Secular variation from monthly means from Apia and Amberley magnetic observatories

Abstract

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 rather 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 t o be at least as suitable as those based on the AE and DSt indices.Time series of monthly means are developed for all components a t both observatories from 1921 (Apia) and 1923 (Amberley). Cubics provide good fits t o the long-term variations, but the residuals t o 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 t o be in phase with the double sunspot cycle by Beagley & Bullen, have not continued in phase.The magnetic field is widely believed t o 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 t o investigate the duration of this event. The resolution is limited t o 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 D. Gubbins and L . Tomlinson useful one for a global study of the phenomenon. Another jerk, believed to occur in 1978, is revealed in the Apia record a t 1978.1.

The Geomagnetic Field 1900-1995, Including the Large-Scale Field from Magnetospheric Sources, and the NASA Candidate Models for the 1995 Revision of the IGRF

“…Discernment of jerks has also been claimed at other epochs, though it should be noted that the definition of a jerk is debated. Among the most credible claims are that jerks occurred near 1913 (Ducruix and Le Mouel, 1983;Gire et al, 1984), and near 1978 (Gavoret et al, 1986;Gubbins and Tomlinson, 1986). Alexandrescu et al (1995) have proposed a test for jerks using wavelet analysis that seems to work well.…”

Local time features of geomagnetic jerks

“…Alldredge (1984) and Alldredge (1985), however, questioned the data selection and the method of analysis by Malin and Hodder (1982), and insisted that an external current system could generate the observed jerks. Despite of many researches on the jerks, whether their sources are internal or external is still controversial (e.g., Kerridge and Barraclough, 1985;McLeod, 1985;Gavoret et al, 1986;Gubbins and Tomlinson, 1986;Thompson and Cain, 1987;Whaler, 1987;Golovkov et al, 1989;McLeod, 1992). It is suggested that the jerks may have some correlations with abrupt changes in decadal length-of-day variation (e.g., Courtillot et al, 1978;Le Mouël and Courtillot, 1981;Davis and Whaler, 1997;Mandea et al, 2000) and with motion of fluid flow at the top 0000-0100LT 0600-0700LT 1200-1300LT 1800-1900LT Fig.…”

Geomagnetic Secular Variation at the Indian Observatories.

“…A jerk around this period was also seen in Portugal (Pais and Miranda, 1995) and in Peru (Rangarajan et al, 1996). Gubbins and Tomlinson (1986) detected the signature of the 1969-70 impulse in New Zealand too, but found that the epochs for the jerk in the Y component at Apia and Amberley differed by 2 years. Whether the causes for the impulsive changes are internal (Courtillot et al, 1978) or external (Alldredge, 1979;Nevanlinna and Sucksdorff, 1981) is still debatable (Malin and Hodder, 1982).…”

“…Two straight-line segments with distinctly different slopes are indicative of a possible jerk. If the first derivative is fairly linear in time before and after the jerk, the cusp in the derivative is easy to detect (Gubbins and Tomlinson, 1986).…”