| The auroral substorm |
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The most common auroral forms are arcs, bands and rays. At times they change shape rapidly, advance, retreat or bulge out in a violent fashion, and they also get quite bright.
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Scientists call such an active, violent outburst an auroral substorm. We still study the release of particle energy, far in space, which causes it. If you are lucky you may also see a "corona" - a burst of rays radiating in all directions right overhead. |
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An active auroral breakup – called an auroral substorm, shows a large variation in shapes, intensity, colours and particular dynamics – very rapid motions are observed. It took several years before scientists in the 1960s recognised that an auroral substorm follows a fixed pattern, with four scenes or movements (Akasofu, 1968).
For a night-time observer near the oval, the display usually starts with one or more quiet, homogeneous arcs of fairly low intensity (< 5kR) elongated approximately in the geomagnetic east-west direction. Within a couple of hours, these arcs start to move equator-ward;, increases in intensity are observed, and they develop ray structure and takes the form of rayed auroral bands.
The animation above shows sequence of images captured by the Ultraviolet Imager on NASA's Earth-orbiting Polar satellite shows an auroral substorm over northern Asia on February 24 2000. Maximum activity, denoted by dynamic yellow blobs in the aurora oval, occurs around 1400 UT. Because it records ultraviolet light, Polar's UVI camera can see aurorae from space on both the day and night sides of Earth. |
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Then, suddenly, the whole sky explodes, The arcs and bands lose their well-oriented forms and change into bundles of rays, draperies, curls and corona and the auroras spreads over the entire sky often with an intense shades of red along the lower edge. Simultaneously, the aurora moves rapidly, with change in form and intensity, at times increasing to several 100 kR. Individual structures may show apparent speeds in an easterly or westerly direction of several 10 km/s. This break up phase normally lasts less than ~ 10 minutes. Then the recovery phase starts, which may last between 0,5 and 1 hour when the auroral activity gradually decreases back to quiet level. For disturbed periods, two or more auroral break-ups may occur in one night.
An illustration of the development of an auroral substorm in the dipole local-time coordinate system is shown in Figure 5.13. A substorm must be considered within a fixed Earth-Sun frame of reference, as is true for all auroral phenomena. A ground-based observer thus must remove the effect of the Earth’s rotation in tracing auroral motion in his reference frame.
Figure 5.13: An illustration of the development of an auroral substorm is shown. |
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