Solar Cycle and the Aurora

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(Cover story of Nature, published May 28, 1998)

Background:

Like the Earth, the sun has a magnetic field, but unlike the Earth, the direction of the sun's field reverses every 11 years. The number of spots on the sun and the ultraviolet (UV) rays emitted by the sun vary accordingly. Since sunspots are easily seen with simple equipment (don't try this at home!) the 11-year variation was discovered in the 18th century. Ever since a cottage industry has attempted to relate various elements of the terrestrial environment to the solar cycle. Everything from the stock market to skirt lengths to the longevity of congressmen has been correlated -- needless to say, most such correlations have proven spurious.

The first relationship which seemed real was that aurora are more frequent after solar maximum (specifically the peak is two years after solar maximum) than at solar minimum. This determination is based on historical records of large magnetic storms which cause the aurora to reach populated latitudes. These records, kept by dedicated early scientists at isolated points around the world, typically count a few or at most a score or two of aurora a year. However intense aurora actually occur pretty much every night at high latitudes (the ideal place to look is about 2500 to 2800 km away from the magnetic pole, which lies near Resolute Bay, Canada).

Although much has been learned about the aurora, no one theory of arc formation is yet widely accepted. One promising theory is the ionospheric conductivity feedback mechanism. It predicts that as ionospheric conductivity rises -- as it does at solar max, because of the increased solar uv -- fewer intense aurora should occur. Since this theory had already correctly predicted that fewer aurora occur in sunlight than darkness ({story 1}), we decided to test this surprising prediction.

Probably the only way to measure the global occurrence rate of aurora over a whole solar cycle, in both sunlight and darkness, is using the particle detectors on the Air Force series of satellites called the Defense Meteorological Satellite Program (DMSP). The immediate cause of aurora is accelerated electrons from space. The acceleration process leaves a distinctive signature in the electron population, making it easy to identify intense aurora.

Results:

The global frequency of aurora (left) for 1984, around solar minimum and (right) for 1991 shortly after solar maximum. This figure is under sunlit conditions only, when the sun's ultraviolet rays cause increased ionospheric conductivity. (Click on either image to view larger version.)

When the ionosphere was sunlit (which at high latitudes and the local times of interest -- dusk to midnight -- means summer) we found fewer intense aurora at solar maximum than minimum. However under conditions of darkness, no real trend was clear.

These results can be made somewhat more quantitative by plotting auroral frequency as a function of F10.7 number (the solar flux at 10.7 cm). This wavelength is not energetic enough to cause ionization, but it is easily measured, and has been shown to be well correlated with ionizing UV.

The global frequency of aurora as an explicit function of solar ionizing radiation (which is highest at solar maximum). Left: Under sunlit conditions, auroral frequency decreases as solar UV increases. Right: In darkness, solar UV has no effect on auroral frequency. (Click on either image to view larger version.)

You might wonder what caused the common belief that aurora are more common at solar maximum. The answer is that the solar wind average conditions do not change much between solar minimum and maximum, but the solar wind is more variable at and following solar max. This great variability means that the rare extreme conditions which cause huge auroral storms which can bring the aurora down to populated latitudes are more common following solar maximum. If you live in the U.S. outside of Alaska, you will not see the aurora very often, but your chances would be best following solar maximum. If you live in Alaska you probably see the aurora very often (if you bother to look!) but you will not see any more aurora at solar maximum than minimum.

You can read more about the aurora and the ionospheric conductivity feedback mechanism in the next story ( "Is the aurora there when no one is looking?").

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