Lewis and Clark College

December 14, 1999

Sunspot Cycle at the Millennium

PORTLAND, Ore. -- As the millennium draws near, the Internet is full of wild predictions about the sun, ranging from misconceptions about coronal mass ejections to suggestions of a solar megacycle.

"Despite these dire predictions, this sunspot cycle (Cycle 23) will be average in strength," states Herschel Snodgrass, professor of physics at Lewis & Clark College in Portland, Ore., and an expert on the solar magnetic activity cycle and the solar torsional oscillation.

"Cycle 23 is reaching its maximum earlier than expected, but it is not a strong maximum," he said.

Professor Snodgrass will explain why the torsional oscillation may be the key factor to making reliable predictions of upcoming solar cycles at a press conference of the American Geophysical Union, 8:30 a.m., Thursday, Dec. 16, 112 Moscone Convention Center, in San Francisco. Title of the press conference is "The Solar Max: New Thoughts on Why and When." Torsional oscillations are jet-stream-like patterns that appear to link the polar fields of one cycle to the sunspot fields of the next.

Reliable predictions are important because the sun's magnetism, manifest by sunspots and other disruptions of the Sun's surface, affects the Earth's climate and can seriously disrupt satellite communication, telephones, power grids and space flight.

Some observers, Snodgrass notes, find evidence that the Cycle 23 has peaked already, ahead of schedule. But most indicators suggest it will peak in mid-2000. Cycle 19, the strongest on record, peaked around 1960. Cycle 20 was average, and Cycles 21 and 22 were both stronger than average.

"We expected to see the torsional oscillations begin in 1994, but instead the pattern didn't begin until late 1995," Snodgrass said. "Its late and weak start suggests that Cycle 23 will be average or slighly stronger than average, but definitely weaker than Cycles 21 and 22."

"This does not mean that there will not be significant flares and so forth," he said. "There always are, especially during the declining phase of the cycle when the active regions become large. But at present, it looks as though the next thousand years will dawn under the Sun's rather gently smiling face."


Torsional Oscillations

Detailed mapping of the Sun's surface motions and magnetic fields show connections between patterns in the magnetic field and patterns of large-scale motion of ionized solar gases, called plasma, according to Snodgrass.

The sun rotates on its axis as does the Earth, but unlike the Earth, the Sun does not rotate rigidly. The poles take more time to make a full revolution than does the equator. This differential rotation is possible because the Sun is not a solid but gaseous throughout. Scientists believe this differential rotation twists and intensifies the magnetic fields. In 1980, scientists at Mount Wilson Observatory discovered a weak but large-scale fluctuation in this differential rotation and dubbed it the torsional oscillations. The fluctuation looks like a wave in which the rotation is slower than normal in one zone of latitudes and faster than normal in the adjacent zone closer to the equator.

"It was as if the Sun had, in each hemisphere, two jet stream-like winds blowing in opposite directions. The higher latitude band flows in the opposite direction of the Sun's rotation, and the lower-latitude band flows in the same direction as the rotation," according to Snodgrass.

"The remarkable thing about this pattern of motion," he said, "is that it migrates in latitude in a way that is linked with solar activity. It begins in high latitudes, following the reversal of the polar fields, and moves gradually toward the equator and continues to be present through solar minimum. When the sunspots appear, they do so along the path of this migration, and the bands of flow persist to the end of the cycle.

"This suggests that the torsional flow forms a link between the polar field reversal of one cycle and the lower latitude activity of the following cycle," Snodgrass said. "Based on evidence found in 1998 by SOHO, an orbiting solar observatory, we now know that the flow extends deeply beneath the Sun's surface and, therefore, that it may serve to channel the fields into the equatorward-migrating zone where they intensify."

Sunspots and the solar activity cycle

The Sun's magnetism undergoes a fascinating sequence of changes that take 20 to 24 years to go full cycle. The activity of the cycle is maximum when the number of sunspots is the greatest and minimum when the number is the least, usually near zero. For every full 20- to 24-year cycle, there are two 10- to 12-year sunspot cycles, Snodgrass explains.

