Geomagnetism Part II: The Earth’s Magnetic Field

Magnetic Field of the Earth The earth's magnetic field is similar to that of a bar magnet tilted 11 degrees from the spin axis of the earth. The problem with that picture is that the Curie temperature of iron is about 770 C . The earth's core is hotter than that and therefore not magnetic. So how did the earth get its magnetic field? Let’s see what it looks like first – that will give us some clues.

The Magnetic Field Measurement We measure a vector at the surface using two angles: inclination and declination Inclination – like dip (w.r.t. horizontal) Declination – like azimuth; angle w.r.t. true north.

Where does that field come from? External and Internal Sources. A lot can be determined just from surface observations and a theoretical expansion that quantifies what internal and external contributions are (spherical harmonics). Fit this expression, and find that nearly all is internal – has to do with rate of decay with r. Also, we can determine that it is predominately a dipole.

EXTERNAL FIELD: Different from dipole on large scale due to interaction with charged particles from Sun (Solar wind). Elements of the External Field: Magnetosphere The whole enchilada Bow Shock region Particles pile up in something akin to a shock wave Megnetosheath Region of deflection Magnetotail The downwind part of the field Magentopause Transition region Van Allen belts Charged particles that manage to penetrate the magnetopause get caught in these.

Main Elements of the External Field

James A. Van Allen (1914- )

Ionosphere Gamma and X rays from the Sun ionize the N and O in the atmosphere. Thicker on day side of the earth. Used by radio folks to bounce radio waves. Transmit on day side and bounce to night side of Earth. So best listening is at night. Several layers – including the Heaviside layer at 80- 100 km (layer D) especially good for communication. Diurnal variation Charged particles in the ionosphere make their own mag field that accounts for daily variations on the order of 10-30 nT. Mag Storms Solar flares with cycles of about 11 years ~ disturbances of 1000 nT. Interferes with radio transmission, and with Geo mag work (go do something else).

Oliver Heaviside Heaviside was able to greatly simplify Maxwell's 20 equations in 20 variables, replacing them by four equations in two variables. Today we call these 'Maxwell's equations' forgetting that they are in fact 'Heaviside's equations'.

Two representations of the Heaviside layer

Sunspot Activity

Interaction of the terrestrial magnetic field with particles from the solar wind sets up the conditions for the aurora phenomena near the poles.

INTERNAL FIELD Expansion of field in terms of poles: 95% is like a dipole. Secular Variations: Position of pole is moving, but magnitude is decreasing by about 10% per year over the past 150 years! Young Earth? Are we on the verge of a polarity shift? Where does it come from? Spherical harmonic analysis shows must be from deep in the Earth – fluid core, lots of metal. Seems like a natural – electric currents in a conductive core. Big problems though – should decay in about 10,000 years unless it is sustained somehow. How sustained? Heat engine again! This is the Geodynamo idea.

Simply stated: The core convects to loose heat, just like the mantle (source of heat? radioactivity or latent heat from inner core formation). Material rises up to the CMB. Because of differences rotational speed and distance from center, the conservation of angular momentum in the core requires that the inner layers rotate faster than the outer layers. This stratification causes a rotational component and eventually a current loop, which generates the field. So – liquid core, heat and rotation of the earth all play a part in generating the field.

Top: A simulation of Earth's magnetic field structure Top: A simulation of Earth's magnetic field structure. Bottom: An image of what Earth's magnetic field might look like during a reversal, something humans may have to worry about.

Although the details of the dynamo effect are not known in detail, the rotation of the Earth plays a part in generating the currents which are presumed to be the source of the magnetic field. Mariner 2 found that Venus does not have such a magnetic field although its core iron content must be similar to that of the Earth. Venus's rotation period of 243 Earth days is just too slow to produce the dynamo effect.

Declination: Then and Now

Migration of the North Pole