On the internal structure and magnetic fields of

First, a sequence of four-zone models for the interior of Venus is constructed under the assumption of hydrostatic equilibrium. While the equation of state for each zone is taken to be the Bullen's relation with its coefficients consistent with the PREM Earth model (Dziewonski and Anderson, 1981), the position of core-mantle boundary is determined by matching solutions of the Emden's equation in different regions. The results of hydrostatic models indicate the presence of a reasonably large molten iron core in Venus, broadly similar to the Earth. It is also found that the position of the core-mantle interface is nearly model-independent. Second, we focus on the question why Venus does not possess a significant global magnetic field and on what we can learn from this fact. Solutions of magnetohydrodynamic equations appropriate for the molten core of Venus are discussed. It is argued that, because the Elsasser number measuring the relative importance of Coriolis and Lorentz forces satisfies 1, equations for the problem of thermal convection in the Venusian fluid core must be nearly uncoupled with the dynamo equation. The existence of a global magnetic field, though small, then suggests that the size of the magnetic Reynolds numberR _m must beR _m =O(10), sustaining a dynamo action near its marginal state but not an active dynamo in the Venusian molten core. On the basis of asymptotic relations for finite amplitude convection, a useful constraint on important physical parameters for the liquid core of Venus is derived and discussed.     

The point spread function of the soft X-ray telescope aboard Yohkoh

The point spread function of the SXT telescope aboardYohkoh has been measured in flight configuration in three different X-ray lines at White Sands Missile Range. We have fitted these data with an elliptical generalization of the Moffat function. Our fitting method consists of ² minimizationin Fourier space, especially designed for matching of sharply peaked functions. We find excellent fits with a reduced ² of order unity or less for single exposure point spread functions over most of the CCD. Near the edges of the CCD the fits are less accurate due to vignetting. From fitting results with summation of multiple exposures we find a systematic error in the fitting function of the order of 3% near the peak of the point spread function, which is close to the photon noise for typical SXT images in orbit. We find that the full width to half maximum and fitting parameters vary significantly with CCD location. However, we also find that point spread functions measured at the same location are consistent to one another within the limit determined by photon noise. A best analytical fit to the PSF as function of position on the CCD is derived for use in SXT image enhancement routines. As an aside result we have found that SXT can determine the location of point sources to about a quarter of a 2.54 arc sec pixel.This paper is dedicated to the memory of our friend and colleague Kermit Smith, who died in June 1993, after a valient fight with leukemia.     

The orion-IRc2 outflow observed in COJ=4-3

A CO J=4-3 map of the Orion-IRc2 region (size 80, beam 15) is presented. The outflow has a bipolar structure observed nearly pole-on. We interpret our observations in terms of an hierarchy of tubes where the fastest gas proceeds through the narrowest, innermost cone. The gas has an inhomogeneous temperature distribution and must be highly clumped. Most of the properties derived comply with the picture developed for the few known extreme high velocity outflows; however, this outflow contains exceptionally dense gas ( > 10⁶ cm^–3).     

Phase transitions in the ISM a source of dissipative behaviour

By considering a simple fluid model, we investigate the role of phase transitions in the ISM on the galaxy- scale gas dynamics. Cooling and heating timescales in the ISM are typically shorter than typical galactic rotation timescales, so the individual phases in the ISM can be assumed to be in temperature equilibrium with the radiation field. Using this approximation we can construct an equation of state which depends upon the average density and mass fractions in the individual phases. Previous studies suggest that there is an equilibrium phase fraction as a function of pressure. We incorporate evolution towards this equilibrium state as a relaxation term with a time to obtain equilibrium . We derive a condition in terms of a critical Mach number when one dimensional shocks should be continuous. For small values of the relaxation time we show that the relaxation term acts like a viscosity. We show with one dimensional simulations that increasing causes shocks to become smoother. This work suggests that phase changes can strongly effect the gas dynamics of the ISM across spiral arms and bars.     