At minimum, the Sun's magnetic field is similar to the Earth's, with magnetism concentrated toward the poles. As the cycle progresses, the number of sunspots increases. Individual spots last from a few days to a year or so. At first, sunspots appear in each hemisphere at mid-latitudes. But as time goes on, they appear closer to the equator, and the Sun's magnetic field becomes more concentrated in these spots than at the poles.

After about four years, the activity cycle reaches its maximum, and a year or so later, the fields at the poles reverse sign: what was a north magnetic pole becomes a south magnetic pole, and vice versa. This begins the declining phase: the spots become larger but fewer in number, and in the course of five to seven more years, the cycle of activity reaches its next minimum. This minimum is like the previous one, except the fields at the pole now have the opposite sign. The second half of full cycle begins with reversed-polarity magnetic fields and ends with another minimum with the polar fields back to the same signs they started with. Thus, for every full 20- to 24-year cycle, in which everything returns to its initial state, there are two 10- to 12-year sunspot cycles.

Sunspots have been coming and going in these 10- to 12-year cyclic patterns since the early 18th century. Prior to that, there was a 75-year period during which the cycle was dormant, and the Earth underwent a little ice age. When spots are not present on the sun its surface is overall slightly cooler.

Space Science News, 14 October, 1999

Solar Cycle Update - Updated predictions from NASA scientists place the solar maximum in mid-2000. As activity on the Sun begins to increase toward this broad maximum, we can expect more auroral displays, radio disruptions and power fluctuations.

October 19: Sunspot activity increases - As the sun nears solar maximum, NASA scientists report that the sunspot cycle is closely following their prediction.

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June 25, 1998

The Solar Maxiumum is Coming!

SUNSPOT, N.M. -- The Solar Maximum is the peak of a cyclical phenomenon on the sun that occurs every eleven years; it is a period of maximum solar activity that lasts for two or more years.

Being able to predict when the sun will produce the activity which can create more auroras, as well as generate highly energetic particles and geomagnetic activity, is crucial because this heightened activity can interfere with communication satellites, and in extreme cases, destroy them. Also, it can interfere with ground-based radar and radio communications.

Usually, radio and radar communication can bounce off of the ionosphere as if it were a mirror reflecting a signal. These transmissions off of a "flat" surface can be predicted. When Solar Maximum occurs, the ionosphere acts like a corrugated surface, and predicting where a signal will land becomes difficult. The ionosphere can also become opaque to certain frequencies, making communications with satellites difficult or impossible.

Air Force Research Laboratory (AFRL) scientists have found a new technique to forecast the Solar Maximum.

In a paper presented at the spring meeting of the American Geophysical Union in Boston last month, the Space Hazards Branch of the Battlespace Environment Division of the Space Vehicles Directorate identified the first reliable precursor to the maximum of solar activity that will occur near the turn of the century.

A study of the sun's long-term variation of emission features seen in Fe XIV, an ion that is found throughout the solar corona at temperatures around 3 to 4 million degrees Fahrenheit, has shown that, prior to Solar Maximum, emission features appear near 55 degrees latitude in both hemispheres and begin to move toward the poles at a rate of 9 to 12 degrees of latitude per year.

This motion is maintained for a period of 3 or 4 years, at which time the emission features disappear at the poles. This phenomenon, which represents the fastest global motion of any kind on the sun that is sustained for such an interval, has been referred to as the "Rush to the Poles."

Looking at these measurements, the maximum of solar activity, as represented by the number of sunspots on the sun, occurs approximately 14 months before the features reach the poles.

In early 1997, emission features appeared near 55 degrees latitude, and subsequent observations have shown that these features are moving toward the poles. This then is the Rush to the Poles that heralds the next Solar Maximum.

Based on previous observations, these features will reach the poles sometime between March 2000 and January 2001, which results in a prediction for Solar Maximum of between January and November 1999, substantially earlier than some other predictions.

Predicting this phenomenon is important for the Air Force and the public in general. Since the last Solar Maximum in 1989, we rely more than ever on satellites and the information they provide.

"In addition to the disruption to Air Force radar, communications and satellite operations, solar activity can and has produced electrical blackouts that affect millions of people," says Dr. Dick Altrock, AFRL's astrophysicist at the National Solar Observatory, in Sunspot, N.M.

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