doi:10.1038/nindia.2012.164 Published online 5 November 2012
A theoretical model developed by scientists of the Indian Institute of Science (IISc) in Bangalore explains why sunspots sometimes disappear for almost a century many times in the past1.
Sunspots that can be as large as 80,000 kilometres across are temporary phenomenon on the Sun's surface (photosphere). They appear darker in comparison to their surroundings. The number of sunspots increases and decreases in cycles of approximately 11 years time period.
But one of the most striking aspects of the 11-year sunspot cycle is that there have been times in the past when some cycles went missing for several decades resulting in the so called 'grand minima'. Modern research found indirect evidence for 27 such grand minima in the last 11,000 years implying that about 2:7% of the solar cycles had conditions appropriate for forcing the Sun into grand minima.
These grand minima — that result when the Sun takes a break from its customary cycle of sunspot activity — may influence space weather and the Earth's climate, as evidenced by a cold spell during the 'Maunder minimum' (1645-1715) when very few sunspots appeared on the face of the Sun. Therefore, scientists consider it very important to understand the origin of the grand minima and the probability of their occurrence.
It is generally believed that the Sun's magnetic field is generated by a magnetic dynamo within. Research in the past suggested that a grand minimum may arise due to weakening in the Sun's magnetic field and circulation. Arnab Rai Choudhuri and Bidya Karak of the IISc have now uncovered the physics underlying these sunspot variations and developed a model to predict how often the grand minima can occur.
"Sunspots are regions of strong magnetic field embedded in the plasma," the authors report in their paper. "A plasma process, known as the 'dynamo process', is responsible for producing the 11-year sunspot cycle, which is basically the magnetic cycle of the sun," they say.
In their work, highlighted by the journal, the researchers studied the fluctuations of the Sun's magnetic field and circulation in its outer layers and have shown that sufficiently large fluctuations present in the dynamo process can sometimes push the sun into grand minima. "We show from our flux transport dynamo model that about 1–4% of the sunspot cycles may have conditions suitable for inducing grand minima," they report.
Using their theoretical model, they have calculated the frequency of occurrence of grand minima and find that their simulations typically produce about 24–30 grand minima in 11,000 years — remarkably close to the 27 grand minima actually observed in that period. In another important result, they find that the Sun spends about 10–15% of the time in a grand minimum state, which is very close to the observed 17%.
"We are amazed that the observational data could be reproduced so well (by our model)," the scientists say. They conclude that the irregularities of solar cycles, including the grand minima, are produced by fluctuations in magnetic field generation and in 'meridian circulation' which is the flow of material from the Sun's equator toward the poles at the surface and from the poles to the equator below the surface.