Auroral displays are due to the interaction of high energy particles from the Van Allen radiation belts with the upper atmosphere. The effect is similar to the excitation of the low pressure gas in a neon tube by high energy electrons.
Particles (mostly protons and electrons) spiral along field lines. As they approach the atmosphere at high latitudes, the magnetic field intensifies and the particles spiral more tightly - eventually reflecting - and returning to the other pole.
Individual collections of particles make the return journey repeatedly every few seconds. Interaction with the low pressure gas, in the upper atmosphere, excites oxygen and nitrogen atoms. Particles lose a little energy on each encounter and are gradually removed from the radiation belts which are donut shaped regions circling the equator at several Earth radii. New particles are continually supplied to the radiation belts by the solar wind.
The radiation levels and particle distributions vary constantly with variations in solar activity. The stylized diagram above shows the main features of the belts during a quiet solar period. The lower belt is particularly dangerous for spacecraft. Communications satellites must incorporate good (2 mm Al) shielding for the electronics on board if sustained operations are required within the lower belt.
The movie clip below shows a highly active auroral display over Anchorage Alaska.
The Aurora are absent from the poles and are most active in two rings at magnetic latitude 67° North and South. During periods of high sunspot activity, the displays are brighter and they occur at lower latitudes. The editor remembers seeing hanging pink curtains in the southern sky on the West Coast of the South Island of New Zealand, at magnetic latitude 45° South, at some time during the solar maximum in 1957. During the 2001 solar maximum, red aurora were again seen in southern New Zealand, at magnetic latitude 53° South, and even in Texas, magnetic latitude 43° North.
The image taken from the Space Shuttle (at right) shows the structure of an active aurora. The excitation is due to high energy particles streaming down magnetic field lines. The very high altitude red emission is from excited atomic oxygen. The emission is all well above the 100 km level.
The locality most famous for auroral displays is Anchorage Alaska which has a University, an active auroral research program, and a location some distance from the Magnetic Pole. Because Aurora are caused by charged particles streaming down magnetic field lines from the Earth's radiation belts, they occur in rings around the magnetic poles. Since most particles trapped in the radiation belts complete many return journeys from North to South, identical aurora are seen at the same times at opposite ends of the field lines. The identical nature of aurora at opposite ends of magnetic field lines was established in the early 1970's, by an American expedition that flew planes north from Anchorage Alaska and south from Christchurch New Zealand. The planes were equipped with Image Orthicon cameras - the fore-runners of today's digital video cameras - that were fast enough to record changing all sky images in real time.
A second image shows an auroral ring due to incident protons centered on the Magnetic pole and an intense spot (lower left) due to protons entering directly from the sun through a 'crack' on the magnetosphere. The image of the proton aurora is in false color, adapted from an Astronomy Picture of the Day original.
A third image, also from Astronomy Picture of the Day, shows a view taken from above the North pole in UV light. The Earth's shadow can be seen pointing away from the Sun (which was at the lower right hand corner). Two bright auroral rings, visible as northern lights from the ground, are seen in the center of the image.
Bangkok
Aurora are not observed in Bangkok, at 4° magnetic latitude.
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Planets with magnetic fields have radiation belts and aurora. The rings on Saturn and Jupiter are more than 1,500 km above the clouds. In the false-color Saturn image, the red Auroras are due to UV emission from atomic hydrogen, while the more concentrated white areas are due to emission from hydrogen molecules.
[Red/green visible light auroral colors of Earth aurora are absent, because there is little or no nitrogen or oxygen present in the upper atmospheres of the giant planets.]