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?
 | Magnetic fields surround electric currents, so we surmise that circulating electic currents in the Earth's molten metalic core are the origin of the magnetic field. A current loop gives a field similar to that of the earth. The magnetic field magnitude measured at the surface of the Earth is about half aGauss and dips toward the Earth in the northern hemisphere. The magnitude varies over the surface of the Earth in the range 0.3 to 0.6 Gauss. |
The Earth's magnetic field is attributed to a
dynamo effect of circulating electric current, but it is not constant in direction. Rock specimens of different age in similar locations have different directions of permanent magnetization. Evidence for 171 magnetic field reversals during the past 71 million years has been reported.
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.
Interaction of the terrestrial magnetic field with particles from the
solar wind sets up the conditions for the
aurora phenomena near the poles.
The north pole of a compass needle is a magnetic north pole. It is attracted to the geographic North Pole, which is a magnetic south pole (opposite magnetic poles attract).
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 loopcould sustain the magnetic dipole type magnetic field of the earth. Large-scale computer models are approaching a realistic simulation of such a geodynamo.
Magnetic Field of Current Loop
| Examining the direction of the magnetic field produced by a current-carrying segment of wire shows that all parts of the loop contribute magnetic field in the same direction inside the loop. |
 |
| Electric current in a circular loop creates a magnetic field which is more concentrated in the center of the loop than outside the loop. Stacking multiple loops concentrates the field even more into what is called a solenoid. |  |
Magnetic Field Units
The standard SI unit for
magnetic field is the Tesla, which can be seen from the magnetic part of the
Lorentz force law F
magnetic = qvB to be composed of (Newton x second)/(Coulomb x meter). A smaller magnetic field unit is the Gauss (1 Tesla = 10,000 Gauss).
The magnetic quantity B which is being called "magnetic field" here is sometimes called "magnetic flux density". An older unit name for the Tesla is Webers per meter squared, with the Weber being the unit of magnetic flux.
The Solar Wind
The sun gradually loses mass in the form of high speed protons and electrons leaking away from the sun's out layers. This flux of particles is called the solar wind. It can be thought of as a kind of "evaporation" of particles from the corona. The corona reaches a temperature of about a million Kelvin at a distance of 10,000 km above the photoshpere. Such a hot gas would have a
thermal energyof about 130
electron volts, and the mean speed for hydrogen nuclei in such a gas if viewed as having a
Maxwellian speed distribution is about 145 km/s. The escape velocity from the surface of the sun is about 618 km/s, so those hydrogen atoms with average speed would not escape. Considering the nature of the speed distribution would show that there will be a few with speed above the escape velocity. Chaisson & McMillan characterize the mass loss as being about a million tons of solar matter per second. They note that at this rate, less than 0.1% of the Sun has been lost through this mechanism in its 4.6 billion year lifetime.
If a planet has a magnetic field, it will interact with the solar wind to deflect the charged particles and form an elongated cavity in the solar wind. This cavity is called the magnetosphere of the planet.
In the vicinity of the earth, the particles of the solar wind are traveling about 400 km/s. They are slowed by the interaction with the earth to produce a bow shaped shock wave around the earth.
Inside a boundary called the magnetopause, the earth's magnetic field is dominant over the effects of the solar wind. The small fraction of the charged particles which do leak through the magnetopause are trapped in two large doughnut-shaped rings called the
Van Allen radiation belts.
The solar wind was first detected directly by the spacecraft
Mariner 2. It has been studied in more detail by the
SOHO satellite.
Van Allen Belts
The earth satellite
Explorer 1 carried a Geiger counter which detected bands of radiating particles surrounding the earth. James Van Allen headed the team of scientists who investigated these bands, and they were named the Van Allen belts. One motivation for naming the belts after Van Allen was that he was the one who insisted that the satellite carry a Geiger counter for particle detection. The two huge doughnut-shaped rings contain charged particles collected from the
solar wind. The inner Van Allen belt extends over altitudes from about 2000 to 5000 kilometers and contains mainly protons. The outer Van Allen belt is about 6000 kilometers thick centered at about 16000 km from the earth. It contains mostly electrons. The outer belt was discovered by the
Pioneer spacecraft.
