The Auroral Substorm
It has long been known that auroral activity undergoes a systematic change of forms during the course of a night. Until the early 1960s, most auroral scientists believed that there was a fixed pattern of auroral forms under which the earth and an observer on it rotated once a day. The fixed pattern consisted of quiet curtain-like forms in the evening sky, curtains with folds and waves around midnight, and isolated rays and patches in the morning sky.
Beginning in 1961, S.I. Akasofu, a young researcher at the Geophysical Institute (and now holder of the Sydney Chapman chair at UAF), began the study of a large number of auroral photographs taken simultaneously at widely scattered locations in Siberia, Alaska and Canada. Some of the things that he noticed led him to question the concept of a nightly fixed pattern of auroral forms.
What Akasofu discovered was that during a quiet period, the aurora can have a simple curtain-like form over the entire dark hemisphere of the earth. However, during an active period, the auroral features can run the entire gamut of forms from quiet curtain to rapid wavy motion to breakup in as short a time as from one to three hours. He named these cycles "auroral substorms" in a paper which he published in 1964. In years to come, this paper was to become a classic in the field and one of the most often-cited references in space science literature.
At the time, however, most workers in the field held to the traditional line. Even Christian T. Elvey, then director of the Geophysical Institute (and namesake of the present Institute building) adhered to the notion that these changes in the aurora predictably took the same course night after night. There was even a wager made between Akasofu and Elvey as to which model would eventually prove to be the right one.
Akasofu got the chance to prove his point in 1968 and 1969, when the NASA Convair 990 research jet Galileo was committed to a NASA project entitled the Airborne Auroral Expedition. As part of the program, the high-speed jet was flown westward from Churchill in Canada to Fairbanks against the direction of the earth's rotation, but at almost the same speed. In doing so, it remained in essentially the same spot relative to the sun on the opposite side of the earth for a period of several hours. As Akasofu had predicted, the aurora in the midnight sky sometimes underwent the quiet phase, brightening, development of wavy structure, and breakup that had been thought to take place only over the course of an entire night. This meant that the old notion of the particular stage of the aurora always being related to the hour was incorrect.
Elvey was delighted to see the pictures taken from the airplane which provided documentary proof of Akasofu's hypothesis, and he graciously conceded the bet. However, it was to remain for polar-orbiting satellites in the years ahead to provide the final body of evidence to corroborate the findings that Akasofu had set forth over a decade earlier.
To summarize the auroral activity during a substorm, the following paragraph is excerpted from Akasofu's book, Aurora Borealis, The Amazing Northern Lights, published by the Alaska Geographic Society. Akasofu's description relates to only one substorm cycle although he has observed as many as four to occur during a single night.
"In the most common auroral activity, the initial action is seen as a sudden brightening of an auroral curtain in the midnight sky at the observer's meridian. As its brightness increases, the ray and wavy structures predominate. In a matter of minutes, a northward motion begins. This northward motion produces a large-scale wave structure, called the westward traveling surge, which moves rapidly to the west along the auroral curtain. Often such a surge culminates in a spectacular curling motion of the auroral curtain accompanied by a crimson tint. Meanwhile, the midnight sky is filled with 'broken curtains,' a phenomenon also often referred to as the 'break- up.' Finally, in the morning sky, the auroral curtain disintegrates into patchy luminosity. Eventually, all auroral activity ceases, but a new chain of activity will eventually be repeated in the same way."
