Geomagnetic Models Research Articles

Blake polarity episode in two cores from the Greater Antilles Outer Ridge

Two large-diameter cores from the Greater Antilles Outer Ridge have confirmed the Blake episode 0.1 m.y. B.P. as a genuine paleomagnetic reversed interval at least in that region. The feature is clearly defined in more than 125 stably magnetized specimens of Last Interglacial abyssal brown clay, but its precise age and duration cannot yet be estimated reliably from these or other available data. During the reversed period, positions of the virtual geomagnetic pole lay about 20° from the south geographic pole, with polarity transitions traversing paths in the eastern hemisphere. Each core also recorded a normally polarized excursion within the Blake feature. Because the most acceptable Blake episode data presently span only a small geographic area, they are insufficient for distinguishing between global and local geomagnetic models for the feature.

Error enhancement in geomagnetic models derived from scalar data

An investigation conducted by Backus (1970) regarding the possible existence of two harmonic functions of certain characteristics in three-dimensional space is considered. The derivation of a model of the main geomagnetic field from scalar data is discussed along with a numerical simulation study. It is found that experimental discrepancies between vector field observations and the predictions of the model may have a mathematical origin, related to the work of Backus.

The Laschamp geomagnetic ‘event’

SHORT lived geomagnetic ‘events’ provide useful stratigraphic markers; they may enhance climatic and evolutionary changes1,2 and they provide stringent parameters for geomagnetic models. During such events the virtual geomagnetic pole undergoes excursions outside the usual range of secular variation but they are usually brief, generally lasting about 105 yr, so that their magnetic record in deep sea sediments is often lost because of post-depositional remagnetisation3 and because of biological activity in the upper few decimetres of the sediments4. In lake sediments, where sedimentation is more rapid, the magnetic record is better preserved. Dating is, however, difficult because secular variations in atmospheric carbon isotopes and irregular distributions in the environment are known to produce errors. Furthermore, geomagnetic events, the levels of which may have been displaced by bioturbation4, are usually dated by assuming uniform sedimentation between 14C dated horizons. The Laschamp event, in particular, seems to have been detected at 12 locations although the reported age (Table 1) varies between 7,000 and 17,000 yr BP, a range outside the quoted error for individual determinations, but within the realistic error in dating.

Geomagnetic models from satellite surveys

The use of satellite scalar magnetic field data to produce geomagnetic field models is reviewed. There have been nine separate spacecraft that have acquired observations of the geomagnetic field from low (<1500 km) satellite altitudes since 1958. The magnetic field models produced from such data have not been sufficiently compared with surface vector data to firmly, establish their validity. One comparison has indicated that satellite‐derived models are as valid over‐all as those produced from surface data. Variations in the field components are shown to be larger than those in scalar field by factors of 4–10. Recent satellite models indicate an increase in dipole decay to −27 γ/year, a value that may be plausible, judging from extrapolation of earlier surface‐derived models. Also noted is a recent slowing of eccentric dipole westward drift.

Charged-particle variations in the outer Van Allen zone during a geomagnetic storm

Comments and data are preserted on the phenomena observed by instruments on the Explorer VI satellite. The data associated with the intense and unusual fluctuations in the particle intensity of the August 16, 1959, geomagnetic storm were compared with the data obtained by the spin coil magnetometer. This comparison revealed that a 180 deg out-of-phase correlation existed between the particle intensity and the magnetic field strength. The storm was accompanied by an increase in the scintillation count rate in the outer zone within 12 hr following the sudden commencement of the storm, whereas the instruments sensitive to bremsstrahlung from electrons showed a decrease. These observations are consistent with the geomagnetic storm model of Dessler and Parker, and the confirmation of the increase of energetic protons in the geomagnetic field during a storm makes plausible the establishment of the field stresses which accourt for the main phase of a geomagnetic storm in that model. (B.O.G.)