Observations of solar-cycle effects by local- area helioseismology

The local-area helioseismology is a set of new developing tools for 3D acoustic imaging of the Sun's interior structure and dynamics. It considerably extends our capability for observing and understanding solar-cycle effects and dynamo processes of various scales inside the Sun. It also helps to interpret the solar-cycle variations that are observed in oscillation frequencies and other properties of global modes. Three basic tools: the ring-diagram analysis, the time-distance helioseismology and the acoustic holography, provide complementary diagnostics of the internal processes associated with solar activity. The ring-diagram analysis based on measurements of the local frequency-wavenumber dispersion relation for high-degree sound waves provides synoptic maps of large-scale subsurface flows. The time-distance helioseismology is capable of providing the synoptic maps for the interior sound-speed distribution in addition to the flow maps. This technique has also revealed intriguing images of developing active regions and sunspots and associated flow patterns in the upper convection zone. The method of acoustic holography has proved to be very efficient for mapping active regions on the far side of the Sun. Most results of the local-area helioseismology obtained so far deal with the solar-cycle effects in the upper half of the convection zone. However, there has been an attempt to obtain longitudinal synoptic maps of sound-speed variations in the tachocline by the time-distance technique. The recent major achievements of the local-area helioseismology also include studies of the meridional circulation and its variation with the solar cycle and an investigation of the relation between the torsional oscillations and large-scale flows around active regions. These results are obtained using Doppler-shift data from the MDI instrument on SOHO (Scherrer et al. 1995).