from space and this is the approach taken by the SOlar and Heliospheric Observatory (SOHO) spacecraft, which was launched in 1995. Sun-as-a-star observing programs onboard SOHO include the Global Oscillations at Low Frequencies [GOLF; @1995SoPh..162...61G] instrument, which also measures line-of-sight velocity, and the Variability of solar IRrandiance and Gravity Oscillations [VIRGO; @1995SoPh..162..101F], which measures the changes in intensity of the Sun.

VIRGO is also capable of making resolved observations of the Sun, as was the Michelson Doppler Imager [MDI; @1995SoPh..162..129S], also onboard SOHO. Global Oscillations Network Group (GONG) is a ground-based network of 6 sites [@1996Sci...272.1284H], which has been making resolved observations of the Sun since 1995. More recently, the Helioseismic and Magnetic Imager [HMI; @2012SoPh..275..229S] and the Atmospheric Imaging Assembly [AIA; @2012SoPh..275...17L] onboard the Solar Dynamics Observatory (SDO) have been used for helioseismic studies [@2011JPhCS.271a2061H; @2011JPhCS.271a2058H]. SDO was only launched in 2010 and so these instruments cannot be used to study previous solar cycles. They are, however, likely to play an important role in helioseismic studies of cycle 24, and possibly beyond.

Local helioseismology {#section[local intro]}
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Since the advent of helioseismic observations at high spatial resolution, a number of different data analysis techniques known collectively as “local helioseismology” 