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SOI/MDI After One Year

A presentation to NASA and ESA by Philip H. Scherrer
16 January 1997


The following is a discussion of SOI/MDI initial results and the benefits of extending the SOHO mission from the initial two to at least six years.


  1. Scientific Case for Mission Extension

    NASA, ESA, and the European member countries have long term goals that require a better understanding of the Sun, Solar variability on multiple scales, and the "Space Weather" that impacts the terrestrial environment. These goals are described in a number of long range plans and need not be further described here. The present question is to see how SOHO is an asset that can contribute to these goals as well as expand and complement the goals that provided the imperative to develop SOHO in the first place.

    SOHO as a whole was developed to address a number of interrelated fundamental questions about the nature of solar physical processes. A number of these phenomena are most simply observed at or near activity minimum. The Sun has been quite obliging in providing a nice quiet minimum during the first year of SOHO operations. Some of the results and studies in progress are described below. A key fact often under-emphasized is that SOHO is working phenomenally well. The spacecraft systems, instruments, and operations teams and facilities are all working as well or better than could be expected. This fact not only enables but provides an imperative for the discussion of how to best use these assets in the coming Solar maximum years.

    The MDI instrument is providing data that often exceeds our expectations. While after only eight months of normal operations the science investigation is by necessity still in the early stages of development, it is not too early to consider the opportunities for progress that would be enabled by continued operation to and through the coming activity maximum. The Solar Oscillations Investigation using the MDI instrument has a number of science goals. Some of these require only short duration observations while others need very long duration observing sequences.

    Some of the primary SOI science objectives require several years of continuous observations to achieve even the original goals. This is due to the nature of making exceedingly accurate measurements of oscillation mode frequencies. Accuracies of a few tens of nano-Hz are required to make reliable inferences about the structure and rotation of the solar interior and that simply takes several years. While a mission of only two years is justifiable in its expected gains, longer duration time series will certainly provide better limits to the structure of the star. The region of the Solar interior that most needs long-baseline observations is the energy generating core where the present limits of accuracy provide tantalizing hints of mixing.

    Other of the primary objectives need continuous sequences of days to months. These are the goals that can be addressed by the new science of Local Helioseismology. These techniques allow imaging of the motions in the interior of the star. Solar minimum is the optimum time to begin this study since the Sun provides its phenomena in the simplest configurations. Early progress has convinced us that the developing techniques will indeed work and we will be able to watch the development of the magnetic cycle from the inside. MDI provides a unique tool for this study. It is the only tool available with the required spatial and temporal resolution to study the upper convection zone where activity evolves as the cycle progresses. If, as we believe, local helioseismology provides a tool that can allow accurate prediction of active region growth, continued MDI operation through to Solar maximum can provide the observational foundation to enable the development of a predictive capability.

    In addition to helioseismology observations, MDI can and does provide observations essential for complementary view of solar processes. We have demonstrated the ability to follow the generation and evolution of magnetic fields on the granulation to global scale with a spatial and time resolution sufficient to, for the first time, follow the life-cycle of magnetic fields. MDI also provides measurements of relative brightness of active and quiet regions to enable the better understanding of the radiative flux balance. It also provides direct measurements of surface motions from meso-granulation through to global scales. These studies continued through maximum would provide a unique view of the process that links interior dynamics to the phenomena that affect the Earth’s space environment.

    SOHO as a whole, combining the unique capabilities of all the instruments, has the capability to provide the answers to many of the fundamental questions of solar variability. It would be unconscionable to not use this resource so long as it is functioning.

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  2. Expected Instrument Performance during Extended Mission

    The MDI instrument is presently operating as designed. There are no known life-limiting elements. There are several areas we are watching. These include the front window temperature which affects the ability to keep the high resolution field in focus. We have no further adjustment range in the direction needed to correct for increased temperature. Front window contamination could cause an increase in temperature which would reduce the quality of the high resolution data. We have seen no evidence of such a trend. The total MDI throughput has not changed. The aging drift of Michelson tuning has slowed from many m/s per day to about 2 m/s per day. Since this drift appears to be limited to the Michelsons only it can continue to be handled by adjusting the tuning every several months with no degradation of science performance. There has been a measurable but negligible increase in CCD dark level. At the present rate, the CCD will outlive the SOHO fuel supply.

