Predicting Auroras

Despite all that is known about the aurora, predicting when the Northern Lights will appear still is a dicey business.

One well-known trend is the 11-year solar cycle auroras are more numerous in years of maximum numbers of sunspots. Right now we are on the decline from a maximum that occurred a year or so ago.

Another cyclic trend relates to the 27-day period of the sun's rotation on its own axis. Sometimes sunspots last for months. Hence one that sprays the earth with aurora-generating solar wind particles on one solar rotation may well do so 27 days later.

There also seems to be a slight tendency for more auroras to occur in spring and fall rather than in summer and winter, owing to the changing annual relationship of earth to the plane of the sun's equator.

When it comes to predicting whether or not spectacular auroras will be seen on any given night, the best bet is to notice what happened the night before. If there was little aurora last night, the chances are that the aurora will be no better tonight. That scheme sometimes fails miserably because a period of relatively quiet must sometime get interrupted by new activity. That new activity can only be predicted by recalling the 27-day solar cycle and, better yet, being able to observe the new appearance of sunspots or to make direct instrumental observations of the solar wind with a satellite flying high above the earth and off in the direction of the sun.

When new activity does commence, it usually last several days. The first night is usually the best, with a decline in activity each night thereafter--unless a new sequence of activity shows up.

Within any one night there are cycles lasting an hour or two. On a particularly active night the cycles follow one right after the other, but sometimes only one or two will appear.

Sound hopeless? Almost, but a little regular experience watching the night sky gives anyone some predictive ability.