[Paper Review] What does helioseismology tell us about solar cycle related structural changes in the Sun?

Sarbani Basu|arXiv (Cornell University)|May 3, 2002

Solar and Space Plasma Dynamics1 references3 citations

TL;DR

This paper uses helioseismic data from GONG and SOHO/MDI to investigate solar cycle-related structural changes in the Sun. It finds that frequency and splitting coefficient variations are primarily due to near-surface magnetic activity, not deep interior changes, with no significant structural evolution detected in the solar interior up to 0.95 R⊙, despite measurable changes in surface and near-surface dynamics.

ABSTRACT

Solar oscillations frequencies show a distinct change with solar activity. The changes in frequencies can be used to study the time variation of solar structure. We discuss constraints on the changes in solar structure with time as obtained with helioseismic data covering the last six years. The frequency variations appear to be dominated by changes in the near-surface layers rather than by changes in the structure of the deeper layers.

Motivation & Objective

• To determine whether solar cycle variations induce measurable structural changes in the solar interior using helioseismic data.
• To assess the role of near-surface magnetic fields and dynamics in modulating oscillation frequencies and splittings.
• To test whether changes in solar structure are detectable via inversion of frequency data across multiple solar activity cycles.
• To evaluate the reliability of current inversion techniques in resolving shallow structural changes linked to solar activity.

Proposed method

• Analysis of solar oscillation frequencies and splitting coefficients from GONG and SOHO/MDI data spanning six years of solar activity.
• Use of a-mode decomposition and splitting coefficient formalism (a_j(n,l)) to parameterize frequency splittings and detect deviations from spherical symmetry.
• Inversion of frequency data to reconstruct time-dependent sound speed and flow profiles, focusing on radial depths up to 0.95 R⊙.
• Comparison of surface term corrections with observed surface magnetic field distributions to isolate near-surface effects.
• Application of frequency scaling to remove mode inertia effects, enabling clearer identification of structural changes.
• Use of time-series analysis to correlate frequency shifts with solar activity indices such as 10.7 cm radio flux and sunspot number.

Experimental results

Research questions

• RQ1 Do solar oscillation frequencies and splittings vary systematically with the solar cycle, and if so, what do these variations imply about internal structural changes?
• RQ2 Are changes in helioseismic coefficients driven by deep interior structural evolution or by near-surface dynamics and magnetic fields?
• RQ3 To what extent can current inversion techniques detect structural changes in the solar interior, particularly in the convection zone and near the base of the photosphere?
• RQ4 Is the surface term in frequency inversions correlated with surface magnetic field distribution, suggesting a direct role of magnetic fields in frequency shifts?
• RQ5 Do solar flows (zonal and meridional) exhibit time-dependent changes correlated with solar activity, independent of structural changes?

Key findings

• Frequency variations are strongly correlated with solar activity indices, with peak-to-peak amplitude of 0.46 ± 0.106 μHz observed in earlier BiSON data.
• Changes in even-order splitting coefficients (a_j) are linearly correlated with the corresponding components of the solar magnetic field, indicating direct magnetic influence.
• The surface term derived from inversions shows a clear correlation with the latitudinal distribution of surface magnetic field, confirming near-surface origin of frequency shifts.
• No significant structural changes are detected in the solar interior up to 0.95 R⊙, as shown by inversions of sound speed and flow profiles.
• Latitudinal sound speed profiles remain unchanged over time, indicating no large-scale structural evolution in the convection zone.
• Zonal and meridional flows show time-dependent behavior, including equatorward migration of low-latitude bands and decreased meridional flow speed during high-activity periods.