Aurora
When energetic charged particles enter the earth's atmosphere from the
solar wind, they tend to be channeled toward the poles by the
magnetic force which causes them to spiral around the
magnetic field lines of the earth. They are energetic enough to ionize air molecules, so a considerable number of atoms and molecules are elevated to excited states. When they make the transition back to their ground states they
emit light characteristic of the atoms and molecules. Red and green light emitted from oxygen atoms is a constituent of the light seen at the poles. Atmospheric nitrogen also plays a role. An example of the colors that might be visible can be found by observing the
nitrogen spectrum. Near the north pole the light show is called the aurora borealis and near the south pole it is called aurora australis.
Atomic Spectra
Nitrogen spectrum | |
The nitrogen spectrum shown above shows distinct bands throughout the visible range. The blue lines in the above were brightened for greater visibility in the image.
Aurora at South Pole
NASA image taken by Polar satellite, November 2004The Sun produced at least five major "halo" coronal mass ejections (CMEs) over the period of Nov. 4-8, 2004, an unusually fast pace for solar activity.The Polar spacecraft saw the aurora australis (southern lights) expanding and brightening on Nov. 8. A "halo" CME occurs when a CME produces an expanding circle of particles all around the Sun. When observers see this they know the CME is heading directly towards or away from Earth. In this case, all were headed in our direction, bringing the auroral light show with them. The source of storms was a group of sunspots called Active Region 696. The area also produced powerful solar explosions called flares. Credit: NASA/UC Berkeley
From space, the aurora is a crown of light that circles each of EarthÕs poles. The IMAGE satellite captured this view of the aurora australis (southern lights) on September 11, 2005, four days after a record-setting solar flare sent plasmaÑan ionized gas of protons and electronsÑflying towards the Earth. The ring of light that the solar storm generated over Antarctica glows green in the ultraviolet part of the spectrum, shown in this image. The IMAGE observations of the aurora are overlaid onto NASAÕs satellite-based Blue Marble image. From the EarthÕs surface, the ring would appear as a curtain of light shimmering across the night sky.
Though scientists knew that the aurora were caused by charged particles from the Sun and their interaction with the EarthÕs magnetic field, they had no way to measure the interaction until NASA launched the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) satellite in 2000. The satelliteÕs mission was to collect data that would allow scientists to study the structure and dynamics of the EarthÕs magnetic field for the first time. Designed to operate for two years, IMAGE sent its last data to Earth in December 2005 after a highly successful five-year mission.
Since 2000, IMAGE has provided insight into how the EarthÕs powerful magnetic field protects the planet from solar winds. Without the shield the magnetic field provides, the upper atmosphere would evaporate into space under the influence of solar winds. IMAGE has shown scientists what sort of changes the magnetic field undertakes as it diverts solar winds from the Earth. Image and description courtesy NASA
Heat Convection
Convection is
heat transfer by mass motion of a fluid such as air or water when the heated fluid is caused to move away from the source of heat, carrying energy with it. Convection above a hot surface occurs because hot air expands, becomes less
dense, and rises (see
Ideal Gas Law). Hot water is likewise less dense than cold water and rises, causing convection currents which transport energy.
 | Convection can also lead to circulation in a liquid, as in the heating of a pot of water over a flame. Heated water expands and becomes more buoyant. Cooler, more dense water near the surface descends and patterns of circulation can be formed, though they will not be as regular as suggested in the drawing. |
 | Convection cells are visible in the heated cooking oil in the pot at left. Heating the oil produces changes in theindex of refraction of the oil, making the cell boundaries visible. Circulation patterns form, and presumably the wall-like structures visible are the boundaries between the circulation patterns. |
| Convection is thought to play a major role in transporting energy from the center of the Sun to the surface, and in movements of the hot magma beneath the surface of the earth. The visible surface of the Sun (the photosphere) has a granular appearance with a typical dimension of a granule being 1000 kilometers. The image at right is from the NASA Solar Physics website and is credited to G. Scharmer and the Swedish Vacuum Solar Telescope. The granules are described as convection cells which transport heat from the interior of the Sun to the surface. |  |
In ordinary heat transfer on the Earth, it is difficult to quantify the effects of convection since it inherently depends upon small nonuniformities in an otherwise fairly homogeneous medium. In modeling things like the
cooling of the human body, we usually just lump it in with conduction.
