By making use of the analogy between the network quantities and the electromagnetic fields, a numerical method is employed to investigate the possible topographic effect in the earth's EM fields for several simple models in two dimensional problems. It is found that various topographies may give rise to distortions in the earth's EM fields, and some EM anomalies observed on the ground surface may be simply due to such a topographic effect.Pour etudier les effets possibles de la topographie sur les champs Clectromagnetiques terrestres, pour des modeles simples en deux dimensions, on utilise une rnethode numerique basee sur l'analogie entre les valeurs du reseau et les champs electromagnetiques. On decouvre que diverses topographies peuvent produire des distortions dans les champs Clectromagni.tiques terrestres et que certaines anomalies Clectromagnetiques obsemees a la surface peuvent resulter de tels effets topographiques.[Traduit par le journal]

The technique of Fourier analysis is reviewed and the equivalence and relative advantages of convolution filtering in the space domain and multiplication filtering in the frequency domain are demonstrated with actual field examples. We discuss the design of ideal filters in terms of the relationships between the main lobe and the side lobes. Cut‐and‐try methods appear to favor the hanning window or the hamming window, since these windows minimize the Gibbs phenomenon associated with the downward continuation or high‐pass filtering operation. New sets of coefficients for convolution filtering, based upon Fourier transform theory and the sampling theory, are derived.

Six-component recording (three Earth current components and three magnetic field variation components) in the period range 10-200s has been carried out for a continuous interval of 336 h at Mont St Hilaire, a mid-latitude station near Montreal, Canada. Power density spectra of all six components for consecutive hourly intervals for the total recording time have been computed on an analogue computer. Paper chart records were compared with the computer results. The analysis indicates that:(a) The distribution of power among the periods is not necessarily the same for all six components and there are often abrupt changes in the dominant periods of the six components from hour to hour. The changes from hour to hour in the power spectra indicate that there is no gradual change in the period, but instead changes in the relative power among the periods present.(b) There is a diurnal variation indicating a maximum near local noon in the continuity of events and in the relative power of the dominant periods, but there is no clear indication of a diurnal variation in the dominant periods of micropulsations.(c) The period of the dominant activity depends upon the three-hour range index Kp. The nature of the dependence varies among the six components.(d) The micropulsation activity in the period range 10-200s at the mid-latitude station of Mont St Hilaire is divided into two main period bands: 20-80 s, and 120-200 s. The pulsations in the longer period band usually contain much more power than those of the shorter period group.

A n analogue method was used to cornpute t h e poxvcr spectra of three orthogonal micropulsation components recordecl a t M o n t St. I-Iilaire, Quebec, Canada the X and I' components changes \\,ith period. Thus, information concerning the distribution of energy among the three components is important and cannot be obtained from either of these methods.An analogue computer \\-as used for statistical computation. Paper chart records are irnportailt in order to observe how the period varies from cycle to Canadian Journal of Physics. Volume 45 (1967) 3743 Can. J. Phys. Downloaded from www.nrcresearchpress.com by University of Tennessee on 12/28/14For personal use only.

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