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.
Credit: Andrés Muñoz-Jaramillo of the Harvard CfA
MARYLAND: In 2008-2009, sunspots almost completely disappeared for two years. Solar activity dropped to hundred-year lows; Earth's upper atmosphere cooled and collapsed and the Sun’s magnetic field weakened, and now scientists have figured out why.
Reporting in Nature, an international team have described the development of a new computer model that can help us to understand and predict solar minimum.
"Plasma currents deep inside the sun interfered with the formation of sunspots and prolonged solar minimum," said lead author Dibyendu Nandi of the Indian Institute of Science Education and Research in Kolkata. "Our conclusions are based on a new computer model of the Sun's interior."
Birth of a new solar cycle
For years, solar physicists have recognised 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 and voila! A new solar cycle is born.
For the first time, Nandi’s team believes they have developed a computer model that gets the physics right for all three aspects of this process - the magnetic dynamo, the conveyor belt, and the buoyant evolution of sunspot magnetic fields.
Conveyer belt dragging sunspot corpses
"According to our model, the trouble with sunspots actually began in back in the late 1990s during the upswing of Solar Cycle 23," said co-author Andrés Muñoz-Jaramillo of the Harvard-Smithsonian Centre for Astrophysics in Massachusetts. "At that time the conveyor belt sped up."
The fast-moving belt rapidly dragged sunspot corpses down to sun's inner dynamo for amplification. At first glance, this might seem to boost sunspot production, but no. When the remains of old sunspots reached the dynamo, they rode the belt through the amplification zone too hastily for full re-animation. Sunspot production was stunted.
Later, in the 2000s, according to the model, the Conveyor Belt slowed down again, allowing magnetic fields to spend more time in the amplification zone, but the damage was already done. New sunspots were in short supply. Adding insult to injury, the slow moving belt did little to assist re-animated sunspots on their journey back to the surface, delaying the onset of Solar Cycle 24.
