All issues
Successor
Volume 32 (1980)
- Issue 12 Pages 721-
- Issue 11 Pages 671-
- Issue 10 Pages 585-
- Issue 9 Pages 507-
- Issue 8 Pages 431-
- Issue 7 Pages 365-
- Issue 6 Pages 309-
- Issue 5 Pages 265-
- Issue 4 Pages 181-
- Issue 3 Pages 129-
- Issue 2 Pages 67-
- Issue 1 Pages 1-
- Issue Supplement3 Pag･･･
- Issue Supplement2 Pag･･･
- Issue Supplement1 Pag･･･
Volume 44, Issue 6
Displaying 1-7 of 7 articles from this issue
- |<
- <
- 1
- >
- >|
-
J. F. HERMANCE, W. WANG1992 Volume 44 Issue 6 Pages 373-389
Published: 1992
Released on J-STAGE: September 14, 2010
JOURNAL FREE ACCESSTraditional approaches to the global induction problem have often determined the conductivity structure of the earth for specific modes of the external source field. In the case of “disturbed time” (or Dst) epochs, workers have tended to use the P10 term, or, in the case of “quiet time” (or Sq or L) epochs, workers have used various low-order Pnm terms. Each of these “modes” has tended to lead to a distinct earth model which differs from those derived from other modes. Since one and only earth model should jointly “fit” any and all modes simultaneously, it is not clear what these separate models represent—we need an approach which is essentially “mode-blind”. We address this problem by applying an extension of the magnetic gradiometric technique to data from 31 standard magnetic observatories in the European sector. We employ the two years of hourly values culled by WINCH (1981) to 1) characterize the dominant statistical properties of the primary data set and reject outliers; 2) construct low order polynomial forms over the array of observatories; 3) estimate gradiometric response parameters; 4) explore model space for classes of earth models which account for all modes simultaneously, and which fit the data while minimizing the resistivity structure at depth.View full abstractDownload PDF (1756K) -
Ebun ONI, J. A. SOGADE1992 Volume 44 Issue 6 Pages 391-402
Published: 1992
Released on J-STAGE: September 14, 2010
JOURNAL FREE ACCESSThe theoretical problems associated with the use of non-uniform electromagnetic source field propagated in low latitudes, in the study of Earth models was presented. In this we present recent results of the theoretical modelling of non-uniform electromagnetic source field interaction with 1 and 2 dimensional Earth models.View full abstractDownload PDF (766K) -
Ebun ONI, J. A. SOGADE1992 Volume 44 Issue 6 Pages 403-414
Published: 1992
Released on J-STAGE: September 14, 2010
JOURNAL FREE ACCESSRecently we have been trying to appropriately model the spectrum of electromagnetic waves propagated in the Earth-Ionosphere waveguide in low latitudes. We shall present the latest models of such fields and the results of their interactions with 2-dimensional earth models.View full abstractDownload PDF (742K) -
Bruce Alan HOBBS1992 Volume 44 Issue 6 Pages 415-431
Published: 1992
Released on J-STAGE: September 14, 2010
JOURNAL FREE ACCESSHourly values of the earth's magnetic field recorded at world-wide magnetic observatories have traditionally been used to study Sq variations and this can lead in particular to the construction the earth's response function at the daily period and its sub-harmonics. As an alternative to least squares analyses of the Sq potential, PRICE and WILKINS (1963) proposed a method involving interpolation, line integral constraints and surface integral separation of the potential into internal and external parts. The philosophy of the first two steps of their procedure is adopted here and these steps are rendered automatic. Spherical spline interpolation is used to convert the observatory X, Y, Z values to those over a regular surface grid and the use of line integral constraints to ensure the X, Y values represent a true potential is cast as a linear inverse problem with an exact solution. With regular data now available, stable spherical harmonic analysis can be applied to separate the total field into internal and external parts and the ratio of these taken to construct the Q-response. The method is illustrated on synthetic data and is then applied to the Price-Wilkins observatory data from the 1932-3 International Polar Year. Several estimates of the Q-response at periods of 24, 12 and 8 hours are converted to Schmucker's c-response and compared to longer period data from other analyses.View full abstractDownload PDF (1242K) -
Nils OLSEN1992 Volume 44 Issue 6 Pages 433-447
Published: 1992
Released on J-STAGE: September 14, 2010
JOURNAL FREE ACCESSThe C-responses of selected observatories are determined for 6 frequencies (1 cpd-6 cpd) using the Z: Y-method: Horizontal components of the magnetic variations are expanded into series of spherical harmonics to achieve a global representation of the unseparated magnetic potential. The C-response of each site is evaluated by comparison with the locally observed Z-variation.
The spherical harmonic analysis is accomplished for each day except D-days separately and coefficients with n=1, ..., 10 and m=-n, ..., n (120 coefficients) are determined. A damping constant α2-n8 is added to the diagonal elements of the normal equations to avoid oscillations (after Marquardt). Mean values of the C-responses are estimated for each observatory using robust methods.
The results using 54 months of world-wide data (≈100 observatories) are consistent for different data sets and adjacent observatories but show a clear regional trend in Europe with decreasing resistivity from the West to the East.View full abstractDownload PDF (1138K) -
Wen-Yao XU1992 Volume 44 Issue 6 Pages 449-458
Published: 1992
Released on J-STAGE: September 14, 2010
JOURNAL FREE ACCESSThe magnetic fields caused by the magnetospheric and ionospheric current systems are calculated. These current systems are Chapman-Ferraro current system (C-F), magnetotail current system (MTL), symmetric ring current system (SRC), region 1 field-aligned current system (FAC1), partial ring current-region 2 field-aligned current-ionospheric current system (PRFI), and the ionospheric dynamo current system (IDC). The variable magnetic fields recorded at the ground surface are the superposition of these fields and other disturbance fields. The spatial-temporal characteristics of these fields are comparatively studied, and a method for separating dynamo-field from the total field is developed. In order to describe conveniently dynamo-field intensity, a new magnetic index, “Sq index”, is proposed.View full abstractDownload PDF (845K) -
W. H. CAMPBELL, E. R. SCHIFFMACHER, B. R. ARORA1992 Volume 44 Issue 6 Pages 459-480
Published: 1992
Released on J-STAGE: September 14, 2010
JOURNAL FREE ACCESSThis paper describes a technique for obtaining the quiet-time geomagnetic field variation expected for all days of the year and distribution of latitudes from a limited set of selected quiet days within a year at a discrete set of locations. We used a data set of observatories operated by Indian and USSR scientists in 1976 and 1977 near 75°E longitude as illustration. Our method relies upon spatial smoothing of the decomposed spectral components. An evaluation of the fidelity of the resulting model shows correlation coefficients usually above 0.9 at the lower latitudes and near 0.7 at the higher latitudes with variations identified as dependent upon season and field element.View full abstractDownload PDF (1938K)
- |<
- <
- 1
- >
- >|