This visible-light photograph, taken in 2008 by NASA’s Solar and Heliospheric Observatory (SOHO) spacecraft, shows the Sun’s face free of sunspots. The Sun experienced 780 spotless days during the unusually long solar minimum that just ended. New computer simulations imply that the Sun’s long quiet spell resulted from changing flows of hot plasma within it. Credit: NASA/SOHO |
The Sun has been in the news a lot lately because it’s beginning
to send out more flares and solar storms. Its recent turmoil is particularly
newsworthy because the Sun was very quiet for an unusually long time.
Astronomers had a tough time explaining the extended solar minimum. New
computer simulations imply that the Sun’s long quiet spell resulted from
changing flows of hot plasma within it.
“The Sun
contains huge rivers of plasma similar to Earth’s ocean currents,” says
Andrés Muñoz-Jaramillo, a visiting research fellow at the Harvard-Smithsonian Center
for Astrophysics (CfA). “Those plasma rivers affect solar activity in ways
we’re just beginning to understand.”
The Sun is
made of a fourth state of matter—plasma. Flowing plasma creates magnetic
fields, which lie at the core of solar activity like flares, eruptions, and
sunspots.
Astronomers
have known for decades that the Sun’s activity rises and falls in a cycle that
lasts 11 years on average. At its most active, called solar maximum, dark
sunspots dot the Sun’s surface and frequent eruptions send billions of tons of
hot plasma into space. If the plasma hits Earth, it can disrupt communications
and electrical grids and short out satellites.
During solar
minimum, the Sun calms down and both sunspots and eruptions are rare. The
effects on Earth, while less dramatic, are still significant. For example,
Earth’s outer atmosphere shrinks closer to the surface, meaning there is less
drag on orbiting space junk. Also, the solar wind that blows through the solar
system (and its associated magnetic field) weakens, allowing more cosmic rays
to reach us from interstellar space.
The most
recent solar minimum had an unusually long number of spotless days: 780 days
during 2008-2010. In a typical solar minimum, the Sun goes spot-free for about
300 days, making the last minimum the longest since 1913.
“The last
solar minimum had two key characteristics: a long period of no sunspots and a
weak polar magnetic field,” explains Muñoz-Jaramillo. (A polar magnetic
field is the magnetic field at the Sun’s north and south poles.) “We have
to explain both factors if we want to understand the solar minimum.”
To study the
problem, Muñoz-Jaramillo used computer simulations to model the Sun’s behavior
over 210 activity cycles spanning some 2,000 years. He specifically looked at
the role of the plasma rivers that circulate from the Sun’s equator to higher
latitudes. These currents flow much like Earth’s ocean currents: rising at the
equator, streaming toward the poles, then sinking and flowing back to the
equator. At a typical speed of 40 miles per hour, it takes about 11 years to
make one loop.
Muñoz-Jaramillo
and his colleagues discovered that the Sun’s plasma rivers speed up and slow
down like a malfunctioning conveyor belt. They find that a faster flow during
the first half of the solar cycle, followed by a slower flow in the second half
of the cycle, can lead to an extended solar minimum. The cause of the speed-up
and slowdown likely involves a complicated feedback between the plasma flow and
solar magnetic fields.
“It’s
like a production line—a slowdown puts ‘distance’ between the end of the last
solar cycle and the start of the new one,” says Muñoz-Jaramillo.
The ultimate
goal of studies like this is to predict upcoming solar maxima and minima—both
their strength and timing. The team focused on simulating solar minima, and say
that they can’t forecast the next solar minimum (which is expected to occur in
2019) just yet.
“We can’t
predict how the flow of these plasma rivers will change,” explains lead
author Dibyendu Nandy (Indian Institute of Science Education and Research,
Kolkata). “Instead, once we see how the flow is changing, we can predict
the consequences.”
Their findings
appear in Nature.