by Bob Henson and David Hosanskey
If you like a lot of feedback on your job performance, don't become a solar-cycle forecaster. Operational weather forecasters get to test their skills every day; even scientists who forecast climate have a chance to evaluate their prognostications at least once a year. But a person who forecasts solar cycles may get only four or five events to predict in an entire career.
NCAR scientists Mausumi Dikpati (left), Peter Gilman, and Giuliana de Toma examine results from the Predictive Flux-transport Dynamo Model. The backward C shapes are the modeled solar convection zone (about the outer one-third of the Sun); colors indicate the presence of magnetic signatures of sunspots from preceding solar cycles. The team used the model to simulate the evolution of the Sun's large-scale magnetic fields. (Photo by Carlye Calvin, UCAR.)
Scientists have recognized since the 1840s that solar activity rises and falls on a cycle that averages 10–12 years long. Each peak boosts the amount of solar activity by tenfold or more, increasing the odds of phenomena ranging from sunspots to X-ray flares and cosmic rays. As our high-tech society becomes more vulnerable to solar storms, there's growing interest in predictions of how strong the next peak will be and exactly when it will arrive.
This year, as the cycle approaches its maximum, forecasters are placing bets on how the next solar peak will unfold. So far, their outlooks aren't converging. A group led by Leif Svalgaard of ETK (a Houston-based consulting firm) predicts the weakest peak in over a century. Using a different technique, David Hathaway and Robert Wilson of NASA are calling for the strongest cycle since the 1950s.
Now a group of scientists from NCAR's High Altitude Observatory (HAO) are joining the fray. The NCAR team, led by Mausumi Dikpati, is basing its first-ever outlook on a new model of the solar dynamo that replicates past cycles remarkably well. "This is a significant breakthrough with important applications, especially for satellite-dependent sectors of society," says NCAR scientist Peter Gilman, one of the developers of the new model.
Whether the next solar cycle is strong or weak makes a huge difference to satellite operators, who plan their launches many years in advance. Each solar peak heats and expands the outer atmosphere, which in turn increases the drag on satellites, especially those in low-Earth orbit (below about 1,600 kilometers or 1,000 miles in altitude). The satellites can be thrust into a higher orbit to reduce drag, but that adds to the cost of a launch. Satellite planners thus time their missions and adjust orbital heights to take advantage of weak solar activity whenever possible. If a solar peak occurs earlier or later than expected, or if it's unexpectedly strong, the atmospheric drag can pull a satellite out of its orbit prematurely, cutting a year or more out of its useful life.
Solar-cycle forecasts also help a variety of other industries to prepare for possible impacts from solar storms, which can jeopardize power grids on Earth and corrupt satellite communication systems. Solar storms can occur any time—in fact, some of the most powerful ones erupt as a solar cycle is declining. However, they're most likely when the cycle is near its peak.
Sunspots and other statistics
One simple and time-tested way to measure the solar cycle is by counting sunspots, which have been observed systematically since Galileo began the practice in 1610. The International Sunspot Number, a daily record of grouped sunspot activity, extends from 1749 to the present. Although much more sophisticated observing tools have since come on line, the length of the sunspot record makes it uniquely useful. "Some researchers discredit sunspot numbers as archaic and of little value. However, the sunspot numbers track modern measures of solar activity very well," says NASA's Hathaway.
By carefully analyzing the long-term sunspot record, scientists have learned how to estimate the strength and timing of upcoming solar cycles based on statistical relationships between one cycle and the next. Among the most important correlations:
• Strong cycles tend to follow short ones.
• Strong cycles generally reach their peaks more quickly than weak ones.
• High minima are typically followed by high maxima.
This graphic shows how magnetic fields are recycled to produce sunspots within the solar convection zone (the top 30% of the solar interior, shown in white, surrounding the radiative core, in orange). Because the sun rotates faster at the equator than the poles, the north-south (poloidal) magnetic field (a) gets twisted into an east-west (toroidal) field (b). Pockets of enhanced toroidal field rise to the surface, twisting in the process, and emerge to create sunspots (c, upper right). Magnetic flux emerges and spreads outward as the spots decay. Panels (d) and (e) show the conveyor belt of plasma flow (yellow) carrying the surface magnetic flux toward the poles—reversing the polar field—and eventually downward and back toward the equator. New sunspots eventually form in the poloidal field (f), which is now reversed from that in (a). (Figure by Mausumi Dikpati, NCAR.)
These relationships are fairly reliable, but they have their limits. Much like the rule-of-thumb weather predictions common in the 1800s, statistical techniques rely on correlations rather than being rooted in physical understanding. Thus, sooner or later, exceptions come along.
Solar cycle number 23—the current cycle, which is now winding down—is a good case in point, says NCAR's Dikpati. Statistical forecasts in the mid-1990s called for the cycle to be a strong one; for instance, a panel of experts predicted a maximum sunspot number of 160, plus or minus 30. However, the actual maximum number (which occurred in April 2000) was only 121, compared to an average for the last century of about 100. This was the first time in more than 80 years that an odd-numbered cycle fell short of the strength of the preceding even-numbered cycle.
In the 1970s, Kenneth Schatten (now at a.i. solutions) pioneered an alternative to statistical forecasting called the precursor method. It employs observations of magnetism near the Sun's polar surface together with a theory of how sunspots evolve. The idea is that the seeds of each solar cycle appear in magnetic fields that intensify near the poles as the preceding cycle wanes. By measuring the strength of the polar fields, Schatten's method claims, one can assess how strong the upcoming cycle might be. Like the statistical methods, though, this technique overshot the strength of cycle number 23. The precursor method is also limited in its ability to "hindcast" past events because the magnetism data on which it relies have only been collected for the last three decades.
