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Making sense of the solar cycle

THE NUMBER OF SUNSPOTS increases and decreases over time in a regular, approximately 11-year cycle, called the sunspot cycle. The exact length of the cycle can vary. It has been as short as eight years and as long as fourteen, but the number of sunspots always increases over time, and then returns to low again.

Sunspots look dark only because they’re a bit cooler than the surrounding solar surface. In reality, sunspots are still intensely hot—around 3,000 to 4,000 degrees Celsius.

More sunspots mean increased solar activity, when great blooms of radiation known as solar flares or bursts of solar material known as coronal mass ejections (CMEs) shoot off the Sun’s surface.

The highest number of sunspots in any given cycle is designated “solar maximum,” while the lowest number is designated “solar minimum.”

A sunspot group

Sunspot numbers climb and fall over an 11-year cycle.

Each cycle, varies dramatically in intensity, with some solar maxima being so low as to be almost indistinguishable from the preceding minimum.

The last solar maximum was in the year 2000, and the next is expected to occur in early 2013.

Solar cauldron

Sunspots are a magnetic phenomenon. The entire Sun is magnetised with a north and a south magnetic pole just like a bar magnet. The comparison to a simple bar magnet ends there, however, as the Sun’s interior is constantly on the move.

By tracking pressure waves that course through the centre of the Sun, an area of research known as helioseismology, scientists can gain an understanding of what’s deep inside the Sun.

They have found that the magnetic material inside the Sun is constantly stretching, twisting, and crossing as it bubbles up to the surface.

The exact pattern of movements is not conclusively mapped out, but over time they eventually lead to the poles reversing completely.

The Sun flips out

The sunspot cycle happens because the magnetic poles flip—north becomes south and south becomes north—approximately every 11 years. Some 11 years later, the poles reverse again back to where they started, making the full solar cycle actually a 22-year phenomenon.

The Sun behaves similarly over the course of each 11-year cycle no matter which pole is on top, however, so the shorter cycle tends to receive more attention.

Adapted from information issued by NASA / Goddard Space Flight Centre. Images courtesy NSO and SOHO (ESA & NASA).

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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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Sun’s size surprises scientists

The Sun’s disc

The Sun’s disc showing active region 10486, which became the largest sunspot seen by the SOHO satellite.

  • Sun’s size normally varies
  • But has stayed the same for 12 years

Astronomers have found that in recent times the Sun’s size has been remarkably constant. Normally its diameter varies slightly, but the scientists have found that it has changed by less than one part in a million over the last 12 years.

“This constancy is baffling, given the violence of the changes we see every day on the Sun’s surface and the fluctuations that take place over an 11-year solar cycle,” said Dr Jeff Kuhn, associate director of the University of Hawaii Institute for Astronomy (IfA) who is responsible for Haleakala Observatories.

Kuhn’s work is part of worldwide efforts to understand the influence of the Sun on Earth’s climate. “We can’t predict the climate on Earth until we understand these changes on the Sun,” he said.

Kuhn and his colleagues used NASA’s long-lived Solar and Heliospheric Observatory (SOHO) satellite to monitor the Sun’s diameter, and they will soon repeat the experiment with much greater accuracy using NASA’s new Solar Dynamics Observatory (SDO), which was launched on February 11.

According to Kuhn, the ultimate solution to this puzzle will depend on probing the smallest observable scales of the solar surface using the Advanced Technology Solar Telescope (ATST), which is scheduled for completion on Haleakala in 2017.

“To be able to predict what the Sun will do, we need both the big picture and the details,” said Kuhn. “Just as powerful hurricanes on Earth start as a gentle breeze, the analogues of terrestrial storms on the Sun start as small kinks in the Sun’s magnetic field.”

Adapted from information issued by the University of Hawaii at Manoa’s Institute for Astronomy / SOHO / MDI consortium.

Sun begins a new cycle

Loops in the Sun's magnetic field

Loops in the Sun's magnetic field are often associated with sunspots.

Scientists from Geoscience Australia will be watching changes to Earth’s magnetic field over the coming few years with evidence that the Sun has begun its latest cycle of sunspot activity.

The most recent so called sunspot maximum occurred in 2000 and was followed by a period of decreasing sunspot numbers which was slightly longer than usual. However, scientists are now seeing evidence that the new solar cycle has begun and the number of sunspots may be starting to rise again. If the normal pattern continues they should peak around 2013.

One of the significant effects of sunspots and associated solar flares is the impact on Earth’s magnetic field. The rapid field changes caused by sunspots affect satellite and high-frequency radio communications, telephones and powerlines. They also degrade the accuracy of GPS positions and disrupt magnetic surveying operations.

Another consequence of increased solar activity is auroral displays which are caused by charged particles from the Sun entering Earth’s magnetic field and colliding with gas particles in the atmosphere. Auroras occur more commonly in polar regions, but the phenomenon can sometimes also be seen nearer the equator during periods of intense magnetic activity.

Working in conjunction with the Australian Space Weather Agency, IPS Radio and Space Services, Geoscience Australia will record any increased activity in Earth’s magnetic field at its geomagnetic monitoring stations at Kakadu and Alice Springs in the Northern Territory, Learmonth and Gnangara in Western Australia, Charters Towers in Queensland, near Canberra in the ACT and in Antarctica at Casey, Mawson and Macquarie Island.

Adapted from information issued by Geoscience Australia. Image courtesy TRACE / NASA.