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  3. Main Scientific Achievements (so far)

    The primary MDI science data collection began between 24 April and 23 May depending on the observable. For the primary helioseismology datasets we have now examined the first 5 months (end May through end October) and we have completed preliminary quality checks of the 2-month continuous run. Initial results for the first 60 days were reported at the IAU Symposium 181 in Nice in October (20 papers). Additional results for the first 150 days were reported at the December AGU meeting in San Francisco (8 papers). Additionally special observations obtained during the commissioning phase of the mission (January through mid April) have also resulted in presentations and publications. First result papers have also been submitted to Solar Physics for inclusion in the SOHO First Results issues. All together MDI observations have been reported in at least 50 presentations with 23 papers submitted or in press, and 4 articles in less technical journals. Some of these results are described here.

    The science productivity of SOI/MDI can be discussed in four parts: Global Helioseismology, Local Helioseismology, Associated Objectives, Collaborative Science. The global and local helioseismology together address the primary SOI science objectives but are addressed separately due to the differing data requirements. In the discussion below the names of the individual investigators have been omitted for brevity. The work has been accomplished by members of the SOI investigators team which includes people at many institutions in the US and Europe. Please see the attached lists of publications and presentations for the names of the particular investigators responsible for the results described below.

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  4. Operations

    The MDI instrument can be operated in several modes depending on telemetry opportunities. These modes provide observations suitable for differing science objectives. These modes, in order of their priority for SOI are:

    1. Two- to three-month long "Dynamics" mode operations. This mode uses the VC2/VC3 combination to achieve nearly continuous (>95% ) coverage for 60- 90 days. This is the prime helioseismology opportunity for studies of the convection zone, dynamics of convection, magnetic active region formation, etc.

    2. Continuous "Structure Program" mode in the 5 kbps channel. This mode provides data required for a number of the SOI science objectives including global structure and rotation, limb shape, flux budget, low frequency oscillations, etc. This mode is nearly "free" since the telemetry recovery is handled along with the rest of SOHO and adds no requirements beyond those of the other helioseismology experiments.

    3. Two- to three-day long "extended campaign" or "dynamics" mode operations. This mode uses the VC2/VC3 combination to provide nearly continuous coverage for typically 80 hours. This provides opportunities for local helioseismology and associated science studies using the high resolution field of view. The multi-day coverage allows tracking of features during their transit across the high resolution field of view and provides several 16-hour contiguous runs for local helioseismology "time-distance" and "rings" analyses.

    4. Synoptic magnetic program. This mode provides the 96-minute cadence of full disk 2-arc-sec magnetograms. The magnetograms are proving highly useful for correlative data for the helioseismology, for studies of magnetic field development, and as correlative data for other experiments. This mode requires 10 minutes of VC2/VC3 during each DSN pass.

    5. Daily 8-hour "Campaign" mode operations. This mode uses VC2/VC3 telemetry for intervals of 8 to 12 hours each day. This mode provides the primary opportunity for the SOI "associated" science objectives. Several combinations of observables, resolution, sub-rastering, etc. have been developed and are available for SOI associated science and for collaborative investigations.

    The mix of modes actually to be used during an extended mission would depend on the funding available for operations and for science analysis and on available DSN support for VC2/VC3. The optimum science return can be achieved by continuing operations in the same mode as at present. If the extended mission will be supported at nearer half the present funding level than the present level (as has been suggested by NASA) the operations of MDI would be severely restricted. The operations requirements for both scenarios are described below.

    MDI Operations requirements for optimum extended mission

    The optimum use of the SOI resources (MDI, the SOI data facilities and the SOI operations team) would be to continue to operate MDI nearly as it is now operated. We expect the software development efforts for science operations (including data processing through to calibrated datasets) to have ceased long before the extended mission interval would begin. While continued maintenance will be necessary there will be some significant cost reductions in science operations even while retaining full use of MDI. Some of the savings will come from freezing the development of new “observations” with the associated testing and processing code development. We are presently evaluating the efforts involved with taking VC2/VC3 data directly from JPL which would add data reformatting and data capture tasks but perhaps simplify the bookkeeping tasks. The costs of extended operations in a full-use of MDI scenario will be studied over the next 6 months.

    1. Minimum hours/day of NRT commanding required

      Under normal circumstances, the bulk of MDI commanding can be accomplished in 30 to 60 minutes of NRT commanding per day. Software reloads and calibrations typically take 2 to 4 hours to accomplish, but can be scheduled well in advance of the planned activity.

    2. Minimum hours/day of NRT commanding acceptable

      For nominal operations, much of the MDI commanding could be accomplished by Delayed Commanding. This change would require several months of development and testing by the MDI operations team to insure that all activities can be adequately supported. In addition, this approach would severely decrease the MDI operational flexibility.

    3. Instrument team presence at the EOF

      Continue as now.

    4. Instrument team presence at the EAF

      A minimum of two offices are required; one for Julia Saba and one for Craig DeForest. An additional office would be useful for visitors.

