GONG instrument and science

Abstract

The Global Oscillation Network Group (GONG) is an international project to study the solar interior using helioseismology. The major goal is to accurately measure those modes of solar acoustic oscillations that have global characteristics in order to diagnose solar structure and dynamics throughout most of the interior. Starting in October 1995, observations of solar oscillations have been obtained with a duty cycle averaging 90% using a network of six identical instruments sited around the world. These instruments produce full-disk images with a pixel size of about 8 arc seconds every 1/6 second. The Doppler shift of a photospheric spectrum line is encoded into three intensity images as the phase of a sinusoidal intensity variation. Calibration of instrumental effects is difficult, but is adequate for mesurements of the acoustic oscillation. The instrumental noise level is less than solar sources at all temporal and spatial scales necessary to meet the main science goal. The statistical properties of individual modes of oscillation make it difficult to accurately measure characteristic frequencies, amplitudes, etc. without averaging over some domain. GONG is providing large quantities of unaveraged mode characteristics for the first time. The properties of the interior inferred using the unaveraged GONG data agree with previous determinations in general, but there are some interesting differences.