2.1 Data for local helioseismology

The fundamental data of modern helioseismology are high-resolution Doppler images of the Sun’s surface. Local helioseismology started with observations of acoustic absorption by sunspots using data from the Kitt Peak vacuum telescope (Braun et al., 1987

, 1988

). Observations obtained by Hill (1988

, 1989 ) at the Sacramento Peak vacuum tower telescope demonstrated that local spectra of solar oscillations provide measurable information about internal horizontal flows. Continuous data from the 1988 south pole expedition lead to direct measurements of local travel times of acoustic waves (Duvall Jr et al., 1993

). Today, the development of local helioseismology is fueled by high-quality data from space and ground based networks. The main datasets are provided by the Taiwan Oscillation Network (TON), the Global Oscillation Network Group (GONG), and the Michelson Doppler Imager (MDI) aboard the ESA/NASA SOHO spacecraft in a halo orbit around the $L1$ Sun-Earth Lagrange point. The TON consists of six identical telescopes at appropriate longitude around the globe. The data are series of $1080 × 1080$ full-disk $Ca+ K$ -line intensity images recorded at a rate of one image per minute. A description of the TON project is given by Chou et al. (1995 ). The TON data may be requested by contacting the Principal Investigator, Dr. Dean-Yi Chou (chou@phys.nthu.edu.tw).

The GONG is an international network of six extremely sensitive and stable solar velocity imagers that provide nearly continuous observations of solar oscillations (Leibacher, 1999 ). The GONG instruments, which are Michelson-interferometer-based Fourier tachometers, observe the $º Ni I 6768 A$ line. In addition to Doppler and intensity images every minute, GONG provides full-disk magnetograms nominally every 20 minutes. The system became operational in October 1995, and will operate for at least an eleven-year solar cycle. The observation duty cycle has averaged about 90%. The original instruments used $256 × 256$ pixel CCD cameras, which where replaced in 2001 by $1024 × 1024$ square-pixel cameras. The GONG data products can be accessed at the project’s website (GONG, 2002 ).

The MDI has provided line-of-sight Doppler velocity images since 1996 with an excellent duty cycle (Scherrer et al., 1995 ). MDI Dopplergrams are obtained by combining 4 filtergrams on the wings and core of the $Ni 6788 ºA$ absorption line, formed just above the photosphere. Dopplergrams are available at a one minute cadence. MDI operates under several observing modes. The Dynamics Program runs for 2 to 3 months each year and provides $1024 × 1024$ full-disk Doppler images; the plate scale is $'' 2$ per pixel, or $0.12$ heliographic degrees ( $1.45 Mm$ at disk center). The Structure Program provides continuous coverage: full-disk images are binned onboard into a set of about 20,000 regions of roughly similar projected areas on the Sun to make use of the narrow telemetry channel. The Structure Program data are used to measure mode frequencies up to spherical harmonics degrees of 250. MDI can also operate in High-Resolution mode by zooming on a $' 11$ square field of the Sun with a plate scale of $'' 0.625$ per pixel and a diffraction-limited resolution of $'' 1.25$ . MDI data can be accessed at the project’s website (MDI, 1997 ).