The aim of the VIRGO investigation (Variability of solar IRradiance and Gravity Oscillations) on SOHO (SOlar and Heliospheric Observatory) is to determine the characteristics of pressure and internal gravity oscillations by observing irradiance and radiance variations, to measure the solar total and spectral irradiance and to quantify their variability over periods of days to the duration of the mission. VIRGO contains two different active-cavity radiometers for monitoring the sol ar 'constant' (DIARAD and PM06-V), two three-channel sunphotometers (SPM) for the measurement of the spectral...

The SOLAR-A spacecraft was launched by the Institute of Space and Astronautical Science, Japan (ISS) in August 1991, and, following and ISAS tradition, was renamed YOHKOH. This mission is dedicated principally to the study of solar flares, especially of high-energy phenomena observed in the X-ray and gamma-ray ranges. With a co-ordinated set of instruments including hard X-ray and soft X-ray imaging telescopes as well as spectrometers with advanced capabilities, it is expected to reveal new aspects of flares and help better understand their physics, supporting international collaborations...