# A Helioseismic Perspective on the Depth of the Minimum Between Solar Cycles 23 and 24

## TL;DR

This study uses helioseismology to analyze the unusually deep solar minimum between Cycles 23 and 24, revealing a weaker global magnetic field during that minimum through frequency shifts in oscillations.

## Contribution

The paper demonstrates that helioseismic oscillation frequencies are sensitive indicators of the Sun's magnetic field, providing new insights into the magnetic conditions during solar minima.

## Key findings

- Helioseismic frequencies were systematically lower after Cycle 23.
- The frequency shifts suggest a ~1G weaker magnetic field during the minimum after Cycle 23.
- Frequency dependence of shifts indicates near-surface magnetic perturbations.

## Abstract

The solar-activity-cycle minimum observed between Cycles 23 and 24 is generally regarded as being unusually deep and long. That minimum is being followed by one of the smallest amplitude cycles in recent history. We perform an in-depth analysis of this minimum with helioseismology. We use Global Oscillation Network Group (GONG) data to demonstrate that the frequencies of helioseismic oscillations are a sensitive probe of the Sun's magnetic field: The frequencies of the helioseismic oscillations were found to be systematically lower in the minimum following Cycle 23 than in the minimum preceding it. This difference is statistically significant and may indicate that the Sun's global magnetic field was weaker in the minimum following Cycle 23. The size of the shift in oscillation frequencies between the two minima is dependent on the frequency of the oscillation and takes the same functional form as the frequency dependence observed when the frequencies at cycle maximum are compared with the cycle-minimum frequencies. This implies that the same near-surface magnetic perturbation is responsible. Finally, we determine that the difference in the mean magnetic field between the minimum preceding Cycle 23 and that following it is approximately 1G.