Journal of geomagnetism and geoelectricity

Recent Advances in the Study of Electric and Magnetic Fields in the Ionosphere and Magnetosphere

Gordon ROSTOKER

1980 Volume 32 Issue 8 Pages 431-470
Published: 1980
Released on J-STAGE: March 12, 2010

DOIhttps://doi.org/10.5636/jgg.32.431

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In the past two years there have been major advances in two areas. Firstly the nature of the DPY current system in the region of the polar cleft (cusp) has been more clearly defined through the use of the polar orbiters Triad and ISIS 2. It was now clear that field-aligned currents bound an ionospheric electrojet whose direction of the flow is regulated by the polarity of the B_y component of the interplanetary magnetic field. A second major advance has been in the development of comprehensive three-dimensional current system models which are capable of reproducing high latitude magnetic field perturbations. Work of this nature has been carried out in the U. S. S. R. and Canada, while investigations as to the driving mechanism for the current systems have been carried out in the U. S. A. and in Japan. Considerable progress continues to be made in defining the phenomonology of parallel electric fields in the altitude range 2, 000-10, 000km and in the development of the theory for explaining the acceleration mechanisms. The STARE radar system has now provided a powerful tool for the study of spatial and temporal variations of ionospheric electric fields; the use of the STARE data together with data from the European magnetometer arrays operating in Scandinavia during the IMS provides an outstanding opportunity to make major breakthroughs in the next few years.

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Source-Field Bias in Geomagnetic Transfer Functions: A Case History

D. I. GOUGH, J. H. DE BEER

1980 Volume 32 Issue 8 Pages 471-482
Published: 1980
Released on J-STAGE: March 12, 2010

DOIhttps://doi.org/10.5636/jgg.32.471

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A magnetometer array study in 1971 discovered the Southern Cape Conductive Belt beneath the Karroo Basin and Cape Folded Mountains of South Africa. A second array in 1977 has mapped this Conductive Belt in some detail, by means of anomalies in maps of Fourier transform amplitudes and phases. Transfer functions from horizontal to vertical components were calculated from data recorded by each array, and used to map induction vectors. The in-phase induction vectors from the 1971 array are in good agreement with the current locations shown by the Fourier transform anomaly maps. The 1977 array data give induction vectors which agree with neither those of 1971, nor the anomaly maps. After other possible sources of systematic error had been eliminated, it has been shown that the problem arises from source-field bias introduced by cross-correlation between the horizontal components of the normal field. Two tests are suggested, in the selection of events for calculation of valid transfer functions. Errors from source-field bias could be important in work with small numbers of magnetometers, in which anomalies cannot be mapped from simultaneous observations of events by a large array.

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Interpreting Regional Geomagnetic Anomalies and Their Secular Variation with the Aid of Radial Dipoles and Current Loops

H. NEVANLINNA

1980 Volume 32 Issue 8 Pages 483-506
Published: 1980
Released on J-STAGE: March 12, 2010

DOIhttps://doi.org/10.5636/jgg.32.483

JOURNAL FREE ACCESS

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By adjusting the depth and strength of a radial magnetic dipole so that the intensity of the field at the focal point of Z and the distance between the positive and negative focal points of the X and Y isolines correspond to those around a regional anomaly obtained from non-dipole maps, it is demonstrated that the dipole is located at a radial distance of 0.25 R_e from the geocentre. A circular current loop at the core-mantle boundary corresponding to this dipole has a radius of 0.35 R_e.
Using a conducting Earth model, it was found that the effect of the conductivity of the mantle is negligible if the period of the source dipole is more than 50 years.
The secular change in Europe, Asia, Africa, and part of North America was found to follow the change of two radial dipoles, one lying under the Asian anomaly and the other under the African anomaly. As interpreted by this two-dipole model, the strong centres of the secular variation observed in the Atlantic Ocean are caused by the rapid westerly drift of the African anomaly. In Asia, the change of the large positive Z-cell to a negative one during the last 30 years was interpreted as having been caused by changes in the intensity of the Asian anomaly.
The observed secular change at 29 observatories in Eurasia and part of North America for the epoch 1965.0 can be explained by a two-dipole model with an rms error of 7.1nT/y, which is clearly better than the rms error of the IGRF (14.5nT/y), and roughly the same as that of the definitive model for 1965 (6.1nT/y) obtained.

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