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WHAT'S DOWN WITH THE SUN?
MAJOR DROP IN SOLAR ACTIVITY PREDICTED
A missing jet stream, fading spots, and slower activity near the poles
say that our Sun is heading for a rest period even as it is acting up
for the first time in years, according to scientists at the National
Solar Observatory (NSO) and the Air Force Research Laboratory (AFRL).
As the current sunspot cycle, Cycle 24, begins to ramp up toward
maximum, independent studies of the solar interior, visible surface,
and the corona indicate that the next 11-year solar sunspot cycle,
Cycle 25, will be greatly reduced or may not happen at all.
The results were announced at the annual meeting of the Solar Physics
Division of the American Astronomical Society, which is being held
this week at New Mexico State University in Las Cruces:
"This is highly unusual and unexpected," Dr. Frank Hill, associate
director of the NSO's Solar Synoptic Network, said of the results.
"But the fact that three completely different views of the Sun point
in the same direction is a powerful indicator that the sunspot cycle
may be going into hibernation."
Spot numbers and other solar activity rise and fall about every 11
years, which is half of the Sun's 22-year magnetic interval since the
Sun's magnetic poles reverse with each cycle. An immediate question is
whether this slowdown presages a second Maunder Minimum, a 70-year
period with virtually no sunspots during 1645-1715.
Hill is the lead author on one of three papers on these results being
presented this week. Using data from the Global Oscillation Network
Group (GONG) of six observing stations around the world, the team
translates surface pulsations caused by sound reverberating through
the Sun into models of the internal structure. One of their
discoveries is an east-west zonal wind flow inside the Sun, called the
torsional oscillation, which starts at mid-latitudes and migrates
towards the equator. The latitude of this wind stream matches the new
spot formation in each cycle, and successfully predicted the late
onset of the current Cycle 24.
"We expected to see the start of the zonal flow for Cycle 25 by now,"
Hill explained, "but we see no sign of it. This indicates that the
start of Cycle 25 may be delayed to 2021 or 2022, or may not happen at
In the second paper, Matt Penn and William Livingston see a long-term
weakening trend in the strength of sunspots, and predict that by Cycle
25 magnetic fields erupting on the Sun will be so weak that few if any
sunspots will be formed. Spots are formed when intense magnetic flux
tubes erupt from the interior and keep cooled gas from circulating
back to the interior. For typical sunspots this magnetism has a
strength of 2,500 to 3,500 gauss (Earth's magnetic field is less than
1 gauss at the surface); the field must reach at least 1,500 gauss to
form a dark spot.
Using more than 13 years of sunspot data collected at the
McMath-Pierce Telescope at Kitt Peak in Arizona, Penn and Livingston
observed that the average field strength declined about 50 gauss per
year during Cycle 23 and now in Cycle 24. They also observed that spot
temperatures have risen exactly as expected for such changes in the
magnetic field. If the trend continues, the field strength will drop
below the 1,500 gauss threshold and spots will largely disappear as
the magnetic field is no longer strong enough to overcome convective
forces on the solar surface.
Moving outward, Richard Altrock, manager of the Air Force's coronal
research program at NSO's Sunspot, NM, facilities has observed a
slowing of the "rush to the poles," the rapid poleward march of
magnetic activity observed in the Sun's faint corona. Altrock used
four decades of observations with NSO's 40-cm (16-inch) coronagraphic
telescope at Sunspot.
"A key thing to understand is that those wonderful, delicate coronal
features are actually powerful, robust magnetic structures rooted in
the interior of the Sun," Altrock explained. "Changes we see in the
corona reflect changes deep inside the Sun."
Altrock used a photometer to map iron heated to 2 million degrees C
(3.6 million F). Stripped of half of its electrons, it is easily
concentrated by magnetism rising from the Sun. In a well-known
pattern, new solar activity emerges first at about 70 degrees latitude
at the start of a cycle, then towards the equator as the cycle ages.
At the same time, the new magnetic fields push remnants of the older
cycle as far as 85 degrees poleward.
"In cycles 21 through 23, solar maximum occurred when this rush
appeared at an average latitude of 76 degrees," Altrock said. "Cycle
24 started out late and slow and may not be strong enough to create a
rush to the poles, indicating we'll see a very weak solar maximum in
2013, if at all. If the rush to the poles fails to complete, this
creates a tremendous dilemma for the theorists, as it would mean that
Cycle 23's magnetic field will not completely disappear from the polar
regions (the rush to the poles accomplishes this feat). No one knows
what the Sun will do in that case."
All three of these lines of research to point to the familiar sunspot
cycle shutting down for a while.
"If we are right," Hill concluded, "this could be the last solar
maximum we'll see for a few decades. That would affect everything from
space exploration to Earth's climate."
# # #
These results have been presented at the current meeting of the
16.10: "Large-Scale Zonal Flows During the Solar Minimum -- Where Is
Cycle 25?" by Frank Hill, R. Howe, R. Komm, J. Christensen-Dalsgaard,
T.P. Larson, J. Schou & M. J. Thompson.
17.21: "A Decade of Diminishing Sunspot Vigor" by W. C. Livingston, M.
Penn & L. Svalgard.
18.04: "Whither Goes Cycle 24? A View from the Fe XIV Corona" by R. C.