CfA: Solar Mystery Solved

bystander · Post by **bystander** » Fri Mar 25, 2011 9:35 pm

Solar Mystery Solved
Center for Astrophysics | 2011 Mar 02

Spotless Sun (2008) [i](Credit: NASA/SOHO)[/i]
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, in which negative electrons and positive ions flow freely. 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."

The Mystery of the Absent Sunspots
ScienceShot | Edwin Cartlidge | 2011 Mar 02

[i]Credit: Andrés Muñoz-Jaramillo/CfA and Tom Bridgman/NASA[/i]
The sun is usually a predictable beast, at least as far as its sunspot cycle goes. Every 11 years or so, the sun's magnetic activity peaks and then troughs, resulting in relatively high and then low numbers of dark spots and flares on the solar surface. But in the cycle that has just finished, the trough went on for much longer than normal, with more than twice as many days without sunspots compared with previous cycles. To figure out what caused this, researchers used a computer simulation of the churning hot plasma inside the sun. As each cycle progresses, the movement of this plasma (black loop) shifts the solar magnetic fields (gold strands)—from which sunspots erupt—from the sun's midlatitudes to its equator. An extended minimum occurs whenever the plasma moves quickly at the beginning of a cycle—preventing a large buildup of magnetic fields—but then slows down toward the end, delaying the onset of the next cycle, the team reports online today in Nature. This knowledge won't help in predicting individual solar storms, the researchers say, but it should give scientists a better idea of how stormy the sun will be in the years to come. And that should help to limit the worst effects of storms, be it damage to satellites in orbit or harm to people flying close to the poles.

Researchers Crack the Mystery of the Missing Sunspots
NASA Science News | Dr. Tony Phillips | 2011 Mar 02

Click to play embedded YouTube video.
In 2008-2009, sunspots almost completely disappeared for two years. Solar activity dropped to hundred-year lows; Earth's upper atmosphere cooled and collapsed; the sun’s magnetic field weakened, allowing cosmic rays to penetrate the Solar System in record numbers. It was a big event, and solar physicists openly wondered, where have all the sunspots gone?
...
For years, solar physicists have recognized the importance of the sun's "Great Conveyor Belt." A vast system of plasma currents called ‘meridional flows’ (akin to ocean currents on Earth) travel along the sun's surface, plunge inward around the poles, and pop up again near the sun's equator. These looping currents play a key role in the 11-year solar cycle. When sunspots begin to decay, surface currents sweep up their magnetic remains and pull them down inside the star; 300,000 km below the surface, the sun’s magnetic dynamo amplifies the decaying magnetic fields. Re-animated sunspots become buoyant and bob up to the surface like a cork in water—voila! A new solar cycle is born.
...

The unusual minimum of sunspot cycle 23 caused by meridional plasma flow variations - D Nandy et al

Nature 471 80 (03 Mar 2011) DOI: 10.1038/nature09786