Researchers using the Birmingham Solar-Oscillations Network, known as BiSON, have detected subtle structural differences between successive solar minima that point to hidden changes in the Sun’s outer layers. These shifts, measured through oscillation frequencies and sound-speed variations, offer a new explanation for why some solar cycles produce far fewer sunspots and flares than others. The findings arrive as scientists work to improve forecasts for solar storms that can disrupt satellites, power grids, and communications systems on Earth.
What BiSON Found Beneath the Surface
The Sun vibrates constantly, and those vibrations carry information about conditions deep inside the star. By tracking low-degree pressure-mode oscillation frequencies, or p-mode frequencies, across four consecutive solar minima, the BiSON team identified measurable differences in internal structure from one quiet period to the next. The study, published in the journal Monthly Notices, quantifies what the researchers call “seismic diversity” among minima, meaning that the Sun’s interior does not simply reset to the same baseline state each time activity winds down. Instead, each minimum carries a slightly different internal fingerprint that can be read through helioseismology.
Two techniques drove the analysis. First, the team examined the signature of the He II ionization zone, a layer where helium atoms lose their second electron. This feature, sometimes called the “helium glitch,” leaves a detectable imprint on oscillation frequencies, and its variation between minima suggests real physical changes in temperature or composition at that depth. Second, the researchers used inversions for sound-speed changes to map how quickly acoustic waves travel through different layers. Together, these measurements show that the Sun’s outer structure shifted in ways that standard solar models do not predict between cycles. A freely accessible preprint of the work reproduces the full set of figures, tables, and methodological details, providing transparency on the helioseismic analysis, glitch fitting, model comparisons, and inversion techniques.
Why Solar Cycle 24 Was Unusually Quiet
Solar activity follows an 11-year cycle of rising and falling sunspot counts, but not every cycle reaches the same peak. Solar Cycle 24, which peaked around 2014, was notably subdued compared with its immediate predecessors. A 2015 analysis indexed in NASA’s Astrophysics Data System described the situation plainly: the ongoing cycle was “considerably less vigorous than the three cycles before,” highlighting a mismatch between expectations and reality. That weakness caught forecasters off guard and raised questions about whether something fundamental had changed inside the Sun, rather than merely on its visible surface.
The BiSON results suggest a mechanism. If the Sun’s internal structure genuinely differs from one minimum to the next, then the starting conditions for each new cycle are not identical. A minimum that precedes a weak cycle may carry distinct sound-speed profiles or helium-zone signatures compared with one that precedes a strong cycle. This reframes the weak-cycle problem: rather than looking only at surface magnetism or sunspot counts, scientists may need to monitor the Sun’s acoustic fingerprint during quiet periods to anticipate what comes next. Large-scale plasma flows, including the meridional circulations that redistribute magnetic flux through the convection zone, likely help set those internal conditions by slowly reconfiguring the outer layers over many years.
The Dynamo Starts Near the Surface
A separate line of research adds weight to the idea that the Sun’s outer layers hold the key to cycle strength. A recent study in Nature found that key solar-cycle-related flow signatures, particularly low-latitude torsional oscillations, originate surprisingly close to the surface rather than deep in the convection zone. Using helioseismic measurements, the authors traced these bands of faster and slower rotation to shallow depths, challenging older models that placed the solar dynamo’s primary action near the base of the convection zone. If the engine driving the 11-year cycle sits closer to the surface than previously assumed, then the structural variations BiSON detected in the Sun’s outer layers become far more significant for understanding how magnetic fields are generated and amplified.
Scientists now view these near-surface signals as essential pieces of the solar-cycle puzzle. Independent research coverage has highlighted how subtle subsurface dynamics affect solar rotation and, in turn, the evolution of magnetic structures that ultimately emerge as sunspots. The convergence of these findings, one showing structural diversity between minima and the other relocating crucial dynamo-related flows to shallow layers, points toward a unified picture. The quiet periods between cycles are not dead time. They are windows into the physical setup that determines whether the next cycle will be fierce or feeble, with the outer few percent of the Sun’s radius acting as a sensitive staging ground for future activity.
What This Means for Solar Storm Forecasts
Predicting solar activity has long relied on expert panels that assess sunspot trends and magnetic-field data. NOAA’s Space Weather Prediction Center, for example, convened a 2019 group to produce a baseline forecast for Solar Cycle 25 and later issued a revised outlook projecting a quicker, stronger peak than initially expected. That revision itself illustrated the difficulty: even with decades of observational data, forecasters had to update their predictions significantly after the cycle was already underway. The unexpectedly low amplitude of Solar Cycle 24 had skewed early expectations downward, underscoring how sensitive forecasts are to assumptions about the Sun’s internal state at minimum.
The BiSON findings could sharpen future predictions by giving forecasters a new diagnostic tool. Instead of waiting for sunspots to appear and counting them, scientists could analyze p-mode frequencies and helium-glitch signatures during the minimum phase to assess the Sun’s structural readiness for the next cycle. If a particular pattern of sound-speed anomalies or helium-zone depth correlates reliably with stronger or weaker cycles, those helioseismic markers could be folded into operational forecasts alongside traditional magnetic indices. In principle, this would allow space-weather centers to adjust their expectations earlier, giving satellite operators, grid managers, and other vulnerable sectors more lead time to prepare for either heightened storm risks or a relatively calm decade.
Building the Data Infrastructure for Better Models
Turning these scientific insights into practical forecasting tools depends on long-term, stable data streams. Networks such as BiSON must continue to operate across multiple cycles to capture subtle structural trends, while complementary instruments on spacecraft and ground-based observatories refine measurements of flows and fields in the solar interior. Much of the underlying research circulates through open repositories that allow rapid sharing of helioseismic analyses and dynamo simulations. The preprint server arXiv, supported by institutional membership, plays a central role by giving solar physicists a platform to disseminate results quickly, compare models, and iterate on forecasting techniques in near real time.
Sustaining that ecosystem also requires direct backing from the broader community. Voluntary contributions help keep open repositories running, ensuring that data products, analysis codes, and early-stage manuscripts remain widely accessible to researchers in different countries and at institutions with varying resources. As helioseismic diagnostics like those from BiSON become more tightly woven into operational space-weather forecasting, the link between basic research infrastructure and practical risk mitigation will grow clearer. Investments in long-baseline observations and open data sharing are not just about understanding the Sun for its own sake; they are part of building a more resilient technological society under an active, sometimes unpredictable, star.
More from Morning Overview
*This article was researched with the help of AI, with human editors creating the final content.
