  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.
