[ RadSafe ] Article: Predicting solar storms and radiation effects

[John Jacobus]

>From Nature 441, 402-404 (25 May 2006)
Published online 24 May 2006 at
http://www.nature.com/nature/journal/v441/n7092/full/441402a.html

The dark side of the Sun
Stuart Clark1
Abstract: The Sun occasionally hurls streams of
particles towards Earth, where they can wreak havoc
with satellites. Predicting these solar storms is
hard, but some physicists believe we're about to face
the biggest bout of solar flares in years. 

Halloween is supposed to be a time of weird phenomena
and spooky events. But by any standards what happened
in late October 2003 was unusual. Telecommunications
around the world were disrupted, half of NASA's
satellites malfunctioned, 50,000 people in Sweden were
left without electricity, and the global airline
industry lost millions of dollars.

The link between these events was not supernatural, it
was something far more familiar: the Sun. The chaos
was caused as our star went through one of the more
active moments in its 11-year cycle. And according to
some predictions, what happened that October is
nothing compared with what is going to occur in five
or six years' time.

"Solar activity in the next cycle could be more of a
problem than ever," warns Peter Gilman, a physicist at
the National Center for Atmospheric Research (NCAR) in
Boulder, Colorado. And it's not just satellites and
telecommunications that face problems. Some
researchers claim that the Sun's behaviour affects
Earth's atmosphere — in particular influencing cloud
formation. This claim has attracted global-warming
sceptics, who argue that the Sun has greater influence
than human activities on our changing climate.

The Sun's 11-year cycle is driven by its magnetic
field, and generates a flow of charged particles known
as the solar wind. At the quieter parts of the cycle,
activity is fairly low and the solar wind is
reasonably uniform. But at the 'solar maximum',
sunspots — dark patches caused by the magnetic field
twisting at the surface — appear on the Sun's face.
Huge solar flares explode above these spots causing
turbulence in the solar wind and sending streams of
charged particles hurtling through space.

The most recent solar cycle was fairly moderate when
measured by the number of sunspots (see graphic). Yet
in late October 2003, three years after the cycle's
peak, two monstrous sunspots appeared, each more than
ten times the diameter of Earth. Both were in a state
of almost constant eruption, spewing out billions of
tonnes of electrically charged particles.

These were the particles that caused such havoc when
they hit Earth's atmosphere. The global maritime
emergency call system blacked-out, contact was lost
with expeditions on Mount Everest, and the accuracy of
the global positioning system was impaired. As well as
NASA's satellite malfunctions, the Japanese lost
contact with one of their weather satellites
altogether. The cost to the airline industry arose as
planes were re-routed to lower altitudes, congesting
the airways and burning more fuel.

Lucky escape
The sunspots bombarded Earth, on and off, for two
weeks as the Sun's rotation carried them across its
face. On 4 November, as the second sunspot was about
to be lost from sight, it let loose another tremendous
explosion. Solar physicists calculated that it was one
of the largest solar flares in recorded history1. By
sheer luck it exploded into deep space, catching Earth
only in the side wash. Those who saw it breathed a
sigh of relief and wondered what the damage might have
been if such a flare had exploded facing Earth. If the
latest prediction comes true for the next solar cycle,
we may yet find out.

Predicting the timing and strength of such solar
eruptions is clearly important, but it is hampered by
the fact that scientists know relatively little about
the Sun's inner workings. So to coincide with the
start of the next solar cycle, the largest coordinated
study of the Sun will be launched next year. Known as
the International Heliophysical Year (IHY), the
initiative hopes to build awareness of the Sun's
possible influence on Earth's climate and to bring
researchers from different disciplines together to
study solar activity.

Currently, the Sun is at a solar minimum, and most
predictions suggest that the next solar maximum in
five or six years' time will be weak. But the most
recent forecast, the first to be based on a completely
physical model of the Sun, suggests otherwise.

This forecast has been generated by Mausumi Dikpati
and her team at the NCAR2. They have developed a
computer simulation that mixes the Sun's internal
magnetic dynamo with theories about how solar plasma
circulates near the surface. And they have reached a
sobering conclusion. "We expect between 30% and 50%
more sunspots and solar activity than the cycle just
ending," says Gilman, who is a member of Dikpati's
team.

