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Stanford University researchers have found a way to detect sunspots such as these two days before they reach the surface of the sun. Image: Thomas Hartlep
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Viewed from
the technological perspective of modern humans, the sun is a seething cauldron
of disruptive influences that can wreak havoc on communication systems, air
travel, power grids, and satellites—not to mention astronauts in space.
If disruptions
such as solar flares and mass eruptions could be predicted, protective measures
could be taken to shield vulnerable electronics before solar storms strike.
Now Stanford University researchers have developed a
method that allows them to peer deep into the sun's interior, using acoustic waves
to catch sunspots in the early stage of development and giving as much as two
days' warning.
Sunspots
develop in active solar regions of strong, concentrated magnetic fields and
appear dark when they reach the surface of the sun. Eruptions of the intense
magnetic flux give rise to solar storms, but until now, no one has had luck in
predicting them.
"Many
solar physicists tried different ways to predict when sunspots would appear,
but with no success," says Phil Scherrer, a professor of physics in whose lab
the research was conducted.
The key to the
new method is using acoustic waves generated inside the sun by the turbulent
motion of plasma and gases in constant motion. In the near-surface region,
small-scale convection cells—about the size of California—generate sound waves that travel
to the interior of the sun and are refracted back to the surface.
The
researchers got help from the Michelson Doppler Imager aboard NASA's Solar and
Heliospheric Observatory satellite, known as SOHO.
The craft spent 15 years making detailed observations of the sound waves within
the sun. It was superseded in 2010 with the launch of NASA's Solar Dynamics
Observatory satellite, which carries the Helioseismic and Magnetic Imager.
Using the
masses of data generated by the two imagers, Stathis Ilonidis, a Stanford
graduate student in physics, was able to develop a way to reduce the electronic
clutter in the data so he could accurately measure the solar sounds.
The new method
enabled Ilonidis to detect sunspots in the early stages of formation as deep as
65,000 km inside the sun. Between one and two days later, the sunspots would
appear on the surface. Ilonidis is the lead author of a paper describing the research, published in Science.
The principles
used to track and measure the acoustic waves traveling through the sun are
comparable to measuring seismic waves on Earth. The researchers measure the
travel time of acoustic waves between widely separated points on the solar
surface.
"We know
enough about the structure of the sun that we can predict the travel path and
travel time of an acoustic wave as it propagates through the interior of the
sun," says Junwei Zhao, a senior research scientist at Stanford's Hansen
Experimental Physics Lab. "Travel times get perturbed if there are
magnetic fields located along the wave's travel path." Those perturbations
are what tip the researchers that a sunspot is forming.
By measuring
and comparing millions of pairs of points and the travel times between them,
the researchers are able to home in on the anomalies that reveal the growing
presence of magnetic flux associated with an incipient sunspot.
They found
that sunspots that ultimately become large rise up to the surface more quickly
than ones that stay small. The larger sunspots are the ones that spawn the
biggest disruptions, and for those the warning time is roughly a day. The
smaller ones can be found up to two days before they reach the surface.
"Researchers
have suspected for a long time that sunspot regions are generated in the deep
solar interior, but until now the emergence of these regions through the
convection zone to the surface had gone undetected," Ilonidis says.
"We have now successfully detected them four times and tracked them moving
upward at speeds between 1,000 and 2,000 km per hour."
One of the big goals with forecasting space weather is achieving a three-day
warning time of impending solar storms. That would give the potential victims a
day to plan, another day to put the plan into action and a third day as a
safety margin.
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