Effects of hall current on the hydromagnetic free convection with mass transfer in a rotating fluid

An exact analysis of Hall current on hydromagnetic free convection with mass transfer in a conducting liquid past an infinite vertical porous plate in a rotating fluid has been presented. Exact solution for the velocity field has been obtained and the effects ofm (Hall parameter),E (Ekman number), andS _c (Schmidt number) on the velocity field have been discussed.Nomenclature C species concentration - C _w concentration at the porous plate - C species concentration at infinity - C _p specific heat at constant pressure - D chemical molecular diffusivity - g acceleration due to gravity - E Ekman number - G Grashof number - H ₀ applied magnetic field - j _x, j_y, j_z components of the current densityJ - k thermal conductivity - M Hartman number - m Hall parameter - P Prandtl number - Q heat flux per unit area - S _c Sehmidt number - T temperature of the fluid near the plate - T _w temperature of the plate - T temperature of the fluid in the free-stream - u, v, w components of the velocity fieldq, - U uniform free stream velocity - w ₀ suction velocity - x, y, z Cartesian coordinates - Z dimensionless coordinate normal to the plate. Greek symbols coefficient of volume expansion - ^* coefficient of expansion with concentration - _e cyclotron frequency - dimensionless temperature - ^* dimensionless concentration - v kinematic viscosity - density of the fluid in the boundary layer - coefficient of viscosity - _e magnetic permeability - angular velocity - electrical conductivity of the fluid - _e electron collision time - _u skin-friction in the direction ofu - _v skin-friction in the direction ofv     

Solar noise storms coordinated observations: May 16–24, 1981

We present the main results obtained from the coordinated observations of solar radio noise storms organized by C.E.S.R.A. in May 1981. They concern the structure and polarization of radio sources, and the relation with the photospheric magnetic field. A model of coronal magnetic field, accounting for the observations, is briefly discussed.     

On the possibility of submillimetric interstellar lines detection by means of laser heterodyne techniques

The search for extraterrestrial and, in particular, interstellar molecules has suddenly increased in the last ten years after the discovery of many complex organic molecules by radioastronomy. Since some very interesting lines are not detectable in the radio region, but only in the middle and far infrared, heterodyne techniques seem to be a powerful tool for interstellar line detection in the submillimetric region and the detection of CO at 870 in Orion (Philipps and Huggins, 1977) represents an encouraging success for further efforts in this new field of research. In this work the possibility of observing interstellar lines at heterodyne laser wavelengths already available in laboratory has been considered. We also computed relative intensities for some representative rotational lines of interstellar molecules at three typical temperatures (10, 20, 50 K) assuming thermodynamic equilibrium and optically thin regions. The results achieved with heterodyne laser techniques developed in the laboratory are discussed here in order to study the possibility for ground and space astronomical observations.     

The satellites of Neptune and the origin of Pluto

Pluto and the chaotic satellite system of Neptune may have originated from a single encounter of Neptune with a massive solar system body. A series of numerical experiments has been carried out to try to set limits on the circumstances of such an encounter. These experiments show that orbits very much like those of Pluto, Triton, and Nereid can result from a single close encounter of such a body with Neptune. The implied mass range and encounter velocities limit the source of the encountering body to a former trans-Neptunian planet in the 2- to 5-Earth-mass range.     

Potential and inductive electric fields in the magnetosphere during auroras

During quiescent auroras the large-scale electric field is essentially irrotational. The volume formed by the plasma sheet and its extension into the auroral oval is connected to an external source by electric currents, which enter and leave the volume at different electric potentials and which supply sufficient energy to support the auroral activity. The location of the actual acceleration of particles depends on the internal distribution of electric fields and currents. One important feature is the energization of the carriers of the cross-tail current and another is the acceleration of electrons precipitated through relatively low-altitude magnetic-field-aligned potential drops.Substorm auroras depend on rapid and (especially initially) localized release of energy that can only be supplied by tapping stored magnetic energy. The energy is transmitted to the charged particle via electric inductive fields.The primary electric field due to changing electric currents is redistributed in a complicated way—but never extinguished—by polarization of charges. As a consequence, any tendency of the plasma to suppress magnetic-field-aligned components of the electric fields leads to a corresponding enhancement of the transverse component.     

Convective collapse of flux tubes

Flux tubes of constant extending vertically through the solar convection zone are unstable to a convective instability if the surface field strength is less than 1270 G. By downward displacement of matter along the tube an unstable tube can transform into a new equilibrium state with lower energy which has a higher field strength. Numerical calculations of these collapsed states are presented. If the collapse starts in a field with a strength corresponding to equipartition with kinetic energy in the convection zone, it yields a surface field strength of about 1650 G. It is proposed that the small scale magnetic field in active regions consists of such tubes. The collapsed state is not in thermal equilibrium. In the deeper layers the heat exchange following the collapse is very slow but the surface layers return rapidly to temperature equilibrium. It is argued that during the gradual thermal evolution of the collapsed state its surface layers may start an overstable oscillation. A brightness-velocity correlation in this oscillation could account for the observed downdraft in the tubes.The National Center for Atmospheric Research is sponsored by the National Science Foundation.