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fig. 6.6 A small segment of a ridge in a diagnostic diagram that was constructed from three years of GONG data. The degree L is plotted vertically, and the frequency (from 1750 to 2000 microhertz) is plotted horizontally. The splitting of L-modes into M-modes is easily seen. The darkness of a line indicates the strength of the mode.

ertheless, helioseismologists have hungered for a satellite of their own since the early 1980s.

They finally got a share of one in the Solar and Heliospheric Observatory (SOHO) (fig. 6.7). SOHO is an international project led and built by the European Space Agency. NASA launched it on December 2, 1995, and the craft now orbits about the point in space (the so-called Lagrange point) about a million miles toward the Sun, where the gravitational pulls of the Sun and Earth are balanced. From there, the 1850-kilogram satellite has an uninterrupted view of the Sun. It has enough rocket fuel onboard to maneuver it for twenty years, but no decision has been made on how long to continue to take data. SOHO carries twelve instruments. Of these, nine carry out coordinated studies of the corona and solar wind. The remaining three experiments were built to study solar oscillations. Each experiment has a Principal Investigator (PI), who is the leader and spokesperson for a large team of scientists, engineers, and technicians. He or she won a place on SOHO through a rigorous competition of ideas and designs. To get onboard, each PI must agree to share the reduced and calibrated data with anybody who asks for it. In practice, the data are generally available over the World Wide Web, but one has to know what to ask for. This policy of instant availability is a departure from the old practice of reserving the data for the experimental team alone for at least a year, so that it could be the first to publish scientific results.

These experiments are not cheap. If an instrument fails in orbit, it can seldom be resurrected, and thousands of man-years of work are then lost. So absolute reliability, with redundancy, must be built in. The quality controls are slow, exacting, and expensive. A typical price tag reads "ten million dollars." Let's have a look at these crown jewels.

Global Oscillations at Low Frequencies (GOLF) is essentially a resonance-scattering experiment, similar in concept to the potassium vapor cells favored by the Birmingham group for their BISON network (see details in note 4.2). It views the Sun as a star, with no spatial resolution over the solar disk. Its main purpose is to probe the solar core with long-period sound waves and, hopefully, gravity waves. GOLF was designed to record low-degree oscillations with periods as short as two minutes and as long as one hundred days.

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