The last time solar activity occurred on this scale
was in 1958, when there was little technology in
orbit. Now things are very different: Earth is
surrounded by thousands of active satellites.

Satellite operators rely on predictions of solar
activity to estimate the lifetime of space missions.
The solar wind heats Earth's thin upper atmosphere,
increasing atmospheric density and causing more drag.
Gilman estimates that a 30% increase in activity will
almost double the atmospheric density at an altitude
of 300 km, affecting low-altitude satellites.

Mission planners looking ahead to 2012 may want to
boost their spacecraft to higher orbits, or accept a
shorter operational lifetime. Even above 800 km, where
satellites are safe from atmospheric drag, other
dangers remain. The solar wind can cause a build up in
electrical charge, which then short-circuits and burns
out sensitive equipment. This is the suspected fate of
the Japanese Midori 2 satellite, lost during the 2003
flares. And as more satellites die in orbit, operators
have to worry about dodging 'space junk'. In the
aftermath of a large solar storm, the change in
atmospheric drag can shift the orbit of space debris,
endangering active satellites.

The Sun's influence over space hardware is only one
aspect of the latest drive to understand the star. The
possible effects of the solar cycle on our climate,
especially cloud formation, are also receiving a lot
of attention. A link between the two was suggested in
1997, when meteorologists Henrik Svensmark and Eigil
Friis-Christensen, both at the Danish Meteorology
Institute in Copenhagen, analysed weather satellite
records for 1979 to 1992. They found that during solar
minima, Earth was 3% cloudier than at solar maxima3.
They also noticed that the influx of high-energy
particles reaching Earth from deep space, phenomena
known as cosmic rays, was up to 25% higher at solar
minima, hinting that they might seed cloud formation.
The pair called their finding a "missing link in
solar–climate relationships".

Climate sceptics who argue that human activities are
not responsible for global warming have seized on
these results. They claim it shows that the Sun is
largely responsible for variations in our climate. So
convinced are they that last year two Russian sceptics
placed a $10,000 bet that global temperatures will
show an average fall for 2012–17 — on the assumption
that the next solar cycle will be weak4.

But most proponents of the solar–climate link are
proceeding more carefully. "We're not suggesting that
all clouds are formed by solar activity, merely that
the process might be modulated by solar activity,"
says Robert Bingham, a physicist at the Rutherford
Appleton Laboratory in Didcot, UK. He is part of an
international experiment known as CLOUD, or Cosmics
Leaving Outdoor Droplets. This will use CERN's
particle accelerator on the French–Swiss border to
fire charged particles through a chamber holding gases
to simulate Earth's atmosphere and determine whether
'clouds' are created.

Global network
To take advantage of the next solar cycle more
directly, the United Nations is heading an initiative
to install radio receivers in all 191 of its member
states. For the first time, the upper atmosphere's
response to the continual collision of solar radiation
would be monitored on a global basis. Although space
officially starts at an altitude of about 100 km,
scientists know little about this region because it is
difficult to study.

The UN project is one of the planned elements in the
IHY. Although it has no dedicated research budget, the
IHY has initiated a call-for-proposals aimed at making
it easier for scientists from any discipline to gain
access to solar instruments and data. "We are inviting
ideas from the community," says Rutherford's Richard
Harrison, the joint UK coordinator for the IHY.

Certainly 2007 will put at scientists' disposal the
largest-ever fleet of space missions for studying
solar–terrestrial interactions. A dozen spacecraft
that track solar activity are already in orbit, and
another three should launch this year, including the
most sophisticated solar watchdog yet. NASA's Solar
Terrestrial Relations Observatory (STEREO) consists of
two nearly identical craft that will watch the Sun
from different locations, one preceding Earth in its
orbit and the other following behind. This will allow
them to take stereoscopic images of the Sun and to
track the three-dimensional structure of particle
eruptions. In this way, STEREO might be able to supply
advance notice of the speed and direction of eruptions
as they head towards Earth.

Such information should help satellite operators
respond to imminent dangers, but for proper planning
they will need long-term forecasts of solar activity.
Some researchers, such as Harrison, believe that
scientists don't yet understand the Sun enough to make
meaningful long-term predictions. Certainly, past
forecasts have relied on tracking signposts of future
solar activity, without worrying too much about the
mechanisms behind them.

