Two instruments on SOHO report on the condition of the Sun's bright face:
- MDI (Michelson Doppler Imager)
- VIRGO (Variability of Solar Irradiance and Gravity Oscillations)
MDI (Michelson Doppler Imager)
MDI gauges the brightness at a million points and monitors the dark sunspots and bright faculae. It measures magnetic fields, too, across the visible surface, producing the best maps and movies of solar magnetism ever made. These show that the pattern of surface magnetism changes completely in less than two days. MDI also sees shock waves radiating outwards from the scene of a solar flare, like the ripples seen when a stone falls into a pond.
VIRGO (Variability of Solar Irradiance and Gravity Oscillations) measures the Sun's total brightness, or irradiance, and its intensity in red, green and blue light. It records many small variations in sunlight from minute to minute and day to day. After SOHO's launch in 1995, when the count of sunspots was at a minimum, the average intensity increased towards the year 2000 and the sunspot maximum. The brightening seen by VIRGO confirmed variations reported by previous spacecraft. The change was less than 0.1 per cent, and no long-term change in brightness is yet apparent in data from space.