Images from the Solar Oscillations Investigation - Michelson Doppler Imager

Dr. Rock Bush, Stanford University

Presented at the Joint Users Resource Allocation Planning Meeting
20 June 1996

Figure 1: MDI Single Dopplergram

The MDI instrument is designed to observe the line-of-sight motion of the Sun's photosphere, and to produce a velocity image or dopplergram once per minute. This figure illustrates a typical MDI dopplergram, with the dominant feature being the solar rotation, which appears as a shift from dark to light across the Sun's disk. The dark colors indicate motion toward the observer and lighter colors indicate motion away from the observer.

Figure 2: MDI Single Dopplergram Minus 45 Image Average

Subtracting an average solar velocity image observed over 45 minutes from a single velocity image reveals the surface motions associated with sound waves traveling through the Sun's interior. The small-scale light and dark regions represent the up and down motions of the hot gas near the Sun's surface. The pattern falls off towards the limb because the acoustic waves are primarily radial.

Figure 3: MDI 45 Image Average Dopplergram Minus Polynomial Fit

Subtracting the average solar rotation signal from a 45 image average of full disk dopplergrams enhances the surface motions associated with solar convection. Convective flow transports material and energy from the Sun's interior along narrow plumes. At the surface, the upwelling material then spreads out horizontally in the granulation pattern seen in this image. Little of this pattern is seen at the center of the solar disk because the motion is perpendicular to the line of sight.

Figure 4: Interior Solar Cutaway and Mode Diagram

This sketch illustrates how sound waves propagate through the Sun's interior. These sound waves last long enough to set up standing waves that have well defined periods and wavelengths. The diagram at the right shows the relationship between period and wavelength for sound waves in the Sun.

Figure 5: MDI Observed Oscillations

This figure is the l-nu (period versus wavelength) diagram determined from observations made by the MDI instrument during a continuous 83-hour observing run in April 1996. The x axis is the inverse spatial wavelength, with the right end representing waves on the order of 10,000 km (l=400). The y axis represents the wave frequency up to the instrument Nyquist cutoff (1 minute or 0.016 Hz).

Figure 6: MDI Partial High Resolution Dopplergram

This image is a portion of a MDI high-resolution dopplergram and shows about 4% of the solar disk. The large-scale rotation signature has been removed to clarify the smaller-scale surface motions.

Figure 7: Acoustic Wave Paths for Time-Distance Studies

Sound waves propagate into the solar interior along ray paths illustrated in this diagram. The waves are refracted because of changes in the sound speed due to varying density and temperature. The propagation path depends on the initial direction the sound waves travel.

Figure 8: MDI Observed Solar Surface Flows

Variations in the solar surface velocity will show up at other locations depending on the propagation path taken by the sound waves. Correlating the time delay as a function of distance provides information on the subsurface temperature and flows. This diagram illustrates the flow at the Sun's surface deduced from a time-distance analysis. The dark regions are magnetic fields simultaneously measured by the MDI instrument.

Figure 9: Solar Subsurface Flows Deduced from MDI Observations

By combining the time-distance analysis with inversion models, the subsurface flows can be calculated. This picture indicates the inferred convective flow in the solar interior near the Sun's surface.

Figure 10: MDI 160 kbps Telemetry Coverage during April 1996

This diagram indicates the coverage of the 160 kbps MDI telemetry from the SOHO spacecraft for the month of April 1996. The light gray indicates data that has been received and processed at Stanford, the medium gray indicates data that is missing, and the dark gray is data that has been received but needs to be reprocessed. The 83-hour continuous contact from April 24th to 28th resulted in 97% recovery of all MDI 160 kbps packets and 94.5% recovery of complete dopplergrams.

Figure 11: MDI 160 kbps Telemetry Coverage during May 1996

The MDI 60-day continuous coverage started on May 23rd. This diagram illustrates data recovery and processing of the May 1996 data through the last week in June 1996.

Please address comments and questions to the author.

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Last Modified by Amara Graps, SOI, on 15 November 1996.