Our sun has been behaving differently lately. It will usually go through an 11-year cycle, from a time of displaying few, if any, sunspots, to having more than a hundred on its surface and then back to a minimum number. At present, the sun is going through a solar minimum, which typically lasts just under 500 days. However, the cycle now in progress seems to be stuck in an extended minimum — so far lasting for more than 650 days.
Sunspots are probably the most striking feature on the sun’s surface. An image of the sun will show that sunspots are brownish to dark-brownish and, considering the size of the sunspot to the size of the sun, are generally larger than Earth. These objects appear dark on the bright solar surface, but if a sunspot could be “plucked” off the sun and seen against the blackness of space, it would be quite bright itself.
Sunspot regions are typically 1,000 to 2,000 degrees cooler than the sun’s 10,000-degree surface. Since individual sunspots can exist for days or weeks, there must be some mechanism acting on the sun’s surface that prevents its hot gas from mixing with the sunspot’s cooler gas. Analysis of the light from sunspots has shown that this mechanism is magnetic fields.
A magnetic field is, of course, invisible, but to understand its effect on the sun’s surface, the magnetic field can be visualized. One classic science demonstration is to trace the field by sprinkling filings from a piece of iron around a magnet. These iron filings mark out lines along the magnetic field. These field “lines” are closer together where the magnetic field is stronger and farther apart where the field is less intense.
When a gas becomes very hot, as with the sun, it becomes electrically charged and can only move along these field lines like beads on a string.
Where the magnetic field pierces the sun’s surface, the charged gas is hindered from moving sideways and mixing with the surrounding gas. This “magnetically confined” gas radiates its energy to space and cools, producing a darker region on the sun’s surface — a sunspot. Eventually, within a few days or weeks, the field weakens, the gas remixes and the sunspot disappears.
The sunspot record — the recorded number of sunspots taken over many years — is not very lengthy as far as solar science is concerned. Galileo was the first person to use a telescope to observe, sketch and make systematic counts of sunspots starting in 1609. Then in 1826, Heinrich Schwabe, a German amateur astronomer, recognized that the sun produced sunspots on a regular cycle that averages about 11 years.
However, within a few decades of Galileo’s observations, the sun went into a deep sunspot minimum. From 1645 to 1715, there were virtually no sunspots on the sun — and Earth cooled. During that time, glaciers expanded in Europe and warm summers disappeared. At the worst period, the Northwest Passage did not thaw and the Thames River in England froze. This period has now been named the Little Ice Age.
Two centuries before Galileo, tree-ring data and weather records suggest there was another long solar minimum with an accompanying cold period lasting about 150 years. However, the same data also show that 400 years before the Little Ice Age, there was an extended solar maximum together with higher temperatures on Earth.
Signs of recovery
The coming solar cycle, called Solar Cycle 24 because it is the 24th sunspot cycle for which there are reliable records, is in a lull. In the past year-and-a-half, there have been small, short-lived sunspots, a weak flow of charged particles streaming from the sun, and a lower than normal solar energy output. Further, the sun has gone more than two years without a significant solar flare.
Solar scientists who forecast that the current solar minimum would end in March 2008 have now revised their calculations and think that the sun will remain calm for another year then build to a May 2013 maximum. Nonetheless, this maximum will have a below-average number of sunspots with a peak of 90 per day. This would be the lowest count since Solar Cycle 16 peaked at 78 sunspots per day back in 1928.
There have been an increasing number of indicators that the sun is starting to be active again — the appearance of “proto-sunspots,” small spots at high solar latitudes, and a tiny but significant uptick in radio noise from the sun.
At the June meeting of the American Astronomical Society, solar physicists announced that they have tracked a jet stream of hot gas 4,000 miles below the sun’s surface and that this stream has taken a year longer than usual to reach a critical latitude on the sun for new sunspots to appear.
Finally, on June 30, an area appeared on the sun that developed into a sunspot region, which displayed the magnetic characteristics of a new solar cycle. It seems that Solar Cycle 24 has begun. Stay tuned!
To keep track of our sun’s behavior, go to spaceweather.com for a daily image of the solar surface and other relevant information about its performance.
Saturn’s visibility in the evening sky will diminish in August as its motion swings it closer to where the sun appears in the sky. After the second week of the month, Saturn will be situated in the western sunset glow and binoculars will be necessary to find the Ringed World in the twilight sky.
Jupiter is opposite to the sun in the sky during August, so that as the sun sets in the west-northwest, Jupiter is rising in the east-southeast.
Wait a few hours for Jupiter to get fairly high above the horizon to get a good view of this giant planet through a telescope.
These views are rewarding because a telescope reveals Jupiter’s darkish atmospheric belts and light atmospheric zones containing spots and festoons. The planet is quite bright and occupies the eastern part of the constellation Capricornus, the sea goat.
The moon, a day past full, will appear near Jupiter on Aug. 6. Before dawn on Aug. 16, a lunar crescent will be seen near Mars in the eastern sky. The next morning, a thinner crescent moon will appear by Venus before daybreak.
Richard Monda is an astronomer living in the Capital Region.