The MDI is part of the SOHO Solar Oscillations Investigation.
The Solar Oscillations Investigation (SOI) uses the Michelson Doppler Imager (MDI) instrument to probe the interior of the Sun by measuring the photospheric manifestation of solar oscillations. Characteristics of the modes reveal the static and dynamic properties of the convection zone and core. Knowledge of these properties will improve our understanding of the solar cycle and of stellar evolution. Other photospheric observations will contribute to our knowledge of the solar magnetic field. The scientific program relies on the coordinated efforts of several teams that pursue specific investigations.
The instrument images the Sun on a 1024*1024 CCD camera through a series of increasingly narrow filters. The final elements, a pair of tunable Michelson interferometers, enable MDI to record filtergrams with a FWHM bandwidth of 100 mÅ. Normally 20 filtergrams centered at 5 wavelenths near the NiI 6768 Å spectral line are obtained each minute and used to calculate the velocity and continuum intensity with a resolution of 4" over the whole disk. Alternatively, 10 of the filtergrams can be used on a smaller region with 1.25" resolution centered about 160" north of the equator. Several times each day polarizers will be inserted to determine the line-of-sight magnetic field. An extensive calibration program has verified the end-to-end performance of the instrument.
Complete velocity and intensity images can be relayed to Earth each minute about half of the time, including a continuous 60-day interval each year. To provide continuous observation of the longest-lived modes, a carefully selected set of spatial averages are computed and downlinked at all times. Various observing campaigns will be developed to make the best use of the daily high-rate telemetry intervals available for eight hours per day during the rest of the year. Ten of the 20 filtergrams each minute can be used for these campaigns.
MDI operations will be scheduled well in advance and will vary only during the daily 8-hour campaigns. Quick-look and summary data, including magnetograms, will be available at the Experiment Operations Facility (EOF) at Goddard. Most high-rate data will be delivered directly to the SOI Science Support Center (SSSC) at Stanford, where a processing pipeline will produce 3 Terabytes of calibrated data products each year. These data products will be analyzed using the SSSC and the distributed resources of the coinvestigators.
MDI is one of twelve experiments onboard SOHO, which was launched at 12:08 AM PST on December 2, 1995.
More information on the MDI can be obtained from : http://soi.stanford.edu/
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