Sunspot Mystery Cracked

We have started to unravel the behaviorof the solar conveyor belt whose fluctuations are clearly the driver behind theobserved 11 year sunspot cycle.  It isdescribed as akin to a conveyor belt that occasionally sticks a bit and thenreleases and hurries to catch back up.

The model also provides explicationfor the Maunder minimum.  The linkage toclimate is still not entirely pinned down and we see it is a mechanism drivenby strong changes in the magnetic flux that changes our upper atmosphere in acyclic manner through the eleven years and could well affect temperatures.  It really one more significant factor that ischallenging to fit in.

We now know what we are lookingat and what we need to monitor closely in order to refine our working model.  It really looks good.  The sunspots are a small part of a hugesystem and we now understand how they fit in.

Researchers Crack the Mystery of the Missing Sunspots

March 2, 2011: In 2008-2009, sunspots almost completelydisappeared for two years. Solar activity dropped to hundred-year lows; Earth's upper atmosphere cooled and collapsed; the sun’s magnetic fieldweakened, allowing cosmic rays to penetrate the Solar System in record numbers.It was a big event, and solar physicists openly wondered, where have allthe sunspots gone?  

Now they know. An answer is being published in the March 3rd edition of Nature.


In this artistic cutaway view of the sun, the Great Conveyor Beltappears as a set of black loops connecting the stellar surface to the interior.Credit: Andrés Muñoz-Jaramillo of the Harvard CfA.

"Plasma currents deep inside the sun interfered with the formationof sunspots and prolonged solar minimum," says lead author Dibyendu Nandiof the Indian Institute of Science Education and Research in Kolkata. "Ourconclusions are based on a new computer model of the sun's interior."

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 thesun's surface, plunge inward around the poles, and pop up again near the sun'sequator.  These looping currents play a key role in the 11-year solarcycle.  When sunspots begin to decay, surface currents sweep up theirmagnetic remains and pull them down inside the star; 300,000 km below thesurface, the sun’s magnetic dynamo amplifies the decaying magneticfields.  Re-animated sunspots become buoyant and bob up to the surfacelike a cork in water—voila!  A new solar cycle is born.

For the first time, Nandi’s team believes they have developed acomputer model that gets the physics right for all three aspects of thisprocess--the magnetic dynamo, the conveyor belt, and the buoyant evolution ofsunspot magnetic fields.

"According to our model, the trouble with sunspots actuallybegan in back in the late 1990s during the upswing of Solar Cycle 23,"says co-author Andrés Muñoz-Jaramillo of the Harvard-Smithsonian Centerfor Astrophysics. "At that time, the conveyor belt sped up."

The fast-moving belt rapidly dragged sunspot corpses down to sun'sinner dynamo for amplification. At first glance, this might seem to boostsunspot production, but no. When the remains of old sunspots reached thedynamo, they rode the belt through the amplification zone too hastily for fullre-animation.  Sunspot production was stunted.

Sunspot cycles over the last century. The blue curve shows the cyclicvariation in the number of sunspots. Red bars show the cumulative number ofsunspot-less days. The minimum of sunspot cycle 23 was the longest in the spaceage with the largest number of spotless days. Credit: Dibyendu Nandi et al.

Later, in the 2000s, according to the model, the Conveyor Belt sloweddown again, allowing magnetic fields to spend more time in the amplificationzone, but the damage was already done.  New sunspots were in shortsupply.  Adding insult to injury, the slow moving belt did little toassist re-animated sunspots on their journey back to the surface, delaying theonset of Solar Cycle 24.

"The stage was set for the deepest solar minimum in acentury," says co-author Petrus Martens of the Montana State UniversityDepartment of Physics.

Colleagues and supporters of the team are calling the new model asignificant advance.
"Understanding and predicting solar minimum is something we’venever been able to do before---and it turns out to be very important,"says Lika Guhathakurta of NASA’s Heliophysics Division in Washington, DC

Three years ago on March 2, 2008, the face of the sun wasfeatureless--no sunspots. Credit: SOHO/MDI

While Solar Max is relatively brief, lasting a few years punctuated byepisodes of violent flaring, over and done in days, Solar Minimum can grind onfor many years. The famous Maunder Minimum of the 17th century lasted 70years and coincided with the deepest part of Europe's Little Ice Age. Researchersare still struggling to understand the connection.

One thing is clear: During long minima, strange things happen. In2008-2009, the sun’s global magnetic field weakened and the solar windsubsided.  Cosmic rays normally held at bay by the sun’s windy magnetismsurged into the inner solar system.  During the deepest solar minimum in acentury, ironically, space became a more dangerous place to travel.  Atthe same time, the heating action of UV rays normally provided by sunspots wasabsent, so Earth’s upper atmosphere began to cool and collapse.  Spacejunk stopped decaying as rapidly as usual and started accumulating in Earthorbit.  And so on….

