Historical Auroras

Written records telling of the observation of great auroral displays extend back to Roman times in the western world and to well beyond that in the Orient. Even as far back as the seventh century, there are written accounts of aurora being seen at middle latitudes in both the eastern and western worlds.

All this is of more than casual interest because the written record of great auroral displays over the past 1,500 years indicates changing levels of auroral activity. The change can be the result of long-term variations in sunspot activity and can also be due to long-term variations in the earth's magnetic field. Very likely, solar variability is the main cause. Since solar variability also affects weather, any insight into that variability acquired through study of auroral history can have consequence beyond just learning more about the aurora.

Another line of evidence on variation in solar activity appears in the content of radioactive carbon (Carbon-14) contained in trees. The solar activity at any time determines how much carbon-14 is available in the atmosphere for plants to take up. Hence, using tree-ring dating and carbon-14 measurement, one has another means to observe solar variability over past centuries. Fair agreement pertains between the carbon-14 content and auroral history methods.

Written auroral records show that the present is an era of auroral maximum that began about the middle of the nineteenth century. A definite low, known as the Maunder Minimum, began in the mid-1600s and ended in 1716. During that minimum few if any auroras were seen at middle and low latitudes. Other minimum auroral periods existed in the seventh and fifteenth centuries.

A particularly good time for seeing auroras was the twelfth century, a period called the Medieval Maximum. The century preceding the birth of Christ also was good; it is called the Roman Maximum. Prior to that time, the studied record of observed auroral displays becomes too sparse to indicate the level of activity which existed at any particular era.

Those who are studying records of historical auroras are of the opinion that further systematic investigation will reveal even more information, enough to obtain a rather detailed picture of the auroral history of the past 2,000 years. To the non-historian that idea is somewhat surprising because it would seem that all the past records might have been examined by now. That this may not be so reminds one of how recent in human history is the practice of systematically examining those records of the past obtained around the globe, many of which are still being uncovered after centuries of storage.

One interesting result of the studies of past auroras is the indication that our familiar 11-year sunspot cycle may have been somewhat different about a thousand years ago. At that time it seems that this cycle had a period closer to 10 years than 11 years. Though the 11-year cycle has much influence over the numbers of auroras seen at middle and low latitudes, it is not as noticeable at high latitudes where auroras are common regardless of time within the cycle. Similarly, during those prolonged periods of auroral quiet of past centuries, there probably was plenty of aurora for northerners to see.