Secular Variation of the Earth's Magnetic Field.

Abstract

There have been many models of the secular variation (SV) of the Earth's magnetic field proposed over the past few decades. Most if not all of them are models of Earth's magnetic field during so-called normal times, in other words, times during which the field is not reversing or undergoing an excursion. However, in studying data from Icelandic lava flows and from other areas, it has become obvious that there is no real distinction between so-called normal SV and the larger deviations of the Earth's field which are classified as excursions and/or reversals, except by the magnitude of the deviation, in which there is a continuous variation. Recently, reversal patterns have been studied in more detail, and it has been found that the exact start or end of a reversal is frequently only fuzzily defined, there being precursors to reversals, or events following reversals, which cannot be described either as belonging to the reversal per se or being a part of the normal SV pattern. In this paper, information from spherical harmonic models of the field during the past three centuries will be used to show that the field today has an abnormally large latitudinal variation of virtual geomagnetic pole (VGP) angular standard deviation, and should not be used to constrain SV models. Data for lava flows and other rock types from a number of different areas will be used to determine the scatter of the field at various observation latitudes, using VGPs from all latitudes, so that eventually a SV model may be produced which does not reject low latitude VGP information. Other factors will be described which any SV model should be able to produce. IntroductionInterest in the SV of Earth's magnetic field has been present since the time of Gellibrand (1635) who observed that the magnetic declination changed with time. Modern models of the SV depend on measurements made directly on the magnetic field as well as on the magnetization direction in rocks, sediments and archeological samples.Model A (Irving and Ward, 1964) consisted of a geocentric axial dipole field of fixed moment which is perturbed by a randomly directed vector of constant magnitude. Model B (Creer et al., 1959;Creer, 1962) consisted solely of a dipole wobble with a Fisherian distribution. It was checked out by comparing the latitudinal scatter in field direction with that produced by rotating the present day field around the geographic pole. It is now common to compare the latitudinal scatter of VGPs produced by the model with those observed from the present day field or from paleomagnetic observations, rather than looking at the latitudinal variation in the scatter of field directions. Model B gives no latitudinal variation of VGP scatter, and so was inadequate because both the present field and paleomagnetic observations give a large latitudinal variation of VGP scatter.Model C (Cox, 1962) consisted of contributions from both the dipole and the non-dipole field and relied completely on an analysis done of the present day field. Model D (Cox, 1970) included wobble of t...

Latitudinal signature of Earth's magnetic field variation over the last 5 million years

“…[31] The pattern of secular variation described in this paper shows an increase in ASD of the Fisher distribution representing the low colatitude VGP results as a function of observation latitude, as has been shown in many other papers (e.g., Constable and Johnson [1999], Kono and Tanaka [1995], McElhinny and McFadden [1997], and other references in the work of Harrison [2007]). Even with the more restrictive data set used here, the latitudinal variation seems to remain about the same.…”

“…One of the points of their paper was that variations of secular variation merged into variations associated with reversals without any break in data, as exemplified in the title of their paper. Harrison [2004Harrison [ , 2007 used vertical dipoles within the core, concentrated toward the geographic poles, to model the nondipole field including all data and found excellent agreement with observations, including intensity of magnetization as a function of VGP latitude for Icelandic data.…”

“…Large numbers of models of secular variation of Earth's magnetic field have been proposed throughout the years. For a review of these models, see Harrison [2007]. These models attempt to describe the variation of the scatter of VGPs or directions as a function of observation latitude and to explain why this variation exists.…”