Nandi notes that their new computer model explained not only theabsence of sunspots but also the sun’s weakened magnetic field in 08-09. "It's confirmation that we’re on the right track."

Next step:  NASA’s Solar Dynamics Observatory (SDO) can measurethe motions of the sun’s conveyor belt—not just on the surface but deep inside,too. The technique is called helioseismology; it reveals the sun’s interior inmuch the same way that an ultrasound works on a pregnant woman.  Byplugging SDO’s high-quality data into the computer model, the researchers mightbe able to predict how future solar minima will unfold.  SDO is justgetting started, however, so forecasts will have to wait.

Indeed, much work remains to be done, but, says Guhathakurta,"finally, we may be cracking the mystery of the spotless sun."

Credits:  This research was funded by NASA’s Living With a Star Programand the Department of Science and Technology of the Government of India.

Missing sunspots: Solar mystery solved

March 2, 2011

This visible-light photograph, taken in 2008 by NASA's Solar andHeliospheric Observatory (SOHO) spacecraft,shows the sun's face free of sunspots. The sun experienced 780 spotless daysduring the unusually long solar minimum that just ended. New computer simulationsimply that the sun's long quiet spell resulted from changing flows of hotplasma within it. Credit: NASA/SOHO

The Sun has been in the news a lot lately because it's beginning tosend out more flares and solar storms. Its recent turmoil is particularlynewsworthy because the Sun was very quiet for an unusually long time.Astronomers had a tough time explaining the extended solar minimum. Newcomputer simulations imply that the Sun's long quiet spell resulted fromchanging 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 Center for Astrophysics (CfA)."Those plasma rivers affect solar activity inways we're just beginning to understand."

The Sun is made of a fourth state of matter - plasma, in which negativeelectrons and positive ions flow freely. Flowing plasma creates magneticfields, which lie at the core of solar activity like flares, eruptions, andsunspots.

Astronomers have known for decades that the Sun's activity rises andfalls in a cycle that lasts 11 years on average. At its most active, calledsolar maximum, dark sunspots dot the Sun's surface and frequent eruptions sendbillions of tons of hot plasma into space. If the plasma hits Earth, it candisrupt communications and electrical grids and short out satellites.

During solar minimum, the Sun calms down and both sunspots anderuptions are rare. The effects on Earth, while less dramatic, are stillsignificant. For example, Earth's outer atmosphere shrinkscloser to the surface, meaning there is less drag on orbiting space junk. Also,the solar wind thatblows through the solar system (and its associated magnetic field) weakens,allowing more cosmicraysto reach us from interstellar space.

The most recent solar minimum had an unusually long number of spotlessdays: 780 days during 2008-2010. In a typical solar minimum, the Sun goesspot-free for about 300 days, making the last minimum the longest since 1913.

"The last solar minimum had two key characteristics: a long periodof 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 southpoles.) "We have to explain both factors if we want to understand thesolar minimum."

To study the problem, Munoz-Jaramillo used computer simulations tomodel the Sun's behavior over 210 activity cycles spanning some 2,000 years. Hespecifically looked at the role of the plasma rivers that circulate from theSun's equator to higher latitudes. These currents flow much like Earth's oceancurrents: rising at the equator, streaming toward the poles, then sinking andflowing back to the equator. At a typical speed of 40 miles per hour, it takesabout 11 years to make one loop.

Munoz-Jaramillo and his colleagues discovered that the Sun's plasmarivers speed up and slow down like a malfunctioning conveyor belt. They findthat a faster flow during the first half of the solar cycle, followed by aslower flow in the second half of the cycle, can lead to an extended solarminimum. The cause of the speed-up and slowdown likely involves a complicatedfeedback between the plasma flow and solar magnetic fields.

"It's like a production line - a slowdown puts 'distance' betweenthe end of the last solar cycle and the start of the new one," saysMunoz-Jaramillo.

The ultimate goal of studies like this is to predict upcoming solarmaxima and minima - both their strength and timing. The team focused onsimulating solar minima, and say that they can't forecast the next solarminimum (which is expected to occur in 2019) just yet.

"We can't predict how the flow of these plasma rivers willchange," explains lead author Dibyendu Nandy (Indian Institute of ScienceEducation and Research, Kolkata). "Instead, once we see how the flow ischanging, we can predict the consequences."

Provided by Harvard-Smithsonian Center for Astrophysics.

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