1908-1958

Van Allen belts

• Kristian Birkeland ran expeditions to northern Norway
• and established a network of observatories
• to see how the magnetic field related to the northern lights.
• He proposed in 1908 that Earth’s magnetic field
• guided electrons ejected from the sun toward the poles
• where they interacted with currents, now known as Birkeland currents,
• that flow along geomagnetic field lines from the north pole.
• Birkeland’s theories were ridiculed
• until rockets were deployed
• to permit direct measurements from space.
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• Birkeland died in 1917 but Carl Størmer
• was inspired by Birkeland to study
• the motion of charged particles in Earth’s magnetosphere
• and develop mathematical models of them,
• predicting orbits of charged particles
• that were later confirmed by James Van Allen.
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• James Van Allen designed the scientific instrumentation
• for Explorer 1, the first satellite launched by the United States,
• and the first spacecraft to detect a Van Allen radiation belt.
• Van Allen’s instrumentation included
• an omnidirectional Geiger-Müller tube
• to detect cosmic rays
• five temperature sensors
• an acoustic detector and a wire grid detector
• to detect impacts of cosmic dust
• packed into a cone and cylinder a yard long
• and six inches in diameter.
• Charged particles trapped by Earth’s magnetic field
• overwhelmed the Geiger counter so that it read zero
• as Explorer 1 passed through the inner Van Allen belt.

Shields

• The inner Van Allen belt
• contains mainly high-energy electrons
• trapped by earth’s magnetic field.
• The outer Van Allen belt
• contains high-energy electrons and ions
• and fluctuates in response
• to geomagnetic storms.
• A transient third belt just outside the atmosphere
• contains unrelativistic particles
• that merge with the outer belt.
• Van Allen belts protect the atmosphere
• against the solar wind; however,
• charged particles in the belts
• cause problems for satellites.

Auroras

• Winds of charged particles from the sun
• batter the magnetosphere of the earth
• so both rain charged particles
• into the upper atmosphere at high latitudes.
• which excites atmospheric gases
• to emit a diffuse glow or folded curtains of light
• red from atomic oxygen at high altitudes
• green at lower altitudes from nitrogen
• transfering energy to oxygen
• blue and red from nitrogen at lower altitudes
• combine for yellow and pink.