Open AccessDynamo waves and palaeomagnetic secular variation
Author(s) -
Olson Peter,
Hagee V. Lee
Publication year - 1987
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.1987.tb01373.x
Subject(s) - geophysics , geology , dynamo , dynamo theory , outer core , secular variation , geomagnetic secular variation , convection , core–mantle boundary , mantle (geology) , earth's magnetic field , inner core , physics , mechanics , magnetic field , geomagnetic storm , quantum mechanics
Summary A dynamic model is proposed for large amplitude geomagnetic secular variation with periodicity in the range of 1000‐3000 yr. Thermal convection in the lower mantle imposes a spatially variable heat flux at the core–mantle boundary. If the total heat loss from the core exceeds approximately 3 times 10 12 W, then both (i) a convective layer and (ii) large scale, quasi‐geostrophic shear flows are likely to exist beneath the core–mantle boundary. The combined effects of convection and shear at the top of the outer core can generate three‐dimensionally propagating magnetic field disturbances called αω‐dynamo waves. An analysis is made of the surface magnetic field disturbances due to dynamo wave propagation in the outer core in the presence of turbulent convection and an east‐west shear flow. Non‐axisymmetric disturbances exhibit the following characteristics: (i) dynamo waves propagate in each hemisphere with a poleward or equatorward component, depending on the sign of the α‐effect; (ii) both eastward and westward propagation is allowed although westward propagation is preferred. The apparent velocity at the earth's surface is the sum of propagation and drift velocities in the core; (iii) dynamo wave disturbances are transient, with exponential growth and decay phases; (iv) traces of inclination versus declination of individual Fourier components obey Runcorn's circularity rule. A comparison is made with lake sediment secular variation records from central North America measured by Lund & Banerjee. The band‐passed time series and the major inclination/declination loops can be modelled from 2600 to 10 000 yr BP by a single dynamo wave, periodic in 2400 yr, propagating under the site along an azimuth of 345°. Between 0 and 2600 yr BP, the record can be modelled by addition of a wave with 1200 yr period, propagating along an azimuth of 30°. This indicates that the palaeomagnetic secular variation record in North America to 10 000 yr may be dominated by poleward propagating wave‐like disturbances.