The MDI Resident Data ArchiveThe MDI operational principle is to combine multiple filtergrams taken in several very narrow passbands selected by a tunable filter across and slightly beyond the wavelength width of the absorption line in order to estimate the Doppler and/or Zeeman shift of the line as well as the line depth and the continuum intensity in the immediate vicinity of the line. For each observation, filtergrams from four tuning positions across the line are used. A continuum intensity proxy is estimated in one of two ways: either as the base of the overall line profile fit for the Doppler shift, or simply as the intensity of a filtergram in a fifth tuning position further from line center than the others. There are thus a total of five “observables”: Doppler shift (V), Zeeman shift (M), Line depth (Ld), fit continuum intensity (Ic), and off-line filtergram intensity (I0).
The MDI telescope has an optical resolution of about 1.4 arc-sec. Critical sampling at this resolution (0.7 arcsec/pixel) would require a camera diameter of nearly 3000 pixels to record individual filtergrams of the entire photospheric image. MDI has a 1024×1024 pixel camera. The camera can be fed from either of two light paths, however. In one, the image is demagnified so that the entire disc of the Sun fits within the CCD, with a resolution of approximately 2 arc-sec per pixel. This is called “full-disc” (fd) mode. In the other, the image is at full resolution, about 0.7 arc-sec per pixel, but a region of only about 700 arc-sec squared, about 10% of the disc, is observed. This is called the “high-resolution” (high-res; hr) mode. The location of the high-resolution region is fixed. When the spacecraft is oriented in its nominal position with one axis aligned with the solar (Carrington) north axis, the high-resolution region is approximately centered in the solar east-west direction, but displaced to the north so that about 75% of the region is in the northern hemisphere. Of course this flips when, as occurred for half the time during the last nine years of the mission, beginning in the summer of 2003, the spacecraft is aligned with the south axis. (This was due to a failure of the spacecraft's high-gain antenna pointing mechanism.)
The MDI camera acquires one filtergram approximately every 3 sec. The telemetry bandwidth was vastly lower than needed to transmit each filtergram, so it was necessary to perform the observables calculations onboard. In a “high-rate” telemetry mode of 160 kbps, normally available for only about 8 hours per day, it was possible to download two observables for the full disc or most of the high-res field per minute. A “low-rate” telemetry mode of 5 kbps, nominally available continuously either directly or from dumps of recorded data, allowed continuous acquisition of selected observables at either greatly reduced spatial or temporal resolution, or both, via onboard binning or weighted averaging. Six so-called “Structure Program” products are available essentially continuously throughout the mission, with the obvious exceptions of the two extended spacecraft outages:
The onboard processing capability for extraction, spatial binning, and temporal summing also made it possible to design additional high-rate data products in order to enhance the number or extent of the observables acquired in a single minute. In particular, there are versions of several of the full-disc and high-res observables involving onboard 2×2 binning to effectively reduce the nominal pixel resolution by a factor of 2, to 4 or 1.4 arcsec/pixel, respectively. Additionally, there are various data products for which only a rectangular subgrid of the original observable image is extracted, the location and size possible varying from image to image. Finally, there are a few data products characterized by distinct sampling frequencies as discussed below.
Most of the observables can be produced from appropriate combinations of individual filtergrams in different wavelengths and polarizations acquired over an interval of either 30 or 60 sec. (The individual filtergram exposure times are less than a second.) During the regular mission (Phase 1 onward), when the Structure Program was running, the “upper half” of each minute, i.e. from 15 sec before the minute to 15 sec after, was always devoted to the observations required to produce the relevant observables (V, Ic, and Ld), that is, filtergrams in linear polarization at the standard wavelengths and in full-disc mode. The lower half of each minute could be devoted to either the same observations, or to those required for magnetograms (using the circular polarizer), or to any observation in high-res mode, or to calibration measurements. When the circular polarizer was in use during the lower half of the minute, Doppler and intensity measurements, even if made in full-disc mode, were not used for the Structure Program observables, because of optical displacements caused by the circular polarizer, degrading the per pixel onboard integrations.
