Computer modeling deepens scientists' understanding of solar cycle

Phenomenon underpinning aurora has eluded explanation as we approach 11-year flip

Skywatchers this month feasted their eyes on ramping up of the aurora borealis, a light show caused by the interaction between the solar wind of charged particles and Earth's magnetic field.

The auroras in May are believed to have possibly been the strongest for some 500 years, according to NASA.

While observers have understood for centuries that the peak of the phenomenon – when we see more solar flares as well as displays of northern and southern lights – occurs after a cycle of roughly 11 years, scientists have struggled to come up with a consistent theory explaining this.

In the science journal Nature this week, a team led by Edinburgh University's Geoff Vasil published a paper that might hold some clues. Using numerical analysis, they propose that a physical phenomenon dubbed magnetorotational instability plays a central role whereby the magnetic fluid slows as it gets further from the center.

The Sun's high-energy radiation cycle is strong enough to affect the behavior of spaceborne instruments, such as communications satellites, said Ellen Zweibel, astrophysics professor at Wisconsin University, in an accompanying article. "This cycle is related to the Sun's magnetic field, but despite decades of observational and theoretical progress, a consistent explanation for many aspects of solar magnetism's most basic features remains elusive."

Although the Sun's magnetic field is a bit like Earth's in that it has two poles aligned roughly with the rotational axis, the field is skewed such that the lines also "have a 'toroidal' component, which runs parallel to the Sun's equator." Dark spots are visible where the toroidal lines emerge from the surface, Zweibel added, and scientists are able to track those spots over the 11-year cycle during which the polarity flips and the "toroidal component migrates from mid to equatorial latitudes."

Researchers have previously pointed out that the solar cycle is linked to changes in the Sun's rotation at the surface and associated this with the magnetic field, but Vasil and colleagues are the first to describe the underlying physics.

"Both are manifestations of the same underlying phenomenon, known as the magnetorotational instability (MRI), which arises when an electrically conducting fluid in a magnetic field spins faster near its center of rotation than it does farther away," Zweibel said.

The researchers used code from Dedalus, an open source project for numerical simulation written in Python. Their model supported the idea that MRI was driving the solar cycle, the paper said.

"The dynamo resulting from a well-understood near-surface phenomenon improves prospects for accurate predictions of full magnetic cycles and space weather, affecting the electromagnetic infrastructure of Earth," they said.

While it was a step in the right direction, Zweibel pointed out that there was more work to be done.

"The authors' model is highly simplified, especially in its treatment of thermal convection, and the existence of the near-surface shear layer is not explained. However, Vasil and colleagues' initial results are intriguing. They could well furnish an interpretative framework for more elaborate models, and they are sure to inspire future studies," she said. ®

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