Internet resources

NOAA's National Geophysical Data Center's FAQ page (highly recommended)

The U.S. Geological Survey's National Geomagnetism Program

How to construct a simple soda bottle magnetometer

Basics

• Earth acts like a bar magnet (which is slightly off axis of its rotational axis) with its south pole directed at Canada and its north pole at Antarctica.
  Location of the magnetic dip poles.

• The magnetic field at the surface of the Earth is largely determined by processes within Earth's core. The field generated by these processes is called the Main Field.

• However, about 10% of the surface magnetic field is due to processes in Earth's atmosphere. This contribution is called the External Field.

• Charge particles (electrons and ions) can simultaneously undergo three types of motion as a result of Earth's magnetic field.
  • gyration: circular motion or cyclotron motion [on the order of thousandths of seconds (milliseconds)]
    
  • latitudinal oscillation: bounce or mirroring between the poles [on the order of seconds]
    
  • longitudinal drift: revolution around the Earth [on the order of hours]

• Moving charged particles (electrical currents) generate magnetic fields

• Note: moving electrons usually determine the direction of an electrical current, but by convention, the direction of the electrical current is in the direction opposite to the flow of electrons

Historical outline

Internet resources

Windows to the Universe Radiation Belt page

A perspective of military exploitation from the Global Policy Forum about Argus, Starfish and HAARP

Basics

• The Van Allen Belts are toroidal (doughnut shaped) regions of magnetically trapped highly energetic charged particles
• "Radiation" refers to the energetic particles; the Belts are not radioactive, however it is release of similarly energetic particles that makes radioactivity dangerous
• These particles coexist with the much more numerous lower energy particles of the plasmasphere
• The outer belt
  
  • largely populated with electrons originating from the solar wind or the ionosphere that have become accelerated by magnetospheric processes
    
  • maximum population typically around L = 4 (meaning the geomagnetic field line that is at 4 Earth radii at the geomagnetic equator) corresponding to an altitude of about 12,000 miles (19,000 km) above the equator; the altitude decreases as one moves toward the poles
    
• The inner belt
  
  • largely populated with protons originating from decay of neutrons liberated from the upper atmosphere by the impact of galactic cosmic rays (GCR). Some more about cosmic rays.
    
  • typically centered around L = 1.5 corresponding to about 2000 miles above the geomagnetic equator
    
  • extends down to about 250 miles altitude (400 km) over the South Atlantic Ocean
    
  • trapped particles originating as anomalous cosmic rays (ACR) have also recently been identified to populate the inner belt region
• The slot
  
  • a region of reduced occurrence of energetic particles between the inner and outer belt
    
  • typically around L = 3 (around 8000 miles above the surface at the dip equator)
    
  • can disappear during active solar periods so that the inner and outer belts overlap.

Historical outline