Earth's Magnetic Field

The Dynamo Effect

The simple question "how does the Earth get its magnetic field?" does not have a simple answer. It does seem clear that the generation of the magnetic field is linked to the rotation of the earth, since Venus with a similar iron-core composition but a 243 Earth-day rotation period does not have a measurable magnetic field. It certainly seems plausible that it depends upon the rotation of the fluid metallic iron which makes up a large portion of the interior, and the rotating conductor model leads to the term "dynamo effect" or "geodynamo", evoking the image of an electric generator.

Convection drives the outer-core fluid and it circulates relative to the earth. This means the electrically conducting material moves relative to the earth's magnetic field. If it can obtain a charge by some interaction like friction between layers, an effective current loop could be produced. The magnetic field of a current loop could sustain the magnetic dipole type magnetic field of the earth. Large-scale computer models are approaching a realistic simulation of such a geodynamo.

1838, Carl Friedrich Gauss proved 95% of Earth's magnetic field is internal, 5% external
Earth's field is sum of 3 parts:
- Main magnetic field
  - produced by electrical currents in outer core
  - steady on time scale of days, but variable over years
  - approximately 50,000 nT (0.5 Oe)
  - accounts for large regional variations in intensity and direction
- External magnetic field
  - produced by electrical currents in ionosphere consisting of particles ionized by solar radiation
  - usually variable over hours, at tens of nT
  - occasionally variable over minutes, at hundreds of nT (magnetic storm)
- Anomalous, induced magnetic field
  - magnetization induced in crust by Main Field (and External Field) or remanent magnetization ("permanently induced")
  - limited to upper crust (Curie T)
  - limited to ferr(-o, -i)magnetic materials

Spherical Harmonic Analysis of Earth's Field

if Earth's field were dipolar and aligned with geographic north (i.e., with coordinate system for spherical harmonic analysis), then g₁⁰ would be only non-zero term
if dipolar, but tilted, g₁⁰, g₁¹, h₁¹ would be only non-zero terms

International Geomagnetic Reference Field (IGRF)

often subtract Earth's main field from magnetic data:
- removes regional field
- removes secular variation in main field, important when combining data acquired years apart
field evaluated every 5 years; coefficients of field, and secular variation in field, published
example: to find field in June, 2003 (2003.5), extrapolate by using IGRF2000 values for 2000.0 plus secular terms for 2000.0 times 3.5; to find field in 1994, interpolate between 1990.0 and 1995.0 coefficients

Main Magnetic Field

can be thought of as consisting of dipolar and non-dipolar components

Dipole Field

best-fit (tilted) dipole field, at r = a (radius of Earth)
N pole presently at 79 deg N, 70 deg W
S pole presently at 79 deg S, 110 deg E

Dip poles

based on Earth's actual, total field, dip poles are where inclination = 90 deg
magnetic equator is 0 deg inclination contour line
"current" dip poles:

Non-Dipole Field

total field - dipole field
shows four major anomalies (quadrupolar)
can be constructed from spherical harmonic coefficients

Secular Variation

data go back to at least 1500s (London Observatory)
change about 50 - 150 nT/year most places mostly related to non-dipole field
non-dipole field changing also; decreasing 1/2000 per year
westward drift: about 0.2 deg longitude per year
- produces decreases and increases at some localities
- indicates that core rotating more slowly than mantle/crust

External Field

about 1% of total field
diurnal variation
- tens of nT
- strongest at equator
- seasonal: diurnal variation greatest in summer
- all suggest role of sun
  1. EM (UV and x-rays) ionizes particles in ionosphere
  2. Sun's tidal force produces cyclic wind currents in ionosphere, which in turn produces electrical currents
- ionosphere directly produces 2/3 of observed diurnal effect; induced currents in Earth contribute about 1/3
magnetic storms
- from 100s to 1000s of nT
- last hours to days
- caused by sunspot activity
- 27 day cycles (sunspots rotate with Sun)
- fixed-station recording magnetometers provide data on field variability (e.g., Alaska)

Field Elements

Magnetic Reversals

CNN Article on Reversals (4/7/04)

Illustrations of Earth's Magnetic Field

Magnetic field of Earth based on IGRF 1990 (Blakely):

(a) Isodynamic map showing total intensity, contour interval 2,500 nT

(b) Isoclinic map showing constant inclination, contour interval 10^o

Present-day total magnetic field

Secular variation in vertical field

Magnetic field power spectrum

Field values based on 2000 IGRF