I. Contents

• I. Contents
• II. Previous Senior Review
• III. MDI Science Contributions
• IV. Proposed Investigations
• V. MDI Instrument Status
• VI. MDI Data Processing
• VII. MDI Publication Record
• VIII. MDI EPO Activities

II. Previous Senior Review

The SOHO Prime Mission was from April 1996 through April 1998. The SOHO program was reviewed along with other operating missions in the NASA SEC Theme in June 1997. The result of that review was approval to continue SOHO operations and science analysis for what was called the SOHO Solar Maximum Science Program. Part of the reccommendation was for the instrument team co-investigators to be funded via an expanded Guest-Investigator program. While the SOHO instrument teams were funded at or near the necessary levels to accomplish the goals of the solar maximum program the Guest Investigator program was not adequately funded. As a result, a number of the analysis projects we counted on proceeded on shoestring budgets with fewer results than had been hoped. Nevertheless, we as a team, have made significant progress toward the goals outlined in 1997 and have made discoveries in areas not anticipated in the prior review. The near loss of SOHO in 1998 and resultant intermittent operations into Feb 1999 also caused a significant loss in momentum and a break in continuity that delayed some of the investigations requiring continuity such as the g-mode search and the cycle evolution of fields and flows.

The Previous Review Links contain the SOHO Solar Maximum Science Program proposal and the report and reccommendations of the Senior Review Panel. The proposal outlined major accomplishements for helioseismology as:

• Helioseismology
  • The first ever image of the convection zone of a star shows that convective cells in the Sun are shaped like pancakes, not spheres as assumed in mixing-length theory
  • Differential rotation continues in depth to the bottom of the convection zone.
  • Below the convection zone the Sun rotates as a solid sphere.
  • There is a narrow shear zone at the bottom of the convection zone where the interior is very turbulent - this is the most likely region for the generation of the solar dynamo.

• Helioseismology
  • Determine whether we can probe the deepest interior of the Sun with g-modes and low-l p-modes
  • Observe how the interior and convective transport evolve with the solar cycle and study the interaction between the magnetic cycle and the shear zone
  • Understand where and how the dynamo really works

III. MDI Science Contributions

MDI successfully provided observations leading to discoveries and progress in understanding of a number of topics during the SOHO prime mission and extended phases. MDI data has contributed to at least 12 PhD dissertations, made significant contribution to at least 15 symposia and workshops with refereed proceedings, and provided data for at least 458 papers as of March 2001. Of the papers, there were 13, 43, 142, 98, 120, and 42 in 1996-2001 respectively.

Since the 1997 Senior Review MDI contributed to several new discoveries. There is not space here to review them all so only a few are listed here. The full list of papers is linked below.

• New constraints on interior rotation and sound-speed profiles (Schou et al., 1998; Birch & Kosovichev, 1998; Dziembowski et al, 1998; Kosovichev et al., 1997, 1998; Toutain et al. 1998).
• Migration of zonal flows (Schou 1999, Howe et al, 2000b).
• Zonal flows with depth (Kosovichev and Schou, 1997; Schou et al., 1998).
• Slow polar rotation (Birch and Kosovichev, 1998).
• Tachocline shear variations (Howe et al, 2000)
• Meridional flow with depth (Giles et al., 1997; Schou & Bogart, 1998; González Hernández, 1998).
• Meridional flow varies with cycle (Giles dissertation, 1999).
• Precise measurement of the seismic radius (Schou et al., 1997).
• Shape of limb (Kuhn et al, 1998)
• Assymmetries of p-mode spectrum lines, source of oscillations ( e.g. Nigam, et al., 1997, Nigam et al, 1998, Kumar and Basu 1999)
• Relativistic corrections to the equation of state (Elliott & Kosovichev, 1998).
• Studies on nature of supergranulation (Hathaway et al, 2000, Beck & Schou 2000, Beck & Duvall 2001)
• Detection of Giant Cells (Beck et al.,1998).
• Detection of seismic response to flares (Kosovichev & Zharkova, 1998).
• Thermal structure beneath sunspots (Kosovichev, Duvall & Scherrer, 1997).
• Flows beneath sunspots (e.g. Kosovichev et al, 2001).
• Accoustic glories etc. (Donea et al, 2000)
• Farside imaging (Lindsey & Braun, 2000)
• Ordered large scale flows toward active regions (Gizon et al., 2001).
• Upper limits to g-mode amplitudes (Appourchaux et al. 2000)

A number of MDI results have produced interesting visual results. Some of these are available in the MDI "Nice" Image Collection for use as needed by the MDI community.

IV. Proposed Investigations

There are many science investigation topics presently proceeding with the support of MDI observations. The range of topics presently under investigation is similar to the topics in the hundreds of papers published to date. Some sample studies that require continued MDI observations in the declining and near minimum phases of the present cycle are described here.

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• We have evidence of large scale organized subsurface (near surface) flows toward active regions. We have samples from 97, 98, 99 from MDI dynamics where there was data and activity. 96 was too quiet and 2000 had not enough data to make a clean statement. These samples are only 2 rotations each so we do not yet know how these flows evolve and relate to the growth/decay phases of activity. But some samples in the declining phase are important to obtain - else we have to wait until 2012 to get that info - and it may be very helpful to understand the emergence and decay processes of active regions.

