The Analysis of Daily Variations Recorded by Magnetometer Arrays
Frederick E. M. Lilley · Geophysical Journal International · 1975
A procedure is proposed for the analysis of daily variations recorded by magnetometer arrays, based upon the analysis of each quiet day as an individual and complete event. Such an approach is justified by the argument that any particular observatory has no premonition of variation to occur on the next day, and records only an insignificant signal from the variation of the day before. Many of the factors affecting the variation on an individual day are independent of the rotation of the Earth, and are also therefore independent of the strong 24-hr, 12-hr, 8-hr and 6-hr periodicities usually associated with daily variation analyses. Data for individual days are transformed into the frequency domain and plotted on polar diagrams. For an array area, the separation of anomalous fields from regional fields is based upon determining the latter as a smooth curve on a polar diagram. If a sufficient number of days are available for analysis, and they are sufficiently varied, all nine elements of a transfer function matrix may be sought, relating locally anomalous fields to regional fields.