When the solar maximum proved to be unexpectedly weak, Dikpati was as surprised as other solar researchers. Yet she and her colleagues were already working on a model that can now reproduce the character of the last peak well. According to that model, each solar maximum is linked not just to the previous cycle but to the last several. In Dikpati's words, "The seeds for cycle 23 came from cycles 20, 21, and 22."
A conveyor belt of sunspots
Dikpati began modeling solar cycles for her doctoral dissertation at the Tata Institute in Bangalore, India. After graduating in 1996, she joined NCAR as a postdoctoral fellow and continued her line of research. In 1999, she and HAO colleague Paul Charbonneau (now at the University of Montreal) completed the first version of their groundbreaking solar-dynamo model. Dikpati spent years improving the model with Charbonneau, Gilman, Keith MacGregor, Matthias Rempel, Giuliana de Toma, and other colleagues in HAO. Recently she carried out simulations that extend into the next solar cycle. The team published its forecast in Geophysical Research Letters in March. They agree with Hathaway that cycle 24 will be a strong one: 30–50% more intense than the last peak, and perhaps second only to the 1957–58 peak as the strongest on record.
One of the innovative aspects of the new HAO model, known as the Predictive Flux-transport Dynamo Model, is its three-dimensional perspective. It tracks motion not only on the Sun's surface but throughout the depth of its convection zone, which extends nearly a third of the way toward the solar core. The model traces the looping path of plasma, or electrified gas, as it flows toward the poles, descends to the base of the convection zone, and then returns toward the equator, where it rises and completes the circuit.
The dense subsurface flow is the sunspot producer. It moves toward the equator at a top speed of only about 1–2 meters per second (2–4 mph). Along the way, the magnetic field within this conveyor belt gets twisted because the Sun's rotation rate is higher at the equator than at the poles (see graphic). When a set of coiled-up magnetic field lines erupts at the surface, it forms a sunspot. Eventually the sunspot decays and imprints the surface plasma with a type of magnetic signature. These signatures—the memory of a solar cycle—get recycled years later as they reach the pole and sink. In 17–22 years, the remnant fields move from the high-latitude solar surface downward, equatorward to the midlatitudes, and upward to the surface again, where they begin to generate fresh sunspots.
The HAO model relies heavily on data from helioseismology, a discipline that emerged from NASA- and NSF-funded instruments in the 1990s. Helioseismology involves tracking sound waves reverberating inside the Sun to reveal details about the interior, much as a doctor might use an ultrasound to see inside a patient. New data from this field allowed the HAO team to infer the leisurely pace of the subsurface flow and the sunspot seeds it carries. Variations in the speed of this flow appear to determine the length of time between cycles, while the presence or absence of magnetic remnants from the last three cycles helps determine the strength of the next one.
Because helioseismological data on plasma flow only extend back to 1996, Dikpati and colleagues held the modeled flow steady in order to test their model against the past eight solar cycles. It performed impressively, simulating the strength of those cycles with more than 98% accuracy, including the 1957–58 record maximum. Hathaway points out that, although any forecasting technique could have predicted that the 1957–58 would be a bit bigger than average, for the HAO model "to say that it's going to be the biggest one is quite amazing. It says volumes about the validity of Mausumi's model."
Already, the new flux-dynamo model has inspired a look at thermosphere density for the next solar cycle. HAO's Stanley Solomon and Liying Qian produced an outlook as a way to illustrate how the new cycle could affect satellites. Starting with Dikpati's forecast, Solomon and Qian added the kind of short-term solar variations one might see embedded in a typical cycle. This was then fed into the Thermosphere Ionosphere Electrodynamics General Circulation Model. Typically run for short periods, the TIEGCM takes solar input and simulates its effects on the upper atmosphere. Solomon and Qian produced the equivalent of a TIEGCM marathon—a 13-year run—and plan to publish the results shortly. "This is the first solar density forecast produced from a physics-based model," says Solomon.
Crafting a consensus
X-ray flares like this (upper right), coronal mass ejections, and other phenomena that occur at solar maximum can wreak havoc on satellite-dependent industries. (Image courtesy NASA.)
More than a dozen papers now offer predictions on the strength of the next solar cycle, with more than a twofold difference among them. "That's a tremendous range," says William Murtagh, a space weather forecaster at NOAA's Space Environment Center (SEC). Satellite launch teams and other end users aren't always equipped to sort through all the conflicting guidance. "Coming up with a single official prediction is challenging but important," says Murtagh.
That job falls to SEC, which issues an official forecast shortly after the start of each cycle. "We're viewed as an independent party that doesn't have a stake in a particular prediction," says SEC research scientist Doug Biesecker. As part of its Space Weather Week in April, the SEC convened a panel of 12 solar-cycle forecasters to determine the best process for reconciling the various predictions. The final NOAA outlook will be issued sometime in early 2007.
In the meantime, all eyes will be on the Sun's lower midlatitudes, around 25–30°, where the first sunspots of the new cycle will appear. As each solar cycle unfolds, sunspots become more prevalent, appearing at lower latitudes over time as the subsurface plasma continues its slow flow. The cycle wraps up with the last sunspots occurring near the equator; cycle number 23 is in that phase now. However, solar cycles usually overlap by a year or two, so the first spots of cycle 24 could appear in the 25–30° latitude band at any time.
ETK's Svalgaard, who uses a precursor technique, is sticking with his forecast of an unusually quiet solar cycle. He now calls for a peak sunspot number of only around 75, which would be a drop of close to 40% from the last solar maximum. "It's very interesting that our two techniques [his and HAO's] show very different predictions. That gives us a real chance of telling one from the other," he says. "Of course, nature can be really perverse, and we could get a result smack in the middle. We'll know that pretty soon, though."