For example, in the 1970s, astronomers recognized that
the build up of magnetism at the Sun's poles after the
cycle has peaked has a bearing on the strength of the
next cycle. Just last year, one of the pioneers of
this method, Leif Svalgaard, used the Sun's polar
magnetic field to predict that the next solar cycle
will be the weakest for a century5. Other 'signpost'
methods, such as those looking at the amount of
10.7-centimetre radio waves coming from the Sun or the
number of bright patches near the Sun's poles, also
forecast a weak cycle.

The only signpost method to predict a strong cycle
comes from solar physicists David Hathaway and Robert
Wilson at NASA's Marshall Space Flight Center in
Huntsville, Alabama. In 2004, they noticed that a
solar cycle's strength correlates to the number of
sunspots two cycles before. Applying that rule of
thumb to the next cycle, they have predicted strong
activity in 2012 (ref. 6). Dikpati's model agrees with
this forecast and, crucially, puts the reason for it
on a physical footing.

In the past decade, physicists have discovered a vast
conveyor belt of plasma on the Sun that seems to flow
from the equator to the poles in each hemisphere at
around 30–65 kilometres per hour. Sunspots are
typically active for just a few weeks before fading
from view, but their magnetic fields linger on. These
weak fields are carried by the flow and accumulate at
the poles before being submerged below the surface,
where they presumably flow back towards the equator7.

Dikpati's work combines sunspot observations dating
back to the 1900s with a computer simulation of the
Sun's magnetic dynamo and the conveyor belt (see
graphic). In the simulation, the conveyor belt sweeps
along old sunspots, submerging them at the poles.
During the deep return flow, the Sun's rotation
rejuvenates the old magnetic fields, creating new
sunspots and fresh areas of solar activity.

It is the only prediction in which every step uses a
physics-based computer model, which is why it is being
taken seriously by solar physicists. "The solid
physics of Dikpati's model is a high hurdle for the
other techniques to get over," says Hathaway.

Solar memory
The key to Dikpati's forecast is how fast the Sun's
conveyor belt runs. The deep return flow is
unmeasurable but the model suggests that it is slower
than the surface flow, perhaps just 5 kilometres per
hour. If so, the return leg of the journey would take
a couple of decades. "This shows that the Sun retains
a memory of its magnetic field for about 20 years,"
says Dikpati. So in her model, the Sun's activity is
not based solely on the previous cycle's magnetic
field but on the interplay with earlier cycles. In
contrast, most 'signpost' prediction methods assume
that the previous solar cycle immediately kicks off
the activity of the next. "It is good for science that
the predictions are now diverging," says Svalgaard,
although he disagrees with Dikpati's conclusions.

Solar physicists are now waiting to see if this
physics-based forecast is right. And they may not have
to wait for the peak of activity in six years' time to
find out. All methods predict only the average number
of sunspots, but records show that large cycles have
always begun early and raced to their peak. That means
that the telltale signs of a large solar cycle should
be evident within just three or four years from now.

"We must now let Mother Nature tell us who is right,"
says Svalgaard. But Dikpati and her team are refining
their model to see whether it can predict features
such as an early start. Either way, there will be
plenty of sun watchers — from mission planners to
climate sceptics — tracking the way the solar wind
blows.

References
1. Tsurutani, B. T. et al. Geophys. Res. Lett. 32,
L03S09 (2005). 
2. Dikpati, M. , de Toma, G. & Gilman, P. A. Geophys.
Res. Lett. 33, L05102 (2006). 
3. Svensmark, H. & Friis-Christensen, E. J. Atmos.
Solar-Terr. Phys. 59, 1225–1232 (1997). 
4. Giles, J. Nature 436, 897 (2005). 
5. Svalgaard, L. , Cliver, E. W. & Kamide, Y. Geophys.
Res. Lett. 32, L01104 (2005). 
6. Hathaway, D. H. & Wilson, R. M. Solar Phys. 224,
5–19 (2004).
7. Hathaway, D. H. et al. Astrophys. J. 589, 665–670 (2003).

+++++++++++++++++++
"You get a lot more authority when the workforce doesn't think it's amateur hour on the top floor."
GEN. MICHAEL V. HAYDEN, President Bush's nominee for C.I.A. director.

-- John
John Jacobus, MS
Certified Health Physicist
e-mail:  crispy_bird at yahoo.com

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