An Opportunity for High-Resolution Observations Near the Limb

SOI TN 96-129

R. S. Bogart

Table of Contents


At the January monthly planning meeting for SOI/MDI operations, an interesting possibility of a unique opportunity for special MDI observations arose. As part of the special calibration observation sequences planned during January, the MDI mounting legs will be used to offset the image by more than the range of the PZT's, on the order of the solar radius, in order to obtain information needed for optimum flat-fielding. The required observations are expected to last throughout most of an observing day of 8 hours high-rate telemetry.

Depending on the schedule (and there are big question marks), it may be possible to leave the legs in a location of large displacement through the next observing day, rather than recentering the image immediately at the end of the flat-field observation sequence. It is possible that this would leave the high-resolution field of MDI, which is otherwise forever fixed near disc center, near or at the limb, possibly (preferably we would think) at one of the poles. Because leg motions are virtually forbidden at any other time during the mission, due to the danger of failure and the inability of the Image Stabilization System to function when the image is not centered, this must be viewed as a one-time only opportunity.

The question was posed to the members of the team: what observations should be made in case this opportunity arises, and what is the justification and requirements for any proposed observations? The nominal opportunity was described as an 8-hour period of high-rate telemetry with the high-resolution field at some point TBD far from disc center, no image stabilization, and the ability to make Dopplergrams, Magnetograms, and continuum photograms, but no complicated campaigns (and certainly no possibility of using the low-rate telemetry channel for anything special).

Observational Constraints

The high-resolution field has a projected width of about 680 arc-sec (11+ arc-min). It is displaced relative to the center of the full-disc field so that when the solar image is centered, it extends ±340 arc-sec east-west of center, and from 200 arc-sec south of center to 480 arc-sec north of center.

During the month of January, the heliographic latitude of Earth increases from -3 degrees to -6 degrees, at which point the south pole is almost 9 high-resolution pixels (5 arc-sec) in from the limb. (The inclination of the SOHO orbit to the ecliptic is of negligible consequence.) The projected solar radius is of order 975 arc-sec.

The range of leg motions permits the center of the field to be displaced anywhere within a rhombus of about ±1060 arc-sec east-west of the nominal center, ±800 arc-sec north, and ±850 arc-sec south of nominal center. (These are the extremes of the range; we do not want to drive the legs to the limits obviously.) The nominal center is the center of the field originally observed when the door was first opened, before the legs were displaced to center the full-disc image. (See the first light image on the SOI home page.) This is approximately 85 arc-sec east and 150 arc-sec south of the center of the solar disc with the present pointing of SOHO. SOHO will be repointed, however, almost certainly before the observations contemplated here, to align one of the other instruments. It is expected that this repointing will make the nominal leg center nearly coincident with the disc center, thereby making the range of leg motions nearly symmetric about disc center. With the present pointing only the limb near the north pole can be reached in the high-resolution field; the south pole misses by at least 1 arc-min. It is possible that ths south pole may be reachable if the pointing restores nominal center to disc center.

There is no campaign sequence at this time that can be used to send more than one full-field image at a time at a one-minute cadence. Up to 14 images can be made quickly (within 30 seconds) and stored in memory, but the time to download the data is of the order of one image per minute. Simultaneous Dopplergrams and continuum images can be downlinked once per minute if they are cropped in the circular pattern appropriate to the full-disc observations; I do not know whether this mode of operation can be used with the high-resolution field, although there is no reason in principle why it could not.

The schedule for the leg flat-field observations has not yet been set. It is likely to take place sometime in the last half of January. The level of solar activity at this time is fairly low. There is one new active region that appeared on Jan 3 that should reappear at the east limb around Jan 23, at a fairly low northern latitude.

Suggested Observations

Suggestions for observations were received from Rick Bogart, Alessandro Cacciani, Gary Chapman, Tom Duvall, Jack Harvey, Dave Hathaway, Sasha Kosovichev, Jeff Kuhn, Rob Rutten, Dick Shine, George Simon, and Ellen Zweibel. The proposals for high-resolution observations are summarized below. They are numbered only for reference, not ranked. I have rearranged and reworded some of the proposals to reduce overlap.
  1. A time series of continuum images at the limb (any azimuth), together with small PZT offsets from the mean position, to yield an improved limb darkening profile and improved flat-field data near the limb that would help to improve the analysis of limb oscillations. The measurements with PZT offsets could in principle be incorporated in the flat-field observations of the planned leg-moving sequence.
  2. A series of Dopplergrams including the area around the south pole, long enough to be able to detect evidence for a polar vortex in the mean surface flow.
  3. A time series of Dopplergrams including the area around pole, to be used for time-distance helioseismology, together with periodic magnetograms; together these could be used to study the dynamics of the network.
  4. Approximately 8 hours of Doppler observations at any azimuth (preferably at the pole), including the region from 0.6 to 0.9 R where supergranules show up best, to study the evolution of convection patterns and magnetic field.
  5. Continuum images near the limb that could be used to place limits on the ability to identify and track highly foreshortened granules.
  6. Several sets of filtergrams, preferably including steps intermediate to the normal tuning sequence, that could be used to establish detailed variations in the line profile with height very near the limb. This would be most interesting if it could include a magnetic active region near the limb, which would require a near-equatorial azimuth (and serendipity), but would still be worthwhile at any azimuth.
  7. One or more deep continuum or filtergram images including the area just off the limb that could be used to improve the narrow-angle component of the light scattering matrix. (These could be incorporated as part of the leg flat-field observations.)
  8. A time-series of intensity observations in line-center (or line depth?) for seismic analysis, to counter the fallof in Doppler oscillation signal near the limb.
  9. An oscillation sequence to elucidate how center-to-limb variations of granule contrast and line profile affect the Doppler signal and to study latitudinal and limb variations in the properties of oscillations.
  10. Continuum intensity and line depth observations that could be used to search for and study polar faculae. A time-lapse series would be desirable to study lifetimes and migration.
  11. A time series of rapidly interspersed magnetic and Doppler images of an active region near the limb that could be analyzed for covariance between the two signals to locate Alfvén waves.
  12. A series of interspersed Doppler, magnetic, and continuum images with sufficient time resolution to follow flows and magnetic evolution, and to study the statistics of flows and fields, preferably near a pole.
  13. A seismic Dopplergram sequence in an area including an active region or plage to study the enhancement of horizontal relative to vertical velocity in the p-mode eigenfunction by magnetic fields.
  14. Take some deep-continuum observations off the limb to try to detect spicules, and high-resolution magnetograms in the neigboring photosphere to see if the spicules can be traced to the near-limb network.

This page last revised 10 January, 1996.

Please address comments and questions to the author(s).

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