On the flow magnitude and field-flow alignment at Earth's core surface

Chris Finlay, H. Amit

Research output: Contribution to conferenceConference abstract for conferenceResearchpeer-review

Abstract

We present a method to estimate the typical magnitude of flow close toEarth's core surface based on observational knowledge of the maingeomagnetic field (MF) and its secular variation (SV), together withprior information concerning field-flow alignment gleaned from numericaldynamo models. An expression linking the core surface flow magnitude tospherical harmonic spectra of the MF and SV is derived from the magneticinduction equation. This involves the angle gamma between the flowand the horizontal gradient of the radial field. We study gamma in asuite of numerical dynamo models and discuss the physical mechanismsthat control it. Horizontal flow is observed to approximately followcontours of the radial field close to high-latitude flux bundles, whilemore efficient induction occurs at lower latitudes where predominantlyzonal flows are often perpendicular to contours of the radial field. Weshow that the amount of field-flow alignment depends primarily on amagnetic modified Rayleigh number Raeta = alpha g0 Delta T D / eta Omega , which measures the vigorof convective driving relative to the strength of magnetic dissipation.Synthetic tests of the flow magnitude estimation scheme are encouraging,with results differing from reference values by less than 8%.Application to a high quality geomagnetic field model based on satelliteobservations (the xCHAOS model in epoch 2004.0) leads to a flowmagnitude estimate of 11 - 14 km / yr, in accordance with previousestimates. When applied to the historical geomagnetic field model gufm1for the interval 1840.0 - 1990.0, the method predicts temporalvariations in flow magnitude similar to those found in earlier studies.The calculations rely primarily on knowledge of the MF and SV spectra;by extrapolating these beyond observed scales the influence of smallscales on flow magnitude estimates is assessed. Exploring three possiblespectral extrapolations we find that the magnitude of the core surfaceflow, including small scales, is likely less than 50 km / yr.
Original languageEnglish
Publication date2012
Number of pages1
Publication statusPublished - 2012
Event2012 AGU Fall Meeting - San Francisco, United States
Duration: 3 Dec 20127 Dec 2012
Conference number: 45
http://fallmeeting.agu.org/2012/

Conference

Conference2012 AGU Fall Meeting
Number45
CountryUnited States
CitySan Francisco
Period03/12/201207/12/2012
Internet address

Keywords

  • Geomagnetism and Paleomagnetism
  • Core processes
  • Dynamo: theories and simulations

Cite this

Finlay, C., & Amit, H. (2012). On the flow magnitude and field-flow alignment at Earth's core surface. Abstract from 2012 AGU Fall Meeting, San Francisco, United States.