The aurora is a permanent feature of the earth's upper atmosphere. It is actually an oval centered on the geomagnetic poles.

The size and shape of the auroral oval change depending on how hard and how fast the solar wind is blowing. When the sun is quiet and the solar wind is calm, the auroral oval is small and thin. When the sun becomes more active and the solar wind hits the earth's magnetic field with strong gusts, the auroral oval becomes wider and stretches south. So, the aurora is always present, but for most people in the U.S. and Europe, it rarely stretches far enough south for them to see.

For a given level of solar activity, the thinnest part of the auroral oval is always on the sunny side of the earth, while the thickest part of the oval is over the nighttime side of the earth. If you live near the northern auroral oval, you will often see aurora along the northern horizon early in the evening, and the aurora will gradually move south. The gradual southward movement is caused by your location on earth rotating under the thicker part of the auroral oval.

So, the best time to watch for the aurora is late at night or in the early morning hours, from about 10 PM to 3 AM.

Solar Activities

People have known about sunspots for thousands of years. Large sunspots are easily seen with the naked eye when the sun is partially obscured by clouds or fog. (Never look directly at the sun, not even with dark glasses or filters...you could permanently damage your eyes!)

When people began recording how many sunspots were visible about 300 years ago, a pattern in their records began to appear: large numbers of sunspots would appear about every 11 years.

We now know that when there are a lot of sunspots, the sun is very active and pumps out vast amounts of energy in flares, x-rays, and other forms. As a result, the aurora becomes more active, and spectacular displays are more frequent.

The last time of maximum solar activity was in 1989. The minimum level of activity was seen in 1995, and activity is now increasing again. The next peak of activity is expected to take place around the year 2000.

In addition to the 11 year cycle of solar activity, the sun also rotates. That means that an active region on the sun which is spewing out charged particles swings into view each time the sun rotates. During the quiet part of the solar cycle, there are only a few active regions on the sun at a given time, so their effects such as active auroras appear once in each solar rotation. The diagram below shows times of auroral activity (bright areas) for 1995. Can you figure out what the solar rotation period is by looking at the pattern in this diagram?

The solar rotation period is approximately 27-28 days.

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