Search

Using Australian records, we studied six isolated geomagnetic storms for the regional effects of their associated magnetic fields, emphasizing the linear regression correlation coefficient between station locations and indices. An innovation used here was to consider both the full magnetic disturbance field and a form of this field with the smoothed storm-time Dst values removed to feature the small variations. Magnetic H-component data from paired stations throughout the continent showed correlation values of +0.90 to +0.97. These results indicate the adequacy of the present density of observatories in Australia and their value for use with aeromagnetic surveys. The planetary magnetic ap index was found to have an inconsistent relationship to Australian H-component fields. The planetary magnetic Dst index was successfully represented by data from four Australian observatories. The discovered capability of Australian magnetic observatories to represent the magnetic storm Dst index in real time, combined with the established lognormal-form characteristic of Dst, means that Australian solar-terrestrial disturbance centers should be able to predict the time to recovery of quieter fields once an on-line Austrialian magnetic field Dst mainphase computation has been made.

This study concerns the use of selected geomagnetic field records to establish the 1990 quiet-day current system (Sq) for Australia and to use the ionospheric current source of Sqfor a determination of the Earth’s deep electrical conductivity. The primary data set came from a chain of eight, three-component magnetometer stations that was operated along a north-south line in central Australia. Additional records, necessary for boundary conditions, were added to the data set. A regional spherical harmonic analysis (SHA) allowed the separation of the internal and external field contributions to the Sqvariations. Mapping of the equivalent ionospheric current from the external field showed that the Sqcontour focus passed near the —30° geomagnetic latitude of central Australia with a 5° latitude variation between winter and summer and a corresponding change from about 80 to 200 kA in strength. A special transfer function allowed the computation of an equivalent conductivity-depth profile of central Australia from the paired external and internal coefficients of the SHA. A regression line through the conductivity estimates gives a profile that starts at 0.025 S/m for a depth of 130 km, rising gradually to about 0.045 S/m at 250 km, then steepens to 0.11 S/m at 360 km and rises moderately to 0.13 S/m at 470 km near the base of the upper mantle. No data were obtained through the mantle transition zone. Computations gave 0.18 S/m in the region of 800 km depth. Previous conductivity models for the upper mantle beneath central Australia, although less specific in values, are consistent with our profile. At depths greater than 500 km, the regression profile is in agreement with the conductivity distribution beneath the Tasman Sea determined from seafloor magnetotellurics, although both measurements lack high resolution at such depths.

Late Eocene-early Oligocene (42–35 Myr) sediments cored at two DSDP sites in the south-west Pacific contain evidence of a pronounced increase in local volcanic activity, particularly in close association with the Eocene–Oligocene boundary. This pulse of volcanism is coeval with that in New Zealand and resulted from the development of an Indo- Australian / Pacific Plate boundary through the region during the late Eocene. The late Eocene / earliest Oligocene was marked by widespread volcanism and tectonism throughout the Pacific and elsewhere, and by one of the most important episodes of Cenozoic climatic cooling.

Acquisition of magnetic remanence in slurries of fme grained organic muds settling in long tubes is investigated using a cryogenic magnetometer. The average settling behaviour of remanence carrying grains relative to the whole sediment gives information about the relative magnetic grain size spectrum, whereas the response of settled deposits to vibration gives an indication of the degree of alignment of particles and their average shape. Two classes of behaviour are apparent in both the time and field dependence of detrital remanent magnetization (DRM) acquisition. Dilute slurries settling in the Earth's field (analogous to detrital sedimentation) acquire a remanence which reaches a maximum after about 2 day, whereas for concentrated slurries (analogous to slumped or bioturbated sediments) this takes only a matter of minutes. The field dependence of DRM in dilute slurries is in plausible quantitative agreement with Stacey's extension of the classical Langevin expression for the susceptibility of a paramagnetic gas, whereas concentrated slurries show a quasi-linear dependence of DRM on the applied field. Inclination errors are generally absent, but do appear in fields less than about 0.5 Oe, and when the magnetic fraction settles out preferentially. A weak negative dependence of DRM on temperature is found, but the results are too crude to provide a further test of Stacey's theory. Remanence acquisition in slurries settled in zero field indicates that short term post-depositional magnetization processes are relatively unimportant in slurries that have ceased to compact rapidly. A large increase in remanence from naturally occurring sediments to slowly redeposited long cores, to rapidly settled slurries is best explained as a feature of compaction. Unit conversion between the cgs system used throughout this paper and the Systéme International (SI) are: Magnetic flux density (B): 1 mT = 10-3 Tesla (Weber m-2) = 10 Gauss (G); 1 nT = 10-9 Tesla = 1 gamma. Magnetic field strength (H): (1O3/4π) A turns m-2 = 1 Oersted (Oe). (In a vacuum, a flux density of 1 mT causes a magnetic field of strength 10 Oe.) Intensity of magnetization (J): 1 mA m-1 = 10-6 emu cm-3 = 1 μG.

Detailed within-lake magnetic stratigraphies enable composite declination and inclination versus depth curves to be derived for sets of 6 m cores from each of three volcanic crater lakes in western Victoria. Comprehensive radiocarbon chronologies for each sequence enable reliable time-scales to be assigned to the variations in direction of the remanence. The resultant time-scaled curves are in satisfactory agreement and are therefore thought to provide an estimate of the geomagnetic secular variation in SE Australia during the last 10 000 yr. Amplitude attenuation occurs as an increasing function of age, and probably results from long time-scale remagnetization processes. Mean directions coincide with the geocentric axial dipole field directions at the three sites. Inclination oscillations are stronger and better resolved than those in declination, whereas the opposite tends to be true for the European records. Reduction of records to a common ‘virtual’ site suggests that the difference is largely of geometrical origin. Comparisons of VGP paths do not reveal a common dipole wobble component. The sense of looping of the magnetic vector favours westward rather than eastward drift of features of the non-dipole field. Relative field strength estimates based on normalization of NRM with respect to measures of the magnetic mineral content do not agree with Australian and worldwide archaeointensity results.

A 10 000 yr continuous secular variation record from intensively dated lake sediments in SE Australia has been subjected to periodogram and maximum entropy method analysis. Tests on synthetic data reveal some of the limitations of the latter method, particularly when applied to complex number series. Anticlockwise precession of the magnetic vector at a period of 5000 ± 1000 yr is tentatively ascribed to dipole precession, and clockwise precession at a period of about 2800 yr is probably due to westward drift of features of the non-dipole field. The effect of calibrating the radiocarbon time-scale is important and results in periodicity shifts of up to 25 per cent. Even for well-dated lacustrine sequences power spectra are poorly constrained: it is thus possible that the geomagnetic secular variation on a time-scale of thousands of years is more uniform than often supposed. Mismatches between declination and inclination spectra can arise as a natural consequence of certain types of source mechanism and should not be simply attributed to figments of the analysis employed.