Scientific Rationale

Helio- and asteroseismology have received a tremendous amount of new information from space missions and ground-based networks. In particular, SOHO and GONG have observed the Sun for longer than one solar cycle; Hinode has started sending us high-resolution data; MOST and COROT have provided unique asteroseismology observations; and SDO/HMI is expected to be launched and provide initial results by the time of the IAU General Assembly. Thus, this Joint Discussion will become a timely meeting for solar physicists and astrophysicists to share new discoveries and advances in our understanding the interior structure, dynamics and activity of the Sun and other stars. The Sun has an activity cycle of 11 years, and GONG and SOHO have observed the Sun for more than one solar cycle. The temporal variations of solar interior structures, rotational rates, and meridional flows, particularly at the tachocline area, are of great importance for better understanding what causes the solar magnetism and how the solar dynamo operates. It is also essential to know how the whole solar cycle observations will help to improve the dynamo simulations for a better prediction of next solar cycle activity level. Using more than 10 years' observation from SOHO/GOLF, helioseismologists have made new attempts to find signals of the long-sought solar g-modes, probably opening a new window for studying the solar core structure and dynamics.

Local helioseismology has made significant advances in inferring solar interior structures and flow fields on the scales of supergranulation, active regions and sunspots. The new high-resolution observations available from Hinode and anticipated from the Solar Dynamics Observatory will help us to greatly improve our knowledge of the Sun's local structures and dynamics. On the other hand, the recent advances in numerical simulations in solar upper convection zone have given us unprecedented opportunities to evaluate and improve the diagnostic methods. The interaction between solar magnetic field and acoustic waves is particularly important for detecting structures beneath sunspots, and the magnetoseismology investigations have made significant progress in the past a few years.

The advances in helioseismology are complemented by the tremendous progress in asteroseismology, in particular by the new results from the space missions MOST, COROT, WIRE and SMEI. Seismology of distant stars, particularly, of solar-type stars, is extremely challenging and opens new opportunities to measure gross properties of many stars in their various evolutionary stages with unprecedented accuracy. It requires huge coordinated efforts among space-borne and ground-based projects. The space missions provide high-precision photometric data, and it is important to combine these with high S/N, high time-resolution, and high wavelength-resolution spectroscopic data. The new asteroseismic observations have discovered a rich world of stellar oscillations in various type of stars and provide new opportunities to bring our understanding of the theory of stellar structure and evolution to a new level of sophistication.

The Joint Discussion of the recent results and new approaches in helio- and asteroseismology will help to advance our understanding of the mechanism of solar and stellar oscillations and the physical processes inside the Sun and other stars.