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THE GLOBAL Oscillation Network Group, GONG, aims to monitor the Sun’s
oscillations continuously, using sites scattered around the world. Provided
that interruptions are random, rather than regular, and take up less than
95 per cent of the observation time, researchers can remove their effects
from the data to reveal the genuine solar vibrations.

In principle, this needs three sites, spaced 120 degrees apart on the
Earth’s surface, skies that never cloud, and instruments that never break
down. In practice, GONG will use six sites, spread as evenly as possible
around the world.

The researchers are testing 10 sites, at Mauna Kea and Haleakala on
Hawaii, Mount Wilson and Big Bear in California, Yuma in Arizona, Cerro
Tololo and Las Campanas in Chile, Izana in the Canary Islands, Udaipur in
India, and Learmonth in Western Australia. In addition, there is a ‘reference
instrument’ at the US National Solar Observatory in Tucson, Arizona, which
is the parent site for the whole project. The astronomers testing the instruments
have already had several runs of a few days, and one of more than a fortnight,
when the Sun never set on their prototype network.

At each site, there is an automatic 50-millimetre instrument that will
take a snapshot of the Sun every minute. The aperture on this instrument
is so modest that astronomers hesitate to call it a telescope. The design
philosophy of the network resembles that for a space mission, using simple
rugged instruments that work without any help from a human operator. Two
instruments will look at the Sun at any one time, to cover for the inevitable
cloudy weather or breakdowns. They will measure the speed of the Sun’s surface
as it ‘breathes’ in and out using a Fourier tachometer, which can measure
doppler shifts as small as one part in a billion.

The unique feature of the instruments in the GONG network is that they
measure velocity at about 65,000 points across the surface of the Sun simultaneously.
This will make it possible for researchers to study tens of thousands of
different modes of vibration.

The whole network should become fully operational in 1991, when it will
generate a gigabyte of data every day. The raw data will be stored on tape,
then transferred to optical discs after initial processing. As a hedge against
disaster, the GONG team will keep master copies of the raw and processed
data at Tucson and some other site, not yet chosen.

Handling and analysing the data will be at least as big a task as setting
up the network itself, and the project has already been delayed by funding
problems. So far, it is funded for only three years. The core money comes
from the US National Science Foundation, but the project involves more than
150 scientists from 61 research centres in 15 countries. To them, the guaranteed
three-year life of the project barely scratches the surface of what could
be done.

Ironically, the complete GONG network will be up and running just too
late to study the next peak in the Sun’s activity, expected during this
year. And having established a global network to monitor solar seismology,
it seems worthwhile to keep it running long enough to gather data over one
complete, 11-year, solar cycle. That will need money for a further eight
years of gathering data, and a comparably large and expensive effort to
reduce those data to useful information about our nearest star.