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Stunning solar eruption

NASA’S SOLAR DYNAMICS OBSERVATORY (SDO) spacecraft captured an enormous plasma ‘filament’ collapsing on the Sun on August 31. A filament is a type of prominence – a loop of plasma (ionised gas) extending up from the Sun’s visible surface – seen in silhouette against the solar disc.

Solar prominences reach up from the photosphere (the visible surface) into the corona, the outer atmosphere of the Sun which contains extremely hot gases. The corona is so hot that it radiates energy beyond the wavelengths of visible light, so it is not normally seen. Prominences are made of cooler ionised gas, so they are visible even through they extend into the corona.

The video above shows a filament loop collapsing, seen partly silhouetted against the solar disc.

The SDO spacecraft, launched in February 2010, studies the Sun continuously from its orbit around the Earth.

More information:

Solar Dynamics Observatory

The Sun

Story by Jonathan Nally. Video and image courtesy NASA/SDO.

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Spotless Sun

SOHO image of the Sun with no sunspots

Periods of inactivity are normal for the Sun, but the recent period has gone on longer than usual. New computer simulations imply that the long quiet spell resulted from changing flows of hot plasma within the Sun. Image courtesy NASA / SOHO.

  • Sunspots come and go in an 11-year cycle from one minimum to the next
  • Recent minimum period went on for much longer than usual
  • Scientists say the cause is disruptions in the Sun’s ‘plasma rivers’

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. But 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 Andres Munoz-Jaramillo, a visiting research fellow at the Harvard-Smithsonian Centre 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.

Spotless days

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.

Sunspot cycles over the last century

Sunspot cycles over the last century. The blue curve shows the cyclic variation in the number of sunspots. Red bars show the cumulative number of sunspot-less days. The minimum of sunspot cycle 23 was the longest in the space age, having the largest number of spotless days. Image courtesy Dibyendu Nandi et al.

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 most recent 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 Munoz-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.”

Plasma rivers

To study the problem, Munoz-Jaramillo used computer simulations to model the Sun’s behaviour 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.

Cutaway diagram of the Sun showing the Great Conveyor Belt

In this artistic cutaway view of the Sun, the Great Conveyor Belt appears as a set of black loops connecting the stellar surface to the interior. Image courtesy Andrés Muñoz-Jaramillo / Harvard CfA.

At a typical speed of 65 kilometres per hour, it takes about 11 years to make one loop.

Munoz-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 Munoz-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.”

Adapted from information issued by the Harvard-Smithsonian Centre for Astrophysics.

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Stretchy “conveyor belt” slows the Sun

Full disc image of the Sun

The Sun goes through an 11-year cycle of high and low activity. The quiet phase of solar cycle 23, just passed, was unusually long.

  • Last solar cycle had longer “quiet phase” than usual
  • Due to changes in the Sun’s equator-to-pole plasma flow
  • The flow reached the poles instead of turning back earlier

A new study of the unusually long solar cycle that ended in 2008 suggests that one reason for its length could be a stretching of the Sun’s “conveyor belt”…a current of plasma that circulates between the Sun’s equator and its poles.

The Sun goes through cycles lasting approximately 11 years that include phases with increased magnetic activity, more sunspots, and more solar flares, and then phases with less activity.

The level of solar activity can affect navigation and communications systems on Earth.

Diagram showing equator-to-pole plasma flow on the Sun

Looking into the Sun's surface layer. Normally, the plasma flow from the equator turns back before reaching the poles (left), but in solar cycle 23 it reached practically all the way to the pole.

Puzzlingly, solar cycle 23, which ended in 2008, lasted longer than previous cycles, with a prolonged phase of low activity that had scientists baffled.

The study was conducted by Mausumi Dikpati, Peter Gilman, and Giuliana de Toma, all scientists with the High Altitude Observatory of the National Centre for Atmospheric Research (NCAR), and by Roger Ulrich at the University of California, Los Angeles.

The NCAR analysis suggests that one reason for the long cycle could have been changes in the Sun’s conveyor belt.

Just as Earth’s global ocean circulation transports water and heat around the planet, the Sun has a conveyor belt in which plasma flows along the surface from the equator toward the poles, sinks, and returns toward the equator, transporting magnetic energy along the way.

Recent measurements gathered and analysed by Ulrich and colleagues show that in solar cycle 23, the poleward flow extended all the way to the poles, while in previous cycles the flow turned back toward the equator at about 60 degrees latitude.

Furthermore, the return flow was slower in cycle 23 than in previous cycles.

In 2004, the NCAR team’s computer model successfully predicted that cycle 23 would last longer than usual.

Adapted from information issued by NCAR / UCAR / Big Bear Solar Observatory.

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