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 Ni I 6768 Å 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 Å 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).
© Max Planck Society and the author(s)