J. O. Murphy

On Magnetic Inhibition of Thermal Convection

The effect of an imposed vertical magnetic field on convective transfer in a horizontal Boussinesq layer of fluid heated from below is studied in the mean field approximation. Solutions are found over a wide range of conditions, for free boundaries, by a combination of numerical and analytic techniques. Quantitative estimates are made of the significant modifications to the heat transfer which are brought about by the presence of the magnetic field. It is found that the general properties of nonlinear steady cellular convection seem to persist in the face of magnetic inhibition.

Australian Tree Ring Chronologies as Proxy Data for Solar Variability

Data from the Solar Maximum Mission satellite suggests that there is a 0.1 percent variation in solar luminosity over the 11-year solar cycle with maximum output corresponding to maximum sunspot number. Tree ring chronologies can be utilized as proxy data to compile a record of the magnitude of past solar variability. Spectral analysis of tree ring series compiled from a site in Tasmania has established, among others, significant periodicities of about 90 and 11 years. The application of band pass filter techniques shows that the 11-year cycle present in some tree ring series correlates with the Zurich sunspot numbers over the period from AD1700, the extent of accurate sunspot records, with a time lag of about three years.

The Effect of Rotation on Nonlinear Thermal Convection

The effect of rotation on nonlinear thermal convection is investigated, in particular at high Rayleigh number. The Boussinesq approximation is adopted in the basic equations and the free boundary conditions are applied. The results derived from asymptotic and perturbation methods are found to be in very good agreement with those obtained by numerical integration.

Physical Structure of a Sequence of Two Zone Polytropic Stellar Models

Expressions for the physical structure of a sequence of two zone polytropic stellar models, based on composite analytical solutions of the Lane-Emden equation with indices n = 1 and n = 5, hâve been determined. The coefficients of vibrational stability for radial oscillations of this sequence of models have also been calculated and it is found that increasing the extent of the n = 5 outer zone has a stabilizing effect.

Time-Dependent Magneto-Convection with Hexagonal Planform

High-resolution observations indicate that very strong, small scale magnetic fields in the solar photosphere are concentrated into ropes which emerge through it. The scale of these ropes is only a few hundred kilometres across (Stenflo 1976) and their strength is estimated to vary between 1,000 and 2,000 G (Harvey 1977). These features are closely related to photospheric granular convection. The flux is observed as X-ray bright spots and sheet-like crinkles in the dark intergranular lanes, and is buffeted and shifted about by the granules (Dunn and Zirker 1973). Further, the crinkles can be resolved into separate features which outline small micropores as though a flux sheet at the end of a convection cell has separated into several isolated tubes (Galloway and Weiss 1981).

The Effect of Rotation on Thermal Convection

An investigation into the influence of rotation on thermal convection has some applicability in the study of the solar convection zone. Of particular interest is the effect of rotation on the total heat transport and the cell size for maximum heat transport at high Rayleigh number, which is estimated to be as high as 10 20 for the Sun.

Reconstruction of the Annual Variation in Solar Radio Flux and the Catania Sunspot Area from Tree Ring-index Time Series

Four tree ring-index site chronologies, representing standardised annual growth rates for spruce trees growing at high altitude sites in Colorado, have been employed as proxy data in a regression model for the annual variation of solar radio flux at 2800 MHz ( F 10·7 ) and the Catania sunspot area ( A c ). These dendrochronological time series all exhibit significant power spectrum peaks at about 11 years and separately correlate with the annual values of R z , F 10·7 and A c , as solar activity indicators. The two models constructed give the cyclic variation of F 10·7 and A c back to AD1673.