Waves in the Earth’s Core
(Understanding historical evolution of the Earth’s magnetic field)
Student:
Chris Finlay
Supervisors: Dr. A. Jackson
Prof. D. Gubbins (FRS)
Background
•
•
Seismology:
Outer core is liquid (mostly iron)
Geomagnetic Observations: Field changes on a range of time scales
•
Dynamo Theory:
Earth’s magnetic field is generated by convection
driven fluid motion which produces currents in the
outer core
Observations
•
Geomagnetic field has been measured for the past 400 years by:
–
–
–
–
marine navigators
network of magnetic observatories (since 1830’s)
magnetic surveys
satellites (late 20th century)
Mapping Field at Core-Mantle
Boundary
•
•
If the mantle is a poor conductor, the magnetic field at CMB is
found by downward continuation of surface measurements
Small scale features are preferentially amplified
Field at CMB from 1590-1990
•
•
Time evolution of features is observed in movies constructed from
maps from different epochs
Movies show both stationary and time dependent (drifting) features
Causes of magnetic field
change
•
Highs and lows seen in the maps at the CMB correspond to
regions of high and low density of magnetic field lines
•
To understand the evolution of these features we must consider
mechanisms which change the position of magnetic field lines
(i) Advection by fluid flows
(ii) Magnetic wave propogation
•
It is likely that both mechanisms occur in the Earth’s core
•
This project aims to elucidate the properties of the magnetic
waves by observational analysis and theoretical modelling
Planetary waves
• Waves are the natural response of a fluid to disturbance
• Energy and momentum are transmitted by waves - this is how
different parts of large fluid systems influence each other
• Waves have been predicted and observed in
several planetary scale rotating fluid systems:
– Earth’s atmosphere
– Earth’s oceans
– the Sun
• We plan to apply techniques developed
in these fields to the geomagnetic problem
Project plan
• Use of satellite data from current missions
OERSTED, CHAMP and new high resolution
imaging techniques to identify magnetic waves
• Time series analysis of historical observations to
characterise magnetic wave properties (phase
speed, dispersion relation, typical scale)
• Theoretical modelling of wave dynamics
• Analysis of dynamo models for evidence of waves
Summary
• Magnetic wave propagation could help explain the
evolution of the Earth’s magnetic field
• No previous study has unambiguously identified
magnetic waves in the core
• Techniques for planetary wave characterisation exist
and could be applied to our CMB magnetic maps
• Identification and characterisation of magnetic waves
would be an advance in our understanding of core
dynamics