by Bob Henson and David Hosanskey
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.
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.)
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
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.
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
Sunspots and other statistics
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.
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.
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.)
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.
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
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.
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
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.
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.
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.
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."
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
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.)
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.
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."