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:
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.
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.
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.
|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)|
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.
|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.
|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.|
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).
|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.
|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).|
CMEs are only geoeffective when they contain or drive several hours of southward Bz when they arrive at Earth. Estimates of the orientation of fields pushed by CMEs show that the pre-existing field estimates for the overlying corona make good predictions of the Bz when the locations of Earth directed CMEs are known (e.g. Zhao & Hoeksema, 1997). Recent developments in processing of MDI magnetograms has led to the concept of "synoptic frame" where synoptic whole-Sun Carrington grids of magnetic field are suplemented by rapidly updated magnetic information for the visible disk. Initial analyses of these data are promising and may lead to better estimates of the sign and strength of the Bz component of the field carried by CMEs. Tools to use this data is presently being developed at UC Berkeley. See: sample use. The synoptic maps using the 96minute magnetograms are prepared daily for planning purposes. Some notes on the geo-effectiveness of CME's.
Evidence shows the visible part of the cycle is something like 18 years with a 6-8 year overlap between cycles. Altrock (Sol. Phys., 170, 411, 1997) reviews the literature and adds new evidence for the 19-20 year duration of each 11-year cycle. Each new cycle can be detected in magnetic polarity at high latitude just after max, both in ephemeral regions (e.g. Martin and Harvey) and in large scale measure of torriodal field (Schrauner & Scherrer). [gee it sure would be nice to update the WSO analysis. hmmm - ah, it would take a few days, but which few?] The extended cycle is also seen clearly in the large scale zonal rotation residuals also known as "torsional oscillations". We now see the zonal flows are in at least the top third of the convection zone (see e.g. Giles dissertation, and Howe et al. (ApJ, 533, L163-L166, 2000)) and can watch for the first time to see how they evolve and relate to the appearance of new cycle fields.
|This figure shows the residuals in rotation after removing a smooth equator-to-pole variation. The panels are centered at progressively deeper layers. From Howe et al.(ApJ 533, L163-L166, 2000).|
There are at least three aspects to the extended cycle that can be examined with continued MDI observations into the declining and minimum phases of the cycle.
Many if not most 11-year cycles have a second peak or a delayed peak in geo-effective solar disturbances to the geomagnetic field. The 27-day recurring geomagnetic storms that are common in the declining toward minimum phase of the cycle are the result of low latitude extensions of the polar coronal holes apparently combined with the conditions to create southward Bz at the Earth. While perhaps not an MDI topic, the availability of EIT and LASCO data, combined with MDI magnetograms, can allow study of the solar side of this interesting phase of the cycle.
|This figure shows the number of days per year where the geomagnetic activity index "Ap" exceeded a value of 50. The Ap data is plotted over the sunspot number index. It can be seen that geoeffective solar events are more prevalent in the declining years of the sunspot cycle. Figure is from Joe Hirman at NOAA via Xue Pu Zhao|
After 5 years of operation, MDI is basically working very well. MDI has made about 50 million images. After on-board computations, it has delivered about 8 million raw data images to the ground.
At the end of March 2001, the MDI data processing system, SSSC (SOI Science Support Center), had archived 1.015 million datasets containing 53.6 Terabytes of raw and processed data. The MDI data center has responded to more than 6000 individual requests for data amounting to about 8 Terabytes. Most requests are for multiple datasets. Requests for MDI data processed through the SOHO archives at GSFC and the remote SOHO archives in Europe are not shown here.
Left: Requests for MDI Data exports processed since Sept 1997.
Right: Cummulative volume of data exported from the MDI archive via the Web interface at http://soi.stanford.edu/data
For Dissertations see: Dissertation List
For list of Proceedings see: Conference Proceedings List
For list of Papers see: Paper List
Education and Public Outreach are important adjunct activities of the MDI team. These activities include: