Open AccessSecular variation from monthly means from Apia and Amberley magnetic observatories
Author(s) -
Gubbins D.,
Tomlinson L.
Publication year - 1986
Publication title -
geophysical journal of the royal astronomical society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.302
H-Index - 168
eISSN - 1365-246X
pISSN - 0016-8009
DOI - 10.1111/j.1365-246x.1986.tb03846.x
Subject(s) - earth's magnetic field , secular variation , quiet , geodesy , sunspot , climatology , residual , series (stratigraphy) , meteorology , geology , environmental science , magnetic field , geography , geophysics , mathematics , physics , astronomy , paleontology , algorithm , quantum mechanics
Summary. The availability of data from magnetic observatories in machinereadable form means that we can now study secular change (internal variations of the magnetic field) by analysing time series with a rapid sampling rate lather than annual means. The major task is to identify and remove fields due to external sources: the major benefit is finer time resolution, which is particularly important because of the recent interest in sudden phenomena such as geomagnetic impulses and jerks. Apia and Amberley observatories provide good data for experimentation. Many external fields are satisfactorily excluded by taking averages of hourly values at local midnight for the five international quiet days of the month and removing annual and semi‐annual lines by least squares. Quiet days based on the K ‐index are found to be at least as suitable as those based on the AE and D st indices. Time series of monthly means are developed for all components at both observatories from 1921 (Apia) and 1923 (Amberley). Cubics provide good fits to the long‐term variations, but the residuals to these cubics are highly significant. An 11 yr variation is noticeable, showing that we have not succeeded in removing this external source of noise. Residual curves for both observatories, which are some 30° apart, follow each other quite closely. The large swings, with a period of approximately 22 yr, that were noted to be in phase with the double sunspot cycle by Beagley & Bullen, have not continued in phase. The magnetic field is widely believed to have undergone a change around 1969–70. This change is now known as the geomagnetic jerk. The finer time resolution afforded by monthly means is used to investigate the duration of this event. The resolution is limited to 6–18 month by noise in the measurements. A rapid change in the Y ‐component occurs effectively instantaneously at 1970.6 for Apia and 1972.4 for Amberley. We propose our method as a useful one for a global study of the phenomenon. Another jerk, believed to occur in 1978, is revealed in the Apia record at 1978.1.