Limb Figure Detailed Specifications

Team Coordinator: Rick Bogart

Observable

We use the Continuum Intensity (white light) proxy Ic, as described in XXX, obtained in full-disk observing mode.

Cadence & Duration

Data are obtained once per minute continuously for at least the duration of the nominal SOHO mission, 2 years

Spatial Filtering

We use only values within an annular mask chosen so to include all pixels containing the expected solar limb of epoch, to have an average thickness of 6 pixels (12 arcsec) with no thickness deviations exceeding 1.5 pixel (3 arcsec), and to have approximately equal numbers of pixels inside and outside the limb. (The ratio of the outer and inner radii of the mask to the limb are about 1.006 and 0.994 respectively, so the ratio of numbers of pixels centered outside and inside the limb would be about 1.012 if the limb were centered by radius; the distinction is probably not important.) The mask is calculated onboard from a given radius and thickness according to the algorithm to be supplied. The mask is recalculated sufficiently frequently to meet the thickness specifications and keep the limb centered. When a new mask is used, both sets of masked observations must be used in the temporal filters (see below).

Temporal Filtering

The data are averaged weighted by a Gaussian truncated to 0 at tc +/- 12 min (thus, only 23 one-minute samples are included, and 2 accumulation buffers are required). The Gaussian weights, normalized to unity at tc, correspond to a standard deviation of 204 seconds (3.4 minutes) as follows:

     tc              1.000000        tc +/- 6        0.210748
     tc +/- 1        0.957669        tc +/- 7        0.120106
     tc +/- 2        0.841129        tc +/- 8        0.0627770
     tc +/- 3        0.677549        tc +/- 9        0.0300931
     tc +/- 4        0.500553        tc +/- 10       0.0132301
     tc +/- 5        0.339149        tc +/- 11       0.00533448

The actual integer weights will be adjusted to avoid overflow in the temporal sums (TBD).

The specified temporal window provides a low-pass filter with a smooth roll-off to a value of 0.1 (-20 db) at 1.7 mHz, 0.01 at 2.4 mHz, first zero at 3 mHz, and transmission < -60 db (0.001) for all frequencies > 2.8 mHz. With this filter no more than about 40% of the power below 1 mHz should be due to aliasing of higher-frequency p-mode power. This is the same temporal filtering as that applied to the Continuum and Line Depth proxies used for the SOI/GOLF Intercomparison and Flux Budget programs.

Calibration

No calibration, flat-fielding, or bad pixel correction is performed on board. Flat-fielding and bad pixel correction must be performed on the ground as Level-1 processing of the data. For the purpose of determining the correct flat field values, TBD-amplitude instrument rolls must be performed at a minimum frequency of TBD.

Data Organization

The data stream for the time-averaged Limb-Figure data in the MDI Low Rate Telemetry channel (LRT) is set off by one of a number of unique Data Product Codes (DPC's), 32-bit numbers appearing at defined locations in the data headers of the overall data stream, followed by data sections containing the data in a pixel order TBD. Each mask used corresponds to a unique DPC. The DPC's for Limb Figure data are chosen so that the lower two bytes contain the information on radius and thickness used by the onboard Dedicated Experiment Processor (DEP) to calculate the mask. The high two bytes of all DPC's for Limb Figure data are unique up to a few reserved bits used to distinguish real flight data from simulated or ground data. In principle, the mask and pixel order can be recovered from the DPC of each data packet by simulating the behavior of the DEP. From time to time the actual pixel list used will be downlinked for verification.

Raw data from all telemetry frames sharing the same DPC within a defined mission day will be assembled into a single dataset. This dataset will contain up to 120 data records representing each of the 12-minute samples within a 24-hour period, plus one additional data record containing the pixel location information. Whenever the DPC changes during the course of a mission day, a new dataset is created.

Data Representation and Precision

Flat Fielding

Mapping and Interpolation

The of the flat-fielded data pixels are mapped to a uniform grid in the two coordinates radial distance (height) and azimuthal angle. Need to describe interpolation scheme, resolution. The position angle (roll angle) of the instrument needs to be known (to what precision)