-symmetric jet are described. The instabilities within the jet are characterized by growing internal body waves and their coupled surface waves that are also predicted in linear perturbation theory. The characteristic theory of fluid dynamics is used to help interpret the wave morphologies. It is demonstrated that these waves can be excited by imposing an arbitrary disturbance. From the numerical simulations, it is found that the sound waves propagating against the flow slow down as they propagate outward, and they grow in amplitude. These waves eventually disrupt the jet at a certain length. This disruption length is related to the jet Mach number and the perturbation intensity. Thus, the Mach number of a jet observed with a radio telescope can be estimated by measuring the disruption length and estimating the perturbation intensity. The jet Mach numbers in radio tailed sources determined in this way agree quite well with estimates from ram pressure bending arguments. The wiggles and flares observed in many extragalactic jets, especially in tailed radio sources, appear to be intimately related to instabilities and the jet disruption process.

Dispersal of gases generated near a lunar outpost

The extremely low density of the present lunar atmosphere provides an ideal environment for activities such as high-vacuum materials processing and high resolution astronomy. The aim of this work is to study the dispersal of gases arising from operations on a future lunar outpost and to predict its effects on these activities. The dispersal is modeled analytically using continuous (e.g., mining and habitat venting) and impulsive (e.g., rocket exhaust) injection mechanisms and assuming a collisionless, isothermal atmosphere. In the impulsive injection case, the neutral atmosphere and associated ionosphere both decay on time scales of about 20 min. In the continuous injection scenario, the atmosphere near the outpost grows and reaches a steady state after approximately 20 min. For a moderate injection rate (1 kg/s), any significant atmosphere is limited to within 1 km of the source. The resulting ionosphere impacts radio astronomical observations only within 10 km of the source. Both direct transport and diffusive transport (i.e., repeated bounces off of the lunar surface) are considered. It is concluded that at these injection rates and within the constraints of our assumptions, an artificial lunar atmosphere is not a serious detriment to astronomical observations and high-vacuum materials processing.

Uses of continuum radiation in the AXAF calibration

X-ray calibration of the Advanced X-ray Astrophysics Facility (AXAF) observatory at the MSFC X-Ray Calibration Facility (XRCF) made novel use of the x-ray continuum from a conventional electron-impact source. Taking advantage of the good spectral resolution of solid-state detectors, continuum measurements proved advantageous in calibrating the effective area of AXAFs high resolution mirror assembly (HRMA) and in verifying its alignment to the XRCFs optical axis.

X-ray source system at the Marshall Space Flight Center X-ray calibration facility

In preparation for calibrating the Advanced X-ray Astrophysics Facility, a multicomponent x- ray source system has been assembled for use at the x-ray calibration facility at MSFC. The system consists of an electron impact point source with filters, a penning gas discharge source, and two monochromators fed by rotating anode x-ray generator. The purpose and predicted performance characteristics of these elements are described as they apply to the AXAF calibration. The planned source characterizations, performed in June 1995 through June 1996 time period, are also described.

AXAF-mirror effective area calibration using the C-continuum source and solid state detectors

The AXAF X-ray mirrors underwent thorough calibration using the X-ray Calibration Facility (XRCF) at the Marshall Space Flight Center in Huntsville, AL from late 1996 to early 1997. The x-ray calibration made novel use of the x-ray continuum from a conventional electron-impact source. Taking advantage of the good spectral resolution of solid-state detectors, continuum measurements proved advantageous in calibration the effective area of AXAFs High-Resolution Mirror Assembly (HRMA) for the entire AXAF energy band. The measurements were made by comparing the spectrum detected by a SSD at the focal plane with the spectrum detected by a beam normalization SSD. The HRMA effective area was calibrated by comparing the measurements with the HRMA raytrace model. The HRMA on-orbit performance predictions are made using the calibration results.