What helioseismology teaches us about the Sun

Helioseismology uses observations of oscillations of the solar surface to determine the internal structure and dynamics of the Sun, providing critical knowledge about the mechanisms of solar variability and activity cycles. The recent advances based on observations from SOHO spacecraft and GONG network have allowed us to study both long-term changes of the global structure and circulation and short-term variations associated with developing active regions, sunspots and coronal activity. In particular, the global helioseismology results have revealed 1.3-year variations of the rotation rate in the tachocline, but found no indication of 11-year variations. Studies of the meridional circulation have shown formation of additional meridional cells of flows converging toward the activity belts, thus, questioning the flux-transport theories of the solar cycle. It is found that sunspots as cool objects appear to be only 4-5 Mm deep, but accumulate significant heat in the deeper layers, and also form converging downflows. Large active regions are formed as a result of multiple flux emergence, and no evidence of large-scale emerging Ω-loop has been found. This paper presents a brief review of these and some other results of helioseismology, analysis techniques, and perspectives.