Palaeomagnetic secular variation as a function of intensity

Abstract

We seek to establish whether or not secular variation, the rate at which the magnetic eld is changing in time, is a function of eld strength. Towards that end we examine a database consisting of palaeomagnetic directions and absolute intensities from piles of extruded lava ®ows, many of which record polarity transitions. We nd that directions from stratigraphically adjacent lava ®ows are most (least) correlated when the local eld strength is high (low). Since volcanic activity is unrelated to, and therefore uncorrelated with, magnetic secular variation, this relationship between angular correlation and intensity indicates that angular secular variation is quiet (enhanced) when and where the local eld strength is high (low). Our conclusion is consistent with some aspects of the recent behaviour of the modern eld and is qualitatively consistent with sedimentary data recording reversals. Although we nd a simple relationship between angular di¬erence and intensity, a corresponding relationship for relative intensity di¬erences has proved to be more elusive; its possible resolution will bene t from the continued collection of full vectorial palaeomagnetic data from lavas. Statistical models of secular variation need to incorporate the information content of serially correlated stratigraphically ordered data if the lava data are to be fully exploited. We suggest that the apparent inverse relationship between angular secular variation and local eld strength could be the result of electromagnetic coupling between the solid inner core and the liquid outer core, with the inner core tending to stabilize core convection, and hence the eld, when the intensity is high (as has been hypothesized).

The Cretaceous superchron geodynamo: Observations near the tangent cylinder

“…Numerical simulations that vary the pattern of heat flux across the core-mantle boundary result in a stable geodynamo with a highest dipole moment (2) qualitatively similar to our observations. Our analyses further suggest that geomagnetic reversals, field morphology, secular variation, and intensity, which are sometimes considered to be isolated phenomena, may instead be related on time scales of thousands (45) to hundreds of millions of years (7,8).…”

Paleomagnetism and 40Ar/39Ar ages from La Palma in the Canary Islands

“…First, the data set includes directions that areintermediate between normal and reversed and should be excluded from the timeaveraged field data base. Second, these data come from continuously sampledsequences of lava flows (and sills) whose age relations are poorly constrained.It is difficult to avoid overemphasis of particular geomagnetic field statesand to ensure that each data point represents a distinct estimate of the geomagneticfield, and efforts at combining data from sequential lava flows are fraughtwith difficulty [e.g., Love , 2000]. In order to avoid such difficulties, Johnson andConstable [1998, p. 95] suggested that data for paleosecularvariation studies should have “sufficient information in the referenceto establish temporal independence of the flows.” Hence these data tooshould not be included in TAF models.…”

Discrete scale invariance connects geodynamo timescales

“…The geodynamo can therewith be considered a single scale‐free system with extremely long‐term memory, pervaded by DSI‐modulated power laws that jointly span over seven orders of magnitude in time, possibly more. Specific events such as geomagnetic jerks, core spots, archeomagnetic jerks, mean zonal flows, dipole intensity fluctuations, reversals and even superchrons can thereby be interpreted as internally controlled units of secular variation associated with a particular threshold, preferred interval and physical size (Sarson & Jones 1999; Coe et al 2000; Love 2000; Hulot & Gallet 2003; Narteau & Le Mouël 2005). Together, these characteristic scales provide a quantified profile with which to constrain the relevant physics.…”