Why was the Sun spotless for long?
doi:10.1038/nindia.2011.33 Published online 2 March 2011
Scientists have been able to provide the first consistent explanation of the long absence of sunspots — visibly dark, strongly magnetised regions of the Sun — during the end of the last solar cycle which peaked in 2001.
They have also been able to attribute reasons to the very weak large-scale magnetic field of the Sun in the solar system during the deep and prolonged lull in solar activity at the end of solar cycle 23.
The number of sunspots on the solar surface changes in time, going through periods of highs and lows in a cyclic fashion with an average periodicity of 11 years. This phenomenon is known as the solar magnetic cycle, or simply the sunspot cycle.
Through computer simulations, a team of Indian and American scientists has found that variations in a large-scale flow of plasma in the Sun's interior (called meridional circulation) caused this unprecedented solar minimum.
"The meridional circulation is a loop-like flow that stretches from the Sun's equator to the poles near the surface, and closes in on itself, flowing from the poles to the equator deeper within the Sun," says lead researcher Dibyendu Nandy of the Indian Institute of Science Education and Research, Kolkata.
Nandy and co-researchers Andrés Muñoz-Jaramillo, currently with the Harvard Smithsonian Center for Astrophysics, and Petrus C. H. Martens of the Montana State University, have shown through simulations that a fast plasma flow during the early half of a sunspot cycle, followed by a slower flow created the weak large-scale field of the Sun.
"This generated a solar minimum with a large number of spotless days," Nandy told Nature India. The team predicts that very deep solar minima with a large number of spotless days are bound to be associated with weak large-scale solar dipolar field, thereby resulting in a low magnetic field in the heliosphere.
Sunspots play an important role in space and on Earth. Solar magnetic storms, the largest explosions in the solar system, originate from sunspots and carry vast amounts of plasma (charged particles) into space generating what is known as space weather.
When directed towards Earth and its orbiting satellites, these solar storms disrupt operations of sensitive equipment in spacecrafts, affect telecommunication systems, and pose a hazard to air-traffic on polar routes — the preferred route for long distance carriers.
Sunspots also control the amount of total energy being radiated from the Sun; more sunspots mean more incident solar energy on Earth – which is the primary natural driver of the climate system. The magnetic field from Sun is also carried by the solar wind and permeates the solar system and this magnetic field is the primary modulator of the cosmic ray flux on Earth.
Cosmic ray flux is believed to be important in climate dynamics because it seeds cloud formation in the Earth's atmosphere. Therefore, variations in the number of sunspots, including their long absence, can affect the Earth's climate.
This opens up the possibility of predicting fair weather in space.
The findings by the Indo-American team demonstrate how the interactions of magnetic field with plasma flows deep within Sun can control its magnetic and energetic output. Also, how this determines the environment in space and affects climate on Earth. The research was supported by the Ramanujan Fellowship of India's department of science and technology and NASA 'Living With a Star' grant.
"This opens up the possibility of predicting fair weather in space, based on computer simulations of solar activity driven by observations of its plasma flows," Nandy says.
Such predictions are important for planning space missions, estimating the life-time of satellites and scheduling air-traffic on polar routes and are relevant for space weather forecasting – a multi-billion dollar industry worldwide.
- Nandy, D. et al. The unusual minimum of sunspot cycle 23 caused by meridional plasma flow variations. Nature. 471, 80-82 (2011)