  We derive formulae connecting the frequency variations in the spectrum of
solar oscillations to the dynamical quantities that are expected to change over
the solar activity cycle. This is done for both centroids and the asymmetric
part of the fine structure (so-called even-a coefficients). We consider the
near-surface, small-scale magnetic and turbulent velocity fields, as well as
horizontal magnetic fields buried near the base of the convective zone. For the
centroids we also discuss the effect of temperature variation. We demonstrate
that there is a full, one-to-one correspondence between the expansion
coefficients of the fine structure and those of both the averaged small-scale
velocity and magnetic fields. Measured changes in the centroid frequencies and
the even-a's over the rising phase solar cycle may be accounted for by a
decrease in the turbulent velocity of order 1%. We show that the mean
temperature decrease associated with the net decrease in the efficiency of
convective transport may also significantly contribute to the increase of the
centroid frequencies. Alternatively, the increase may be accounted for by an
increase of the small-scale magnetic field of order 100 G, if the growing field
is predominantly radial. We also show that global seismology can be used to
detect a field at the level of a few times 10^5 G, if such a field were present
and confined to a thin layer near the base of the convective envelope.
