Magnetic Substorms

Many years ago people gave the name magnetic storms to those intervals lasting a few days wherein there appear the most extensive auroral displays and during which marked variations in the earth's magnetic field occur. A more recent discovery, made some years ago by Dr. Syun-Ichi Akasofu of the Geophysical Institute, is that magnetic storms contain shorter periods of increased activity, called magnetic substorms or auroral substorms.

Substorms typically last about an hour or so. When a substorm begins, the number of auroras seen in the sky may increase, and those that exist will brighten perceptibly. Rapid motions within the auroral forms and violent motion of the forms themselves occur. In a few minutes' time, the aurora can change from barely noticeable to a brilliant display extending across the visible sky.

Observations from satellites and from widely-spaced observing points on the earth's surface show that substorms typically originate at the auroral zone on that part of the earth opposite the sun. In other words, the substorm usually begins near local midnight at the auroral zone. The enhanced activity expands to the west and to the east away from the midnight meridian. For that reason an observer often sees enhanced activity move his direction from the eastern horizon in the hours before local midnight. If the observer's local time is post-midnight, he often sees enhanced activity moving toward him across the western horizon. Also, an observer located north of the auroral zone, for example on the northern coast of Alaska or Canada, often observes the region of enhanced activity expanding toward him from the south.

Thus, while a substorm usually starts in one region, the midnight region of the auroral zone, it quickly expands east and west, and also somewhat poleward, to spread around the earth. Consequently, it is a global, not a local phenomenon. When a substorm is in progress, all of the earth's auroras are affected.

During the substorm there are rapid variations in the earth's magnetic field. Then, the magnetic compass needle fluctuates in direction, sometimes by several degrees.

After several tens of minutes the substorm decays, and the aurora grows weaker. Some hours may pass before a new substorm starts, but during particularly large magnetic storms the substorms may follow one after the other.

The substorm behavior explains why auroras come and go on any particular night. On some nights there may be only one substorm, and on others several may occur. If one happens to walk outside to look at the sky between substorms he might not see much aurora, even on a night that is generally active. But an hour or two later, after a new substorm has started, the sky might be full of aurora.