  Flows Toward Active Regions

  Horizontal flow map obtained for part of rotation 1949 (in 1999) after subtraction of a smooth rotation background. Arrows are plotted every 3.84 degrees. Mean travel times shorter than average (dark shade, up to 3% relative change) are associated with magnetic activity. (From Gizon, IAU 203, 2000)

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• The structure beneath surface fields

  The increase in wave speed beneath magnetic fields is now well established (Kosovichev, et al., 1997, Lindsey & Braun 1999). First seen in pre-SOHO observations by Duvall et al. (1993), and familiar to many via the "Coffee-Cup Picture" featured on the early NASA Living With A Star documents, the faster wave speed is seen beneath spots, plage, and pores.

  Wave Speed Beneath A Sunspot

  The accoustic wave speed beneath the spot in the high resolution field of view in June 1998. Redder colors are faster and bluer colors are slower. Detailed views show "fingers" of faster speed connect nearby same-polarity pores at a depth of about 3 Mm. This image is also available in postscript.

  Work done by Junwei Zhao confirms suggestions from Duvall and Kosovichev that there is an ordered flow beneath large spots. This data is from J. Zhao, A. Kosovichev, and T. Duvall (2001) from a new analysis. It is consistent with a previous analysis using a less robust technique bu Kosovichev et al (2000) which resulted in the "coffee cup" figure.

  Flows Beneath A Sunspot

  Motion beneath a spot. These figures show material flows at the depth of 0-3 Mm (a) and at 6-9 Mm (b). The data is from the 19 June 1998 spot in the MDI high resolution field. The outline of the sunspot umbra and penumbra is shown. The colors are vertical motion with positive values (red) corresponding to downflow. The arrows are horizonatl motion. The longest arrows correspond to 1.0 km/s in (a) and 1.6 km/s in (b). Also in postscript.

  Vertical cuts through the 19 June 1998 sunspot. The arows show flow speeds with the longest arrows representing 1.4 km/s. The combination of figure 2 and 3 show a converging flow pattern in the first 3-4 Mm beneath the spot and a diverging flow from 6 to 9 Mm. There is a downflow centered on the umbra near the surface turning into an upflow below about 9 Mm. A weak upflow is also seen at the edge of the moat flow in the upper layers. Also in postscript.

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• We have evidence of variations in the shear at the tachocline and some suggestions of variations in poleward flow and perhaps longitudinal variation in the near surface shear. These are all important to understand the cycle and may vary in different phases of the cycle. Again, we would very much like to see what happens in the declining phase/begining of next cycle which happens in the next 2-4 years.

  The tachocline (shear layer at the base of the convection zone) near the equator shows variations in the strength of the shear with about a 1.3 year period. This variation is not yet understood and its implications are not yet understood. It has withstood a number of tests of reality and the result remains intact. We await more years to see what happens in the declining phase. The figure below is from Howe et al (2000).

  Variations in Shear at the Tachocline

  The cutaway shows rotation shear strength. In the inner region red and blue show faster and slower rotation while in the surface layers red and green are used to show slower and faster. The variations near the base of the convection zone are up to 20% of the shear. A mpeg movie shows this variation in time.

  The existance of a poleward flow of 10-20 m/s has been known for 2 cycles but only with MDI observations has it been well characterized and observed to extend at least 10% into the convection zone. See e.g. Giles et al. (1998) or Hernández et al. (1998) for details. There is a suggestion in the data that the poleward flow extends only to lower latitudes in the years nearer to maximum. This possible variation with the cycle is a topic of current investigation.

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• We know that the surface and near surface rotation at the poles shows some variation over the cycle - from comparing Big Bear helioseismology data with MDI data. What we can now learn is just what this change is and how it relates to the uniformity of the polar fields. This is in part related to the zonal flow study in the upper part of the convection zone - accessible to MDI but not from the ground. Birch and Kosovichev (1998) have examined the near-pole rotation and look forward to extending the study into the declining phase. (GONG+ may also be able to contribute here.)

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• Lindsey and Braun (2000) demonstrated that phase sensitive accoustic imaging techniques (essentially the same as the time-distance method) can be used to image magnetic regions on the farside of the Sun. We have just begun to experiment with making farside flux estimates on a daily basis using MDI medium-l continuous Dopplergram data. This data is available each day in near real time and can be used to detect changes in activity a week or more before it rotates onto the Earth facing solar hemisphere. The images here show the whole Sun 27 and 13.5 days before region 9393 was at disk center. It is clear that this region, which produced a strong geo-effective CME and the largest flare in more than a decade (aurora in Mexico), was visible for more than a week before it rotated onto the disk. There was little hint of it when that longitude was Earth-facing in the prior rotation. We would like to learn how to make these predictive images better and to study if they can be of use in planning for observations as well as learn if they are useful for projecting activity levels for other purposes.

  Farside Image of March 2001 Region 9393

  	1 March 2001. Carrington map of magnetic flux (Earthside) and inferred flux (farside). Region 9393 will be at Carrington longitude 154 Latitude 17N. There is no sign of the large region on this date.
  	15 March 2001. Carrington map of magnetic flux (Earthside) and inferred flux (farside). Region 9393 was at Carrington longitude 154 Latitude 17N. The region was first detected on the farside on 12 March and could be seen to be large on the 13th.
  	30 March 2001. Carrington map of magnetic flux (Earthside) and inferred flux (farside). Region 9393 was at Carrington longitude 154 Latitude 17N. The region contained the largest spot complex of the cycle (to date).