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).
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.
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.
- 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.
- 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
- 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.
- 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
- Continuum images near the limb that could be used to place limits on
the ability to identify and track highly foreshortened granules.
- 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.
- 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.)
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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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