GEOMAGNETISM: a dynamo at the centre of the Earth - PowerPoint PPT Presentation

1 / 32
About This Presentation
Title:

GEOMAGNETISM: a dynamo at the centre of the Earth

Description:

OVERVIEW Lateral variations in temperature in the lower mantle determine the heat flux across the ... Persistent path selection requires departures from axial ... – PowerPoint PPT presentation

Number of Views:83
Avg rating:3.0/5.0
Slides: 33
Provided by: DavidGu154
Category:

less

Transcript and Presenter's Notes

Title: GEOMAGNETISM: a dynamo at the centre of the Earth


1
GEOMAGNETISM a dynamo at the centre of the Earth
  • Lecture 4
  • Thermal Core-Mantle Interaction

2
OVERVIEW
  • Lateral variations in temperature in the lower
    mantle determine the heat flux across the
    core-mantle boundary
  • this affects core convection
  • e.g. by promoting downwelling beneath cold
    mantle, upwelling beneath hot regions...
  • ...this affects the magnetic field
  • e.g. by downwelling concentrating magnetic flux,
    upwelling dispersing it

3
(No Transcript)
4
(No Transcript)
5
The core-mantle boundary
6
Thermal Core-Mantle Interaction
(hot)
(cold)
7
  • Downwelling below cold mantle concentrates flux
  • Upwelling below hot mantle disperses flux

8
Lateral variations in heat flux across the CMB
can
  • Drive thermal winds
  • lock core convection
  • Force a lateral scale on the core convection
  • none of the above!

9
(No Transcript)
10
Critical Rayleigh number for magnetoconvection
E10-9
11
DYNAMO CATASTROPHE
  • The Rayleigh number is fixed
  • The critical Rayleigh number depends on field
    strength
  • Vigour of convection varies with supercritical
    Ra
  • So does the dynamo action
  • If the magnetic field drops, so does the vigour
    of convection, so does the dynamo action
  • The dynamo dies

12
(No Transcript)
13
(No Transcript)
14
The Tangent Cylinder
15
Z2 on core surface 1980
16
GEOMAGNETIC FIELD AND LOWER MANTLE VS AS
FUNCTIONS OF LONGITUDE
17
THE TIME-AVERAGED PALEOMAGNETIC FIELD LAST 5Ma
18
(No Transcript)
19
Reversal transition paths (1)
  • Many reversal transition fields have now been
    captured in lava flows and sediments
  • These are plotted for each site by mapping into a
    virtual geomagnetic pole
  • The VGPs trace a path from north to south or
    south to north that depends on the site and the
    transition field
  • Many paths tend to lie in the Americas or in
    Asia, the other half of a great circle

20
(No Transcript)
21
Transitions reversals and excursions(Love 1998)
22
REVERSAL SCHEME OF GUBBINS SARSON (1994)
  • Small changes in fluid flow change a steady field
    to an oscillatory one, a dynamo wave
  • Flows generating oscillatory solutions are rare
  • Fluctuations in fluid flow can move the dynamo
    between steady and oscillatory regimes
  • Both oscillatory and steady fields have flux
    concentrated by downwellings induced by cold
    mantle
  • The oscillatory fields give persistent VGP paths
    lying close to the longitudes of downwelling

23
(No Transcript)
24
(No Transcript)
25
VGP PATHS
26
(No Transcript)
27
Reversal transition paths (2)
  • Persistent path selection requires departures
    from axial symmetry, as in the boundary
    conditions
  • The path is site-dependent unless the transition
    field is a perfect axial dipole
  • If some sites record an American path and some an
    Asian path for the same transition, other sites
    could be expected to record neither path
  • A site recording an Americas path for a
    normal-reverse transition must record an Asian
    path for a reverse-normal transition

28
Matuyama-Brunhes, after Love Mazau (1997)
29
(No Transcript)
30
(No Transcript)
31
Matuyama-Brunhes VGPsmodified
32
CONCLUSIONS
  • Thermal interactions can influence core
    convection and the magnetic field.
  • The same mechanism of flux concentration can
  • explain the present-day morphology
  • produce a non-axisymmetric time average
  • explain persistent VGP reversal transition paths
  • explain persistent low secular variation in the
    Pacific
Write a Comment
User Comments (0)
About PowerShow.com