Local helioseismology: Three-dimensional imaging of the solar interior

The Sun supports a rich spectrum of internal waves that are continuously excited by turbulent convection. The Global Oscillation Network Group (GONG) network and the SOHO/MDI (Solar and Heliospheric Observatory/Michelson Doppler Imager) space instrument provide an exceptional database of spatially resolved observations of solar oscillations, covering more than an entire sunspot cycle (11 years). Local helioseismology is a set of tools for probing the solar interior in three dimensions using measurements of wave travel times and local mode frequencies. Local helioseismology has discovered (a) near-surface vector flows associated with convection, (b) 250 m s^-1 subsurface horizontal outflows around sunspots, (c) ̃50 m s^-1 extended horizontal flows around active regions (converging near the surface and diverging below), (d ) the effect of the Coriolis force on convective flows and active region flows, (e) the subsurface signature of the 15m s^-1 poleward meridional flow, ( f ) a ±5 m s^-1 time-varying depth-dependent component of the meridional circulation around the mean latitude of activity, and ( g) magnetic activity on the farside of the Sun.