Effects of rotation on Rayleigh-Taylor instabilities of an accelerating,compressible, perfectly conducting plane layer

The stability of a plane-stratified slab of perfectly conducting, rotating, compressible, inviscid plasma accelerated by a magnetic field is considered. Exact solutions for the growth rates of the unstable modes are determined for a -law gas when the undisturbed equilibrium is an isothermal atmosphere of semi-infinite extent with no frozen-in field. It is shown that the rotation has no effect on the region of unstable modes which has wavelengths long compared with the atmospheric scale height. On the other side, the growth rates in the presence of rotation are less than those in the absence of rotation for unstable modes.     

A catalogue and finding list of galactic novae

A catalogue of galactic novae and an atlas of finding charts are under preparation and will be published in 1984. The status of the project is described.Paper presented at the Lembang-Bamberg IAU Colloquium No. 80 on Double Stars: Physical Properties and Generic Relations, held at Bandung, Indonesia, 3–7 June, 1983.     

Differential rotation in a solar-driven quasi-axisymmetric circulation

We consider the concept of a quasi-axisymmetric circulation to explore the global scale dynamics of planetary atmospheres. The momentum and energy transport processes in the smaller scales are formulated in terms of anisotropic eddy diffusion. In the early work of Williams and Robinson (1973) these concepts have been introduced to describe the Jovian circulation. Our study differs in that we adopt a spectral model (with vector spherical harmonics) and consider a linear system; we are also examining a different parameter regime. The troposphere of Jupiter is assumed to be weakly convectively unstable, and the circulation is driven by the fundamental component of solar differential heating with a broad maximum at the equator. Mode coupling arising from the Coriolis action is considered in self consistent form, and momentum and energy are allowed to cascade from lower to higher order modes. With a limited number of spherical harmonics, up to order 40, and with homogeneous boundary conditions, the conservation equations are integrated between the 25 and 10^–5 bar pressure levels. In addition, a simplified single layer model is discussed which, even though heuristic in nature, elucidates and complements the numerical results. Our analysis leads to the following conclusions: (a) For a negative stability, S ₀ = T ₀/r + , the energy transports arising from large scale advection by the meridional circulation can amplify the response to the external heating. This crucially depends on the latitudinal structure of the circulation, so that banded wind fields with equatorial zonal jets are preferentially excited. (b) With a negative stability of order S ₀ ~ – 10^–6 K cm^–1, the computed number of positive (and negative) zonal jets is similar to that observed on Jupiter. (c) The observed magnitudes in the zonal wind velocities require that the vertical eddy diffusion coefficient is of the order K _r ~ 3 × 10⁵ cm² s^–1, which in turn is consistent with the observed outward flux of energy from the planetary interior (F K _r S ₀ ); this diffusion rate is also of the right order of magnitude to obey mixing length theory. (d) The ratio between the horizontal and vertical eddy diffusion coefficients (relative mixing factor) is of critical importance. If it is too large ( 10⁵), differential rotation or alternating zonal jets cannot be maintained; if it is too small ( 10⁴), the equator tends to corotate. The intermediate value of order R ~ 5 × 10⁴ is again consistent with mixing length theory. (e) With the above constraints on the transport coefficients, the flow is quasigeostrophic. (f) The meridional circulation is multicellular and of the Ferrel-Thomson type. It is consistent with the observed cloud striations in the Jovian atmosphere. (g) In the stable stratosphere at higher altitudes the fundamental component, directly driven by the Sun, dominates. The circulation degenerates, and broad, positive zonal jets develop at middle latitudes, resembling the observed wind field in the visible cloud cover of the Venus atmosphere.Applied Research Corporation, Landover, Maryland, U.S.A.     

Oscillations of the Hα emission in solar prominences

The time dependence of Doppler shift and line-center intensity is simultaneously observed for the H emission of three solar prominences, each one during about two hours. Doppler oscillations with periods near one hour and amplitudes between 1 and 2 km s^–1 are conspicuously visible in the recordings of all three prominences. Fourier analysis yields periods of 50, 60, and 64 min, as well as slight indications of short periods near 3 and 5 min. No oscillations are found in the line-center brightness.     

Properties of metre-wavelength solar bursts associated with interplanetary type II emission

A statistical analysis is used to determine the properties of metre-wavelength events which are associated with interplanetary type II bursts. It is found that the likelihood of an interplanetary type II burst is greatly increased if: (a) an associated metre-wavelength type II has a starting frequency less than 45 MHz; (b) a strong metre-wavelength continuum is present; (c) the type II contains herringbone fine structure; and (d) the metre-wavelength activity is accompanied by strong, long-lasting H and soft X-ray events.Visiting scientist at Division of Radiophysics, January 1983; previous address - NASA/Goddard Space Flight Center, Greenbelt, Maryland.     

