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
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
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
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(s).
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