    5. Minimum acceptable data recovery rate

      The nominal data recovery rate for VC0/VC1 telemetry should remain better than 99.5%, or the MDI helioseismology studies will suffer. The data recovery for the MDI VC2/VC3 telemetry should be better than 95%; the limiting factor has been recovery of all the data collected by the DSN sites.

    6. Is it acceptable to reduce PACOR Quick-Look/Real Time data requirements?

      It depends which requirements are being considered. At this time we are investigating whether the MDI high rate telemetry (160kbps channel) can be delivered directly from JPL to Stanford. This has certain implications on the data recovery and the effort at Stanford to insure that data is received in a timely fashion. The need for PACOR processing of the VC2 data might be eliminated. The need for Real Time data during contact times would remain.

    7. MDI only: VC2/3 requirements

      a) Routine: The present mode with c. 10 minutes VC2 on short passes and 1 hour during long passes would continue. The remainder of the long pass coverage would be VC3.

      b) Campaign: For continuous contact times, i.e., yearly 2-3 month intervals and monthly 3-day intervals, the VC2 requirement could reduce to 10-20 minutes twice per day.

    MDI Operations requirements for a severely restricted extended mission

    As described above, the SOI primary science objectives would have priority to the exclusion of associated objectives. At present about 40% of the SOI support covers instrument operations and data processing through to archived calibrated datasets. If the present full-use mode of operations were to continue but the total funds were only half that available now the funds for science analysis would be about 1/6 of the present level. It would be pointless to operate the instrument at the full level but do only 1/6 of the science analysis, so in the underfunded case operations would be scaled back to allow science analysis for the primary science objectives to continue at least at a reduced level. In the reduced mode there would be no support of JOPS unless they required simply the primary SOI data. There would be no further development of observing sequences. This would allow shutdown of the MDI simulator presently used for verification of command sequences as well as simplified operations. In terms of the MDI operating modes described above, 1 through 3 above would have priority with modes 4 and 5 likely abandoned. The MDI instrument can operate nearly unattended in mode 2. We would support operations from the EOF only during intervals of continuous multi-day contacts. The MDI "science operations" would be handled from Palo Alto so there would be no scientist presence at the EOF. In this mode the science output of MDI would be a tiny fraction of the potential but at least the most unique helioseismology capabilities would continue to be utilized.

    1. Minimum hours/day of NRT commanding required

      Much of the MDI commanding would be accomplished by Delayed Commanding. NRT commanding would be required only to deal with anomalous situations.

    2. Minimum hours/day of NRT commanding acceptable

      NRT commanding would only be used in anomalous situations.

    3. Instrument team presence at the EOF

      Remote operation from home institutes.

      EOF presence would be only to handle severe anomalies. We would leave a workstation but would have no normal presence.

    4. Instrument team presence at the EAF

      There would be no SOI presence at the EAF.

    5. Minimum acceptable data recovery rate

      The nominal data recovery rate for VC0/VC1 telemetry should remain better than 99.5%, or the MDI helioseismology studies will suffer. VC2/VC3 recovery during continuous contact times should be better than 95% as it is now. There would be no VC2/VC3 except during continuous contact times.

    6. Is it acceptable to reduce PACOR Quick-Look/Real Time data requirements?

      See note in normal operations section, but note the VC2 data volume would be limited to 10 minutes each pass.

    7. MDI only: VC2/3 requirements

      a) Routine: VC2 would be required for c. 10 minutes on each pass. There would be no MDI VC3 use on long passes.

      b) Campaign: For continuous contact times, i.e., yearly 2-3 month intervals and monthly 3-day intervals, the VC2 requirement could reduce to 10-20 minutes twice per day.

  5. Education and Public Information The SOI project recognizes the importance and responsibility to provide general interest and educational material. As the ultimate source of energy for our environment, the Sun affects each of us every day. Observations of our closest star can be appreciated at many levels. Discoveries in solar physics are exciting and relevant to the general public and can be a fascinating area of investigation for students of various ages.

    SOHO has been observing the Sun continuously in unprecedented detail since December 1995. SOHO images of the Sun and its atmosphere are available on the world wide web almost as soon as they are transmitted to Earth. In particular, images from the MDI show changes in the Sun's magnetic field, surface motions, and brightness. These and related SOHO data are ideally suited for development of a web-based educational outreach program.

    We are developing graphical and video material and a set of web-based activities to interest, inform, and involve people outside the project in the study of the Sun. The target audiences include elementary and secondary school students as well as the general public.

  6. Publications of Results from SOI/MDI Observations

  7. Presentations of Results from SOI/MDI Observations

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