Scientists sneaked a peek into the Sun’s interior, but what they saw contradicts the assumptions made by models predicting solar activity.
Like a toddler, the Sun can be changeable. Once you learn to read the signs, tantrums — or in the Sun’s case, radiation- and particle-spewing fits — can become easier to predict. But much as parents look forward to the day their children can verbalize their thoughts, solar scientists look forward to forecasting activity based on what the Sun tells them about its own inner churnings.
A new study published in The Astrophysical Journal Letters brings that goal one step closer — and yet, another, possibly bigger step farther away. Junwei Zhao (Stanford University) and colleagues used the Solar Dynamics Observatory to watch the gas in the Sun’s photosphere bulge and retract in response to sound waves passing through the interior. By timing the arrival of waves sloshing against the surface, the team indirectly glimpsed gas flows deep inside the Sun.
And what the astronomers saw flies in the face of current models that attempt to describe the Sun’s behavior.
The Sun’s magnetic field is like a bar magnet, generated as charged particles inside the Sun circulate like water boiling inside a spherical pot. The bar-magnetic field brings relative calm, but as ever, calm precedes the storm. Because the Sun’s equator rotates faster than its poles, the rotation twists and disrupts the magnetic field, creating sunspots, magnetic loops of charged gas, and massive (and sometimes Earth-directed) ejections of charged particles and radiation. It takes another current, the meridional flow, to carry tangled magnetic flux from the equator to the poles and reset the field. The 11-year cycle that ensues governs solar activity.
Models of how the Sun creates its magnetic field have assumed the meridional flow follows a single circulation cell, traveling along the Sun’s surface from the equator to the poles before sinking deep inside for the ride back. The return flow was supposed to be so deep and so slow, it would take roughly 11 years to cycle back to the equator, explaining the length of the solar cycle.
For decades, solar-cycle forecasters had used signals from the Sun’s surface, rather than a true physics-based understanding of the Sun’s inner machinations. But when the first physics-based models predicted activity for Cycle 24, they failed miserably. They predicted a cycle 50% stronger than the preceding one; instead, Cycle 24 turned out to be the weakest solar cycle in 100 years.
The observations reported by Zhao’s team might explain why the physics-based models failed: the team measured not just one, but two circulation cells for the meridional flow.
The first return flow from the pole back to the equator is relatively shallow, 62,000 km (39,000 miles) below the Sun’s surface — half as deep as models had assumed. But the flow turns poleward again at 125,000 km down, indicating a second cell stacked beneath the first. The authors note that even more cells could be hidden below these.
David Hathaway (Marshall Space Flight Center), who has done extensive research on the meridional flow and its role in predicting the solar cycle, says the results are “catastrophic” for current theory. “It indicates the need for revolutionary changes in our dynamo models for the sunspot cycle.”