At about the same time as Galileo published his description of his construction of the Dutch invention of the telescope, natural philosophers throughout Europe began noting that the sun had imperfections, “spots” on it. This was far easier to watch with a lens, since you could project an image of the sun onto a white sheet, and observe it without destroying your eyes. The novelty led several natural philosophers to begin a program of noting the sunspots on a regular basis. Therefore, we have an excellent European record of the number of sunspots starting with Galileo’s first such observation in 1610.
This notion of the imperfection of the sun would have come as no great surprise to the court astronomers of China and Korea. In the court logs of the observations of those staff astronomers, there are sunspot records made with the naked eye going back another millennium and a half. Using those records, we can trace the sunspot number from about 28 BCE, using a reasonable relationship between the capabilities of naked-eye astronomers and those using projections and lenses.
For all of this two thousand year period of recorded observations, the number of sunspots on the surface of the sun has varied in an eleven-year cycle. As of this writing, in 2003, we are near the falling side of the peak of the current cycle with sunspots near the historic high of over 200. This extreme activity has resulted in spectacular auroras being seen as far south as 30 degrees north (Oklahoma City). At the other end of the measure, the lows have had sunspot numbers in the low teens to the mid-20s. The “average” low is between 20 and 30.
For reasons that no one understands, starting in about 1610, the number of sunspots plummeted. By 1632, which should have been a peak year, the sunspots were down to the mid-teens, and by 1640, had dropped to zero. (There is an anomalous high data point in 1639.) The 11-year cycle did continue, with peaks as high as 8 or 9 between 1645 and 1700. Then, again for reasons that no one understands, starting in 1710, the numbers went back up, and have continued quite regularly for the last three hundred years. This is not a “lack of observations” artifact, since the court observations in China and Korea correlate quite well with the western records. This is real.
Recent work by observational astronomers using a combination of new techniques by really really smart people on type G2V stars like our sun have figured out a way to measure the sunspot number of a star even though we cannot “image” the star. This work indicates that G2 stars may typically spend as much as 20% of their time in this “quiescent” mode. It could start again tomorrow. No one has any models for why it happens, or what causes it, or why it stopped. It’s all quite confusing.
So what, you say? Well, it turns out that the number of sunspots is very highly correlated with the thickness of the upper layers of the ionosphere. There are several “layers” in the upper atmosphere, which get ionized for different reasons. These are labeled, from the outside in, A through F.
The innermost F, E, and D layers are caused each day by the action of ultraviolet light on the earth’s upper atmosphere. This is the same action that splits O2 apart and gets the free oxygen that can combine into O3, to form the “ozone” layer. These ionization layers form every morning at sunrise, and thicken throughout the day, and then begin to fade at sunset. The combination of their chemistry and their electrical properties causes them to absorb radio waves longer than about 4 MHz. That’s why, during the day, you can only hear your local AM radio station; but at night, you can pick up the one from the other side of the country.
Shorter wavelengths pass right through the DEF layers.
Further out, the ABC layers are ionized not by UV light, but by the action of the solar wind on the outer layers of the earth’s atmosphere and its interaction with the earth’s magnetic fields. During periods when there are lots of sunspots, the sun puts out a lot of particles, and these ionization layers are quite thick and robust. Without the solar action, during sunspot minima, the ABC layers are thinner and weaker.