Maunder Minimum

The Maunder minimum is the name given to a period of extreme solar inactivity that occurred between 1645 and 1710. Of particular interest is that this period of inactivity corresponds closely to one of the coldest periods of the so-called "Little Ice Age" in Europe, a time of long, cold winters that caused severe hardships in the pre-industrial revolution world. This has led scientists to extensively study the possible influences of solar activity on terrestrial climate, as well as examine other stars for evidence of activity cycle behavior similar to the Sun's.

Some of the first telescopic observations were made by Galileo in 1611, and he immediately noted the presence of dark blemishes on the Sun's surface; these were the now well-known sunspots. (Several of Galileo's contemporaries saw sunspots as well, but Galileo is the most famous and usually gets the credit for "discovering" sunspots.) Today we know that the number of sunspots rises and falls in a roughly 11-year cycle; this is one of the most obvious manifestations of the solar activity cycle.

Although sunspots were observed telescopically in 1611, it was not until 1843 that an amateur German astronomer, Heinrich Schwabe, noticed a periodic rise and fall in their numbers. That it took over 200 years for astronomers to notice something so seemingly obvious is some cause for wonder, but it may be partly explained by the nearly complete absence of sunspots for 70 of those years, between 1645 and 1715.

For reasons not yet understood, the solar cycle operated at a greatly reduced amplitude during that time. Evidence suggests it did not cease entirely, but the sunspot number—an index representing the total level of sunspot activity at a given time—during the late 1600s was reduced by a factor of 10-20 from its typical value during "normal" cycles. This perplexing aspect of the sunspot record was formally pointed out by the astronomers F. W. G. Sporer and E. H. Maunder in 1890, and it is now known as the Maunder minimum.

The existence of the Maunder minimum is interesting on purely astrophysical grounds, because it suggests that the regular rise and fall of sunspots observed from 1715 all the way through to the present day may not be a permanent, or even typical, aspect of solar behavior. It is possible to create a rough reconstruction of the sunspot record prior to the invention of the telescope, using indirect indicators of solar activity, and there is evidence for other Maunder minimum-like periods intermittently from about A.D. 1250 through 1715. The solar cycle as observed today, is therefore not the state in which the Sun spends all—or even most—of its time. Having only observed one Maunder minimum, we have no idea whether the Sun spends 10%, 50%, or 90% of its time in such a state.

Even the "normal" 11-year cycle seems to have longer-term behavior. Different cycles have different strengths, with some of them showing more sunspot activity than others. The strengths of the cycle peaks seem to follow a roughly 80-year period of very strong cycles, slightly weaker ones, then back to stronger ones, and so forth. With detailed sunspot records extending only a few hundred years, it is difficult to confirm or disprove this hypothesis. Combined with evidence for multiple periods of nearly complete inactivity, it becomes impossible to say whether the solar activity cycle, so extensively studied in the last 30 years, is normality or an aberration.

The seemingly erratic behavior of the solar cycle has led a number of astronomers to spend the better parts of their careers studying activity cycles on other stars, the idea being that if those stars show activity cycles or Maunder minimum-like characteristics, we might be better able to understand our own star. Most of this pioneering work has been carried out at the Mt. Wilson Observatory, near Los Angeles. Observations of solar-like stars have been underway at Mt. Wilson since 1963, and the program has accumulated a vast database of solar activity data. The result has been the discovery of a veritable zoo of activity cycles. Some stars have well-behaved cycles with periods comparable to our own Sun's 11-year cycle; these are of particular interest for comparison to the Sun. Other stars have highly variable cycles, while still others vary wildly but with no discernible, regular period. Finally, there are stars that show a complete absence of any activity cycle. Some of them appear to show no cyclic activity at all, while others exhibit tantalizing evidence of having "turned off" midway through the 30 years they have been observed from Earth. Whether or not these stars are truly in a Maunder minimum phase has not been answered, because it is very difficult to tell if they have low-amplitude cycles or no cycles at all, and it is even more difficult to study their finer characteristics in detail. However, there is no doubt that pronounced, fairly regular activity cycles like the Sun's are not universal either for the Sun or its stellar cousins.

Examinations of the solar activity cycle and the unusually cold weather of the Maunder minimum period have spurred significant controversy among astronomers, atmospheric scientists, and climatologists. The period from about 1300-1715 is known as the "Little Ice Age" in Europe, a period characterized by unusually long and cold winters. This period coincides closely with the time during which the Sun is known to have had time of inactivity, with some of the worst weather occurring squarely during the Maunder minimum.

In 1991, a pair of Danish meteorologists published a paper in which they pointed out a remarkably strong correlation between the length of the solar activity cycle and the global mean temperature in the northern hemisphere. Not all activity cycles are the same length, with longer cycles (12-14 years) seeming to indicate cooler global temperatures than the short (9-10 year) cycles. It is very difficult to assess the effect of even recent solar cycles on global climate, let alone those from the Maunder minimum period, because of the relatively short time span for which detailed observations exist, and because climate records become sparse to nonexistent as one looks back more than a century or so.

Despite the ongoing controversy, for which there is decidedly no definitive answer as of the year 2000, there is no doubt the Maunder minimum years were a time of significant misery in Europe, with the long, harsh winters leading to shortened growing seasons, failed crops, and widespread famine. Whether, or to what degree, the Sun is responsible for this, is an important question for atmospheric scientists and astronomers to tackle over the next few decades.