  We describe systematic changes in the centroid frequencies and the splitting
coefficients as found using data from MDI on board SOHO, covering cycle 23. The
data allow us to construct a seismic map of the evolving solar activity --
covering all latitudes. At lower latitudes, the temporal evolution closely
tracks that of {\it butterfly diagram}. The additional information from higher
latitudes in the map is of a significant activity in the polar region, peaking
at activity minimum in 1996. The most plausible source of solar oscillation
frequency changes over the solar cycle is the evolution of the radial component
of the small-scale magnetic field. The amplitude of the required mean field
changes is about 100 G at the photosphere, and increasing going inward.
