THE SOLAR OSCILLATIONS INVESTIGATION - MICHELSON DOPPLER IMAGER

The Solar Oscillations Investigation (SOI) uses the Michelson Doppler Imager (MDI) instrument to probe the interior of the Sun by measuring the photospheric manifestations of solar oscillations. Characteristics of the modes reveal the static and dynamic properties of the convecti on zone and core. Knowledge of these properties will improve our under standing of the solar cycle and of stellar evolution. Other photospher ic observations will contribute to our knowledge of the solar magnetic field and surface motions. The investigation consists of coordinated efforts by several teams pursuing specific scientific objectives. The instrument images the Sun on a 1024(2) CCD camera through a series of increasingly narrow spectral filters. The final elements, a pair of tu nable Michelson interferometers, enable MDI to record filtergrams with a FWHM bandwidth of 94 Angstrom. Normally 20 images centered at 5 wav elengths near the Ni I 6768 spectral line are recorded each minute. MD I calculates velocity and continuum intensity from the filtergrams wit h a resolution of 4 '' over the whole disk. An extensive calibration p rogram has verified the end-to-end performance of the instrument. To p rovide continuous observations of the longest-lived modes that reveal the internal structure of the Sun, a carefully-selected set of spatial averages are computed and down-linked at all times. About half the ti me MDI will also be able to downlink complete velocity and intensity i mages each minute. This high rate telemetry (HRT) coverage is availabl e for at least a continuous 60-day interval each year and for 8 hours each day during the rest of the year. During the 8-hour HRT intervals, 10 of the exposures each minute can be programmed for other observati ons, such as measurements in MDI's higher resolution (1.25 '') field c entered about 160 '' north of the equator; meanwhile, the continuous s tructure program proceeds during the other half minute. Several times each day, polarizers will be inserted to measure the line-of-sight mag netic field. MDI operations will be scheduled well in advance and will vary only during the daily 8-hour campaigns. Quick-look and summary d ata, including magnetograms, will be processed immediately. Most high- rate data will be delivered only by mail to the SOI Science Support Ce nter (SSSC) at Stanford, where a processing pipeline will produce 3 Te rabytes of calibrated data products each year. These data products wil l be analyzed using the SSSC and the distributed resources of the co-i nvestigators. The data will be available for collaborative investigati ons.