Helioseismology is the science of studying the Sun’s interior by analyzing its natural oscillations—primarily sound waves (acoustic waves) that propagate through its plasma, much like how geoseismology uses earthquake waves to probe Earth’s interior.

These oscillations manifest as tiny Doppler shifts in spectral lines on the solar surface (velocities of order ~cm/s). By measuring their frequencies, amplitudes, and travel times, scientists infer temperature, density, composition, rotation, flows, and magnetic structures deep inside the Sun.

The Sun rings like a bell with millions of resonant modes.

The main types are:

• p-modes (acoustic/pressure modes): Restored by pressure gradients. These dominate observations and travel as sound waves. They penetrate to different depths depending on frequency and degree (lower-frequency, low-degree modes reach deeper).
• g-modes (gravity modes): Restored by buoyancy. Harder to observe as they are trapped in the deep interior and evanescent near the surface (only weak surface signatures).
• f-modes (surface gravity modes): Like surface waves on water.

Modes are characterized by quantum numbers: radial order n, spherical harmonic degree l (total wavenumber), and azimuthal order m (related to longitude). Rotation and magnetic fields split the degeneracy in m (frequency splittings).

Observations rely on Doppler velocity (line-of-sight shifts) or intensity variations in photospheric spectral lines (e.g., Ni I 676.8 nm for GONG, Na D lines for BiSON, Fe I 617.3 nm for HMI). High precision, long-duration, and high duty-cycle data are essential because modes have long lifetimes but small amplitudes.

Researchers from the University of Birmingham and international collaborators analyzed nearly 40 years of data from the Birmingham Solar Oscillations Network (BiSON). They “listened” to the Sun’s internal sound waves (acoustic p-modes) that are affected by magnetic activity.

Solar cycles are driven by the Sun’s magnetic dynamo in its interior. Traditional monitoring relies on surface features, but helioseismology reveals what’s happening inside. These changes point to a possible evolution in the Sun’s “active biorhythm” over decades, not just the usual 11-year cycle.

Implications include better space weather prediction (solar flares, coronal mass ejections that can affect satellites, power grids, etc.). The underlying cause of this shallowing isn’t yet clear—it may reflect a reorganization rather than simply weaker fields overall.

There is a recent scientific study (published May 2026) using helioseismology to probe the Sun’s interior.

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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 exact title of a new peer-reviewed paper by William J. Chaplin (University of Birmingham) and collaborators (including Sarbani Basu, Rachel Howe, and others), published in Monthly Notices of the Royal Astronomical Society (MNRAS) in late May 2026.

Using nearly 40 years (1987–2025) of Sun-as-a-star helioseismology data from the Birmingham Solar Oscillations Network (BiSON), the team analyzed solar p-mode (acoustic) frequency shifts across three frequency bands (low, mid, and high) and compared them to traditional activity proxies like the 10.7-cm radio flux and Sunspot Number (SSN).

Main results:

• Low-frequency modes (< ~2400 μHz): A change in behavior (departure/offset from Cycle 22-scaled proxies) that began in the declining phase of Cycle 23 has persisted through Cycles 24 and 25. This previously indicated structural changes confined to shallower than ~3000 km subsurface.
• Mid-frequency modes: Systematic reduction in sensitivity to activity proxies over Cycles 23–25 compared to Cycle 22.
• High-frequency modes (~2920–3450 μHz, most sensitive to very near-surface layers ~1000 km or less): Striking change in Cycle 25. Frequency shifts are much stronger than expected from surface proxies. Cycle 25 appears as strong as (or stronger than) Cycles 22/23 in this seismic band, despite being noticeably weaker in traditional surface indicators (e.g., SSN peak ~25% weaker than Cycle 22).

Overall conclusion:

Subsurface structural/magnetic changes linked to the 11-year cycles have become progressively more confined to shallower layers just beneath the Sun’s surface over successive cycles (22→25). This is not simply weaker overall fields but a change in radial confinement (geometry/depth distribution) of the activity.

Why This Is Significant

• Helioseismology (studying sound waves that propagate through the Sun) provides a view inside that surface observations miss.
• Traditional proxies (sunspots, radio flux) suggest Cycle 25 is moderate/weaker; seismic data (especially high-frequency) paint a different picture of strong near-surface activity.
• This implies an evolution in the Sun’s magnetic dynamo behavior over decades, potentially a longer-term shift beyond the standard 11-year cycle (speculation in the paper includes possible Hale cycle links, but more data from late Cycle 25 and Cycle 26 are needed).

The paper is short (~5 pages) and openly available on arXiv (arXiv:2605.29528). It builds on the team’s earlier work from Cycles 21–24.

Ongoing BiSON observations will test if this shallowing trend continues. Implications include refined understanding of solar variability, space weather forecasting, and long-term solar behavior.

Published:  Monthly Notices of the Royal Astronomical Society

DOI: 10.1093/mnras/stag847

Provided: University of Birmingham

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.