Bizarre Physics Seen on Sun Could Answer Mysteries of Magnetic Field

James Maynard
Oct 29, 2019 · 4 min read

Sunspots near the surface of the Sun are driven by an unexpected magnetic instability, a new study finds.

What we see of the Sun is a rollicking, moving ball of plasma (ionized gas), rotating as the Sun moves around its axis. Electric currents within the Sun creates a massive magnetic field, which driving sunspot activity. Exactly how this happens, however, remains a mystery.

A new finding suggests an unexplained mechanism may be unfolding within this plasma, potentially playing a critical role in the behavior of the Sun’s magnetic field.

Plasma rising off the Sun.
Plasma rising off the Sun.
Plasma rises from the surface of the Sun sue to the movement of magnetic fields. Image credit: NASA/SDO

An Attractive Idea About Magnetism

The Sun rotates at different speeds at its equator than it does near the poles. Near the solar equator, the Sun rotates once every 25 days, while polar regions take 33–35 days to make one trip around the solar axis. This results in a stretching of magnetic field lines, which can eventually break, forming sunspots.

Sunspots, perhaps the best-known feature of the Sun to non-astronomers, form around 30 degrees north and south of the equator, and slowly migrate toward the center. As time goes on during the 11-year solar cycle, the number of visible sunspots rises, and the features start to form closer to the equator. This means that most sunspots are seen near the equator of the Sun, and this is where the magnetic activity of the Sun is at its greatest.

Plotting the number of sunspots at different latitudes over time creates a butterfly diagram (top), as sunspots become more plentiful as they form closer to the equator. Image credit: NASA

Magnetorotational instability (MRI) is a mechanism which results in instabilities within electrically-conductive fluids and gases. A new study from an international group of researchers suggest a Super HMRI, unrecorded in current theories of the solar dynamo, could answer mysteries of magnetic processes occurring in the Sun.

“This new instability could play an important role in generating the Sun’s magnetic field. But in order to confirm it we first need to do further numerically complicated calculations,” Dr. Frank Stefani of the Institute of Fluid Dynamics at Helmholtz-Zentrum Dresden-Rossendorf (HZDR) explains.

Magnetic instabilities drive many of the processes in natural systems around the Cosmos. The process by which stars and planets form from disks of gas and dust, for example, require such fields for the creation of such bodies. Inequities in the magnetic fields in these clouds cause turbulence, leading to the buildup of mass in one location, forming the core of a star or planet.

As the Sun rotates, the magnetic field of the Sun is carried faster near the equator than closer to the poles due to the differential rotation of the Sun. The fields fall into layers. These layers eventually pile up on each other, with parts of each layer rotating at different speeds, with regions on the inside of the effect moving more quickly than inner regions, a condition which is typically considered to be extremely stable.

However, near the solar equator, plasma behaves in exactly the opposite fashion, as outer layers travel more quickly then inner layers, researchers from Helmholtz-Zentrum Dresden-Rossendorf (HZDR), the University of Leeds and the Leibniz Institute for Astrophysics Potsdam (AIP) report.

The researchers tested if the magnetic instabilities they postulated could form in the unusual conditions thought to exist near the solar equator. Although the experiment was successful, it modeled a version of the Sun without much of the differential rotational rate the actual Sun.

“Astrophysicists and climate researchers still hope to better understand the cycle of sunspots. Perhaps the ‘Super HMRI’ we have now found will take us a decisive step forward. We’ll check it out,” states Prof. Günther Rüdiger of the Leibniz Institute for Astrophysics Potsdam (AIP).

“The sun, — the bright sun, that brings back, not light alone, but new life, and hope, and freshness to man — burst upon the crowded city in clear and radiant glory. Through costly-colored glass and paper-mended window, through cathedral dome and rotten crevice, it shed its equal ray.”
― Charles Dickens, Oliver Twist

It is possible that the magnetic anomaly examined in this study could play a role in the creation of the entire magnetic field of the Sun, which flips poles once every 11 years (approximately). When this happens, the sunspot cycle repeats, with the magnetic poles reversed from the previous cycle. This means that the complete sunspot cycle lasts about 22 years before returning to its initial conditions.

In 2006, this same research team showed that these magnetic instabilities could — theoretically — form in the topsy-turvy conditions they predict form near the solar equator. Now, the team intends to build on that success, designing an experiment — The DREsden Sodium facility for DYNamo and thermohydraulic studies (DRESDYN) — to test their theory.

Analysis of the new study was published in Physical Review Fluids.

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James Maynard

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Writing about space since I was 10, still not Carl Sagan. Mailing List/Podcast:

The Cosmic Companion

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