Every 11 years, the Sun flips its magnetic poles, driving a cycle of sunspot eruptions, solar flares, and coronal mass ejections that can knock out power grids and fry satellites. For decades, physicists believed the engine behind that cycle sat deep inside the star, roughly 200,000 kilometers below the visible surface, near a boundary called the tachocline where the Sun’s rotation speed changes sharply. A study published in Nature in 2024 upended that picture. By listening to sound waves reverberating through the solar interior, researchers concluded that the magnetic dynamo is confined to a surprisingly thin shell: just the outer 5 to 10 percent of the Sun’s radius, a zone only about 50,000 to 70,000 kilometers deep.
To put that in perspective, Earth’s diameter is roughly 12,700 kilometers. The entire magnetic engine of our star may operate within a skin only four to five Earths thick, sitting right beneath the roiling surface we can actually observe.
Listening to the Sun’s interior
The technique behind the discovery is helioseismology, which works much like an ultrasound scan of the human body. Acoustic pressure waves generated by turbulent convection travel through the Sun, and their speed changes depending on the temperature, density, and flow patterns of the material they pass through. By measuring how long these waves take to travel between different points on the solar surface, scientists can build three-dimensional maps of conditions deep inside the star.
NASA’s Solar Dynamics Observatory (SDO) carries the Helioseismic and Magnetic Imager (HMI), whose time-distance analysis pipeline converts millions of wave travel-time measurements into subsurface flow maps. Those maps have long been used to track torsional oscillations: bands of slightly faster and slower rotation that migrate toward the Sun’s equator in lockstep with sunspot activity. The oscillations are a fingerprint of the magnetic cycle, and pinpointing their depth tells you where the cycle’s engine is running.
The Nature study, led by researchers at Northwestern University and first circulated as a preprint on arXiv in April 2024, used both analytic calculations and numerical simulations to argue that the strongest torsional oscillation signals are confined to the near-surface shear layer. The proposed mechanism is a magnetorotational instability (MRI), a process in which differential rotation stretches and amplifies magnetic field lines, already well known in astrophysics as the driver of accretion disks around black holes. A commentary in Nature Astronomy confirmed the same quantitative conclusion: the MRI operating in this shallow zone can, in principle, generate the magnetic activity patterns observed over the solar cycle.
Why the depth matters for Earth
The relocation of the dynamo from deep interior to shallow skin is not just an academic distinction. It has direct consequences for space-weather forecasting.
Today, predicting when and where the Sun will unleash a major flare or coronal mass ejection relies heavily on statistical models and inferences about conditions hundreds of thousands of kilometers below the surface, a region no instrument can observe directly. Forecasters work with proxies: sunspot counts, magnetic field measurements at the photosphere, and historical patterns. The results are better than guessing but far from precise. When a powerful geomagnetic storm struck Earth in May 2024, disrupting GPS signals and painting auroras as far south as Florida, the event underscored how much warning time still depends on catching eruptions after they have already launched from the Sun.
If the dynamo truly lives in a thin shell just below the surface, the flows that build and release magnetic energy are within reach of existing instruments. SDO’s HMI can already track near-surface shear velocities in near-real time. Monitoring changes in those flows could, in theory, flag the buildup of magnetic stress before it erupts, offering hours or even days of additional warning. That prospect has energized the space-weather community, though turning it into an operational forecasting tool will require years of validation against actual eruption data.
The case for a deeper engine has not disappeared
Not everyone is ready to abandon the tachocline. A separate peer-reviewed study published in Scientific Reports in 2025 argues that helioseismic rotation measurements still show cycle-dependent signatures linked to the tachocline. Specifically, the researchers found that a diagnostic derived from rotational splitting coefficients (the so-called a3 coefficient) varies with the solar cycle in ways that a purely shallow dynamo would struggle to explain.
The two findings are not necessarily contradictory. The Sun could host magnetic activity at multiple depths, with a deep-seated process generating large-scale fields and the near-surface shear layer amplifying or reshaping them before they break through the photosphere as sunspots. Many solar physicists suspect the real answer involves coupling between layers rather than a single dominant location.
But the Nature study’s strongest claim, that MRI growth rates in the shallow zone are fast enough to sustain a self-contained dynamo without help from below, has not yet been independently reproduced. The specific SDO/HMI inversion parameters and sensitivity kernels used to confine the torsional oscillations to the outer 5 to 10 percent have not been released in a form that outside groups can replicate. Until at least one other helioseismology team, using a different inversion pipeline, arrives at the same shallow localization, the result will likely be treated as promising but provisional.
There are also technical reasons for caution. Near the photosphere, acoustic wave paths are short, the physics of spectral line formation becomes more complex, and small systematic errors in modeling can shift where the strongest oscillation signals appear. The HMI pipeline has been extensively validated over more than a decade of operation, but subtle choices in how noise correlations are handled or which sensitivity kernels are selected can influence the outcome. These are not fatal flaws; they are the normal uncertainties of pushing an observational technique to its limits.
What to watch as Solar Cycle 25 unfolds
Solar Cycle 25 reached its peak activity around late 2024, and as of mid-2026, the Sun is beginning its gradual decline toward the next solar minimum. That declining phase offers a natural laboratory for testing the shallow-dynamo hypothesis. Two developments would significantly strengthen or weaken the case.
First, independent confirmation of the depth measurement. If other helioseismology groups, using different inversion techniques and different data segments from HMI or from ground-based networks like GONG, reproduce the confinement of torsional oscillation energy to the outer 5 to 10 percent, the shallow-dynamo model gains substantial credibility.
Second, predictive power. If near-surface shear-layer flow speeds and migration patterns consistently anticipate where active regions emerge and when major eruptions occur, that would argue for a leading role for the shallow engine. Conversely, if deep-interior diagnostics like the a3 rotation coefficient prove to be better predictors, the tachocline model retains its relevance.
For now, the most defensible reading of the evidence is that the Sun’s magnetism is more vertically structured than textbooks have assumed. The acoustic heartbeat of the star has revealed something unexpected: a magnetic engine that may sit close enough to the surface to be watched in real time. Whether that engine runs the whole show or shares the work with deeper machinery is a question the next few years of solar observations are well positioned to answer.
More from Morning Overview
*This article was researched with the help of AI, with human editors creating the final content.
