Recent research using helioseismology (“listening” to the Sun) has revealed that solar magnetic activity and structural changes are becoming increasingly confined to shallow layers just beneath the surface, differing from what traditional surface observations (like sunspot counts) suggest.

Solar cycles basics: The Sun follows roughly 11-year cycles of rising and falling magnetic activity, marked by sunspots, flares, and coronal mass ejections. Activity affects space weather, which can impact satellites, power grids, and communications on Earth.

New insights from sound waves: Scientists analyzed nearly 40 years of data (spanning solar cycles 22–25, from ~1987 onward) from the Birmingham Solar Oscillations Network (BiSON). They studied p-mode oscillations—global sound waves (acoustic pressure waves) trapped inside the Sun whose frequencies shift in response to magnetic activity and structural changes.

The hidden shift: Over recent cycles (especially since Cycle 23), solar-cycle-driven changes appear to be “squeezed” into shallower layers near the surface. High-frequency p-modes (probing these shallow regions) show the current Cycle 25 as strong or comparable to earlier strong cycles (22 and 23). However, surface proxies like sunspots and radio flux suggest a weaker cycle.

Implication: The Sun’s “active biorhythm” may be evolving over decades. Magnetic activity and associated structural changes are increasingly shallow, creating a mismatch between interior signals and visible surface activity. This points to longer-term behavioral changes rather than just cycle-to-cycle variation.

Traditional monitoring relies on surface features, which may now underrepresent the full internal dynamics. This could improve space weather prediction if the trends hold. The underlying cause isn’t fully understood—it isn’t simply weaker overall magnetic fields—and it raises questions about whether the Sun is entering a “different mode of behavior.”

Helioseismic p-mode frequency shifts are a key diagnostic in solar physics. They reveal how the Sun’s interior structure and magnetic activity evolve over the ~11-year solar cycle, often in ways not fully captured by surface observations like sunspots.

What Are p-Modes?

p-modes (pressure modes) are acoustic waves — essentially standing sound waves — trapped inside the Sun. Pressure (sound speed) provides the main restoring force.

• They are excited by turbulent convection near the surface.
• Typical periods are ~3–6 minutes (frequencies ~1,500–5,000 μHz, with peak power around 3,000 μHz, the “5-minute oscillations”).
• Low-degree (low-l) modes are global and observable in “Sun-as-a-star” data; higher-degree modes allow more localized probing.
• Waves propagate inward, refract due to increasing sound speed with depth, and return to the surface. Lower-frequency modes penetrate deeper; higher-frequency modes are more sensitive to shallow subsurface layers.

The Sun acts as a resonant cavity, supporting millions of such modes.

Why Do Frequencies Shift with Solar Activity?

p-mode frequencies increase slightly (by ~0.1–0.5 μHz or fractions of a percent) as solar magnetic activity rises. This is one of the earliest robust findings in helioseismology.

Main causes (near-surface effects dominate):

• Solar Activity: Direct (Lorentz force) and indirect (via structural changes like altered sound speed or density). Perturbations in the mean-square magnetic field in the photosphere/sub-photosphere are a leading candidate.
• Thermal/entropy changes: Alter propagation speed and effective path length. These can produce opposing contributions to frequency shifts.
• The shifts are strongest near the surface (upper ~ few thousand km), where magnetic activity is concentrated. Higher-frequency modes show larger relative shifts because they spend more time in these shallow layers.

Frequency shifts correlate tightly with activity proxies (e.g., 10.7-cm radio flux F10.7, sunspot number SSN) but can diverge when subsurface dynamics change.

There is a study published by researchers at the University of Birmingham (with collaborators including Yale) and published in Monthly Notices of the Royal Astronomical Society in late May 2026.

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Subsurface structural changes associated with successive 11-yr solar activity cycles have been progressively more confined near the surface: new helioseismic results on Cycles 22–25 from BiSON 

This is the title of a new research paper published in Monthly Notices of the Royal Astronomical Society (MNRAS) on May 28, 2026, led by William J. Chaplin (University of Birmingham) with collaborators including Sarbani Basu (Yale).

