  Normal modes of oscillation of the Sun are useful probes of the solar
interior. In this work, we use the even-order splitting coefficients to study
the evolution of magnetic fields in the convection zone over solar cycle 23,
assuming that the frequency splitting is only due to rotation and a large scale
magnetic field. We find that the data are best fit by a combination of a
poloidal field and a double-peaked near-surface toroidal field. The toroidal
fields are centered at r=0.999R_solar and r=0.996R_solar and are confined to
the near-surface layers. The poloidal field is a dipole field. The peak
strength of the poloidal field is 124 +/- 17G. The toroidal field peaks at 380
+/- 30G and 1.4 +/- 0.2kG for the shallower and deeper fields respectively. The
field strengths are highly correlated with surface activity. The toroidal field
strength shows a hysteresis-like effect when compared to the global 10.7 cm
radio flux. The poloidal field strength shows evidence of saturation at high
activity.