The acceleration of minor ion species in the solar wind

This paper provides a comprehensive analysis of the dynamics of the flow of minor ion species in the solar wind under the combined influences of gravity, Coulomb friction (with protons), rotational forces (arising from the Sun's rotation and the interplanetary spiral magnetic field) and wave forces (induced in the minor ion flow by Alfvén waves propagating in the solar wind). It is assumed that the solar wind can be considered as a proton-electron plasma which is, to a first approximation, unaffected by the presence of minor ions. In the dense hot region near the Sun Coulomb friction accelerates minor ions outwards against the gravitational force, part of which is cancelled by the charge-separation electric field. Once the initial acceleration has been achieved, wave and rotational forces assist Coulomb friction in further increasing the minor ion speed so that it becomes comparable with, or perhaps even exceeds, the solar wind speed. A characteristic feature of the non-resonant wave force is that it tends to bring the minor ion flow into an equilibrium where the radial speed matches the Alfvén speed relative to the solar wind speed, whereas Coulomb friction and rotational forces tend to bring the flow into an equilibrium where the radial speed of the minor ions equals the solar wind speed. Therefore, provided that there is sufficient wave energy and Coulomb friction is weak, the minor ion speed can be trapped between these two speeds. This inteststing result is in qualitative agreement with observational findings to the effect that the differential flow speed between helium ions and protons is controlled by the ratio of the solar wind expansion time to the ion-proton collision time. If the thermal speeds of the protons and minor ions are small compared to the Alfvén speed, two stable equilibrium speeds can exist because the rapid decrease in the Coulomb cross-section with increasing differential flow speed allows the non-resonant wave force to balance Coulomb friction at more than one ion speed. However, it must be emphasized that resonant wave acceleration and/or strong ion partial pressure gradients are required to achieve radial speeds of minor ions in excess of the proton speed, since, as is shown in Section 4, the non-resonant wave acceleration on protons and minor ions are identical when their radial speeds are the same, with the result that, in the solar wind, non-resonant wave acceleration tends (asymptotically) to equalize minor ion and proton speeds.     

The rotations of 128 Nemesis and 393 Lampetia: The longest known periods to date

Extensive photoelectric lightcurves of the asteroids 128 Nemesis and 393 Lampetia show for both objects extremely long rotational periods, the longest known to date for minor planets. For 393 Lampetia the combined results suggest with high probability a period of 38.7 hr with a maximal amplitude of 0.14 mag; a double-wave characteristic of the lightcurve must be assumed. For 128 Nemesis the complete double-wave lightcurve was observed and a period of 39.0 hr with a total amplitude of only 0.10 mag was deduced. Observations of 128 Nemesis confirm without doubt the presence of small-scale features of amplitude 0.01 to 0.02 mag, corresponding to small topographic features of about 15 km in height and width on the surface.     

Radar observations of asteroid 1580 Betulia

Radar observations of the asteroid 1580 Betulia, made at a wavelength of 12.6 cm, show a mean radar cross section of 2.2 ± 0.8 km² and a total spectral bandwidth of 26.5 ± 1.5 Hz. Combining our bandwidth measurements with the optically determined rotation period sets a lower limit to the asteroid's radius of 2.9 ± 0.2 km.     

Global fracture patterns of a despun planet: Application to Mercury

We have determined the global fracture patterns resulting from combinations of stresses due to tidal despinning and contraction or expansion. We find that Mercury's lineament pattern is consistent with a history of despinning and contraction. According to our model, the observed tectonic pattern implies that the despinning process reached completion before the planet ceased contracting. Our model predicts a stress due to contraction which is up to 1.8 times the maximum despinning stress on Mercury. The maximum contractional stress could be as large as 4 times the maximum despinning stress if the oldest fractures on the planet are N-S thrust faults in the equatorial region.     

Dependence of the correlation of small scale photospheric structures upon resolution

Correlations between continuum intensity, velocity, and equivalent widths of two Mn i lines as observed with two different entrance apertures tend to deteriorate with improvement in spatial resolution. The KPNO multichannel magnetograph was used to make area scans at the center of the disk with entrance apertures 3.5×2.5 arc and 1×1 arc. A coherence analysis shows that this effect is caused by marked differences of fluctuations in temperature and temperature gradients as well as in the velocity structure of photospheric elements of various sizes.