Open AccessOn Backus’ mantle filter theory and the 1969 geomagnetic impulse
Author(s) -
Courtillot V.,
Mouël J. L. Le,
Ducruix J.
Publication year - 1984
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.1984.tb01965.x
Subject(s) - mantle (geology) , geology , geophysics , earth's magnetic field , impulse (physics) , geodesy , physics , mathematics , mathematical analysis , magnetic field , classical mechanics , quantum mechanics
Summary. We present a discussion of Backus’ mantle‐filter theory in relation to the 1969 geomagnetic impulse (or jerk). Backus shows that mixing of harmonic modes may be responsible for the negative value of the squared mantle‐filter smoothing time which is found with European observatory data, a feature that is not mathematically possible for a pure internal harmonic impulse. We show here, and illustrate with a simple numerical experiment, that this negative value may simply be due to a remaining external solar cycle effect. This, together with the observation that the 1969 impulse is both world‐wide and synchronous, leads us to propose that: (1) the smoothing time and zero‐frequency delay of the mantle filter, as defined by Backus, are both likely to be shorter than a few years; (2) in that case, the 1956 elbow observed in the length of day curve would not be related to the 1969 impulse; (3) electrical conductivity is likely to be lower than a few hundred ω ‐1 m ‐1 in more than 97 per cent of the volume of the mantle. We recall our earlier suggestion that the magnetic variations apparently lead the length of day curve by 10‐15 yr, which can be accounted for in terms of a Bullard model of electromagnetic core‐mantle coupling.