Heartbeat of the Ionosphere

The boss has been uncommonly busy lately. Directing the Geophysical Institute is enough to keep anyone hopping, but Director Akasofu also is preparing for an international scientific meeting for which he will play host. It's only fair; the field considered and celebrated at this gathering, the Second International Conference on Substorms, is one Syun-ichi Akasofu virtually invented.

Or, more properly, the subject is a phenomenon that he helped discover and establish. When Akasofu came to Alaska in the late 1950s, it was to study under the guidance of Sydney Chapman, a distinguished British scientist who served as scientific director of the institute for 21 years. This was a logical place for the student from Japan to find a mentor from England, for then as now, interior Alaska was the best place in the world to observe auroras, and both Chapman and Akasofu wanted to understand these mysterious northern lights.

Then as now, professors can assign time-consuming chores to their students. A massive multinational effort to study the polar regions, the International Geophysical Year of 1957-58, had produced mountains of data. Akasofu found himself confronted with the monumental task of analyzing miles of film taken by the 100-instrument circumpolar all-sky camera network during the IGY.

Back then, little was known about how aurora was distributed around the poles and about how auroral activity at one place related to other places. Akasofu's laborious analysis established not only that auroras were distributed along a ring-shaped belt centered around the geomagnetic pole, but also that they undergo simultaneous and systematic changes all along that belt. Chapman helped assess the findings, and coined the term "auroral substorm" for this patterned activity. Akasofu wrote up the results of their work, and submitted the paper to an American journal.

The journal editors promptly rejected the paper, claiming it contained nothing new. Somewhat rewritten, the paper was finally printed in 1964 by a British journal, Planetary and Space Science. It was swiftly recognized as presenting something very new indeed, and--eventually--the auroral substorm became a core concept in scientific understanding of the aurora.

In the 30 years since the world learned of substorms, the whole auroral phenomenon has come to be much better--but still not perfectly--understood. In explaining what we know, Akasofu often uses the analogy of an oscilloscope. Think of one of those TV dramas with a patient in intensive care. Somewhere in the picture, inevitably, is a video-screen type of device with a spiking green line traced across its face. That's an oscilloscope. The line is produced by one of the patient's vital signs, typically (at least on television) the heartbeat. The heart beats; the sensors translate that pulse into an electrical impulse. That impulse becomes an input signal to the oscilloscope. The oscilloscope amounts to an electron gun at one end of a tube and a screen coated with fluorescing material at the other end. The beam of electrons fired at the screen creates a visible trace, and that trace is modified by the input from the patient--hence, a bunch of spikes from a beating heart (or a flat line, if the patient dies).

The aurora is like a celestial oscilloscope. The distant sun is like the beating heart; the electrically charged particles it sends earthward is like the heartbeat signal. The particles interact with the earth's magnetosphere when they get here, in a dynamo-like process that takes the role of the electron gun at the back of the oscilloscope tube. Guided and accelerated by the interaction processes, charged particles shoot into the upper atmosphere, where they strike gas molecules. These molecules emit light, and produce a glowing trace in the sky. Just as the oscilloscope trace reports something of the patient's physical condition, the shifts and swings of the aurora's light tell something about the magnetosphere's electromagnetic condition.

I like the image of aurora as diagnostic device. I just hope the assembled experts will be able to announce that the magnetosphere is healthy.