D. E. Winch

The eccentric dipole

SummaryAn up to date (1965) collection of the geometric parameters of the eccentric dipole is given and a new approach to the theory is introduced visualising the eccentric dipole as a superposition of multipoles. Formulae are also given for the determination of the theoretical north and south dip-poles.

Night-time geomagnetic variations at low latitudes

Abstract Using hourly values of the magnetic elements H , D and Z for 1964, 1965, their variation during night-time hours is examined from both their monthly means and from a previously used harmonic analysis method. The data set used represents quiet magnetic conditions. Consistent changes during the night are often found. Seasonal changes are also examined and it seems necessary to modify the Malin—Isikara hypothesis of a moving ring current by including a seasonal modulation of ring current strength with equinoctial maximum and a local time-varying component or partial ring current. The night-time D component shows considerable asymmetry between North and South hemispheres and this might be due to field-aligned current structure. There is a small amount of evidence for a night-time westward equatorial electrojet enhancement.

Longitudinal effects in geomagnetic disturbances at mid-latitudes

The paper describes the differences in the magnetic disturbance effect on horizontal geomagnetic field (H) and declination (D) at Alibag (India), Lunping (Taiwan), Chichijima (Japan) and Kakioka (Japan). It is suggested that the trapped particles in the earth’s magnetic field lines are guided by the dipole declination and not by the ground declination and follow the direction towards the geomagnetic pole. The disturbance vector is given by sin (ψ - D) where ψ and D are dipole and ground declinations of the station respectively. The mean disturbance daily variation (SD) of H field is similar at all stations but SD of eastward field (Y) is negative at Alibag but positive at Lunping, Chichijima and Kakioka. Similarly, the daily mean values of H field systematically decrease with increasing ring current index (decreasing value of Dst index) for all stations. But declination values are positively related with Dst index at Alibag and negatively related at Lumping, Chichijima and Kakioka. The SSC amplitude in Y field shows negative values at Alibag but positive values at Kakioka. The storm-time variations Dst(H) show negative excursion during the main phase of the storm at Alibag, Lunping, Chichijima and Kakioka. On the other hand, the Dst variations in declination during the main phase show negative excursions at Alibag but positive excursions at Lumping and Chichijima. This shows a very significant longitudinal inequality in storm-time behavior of eastward geomagnetic field at any place on the earth.

Ørsted and Magsat scalar anomaly fields

The scalar anomaly field determined from available Ørsted data is compared with the upward continued scalar anomaly field derived from Magsat data. Two techniques were used to remove the core field from the Ørsted satellite data. In the first method, monthly spherical harmonic core field models of degree and order 13 derived from scalar and vector data were subtracted, and in the second method, along-track high-pass filtering of scalar data only was used. In both methods, the binned residuals were interpolated to a sphere, and subsequently filtered. Monthly degree and order 13 spherical harmonic core field models were removed from Magsat vector data. The binned Magsat vector residuals were interpolated to a sphere, filtered, and upward continued by high degree spherical harmonic analysis. The corresponding Magsat scalar anomaly field at Ørsted altitude was then determined. For latitudes below 50 degrees, removal of the core field by signal processing techniques from presently available Ørsted data led to a scalar anomaly field in better agreement with that determined from Magsat data, than removal by spherical harmonic analysis.

Semi-annual variation of the geomagnetic field

Nighttime hourly mean values of D, H and Z (or X, Y and Z in a few cases) from 113 observatories for the interval 1964.0 to 1966.0 have been analyzed to determine the semi-annual variation. Results from the 84 observatories with dip latitudes between ±60° have been subjected to spherical harmonic analysis to determine the coefficients of the internal and external parts. Only those coefficients that are found to be significantly different from zero at the 5 per cent level have been included.One of the main objectives is to obtain a reliable estimate, with confidence limits, of the internal/external ratio at a very low frequency for constraining estimates of the deep conductivity of the mantle. It is shown that a model that includes only the principal P10 term can lead to a seriously misleading internal/external ratio.

The fourth order geomagnetic multipole: The sedecimupole

SummaryThe nine coefficients of fourth order spherical harmonic functions in the expression for the potential of the main geomagnetic field are used to derive the nine parameters of a fourth order multipole. These nine parameters consist of the strength of the multipole, and four unit vectors or axes. The points of intersection of the axes with the surface of the Earth are called poles. Although movements of the poles over the period 1829 to 1965 are masked by random errors in the spherical harmonic coefficients, all of the axes show eastward components of drift since 1829, but two have drifted westward in later years to return almost to their initial 1829 positions. The strength of the fourth order multipole has shown a general increase, amounting to some 30 per cent of the 1829 value.

The Sq overhead current system approximation

An examination is made of the assumption that rotation of the horizontal component of the quiet day transient geomagnetic variation (Sq) field clockwise through 90°, gives a vector which shows the direction and which is proportional to the intensity of the overhead current system corresponding to the Sq field. Expressing the results in the form of a current function shows that the assumption leads to an error of approximately 10 per cent, at least, of the maximum overhead current circulation for the current system corresponding to the Sq field, with the largest errors occurring in middle and equatorial latitudes. Current functions have been recomputed from the results of an earlier spherical harmonic analysis of the Sq field, and show discrepancies from the current systems first given and subsequently reprinted many times.

Geomagnetic multipoles, 1965.0

SummaryFollowing the recent appearance of spherical harmonic coefficients for the potential of the geomagnetic field for epoch 1965.0, the strengths and axes for a dipole, quadrupole and octupole have been computed from first, second and third order spherical harmonic coefficients respectively. An interesting relation existing between the parameters of the dipole, quadrupole and geomagnetic eccentric dipole is also indicated.

The fifth order geomagnetic multipole: The duotrigintupole

SummaryThe notion of a dipole is generalized to the case of the fifth order spherical harmonic coefficients of the geomagnetic potential. The corresponding five axes and fifth order multipole strength are computed for ten epochs in the interval 1845 to 1965.

Some morphological aspects of geomagnetic secular variation

Abstract Analysis of geomagnetic secular variation data, in the three elements X, Y and Z at five observatories distributed over the Earth, shows that amplitudes obtained for a 50 year cycle by harmonic analysis are highly significant. Amplitudes of higher order harmonics are found to be either not as significant or not significant, indeed, the cycle can be seen directly on the graphs of observational data. These results are confirmed by power spectrum analysis. Using phase angles obtained for the 50 year (semicentennial) cycle from different stretches of data, an improved value for the period is found to be 61 ± 6 years. The approximate semi-centennial values are interesting in view of the theoretical possibility of an oscillation of an 52 S 2 0 field of period 77 years superimposed on the main field.