The Solar Oscillations Investigation

The Solar Oscillations Investigation (SOI), directed by Philip Scherrer, has moved from the development phase to the flight phase. SOI is a project to study the interior of the sun by using the techniques of helioseismology. A primary component of the SOI program is the Michelson Doppler Imager (MDI) instrument which is part of the Solar and Heliospheric Observatory (SOHO), a joint project of NASA and the European Space Agency. SOHO was launched insto space last December and is now located near the Sun-Earth L1 Lagrange point, where it will observe the Sun continuously fort the next several years. The SOI program started in 1987, and the flight instrument was delivered in 1994. Design and fabrication of the MDI instrument was a joint project of the SOI team at Stanford's W.W. Hansen Experimental Physics Laboratory (HEPL) and the Lockheed-Martin Palo Alto Research Lab. The Lockheed group, led by Alan Title (Consulting Professor, Stanford Center for Space Science and Astrophysics), has built and flown a number of very successful solar observing missions during the past two decades. The Stanford team is responsible for the overall project direction, for the data processing, and for the coordination of the SOI science team, which includes Co-Investigators at ten other universities and laboratories as well as an extended team of more than 50 associated investigators in the U.S. and abroad.

Convective Flows Below the Sun's Surface -- A vertical cut through the outer 1% of the sun showing flows and temperature variations inferred by helioseismic tomography.

Helioseismology is a method of studying the interior of the Sun by analysis of observable oscillations in both brightness and velocity in the Sun's surface layers. From the perspective of helioseismology, the Sun is a sphere filled with acoustic waves, propagating throughout its interior. The Sun is essentially "ringing" simultaneously in about 10 million normal modes of oscillation. Each of these oscillation modes has a different frequency and spatial distribution. By measuring the motion of the solar surface of a grid of a million points once each minute for several years, these frequencies can be measured. The observations are made by measuring the Doppler shift as seen in the light of a red absorption line of nickel. Once the motions are measured each minute, the data can be analyzed to determine the mode frequencies. Since each of the modes "samples" a differing region inside the Sun, maps of sound speed (temperature), rotation, relative abundances of the elements, and convective motions can be inferred.

A million numbers per minute for months to years amounts to a lot of data. During the processing to determine the structure and dynamics of the solar interior, many intermediate data sets must be generated, making the data handling problem even worse. Since the MID instrument began continuous observations last April, we have archived more than six million megabytes in about 150 thousand datasets. Some of the initial results were presented last March and June in press conferences at NASA headquarters, at ESA headquarters in Paris, and at the American Astronomical Society meeting in Madison, Wsiconsin. The second figure shows the results reported in Madison. This figure shows the first measurement of temperature variations and convective motions down to 7000 km beneath the solar surface. This is about one percent of the distance to the Sun's center. The convective cells mapped by the arrows are the Earth-sized supergranulation cells that have previously only been observed on the Sun's surface. Continuing analysis of this and other MDI data has shown us a new picture of the evolution of magnetic fields in the Sun. Much work remains to be done both in continuing observations and analysis at Stanford, Lockheed, and other partner institutions. We presently anticipate at least two more years of SOHO operation with many opportunities for research into the operation of our nearby star.

More information, including samples of current observations from MDI and the other eleven experiments on SOHO can be found on the SOI Web page at: http://soi.stanford.edu.

The SOHO spacecraft before final integration and launch. The MDI optical instrument is located on the upper right hand corner of the right face of the spacecraft.

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