Sampling requirements for various types of data are quite different, depending on whether their primary purpose is helioseismology, focused campaign studies, or regular monitoring. Structure Program data were sampled as nearly continuously as possible at their nominal cadence. High-rate data were sampled either continuously in dedicated blocks of time, regularly at rates of one to fifteen per day, or only sporadically. For data expected to be continuous, there exist data records (metadata) for each time step, regardless of whether the associated image data for that time step exist or not. This allows for determination of actual data coverage (duty cycle) during any given interval and for the extraction of continuous metadata parameters not dependent on the presence of actual image data for the target time, such as spacecraft position and velocity. (The exception is that there are no metadata records during the SoHO vacations.) For data that were sampled continuously only in dedicated blocks of time, the metadata are filled continuously only within blocks of a full hour or full day (depending on the series) for which some image data are present. For sporadic observations, metadata records exist only for times at which real data are present. These three cases are denoted by C (continuous), H or D (depending on the block length, hourly or daily), and S (sporadic), respectively,
Table 1: MDI Level 1(.5) Observations
Coverage during mission phases
(For Phase 0, image counts are shown rather than fractional coverage)
series metadata images coverage during mission phase Data Volume fill 0 1 2 3 4 5 (GB) fd_I0 H 46 751 0 0 .0362 .0098 .0009 .0005 33.910 fd_Ic H 284 183 279 .0060 .1291 .0782 .0167 .1073 259.928 fd_Ld H 95 234 0 .1039 .0811 0 .0001 0 83.144 fd_M_lev182 C 2 588 296 0 .0001 .0711 .0684 .4048 .1046 102.830 fd_M_96m_lev182 C 83 700 49 .7584 .9600 .8736 .9216 .7296 (102.830) fd_V C 3 071 162 449 .1077 .3678 .0200 .4354 .0024 2875.291 fd_Ic_30s H fd_V_30s H fd_I0_extract H 25 761 0 0 .0118 .0280 .0016 0 10.413 fd_M_extract H 82 523 0 0 .0468 .1496 .0028 0 88.377 fd_V_extract H 82 595 0 0 .0469 .1493 .0028 0 61.609 fd_Ic_bin2x2_30s H 160 004 0 0 .1440 .0001 .0003 0 40.721 fd_V_bin2x2_30s H 17 262 0 0 .0148 0 .0001 0 4.649 fd_V_bin2x2 H 11 890 0 0 .0107 .0002 0 0 3.332 hr_I0 H 490 279 0 .0344 .1286 .2088 .0536 .0027 250.576 hr_Ic H 985 0 0 .0008 0 0 0 0.522 hr_Ld H 2 059 0 0 .0018 0 0 0 1.321 hr_M H 754 679 0 .0389 .1503 .2132 .0928 .0054 794.322 hr_V H 699 941 25 .0308 .1672 .1926 .0814 .0011 433.529 hr_V_12s H 22 725 22725 0 0 0 0 0 6.645 hr_Ld_bin2x2 H 1 894 0 0 .0016 0 0 0 0.555 hr_M_bin2x2 H 11 727 0 .0332 .0086 .0001 0 0 4.609 hr_V_bin2x2 H 196 997 0 .0051 .0474 .0002 .0236 0 61.789 limb_figure C 607 968 1 .7440 .9792 .9084 .9564 .8616 98.877 loi_Ic C 7 397 013 0 0 .9912 .9045 .9750 .9002 5.125 loi_V C 7 444 451 0 .9692 .9911 .9044 .9750 .9002 5.158 rwbin_Ic C 618 376 1 .7500 .9864 .9156 .9744 .8688 18.219 rwbin_Ld C 618 425 1 .7632 .9864 .9156 .9744 .8688 18.182 vw_v C 7 403 494 0 .6564 .9871 .9161 .9731 .8680 1082.303Note that the series fd_M_96m_lev182 is merely a regular cadence subsample of the series fd_M_lev182, and since the individual magnetograms of the latter series are collected in daily directories, the total data volume counts are identical. Magnetograms during phase 0 were only included in series fd_M_96m_lev182.
An additional five series of “observables” have been defined to include various types of observations acquired for purposes of instrument calibration. They are summarized in Table 2.
Table 2: MDI Level 1.5 Calibration Data
Image counts during mission phases
series metadata images images during mission phase Data Volume fill 0 1 2 3 4 5 (GB) detune S 97 800 0 72 22833 6507 65948 2440 6.103 focus S 15 882 0 91 1701 4796 8814 480 1.873 ice S 30 420 0 177 3029 529 25412 1273 18.888 lp S lp_bin S
| “Resolution / Field of View” | Doppler LOS Velocity | Continuum Intensity |
LOS Magnetic Flux | Line Depth | Calibration |
|---|---|---|---|---|---|
| Full-Disc |
fd_V, fd_V_30s |
fd_I0, fd_Ic, fd_Ic_30s |
fd_M_lev182, fd_M_96m_lev182 |
fd_Ld | |
| Full-Disc extract | fd_V_extract |
fd_I0_extract, limb_figure |
fd_M_extract | ||
| Full-Disc binned |
fd_V_bin2x2, fd_V_bin2x2_30s, vw_V, loi_V |
fd_Ic_bin2x2_30s, rwbin_Ic, loi_Ic |
rwbin_Ld | lp_bin | |
| High-Res (including extract) |
hr_V, hr_V_12s |
hr_I0, hr_Ic |
hr_M | hr_Ld | |
| High-Res binned | hr_V_bin2x2 | hr_M_bin2x2 | hr_Ld_bin2x2 | ||
| mixed / NA |
detune, focus, ice, lp |
|
| this page last modified 1 Sep 2017, 16:32-0700 |