Using nearly 40 years of “Sun-as-a-star” helioseismic data from the Birmingham Solar Oscillations Network (BiSON) (1987–2025, covering solar cycles 22 through 25), the team found that subsurface structural changes linked to the ~11-year solar activity cycle have become progressively more confined to shallow layers just beneath the Sun’s visible surface (photosphere).

Magnetic activity and associated structural perturbations are increasingly “squeezed” into a thinner skin near the surface (on the order of ~1,000 km or less in recent cycles), rather than extending deeper as in earlier cycles.

Methodology: Helioseismology with p-modes

• p-modes (acoustic pressure waves) are global sound waves trapped inside the Sun. Their frequencies shift in response to magnetic activity and near-surface structural changes.
• Researchers analyzed frequency shifts in three bands:
  • Low-frequency (~1,860–2,400 μHz): Probes deeper layers.
  • Mid-frequency (~2,400–2,920 μHz).
  • High-frequency (~2,920–3,450 μHz): Most sensitive to very shallow subsurface layers.
• These were compared to traditional surface activity proxies: 10.7-cm radio flux (F10.7) and sunspot number (SSN).

Key observations from the data:

• Changes in the low-frequency modes (noted starting in Cycle 23) persisted into Cycle 25, indicating structural shifts in shallower layers than in Cycle 22.
• Mid-frequency sensitivity to activity decreased over Cycles 23–25 compared to Cycle 22.
• High-frequency modes in Cycle 25 showed much stronger shifts than expected from surface proxies. Cycle 25 appears as strong as Cycles 22/23 in this shallow-probing band, while surface indicators (sunspots, radio flux) show it as noticeably weaker (~25% weaker peak than Cycle 22 in some measures).

Implications

• This suggests a long-term evolution in how the Sun’s magnetic dynamo operates, with activity becoming more “skin-deep” over decades. It is not simply due to overall weaker fields but a reorganization of where magnetic activity is stored and manifests subsurface.
• Traditional surface-based monitoring and space weather models (calibrated on deeper activity patterns) may need updating.
• The Sun may be entering a “different mode of behavior.” Continued BiSON observations into Cycle 26 will be key to confirming if this is a sustained trend.

The paper is short (5 pages) and available on arXiv (2605.29528). It builds on earlier BiSON work by the same team that first noted divergences starting in Cycle 23.

This highlights the unique value of long-term helioseismology in revealing interior dynamics invisible to surface observations.

Published:  Monthly Notices of the Royal Astronomical Society

DOI: 10.1093/mnras/stag847

Authors: William J Chaplin, 

Sarbani Basu , 

Rachel Howe , 

Yvonne Elsworth , 

Steven J Hale , 

Eleanor Murray

ABSTRACT

We use Sun-as-a-star helioseismology data, collected by the Birmingham Solar-Oscillations Network, to examine the relationship between the solar-cycle-induced frequency shifts of whole-Sun, low-angular degree solar p modes and well-known proxies of global solar activity.

Changes in behaviour between the low-frequency modes and proxies, which in a previous study we found had occurred on the declining phase of Cycle 23, appear to have persisted into Cycle 25.

More striking is a significant change in the relationship for higher-frequency modes, which the new Cycle 25 data now reveal.

The observed mean frequency shifts in Cycle 25 are much stronger than one would expect for these modes based on the relationship between the frequencies and proxies seen in previous cycles, in particular Cycle 22.

In sum, Cycle 25 is as strong as Cycles 22 and 23 when observed in this higher-frequency seismic band, in marked contrast to the relative sizes of the cycles seen in the global activity proxies, where Cycle 25 is noticeably weaker.

When considered alongside a systematic reduction of the sensitivity of the mid-frequency modes to activity over the past three cycles, these results suggest that subsurface structural changes associated with successive 11-yr cycles are becoming ever more progressively confined just beneath the solar surface.